Antisense oligonucleotide-mediated exon skipping strategies as the treatment for rare diseases

Antisense oligomer-mediated exon skipping is being developed as a strategy for personalised treatment of genetic disorders through excising “in-frame” exons carrying small mutations or restoring the reading frame around frame-shifting deletions. The rationale behind this strategy for Duchenne muscular dystrophy is the correlation between dystrophin genotype and clinical phenotype. Null dystrophin mutations (frame-shifting deletions/duplications, nonsense mutations, splice site defects) typically lead to the severe Duchenne muscular dystrophy, whereas deletion of an in-frame exon, or exon blocks, generally leads to the milder Becker muscular dystrophy. The loss of in-frame exon(s) that results in Becker muscular dystrophy indicates non-essential dystrophin coding domains, providing templates for potentially functional dystrophin isoforms. The FDA accelerated approval of Eteplirsen, Golodirsen and Viltolarsen, and the approval of a new drug application seeking accelerated approval for an additional antisense oligomer to address common DMD mutations, have highlighted the therapeutic potential of exon skipping compounds. However, there is lack of genotype-phenotype correlation downstream of DMD exon 55, as deletions in this region are rare and the exonic arrangement is such that most deletions would disrupt the reading frame. Superficial similarities between DMD and PRKN include the very large gene size, very small exon content and the observation that both are deletion prone, and it appears that a similar “redundancy” of particular exons exists in the parkin gene. Parkinson’s patients with the in-frame deletion of PRKN exons 3 and 4 show later onset and milder disease course than patients with only single exon 3 or 4 frame-shifting deletions. This suggests that appropriate PRKN skipping to restore the reading frame and some function of parkin could be developed as a novel therapy for autosomal recessive juvenile-onset Parkinson’s patients. This thesis describes the induction of “Becker muscular dystrophy-like” in-frame dystrophin isoforms by skipping in-frame exon blocks downstream of DMD exon 55 in mice. A similar strategy of exon skipping was used to restore the PRKN reading frame in patient cells carrying a PRKN exon 3 deletion by skipping exon 4. Functional tests confirmed the activities of the induced protein isoforms. In this manner, we hope to expand the application of exon skipping strategies as the treatment of inherited rare diseases, including Duchenne muscular dystrophy and Parkin-type autosomal recessive juvenile-onset Parkinson’s disease.