Duchenne and Becker Muscular Dystrophy

Question 1

Duchenne Muscular Dystrophy (DMD)

• Inheritance
• Prevalence
• Presentation
• Survival

Answer 1

• X-linked recessive
• Affects 1/3500 males
• Delayed motor milestones, progressive weakness and muscle loss; hypertrophic calf muscles; Cardiac and respiratory complications
• Patients usually only live into their early 20s

Question 2

How does Becker Muscular Dystrophy (BMD) compare to Duchenne Muscular Dystrophy (DMD)?

Answer 2

• BMD is a milder allelic variant with later age of wheelchair dependency

Question 3

Dystrophan in Duchenne Muscular Dystrophy (DMD)

• Quantity
• Immunohistochemistry
• Protein Quality

Answer 3

Quantity:
- 0% - 5% present

Immunohistochemistry:
- Complete/almost complete absence (muscle tissue)

Protein Quality:
- Most often severely TRUNCATED Protein is unstable and degraded

Question 4

Dystrophan in Becker Muscular Dystrophy (BMD)

• Quantity
• Immunohistochemistry
• Protein Quality

Answer 4

Quantity:
- 20%-100%

Immunohistochemistry:
- Normal appearing or reduced ± patchy staining

Protein Quality:

• Reduced levels of altered protein
• NORMAL terminal domains with SHORTENED internal domains

Question 5

What is the overarching difference that leads to the different manifestations of Duchenne and Becker Muscular Dystrophy?

Answer 5

The functionality and general presence of dystrophan

Question 6

Why is the DMD gene so susceptible to errors and mutation?

Answer 6

• Its the largest known gene

Question 7

Why does the size of the DMD gene play a role in it being frequently mutated?

Answer 7

• 79 exons
• Spans MULTIPLE recombination HOTSPOTS
• multiple REPETITIVE sequence elements

Question 8

What percentage of simplex cases are the result of a new mutation in muscular dystrophy ?
- Remember why this is the case

Answer 8

• 1/3 or simplex males are new mutations

NOTE: this is the general rule with severely debilitating X-linked recessive:

• 1/3 of X gene pool is in males
• The affected X gene is lost from affected DMD patients
• Prevalence of the disease stays the same overtime so 1/3 of affected people came from new mutation

**otherwise disease prevalence would decline steady as mutated X chromosomes were lost in affected individuals

Question 9

How often do the following mutations occur in the DMD gene:

• Large Deletions (spanning 1 or more exons)
• Large Duplications
• Small insertions/Deletions or point mutations

Answer 9

• 65% large deletions spanning one or more exons
• 10% large duplications
• 25% small insertions/deletions or point mutations

Question 10

Where do breakpoints tend to occur in the DMD gene (5’-UTR, Exon, Intron, 3’-UTR)?

Answer 10

Introns - these breaks usually span 1 or more exons

Question 11

What are the consequences of deleting an exon that is a multiple of 3?

Answer 11

• Fairly small because this does NOT lead to a frameshift mutation

Question 12

What are the risks associated with deleting an exon that is not a multiple of 3? (2 possible outcomes)

Answer 12

1. Frameshift
2. Frame Neutral (no frameshift)
   - this is only the case if you had a (3)n+2 exon and a (3)n+1 exon being ligated the overhang of 2 and 1 would come together to make a readable codon (otherwise its framshift)

Question 13

See Slide 7 or Dr. Park’s DMD/BMD lecture

Answer 13

See Slide 7 or Dr. Park’s DMD/BMD lecture

Question 14

If Frame neutral deletion of an exon in the DMD gene occurred, what disease would you expect the patient to have?

Answer 14

• You would expect the patient to have Becker’s MD because the deletion occurred in a multiple of 3

Question 15

T or F: SIZE of mutation is more important than changes in reading frame

Answer 15

FALSE, a change in the reading frame will certainly lead to a truncated protein

Question 16

What happens to proteins that have a frameshift mutation?

- patient

Answer 16

• Protein is truncated then degraded

- Patient will have full blown DMD

Question 17

T or F: duplications follow the same rules as deletions (aka multiples of 3 are less harmful than frameshifts)

Answer 17

True

Question 18

Is it possible to determine the kind of muscular dystrophy someone has without doing a muscle biopsy to look at the dystrophan protein?

