Clinical Applications Flashcards
1
Q
Clinical diagnosis of genetic diseases
A
- Fragile syndrome is the most common cause of inherited mental retardation and is an example of a disease resulting from dynamic mutations
- hemoglobinopathies are an example of a class of disease with many types of mutations and diseases of global importance
- cystic fibrosis is a common autosomal recessive with very significant allelic heterogeneity
2
Q
Molecular Genetic Diagnostics
A
Purpose is to alter risk:
- carrier testing
- diagnosis
- prenatal diagnosis (PND)
- presymptomatic Test (PST)
- prenatal exclusion-PND
3
Q
Fragile X Syndrome
A
- most common cause of mental retardation known to be inherited
- incidence of ~1/1250 male births, ~1/2500 female births. Not expected for XL Dominant or XL Recssive MOI
- is an X-linked dominant with reduced penetrance (80% in males, 30% in females)
- the fragile site is defined cytogenetically and can be induced by specific media conditions
4
Q
Mental retardation characteristics
A
- broad forehead
- elongated face
- large prominant ears
- strabismus (crossed eyes)
- highly arched palate
- hyperextensible joints
- hand calluses
- pectus excavatum (indentation of chest)
- mitral valve prolapse
- enlarged testicles
- hypotonia
- soft fleshy skin
- flat feet
- seizures
5
Q
Fragile X- hypomorphic mutation
A
- > 99% of cases due to the expansion of a CGG triplet repeat in the first exon and methylation of the FMR1 gene leading to deficiency of protein
- amplification of triplet expansion is detected by PCR, and.or Southern with greater sensitivity than cytogenetically
- in all mentally retarded individuals, the CGG repeats and the adjacent CpG islands are methylated & the FMR1 is inactive
- three mutation class normal, premutation and full mutation
6
Q
Fragile X (FRAXA)
A
- can detect normal transmitting males ( many develop ataxia) and carrier females (prone to premature ovarian failure) by direct molecular analysis
- good correlation of copy number between class and disease status, but too loose within disease class for prognosis
- probability of further expansion is related to size of expansion in female carrier
- anticipation- known as Sherman Paradox, higher incidence in sons than uncles, penetrance of the disease goes up
7
Q
Premutation
A
- females with premutation may have mild symptoms, premature ovarian failure
- “normal transmitting males” misnomer as they may have mild symptoms, over a third of them will develop Fragile-X Associated Tremor/Ataxia syndrome in later life
- FXTAS is rare in females with premutation
- NTMs may have FRAXA affected grandchildren with full mutation, but their daughters will have premutations
8
Q
Fragile X transition
A
- mutation 1 to mutation 2, small increase in copy number with a small chance of further inrease
- mutation 2 to mutation 3, further small increase with high risk of increase, and then to get to the large full mutation there must be transmission from the mother
9
Q
FMR1 gene on Xq 27.3
A
-can detect and size increase in number of CGG repeats by increase in size of fragment containing the repeat as it is recognized by its homology to the probe
-normal- 30 repeats or less
-premutation to affected- about 200 triplets
-Eag I is a methylation sensitive enzyme, so there is a 5.2 Eco RI fragment- the methylated
and 2.8 kb the EagI/EcoRI fragment
10
Q
Number of bands on males/females, normal and diseased
A
Normal male - 2.8kb
Diseased male- 3.1kb
Normal female- 2.8kb, 5.2kb
Diseased female- 2.8 kb, 3.1kb, 5.2kb, 5.5kb
11
Q
Hemoglobinpathies
A
- easily the most common genetic diseases in the world, and cause substantial morbidity
- ~5% of world population are carriers of genes for clinically important disorders of hemoglobin
- ~300,000 severely affected homozygotes or compound heterozygotes born each year
- 5 alpha-like genes clustered on chromosome 16
- 6 beta-like genes clustered on chromosome 11
12
Q
Hemoglobin
A
- a tetrameric alpha2beta2 molecule of adult hemoglobcin
- representation of a molecule of normal adult hemoglobin (Hb A)
- there are two alpha and two beta chains, each associated with a heme moiety
- as hemoglobin shifts from oxygenated to the deoxygenated state, critical movement of the chain occurs
- beta gene activity replaces the gamma gene activity after birth
13
Q
Sickle Cell disease
A
1/600 African americans
- Hb S is a GAG9G) –> GTG(G) in codon 6 of beta globin gene resulting in the substitution of glutamic acid with valine which also results in the destruction of an Mnl I site (GAGG)
- Hb C is a GAG(G) —> AAG(G) results in the substitution of glutamic acid with lysine which also destroys the same Mnl I site
- carriers with sickle cell trait are usually asymptomatic and have a selective advantage conferred from providing some resistance to the Malaria parasite
- homozygotes (Hb S/ Hb S) have sickle cell disease which is often fatal in early childhood but with good medical care life span can be normal
- hemolytic anemia caused by rigid structure of hemoglobin
14
Q
Sickle cell phenotypes
A
Normal- Hb A, alpha2A/beta2A
Sickle cell trait- Hb A, Hb S, alpha2A/beta2a, alpha2A/beta2S
Sickle cell disease- Hb S, alpha2A/ beta 2S
15
Q
Sickle cell disease symptoms
A
- patients generally present in the first 2 years of life with anemia, failure to thrive, splenomegaly, repeated infections, and episodically, painful swelling of the hands or feet
- Hb SC and compound heterozygotes have a milder sickle cell anemia anemia and may have no clinical problems
- electrophoretic studies can detect carriers
- most likely reason for DNA studies are for Prenatal diagnosis in newborns
- Mnl I digest- those with sicklet cell have one line at 75, the heterozygotes have a line at 75, and 60 and 15, because on cuts and one doesn’t. In normal both are cut so two 60 and 15 bp
