Genetics of Triplet Repeat Disorders (Week 4--Grody) Flashcards
What does it mean that trinucleotide repeat expansions are dynamic mutations?
They can change, and are not fixed like single nucleotide mutations
Unusual aspects of inheritance of trinucleotide repeat disorders
Anticipation
Parent-of-origin effects
Skewed X-inactivation
Methylation effects
Incomplete penetrance
Variable expression
Premutation alleles (specific to Fragile X…)
Examples of triplet repeat disorders
Huntington disease: autosomal dominant; CAG; coding region
Fragile X syndrome: X-linked; CGG; 5’ untranslated region (UTR)
Myotonic dystrophy: autosomal dominant; CTG; 3’ UTR
Friedreich ataxia: autosomal recessive; GAA; intron
Spinobulbar muscular atrophy: CAG
Spinocerebellar ataxia 1: CAG
CGG repeat in Fragile X Syndrome
Normal: 6-54 repeats
Premutation: 55-200 repeats
Full mutation: 200 - >1000 repeats
Methylation in Fragile X Syndrome
Hypermethylation causes gene expression to be turned off
Alleles with >200 repeats are hypermethylated (transcriptionally repressed)
Note: patient with expanded triplet repeat but NO methylation does NOT exhibit symptoms of Fragile X Syndrome!
When using Southern blot vs. PCR to look for full mutation, what do you see?
Southern blot: see large smear because different lengths and long fragments
PCR: see no band because too long and also if CGG repeat (Fragile X) too hard to unwind because C’s and G’s hold on via 3 H bonds
Southern Blot for Fragile X
For females will see 2 bands (XX) and for males will see 1 band (XY)
Larger (higher up) band means premutation and even larger means full mutation
Fragile X methylation analysis
Can use BssH2 which is a methylation sensitive restriction enzyme that only cuts DNA if it is unmethylated
This is important because you could see that you have a large fragment that is unmethylated so will not cause Fragile X
Does the length of the premutation affect the incidence of full mutation in offspring?
Yes!
The longer the maternal premutation, the more likely the offspring is to get the full mutation
56-59 premutation –> 13% chance of offspring with full mutation
80-89 –> 73%
100-109 –> 100%
Fragile X Syndrome
X-linked recessive/semi-dominant (some females affected)
CGG repeat in 5’ UTR
Premutation expands when inherited from mother
Full mutation must be hypermethylated to cause disease
>90% have mental retardation, IQ 20-60; children are hyperactive, ADD, autistic features, hyperextensible joints, mitral prolapse; after puberty: macroorchidism, long face, large ears, prominent jaw
Fragile X premutation symptoms
Female: premature ovarian failure
Male: late-onset tremor-ataxia-dementia
Myotonic dystrophy
Autosomal dominant
Myotonia, muscular dystrophy, cataracts, hypogonadism, frontal balding
CTG repeat in 3’ UTR
Mild or severe based on length of repeat
Greatest expansion when inherited from mother (this can produce severe neonatal form)
Friedreich ataxia
Autosomal recessive
GAA repeat in intron; small proportion are compound heterozygotes w/expansion on one allele and point mutation on the other
Gait disturbance in childhood, ataxia, absent reflexes, cardiomyopathy, intellectual decline
Huntington’s disease
Autosomal dominant
CAG repeat in exon (>39); codes for polyglutamine tract in protein product; length of repeat inversely related to age of onset
Greatest expansions when inherited from father
Middle-age onset, choreoathetosis, dementia, psychosis
Childhood onset Huntington’s disease
If repeat 100 or more
Would need expansion through father for many generations
