Genetic Disorders 2 Flashcards
Lysosomal Storage Diseases
Lysosomes are key components of the “intracellular digestive
tract.” They contain a battery of hydrolytic enzymes, which have
two special properties. First, they function in the acidic milieu of
the lysosomes. Second, these enzymes constitute a special
category of secretory proteins that are destined not for the
extracellular fluids but for intracellular organelles. This latter
characteristic requires special processing within the Golgi
apparatus
Similar to all other secretory proteins, lysosomal enzymes (or
acid hydrolases, as they are sometimes called) are synthesized
in the
ER
The phosphorylated mannose
residues serve as an
“address label” that is recognized by
specific receptors found on the inner surface of the Golgi
membrane.
n inherited deficiency of a
functional lysosomal enzyme gives rise to two pathologic consequences (
Catabolism of the substrate of the missing enzyme remains
incomplete, leading to the accumulation of the partially
degraded insoluble metabolite within the lysosomes.
Since lysosomal function is also essential for autophagy,
impaired autophagy
, leading to the accumulation of the partially
degraded insoluble metabolite within the lysosomes. This is
called
“primary accumulation”.
impaired autophagy gives rise to
“secondary accumulation” of
autophagic substrates such as polyubiquinated proteins and old
and effete mitochondria. The absence of this quality control
mechanism causes accumulation of dysfunctional mitochondria
with poor calcium buffering capacity and altered membrane
potentials. This can trigger generation of free radicals and
apoptosis.
Tay-Sachs Disease (GM2
Gangliosidosis:
Hexosaminidase α-Subunit Deficiency)
GM2 gangliosidoses are a group of three lysosomal storage
diseases caused by an inability to catabolize GM2 gan‐
gliosides. Degradation of GM2 gangliosides requires three
polypeptides encoded by three distinct genes
The phenotypic
effects of mutations affecting these genes are fairly similar,
because they result from accumulation of GM2 gangliosides. The
underlying enzyme defect, however, is different for each
the most common form of GM2 gangliosidosis,
results from mutations in the α-subunit locus on chromosome 15
that cause a severe deficiency of hexosaminidase A.
Tay Sachs
This disease
is especially prevalent among
Jews, particularly among those of
Eastern European (Ashkenazic) origin, in whom a carrier rate of
1 in 30 has been reported.
ganglioside
glycosphingolipid ith one or mote sialic acids on the sugar chain
Complex lipids in the brain
cell-cell recognition, adhesion, transductiom
degraded by ceramides and sequential removal of sugar units in oligosaccharide
The hexosaminidase A is absent from virtually all the
tissues, so GM2 ganglioside accumulates in many
tissues
heart, liver, spleen, nervous system), but
the involvement of neurons in the central and
autonomic nervous systems and retina dominates
the clinical picture
histologic examination
neurons are ballooned with cytoplasmic vacuoles,
each representing a markedly distended lysosome
filled with gangliosides (Fig. 5-11A). Stains for fat
such as oil red O and Sudan black B are positive. With
the electron microscope, several types of cytoplasmic
inclusions can be visualized, the most prominent
being whorled configurations within lysosomes
composed of onion-skin layers of membranes (Fig. 5-
11B).
A cherry-red spot thus appears in the
macula, representing accentuation of the normal
color of the macular choroid contrasted with the pallor
produced by the swollen ganglion cells in the
remainder of the retina (Chapter 29). This finding is
characteristic of Tay-Sachs disease and other storage
disorders affecting the neurons.
Clinical Features of TS
The affected infants appear normal at birth but begin to
manifest signs and symptoms at about age 6 months. There is
relentless motor and mental deterioration, beginning with motor
incoordination, mental obtundation leading to muscular
flaccidity, blindness, and increasing dementia.
cherry-red spot appears in the macula of the
eye in almost all patients. Over the span of 1 or 2 years a
complete vegetative state is reached, followed by death at age 2
to 3 years. More than 100 mutations have been described in the
α-subunit gene; most affect protein folding. Such misfolded
proteins trigger the “unfolded protein” response (Chapter 1)
leading to apoptosis
Niemann-Pick Disease Types A and B
Niemann-Pick disease types A and B are two related
disorders that are characterized by lysosomal
accumulation of sphingomyelin due to an inherited
deficiency of sphingomyelinase.
Type A
severe infantile
form with extensive neurologic involvement, marked visceral
accumulations of sphingomyelin, and progressive wasting and
early death within the first 3 years of life
Type B
disease patients have organomegaly but generally no central
nervous system involvement. They usually survive into
adulthood. As with Tay-Sachs disease, Niemann-Pick disease
types A and B are common in Ashkenazi Jews. The gene for acid
sphingomyelinase maps to chromosome 11p15.4 and is one of
the imprinted genes that is preferentially expressed from the
maternal chromosome as a result of epigenetic silencing of the
paternal gene
Although, this disease is
typically inherited as an autosomal recessive,
those
heterozygotes who inherit the mutant allele from the mother can
develop Nieman Pick Disease. More than 100 mutations have
been found in the acid sphingomyelinase gene and there seems
to be a correlation between the type of mutation, the severity of
enzyme deficiency, and the phenotype.
