MSS30 Muscle Disorders

Question 1

Muscle disorders

Answer 1

1. **Progressive muscle weakness
   - -> **Muscular dystrophies
2. Defects in substrate utilization, impairment of energy harvesting pathway
   - -> Exercise intolerance
3. Defects in contractile mechanisms
   - -> Sudden onset of paralysis

Question 2

Muscular dystrophies

Answer 2

Group of muscle diseases with 3 features in common:

1. Hereditary
2. Progressive
3. Each type causes a characteristic, selective pattern of weakness

Question 3

Duchenne Muscular Dystrophy (DMD)

Answer 3

• 1 in 3500 in live male birth

Stages:

• At birth: no physical indication
• 1st year: rare for any delay in development to be noticed
• 18 months - 4 years: become evident
• 8/9 years: weakness progresses rapidly
• 12 years: lose ability to walk independently
• 15-25 years: death

Progressive symptoms:
Shoulder, pelvic areas
–> shortening of muscles and loss of muscle tissue
–> upper trunk and arm muscles

• **Becker Type Muscular Dystrophy (BMD):
• less severe form of DMD
• onset > 7 yo
• ***muscle hypertrophy esp. Calves
• slowly progressive
• failure to walk 16 - 80 yo

Outlier:

• intermediate between DMD and BMD (severity, onset etc.)
• failure to walk 12-16 yo

Question 4

Genetics of DMD

Answer 4

Genetic Mutation (***Point mutation, Deletion)

• Short arm of ***X-chromosome
• locus: Xp21
• -> ***dystrophin gene
• -> affecting ***Dystrophin (protein)

Inheritance: ***X-linked recessive
(male: 條X有問題就得; female: 兩條X都有問題先得)

Carrier mother x Unaffected father
–> ONLY (mostly) males affected

Son:

• -> Son of carrier mother: 50% affected
• -> Son of affected male: ALL unaffected (rmb ***no male-to-male transmission)

Daughter:

• -> Daughter: carrier
• -> Daughter of carrier: 50% carrier
• -> Daughter of affected male: ALL carriers
• -> female relatives of affected males may be carriers
• -> mothers of affected males (esp. > 1 affected male): Carriers
• -> mothers of affected males with NO affected relatives may NOT be carrier: ∵ sons affected by new mutations

Question 5

Function of Dystrophin

Answer 5

Maintain **shape and **structure of muscle fibres

• -> dystrophin (intracellular) contact F-actin
• -> dystrophin glycoprotein complex form a bridge across sarcolemma to laminin in ECM

Question 6

***Diagnosis of DMD

Answer 6

1. Blood test
   - ***Creatine kinase (CK) test: 10-100x normal amount
2. Electromyography (EMG)
3. Muscle biopsy
   - muscle fibres ***shrink and presence of gaps in-between
4. Immunostaining
   - BMD (milder): **reduced dystrophin staining around rim of muscle fibres
   - DMD: **absent of dystrophin staining
5. Western blot
   - ***protein denaturation followed by gel electrophoresis
6. Detection at DNA level
   - DNA blood test to analyse X chromosome
   - PCR
   - Southern blot (combines transfer of electrophoresis-separated ***DNA fragments to a filter membrane and subsequent fragment detection by probe hybridization)
   - DNA sequencing
   - DNA chip

Question 7

Basic principle of immunostaining

Answer 7

1. Addition and incubation of 1st Ab against dystrophin –> wash
2. Addition of 2nd Ab (***enzyme labeled) against 1st Ab –> wash
3. Colour development

Question 8

Therapeutic possibilities

Answer 8

1. Drug therapy
   - prolong walking by 2-3 years
2. Gene therapy
   - gene repair/replacement
   - introduce another copy of “good” dystrophin gene
3. Cell therapy
   - **myoblast transfer
   - **human stem cells (iPS / Induced pluripotent stem cells)
   - -> collect cells (普通cell都得)
   - -> reprogram into iPS cells
   - -> correct mutation
   - -> genetically correct iPS cells differentiate into blood stem cells
   - -> re-transplant into patient