DM1 1 and 2 Flashcards
What is DM1 stand for?
Myotonic dystrophy
What is myotonic dystrophy?
Multisystem NMD, characterized by muscle weakness, wasting, & myotonia, Microsatellite repeat expansion in DMPK 3’ UTR (DM1) or CNBP 1st intron (DM2)
What does the toxic gain of function cause?
Toxic gain-of-function causes widespread dysregulation of pre-mRNA splicing (spliceopathy)
What type of genetic mutation of DM?
autosomal dominant
If one parent has one affected allele what is the percentage kids will be affected?
50%
Prevalence of DM?
DM is second most common MD after DMD, most common adult MD, prevalence = ~1/8,000
What % DM1 and its future?
98% of DM is DM1
• ~15% of DM1 is fetal-onset congenital DM1
• ~10% of DM1 is childhood-onset, beginning 1-10 yrs old
• ~75% of DM1 is adult-onset, with patients developing symptoms in 2nd, 3rd, 4th decade
where is DM highly prevalent?
High prevalence in PQ, particularly in Charlevoix and Saguenay–Lac St-Jean regions, ~1/600
Diagnosis of DM1?
- DM1 patients usually have characteristic thin, long face, hollow temples and, in men, early frontal balding
- DM causes muscle weakness and wasting, usually beginning in face, neck, and distal limbs
- Myotonia = delayed relaxation (prolonged contraction)
- Myotonic myopathy includes action (grip & release) and percussion myotonia
- When clinical signs point to DM1, genetic testing definitive
- Histological evidence of central nuclei, fiber size variability
What causes DM1?
- DM1 caused by CTG repeat expansion in the 3’ UTR of the DMPK gene
- Mutation causes formation of CUG)n in DMPK transcripts, nuclear aggregation of DMPK mRNA (foci), sequestration of RNA binding proteins = DMPK mRNA toxic gain-of-function
- DMPK protein reduced, however this loss-of-function mechanism does not cause DM1
at DNA level what is expanded?
CTG expansion in 3 pime uTR area of DMPK gene
at mRNA level?
CUG is added
In healthy individuals how many CTG repeats do we have? And what is the threshold
5 is most common, ranges from 5-28, largest number of repeats in healthy individual is 37
Do carries of pre-mutation present symptoms?
No or at least very mild sympotoms such as cateracts
Explain correlation b/w repeats, severity and age of onset for DM1
INCREASED repeats = increase servierty and early onset
Important aspects of DM1 pathobiology:
anticipation and instability
Repeat instability
• Variations in CTG)n lengths between tissues can occur during multiple stages of development
• Against expectations, CTG)n more unstable in nondividing cells of muscle, heart, brain, than in proliferating blood cells
• In skeletal muscle, DM1 expansion typically grows to >2,000 repeats by 20 yrs old, and in patients over 40, average repeat length in muscle >4,000, 3- to 25-fold larger than in blood
• Thus, CTG)n expansion size depends on tissue sampled
& age
Genetic anticipation
Genetic anticipation a phenomenon in which disease severity increases and/or age of disease onset decreases from one generation to the next
• In DM1, anticipation often occurs from generation to next, with larger CTG expansions, younger onset, & more severe symptoms in each subsequent
generation
• CTG)n increases >200 repeats per generation
• Anticipation not inevitable, occasionally CTG)n undergoes intergenerational contraction
What do the CUG expansions on mRNA DMPK do?
CUG expansions form nuclear foci caused by folding of CUG)n into double-stranded hairpin structures that sequester muscleblind (MBNL1) & upregulate CELF1 (aka CUGBP1) RNABPs
Microsatellite expansion within DMPK 3’ UTR sequesters MBNL1, what does MBNL1 do?
mitigates MBNL1- dependent splicing, as well as promotes nuclear retention of DMPK mRNA (nuclear foci)
what does PKC activation by expanded CUG do?
induces CUGBP1 (CELF1) hyperphosphorylation, which stabilizes protein & enhances alternative splicing, stability, translation of CUGBP1 targets
In spinal cord sections of DM1 patients (A and B), FISH and IF analyses demonstrates:
nuclear (blue) foci of CUG)n mRNA (red) in ChAT-positive (green) α-motoneurons of anterior horn, in DM1 patients, muscle blind and cug foci are centrally located (seeing how it traps MBNL-1
Fluorescence in situ hybridization (FISH) and immunofluorescence of tibialis anterior muscle section shows:
myonuclear (blue) colocalization of CUG)n mRNA foci (red) with MBNL1 protein (green) in DM1 patients
Pathogenic model of DM1
- Decreased MBNL1 and increased CUGBP1 activity alter splicing of transcripts involved in DM1 pathology, e.g., RNAs encoding chloride channel (CLCN1) and insulin receptor α subunit (IRα)
- Genes responsible for some clinical features, such as cataracts, not yet been identified
What does MBNL1 do?
Exon 7a skipping on CLCN-1 gene in the premrna for adult splicing patterns
what does the reduced CLCN protein look like?
- DM1 patients exhibit reduced CLCN1 protein to < 10% of levels in healthy individuals
- Loss of CLCN1 protein in skeletal muscle results in augmented excitability & myotonia
What leads to the insulin resistance in DM1 patients?
Missplicing of IRa subunit (contains exon 11 and is predominantly expressed in healthy skeletal muscle)
What does missplicing of IRa do?
• Missplicing of IRα results in fetal IRα-A isoform with lower signaling capability, causing
impaired insulin-stimulated glucose uptake and insulin resistance (IR) in DM1 skeletal muscle
• IR in DM1 correlates with failure to express mature, functional IRα-B splice variant
Is splicing in DM1 patients similar or different
Dysregulated alternative splicing in DM1 skeletal muscle (different for everyone)