Morphology of NEIP
In the classic infantile type A variant, a missense
mutation causes almost complete deficiency of
sphingomyelinase. Sphingomyelin is a ubiquitous
component of cellular (including organellar)
membranes, and so the enzyme deficiency blocks
degradation of the lipid, resulting in its progressive
accumulation within lysosomes, particularly within
cells of the mononuclear phagocyte system. Affected
cells become enlarged, sometimes to 90 µm in
diameter, due to the distention of lysosomes with
sphingomyelin and cholesterol
Innumerable small
vacuoles of relatively uniform size are created
imparting foaminess to the cytoplasm
In frozen sections of fresh tissue
the vacuoles stain for
fat. Electron microscopy confirms that the vacuoles
are engorged secondary lysosomes that often contain
membranous cytoplasmic bodies resembling
concentric lamellated myelin figures, sometimes
called “zebra” bodies
The lipid-laden phagocytic foam cells are
widely
distributed in the spleen, liver, lymph nodes, bone
marrow, tonsils, gastrointestinal tract, and lungs. The
involvement of the spleen generally produces
massive enlargement, sometimes to ten times its
normal weight, but the hepatomegaly is usually not
quite so striking.
In the brain
the gyri are shrunken and the
sulci widened. The neuronal involvement is diffuse,
affecting all parts of the nervous system. Vacuolation
and ballooning of neurons constitute the dominant
histologic change, which in time leads to cell death
and loss of brain substance. A retinal cherry-red
spot similar to that seen in Tay-Sachs disease is
present in about one third to one half of affected
individuals.
Clinical manifestations in type A disease
may be present at
birth and almost invariably become evident by age 6 months.
Infants typically have a protuberant abdomen because of the
hepatosplenomegaly. Once the manifestations appear, they are
followed by progressive failure to thrive, vomiting, fever, and
generalized lymphadenopathy as well as progressive
deterioration of psychomotor function. Death comes, usually
within the first or second year of life
Niemann-Pick Disease Type C
Although previously considered to be related to types A and B,
Niemann-Pick disease type C (NPC) is distinct at the
biochemical and genetic levels and is more common than types
A and B combined. Mutations in two related genes, NPC1 and
NPC2, can give rise to NPC,
NPC1
being responsible for
95% of cases. Unlike most other storage diseases, NPC is due to
a primary defect in nonenzymatic lipid transport. NPC1 is
membrane bound whereas NPC2 is soluble. Both are involved in
the transport of free cholesterol from the lysosomes to the
cytoplasm. NPC is clinically heterogeneous
It may present as (NPC}
hydrops fetalis and stillbirth, as neonatal hepatitis, or, most
commonly, as a chronic form characterized by progressive
neurologic damage. The latter presents in childhood and is
marked by ataxia, vertical supranuclear gaze palsy, dystonia,
dysarthria, and psychomotor regression.
Gaucher Disease
Gaucher disease refers to a cluster of autosomal recessive
disorders resulting from mutations in the gene encoding
glucocerebrosidase. It is the most common lysosomal storage
disorder
The affected gene encodes glucocerebrosidase
an enzyme that normally cleaves the glucose residue from
ceramide. As a result of the enzyme defect, glucocerebroside
accumulates principally in phagocytes but in some subtypes also
in the central nervous system
Glucocerebrosides are continually
formed from the catabolism of glycolipids
derived mainly from
the cell membranes of senescent leukocytes and red cells
pathologic changes in Gaucher disease are
caused not just by the burden of storage material but also by
activation of macrophages and the consequent secretion of
cytokines such as IL-1, IL-6, and tumor necrosis factor (TNF)
type I GD
r the chronic nonneuronopathic form. In this type, storage of
glucocerebrosides is limited to the mononuclear phagocytes
throughout the body without involving the brain. Splenic and
skeletal involvements dominate this pattern of the disease. It is
found principally in Jews of European stock. Individuals with
this disorder have reduced but detectable levels of
glucocerebrosidase activity. Longevity is shortened but not
markedly.
Type II GD
acute neuronopathic Gaucher disease, is the
infantile acute cerebral pattern. This form has no predilection
for Jews. In these patients there is virtually no detectable
glucocerebrosidase activity in the tissues. Hepatosplenomegaly
is also seen in this form of Gaucher disease, but the clinical
picture is dominated by progressive central nervous system
involvement, leading to death at an early age
type III GD
intermediate between types I and II.
These patients have the systemic involvement characteristic of
type I but have progressive central nervous system disease that
usually begins in adolescence or early adulthood
Morphology of GD
distended phagocytic cells, known as Gaucher cells, are found in the
spleen, liver, bone marrow, lymph nodes, tonsils, thymus, and Peyer patches. Similar cells may be found in both the alveolar septa and the air spaces in
the lung.
n contrast to other lipid storage diseases
Gaucher cells rarely appear vacuolated but instead
have a fibrillary type of cytoplasm likened to crumpled
tissue paper (Fig. 5-13). Gaucher cells are often
enlarged, sometimes up to 100 µm in diameter, and
have one or more dark, eccentrically placed nuclei.
