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MEDICAL GENETICS
Basic concepts for Medical Students
Aims:
Every medical student should be aware of the impact of hereditary, genetic and other
related disorders as they constitute a big burden on health care providers. Although
not all of them are common, as some are rare, collectively they cause a great deal of
morbidity and mortality as well as difficulty in diagnosis in many occasions.
Learning objectives:
By the end of this chapter, the student must be able to:
1. Value the
2. Classify genetic disorders (including birth defects).
3. Discuss anatomy, physiology and pathology of chromosomes and genes on
subcellular or molecular level.
4. Detect how each type produces the signs and symptoms of diseases or
syndromes.
5. Define autosomal, sex-linked, recessive, dominant and how they are produced.
6. Classify birth defects and discuss its causes, types, and classification.
7. Recognize the basis of genetic counseling.
8. List the main tests available, with indications and interpretation of results.
BEFORE YOU START THIS CHAPTER,
PLEASE REVIEW YOUR
BIOLOGY, EMBRYOLOGY, AND BIOCHEMISTRY
in relation to:
1. Mitosis and meiosis including all steps and stages.
2. RNA and DNA and chromosome structure, DNA transcription & translation,
cell cycle, patterns and laws of inheritance (Mendel's Laws).
3. Symbols of family pedigree
4. Notes on DNA repair system.
5. Protein structure, synthesis, types (enzymes, hormones, etc.) and their
functions, etc.
6. Normal development from fertilization till birth (Review your embryology).
NOTE: What is found in BETWEEN BRACKETS AND IN GREEN is NICE TO
KNOW, the rest is essential to know, i.e. not knowing it fails you.
IN THIS CHAPTER, YOU ARE GOING TO STUDY THE FOLLOWING
TOPICS:
1. Introduction to genetic disorders "genetic vs. acquired disorders +
Significance of genetic disorders "statistics about incidence and impact on
health care providers" + what makes genetic and inherited disorders
different from other disorders? "History, physical examination, family
History, probabilities and recurrence".
2. Classification of genetic disorders.
3. Genetic basis of disease:
a. Chromosomal disorders + examples
b. DNA mutations + examples
c. Multifactorial disorders + examples
d. (Non-classical genetic disorders).
4. Features of AD, AR, X-linked
(and mitochondrial inheritance)
with their
particular family pedigree.
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5. Teratogenesis & congenital malformation (birth defects) with clinically
important examples.
6. Prenatal diagnosis & Genetic counseling.
7. Basics of genetic testing (principles, indications, clinical significance, and
interpretation of results):
a. Chromosomal study,
b. In-situ hybridization and
c. Molecular study (PCR)
8. You may encounter the following topics (and more) in your future clinical study;
they have a great deal of genetic basis and are good examples to understand and
correlate with what you'd known in this chapter:
a. Mental retardation
b. Growth retardation
c. Abnormal sexual differentiation / genital ambiguity
d. Approach to a child with coarse features
e. Approach to a child with a major or minor malformation].
f. Antenatal care for mothers and unborn babies especially for high risk group.
g. Radiological signs in prenatal Dx (US, MRI, CT, etc.)
h. How to inform a family about their child with inherited disorders
i. Genetics as an integral part of all medical specialties.
9. Advances in molecular biology testing and their clinical applications [gene therapy,
DNA recombination, stem cells, and modern diagnostics: real-time PCR (RT-PCR),
microarray, comparative genomic hybridization (CGH), etc.] are nice to know
topics that seem promising in the coming years.
References:
1. Elements of medical genetics. By Emery & Rimoin, 2010.
2. USMLE road map: Genetics. BY George H. Sack, 2008 (or later).
3. Medical cytogenetics. By Hon Fong L. Mark, 2000.
4. An Introduction to Human Molecular Genetics: Mechanisms of Inherited Diseases,
Second Edition. By: Jack J. Pasternak, University of Waterloo, Ontario, Canada,
2005.
5. Other references & websites as referred to in place.
6. Just google your keyword(s) and find out what you'll get!! You'll find a lot of
interesting information. I promise.
My advices to you:
You may find this chapter non-orthodox. I hope you enjoy it as much as you
benefit from it.
The chapter may seem lengthy, but many paragraphs are explanatory and
meant to make you understand an idea, and not intended to be memorized.
Always return to your photos on the CD obtained from the dept. of pathology
when studying this chapter.
Do not hesitate to ask at any time (preferably after the lab. Time).
Always return to your basic knowledge here "and go to other references"
when you encounter a disease with a genetic basis in your clinical years and
after graduation.
Your questions & feedbacks are most welcome / send emails to
(
) Dr. Bassam Musa Sadik. I'll do my best to assist.
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What is meant by Medical Genetics?
Medical genetics is the specialty of medicine that involves the diagnosis and
management
of hereditary disorders, i.e. it refers to the application of genetics to
medical care. So, diagnosis, management, and counseling of individuals with genetic
disorders as well as research on the causes and inheritance of genetic disorders would
be considered part of medical genetics.
Various specialties within medical genetics are interrelated:
1. Clinical Genetics.
2. Cytogenetics.
3. Molecular Cytogenetics.
4. Molecular Genetics
5. Human genetics, molecular biology, genetic engineering, or biotechnology.
Introduction:
Human diseases in general, whether medical or surgical, can roughly be classified into
three categories:
1. Those that are genetically determined.
2. Those that are almost entirely environmentally determined.
3. And those to which both nature and nurture contribute.
However, progress in understanding the molecular basis of many so-called
environmental disorders had tended to blur these distinctions. At one time, microbial
infections were cited as examples of disorders arising wholly from environmental
influences, but it is now clear that to a considerable extent, an individual's genetic
makeup influences his or her immune response and susceptibility to microbiologic
infections.
Despite the complexities of this nature-nurture interplay, there is little doubt that
nature (i.e. the genetic component) plays a major, if not the determining, role in the
occurrence and severity of many human diseases. Such disorders are far more
frequent than is commonly appreciated (which represents only the tip of the iceberg).
Around 20% of pediatric in-patients suffer from disorders of genetic origin.
Up to 50% of spontaneous abortuses during the early months of gestation have
a demonstrable chromosomal abnormality; there are, in addition, numerous
smaller detectable errors and many others still beyond our range of
identification.
About 1% of all newborn infants possess a gross chromosomal abnormality.
Approximately 5% of individuals under age 25 develop a serious disease with
a significant genetic component.
Only those mutations compatible with independent existence constitute the
reservoir of genetic diseases in the population.
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Classification of Genetic disorders (important):
1. Classical Genetic Diseases:
a. Chromosomal (Cytogenetic) disorders.
b. Single gene (or unifactorial) disorders (Mendelian Disorders).
c. Multifactorial disorders.
2. Non-Classical Diseases "or the single gene disorders with atypical pattern
of inheritance":
a. Diseases caused by mutations in mitochondrial genes.
b. Triplet repeat mutations.
c. Uniparental disomy.
d. Genomic imprinting.
e. Gonadal mosaism.
Added to that, is a large group of disorders "malformations" that manifest at
birth, called congenital malformations, that many of them are caused by genetic
disorders.
Genetic / Hereditary Disorders
1. Classical Genetic Diseases
A. Chromosomal Disorders:
Review of what you've already known:
Normal human nucleated cells contain 46 chromosomes arranged in 22
homologous pairs of autosomal chromosomes in addition to one pair of sex
chromosomes that could be similar (i.e. XX) or different (i.e. XY). This arrangement
into pairs, based on the position of the centromere and on the length of the upper and
lower arms of the chromosomes, is known as the Karyotype (Figures 6-1, 2 photos).
It is estimated that about 1 of 200 newborn infants has some form of
chromosomal abnormality. The figure is much higher in fetuses that do not survive to
term. Cytogenetic (also called karyotypic) disorders may result from alterations in the
number or structure of chromosomes and may affect autosomes or sex chromosomes.
So, they could either be Numerical or Structural.
A. Numerical abnormalities
are defined as a gain or loss of one or more whole
chromosome(s) (whether an autosome or a sex chromosome) or a whole set of
chromosomes.
The normal chromosome count is 46 (i.e. 2n = 46) as it is arranged in two sets of
chromosomes. An exact multiple of the haploid number (n) is called euploid (normal
state). Any number that is not an exact multiple of the haploid (n) is aneuploid, and it
is one of the commonest changes that take place in malignant tissues.
A gain of one or more set of chromosomes is known as polyploidy. This
polyploidy may be triploidy when cells have (3n) or tetraploidy when the cells have
(4n). Polyploidy generally results in spontaneous abortions. It results from a
pathological condition known as endoreduplication i.e. when there is failure of
cytoplasmic division that should follow the nuclear division and therefore the original
duplicated two sets will remain in one cell.
