DM1 and DM2

Question 1

Phenotype of DM1

Answer 1

Can be:
mild (cataracts and balding)
classical (adult onset; DISTAL muscle weakness, mytonia, difficulty gripping with hands, dropping eyelids)
Juvenile - same as adult but also cognitive/behavioural phenotype
Congenital - in utero polyhydramnios and poor fetal movements, neonatal feeding difficulties

Question 2

Phenotype of DM2

Answer 2

cataracts, balding, variable myotonia
PROXIMAL muscle weakness
muscle pain

Question 3

Cause of DM1

Answer 3

(CTG)n expansion in 3’UTR of DMPK gene at 19q13.3 (99% cases)

Question 4

Cause of DM2

Answer 4

(CCTG)n expansion in intron 1 of CMPG gene at 3q21

Question 5

Repeat ranges for CTG expansion in DPMK

Answer 5

normal = 5-34
premutation = 35-49
Expanded = 50+

Question 6

Expanded repeat ranges for CTG expansion in DMPK

Answer 6

51-500 = mild
100-1000 = classical/adult
500-2000 = juvenile
1000 - 5000 = congenital

Question 7

Allele expansion patterns in DM1

Answer 7

51-150 CTGs = expand in 75% cases, irrespective of transmitting parent
150+ CTGs = high risk of expansion into congenital range, especially if maternally inherited

Question 8

Repeat ranges for CCTG expansion in CMPG

Answer 8

Normal = <27
Premutation = 27-74 (phenotype and clinical relevance unknown)
Affected = 75-11,000 (actual pathogenic threshold not determined yet!) - shows marked somatic heterogeneity due to instability

Question 9

Molecular pathogenesis for DM1 and DM2 (similar)

Answer 9

RNA MEDIATED GOF MECHANISM:
mRNA trnascripts containing expansions accumulate in ribonuclear foci of cells
Alters regulation and localisation of CUG and RNA binding proteins
Results in aberrant splicing of genes expressed in tissues affected by DM

Question 10

Testing for DM1

Answer 10

1) flourescent PCR specific for repeat tract (only detects normal alelles)
2) triplet primed PCR (used to size repeat tract)

Question 11

Principles of triplet primed PCR (TP-PCR)

Answer 11

3 primers;
Forward - binds to sequence upstream of repeat
Reverse - 3’ end is complementary to repeat, 5’ end is non specific, so sticks up
Tail - specific to 5’ end of R primer

Question 12

Testing for DM2

Answer 12

Testing is for whole complex, not just CCTG (rest of complex highly polymorphic