Periodic acid–Schiff staining is usually intensely
positive. With the electron microscope the fibrillary
cytoplasm can be resolved as elongated,
distended lysosomes, containing the stored lipid in
stacks of bilayers.
In type I disease, the spleen is enlarged
sometimes up to 10 kg. The lymphadenopathy is
mild to moderate and is body-wide. The accumulation
of Gaucher cells in the bone marrow occurs in 70% to
100% of cases of type I Gaucher disease. It produces
areas of bone erosion
Bone destruction occurs due to
he secretion of cytokines by activated macrophages.
In patients with cerebral involvement, Gaucher cells
are seen in the Virchow-Robin spaces, and arterioles
are surrounded by swollen adventitial cells. There is
no storage of lipids in the neurons, yet neurons appear
shriveled and are progressively destroyed. It is
suspected that the lipids that accumulate in the
phagocytic cells around blood vessels secrete
cytokines that damage nearby neurons
Clinical Features of GD
In type I, symptoms and signs first appear in adult life
and are related to splenomegaly or bone involvement. Most
commonly there is pancytopenia or thrombocytopenia secondary
to hypersplenism. Pathologic fractures and bone pain occur if
there has been extensive expansion of the marrow space.
Although the disease is progressive in the adult, it is compatible
with long life
In types II and III, central nervous system
dysfunction, convulsions, and progressive mental deterioration
dominate, although organs such as the liver, spleen, and lymph
nodes are also affected. The diagnosis of homozygotes can be
made by measurement of glucocerebrosidase activity in
peripheral blood leukocytes or in extracts of cultured skin
fibroblast
Mucopolysaccharidoses (MPS)
The MPSs are a group of closely related syndromes that
result from genetically determined deficiencies of enzymes
involved in the degradation of mucopolysaccharides
(glycosaminoglycans)
Chemically, mucopolysaccharides are
long-chain complex carbohydrates that are linked with proteins
to form proteoglycans. They are abundant in the ground
substance of connective tissue. The glycosaminoglycans that
accumulate in MPSs are dermatan sulfate, heparan sulfate,
keratan sulfate, and chondroitin sulfate.
Several clinical variants of MPS, classified numerically
MPS I to MPS VII, have been described, each resulting from the
deficiency of one specific enzyme.
All the MPSs except one are
inherited as autosomal recessive traits
the exception, Hunter
syndrome, is an X-linked recessive trait. Within a given group
(e.g., MPS I, characterized by a deficiency of α-l-iduronidase),
subgroups exist that result from different mutant alleles at the
same genetic locus
Morphology of MPS
The accumulated mucopolysaccharides are
generally found in mononuclear phagocytic cells,
endothelial cells, intimal smooth muscle cells,
and fibroblasts throughout the body. Common sites
of involvement are thus the spleen, liver, bone
marrow, lymph nodes, blood vessels, and heart.
Microscopically, affected cells are MPS
distended and
have apparent clearing of the cytoplasm to create so-called balloon cells. Under the electron microscope,
the clear cytoplasm can be resolved as numerous
minute vacuoles. These are swollen lysosomes
containing a finely granular periodic acid–Schiff–
positive material that can be identified biochemically
as mucopolysaccharide.
Similar lysosomal changes are
found in the neurons of those syndromes
characterized by central nervous system involvement.
however, some of the lysosomes in
neurons are replaced by lamellated zebra bodies
similar to those seen in Niemann-Pick disease.
common threads that
run through all of the MPSs
Hepatosplenomegaly, skeletal deformities,
valvular lesions, and subendothelial arterial
deposits, particularly in the coronary arteries,
and lesions in the brain
In many of the more
protracted syndromes, coronary subendothelial
lesions lead to myocardial ischemia. Thus, myocardial
infarction and cardiac decompensation are important
causes of death.
Hurler syndrome, also
called MPS I-H,
Results from a deficiency of α-l-iduronidase. It is
one of the most severe forms of MPS. Affected children appear
normal at birth but develop hepatosplenomegaly by age 6 to 24
months. Their growth is retarded, and, as in other forms of MPS,
they develop coarse facial features and skeletal deformities.
Death occurs by age 6 to 10 years and is often due to
cardiovascular complications.
Hunter syndrome,
also called MPS
II, differs from Hurler syndrome in mode of inheritance (Xlinked), absence of corneal clouding, and milder clinical course.
Glycogen Storage Diseases (Glycogenoses)
he glycogen storage diseases result from a hereditary
deficiency of one of the enzymes involved in the synthesis
or sequential degradation of glycogen. Depending on the
tissue or organ distribution of the specific enzyme in the normal
state, glycogen storage in these disorders may be limited to a
few tissues, may be more widespread while not affecting all
tissues, or may be systemic in distribution.
A phosphoglucomutase then transforms the glucose-6-phosphate
to glucose-1-phosphate, which, in turn, is converted
uridine diphosphoglucose. A highly branched, large polymer is then built
(molecular weight as high as 100 million), containing as many as
10,000 glucose molecules linked together by α-1,4-glucoside
bonds. The glycogen chain and branches continue to be
elongated by the addition of glucose molecules mediated by
glycogen synthetases.