A gain of one chromosome is a state known as trisomy, e.g. trisomy of some
autosomal chromosomes such as chromosome 21 is called Down‟s syndrome, trisomy
18 is called Edward‟s syndrome, trisomy 13 is called Patau‟s syndrome. Other
autosomal trisomies are not very frequent. Trisomy of the sex chromosome is
exemplified by Klienfelter's syndrome in male (XXY) and triple X (XXX) syndrome
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in female, while a loss of one chromosome is called monosomy. All trisomies and
monosomies are by definitions (aneuploidies).
A gain or loss in the sex chromosomes, especially the X-chromosome, is
compatible with life and is relatively common; while the loss of an autosomal
chromosome is usually non-compatible with life and a fertilized ovum carrying such
a karyotype could not sustain pregnancy to full term and are usually lost early in
pregnancy, i.e. abortion. Monosomy of sex chromosome in a female is called Turner‟s
syndrome (45 XO).
The chief cause of aneuploidy (and thus trisomy and monosomy) is non-
disjunction of a homologous pair of chromosomes at the 1
st
meiotic division or a
failure of sister chromatids to separate during the 2
nd
meiotic division. The latter may
also occur during somatic cell division, leading to the production of two aneuploid
cells. Instead of two homologous chromosomes separating and moving into opposite
poles of the dividing cell, both move to the same pole so that one cell receives both
and the other receives neither. The same may happen to the sister chromatids in
mitosis.
(what is the other less common cause of aneuploidy? find out)
Fertilization of such gametes by normal gametes from the other partner would
result in two types of zygotes: trisomic, with an extra chromosome (2n+1), or
monosomic (2n-1).
A post-zygotic mitotic non-disjunction would result in the production of a
trisomic and a monosomic daughter cell; the descendants of these cells would then
produce a mosaic. Mosaicism affecting sex chromosomes is common, whereas
autosomal mosaicism is not.
(How can you define mosaicism?)
B. Structural abnormalities:
In this case, the cell has a normal number of 46 chromosomes but they are
morphologically or structurally abnormal.
All chromosomes in a normal human karyotype have a long lower arm (known
as q arm) and a shorter upper arm (known as the p arm), so that the centromere is
located at different levels in between (Figure 2).
The human karyotype is divided into various groups of chromosomes according
to the ISCN (International System for Chromosomal Nomenclature) (See Figure 1-1
photo 2).
X chromosome is included within group C, while Y chromosome is included
within group G.
Group D & G chromosomes are called acrocentric chromosomes as they have a
short arm with very small amount of DNA connected to the centromere by a very
slender thread, giving it a satellite or antenna-like appearance. Even when this satellite
is lost, no abnormality appears.
Structural abnormalities usually result from chromosomal breakage followed by
loss or rearrangement of material.
They are of different types as follows:
1. DELETION: involves loss of a piece of a chromosome that could be either:
i. Terminal, in which, there is one break in the chromosome and the portion distal to
this break is lost. This lost piece could be carrying important genes, and its
loss results in signs and symptoms related to the lost genes products (Figure 6-
3 A).
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ii. Interstitial, where the piece of a chromosome between two breaks is lost resulting
in a chromosome that is shorter than the original with the same consequences
e.g. Cri du chat (loss of short arm of chromosome 5) (Fig. 6-3 A & B).
Most of certain genes, known as oncogenes, are lost from the cells through
deletion and their loss will result in development of malignancy of certain tissues, e.g.
loss of retinoblastoma gene by interstitial deletion of the long arm of chromosome 13,
i.e. 13q in which, the newborn carrying this deletion will develop retinoblastoma
(Figures 6-4 A & B). Another gene is lost in deletion of short arm of chromosome 11,
which results in development of Wilm‟s tumor with aniridia (Figure 6-5).
Sometimes, the piece lost carries no gene and therefore it causes no
abnormality. This is due to the fact that the genes are not situated on the chromosome
one besides the other all along the chromosomes but there are parts of the
chromosome carry no genes. In fact, only about 10% of the whole length of DNA
from all chromosomes carries coding sequences.
If individuals, carrying a deleted chromosome, marry, their gametes formed
would be abnormal and results in either normal children, repeated abortions or in the
birth of a child with malformation.
2. INVERSION:
This abnormality results from two breaks in the chromosome and the piece
between the two breaks will rotate 180
o
and is fixed back again in its rotated position.
The breaks may involve either the short arm or the long arm and the inversion is
called paracentric (the breaks are on one side of the centromere), or the breaks may
involve both arms and the inversion is called pericentric (the centromere is involved
in the inversion), which is more severe (Figure 6-6).
Consequences of both types of inversion are the creation of unbalanced
homologues of chromosomes, one carrying a duplicated gene(s) and the other is
deficient in that gene(s). The individual carrying such inversions may, however, be
without apparent signs; yet, his gametes are unbalanced resulting in the birth of an
abnormal fetus or abortion.
3. TRANSLOCATION:
It is defined as an exchange of segments of chromosomes between non-
homologous chromosomes.
(translocation vs. crossing over)
Regular (reciprocal) translocation that takes place between any two
chromosomes [other than the acrocentric group of chromosomes] is one of the main
pathological changes seen in malignancies (Figure 6-7). It has recently been
discovered that these translocations may translocate some oncogenes from their
normal position to a new one where they are induced to function in an uncontrolled
manner leading to override of the body's regulatory control mechanisms, ultimately
leading to malignancy.
Such important genes are usually those coding for the production of a hormone
that is usually needed only in a minute amount and is necessary for the growth
regulating processes in the body. This is usually seen in leukemias and lymphomas,
e.g. Philadelphia chromosome (the product of a translocation between chromosome
22 and 9) seen in chronic myeloid leukemia (Figure 6-8), or a translocation between 8
and 14 seen frequently in Burkitt's lymphoma.
Another type of translocation called Robertsonian (centric fusion) type that
takes place between two acrocentric chromosomes, either one from each group or
two chromosomes of the same group but different number or the pair of a homologue
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where the two acrocentrics lose their short arm and both long arms of the
chromosomes fuse together to form one chromosome (Figures 6-9).
An individual who carries such a translocation have multiple types of gametes.
These gametes when fertilized by the normal gamete of the normal spouse will
result in four possibilities (Figure 6-10):
1. Normal.
2. Normal count (i.e. 46) but carries the translocated chromosome. This
individual or fetus will be trisomic for the acrocentric chromosome, i.e. it
is the source of cases of translocation Down‟s syndrome.
3. Balanced carrier like the original parent with 45 count of chromosome.
4. Monosomic (i.e. 45) and it is eliminated by abortion early in pregnancy as
all autosomal monosomies.
So, an individual who carries a Robertsonian translocation will have three
possibilities of producing a full term baby or a child with one of the 1
st
three
categories listed above.
4. ISOCHROMOSOMES:
This abnormality results from aberrant division of the centromere which is the
last part of the chromosome that divides in the mitosis to separate the two sister
chromatids into individual chromosomes. This aberrant division takes place in a
horizontal way rather than the perpendicular natural way. So, the resulting two
chromosomes are imbalanced, one formed entirely of two short arms and the other of
two long arms. Each of them is called an isochromsome (Figure 6-11).
These isochromosomes may be seen in some cases of Down's or Turner's
syndromes.
5. RING CHROMOSOME:
It results from deletion of both ends of a chromosome and then the ends,
because of the adhesive nature of the exposed DNA, will stick together forming a ring
or a circle chromosome (Figure 6-12).
6. DUPLICATION OF PART OF A CHROMOSOME:
These abnormalities can produce some characteristic signs and symptoms called
syndromes that can be confirmed by studying the number and structure of patient's
chromosomes. This test is called chromosomal study or studying the karyotype.
(karyotype vs. phenotype)
The main indications for karyotype are:
1. Advanced maternal age (>35 years).
2. A family with a previous child known to have a chromosomal abnormality.
3. Clinical suspicion of a syndrome caused by a chromosomal abnormality.
4. Part of males or females infertility screening.
5. To determine sex (gender) of an individual.
6. Unexplained mental retardation.
7. Unexplained growth retardation
8. Some cases of cancer.
9. Miscellaneous conditions.
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Clinical examples of Some Cytogenetic Abnormalities
( Practical Part):
I. Disorders of Autosomal Chromosomes:
1. Cytogenetic types of Down’s syndrome (Karyotype writing):
1. Regular Down’s, which constitutes to 95% of cases of born Down‟s
syndrome. It usually results from non-disjunction of the two sister chromatids
of one homologue or non-disjunction of the „21‟ homologue pair migrating to
one pole, resulting in a gamete with double „21‟ chromosomes (Figures 6-13,
2 photos).
