B6.020 Prework 3: Genetic, Molecular, and Cellular Aspects

Question 1

gene affected in DMD and BMD

Answer 1

DMD

dystrophin; x chromosome

Question 2

types of mutations in DMD

Answer 2

large deletions (60-70%)
large duplications (10%)
point mutations (15-30%)

Question 3

outcome of mutation in DMD

Answer 3

loss of function

severely reduced or absent protein

Question 4

types of mutations in BMD

Answer 4

large deletions (85%)
point mutations (15%)

Question 5

outcome of mutations in BMD

Answer 5

truncated protein retaining partial function

Question 6

dystrophin protein function

Answer 6

links cytoskeleton to ECM via transmembrane dystroglycan protein and associated sarcoglycans
dystrophin binds cytoskeletal actin via its N-terminal domain and syntrophin complex via its C terminal domain

Question 7

how many repeats typically in a dystrophin protein

Answer 7

24 spectrin like repeats in central rod

Question 8

what happens without dystrophin

Answer 8

muscle membrane susceptible to damage
muscle fiber deterioration occurs
tears in membrane result in calcium leakage
cycles of regeneration and degeneration > fibrosis and fatty replacement of muscles

Question 9

can you distinguish BMD and DMD on histo alone?

Answer 9

NOOOOO

Question 10

histo features of both BMD and DMD

Answer 10

rounding fibers and variation in fiber size
hypertrophy and atrophy of fibers
necrosis and loss of fibers
basophilic regenerating fibers
densely stained hypercontracted fibers
increased internal nuclei
proliferation of CT and increased adipose tissue

Question 11

DMD gene significance

Answer 11

largest in human genome

Question 12

BMD relationship with reading frame

Answer 12

DMD mutations THAT PRESERVE OPEN READING FRAME

translation of internally truncated protein with functional C-terminus

Question 13

DMD relationship with reading frame

Answer 13

out of frame deletions
truncated reading frame
causes complete loss of dystrophin

Question 14

what is nonsense mediated decay

Answer 14

premature termination codons > 50 NTs upstream of final exon-intron junction
transcript degraded via nonsense mediated mRNA decay
any surviving transcripts will make truncated protein

Question 15

pathogenesis of DMD

Answer 15

1. deletion or duplications producing frameshifts in genes encoding dystrophin
2. diminished synthesis of the mRNA for dystrophin
3. low levels or absence of dystrophin
4. structural integrity of the muscle is affected
5. contractions stress the muscle cells and they gradually die
6. progressive, usually fatal muscle weakness

Question 16

pathogenesis of BMD

Answer 16

1. deletions of duplications maintaining reading frame in the gene encoding dystrophin
2. synthesis of truncated mRNA for dystrophin
3. synthesis of a partially functional, shorter dystrophin
4. structural integrity of muscle cells is affected
5. contractions stress the muscle cells and they gradually die
6. progressive, usually fatal muscle weakness

Question 17

female disease expression of BMD and DMD

Answer 17

X linked
80% of female carrier show elevated CK but are asymptomatic
occasionally mild muscle weakness encountered in female carriers
-sufficient # of muscle cells have active X chromosome with mutant dystrophin gene
-females with Turner will show disease (single X)

Question 18

DM1 gene affected and normal function of protein

Answer 18

DMPK gene

serine-threonine kinase

Question 19

types of mutations in DM1

Answer 19

CTG trinucleotide expansion in the 3’ untranslated region of DMPK

Question 20

molecular mechanism of DM1 mutation

Answer 20

toxic gain of function by mRNAs
sequester cellular splicing factors
reduced protein level

Question 21

DM2 gene affected and normal function of protein

Answer 21

CNBP

nucleic acid binding protein translation control

Question 22

types of mutations in DM2

Answer 22

CCTG tetranucleotide repeat expansion in first intron of CNBP

Question 23

molecular mechanism of DM2 mutation

Answer 23

toxic gain of function by mRNAs
sequester cellular splicing factors
reduced protein level

Question 24

histo of DM1 and DM2

Answer 24

wide variation in fiber size
multiple internal nuclei
loss of muscle fibers and increasing amounts of fat and fibrous tissue

Question 25

pathogenesis of DM

Answer 25

expanded repeat mRNAs from stem-loop structure

• sequesters splicing and /or other RNA processing factors
• globally alters mRNA splicing: toxic to cell
  specifically: mis-splicing of CLC1 mRNA, encoding chloride channel, results in reduced channel protein and leads to myotonia

Question 26

variations in pathogenesis of trinucleotide repeat disease

Answer 26

expansion can be in any region (intron, exon, UTRs)
mutation DOES NOT always increase number of AAs
-huntington: increased glutamines in protein
-DM: no change in AAs, expansion is in intron (DM1) or 3’ UTR (DM1)
protein function therefore may not be related to disease

Question 27

tissues/cells affected by trinucleotide repeat diseased

Answer 27

depends on:
tissues/cells in which mutated gene is expressed
importance of altered protein to tissue/cells function
ability of cells to tolerate accumulation of unfolded protein

Question 28

what is anticipation

Answer 28

repeat number is greater than normal but not quite pathogenic level
might have minor clinical phenotypes
unstable prone to additional expansion
repeat numbers increase with each generation