Hepatic forms
The liver is a key player in glycogen
metabolism. It contains enzymes that synthesize glycogen for
storage and ultimately break it down into free glucose, which is
then released into the blood. An inherited deficiency of hepatic
enzymes that are involved in glycogen degradation therefore
leads not only to the storage of glycogen in the liver but also to
a reduction in blood glucose concentrations (hypoglycemia)
Deficiency of the enzyme glucose-6-phosphatase
(von Gierke disease, or type I glycogenosis) is a prime example
of the hepatic-hypoglycemic form of glycogen storage disease
Other examples include deficiencies of liver
phosphorylase and debranching enzyme, both involved in the
breakdown of glycogen (Fig. 5-15). In all these disorders
glycogen is stored in many organs, but
hepatic enlargement
and hypoglycemia dominate the clinical picture
Myopathic forms
In the skeletal muscles, as opposed to the
liver, glycogen is used predominantly as a source of energy
during physical activity. ATP is generated by glycolysis, which
leads ultimately to the formation of lactate
If the
enzymes that fuel the glycolytic pathway are deficient
glycogen
storage occurs in the muscles and is associated with muscular
weakness due to impaired energy production.
McArdle disease
type V glycogenosis
Muscle
phosphofructokinase
(type VII glycogen storage disease)
individuals with the myopathic forms
present with
muscle cramps after exercise and lactate levels in
the blood fail to rise after exercise due to a block in glycolysis.
Glycogen storage diseases associated with (1) deficiency of αglucosidase (acid maltase) and (2) lack of branching enzyme
Glycogen storage diseases associated with (1) deficiency of αglucosidase (acid maltase) and (2) lack of branching enzyme
Disorders Associated with Defects in
Proteins That Regulate Cell Growth
Normal growth and differentiation of cells are regulated by two
classes of genes; proto-oncogenes and tumor suppressor genes,
whose products promote or restrain cell growth (Chapter 7). It is
now well established that mutations in these two classes of
genes are important in the pathogenesis of tumors. In the vast
majority of cases, cancer-causing mutations
In approximately 5%
of all cancers,
however, mutations transmitted through the germ
line contribute to the development of cancer. Most familial
cancers are inherited in an autosomal dominant fashion, but a
few recessive disorders have also been described. This subject is
discussed in Chapter 7.
Complex Multigenic Disorders
such disorders are caused by
interactions between variant forms of genes and environmental
factors. A gene that has at least two alleles, each of which
occurs at a frequency of at least 1% in the population, is
polymorphic, and each variant allele is referred to as a
polymorphism. According to the common disease/common
variant hypothesis, complex genetic disorders occur when many
polymorphisms, each with a modest effect and low penetrance,
are co-inherited.
Two additional facts that have emerged from
studies of common complex disorders, such as
type 1 diabetes,
are:
• While complex disorders result from the collective inheritance
of many polymorphisms, different polymorphisms vary in
significance.
of the 20 to 30 genes implicated in
type 1 diabetes
six to seven are most important, and a few
HLA-alleles contribute more than 50% of the risk
Several normal phenotypic characteristics are governed by
multifactorial inheritance
such as hair color, eye color, skin
color, height, and intelligence. These characteristics show a
continuous variation in population groups, producing the
standard bell-shaped curve of distribution
Chromosomal Disorders
Normal Karyotype
As you will remember, human somatic cells contain 46
chromosomes; these comprise 22 homologous pairs of
autosomes and two sex chromosomes, XX in the female and XY
in the male
The study of chromosomes—karyotyping
basic tool of the cytogeneticist. The usual procedure to examine
chromosomes is to arrest dividing cells in metaphase with
mitotic spindle inhibitors (e.g., N-diacetyl-N-methylcolchicine
[Colcemid])
The one most commonly used Stain involves a
Giemsa stain and
is hence called G banding. A normal male karyotype with G
banding is illustrated in Figure 5-17. With standard G banding,
approximately 400 to 800 bands per haploid set can be detected.
The resolution obtained by banding can be markedly improved
by obtaining the cells in prophase. The individual chromosomes
appear markedly elongated, and as many as 1500 bands per
karyotype can be recognized
total number of chromosomes is
given first, followed by
the sex chromosome complement, and
finally the description of abnormalities in ascending numerical
order
the sex chromosome complement, and
finally the description of abnormalities in ascending numerical
order
The short arm of a chromosome is designated
p (for petit)
the long arm is referred to as
q
In a banded karyotype, each arm of the chromosome is divided
into two or more regions bordered by prominent bands
The
regions are numbered (e.g., 1, 2, 3) from the centromere
outward. Each region is further subdivided into bands and subbands, and these are ordered numerically as well
notation Xp21.2 refers to
chromosomal segment
located on the short arm of the X chromosome, in region 2, band
1, and sub-band 2.
Structural Abnormalities of
Chromosomes
The aberrations underlying cytogenetic disorders may
take the form of an abnormal number of chromosomes or
alterations in the structure of one or more chromosomes.
