Repeat expansion disorders 2

Question 1

Myotonic dystrophy

Answer 1

Expansion resulting in gain of RNA function
Impaired muscle relaxation
Muscle wasting
Insulin resistance
Caused by mutations in two different genes

Question 2

DM1

Answer 2

Expansion in CTG repeats in 3’ UTR of DMPK gene
Normal 5-37
premutation 37-50
Pathogenic 50-1000
Congenital form more than 1000 repeats
Maternal expansion bias
Age of onset has inverse correlation with repeat length

Question 3

DM2

Answer 3

CCTG repeats in intron 1 of ZNF9/CNBP gene
Normal length less than 30
Premutation 31-74
Pathogenic 75-11000

Question 4

RNA gain of function disease

Answer 4

Repeat containing RNA forms imperfect hairpins

Sequesters splicing proteins leading to aberrant splicing of other genes such as insulin receptor

Question 5

FMR1 premutation disorders

Answer 5

Fragile X-associated tremor/ataxia syndrome (FXTAS)
40% of males with premutation repeat
Late onset neurodegenerative disorder

Fragile X associated premature ovarian insufficiency (FXPOI)
25% of female premutation carriers

Question 6

Premutation repeats

Answer 6

Leads to increased transcription of FMR1 mRNA
Sequesters RNA binding proteins and dysregulation of proteins normally regulated by them
May lead to FXTAS/FXPOI

Question 7

Amyotrophic lateral sclerosis

Answer 7

Repeat Associated Non-ATG (RAN) translation of repeat containing RNA

Translation initiated without ATG initiation codon
Repeat expansion translated on both transcripts
Toxic RAN peptides produces
RNA toxicity
Loss of function (haploinsufficiency)
Expanded GGGGCC in intron 1 of C9orf72

Question 8

Mechanisms of repeat expansion

Answer 8

Normal alleles may be short repetitive runs (short-normal alleles) or longer repetitive runs with stabilising interruptions (long-normal alleles)

Expansion begins when uninterrupted repetitive run exceeds a threshold of around 100 to 150 bases.
Often due to loss of stabilising interruptions

Question 9

Repeat expansion beyond the threshold

Answer 9

Normally, both upstream and downstream replication origins are active and repeats are replicated from upstream ori. Structure prone repeat is on the leading strand template and the repeat is stable

If upstream ori is inactive, repeats are replicated by downstream ori. Structure prone repeat on lagging strand. Hairpin forms and converted to expanded repeats

Question 10

Hybrid DNA repair pathways

Answer 10

Following oxidative damage, base excision repair occurs.
DNA pol delta initiates repair synthesis. Repeat sequence displaced and forms hairpin
MutSbeta stabilises hairpin and blocks access to Fen1 flap endonuclease
Hairpin incorporation leads to repeat expansion

Question 11

Hairpin translation

Answer 11

RNA polymerase blocked by hairpin stabilised by MutSbeta
Promotes transcription coupled repair (TC-NER)
NER may excise hairpin (contraction) or nick on one side of the hairpin only (expansion)