(Writing the karyotype is nice to know) but interpretation of results are essential
How to write such a karyotype?
Karyotype writing starts 1
st
by (a) mentioning the chromosome number, then (b)
mentioning the sex of the patient (XX or XY), followed by (c) the sign of addition
because there is an extrachromosome, then (d) the number of additional chromosome,
i.e. 47XY, +21 (male with regular Down's) or 47 XX, +21 (female Down's)
2. Translocation Down’s, they constitute to 3% of cases of born Down‟s
syndrome.
Karyotype writing follows the 1
st
two steps above, then (c) after a semicolon
write a letter „t‟ (d) then opening an arc, put the number of the two chromosomes
sharing in the translocation with a slash in between:
i.e. 46XY, t(14/21) or t(15/21) or t(22/21) or t(21/21)
46 XX
in order of decreasing frequency
3. Isochromosome ‘21’
Isochromosome of the long arm of chromosome „21‟ may take place resulting
in Down‟s syndrome.
Karyotype writing: the same 1
st
two steps then (c) followed by letter „i‟ after
the semicolon with the number of chromosome causing the isochromosome, (d)
followed by the symbol of the arm [the short arm „p‟ or long arm „q‟].
i.e.
46XY, i(21q)
or
46XX, i(21p)
4. Mosaics: are individuals who have a mixture of cells in their body of different
proportions of two or more different karyotpyes.
Karyotype writing is to write every line or type of karyotype on its own the
same way as the above principles then separating them by a slash,
i.e. mosaic Down of normal karyotype and another with translocation:
46XY, / 46XY, t(14/21)
46XX, / 46XX, t(14/21)
and so on mixing any type of karyotype.
2. Cytogenetic types of Edward's syndrome (Karyotype writing):
Edward's Syndrome (Figure 6-14) or trisomy of chromosome 18 is the most
common type:
Regular Edward's S.: 47XX, +18
Mosaic type: 46XY / 47XY +18
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3. Cytogenetic types of Patau syndrome (Karyotype writing):
Patau's Syndrome (Figure 6-15) is in most cases a trisomy of chromosome 13:
Regular Patau S.: 47XX, +13
Translocation type: 46XY; t(13/21) or 46XX; t(13/22)
Isochromsome type: 46XX; i(13q)
Mosaic type: 46XY / 47XY +13
and so on …
4. Karyotype writing of a deletion syndrome:
Writing the karyotype of a deletion again following the same 1
st
two steps,
followed then by the number of chromosome being deleted followed by the sign of
minus mentioning the sign of the arm affected,
e.g. Cri du Chat syndrome (cat cry syndrome) is formed by partial deletion
of the short arm of chromosome 5 in males or females:
46XY, 5p-
or 46XX, 5p-
5. Duplications of part of a chromosome:
(Figure 6-16)
II. Abnormalities of Sex Chromosomes:
1. Cytogenetic types of Turner’s syndrome
(Karyotype writing):
By applying the same principles here, we may have:
Regular Turner‟s syndrome 45XO
Or Turner's with long arm isochromosome X
[46XX, i(Xq)]
Or a mosaic Turner's 46XX / 45XO
(Figure 6-17)
2. Karyotype writing of other sex chromosome disorders:
Regular Klinefelter's Syndrome 47XXY or 48XXXY
Mosaic klinefelter's S. (seen in 15% of cases) 46XY / 47XXY
or sometimes 46XY / 48XXXY
3. Karyotype writing of Y chromosome disorders:
47XYY (rare disorder) very tall with azoospermia (Figure 6-18)
N.B.: Needless to mention that many other structural chromosomal abnormalities
are present, some of which are more complex than the examples depicted here, e.g.
dicentric chromosome.
Or Just about everything you can imagine which can happen to that piece of string --
does!
For a review and more examples, see Figures (6-19 9 photos).
Pay attention to the presentation in your labs.
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B. Defects of Single Genes with Large Effect (Unifactorial or Mendelian
Disorders):
Introduction:
The number of known Mendelian disorders has grown to more than 5000.
Although individually many are rare, altogether they account for approximately 1% of
all adult admissions to hospital and about 6-8% of all pediatric hospital admissions.
They represent the most common purely genetic abnormality as compared to other
causes.
They are caused by a mutation in a single gene. A mutation is a disturbance in
the sequence of the nucleotide arrangement in the DNA molecule, or it is simply a
permanent change in the DNA. Mutations may give rise to inherited disorders or may
cause cancer or congenital malformation.
(When, Why and How?)
Review:
A gene is that part of the DNA that codes for a polypeptide chain. Only about
2% of the DNA codes directly for information, 24% is intronic sequence (non-coding
sequences within the genes); the remaining 74% is a non-coding sequence outside the
genes (Figure 6-20). The function of this huge amount of non-coding sequences has
not been fully elucidated.
There are about 30,000 genes (in contrast to what was previously thought of
about 100,000). Genes are not arranged one beside the other along the chromosome or
the DNA. There is a variable length of those non-coding sequences in between and
inside the genes. Those found inside the gene is called the intervening sequences (or
the introns), while the coding sequences of the gene is called exon.
Some genes have few exons and some have many. Some genes are very small in
size but some are very large with so many exons.
Some chromosomes are rich in genes but some have only few, which explains
why some chromosome deletions or even duplication have no or minimal clinical
effect.
Genes may behave as dominant, i.e. when only one of the alleles becomes
mutated it results in a genetic disease, or they may behave as recessive, i.e. the
diseases result only when both alleles (of both the maternal and paternal origin) are
affected by the same mutation. A 3
rd
category of genes are those genes which
determine an autosomal character but they are situated on the sex chromosome, called
sex-linked genes. So we have autosomal dominant (AD) disorders, autosomal
recessive (AR) disorders, and sex-linked disorders.
The question arises why some genes act in a dominant manner while others
behave in a recessive one, i.e. the problem of dominance and recessiveness. To
answer this question, one has to consider the following principles that must be
remembered:
It was thought earlier that a single gene is responsible for the formation of a
single type of protein, but since the protein is made up of units of polypeptides that
could be the same or different in one molecule of protein, the principle becomes:
A single gene is responsible for the formation of a single type of polypeptide,
and if we know that out body structures and functions from the moment of post-
fertilization to the full maturity and later on are determined by proteins. The types of
proteins are varied; they could be structural proteins, like fibrous tissue, elastic
tissue; they could be immunoglobulins, signal transducing proteins, receptors,
enzymes, hormones, etc. Therefore, the action of the gene being dominant or
recessive is determined by the type of protein it produces and its function.
(Codominance means that both alleles of the same gene are fully expressed in the
heterozygote e.g. HLA and ABO blood group antigens are good examples).
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A single gene mutation may lead to multiple phenotypic effects, a phenomenon
called pleitropy [e.g. Marfan's syndrome where there is skeletal, cardiovascular, and
eye defects]; conversely, mutations at several genetic loci may produce the same trait
[e.g. retinitis pigmentosa] in a phenomenon called genetic heterogeneity.
Dominant genes usually produce these types of proteins:
1. Major structural (or non-enzymatic) proteins, which form or are present
in many parts of the body (e.g. collagen, spectrin, fibrillin, elastin, etc.);
examples are cases of achondroplasia & Ehler Danlos syndrome (lax joints
and skin).
2. A key enzyme in a complex metabolic pathway usually under feedback
control (e.g. AD porphyria), or
3. A membrane receptor regulating a metabolic pathway (e.g. AD familial
hypercholesterolemia disease) where the receptors for LDL are mutated.
They are responsible for regulation of LDL in the cells and the circulation.
4. or a membrane transport protein.
To explain the latter example and how the action of this pathway is executed, let
us consider the pathway of circulating LDL.
LDL should enter the cells of the body for building cellular membrane and
nuclear membrane from wear and tear. It could not enter the cells unless it is
complexed with receptors on the cell membrane.
Once it is inside the cell, the complex will be degraded into free cholesterol and
amino acid, which is the remnant of the proteinous coat of the liporptoein. The free
cholesterol in the cell constitutes the cholesterol pool of the cell, which its level is
regulated by three systems of enzymes, the HMG-CoA reductase (3-hydroxy-3-
methylglutaryl coenzyme A reductase), which forms cholesterol from fatty acids,
ACAT (acyl-CoA:cholesterol) transferase, which hydrolyze cholesterol into ester
rendering it inactive and the number of the receptors on the surface of the cell. If the
pool concentration is low, messages are sent to activate the HMG-CoA reductase,
inactivate ACAT and increase the number of receptors on the cell surface. Therefore,
when one of the two alleles responsible for the formation of the receptors protein, half
the number of receptors are formed only, so 50% of LDL which is used to be
internalized inside the cell will remain in the circulation unable to enter the cells and a
state of hypercholesterolemia results with reading of 400-500 iu/dl of cholesterol in
the blood (Figure 6-20).