Any exact multiple of the
haploid number of chromosomes (23) is called
euploid
If an error occurs in meiosis or mitosis and a cell acquires a
chromosome complement that is not an exact multiple of 23, it is
referred to as
aneuploidy
The usual causes for aneuploidy are
nondisjunction and anaphase lag
When nondisjunction occurs
during gametogenesis, the gametes formed have
either an extra
chromosome (n + 1) or one less chromosome (n − 1).
Fertilization of such gametes by normal gametes results in two
types of zygotes—trisomic (2n + 1) or monosomic (2n − 1).
anaphase lag
one homologous chromosome in meiosis or one
chromatid in mitosis lags behind and is left out of the cell
nucleus. The result is one normal cell and one cell with
monosomy. As seen subsequently, monosomy or trisomy
involving the sex chromosomes, or even more bizarre
aberrations
Monosomy
involving an autosome generally causes
loss of too much
genetic information to permit live birth or even
embryogenesis, but several autosomal trisomies do permit
survival.
With the exception of trisomy 21
all yield severely
handicapped infants who almost invariably die at an early age.
mitotic errors in early development give rise to
two or more populations of cells with different chromosomal
complement, in the same individual, a condition referred to as
mosaicism. Mosaicism can result from mitotic errors during the
cleavage of the fertilized ovum or in somatic cells
Mosaicism
affecting the sex chromosomes is
relatively common. In the
division of the fertilized ovum, an error may lead to one of the
daughter cells receiving three sex chromosomes, whereas the
other receives only one, yielding, for example, a 45,X/47,XXX
mosaic
All descendent cells derived from each of these
precursors thus have either a 47,XXX complement or a 45,X
complement
Such a patient is a mosaic variant of Turner
syndrome, with the extent of phenotypic expression dependent
on the number and distribution of the 45,X cells
An error in an early mitotic
division affecting the autosomes
y leads to a nonviable
mosaic due to autosomal monosomy. Rarely, the nonviable cell
population is lost during embryogenesis, yielding a viable mosaic
(e.g., 46,XY/47,XY,+21).
A second category of chromosomal aberrations is associated
with
changes in the structure of chromosomes.
To be visible by
routine banding techniques, a fairly large amount of DNA
approximately 2 to 4 million base pairs), containing many
genes, must be involved. The resolution is much higher with
fluorescence in situ hybridization (FISH), which can detect
changes as small as kilobases.
Structural changes in
chromosomes usually result from chromosome breakage
followed by loss or rearrangement of material. In the next
section the more common forms of alterations in chromosome
structure and the notations used to signify them are reviewed
Deletion
Refers to loss of a portion of a chromosome (Fig. 5-
18). Most deletions are interstitial, but rarely terminal deletions
may occur.
interstitial deletions
occur when there are two
breaks within a chromosome arm, followed by loss of the
chromosomal material between the breaks and fusion of the
broken ends
One can specify in which regions and at what
bands the breaks have occurred One can specify in which regions and at what
bands the breaks have occurred
For example, 46,XY,del(16)
(p11.2p13.1) describes breakpoints in the short arm of
chromosome 16 at 16p11.2 and 16p13.1 with loss of material
between breaks
Terminal deletions result from a
Single break in
a chromosome arm, producing a fragment with no centromere,
which is then lost at the next cell division, and a chromosome
bearing a deletion. The end of the chromosome is protected by
acquiring telomeric sequences.
ring chromosome
is a special form of deletion. It is produced
when a break occurs at both ends of a chromosome with fusion
of the damaged ends (Fig. 5-18). If significant genetic material is
lost, phenotypic abnormalities result. This might be expressed as
46,XY,r(14). Ring chromosomes do not behave normally in
meiosis or mitosis and usually result in serious consequences.
Inversion
refers to a rearrangement that involves two breaks
within a single chromosome with reincorporation of the
inverted, intervening segment
An inversion involving
only one arm of the chromosome is known as
paracentric
breaks are on opposite sides of the centromere
pericentric. Inversions are often fully compatible with normal
development.
Isochromosome
formation results when one arm of a
chromosome is lost and the remaining arm is duplicated,
resulting in a chromosome consisting of two short arms only or
of two long arms (Fig. 5-18). An isochromosome has
morphologically identical genetic information in both arms. The
most common isochromosome present in live births involves the
long arm of the X and is designated i(X)(q10). The Xq
translocation,
a segment of one chromosome is
transferred to another (Fig. 5-18). In one form, called balanced
reciprocal translocation, there are single breaks in each of two
chromosomes, with exchange of material. A balanced reciprocal
translocation between the long arm of chromosome 2 and the
short arm of chromosome 5 would be written 46,XX,t(2;5)
(q31;p14).
robertsonian translocation
a translocation between two acrocentric chromosomes
Trisomy 21 (Down Syndrome)
Trisomy 21 (Down Syndrome)
United States the incidence in newborns is about 1 in 700.
Approximately 95% of affected individuals have trisomy 21, so
their chromosome count is 47. FISH with chromosome 21–
specific probes reveals the extra copy of chromosome 21 in such
cases
Most others have normal chromosome
numbers, but the extra chromosomal material is present as a
translocation.
