Myotonic dystrophy

Question 1

What is the incidence of myotonic dystrophy?

Answer 1

1 in 8000

Question 2

What is the mean age of onset of DM?

Answer 2

20-50 years

Question 3

What is the leading cause of death in DM patients?

Answer 3

Heart failure

Question 4

What proprotion of DM cases do DM1 and DM2 make up?

Answer 4

DM1 = 98%, DM2 = 2%

Question 5

What is the inheritance pattern on DM?

Answer 5

AD with anticipation

Question 6

What is the phenotype of adult-onset DM1?

Answer 6

Muscle weakness and wasting
Myotonia
Cataracts
Cardiac abnormalities, palpitations
Tiredness
IBS
Some dysmorphic features; tented upper lip, droopiness, frontal balding

Question 7

What is the phenotype of DM2?

Answer 7

Similar features to DM1, but milder. More proximal weakness.

Question 8

What is the pathogenic mechanism of DM1?

Answer 8

CTG repeat expansion of 3’UTR of DMPK on chr 19q

Question 9

What is the pathogenic mechanism of DM2?

Answer 9

CCTG repeat expansion in intron 1 of CNBP (prev ZNF9)

Question 10

What is involved in the management of DM patients?

Answer 10

Avoid some anaesthetics, annual eye and heart examinations. Reproductive advice.

Question 11

What is the phenotype of congenital DM1?

Answer 11

Hypotonia at birth, respiratory failure/compromise, generalised weakness.
If patients survive to childhood - developmental delay and dysmorphic features.

Question 12

What is the phenotype of congenital DM in utero?

Answer 12

Decreased foetal movements, polyanhydramnios

Question 13

What is the cause of congenital DM1?

Answer 13

Large repeat expansion that is almost always maternally inherited.

Question 14

What is the % risk of a mother with mild features of DM1 having a child affected with congenital DM1? What is the risk for a mother with established disease?

Answer 14

Mother with mild features = 3-5% risk

Mother with established disease = 40%

Question 15

What are some in the features in the presymptomatic protocol with DM1 testing compared to HD?

Answer 15

Interventions in DM include annual monitoring and anaesthetic advice. No interventions in HD.

Truncated protocol compared to HD (2 sessions)

Do not need to counsel for psychiatric issues/dementia

Question 16

What are the allele classifications in DM1?

Answer 16

Normal 5-35 CTG repeats
Intermediate 36-50
Affected >51

Question 17

What are the requirements of a molecular genetic test for DM1?

Answer 17

Should be able to distinguish between allele classes and homozygous alleles

Question 18

What are the testing methods for DM1?

Answer 18

Repeat sizing PCR, TP-PCR

Question 19

Describe repeat sizing PCR.

Answer 19

Fluorescently tagged primers flank the repeat region. Benzene and DMSO is added to decrease the formation of secondary DNA structures, which would inhibit the PCR reaction.
Fragments are separated by capillary electrophoresis.
Two controls are added per run; allele boundaries + blank

Question 20

What are the advantages and disadvantages of repeat sizing PCR for DM1?

Answer 20

Advantages: Cheap, easy to set-up, quickly determine unaffected patient with two differently sized alleles, accurately sizes normal alleles.

Disadvantages: SNP under primers can give false -ve, unable to size >80 repeats. Cannot differentiate homozygotes.

Question 21

How does TP-PCR work?

Answer 21

Three primer system:
1: F primer flanks repeat region
2. R primer is composed of two parts: 3' end specific to repeat, 5'end non-specific to human genome
3: Tail primers binds to 5' of R primer
Amplification of products 3bp apart

Question 22

Why might bi-directional TP-PCR be required?

Answer 22

Some families have an insertion in the repeat tract. R primer does not bind close enough to the F primer, which gives a false negative in one direction.

Question 23

When should a familial positive control be used?

Answer 23

In PST, if test was performed prior to use of TP-PCR or was performed by another laboratory.

Question 24

Why does TP-PCR not provide accurate allele sizing?

Answer 24

preferential amplification of smaller allele, cannot amplify large full mutation alleles,s

Question 25

What are the advantages and disadvantages of TP-PCR?

Answer 25

Advantages: Cheap, simple, can be run in parallel with repeat size PCR.

Disadvantages: Interruptions/SNPs can give false negative results. Does not accurately size alleles. Sensitive to MCC (5%)

Question 26

How does Southern blot testing work for DM1?

Answer 26

EcoRI digest ofr large expansion. Fragment contains repeat 9kb or 10kb is insertin present.

Expanded alleles produce a smear due to mitotic instability.

PST1 digest allows more accurate sizing of smaller repeats.

Question 27

What are the advantages and disadvantages of Southern Blotting?

Answer 27

Advantages: 2 blots detects all cases, including atypical expansion/insertions. Can size alleles.

Disadvantage - labour intensive, requires lots of DNA, may use radioactive probes.

Question 28

What is the underlying pathological mechanisms for DM1?

Answer 28

RNA toxic gain of function