A defect of one single copy of a dominant gene produces a great deal of
abnormalities that produce signs and symptoms of a disease related to the function of
the deficient protein.
As for recessive genes, they produce enzymes which usually share in catabolic
pathways (usually non-key enzymes); when both alleles are defective, there is no
protein, i.e. no enzyme and therefore the catabolic pathway is obstructed with the
accumulation of the biochemical substrate; while loss of one single copy is usually
compensated for by the remaining active and normal gene (50% protein present).
Examples of those diseases are some of diseases of thalassemia,
mucopolysaccharidosis, lipidosis, phenylketonuria (PKU), and many others.
(What happens if two copies of a dominant gene are defective?)
(What causes the appearance of signs / symptoms to appear in a recessive disorder?)
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Sex-linked diseases: because most of the genes are carried on the “X” and very
few are present on the “Y”, usually sex-linked is used for “X”-linked and we have
both dominant and recessive disorders.
In this type of inheritance, there is a lot of deviation from what is expected from
autosomal recessive pattern. The usual pattern, in clinical practice, is that only males
who carry the mutated gene on their “X” chromosomes are clinically affected while
females are usually silent carriers but they transfer the disease to their sons e.g. G6PD
deficiency (favism), hemophilia A, … etc. as males have only one X-chromosome
while females have two.
It happens that some females may present the disease clinically like males; this
was explained by Lyon’s hypothesis, which states that in a female‟s autosomal cells,
and during early embryogenesis, all the “X” chromosomes will be inactivated except
one which remains active during interphase.
This process of inactivation takes place early in the post-fertilization period,
19-20 post-fertilization days.
The process of inactivation is random concerning the origin of the inactivated
“X”, i.e. paternal “X”, which comes from the father or maternal “X” that
comes from the mother.
In a cell, all the daughter cells that descend from it, the same “X” will remain
inactive.
All inactive X-chromosomes will be condensed as a (Barr Body) (Figure (6-
21).
This means that around 50% of the “X” chromosomes are inactivated but this
does not necessarily involve all the paternal “X” or all the maternal “X”, as in some
areas, the paternal X is inactivated while in others, it is the maternal “X” that is
inactivated. Therefore, the body of the female is a considered as mosaic concerning
the active “X”. The female is considered heterozygote for the X chromosome
(regarding origin of her X-chromosomes).
In a female who carries a mutated gene on “X” chromosome and present
clinically (called manifesting carrier), the manifestations happen by chance that in
most part of her body, the “X” that carries the abnormal “X” gene remain active (or
the normal X is inactivated), which results in deficiency of the product of the gene
disease. This is because of the randomness of the inactivation. Some have more
mutated Xs and some have more non-mutated Xs.
(What other causes of a manifesting carrier female can you count so far?)
Aetiology (Types of mutations in regard to the structural defect):
[[very important]]
All single gene diseases (dominant or recessive, autosomal or sex linked) are
due to mutations, which are of different structural types:
1. Point mutation: in which one single base is changed (substituted), deleted, or
added.
a. Single point substitution which is the commonest type. It usually
results from a change in only one of the nucleotide bases that form the
trios (three bases), each of which codes for a specific amino acid in the
protein molecule.
The possible consequences are the following:
13
1. Silent mutation: A change in one base will result in a code which codes
for the same amino acid in the protein. This is due to a fact known as the
redundancy of the code, i.e. for each of the 20 amino acids; there is
more than one code from the 64 codes formed by the four nucleotide
bases. Therefore, the character of the protein would not change and
nothing results but one can deduce from this fact that there are proteins
formed by DNA sequence that are variable among individuals although
the protein is the same. This variation forms the bases for DNA finger
printing used to identify individuals like the original finger prints.
2. Neutral mutation: when one base is changed into another forming
another code; the new code codes for another amino acid that is
physically and chemically similar (not identical) to the normal one the
function of the new polypeptide chain is unaffected no disease state.
3. Missense mutation: The change in the base will form a new code
coding for a different amino acid in the protein changing its character
and behaviour resulting in disease, e.g. all cases of thalassemia, sickle
cell disease, PKU, etc.
4. If a mutation occurs in one of the three codons (called termination or
stop codons) that usually stop the signal (e.g. termination of gene
transcription) we have 2 possible types:
a. Non-sense mutation: which occurs when a codon is changed
into a stop codon by changing one of its bases the resulting
polypeptide would be shorter than normal called truncated
protein; e.g. some forms of
-thalassemia
b. Or when a stop codon is converted into a coding one, so the gene
transcription will continue behind the termination point adding
more than normal number of amino acids until a second 'stop'
codon is reached, resulting in a longer protein behaving
abnormally disease, e.g. Constant Spring type of haemoglobin
(Hb
CS
), where the termination codon of α-polypeptide of Hb is
mutated resulting in addition of 31 amino acids to the original 14
amino acids of the normal chain.
b. & c. Additional / deletion mutations:
These mutations cause a gain or loss of one single nucleotide base shift in
the reading frame of the trios "codons" changing the whole codons from the
point of mutation onwards creating a completely new type of protein or
sometimes creates a termination code in the center of the molecule. These
types of mutations are called frame-shift mutations (Figure 6-22).
2. The second type of mutations (beside the point mutations) are the Addition or
deletion of more than one nucleotide bases:
A. If 2 bases 2 frame shift mutation (less common cause than a point
mutation).
B. Addition / deletion of 3 bases or the multiple of 3, i.e. 6, 9, 12, 15 …
etc. This will lead to addition / deletion of 1, 2, 3, 4, … etc. amino
14
acid(s) in the protein molecule leading to abnormal protein, i.e.
Frieberg Hb, where 5 amino acids (i.e. 15 bases) is added between
amino acids 78-79 sequence in β-Hb polypeptide.
C. Addition / deletion of a piece of DNA inside the gene (intragenic) or
in between the gene (intergenic). Again this creates variability and
may lead to a disease state but if not, it causes polymorphism in DNA
sequences that can be used for genetic testing and DNA finger
printing.
3. Unequal crossing over:
Usually during crossing over at meiosis, the chromosomes align themselves side
by side. This process should be very precise that one allele would be in front of its
brother allele and even the bases are matched to each other in number and type. So,
when there is a crossing over and exchange of chromosomal segments, no disturbance
occurs in the newly formed chromosomes or genes, but if the alignment is improper,
then the alleles are pushed away for a certain degree. This case takes place in sites of
the DNA where there are grouping of genes of similar DNA structure with very slight
variation so one gene is mistaken for a different gene as being its allele and if crossing
over occurs, a defect will result leading to the formation of two unbalanced
homologues, one containing duplicated sequences and the other deleted sequences.
The more similarity of DNA within the sequences, the more likely unequal
crossing over will occur. Crossing over is the process most responsible for creating
regional gene duplications in the genome. Repeated rounds of unequal crossing over
cause the homogenization of the two sequences. With the increase in the duplicates,
unequal crossing over can lead to dosage imbalance in the genome and can be highly
deleterious.
Summary:
Structurally, gene mutations can be in one of the following types:
1. Point mutations
a. Single base substitution (most common type):
i. Silent
ii. Neutral
iii. Missense
iv. Non-sense
v. Longer protein
b. Single base addition / deletion frame-shift mutation.
2. Addition / deletion of > 1 base:
a. 2 bases frameshift mutation
b. 3 (or multiple of 3)
c. A piece of DNA
3. Unequal crossing over
15
1. Classical genetic disorders
A. Chromosomal abnormalities:
B. Single gene mutations:
C. Multifactorial Inheritance (MFI)
C. Multifactorial Inheritance (MFI)
Multifactorial (also called polygenic) inheritance is involved in many of the
physiologic characteristics (e.g. weight, height, blood pressure, hair color, etc.). A
multifactorial physiologic or pathologic trait may be defined as one governed by the
additive effect of two or more genes of small effect but conditioned by
environmental, non-genetic influences.
Even monozygotic twins reared separately may achieve different heights
because of nutritional or other environmental influences. This form of inheritance is
believed to underlie such common diseases as diabetes mellitus, hypertension, gout,
schizophrenia, bipolar disorders and certain forms of congenital heart disease as well
as some skeletal abnormalities.
In multifactorial inheritance, it is the additive effect of many genes of small
effect PLUS a suitable environment that cause such disorders.