Down syndrome is
meiotic
nondisjunction. The parents of such children have a normal
karyotype and are normal in all respects.
Maternal age has a strong influence on the incidence of
trisomy 21
It occurs once in 1550 live births in women under
age 20, in contrast to 1 in 25 live births for mothers older than
age 45.
In about 4% of cases of Down syndrome,
the extra
chromosomal material derives from the presence of a
robertsonian translocation of the long arm of chromosome
21 to another acrocentric chromosome (e.g., 22 or 14).
robertsonian translocations,
he genetic material normally found
on two pairs of chromosomes is distributed among only three
chromosomes. This affects chromosome pairing during meiosis,
and as a result the gametes have a high probability of being
aneuploid
Approximately 1% of Down syndrome patients are
mosaics, having a mixture of cells with 46 or 47
chromosomes. This mosaicism results from mitotic
nondisjunction of chromosome 21 during an early stage of
embryogenesis. Symptoms in such cases are variable and milder,
depending on the proportion of abnormal cells.
Clearly, in cases
of translocation or mosaic Down syndrome,
maternal age is of no
importance
The diagnostic clinical features of this condition of Downs
flat facial profile, oblique palpebral fissures, and epicanthic
folds (Fig. 5-20)—are usually readily evident, even at birth
flat
facial profile, oblique palpebral fissures, and epicanthic
folds (Fig. 5-20)—are usually readily evident, even at birth
Approximately 40% of the patients
have congenital heart
disease, most commonly defects of the endocardial cushion,
including ostium primum, atrial septal defects, atrioventricular
valve malformations, and ventricular septal defects. Cardiac
problems are responsible for the majority of the deaths in
infancy and early childhood. Several other congenital
malformations, including atresias of the esophagus and small
bowel, are also common.
Children with trisomy 21
have a 10-fold to 20-fold increased
risk of developing acute leukemia. Both acute lymphoblastic
leukemias and acute myeloid leukemias occur. The latter, most
commonly, is acute megakaryoblastic leukemia.
• Virtually all patients with trisomy 21 older than age 40 develop
neuropathologic changes characteristic of Alzheimer disease, a
degenerative disorder of the brain
Other Trisomies
A variety of other trisomies involving chromosomes 8, 9, 13, 18,
and 22 have been described. Only trisomy 18 (Edwards
syndrome) and trisomy 13 (Patau syndrome) are common
hare several karyotypic and clinical features with trisomy
21. Thus, most cases result from meiotic nondisjunction and
therefore carry a complete extra copy of chromosome 13 or 18.
As in Down syndrome, an association with increased maternal
age is also noted
Chromosome 22q11.2 Deletion Syndrome
Chromosome 22q11.2 deletion syndrome encompasses a
spectrum of disorders that result from a small deletion of
band q11.2 on the long arm of chromosome 22.
but it is often missed because of variable clinical
features.
These include congenital heart defects, abnormalities
of the palate, facial dysmorphism, developmental delay, and
variable degrees of T-cell immunodeficiency and hypocalcemia.
Previously, these clinical features were considered to represent
two different disorders—DiGeorge syndrome and
velocardiofacial syndrome
Velocardiofacial syndrome
congenital heart disease involving
outflow tracts, facial dysmorphism,
and developmental delay
Cytogenetic Disorders Involving Sex
Chromosomes
Genetic diseases associated with changes involving the sex
chromosomes are far more common than those related to
autosomal aberrations. Furthermore, imbalances (excess
or loss) of sex chromosomes are much better tolerated
than are similar imbalances of autosomes.
. Regardless of the number of X chromosomes
the presence of a single Y determines the male sex. The
gene that dictates testicular development (SRY: sex-determining
region Y gene) is located on its distal short arm
Klinefelter Syndrome
Klinefelter syndrome is best defined as male
hypogonadism that occurs when there are two or more X
chromosomes and one or more Y chromosomes.
One of
the most frequent forms of genetic disease involving the sex
chromosomes as well as one of the most common causes of
hypogonadism in the male.
Klienfelter
Most patients have a
distinctive body
habitus with an increase in length between the soles and the
pubic bone, which creates the appearance of an elongated body.
Also characteristic are eunuchoid body habitus with abnormally
long legs; small atrophic testes often associated with a small
penis; and lack of such secondary male characteristics as deep
voice, beard, and male distribution of pubic hair. Gynecomastia
may be present. The mean IQ is somewhat lower than normal,
but mental retardation is uncommon. There is increased
incidence of type 2 diabetes and the metabolic syndrome that
gives rise to insulin resistance. Curiously, mitral valve prolapse
is seen in about 50% of adults with Klinefelter syndrome. There
is also an increased incidence of osteoporosis and fractures due
to sex hormonal imbalance.
Klinefelter syndrome is an important genetic cause of
of reduced
spermatogenesis and male infertility. In some patients the
testicular tubules are totally atrophied and replaced by pink,
hyaline, collagenous ghosts.