Answer 18

Yes, blood samples can be taken where we can just look at the GENE SEQUENCE to see if the mutation was FRAMESHIFT (DMD) or FRAME NEUTRAL (BMD)

• This is 90% accurate

Question 19

What 4 types of point mutations can occur at the DMD gene?

*Note: these combined account for 25% of DMD cases

Answer 19

1. Nonsense
2. Splice Site
3. Small Insertions/deletions
4. Missense (rare)

Question 20

What is the result of nonsense mutations in the DMD gene?

DMD point mutation

Answer 20

Truncating - so they more commonly lead to DMD than BMD

Question 21

What are the outcomes of a splice site mutation in the DMD gene?

(DMD point mutation)

Answer 21

• Could be Neutral (BMD) or Truncating (DMD) depending on the exon taken out

Question 22

What is the outcome of small insertions and deletions in the DMD gene?

(close in size to DMD point mutation)

Answer 22

• Depends on whether these occur in multiples of 3

**Note: these are too small to be detected with an array

Question 23

What is the outcome of RARE missense mutations in the DMD gene?

(DMD point mutation)

Answer 23

• Not usually disease causing

Question 24

What type of tests are commonly used in DMD/BMD testing?

Answer 24

1. Scanning Tests (most common)

2. Targeted tests (way less common)

Question 25

What are 2 good reasons that we use a scanning approach to DMD diagnosis?

Answer 25

1. Lots of Allelic Heterogeneity 2. High Mutation Rates Therefore, we need to look through the ENTIRE GENE

Question 26

Why is there lots of Allelic Heterogeneity in individuals with the DMD phenotype?

Answer 26

many different mutations possible so many affected individuals may have different mutations on each chromosome giving rise to the DMD phenotype

Question 27

When is it okay to use a targeted DMD test?

Answer 27

To test for the Presence or Absence of a KNOWN FAMILIAL mutation

Question 28

What type of test would you use if a female had an affected child and wanted to know her chances of having another affected child?

Answer 28

- Targeted, you would see if her genes had the same mutation that is seen in her affected son

Question 29

When you do an array to for DMD testing what genes are included in the array?

Answer 29

DMD genes ONLY

Question 30

Which of the following will a DMD custom array test for? - Exon deletions and Duplications - Gene mutations elsewhere on Chromosome - Insertions and deletions

Answer 30

Yes: - exon deletions and duplications (75% of cases) NO: - gene mutations in other genes - Insertions or deletions

Question 31

See slide 6 from second part of DMD/BMD lectures

Answer 31

See slide 6 from second part of DMD/BMD lectures

Question 32

What do you do if you're pretty sure a patient has DMD but they're array test comes back negative?

Answer 32

Scan the Gene again and SEQUENCE specific parts

Question 33

What parts of the DMD would you sequence if an array test came back negative?

Answer 33

- Intron/Exon Junctions (splice junctions) - Promoter Regions - A few know intronic mutation points

Question 34

Suppose you find a mutation deep in an intron, what do you do with this information?

Answer 34

- Compare it to a database and see if its Pathogenic or not NOTE: few of these are documented so often its hard to say whether or not the small mutation is pathogenic

Question 35

Where do breakpoints in the DMD gene typically occur? | - what accompanies this deletion?

Answer 35

- INTRONS, these deletions are usually accompanied by the deletion of at least 1 exon

Question 36

What are the two most important domains in Dystrophin protein that is encoded by the DMD gene?

Answer 36

- Two Globular Terminal Domains

Question 37

You take a muscle biopsy and no dystrophin is detected. What is the diagnosis?

Answer 37

- DMD, if it were Becker's you would expect to see some dystrophin

Question 38

T or F: although much less common insertions/deletions or point mutations still make up a large portion of mutations leading to DMD or BMD.

Answer 38

True

Question 39

You use a DMD array test to see if a patient has DMD or BMD and no mutations are detected. What's your next move?

Answer 39

Next you'll want to do another SCANNING test that SEQUENCES the entire DMD gene to look for the point mutations etc.

Question 40

A woman had uncle with DMD and has a Child with DMD, but nothing shows up on either DMD test. What can you assume?

Answer 40

She's the 1% whose results don't show up. We know she has the disease because she is an obligate carrier

Question 41

T or F: a chromosomal microarray and a DMD array are the same thing.

Answer 41

False, the DMD array only does the test on one gene