Duchenne and Becker

Question 1

Where do diagnostic referrals tend to come from?

Answer 1

Paeds/neurology/clinical genetics

Question 2

Where do carrier tesring/PND referrals come from?

Answer 2

Clinical genetics

Question 3

Mode of inheritance

Answer 3

X-linked recessive (Xp21.2)

Question 4

Incidence of a) DMD b) BMD

Answer 4

a) 1/3500 male live births

b) 1/18000 male live births

Question 5

How many exons in the dystrophin gene?

Answer 5

79

Question 6

Classes of causative mutations in DMD?

Answer 6

60-65% frameshift deletions removing one or more exons
25-30% nonsense/frameshift mutations
5% exon duplication

Question 7

What is generally the mechanism by which DMD mutations cause lack of dystrophin?

Answer 7

Creation of a premature STOP codon

Question 8

What causes Becker MD?

Answer 8

In-frame mutations which result in reduced levels of dystrophin production (around 10-40% of normal)

Question 9

How can Becker and Duchenne be distinguished?

Answer 9

Muscle biopsy

Question 10

Onset/mean age of death in DMD

Answer 10

Before 5, mean age of death 25

Question 11

Name of the manouevre that children wit DMD use to stand

Answer 11

Gower

Question 12

Symptoms of DMD (5)

Answer 12

Developmental delay and learning difficulties (30-50%)
Chunky calves
Progressive proximal muscle weakness
Lumbar lordosis
Inability to walk beyond age of 12/13

Question 13

Differences between DMD and BMD

Answer 13

Becker is milder; no learning difficulties; survive to middle age and beyond; onset of muscle weakness is around 11 years

Question 14

What cardiac complication are people with DMD at risk of?

Answer 14

Dilated cardiomyopathy

Question 15

Proportion of female carriers with symptoms?

Answer 15

around 5-10%

Question 16

Carrier symptoms?

Answer 16

Muscle cramps

Mild muscle weakness

Question 17

Carriers should have what kind of screening?

Answer 17

5-yearly echocardiograms due to risk of dilated cardiomyopathy

Question 18

Testing for DMD

Answer 18

Creatine kinase
Genetic testing (try to avoid muscle biopsy)
-1st line MLPA for deletion/duplication analysis
-2nd line Sanger for point mutations

Question 19

When would a muscle biopsy be required and what analysis is performed on the tissue?

Answer 19

If all genetic testing negative

Dystrophin immunohistochemistry

Question 20

What proportion of mutations in dystrophin are de novo?

Answer 20

1/3rd

Question 21

What is the rate of gonadal mosaicism for DMD/BMD?

Answer 21

10%

Question 22

What different timepoints can a mutation occur? (3)

Answer 22

In egg at time of conception- no recurrence risk
After conception (proband is somatic mosaic)- no recurrence risk
Early in female germline development i.e. the mother is gonadal mosaic- therefore recurrence risk depends on the proportion of affectec gonadal cells

Question 23

Under what circumstances can females present with classic DMD? (5)

Answer 23

X-autosome translocation disrupting the dystrophin gene; in this case the normal X is inactivated, therefore the female has no functioning dystrophin

Female with Turner syndrome (XO)

Maternal uniparental disomy (if daughter inherits two copies of the chromosome carrying the mutation)

Skewed X inactivation

Father with BMD x carrier female = daughters at risk of inheriting a mutation on both X chromosomes

Question 24

Sensitivity of MLPA for DMD/BMD?

Answer 24

Approx 72% as does not detect point mutation

Question 25

Why do results showing single exon deletions need to be interpreted with caution?

Answer 25

Could be caused by variant under the probe hybridization site; need to be confirmed by another technique e.g. multiplex PCR

Question 26

How can a high-risk chromosome be tracked through a dystrinopathy pedigree?

Answer 26

Linkage using microsatellite markers

Question 27

When is linkage analysis useful?

Answer 27

When the mutation cannot be found; allows carrier and prenatal testing to be performed

Question 28

What is the use of linkage limited by?

Answer 28

Risk of recombination between markers and the actual dystrophin mutation (10% recombination rate across the dystrophin gene)

Question 29

What is required for linkage analysis?

Answer 29

DNA from appropriate family members to define the risk haplotype; need to check that markers are informative

Question 30

Dystrophin mutational hotspots?

Answer 30

Exons 2-20, exons 45-55

Question 31

What resource is available to check deletions/duplications?

Answer 31

Leiden muscular dystrophy website- can check whether in/out of frame

Question 32

What could happen if the first or last exon is deleted?

Answer 32

Deletion could extend into neighbouring gene and alter the phenotype

Question 33

Who can also be tested alongside the proband to ensure the mutation can be picked up?

Answer 33

A positive control

Question 34

Why are only male fetuses tested for PND?

Answer 34

Not possible to predict phenotype of a female and ethical issues around identifying carrier females before the patient can give consent

Question 35

What test can be performed prior to PND?

Answer 35

Free foetal DNA sexing (sexing also performed by QF-PCR or FISH when the pranatal sample arrives)

Question 36

What needs to be excluded in any prenatal sample and how is this done?

Answer 36

Maternal cell contamination | Microsatellite markers

Question 37

How does MLPA work?

Answer 37

Probes consisting of 2 parts that hybridize to adjacent sequences of target DNA; if bind next to each other, they are ligated and form a probe ligation product that can be amplified by PCR Control probes to other chromosomal locations Normal control samples included