L30 - Muscle Disorders

Question 1

Define muscular dystrophies and 3 features?

Answer 1

Progressive muscle weakness

1. Hereditary (passed from generation to generation)
2. Progressive (gradually unable to move = die)
3. Each type causes a characteristic, selective pattern of weakness

Question 2

List the 3 classes of muscular disorders due to genetic defect?

Answer 2

1. Defects in substrate utilization, impairment of energy harvesting pathway = exercise intolerance
2. Defects in the contractile mechanism = sudden onset of paralysis
3. Progressive muscle weakness = muscular dystrophies

Question 3

What is the life expectancy and onset age of DMD?

Answer 3

18 months to 4 years: symptoms become evident

15-25 years: death without proper care

Question 4

Compare Becker type muscular dystrophy with DMD?

Answer 4

Becker type = less severe form of DMD (although mutation in same gene, also male-predominant) + Later onset + slower progression

Becker type = Muscle HYPERtrophy esp. calves

Question 5

What mutation causes DMD?

Answer 5

Point mutation and Deletion

dystrophin gene on Xp21 (short arm of X-chromosome) coding for dystrophin protein

Question 6

Describe the function of dystrophin protein?

Answer 6

Connect F-actin (cytoskeleton) to Laminin in ECM via dystrophin-glycoprotein complex

> > maintains shape and structure of muscle fiber

Question 7

List the 6 tests for DMD, BMD diseases?

Answer 7

1. Blood Creatine Kinase test
2. Electromyography
3. Muscle biopsy
4. Immunostaining
5. Western blot
6. DNA tests: PCR, Southern lot, sequencing…etc

Question 8

What is the expected blood creatine kinase level in DMD?

Answer 8

10-100 times the normal amount

Not specific (just tells there is cellular damage)

Question 9

Which diagnostic tests for DMD are not specific?

Answer 9

Blood test for CK

EMG

Muscle biopsy (can tell shrinking of muscle fibers and dystrophy but not whether it is DMD)

Question 10

List the steps in Immunostaining?

Answer 10

Basic principle:

1) Add and incubate primary antibody against dystrophin
2) wash unbound antibodies
3) Add enzyme-linked secondary antibody against 1st antibody and wash
4) Color development (amount of color proportional to amount of dystrophin protein)

Question 11

How do the test results from immunostaining tell DMD from BMD?

Answer 11

DMD = Absent dystrophin 
BMD = Localized but reduced dystrophin staining
Outlier = severely reduced dystrophin

Question 12

Dx if Western blot shows dystrophin has reduced abundance but normal size?

Answer 12

Becker type

Question 13

Dx if Western blot shows dystrophin has reduced abundance and size?

Answer 13

Becker type

Question 14

Dx if Western blot shows no dystrophin present?

Answer 14

DMD

Question 15

Dx if Western blot shoes severely reduced abundance?

Answer 15

Outlier type

Not DMD or BMD

Question 16

Principle procedures of Southern blot?

Answer 16

Use enzymes to cut into fragments

> > denature double strand

> > Transfer electrophoresis-separated DNA fragments to a filter membrane

> > detect fragment by probe hybridization

Question 17

List 3 treatment options for DMD?

Answer 17

Drug therapy

Gene therapy/ replacement

Stem cell therapy

Question 18

How may gene therapy theoretically help DMD?

Answer 18

CRISPR-Cas9 to edit DNA

or

Transfect cell with ‘good’ dystrophin gene

Question 19

How may stem cell therapy theoretically help DMD?

Answer 19

Reprogram cells into induced pluripotent stem cells

Myoblast transfer and implantation to grow muscle

Question 20

It is common to find a father with DMD to have affected children of both sex. True or False?

Answer 20

False: DMD father + normal mother&raquo_space; unaffected sons and carrier daughters

+ Unlikely for DMD male to be fertile and reproduce

Question 21

What is the inheritance pattern of DMD?

Answer 21

X-linked recessive

Question 22

Which family members of DMD affected male may be carriers?

Answer 22

Female relatives of affected males may be carriers

Mothers of affected males in families with multiple affected males are carriers

Question 23

Mother of affected males with DMD with no affected relatives are carriers. True or False?

Answer 23

False

their sons may have been affected by new mutations

Question 24

The son of a carrier mother of DMD has 100% chance of being affected? True or False?

Answer 24

False

Mother = X'X
Father = XY

Son = either X’Y or XY so son of a carrier mother has 50% chance of being affected

Question 25

The daughter of a carrier mother of DMD has a 25% chance of being a carrier. True or False?

Answer 25

False

Daughter of a carrier has a 50% probability of being a carrier:

Depends on X chromosome inactivation (different in different cells)

> > how many cells have activated normal vs. mutant chromosome

Question 26

The son of an affected male must be affected with DMD too. True or False?

Answer 26

False

Father = X'Y
Mother = XX

Son = XY only
Sons of an affected male must be unaffected

Question 27

The daughter of an affected male with DMD must be a carrier. True or False?

Answer 27

True

Father = X'Y
Mother = XX

Daughter can only be X’X
Must be carriers