In single gene disorders, individuals in regard to the abnormal gene are one of 3
groups: a heterozygote (carrying one mutated and one normal gene and thus affected
in AD and not affected in AR disorders), a homozygote for the mutated gene (and
thus affected in all cases), or a homozygote normal. There is no gradient in between
these 3 groups.
(What do we call a male carrying an x-linked recessive disorder?
A heterozygous or a homozygous? Or something else?)
In MFI, we could group individuals in a community into many different grades,
which have a normal distribution curve (Gaussian distribution) with a threshold point,
which when exceeded, the disorder is expressed.
The facts that [MFI are affected by many genes (not just one) and that the
additive effect of both genes and the environment determine the expression of MF
disorder], are called Genetic Liability (or genetic predisposition) of the individual
and this liability can be measured. So, MFI is only partially genetic (unlike other
types of inheritance) and needs environmental factors to act for the disorder to appear.
You might imagine that the list of “multifactorial” disorders blends in with the
entire list of human diseases. Hence are “all” diseases “genetic”?
• They are Multi-”FACTORIAL”, not just multi-GENIC
• If a disease or condition is scalable, rather than on or off, it is probably multigenic, or
multifactorial, just part of the spectrum of HOMO-zygous diseases being HOMO-
geneous, and HETERO-zygous diseases being VARIABLE.
• Common phenotypic expressions governed by “multifactorial” inheritance
– Hair color
– Eye color
– Skin color
– Height
– Intelligence
– Diabetes, type II
Mention (10) diseases with multifactorial type of inheritance
16
2. Single Gene Disorders with Atypical Patterns of Inheritance
(or Non-Classical Genetic Disorders)
This group includes:
A. Diseases caused by mutations in mitochondrial genes.
B. Triplet repeat mutations.
C. Uniparental disomy / Genomic imprinting.
D. Gonadal mosaicism.
Specific Features of Single Gene Disorders:
Autosomal Dominant (AD) Disorders
Over 3000 conditions or traits shows this pattern of inheritance are identified. Some
are very mild; others are lethal.
AD disorders are manifested in the heterozygous state, so at least one parent of an
index case is usually affected; both males and females are affected, and both can
transmit the condition.
When an affected person marries an unaffected one, every child has one chance
in two of having the disease. In addition to these basic rules, autosomal dominant
conditions are characterized by the following:
With every autosomal dominant disorder, some patients do not have affected
parents. Such patients owe their disorder to new mutations involving either the
egg or the sperm from which they were derived. Their siblings are neither
affected nor at increased risk for developing the disease.
The proportion of patients who develop the disease as a result of a new mutation
is related to the effect of the disease on reproductive capability.
If a disease markedly reduces reproductive fitness, most cases would be expected
to result from new mutations. Many new mutations seem to occur in germ cells of
relatively older fathers.
Clinical features can be modified by reduced penetrance and variable
expressivity. Some individuals inherit the mutant gene but are phenotypically
normal. This is referred to as reduced penetrance.
Penetrance is expressed in mathematical terms: Thus, 50% penetrance indicates
that 50% of those who carry the gene express the trait.
In contrast to penetrance, if a trait is seen in all individuals carrying the mutant
gene but is expressed differently among individuals, the phenomenon is called
variable expressivity.
For example, manifestations of neurofibromatosis type 1 range from brownish
spots on the skin to multiple skin tumors and skeletal deformities.
The mechanisms underlying reduced penetrance and variable expressivity are not
fully understood, but they most likely result from effects of other genes or
environmental factors that modify the phenotypic expression of the mutant allele.
For example, the phenotype of a patient with sickle cell anemia (resulting from
mutation at the β-globin locus) is influenced by the genotype at the α-globin locus
because the latter influences the total amount of hemoglobin made.
The influence of environmental factors is exemplified by familial
hypercholesterolemia. The expression of the disease in the form of
atherosclerosis is conditioned by the dietary intake of lipids.
In many conditions, the age at onset is delayed: symptoms and signs do not
appear until adulthood (as in Huntington disease).
17
Autosomal Recessive Disorders
Autosomal recessive inheritance is the single largest category of Mendelian disorders.
Because autosomal recessive disorders result only when both alleles at a given gene
locus are mutants, such disorders are characterized by the following features:
(1) The trait does not usually affect the parents, but siblings may show the disease;
(2) siblings have one chance in four of being affected (i.e., the recurrence risk is 25% for
each birth); and
(3) if the mutant gene occurs with a low frequency in the population, there is a strong
likelihood that the proband is the product of a consanguineous marriage.
In contrast to those of autosomal dominant diseases, the following features generally
apply to most autosomal recessive disorders:
• The expression of the defect tends to be more uniform than in AD disorders.
• Complete penetrance is common.
• Onset is frequently early in life.
• Although new mutations for recessive disorders do occur, they are rarely detected
clinically. Since the individual with a new mutation is an asymptomatic
heterozygote, several generations may pass before the descendants of such a
person mate with other heterozygotes and produce affected offspring.
• In many cases, enzyme proteins are affected by a loss of function. In
heterozygotes, equal amounts of normal and defective enzyme are synthesized.
Usually the natural "margin of safety" ensures that cells with half their usual
complement of the enzyme function normally.
X-Linked Disorders
1. All sex-linked disorders are X-linked, almost all X-linked recessive. Several genes
are encoded in the "male-specific region of Y"; all of these are related to
spermatogenesis.
2. Males with mutations affecting the Y-linked genes are usually infertile, and hence
there is no Y-linked inheritance.
3. A few additional genes with homologues on the X chromosome have been mapped
to the Y chromosome, but no disorders resulting from mutations in such genes have
been described.
4. X-linked recessive inheritance accounts for a small number of well-defined clinical
conditions.
5. The Y chromosome, for the most part, is not homologous to the X, and so mutant
genes on the X are not paired with alleles on the Y. Thus, the male is said to be
hemizygous for X-linked mutant genes, so these disorders are expressed in the male.
Other features that characterize these disorders are as follows:
• An affected male does not transmit the disorder to his sons, but all daughters
are carriers.
• Sons of heterozygous women have, of course, one chance in two of receiving
the mutant gene.
• The heterozygous female usually does not express the full phenotypic change
because of the paired normal allele. Because of the random inactivation of one
of the X chromosomes in the female, however, females have a variable
proportion of cells in which the mutant X chromosome is active skewed X-
inactivation.
18
Dysmorphology / Teratogens and Congenital Malformations
(Developmental Diseases) / Birth defects
The word dysmorphology is derived by combining three Greek words (dys—bad
or disordered; morph—shape or structure; and ology—the study or science of).
Dysmorphology is a branch of clinical genetics concerned with the study of structural
defects, especially congenital malformations.
• 20% neonatal deaths caused by these
• 3% live births have one or more congenital abnormality
• Even higher prevalence among still births
• 6% one year olds have one or more
• about 3 million fetuses and infants are born each year with major congenital
malformations
• They account for nearly 500,000 deaths worldwide each year.
• However, individual congenital malformations are seen only infrequently by
the individual practitioner.
Introduction:
Congenital malformations or birth defects are common among all races, cultures, and
socioeconomic strata. Birth defects can be isolated abnormalities or part of a syndrome and
continue to be an important cause of neonatal and infant morbidity and mortality.
Of all congenital malformations diagnosed by the end of first year of life, nearly 60%
are identified in the first month and about 80% by the end of 3 months.
With the introduction
of prenatal ultrasound in obstetric care, many major congenital malformations are diagnosed
prenatally, allowing parents to have the option of terminating the pregnancy.
Several pediatric disorders are of genetic origin. However, it must be borne in
mind that not all genetic disorders are present in infancy and childhood, and
conversely, many pediatric diseases are not of genetic origin (e.g. diseases resulting
from immaturity of organ systems).
Three commonly used terms are to be understood: hereditary, familial, and
congenital. Hereditary disorders, by definition, are derived from one's parents, are
transmitted in the gametes through the generations, and therefore, are familial. The
term 'congenital' simply implies "present at birth".
Not all congenital diseases are of genetic origin (e.g. congenital syphilis), and
not all genetic disorders are congenital (e.g. Huntington's disease, is expressed only
after the 3
rd
or 4
th
decade of life).
Definition of CM:
It is a deformation of structure or function of an organ that is present at birth.
It may be on the surface of the body, e.g. cleft lip; or inside the body e.g.
horseshoe kidney.
It may be macrocellular, i.e. the defect affects large group of cells, tissue or
organ, e.g. club foot, or microcellular, i.e. can be identified only by
microscopic examination, e.g. sponge kidney where the defect is abnormal
connection between the collecting tubules and the urineferous tubules leading
to thin microscopical dilatation that gives the kidney a spongy feeling.
