Therapeutic approaches to DMD 1 and 2 Flashcards
What is another therapeutic approach to DMD?
Antisense oligonucleotide-mediated exon skipping
How does Antisense oligonucleotide-mediated exon skipping work?
• Opposite of ASO therapy in SMA – exon exclusion (DMD) instead of exon inclusion (SMA)
• ASO causes “skipping” of exon that neighbours deleted exon, transforming out-of-frame
(nonsense) transcript into in-frame transcript capable of dystrophin production
• Thus, dystrophin lacks some amino acids but retains function, similar to BMD phenotype
How many exons in open reading frame for healthy dystrophin?
- 79 exons fit precisely together
- Start of each exon coincides with position 1, 2, or 3 of codon, which encodes specific AA
- ASO promotes skipping of exon adjacent to mutation [i.e., deleted exon(s)] to restore ORF
What does the ASO specifically target on the exon?
The ESE (part of splicing process)
What is used for the ASO mediated exon 51 skipping?
Exondys 51
What did the study of DMD patients given exondys 51 intravenous for 48 weeks daily show?
- 23% and 52% dystrophin-positive fibers after 24 and 48 weeks of treatment, respectively
- Presence of nNOS, β- and γ-sarcoglycan restored at sarcolemma, indicating DAPC reassembly
What does nNOS do?
Regulates blood flow
What test did they use to test if the DMD patients retained function?
6 minute walk test
What did the 6 minute walk test show?
• In 4 year, open-label study, Exondys-treated patients (n = 12) experienced slower disease
progression than matched historical controls amenable to exon 51 skipping (n = 11)
• Distance lost on 6MWT 160m less over 4 years, and treated boys (2/12) demonstrated lower
incidence of loss of ambulation compared to control group (6/13) during 4-year study
What did they call Exondys 51?
Eteplirsen
what was the 1st treatment approved for DMD
aso exon skipping
What is the ASO that skips exon 53?
Vyondys 53
With both groups; low does and high does what was seen with the study using vyondys 53?
Significant muscle dystrophin production in both cohorts (+5.8% of normal) observed by WB
What tests were used to see improvements of function using vyondys 53?
Timed function tests
What were the results of the tests?
Walked at faster velocity and less than historical controls, quicker to get up from supine.
What are the 4 ASO treatments? And which exon do the skip?
Exondys 51, Vyondys 53, Viltepso (exon 53), Amondys 45
What percentage of DMD patients benefit from exon 51 skipping?
13%
What percentage of DMD patients benefit from exon 53 skipping?
7.7
What percentage of DMD patients benefit from exon 45 skipping?
8.1%
How common are premature termination codon (PTC) mutations (nonsense) in DMD patients?
15%
What does a premature stop codon do to the mRNA?
Prevents translation of mRNA and results in a truncated version of the polypeptide and is degraded
What happens in the presence of a premature stop codon?
There is still an assembly of ribosome complex and early stages of translation occurs but when they reach the stop codon everything stops and disperses.
How many stop codons? Can there be more how so?
3, can be created by nonsense mutations
What does Translarna do?
promotes production of functional dystrophin by enabling
read through of premature stop codon associated with nonsense mutation
• Does not bypass normal stop signal, does not interfere with transcription or mRNA stability
Is translarna a big or small molecule and how is it injected?
a small molecule that can be taken orally
What do tRNA do?
Bring amino acids to ribosome for translation
In the absensce of translarna, what happens when there is a premature stop codon?
Eukaryotic release factors are recruited, and go to ribosmes which cause disassociation and then a trunctaed polypeptide (most of the time dysfunctional and degraded)
With a molecule that promotes read-through like translarna, what happens?
near-cognate tRNA decodes PTC & new AA incorporated in
place of stop codon
New AA forms sense or missense mutation instead of nonsense
what did 28 day oral, 3X/day Translarna in DMD
patients with PTC mutations do in terms of dystrophin and serum ck levels?
- 60% of patients increased dystrophin levels (mean change 11%)
- Reduction in serum CK in 85% of patients
What did 48 week oral, 3X/day Translarna (ataluren) treatment in DMD patients with PTC mutations show functionally and what test did they use?
• Primary endpoint of 6MWD demonstrated significant difference versus placebo (31.3 m)
• Timed function tests of 10 m walk/run, 4-stair ascend and 4-stair descend, also demonstrated
modest preservation of ambulation with low dose Translarna
used the 6 min walk test
Who has approved translarna?
European union
What is the point of micro dystrophin gene that was created?
Dystrophin is so large to give through a virus so they found the minimal size that will result in a functional protein. That fits in the virus
What goes into the virus?
Micro dystrophin, muscle specific promoter
Describe the Systemic viral delivery of micro-dystrophin to DMD patients study
1-year open-label non-RCT, 4 DMD patients aged ~5 years, single IV dose
• Recombinant adeno-associated virus serotype rh74 with micro-dystrophin driven by skeletal
and cardiac muscle-specific promoter with enhanced cardiac expression (MHCK7)
• Skeletal muscle biopsy (gastroc) after 12 weeks for muscle histology, dystrophin IF and WB
What did the study show?
- Treatment was well tolerated with minimal adverse events
* Improvement in function scores and reduction in serum CK for 1 year after dose
What did the staining of muscle biopsy show?
The central myo-nucleus, fibrous and adipose tissue pre and post
What did red staining do?
Red stain binds to collagen fibers so shows fibrosis (shows it less in post treatment), less creatine kinase
What does IF and WB show?
Robust expression & correct localization of micro-dystrophin
Disadvantages to study?
only 4 kids, and one dose
