B6.020 Prework 3: Genetic, Molecular, and Cellular Aspects Flashcards
gene affected in DMD and BMD
DMD
dystrophin; x chromosome
types of mutations in DMD
large deletions (60-70%) large duplications (10%) point mutations (15-30%)
outcome of mutation in DMD
loss of function
severely reduced or absent protein
types of mutations in BMD
large deletions (85%) point mutations (15%)
outcome of mutations in BMD
truncated protein retaining partial function
dystrophin protein function
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
how many repeats typically in a dystrophin protein
24 spectrin like repeats in central rod
what happens without dystrophin
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
can you distinguish BMD and DMD on histo alone?
NOOOOO
histo features of both BMD and DMD
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
DMD gene significance
largest in human genome
BMD relationship with reading frame
DMD mutations THAT PRESERVE OPEN READING FRAME
translation of internally truncated protein with functional C-terminus
DMD relationship with reading frame
out of frame deletions
truncated reading frame
causes complete loss of dystrophin
what is nonsense mediated decay
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
pathogenesis of DMD
- deletion or duplications producing frameshifts in genes encoding dystrophin
- diminished synthesis of the mRNA for dystrophin
- low levels or absence of dystrophin
- structural integrity of the muscle is affected
- contractions stress the muscle cells and they gradually die
- progressive, usually fatal muscle weakness
pathogenesis of BMD
- deletions of duplications maintaining reading frame in the gene encoding dystrophin
- synthesis of truncated mRNA for dystrophin
- synthesis of a partially functional, shorter dystrophin
- structural integrity of muscle cells is affected
- contractions stress the muscle cells and they gradually die
- progressive, usually fatal muscle weakness
female disease expression of BMD and DMD
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)
DM1 gene affected and normal function of protein
DMPK gene
serine-threonine kinase
types of mutations in DM1
CTG trinucleotide expansion in the 3’ untranslated region of DMPK
molecular mechanism of DM1 mutation
toxic gain of function by mRNAs
sequester cellular splicing factors
reduced protein level
DM2 gene affected and normal function of protein
CNBP
nucleic acid binding protein translation control
types of mutations in DM2
CCTG tetranucleotide repeat expansion in first intron of CNBP
molecular mechanism of DM2 mutation
toxic gain of function by mRNAs
sequester cellular splicing factors
reduced protein level
histo of DM1 and DM2
wide variation in fiber size
multiple internal nuclei
loss of muscle fibers and increasing amounts of fat and fibrous tissue
pathogenesis of DM
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
variations in pathogenesis of trinucleotide repeat disease
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
tissues/cells affected by trinucleotide repeat diseased
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
what is anticipation
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