Patients with Klinefelter syndrome have a higher risk for
breast cancer (20 times more common than in normal males), extragonadal germ cell tumors, and autoimmune diseases such as systemic lupus erythematosus
The classic pattern of Klinefelter syndrome is associated with
a
47,XXY karyotype (90% of cases). This complement of
chromosomes results from nondisjunction during the meiotic
divisions in the germ cells of one of the parents. Maternal and
paternal nondisjunction at the first meiotic division are roughly
equally involved
Turner Syndrome
urner syndrome results from complete or partial
monosomy of the X chromosome and is characterized
primarily by hypogonadism in phenotypic females. It is the
most common sex chromosome abnormality in females, affecting
about 1 in 2500 live-born females.
Approximately 57% of TS
are missing an entire X chromosome,
resulting in a 45,X karyotype. Of the remaining 43%,
approximately one third (approximately 14%) have structural
abnormalities of the X chromosomes, and two thirds
(approximately 29%) are mosaics.
The common feature of the structural abnormalities
produce partial monosomy of the X chromosome. In order of
frequency, the structural abnormalities of the X chromosome
include (1) an isochromosome of the long arm, 46,X,i(X)(q10)
resulting in the loss of the short arm; (2) deletion of portions of
both long and short arms, resulting in the formation of a ring
chromosome, 46,X,r(X); and (3) deletion of portions of the short
or long arm, 46X,del(Xq) or 46X,del(Xp).
Hermaphroditism and Pseudohermaphroditism
The term true hermaphrodite implies the presence of
both ovarian and testicular tissue. In contrast, a
pseudohermaphrodite represents a disagreement between
the phenotypic and gonadal sex (i.e., a female
pseudohermaphrodite has ovaries but male external genitalia; a
male pseudohermaphrodite has testicular tissue but female-type
genitalia).
Single-Gene Disorders with
Nonclassic Inheritance
- Diseases caused by trinucleotide-repeat mutations
- Disorders caused by mutations in mitochondrial genes
- Disorders associated with genomic imprinting
- Disorders associated with gonadal mosaicis
Diseases Caused by Trinucleotide-Repeat
Mutations
Expansion of trineuclotide repeats is an important genetic
cause of human disease, particularly neurodegenerative
disorders
he discovery in 1991 of expanding trinucleotide
repeats
cause of fragile X syndrome was a landmark in
human genetics. Since then the origins of about 40 human
diseases (Table 5-8) have been traced to unstable nucleotide
repeats, and the number continues to grow
Fragile x
The causative mutations are associated with the expansion of a
stretch of trinucleotides that usually share the nucleotides G
and C. In all cases the DNA is unstable, and an expansion of the
repeats above a certain threshold impairs gene function in
various ways, discussed later. In recent years diseases
associated with unstable tetra-, penta-, and hexa- nucleotides
have also been found establishing this as a fundamental
mechanism of neuromuscular diseases.
The proclivity to expand depends strongly on the sex of the
transmitting parent. In the fragile X syndrome, expansions
occur during oogenesis, whereas in Huntington disease they
occur during spermatogenesis.
There are three key mechanisms by which unstable repeats
cause diseases
1) Loss of function of the affected gene,
typically by transcription silencing, as in fragile X syndrome. In
such cases the repeats are generally in non-coding part of the
gene (2) A toxic gain of function by alterations of protein
structure as in Huntington disease and spinocerebellar ataxias.
In such cases the expansions occur in the coding regions of the
genes. (3) A toxic gain of function mediated by mRNA as is seen
in fragile X tremor-ataxia syndrome.
fragile X syndrome
fragile X syndrome
Molecular Genetic Diagnosis
The nascent field of molecular diagnostics emerged in the latter
half of the twentieth century, with the application of low
throughput approaches such as conventional karyotyping for
recognition of cytogenetic disorders (e.g., Down syndrome) and
DNA-based assays such as Southern blotting for the diagnosis of
Huntington disease
POLYMERASE CHAIN REACTION
● PCR analysis, which involves synthesis of relatively
short DNA fragments form a DNA template, has
been a mainstay of molecular diagnostic in a few
decades
● Technique that amplifies segments of DNA, uses
Polymerase and a machine called thermal cycler or
PCR machine.
● Revolutionize genetics
● The target DNA is usually less than 1000 base pairs
(these are DNA that you wanted to figure out if
present or not. For example in an infectious disease
(like tuberculosis), the mycobacterium has a
specific gene pattern, like CGAGC (theoretically),
this is the target DNA you are going to look for in
the patient specimen)
Step 1: Denaturation
● As in DNA replication, the two strands in the DNA
double helix need to be separated.
● The separation happens by raising the temperature
of the mixture (>90 degree celcius) , causing the
hydrogen bonds between the complementary DNA
strands to break. This process is called
denaturation
Step 2: Annealing
Primers bind to the target DNA sequences and
initiate polymerisation. This can only occur once the
temperature of the solution has been lowered (55-
65 degree celcius). One primer binds to each
strand.
Step 3: Extension
● New strands of DNA are made using the original
strands as templates. A DNA polymerase enzyme
joins free DNA nucleotides together.
