Week 5 - Duchenne Muscular Dystrophy

Question 1

What is the cause of muscular dystrophy?

Answer 1

• mutation in the gene producing dystrophin
• causes constant muscle contraction / relaxation
• weakens and destroys the muscle

Question 2

What are the types of muscular dystrophy?

Answer 2

becker
- not produce functional dystrophin
- limited function
- so less severe
duchenne
- not produce dystrophin
- no function
- more severe

Question 3

What are the symptoms of DMD?

Answer 3

early
- delayed walking
- waddling
- difficulty standing up
- enlarged calves
later
- difficulty getting up from a chair
- unable to climb stairs
- wide gaited walk
- balance problems
- fatigue
- mental ratardation

Question 4

How can DMD be diagnosed?

Answer 4

blood Creatine PhosphoKinase (CPK) test
- damaged muscles can release kinase into blood
- elevated levels denote muscle injury
ElectroMyoGraphy (EMG)
- electrical signalling to and from muscle
- rules out neurodegenative diseases
muscle biopsy
- analysis of a sample of muscle tissue
genetic testing
- testing for mutations

Question 5

How does a muscle biopsy look for someone with DMD?

Answer 5

• increased size variation of cells
• necrosis: dead cells
• fibrosis: thickening and scarring of connective tissues
• fatty replacements
• irregular architecture when stained with antibody against dystrophin
• female carriers show patchy staining

Question 6

What is MLPA?

Answer 6

Multiplex Ligation-dependent Probe Amplification
- detects copy number changes
- can scan for intragenic deletions and duplications
- target sequence has pair of primers
- hybridise to adjacent sequences
- right probe contains stuffer fragment
- for variable lengths of PCR
- DNA ligase seals the two probes to create continuous sequence
- amplification
- separated by capillary gel electrophoresis
- half peaks suggests heterozygous deletion

Question 7

How does dystrophin work?

Answer 7

• forms structural link between actin filaments of cytoskeleton at N-terminal
• via membrane bound Dystrophin-Associated Protein (DAP) complex
• to ECM at C-terminal
• gives stability to sarcolemma
• contains spectrin-like repeats
• aid in shock absorbance from elastic recoil during contraction/ relxation
• not mechanically fragile muscle

Question 8

What is utrophin?

Answer 8

• forms Utrophin-Associated Protein Complex (UAPC)
• similiar to DAPCs
• lacks spectrin-like repeats 15 an 19
• only binds through N-terminal domain

Question 9

What is the principle of exon skipping?

Answer 9

• Antisense Oligonucleotides (AOs) used to mask exon (51) to be skipped
• to correct reading frame
• truncated but functional dytrophin
• milder symptoms

Question 10

How can gene replacement treat DMD?

Answer 10

• can be done all DMD patients
• ~20% wild type required for correction
• required widespread and efficient delivery to all muscles
• difficult due to high length
• using AAV vectors (Adeno-Associated Virus) but with limited cloning capacity
• immune response to AAV detected

Question 11

How can utrophin modulation treat DMD?

Answer 11

• switched off in maturing fibres when dystrophin is produced
• produced in early stages for repair after fibre damage
• independent of genetic fault
• can treat all DMD patients
• potential to target multiple pathways