16. Other repeats - polyalanine & FSHD Flashcards
What encodes polyalanine repeats?
Imperfect trinucleotide repeats (GCN)
What size are polyalanine repeats expansions?
> 19 repeats
How do polyalanine tracts compare to polyglutamine tracts?
PolyA tracts are smaller than polyQ
PolyA are stable through meiosis/mitosis
PolyA are often de novo
Describe a polyalanine repeat-associated disorder
OPMD (oculopharyngeal muscular dystrophy)
Late onset, neurodegenerative - dysphagia, ptosis, proximal limb weakness
Caused by polyA expansion in PABN1 exon 1
Usually AD
What is the expansion mechanism in polyA tracts?
Unequal crossing over between 2 normal sized alleles
What is the phenotype of FSHD?
Asymmetrical, progressive muscle weakness (including facial muscles), scapular winging
2 phenotypically indistinguishable sub-types (95% FSHD1)
What is the molecular mechanism behind FSHD1 & what are the repeat ranges?
Contraction of D4Z4 repeats on 4q35 sub-telomere
Normal = 11-100 repeats, pathogenic = 1-10
Each repeat contains copy of DUX4 (DUX4, DUX4-like 1 etc.)
What is the role of DUX4 and DUX4L1, 2, 3, etc
DUX4 causes apoptosis of myeloblasts.
Active in early development and in testes of adults, normally inactive in most adult tissue.
DUX4-like genes - epigenetically inactivated
What two events must occur to cause FSHD?
D4Z4 contraction on permissive 4qA haplotype
No disease if contraction occurs on 4qB haplotype
Polymorphism is distal to D4Z4 repeats
What is the effect of the 4qA allele?
Provides a polyadenylation sequence allowing stable expression of DUX4 mRNA –> apoptosis of myoblasts
What is the effect on a reduction in the number of D4Z4 repeats?
Loss of heterochromatin, reduced CpG methylation –> transcription of DUX4 from final repeat unit (if present on 4qA allele)
What genes are involved in FSHD2?
What is the molecular mechanism?
Mutations in SMCHD1 or DNMT3B + at least 1 permissive 4qA allele (digenic inheritance)
SMCHD1 is an epigentic modifier of D4Z4 - LoF variants cause chromatin relaxation and hypomethylation of D4Z4 repeats –> DUX4 expression –> apoptosis of myoblasts
What is the risk to the offspring of an individual with FSHD2?
25-50%, depending on number of pathogenic haplotypes in the family - due to digenic inheritance
How is FSHD1 tested for?
Southern blotting - size repeats and distinguish chr 4 repeats from chr 10 repeats (not associated with FSHD)
Detection with P13E-11 probe - hybridises to ch4 and 10 repeats
Double restriction enzyme digest - cuts proximal and distal to chr4 and 10 repeats. Chr 10 repeats digested completely
4qA/B probes + HindIII digest to detect permissive haplotype
How is FSHD2 tested for?
Bisulphite PCR of 4q locus + quantification of methylation by pyrosequencing
If hypomethylated –> sequence SMCHD1/DNMT3B