● This enzyme is often Taq polymerase, an enzyme
originally isolated from a thermophilic bacteria
called Thermus aquaticus. The order in which the
free nucleotides are added is determined by the
sequence of nucleotides in the original (template)
DNA strand. You continue to add nucleotides to
form a new DNA. The cycle repeats-
SANGER SEQUENCING
● A single PCR product is mixed with a DNA
polymerase, a specific primer, nucleotides, and four
dead-end (di-deoxy terminator) nucleotides (A, T,
G, and C) labeled with different fluorescent tags.
● The ensuing reaction produces a ladder of DNA
molecules of all possible lengths, each labeled with
a tag corresponding to the base at which the reaction stopped due to incorporation of a terminator nucleotide. After size separation by electrophoresis, the sequence is “read” and
compared with the normal sequence to detect mutations.
SINGLE BASE PAIR PRIMER EXTENSION
This is a useful approach for identifying mutations
at a specific nucleotide position. A primer is added
to the PCR product that hybridizes one base
upstream of the target, differently colored
terminator fluorescent nucleotides are added and a
single base polymerase extension is performed.
RESTRICTION FRAGMENT LENGTH ANALYSIS
● This simple approach takes advantage of the
digestion of DNA with endonucleases known as
restriction enzymes that recognize and cut DNA at
specific sequences.
● If the specific mutation is known to affect a
restriction site, the amplified PCR product may be
digested, and the normal and mutant PCR products
will yield fragments of different sizes that are easily
distinguished
AMPLICON LENGTH ANALYSIS
● Mutations that affect the length of DNA (e.g., deletions or expansions) can be easily detected by PCR.
● Two primers that flank the region containing the trinucleotide repeats at the 5′ end of the FMR1 gene
are used to amplify the intervening sequences
REAL TIME PCR
● Variety of PCR-based technologies that use
fluorophore indicators can detect and quantify the
presence of particular nucleic acid sequences in
real time
● It is most often used to monitor the frequency of
cancer cells bearing characteristic genetic lesions
in the blood or in tissues or the infectious load of certain viruses
● PCR test is the most accurate however
disadvantage is the long processing time
On the other hand, ANTIGEN TEST
is rapid and it’s
also relatively inexpensive but you have accuracy
problems, because it is dependent on the viral load,
if the viral load is low there’s a big chance that it will
have false negative result.
ANTIBODY TEST
looking for antibody against the
virus IgM and IgG. IgM can be detected early but
the IgG is late, and there’s a lot of False positives
and False negatives associated with this test. With
covid the antigen and antibody test is not used.
PCR TESTING WITH COVID 19
PCR TESTING WITH COVID 19
● It is called coronavirus because of the spikes
protein in the envelope, there’s also other proteins
embedded in the envelope.
S gene
Spike protein
E gene
Envelope small membrane protein
M gene
Membrane gene
HE gene
Hemagglutinin Esterase
N gene
Nucleoprotein
GENOMIC IMPRINTING
● Studies over the past two decades have provided definite evidence that, at least with respect to some genes, important functional differences exist between the paternal allele and the maternal allele.
● These differences result from an epigenetic
process called imprinting
● In most cases, imprinting selectively inactivates either the maternal or paternal allele.
maternal imprinting
refers to transcriptional
silencing of the maternal allele, whereas paternal mprinting implies that the paternal allele is
inactivated
Maternal Imprinting Example would be
○ Prader-Willi syndrome - all cases the
deletion affects the paternally derived
chromosome 15. Characterized by mental
retardation, short stature, hypotonia, profound hyperphagia, obesity, small hands and feet, and hypogonadism.
○ Angelman syndrome - deletion of the same chromosomal region derived from their mothers: characterized by mentally retarded, but in addition they present with ataxic gait, seizures, and inappropriate laughter. Because of their laughter and ataxia, they have been referred to as “happy puppets.”
LYON HYPOTHESIS
It states that (1) only one of the X chromosomes is
genetically active, (2) the other X of either maternal
or paternal origin undergoes heteropyknosis and is
rendered inactive, (3) inactivation of either the
maternal or paternal X occurs at random among all
the cells of the blastocyst on or about day 5.5 of
embryonic life, and (4) inactivation of the same X
chromosome persists in all the cells derived from
each precursor cell.
Viruses such as HIV and COVID19 have
RNA as
their genetic material (genome). The virus RNA is
surrounded by nucleocapsid protein within the virus
envelope, other proteins are embedded in the
envelope itself
The SARS- CoV-2 genome contains genes that
carry
The directions for making proteins that are
needed to replicate inside the human cell. The
COVID-19 testing is to identify part of the viral
genome in the patient sample
This is usually the N gene,
which carries directions
for making the nucleocapsid protein. In cases
where there is not enough viral RNA to detect
directly from the patient sample, reverse
transcriptase polymerase chain reaction amplifies
many copies of the segment of the N gene
Short single stranded pieces of DNA
called primers recognize unique RNA sequences within the viral
genome that bracket the target region of the N\gene, after the first primer binds, reverse transcriptase synthesizes a single stranded DNA copy of the viral DNA known as complementary
DNA
