Duchenne and Becker Muscular Dystrophy Flashcards
Duchenne Muscular Dystrophy (DMD)
- Inheritance
- Prevalence
- Presentation
- Survival
- 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
How does Becker Muscular Dystrophy (BMD) compare to Duchenne Muscular Dystrophy (DMD)?
- BMD is a milder allelic variant with later age of wheelchair dependency
Dystrophan in Duchenne Muscular Dystrophy (DMD)
- Quantity
- Immunohistochemistry
- Protein Quality
Quantity:
- 0% - 5% present
Immunohistochemistry:
- Complete/almost complete absence (muscle tissue)
Protein Quality:
- Most often severely TRUNCATED Protein is unstable and degraded
Dystrophan in Becker Muscular Dystrophy (BMD)
- Quantity
- Immunohistochemistry
- Protein Quality
Quantity:
- 20%-100%
Immunohistochemistry:
- Normal appearing or reduced ± patchy staining
Protein Quality:
- Reduced levels of altered protein
- NORMAL terminal domains with SHORTENED internal domains
What is the overarching difference that leads to the different manifestations of Duchenne and Becker Muscular Dystrophy?
The functionality and general presence of dystrophan
Why is the DMD gene so susceptible to errors and mutation?
- Its the largest known gene
Why does the size of the DMD gene play a role in it being frequently mutated?
- 79 exons
- Spans MULTIPLE recombination HOTSPOTS
- multiple REPETITIVE sequence elements
What percentage of simplex cases are the result of a new mutation in muscular dystrophy ?
- Remember why this is the case
- 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
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
- 65% large deletions spanning one or more exons
- 10% large duplications
- 25% small insertions/deletions or point mutations
Where do breakpoints tend to occur in the DMD gene (5’-UTR, Exon, Intron, 3’-UTR)?
Introns - these breaks usually span 1 or more exons
What are the consequences of deleting an exon that is a multiple of 3?
- Fairly small because this does NOT lead to a frameshift mutation
What are the risks associated with deleting an exon that is not a multiple of 3? (2 possible outcomes)
- Frameshift
- 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)
See Slide 7 or Dr. Park’s DMD/BMD lecture
See Slide 7 or Dr. Park’s DMD/BMD lecture
If Frame neutral deletion of an exon in the DMD gene occurred, what disease would you expect the patient to have?
- You would expect the patient to have Becker’s MD because the deletion occurred in a multiple of 3
T or F: SIZE of mutation is more important than changes in reading frame
FALSE, a change in the reading frame will certainly lead to a truncated protein
What happens to proteins that have a frameshift mutation?
- patient
- Protein is truncated then degraded
- Patient will have full blown DMD
T or F: duplications follow the same rules as deletions (aka multiples of 3 are less harmful than frameshifts)
True
Is it possible to determine the kind of muscular dystrophy someone has without doing a muscle biopsy to look at the dystrophan protein?
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
What 4 types of point mutations can occur at the DMD gene?
*Note: these combined account for 25% of DMD cases
- Nonsense
- Splice Site
- Small Insertions/deletions
- Missense (rare)
What is the result of nonsense mutations in the DMD gene?
DMD point mutation
Truncating - so they more commonly lead to DMD than BMD
What are the outcomes of a splice site mutation in the DMD gene?
(DMD point mutation)
- Could be Neutral (BMD) or Truncating (DMD) depending on the exon taken out
What is the outcome of small insertions and deletions in the DMD gene?
(close in size to DMD point mutation)
- Depends on whether these occur in multiples of 3
**Note: these are too small to be detected with an array
What is the outcome of RARE missense mutations in the DMD gene?
(DMD point mutation)
- Not usually disease causing
What type of tests are commonly used in DMD/BMD testing?
- Scanning Tests (most common)
2. Targeted tests (way less common)
What are 2 good reasons that we use a scanning approach to DMD diagnosis?
- Lots of Allelic Heterogeneity
- High Mutation Rates
Therefore, we need to look through the ENTIRE GENE
Why is there lots of Allelic Heterogeneity in individuals with the DMD phenotype?
many different mutations possible so many affected individuals may have different mutations on each chromosome giving rise to the DMD phenotype
When is it okay to use a targeted DMD test?
To test for the Presence or Absence of a KNOWN FAMILIAL mutation
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?
- Targeted, you would see if her genes had the same mutation that is seen in her affected son
When you do an array to for DMD testing what genes are included in the array?
DMD genes ONLY
Which of the following will a DMD custom array test for?
- Exon deletions and Duplications
- Gene mutations elsewhere on Chromosome
- Insertions and deletions
Yes:
- exon deletions and duplications (75% of cases)
NO:
- gene mutations in other genes
- Insertions or deletions
See slide 6 from second part of DMD/BMD lectures
See slide 6 from second part of DMD/BMD lectures
What do you do if you’re pretty sure a patient has DMD but they’re array test comes back negative?
Scan the Gene again and SEQUENCE specific parts
What parts of the DMD would you sequence if an array test came back negative?
- Intron/Exon Junctions (splice junctions)
- Promoter Regions
- A few know intronic mutation points
Suppose you find a mutation deep in an intron, what do you do with this information?
- 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
Where do breakpoints in the DMD gene typically occur?
- what accompanies this deletion?
- INTRONS, these deletions are usually accompanied by the deletion of at least 1 exon
What are the two most important domains in Dystrophin protein that is encoded by the DMD gene?
- Two Globular Terminal Domains
You take a muscle biopsy and no dystrophin is detected. What is the diagnosis?
- DMD, if it were Becker’s you would expect to see some dystrophin
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.
True
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?
Next you’ll want to do another SCANNING test that SEQUENCES the entire DMD gene to look for the point mutations etc.
A woman had uncle with DMD and has a Child with DMD, but nothing shows up on either DMD test. What can you assume?
She’s the 1% whose results don’t show up. We know she has the disease because she is an obligate carrier
T or F: a chromosomal microarray and a DMD array are the same thing.
False, the DMD array only does the test on one gene