It could be diagnosed by the naked eye or it may need special procedure for
diagnosis, e.g. congenital heart defects.
19
It may present at birth with signs and symptoms, e.g. duodenal atresia or may
present later in life but the defect is present at birth, the signs and symptoms
arises only later in life, e.g. adult polycystic kidney chronic renal failure.
It may be familial or non-familial, i.e. either there are multiple cases in one
family or it is the only case in the family (sporadic);
And lastly it could be of a genetic origin (cause), or could be a non-genetic,
i.e. caused by environmental causes, e.g. some cases of microcephaly are of
genetic origin while others are due to in utero infection (prenatal infection).
Causes:
Those agents with a potential to induce a structural anatomic anomaly and thus a
congenital malformation in a developing fetus are termed teratogens (Greek: teratos
[monster] and gen [producing]).
Some teratogens act as a mutagen (when affects a
growing fetus) and some as a carcinogen (when affects a mature adult cell type).
Pathological Action of Teratogens:
The effect of a teratogen is most effective on growing tissues with rapid division
rate but not all teratogens affecting different foeti cause the same severity of action,
i.e. the action of one teratogen is variable in different foeti.
This is due to the following factors:
a. Nature of the teratogenic agent.
b. The dose of the teratogen: larger doses of course cause more severe
effects.
c. Timing and duration of the exposure to the teratogen; the earlier and
the more prolonged, the greater the effect would be (Figure 6-26).
d. Genetic constitution (or host susceptibility): some foeti are more
susceptible than others for the same dose and therefore they are more
severely affected.
e. Interaction with other factors, i.e.
presence of concurrent exposures
e.g. environment of the uterus, maternal metabolic state, and so on.
It is likely that the interactions between genes and environmental factors are
responsible for most birth defects related to teratogenic exposures.
Potential Teratogenic mechanisms:
mutational changes in DNA sequences
chromosomal abnormalities leading to structural or quantitative changes in
DNA
alteration or inhibition of intracellular metabolism, e.g., metabolic blocks and
lack of co-enzymes, precursors or substrates for biosynthesis
interruption of DNA or RNA synthesis
interference with mitosis
interference with cell differentiation
failure of cell-to-cell interactions
failure of cell migrations
cell death through direct cytotoxic effects
effects on cell membrane permeability and osmolar changes
physical disruption of cells or tissues.
21
Types of Teratogens, See Figure (6-27):
Genetic factors are responsible for one third of congenital malformations with
known causes.
(A chromosomal abnormality occurs in 1 of 170 live born infants. Among
chromosomally abnormal neonates, 1/3
rd
have an extra sex chromosome, 1/4
th
have
trisomy of an autosome, and the remaining have an aberration of chromosomal
structure such as a deletion or translocation. However, a significant majority of these
infants have no phenotypic manifestations at birth. Nearly 10% of infants with lethal
multiple congenital malformations have abnormal cytogenetic studies. However, this
proportion is likely to be much higher today with advances in genetics.
With better understanding of the human genome and improved techniques in
molecular cytogenetics, more and more structural chromosomal abnormalities are
being identified as a cause of congenital anomalies previously considered to be of
unknown etiology).
Environmental factors also play an important role in the etiopathogenesis of
many congenital malformations. Maternal exposure to certain environmental agents
can lead to disruption of the normal developmental process and result in both minor
and major congenital anomalies.
Classification of Congenital Anomalies
A. Classification based on timing of insult.
Congenital anomalies can be placed into the following three categories on the basis of
developmental stage during which the aberration in development took place.
1. Malformation. A malformation is a morphologic defect of an organ, part of
an organ, or a region of the body due to an intrinsically abnormal
developmental process. The examples include renal agenesis and neural tube
defects (Figure 6-28).
2. Disruption. It results from the extrinsic breakdown of or an interference with
an originally normal developmental process and the resulting anomaly can
include an organ, part of an organ, or a larger region of the body. A disruption
is never inherited but inherited factors can predispose to and influence the
development of a disruption. An anomaly secondary to disruption can be
caused by mechanical forces; ischemia, hemorrhage, or adhesions of denuded
tissues and occur during or after organogenesis. An example of congenital
anomaly caused by disruption is the amniotic band sequence (Figure 6-29).
3. Deformation. Deformational anomalies are produced by aberrant mechanical
forces that distort otherwise normal structures. These anomalies occur after
organogenesis, frequently involve musculoskeletal tissues and have no
obligatory defects in organogenesis. Common causes of deformation are
structural abnormalities of the uterus such as fibroids, bicornuate uterus,
multiple gestation, and oligohydramnios (Figure 6-30). Deformations can be
reversible after birth. These anomalies are unlikely to have a genetic basis, are
often not associated with cognitive deficits, and have a low recurrence risk.
B. Classification of birth defects based on medical consequences.
Based on the medical consequences, a congenital malformation can be classified
as either major or minor.
1. Major malformations. Major malformations are anatomic abnormalities
which are severe enough to reduce life expectancy or compromise normal
function such as neural tube defects, renal agenesis, etc. Major malformations
can be further divided into lethal or severe malformations. A malformation is
21
considered lethal if it causes stillbirth or infant death in more than 50% of
cases. The remaining major malformations are life-threatening without
medical intervention and are considered severe.
2. Minor malformations. Minor malformations are structural alterations which
either require no treatment or can be treated easily and have no permanent
consequence for normal life expectancy. Minor malformations are most
frequent in areas of complex and variable features such as the face and distal
extremities (Figure 6-33). Minor malformations are relatively frequent and a
higher incidence may be noted among premature infants and infants with
intrauterine growth retardation. In general, minor malformations are more
subtle, have low validity of diagnoses, and are not reported consistently. They
are nevertheless significant as they may be an indication of the presence of a
major malformation and may also provide critical clues to the diagnosis. The
risk of having a major malformation increases with the number of associated
minor malformations. For more examples, see Figures (6-34, 5 photos)
Aetiology of birth defects:
1. Genetic factors: > 35% including:
a. Chromosomal anomalies: both numerical and structural.
b. Single gene defects: less common cause of birth defects.
c. Multifactorial: are important cause of birth defects.
2. Environmental teratogens (45-50%):
1. In-utero infection :
Prenatal infection of the foetus with bacteria, viruses, or parasites may occur
during pregnancy due to maternal infection that is transferred to the foetus through the
placenta causing foetal infection. The most common is the viral infection with rubella
virus (German measles), and influenza virus.
I. Viral infections:
a. Rubella congenital rubella syndrome usually causes a more severe
malformation the earlier it infects the fetus, The incidence of congenital
malformation due to infection with rubella decreases as the infection is
contracted later during pregnancy. The modern immunization against rubella
in girls at reproductive age is one of the very successful methods to prevent
this type of malformation. Rubella causes congenital heart defect, mainly
septal defect, microcephaly as it invades the nervous system mental
retardation, or infection of the chambers of the eyes leading to blindness,
cataract, in addition to causing deafness (Figure 6-35).
b. Influenza virus usually causes cleft lip and palate.
c. Mumps, measles, herpes simplex (type I & II) neural tube defects,
microcephaly, or hydrocephaly (Figure 6-36, 6-37) .
d. CMV: usually causes early fetal loss (abortion), but if not severe
malformation like microcephaly and mental retardation.
II. Bacterial infection: The most important in this regard is Treponyma pallidum
congenital syphilis with multiple organ system effects ranging from severe to mild
forms according to the timing of exposure (more severe in early infections and less
severe later in pregnancy).
III. Parasitic infestation:
Toxoplasma gondii is an intracellular parasite, usually contracted from animals,
mostly sheep or cats. It invades the CNS microcephaly, mental retardation and
jaundice (Toxoplasmosis). It has some similarities to rubella in terms of timing of
infection or infestation.
22
2. Physical teratogens :
These could be:
a. Heat, whether sauna bathing, or from weather or fever (if it does not
cause abortion) neural tube defects.
b. Physical (mechanical) pressure that may be caused from inside the
uterus or outside it pressing the growing fetus preventing its proper
growth. These could be the following :
i. Congenitally abnormal maternal uterus like unicornuate uterus
or septate uterus.
ii. Luminal fibroid (large leiomyoma) projecting into the lumen.
iii. Corset wearing for a long period during pregnancy.
iv. Decreased amount of liquor or absence of liquor (Potter's
facies).
