Myotonic dystrophy Flashcards
What is the incidence of myotonic dystrophy?
1 in 8000
What is the mean age of onset of DM?
20-50 years
What is the leading cause of death in DM patients?
Heart failure
What proprotion of DM cases do DM1 and DM2 make up?
DM1 = 98%, DM2 = 2%
What is the inheritance pattern on DM?
AD with anticipation
What is the phenotype of adult-onset DM1?
Muscle weakness and wasting Myotonia Cataracts Cardiac abnormalities, palpitations Tiredness IBS Some dysmorphic features; tented upper lip, droopiness, frontal balding
What is the phenotype of DM2?
Similar features to DM1, but milder. More proximal weakness.
What is the pathogenic mechanism of DM1?
CTG repeat expansion of 3’UTR of DMPK on chr 19q
What is the pathogenic mechanism of DM2?
CCTG repeat expansion in intron 1 of CNBP (prev ZNF9)
What is involved in the management of DM patients?
Avoid some anaesthetics, annual eye and heart examinations. Reproductive advice.
What is the phenotype of congenital DM1?
Hypotonia at birth, respiratory failure/compromise, generalised weakness.
If patients survive to childhood - developmental delay and dysmorphic features.
What is the phenotype of congenital DM in utero?
Decreased foetal movements, polyanhydramnios
What is the cause of congenital DM1?
Large repeat expansion that is almost always maternally inherited.
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?
Mother with mild features = 3-5% risk
Mother with established disease = 40%
What are some in the features in the presymptomatic protocol with DM1 testing compared to HD?
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
What are the allele classifications in DM1?
Normal 5-35 CTG repeats
Intermediate 36-50
Affected >51
What are the requirements of a molecular genetic test for DM1?
Should be able to distinguish between allele classes and homozygous alleles
What are the testing methods for DM1?
Repeat sizing PCR, TP-PCR
Describe repeat sizing PCR.
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
What are the advantages and disadvantages of repeat sizing PCR for DM1?
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.
How does TP-PCR work?
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
Why might bi-directional TP-PCR be required?
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.
When should a familial positive control be used?
In PST, if test was performed prior to use of TP-PCR or was performed by another laboratory.
Why does TP-PCR not provide accurate allele sizing?
preferential amplification of smaller allele, cannot amplify large full mutation alleles,s
What are the advantages and disadvantages of TP-PCR?
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%)
How does Southern blot testing work for DM1?
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.
What are the advantages and disadvantages of Southern Blotting?
Advantages: 2 blots detects all cases, including atypical expansion/insertions. Can size alleles.
Disadvantage - labour intensive, requires lots of DNA, may use radioactive probes.
What is the underlying pathological mechanisms for DM1?
RNA toxic gain of function
