Genetic Disorders 2 Flashcards

1
Q

Lysosomal Storage Diseases

A

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

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2
Q

Similar to all other secretory proteins, lysosomal enzymes (or
acid hydrolases, as they are sometimes called) are synthesized
in the

A

ER

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3
Q

The phosphorylated mannose

residues serve as an

A

“address label” that is recognized by
specific receptors found on the inner surface of the Golgi
membrane.

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4
Q

n inherited deficiency of a

functional lysosomal enzyme gives rise to two pathologic consequences (

A

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

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5
Q

, leading to the accumulation of the partially
degraded insoluble metabolite within the lysosomes. This is
called

A

“primary accumulation”.

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6
Q

impaired autophagy gives rise to

A

“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.

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7
Q

Tay-Sachs Disease (GM2
Gangliosidosis:
Hexosaminidase α-Subunit Deficiency)

A

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

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8
Q

The phenotypic

effects of mutations affecting these genes are fairly similar,

A

because they result from accumulation of GM2 gangliosides. The
underlying enzyme defect, however, is different for each

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9
Q

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.

A

Tay Sachs

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10
Q

This disease

is especially prevalent among

A

Jews, particularly among those of
Eastern European (Ashkenazic) origin, in whom a carrier rate of
1 in 30 has been reported.

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11
Q

ganglioside

A

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

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12
Q

The hexosaminidase A is absent from virtually all the
tissues, so GM2 ganglioside accumulates in many
tissues

A

heart, liver, spleen, nervous system), but
the involvement of neurons in the central and
autonomic nervous systems and retina dominates
the clinical picture

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13
Q

histologic examination

A

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.

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14
Q

Clinical Features of TS

A

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

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15
Q

Niemann-Pick Disease Types A and B

A

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.

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16
Q

Type A

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

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17
Q

Type B

A

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

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18
Q

Although, this disease is

typically inherited as an autosomal recessive,

A

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.

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19
Q

Morphology of NEIP

A

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

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20
Q

Innumerable small

vacuoles of relatively uniform size are created

A

imparting foaminess to the cytoplasm

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21
Q

In frozen sections of fresh tissue

A

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

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22
Q

The lipid-laden phagocytic foam cells are

A

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.

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23
Q

In the brain

A

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.

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24
Q

Clinical manifestations in type A disease

A

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

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25
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,
26
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
27
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.
28
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
29
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
30
Glucocerebrosides are continually | formed from the catabolism of glycolipids
derived mainly from | the cell membranes of senescent leukocytes and red cells
31
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)
32
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.
33
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
34
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
35
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.
36
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.
37
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
38
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
39
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
40
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)
41
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.
42
Several clinical variants of MPS, classified numerically
MPS I to MPS VII, have been described, each resulting from the deficiency of one specific enzyme.
43
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
44
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.
45
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.
46
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.
47
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.
48
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.
49
Hunter syndrome,
also called MPS II, differs from Hurler syndrome in mode of inheritance (Xlinked), absence of corneal clouding, and milder clinical course.
50
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.
51
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.
52
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)
53
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
54
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
55
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
56
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.
57
McArdle disease
type V glycogenosis
58
Muscle | phosphofructokinase
(type VII glycogen storage disease)
59
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.
60
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
61
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
62
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.
63
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.
64
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.
65
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
66
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
67
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
68
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])
69
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
70
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
71
The short arm of a chromosome is designated
p (for petit)
72
the long arm is referred to as
q
73
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
74
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.
75
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.
76
Any exact multiple of the | haploid number of chromosomes (23) is called
euploid
77
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
78
The usual causes for aneuploidy are
nondisjunction and anaphase lag
79
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).
80
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
81
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.
82
With the exception of trisomy 21
all yield severely | handicapped infants who almost invariably die at an early age.
83
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
84
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
85
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
86
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).
87
A second category of chromosomal aberrations is associated | with
changes in the structure of chromosomes.
88
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.
89
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
90
Deletion
Refers to loss of a portion of a chromosome (Fig. 5- 18). Most deletions are interstitial, but rarely terminal deletions may occur.
91
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
92
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
93
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.
94
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.
95
Inversion
refers to a rearrangement that involves two breaks within a single chromosome with reincorporation of the inverted, intervening segment
96
An inversion involving | only one arm of the chromosome is known as
paracentric
97
breaks are on opposite sides of the centromere
pericentric. Inversions are often fully compatible with normal development.
98
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
99
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).
100
robertsonian translocation
a translocation between two acrocentric chromosomes
101
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.
102
Down syndrome is
meiotic nondisjunction. The parents of such children have a normal karyotype and are normal in all respects.
103
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.
104
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).
105
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
106
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.
107
Clearly, in cases | of translocation or mosaic Down syndrome,
maternal age is of no | importance
108
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
109
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.
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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
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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
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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.
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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
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Velocardiofacial syndrome
congenital heart disease involving outflow tracts, facial dysmorphism, and developmental delay
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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.
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. 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
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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.
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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
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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.
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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.
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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 ```
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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
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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.
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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.
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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).
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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).
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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
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Diseases Caused by Trinucleotide-Repeat | Mutations
Expansion of trineuclotide repeats is an important genetic cause of human disease, particularly neurodegenerative disorders
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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
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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.
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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.
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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.
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fragile X syndrome
fragile X syndrome
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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
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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)
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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
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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.
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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-
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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.
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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.
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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
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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
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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
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● PCR test is the most accurate however
disadvantage is the long processing time
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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.
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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.
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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.
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S gene
Spike protein
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E gene
Envelope small membrane protein
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M gene
Membrane gene
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HE gene
Hemagglutinin Esterase
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N gene
Nucleoprotein
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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.
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maternal imprinting
refers to transcriptional silencing of the maternal allele, whereas paternal mprinting implies that the paternal allele is inactivated
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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.”
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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.
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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
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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
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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
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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