These may cause abnormal positioning of the limbs or feet resulting in
congenital hip dislocation or club foot, face compression, etc.
v. Amniotic (fibrous) band, which are fibrosis of the amniotic
membrane due to infection that may turn around a growing
external part of the foetal body causing cut in its blood supply,
ischemia and necrosis, like amputation of a finger or hand or
foot.
c. Ionizing radiation, whether diagnostic or therapeutic. Radiation or
accidents usually causes malformation. Radiation causes embryonal or
fetal death, microcephaly anophthalmia and spina bifida, and
Congenital heart disease (CHD).
The most sensitive time for radiation effect is between 2-4 weeks after
fertilization.
3. Chemical teratogens, these could be :
a. Non-medicinal:
i. like pesticides, insecticide, and household chemical used in
cleaning and industrial chemical. Some of those usually contain
organic phosphorous, which is very toxic. These usually are
ingested accidentally by contaminated food and water. They
usually cause abnormality of the nervous system like tremor
rigidity, and may cause spina bifida.
ii. Alcohol and cigarette smoking are to be mentioned in this
category, both cause intrauterine growth retardation and
delayed mental development in later life after birth with small
stature. Alcohol also results in a typical facial features and
microcephaly (fetal alcohol syndrome).
b. Medicinal chemicals: one should consider that all drugs are not safe
during the early weeks post-fertilization and medication should be
taken cautiously. The most properly documented drugs in causing
congenital malformation are:
i. Thalidomide: a drug used for sedation or treatment of
hyperemesis gravidarum (severe nausea and vomiting during
early months of pregnancy), usually causes amelia and
phocomelia (short deformed limbs) (Figure 6-38).
ii. Anti-convulsants,
especially
phenytoin
causes
ptosis,
microcephaly,
pilonidal
sinus
and
subnormal
mental
23
development, and camptodactyly (flexion contracture of some
or all fingers).
iii. Anticoagulant, especially warfarin neural tube defects.
iv. Cytotoxic drugs (anti-metabolites) as they depress cellular
metabolism, especially the rapidly dividing ones club foot
(Figure 6-39), cleft palate, growth retardation.
v. Hormones: like progesterone (used to maintain pregnancy in
threatened abortion), oestrogen and cortisone. The 1
st
causes
ambiguous genitalia, the 2
nd
causes vaginal carcinoma of the
growing female baby at an early age between 10-15 years,
while cortisone causes cleft lip and palate and NTDs.
Paracelsus once said:
“Poison is in everything, and nothing is without poison.
The dosage makes it either a poison or a remedy.”
4. Maternal disorders :
a. Diabetes mellitus, whether treated or not, and whether it is of type I (insulin-
dependant) or type II (non-insulin dependant), usually causes caudal
regression syndrome (one single fused limb) and femoral hypoplasia with
agenesis of the sacrum (Figure 6-40).
b. Maternal phenylketonuria (PKU), which is a genetic metabolic disease that
could be treated. Females may reach reproduction with normal state but their
high phenylalanine serum level results from their deficiency of phenylalanne
hydroxylase enzyme will destroy the developing brain of the rowing embryo
to cause brain atrophy and consequently mental retardation.
c. Vitamin A maternal state both hypo and hyper: severe limitation of dietary
intake results in hypo state as happen in Iraqi mothers during the embargo;
hyper state results from treatment with the modern creams of vitamin A used
for acne. Both result in microphthalmia (Figure 6-41). Deficiency states
NTDs and cataracts.
d. Endemic goiter: a decrease in thyroid hormones production due to lack of a
proper iodine intake especially during times of stress and increased T3 & T4
demand (pregnancy, puberty, rapid growth) elevated TSH level and thus
it may pass to the fetus and ultimately suppresses its own thyroid function
congenital hypothyroidism (cretinism).
A better understanding of the etiology and pathogenesis of these defects has led
to several prevention strategies over the years. Rubella immunization and avoidance
of teratogenic drugs in women of reproductive age, use of folic acid supplementation
and maintenance of euglycemia in diabetic patients during the periconception period,
premarital and preconception genetic counseling to couples at risk of certain genetic
disorders, and screening for Down syndrome in presence of advanced maternal age
are a few examples of very effective and successful strategies to prevent congenital
malformations in a newborn.
24
Classification of medicines / chemicals in relation to their teratogenic effects
(nice to know this year but essential to know in clinical practice)
Category
Description
A
Medication has not shown an increased risk for birth defects in human studies.
B
Animal studies have not demonstrated a risk and there are no adequate studies in
humans, OR animal studies have shown a risk, but the risk has not been seen in
humans.
C
Animal studies have shown adverse effects, but no studies are available in humans,
OR studies in humans and animals are not available.
D
Medications that are associated with birth defects in humans, however, there
may be potential benefits in rare cases that outweigh their known risks.
X
Medications are contraindicated (should not be used) in human pregnancy,
because of known fetal abnormalities that have been demonstrated in both human and
animal studies.
Drugs known to cause human birth defects
(nice to know this year but keep it as a reference for future use – essential)
ACE inhibitors (eg, captopril, enalapril) - D
Acetohydroxamic acid (AHA) - X
Aminocaproic acid - D
Androgens (eg, Danazol) - X
Angiotensin II receptor antagonists (eg,
losartan, valsartan) - D
Antineoplastics (alkylating agents) - D
Antineoplastics (antimetabolites) - X
– 5-Fluorouracil
– Methotrexate
– Methylaminopterin
– Cytarabine
– Busulfan
– Chlorambucil
– Azathioprine
– Cyclophosphamide
– Mechlorethamine
– Cisplatin
– Bleomycin
Aminoglycosides (eg, gentamicin,
streptomycin) - D
Aspirin - D
Atenolol - D
Benzodiazepines - D and X
– Flurazepam (X)
– Temazepam (X)
– Triazolam (X)
Bromides - D
Carbamazepine - D
Colchicine - D
Corticosteroids - C
Danazol - X
Diethylstilbestrol - Not on market
Ergotamine - X
Finasteride - X
Fluconazole - C
Folic acid antagonists - D (phenytoin) and
X (methotrexate)
Lithium - D
Methimazole - D
Methylene blue - C
Mifepristone, RU-486 - D
Minoxidil - C
Misoprostol - X
Mysoline - D
Penicillamine - D
Phenobarbital or methylphenobarbital - D
Potassium iodine and medications that
effect iodine levels (diatrizoate) - D
Progestins - X (except megestrol and
norethindrone - D)
Raloxifene (Evista) - X
Retinoic acid, isotretinoin (Accutane),
acitretin (Soriatane), etretinate, topical
tazarotene - X
Statins (3-hydroxy-3-methylglutaryl
coenzyme A [HMG-CoA] reductase
inhibitors) - X
Tamoxifen - D
Tetracycline - D
Thalidomide - X
Valproic acid - D
Warfarin - X
25
Diagnosis of Genetic Diseases
Diagnosis of genetic diseases requires the classical sequence of getting
information about the patient like any medical disorder i.e. by history taking, clinical
examination of the patient plus doing some additional laboratory (hematological,
biochemical, serological, hormonal, etc.) or radiological (plain X-ray, CT-scan, MRI,
Echocardiogram, etc.) tests when indicated. If a provisional diagnosis is made or a list
of few differential diagnoses is thought of, a confirmatory test should be sought for to
confirm or rule out that diagnosis.
Cytogenetics (Karyotyping):
A specific diagnosis for the genetic disorders may require a chromosomal study
(a cytogenetic study to diagnose a numerical or structural chromosomal abnormality),
which is the basic tool for any genetic laboratory, and can be used in a wide variety of
gross chromosomal abnormalities (as discussed earlier), but many genetic diseases are
caused by subtle changes in individual genes that cannot be detected by karyotyping.
Molecular cytogenetics:
Sometimes, the defect on the chromosome is more subtle (e.g. deletion of the
largest gene in the whole human DNA i.e. the dystrophin gene of a size of 2.4 M.B.,
is beyond the capability of the light microscope to detect). In this case, in situ
hybridization is used to detect the mutated chromosome.
Hybridization:
This is a procedure used in the diagnosis of genetic and other pathologies as
well as in the diagnosis of cancer.
It is based on the fact that the two DNA strands are not identical but
complementary.
The test is performed by adding a synthetic, single stranded DNA sequence
(called a probe) [that is made complementary to a specific region of DNA under study
and is being labeled with a specific dye] to the double stranded DNA from the patient
(after making it single stranded by a process called denaturation). If the probe found
its complementary region along the patient's DNA, it'll combine (hybridize) to it and
starts emitting a color or "fluoresce". This emitted color can be detected using a UV-
microscope.
Advantages:
This procedure can detect smaller chromosomal defects (mostly structural ones)
and can also detects numerical chromosomal abnormalities during interphase as well
as during metaphase (in contrast to karyotyping that requires the cells to be tested
during metaphase only i.e. dividing cells). It also require less incubation time and
fewer number of tested cells.
This procedure forms the basis of what is known as fluorescent in situ
hybridization (FISH).
Nevertheless, this procedure cannot detect single point mutations or even
addition / deletion of 2 or more nucleotide bases.
So, the technique used for detection of such smaller defects is usually DNA-
based; the most representative and most commonly used one is polymerase chain
reaction (PCR) that revolutionalized the diagnostic ability of genetic testing. Most
new techniques used nowadays are PCR-based.
26
Molecular Genetics
Traditionally the diagnosis of single-gene disorders has depended on the
identification of abnormal gene products (e.g., mutant hemoglobin or enzymes) or
their clinical effects, such as anemia or mental retardation (e.g., phenylketonuria).
Now it is possible to identify mutations at the level of DNA and offer gene diagnosis
for several mendelian disorders. The use of recombinant DNA technology for the
diagnosis of inherited diseases has several distinct advantages over other techniques:
It is remarkably sensitive. The amount of DNA required for diagnosis by
molecular hybridization techniques can be readily obtained from 100,000
cells. Furthermore, the use of PCR allows several million-fold
amplification of DNA or RNA, making it possible to use as few as 100
cells or 1 cell for analysis. Tiny amounts of whole blood or even dried
blood can supply sufficient DNA for PCR amplification.
DNA-based tests are not dependent on a gene product that may be
produced only in certain specialized cells (e.g., brain) or expression of a
gene that may occur late in life. Because virtually all cells of the body of
an affected individual contain the same DNA, each postzygotic cell
carries the mutant gene.
These two features have profound implications for the prenatal diagnosis of
genetic diseases (giving a diagnosis of the yet unborn foetus).
There are two distinct approaches to the diagnosis of single-gene diseases by
recombinant DNA technology: direct detection of mutations and indirect detection
based on linkage of the disease gene with a harmless "marker gene."
DIRECT GENE DIAGNOSIS
It is also called the diagnostic biopsy of the human genome. It is indicated when
the location of the gene being tested is known. Such diagnosis depends on the
detection of an important qualitative change in the DNA. There are several methods
of direct gene diagnosis; almost all are based on polymerase chain reaction (PCR)
analysis, which involves exponential amplification of DNA from small quantities of
starting material. If RNA is used as a substrate, it is first reverse transcribed to obtain
cDNA (using an enzyme called reverse transcriptase) and then amplified by PCR.
This method is often abbreviated as RT-PCR.
To detect the mutant gene, two primers (lengths of a single stranded DNA
made complementary to a desirable length of the tested DNA 'part of the gene to
be tested') that bind to the 3' (called three prime) and 5' (called five prime) ends of
the normal sequence are designed. By using appropriate DNA polymerases (enzymes
that build up DNA strand based on its complementary strand) and thermal cycling,
the DNA between the primers is greatly amplified, producing millions of copies of the
DNA between the two primer sites. The amplified normal DNA and patient's DNA
are then digested with a restriction enzyme that cuts the amplified DNA into pieces of
knwn sizes e.g. the normal DNA yields three fragments (67 base pairs, 37 base pairs,
and 163 base pairs long); by contrast, the patient's DNA yields only two products, an
abnormal fragment that is 200 base pairs (instead of two pairs of 37 and 163 b.p.) and
a normal fragment that is 67 base pairs long. These DNA fragments can be readily
resolved by gel electrophoresis (by which we can separate DNA bands or pieces
according to their molecular weight) and then visualized after staining with ethidium
bromide under ultraviolet light.
INDIRECT DNA DIAGNOSIS: LINKAGE ANALYSIS
Direct gene diagnosis is possible only if the mutant gene and its normal
counterpart have been identified and cloned and their nucleotide sequences are known
27
(its exact locus and size are well known). In a large number of genetic diseases,
including some that are relatively common, information about the gene sequence is
lacking. Therefore, alternative strategies must be employed to track the mutant gene
on the basis of its linkage to detectable genetic markers. In essence, one has to
determine whether a given fetus or family member has inherited the same relevant
chromosomal region(s) as a previously affected family member. It follows therefore
that the success of such a strategy depends on the ability to distinguish the
chromosome that carries the mutation from its normal homologous counterpart. This
is accomplished by exploiting naturally occurring variations or polymorphisms in
DNA sequences.
Molecular diagnosis by linkage analysis has been useful in the antenatal or
presymptomatic diagnosis of disorders such as Huntington disease, cystic fibrosis, and
adult polycystic kidney disease. In general, when a disease gene is identified and
cloned, direct gene diagnosis becomes the method of choice. If the disease is caused
by one of several different mutations in a given gene, and direct gene diagnosis is not
feasible, linkage analysis remains the preferred method.
CLINICAL GENETICS
It is the study of the possible genetic determinants affecting the occurrence of
diseases and disorders.
Role of The Geneticist
• Diagnosis (History, Physical Examination, Investigations (including
karyotyping or DNA diagnostic methods).
• Genetic counseling
1. Discussing the problem with the family or the individual concerned
2. Recurrence risk calculation
3. (?) Treatment
4. (?) Prenatal diagnosis
5. (?) Prevention
Who Might Benefit from Genetic Services?
• Parents of a child born with a genetic disorder (AD, AR, XR), birth defect, or
chromosomal abnormality.
• Parents concerned about certain finding in the family that is recurrent or severe
(deafness, paralysis, short stature, mental retardation, developmental delay,
childhood blindness, etc.)
• Children with developmental delay and unusual features (dysmorphic features)
• Individuals that fail to develop secondary sexual characteristics or children with
ambiguous genitalia
• Couples with a history of difficulty becoming pregnant, repeated pregnancy
losses, stillbirths, or early infant deaths
• Children with short stature that is unusual for the family
• Individuals with a family history of a birth defects or genetic disease.
• Multiple miscarriages, still births, or early infant deaths with congenital anomalies
• Couples who are first cousins or close blood relatives (consanguinity)
• Pregnant woman or their infants who have been exposed to a medication, drug,
radiation, or other environmental agent
• Woman in their mid-30's and 40's who are pregnant or are planning a pregnancy
• Individuals of certain ethnic or geographic groups at increased risk for genetic
disorders
28
To make a diagnosis:
• Information are collected – about family and medical history, family history
sometimes including photographs of family members plus their physical
examination.
• Consultation of other specialist(s)
• Selective laboratory investigations.
• Making a clinical diagnosis.
• Confirmation (whenever possible or feasible) by: chromosome testing, FISH,
DNA testing, or other genetic testing, as needed.
What is Genetic Counseling?
It is a communication process that translates technical and complicated knowledge
into practical information for individuals and families. Genetic Counseling can help
an individual or a family with a condition or a disease to:
• Understand the cause of the condition
• Know the chance of having future children with the condition (recurrence risk)
• Learn about current research, testing procedures, prenatal diagnosis
• Contact community resources, and if possible, other families or patients with
the same condition
• Adjust to the personal and family issues related to a genetic disorder
PRENATAL DIAGNOSIS
It employs a variety of techniques to determine the health and condition of an
unborn fetus. Without knowledge gained by prenatal diagnosis, there could be an
untoward outcome for the fetus or the mother or both. Congenital anomalies account
for 20 to 25% of perinatal deaths.
Prenatal diagnosis is helpful for:
1. Managing the remaining weeks of the pregnancy
2. Determining the outcome of the pregnancy
3. Planning for possible complications with the birth process
4. Planning for problems that may occur in the newborn infant
5. Deciding whether or not to continue the pregnancy
6. Finding conditions that may affect future pregnancies
There are a variety of non-invasive and invasive techniques available for prenatal
diagnosis. Each one can be applied only during specific time periods during the
pregnancy for greatest utility. The techniques employed for prenatal diagnosis
include:
• Ultrasonography
• Fetoscopy
• Amniocentesis
• Chorionic villus sampling
• Fetal blood cells in maternal blood
• Maternal serum (alpha-fetoprotein,
beta-HCG, and estriol)
• Inhibin-A
• Radiography
• Gross Examination
• Microscopic Examination
• Microbiologic Culture & Serology
• Karyotyping
• FISH (on fresh tissue or paraffin blocks)
• DNA Probes
• Flow Cytometry
Example: A commonly used screening test: "Triple" or "Quadruple" screen
Combining the maternal serum assays may aid in increasing the sensitivity and
specificity of detection for fetal abnormalities. The classic test is the "triple screen"
for maternal serum alpha-fetoprotein (MSAFP), beta-HCG, and estriol (uE3). The
"quadruple screen" adds inhibin-A.