Start Making Sense - Exon Skipping & Nonsense Suppression
Exon skipping
Enter two radically different solutions - which are geared towards rescue rather than replacement (or even repair). Instead of introducing and expressing a foreign full-length wild-type gene, both treatments are focused on recovering a functional product from the endogenous mutant gene itself. It now appears possible to bypass a problematic mutation entirely, and induce the 'faulty' gene to express a therapeutic protein of its own. Interestingly, both new strategies for this involve entirely distinct mechanisms and completely different types of molecule. And more promisingly, both have advanced into late-stage clinical development at an encouragingly rapid rate, with DMD as their lead indication, but with many other monogenic disorders in view.
The first of these, known as exon skipping, is an elegant method which allows the cellular transcription process to jump over entire regions of a gene. It was pioneered as a therapeutic approach by Zbigniew Dominski and Ryszard Kole at the University of North Carolina in 1993, and validated in a human patient-derived dystrophin system by Judith van Deutekom's group at Leiden University in 2001. An antisense sequence, administered either as a synthetic oligonucleotide or expressed from a gene therapy vector, binds specifically to splice sites or internal sequences in the particular exon containing the mutation. When mRNA transcription occurs, it reaches the bound antisense sequence and is induced to 'skip' over the mutated exon and resume read-through at a site further downstream, thus generating the remainder of the complete protein sequence. If the bypassed mutation contained a premature stop codon (resulting in a severely truncated protein), or a shift in the reading frame (resulting in the wrong sequence), the advantages of skipping this section and regenerating the correct open reading frame are obvious - even if this occurs at the expense of deleting a small internal region of the protein.
In the case of muscular dystrophy, the near-term goal is to convert severe DMD cases to much milder BMD versions, which have a greatly improved lifetime prognosis - a partially-functional protein is much better than none at all. Most muscular dystrophy mutations occur within a well-defined 'hotspot' region at exons 45-51. The fact that many truncated forms of dystrophin are known to be at least partially functional in BMD patients make DMD a highly attractive initial candidate for the exon skipping approach - and it has been estimated that up to 90% of DMD patients have types of mutations which could be amenable to this therapeutic option.
In late 2007, the first clinical trial pioneering therapeutic exon skipping was successfully completed in four DMD patients, aged 10-13 years. The drug involved was PRO-051, a 2'-O-methyl antisense oligonucleotide targeting exon-51 of the dystrophin gene, where a cluster of mutations affecting 13% of DMD patients has been documented. A single injection into lower leg muscle resulted in novel dystrophin expression in the majority of muscle fibres (65-95%) in each patient, at clinically significant levels, although the effect remained localized. An ascending-dose Phase II trial involving repeated systemic injections in 18 patients aged 5-16 years is now underway.
The developer of this drug is the Dutch company Prosensa (with the name implying its 'pro-sense' approach), which has the aforementioned Leiden University's exon skipping pioneer Judith van Deutekom as its Discovery Director. Prosensa has licensed this flagship PRO-051 project to GlaxoSmithKline (GSK) for development - and GSK also has an exclusive option on 3 follow-up Prosensa DMD drugs, in what seems a perspicacious move to give it a firm foothold in this emerging sector. Prosensa aims to carve itself a niche in commercialization of projects from the exon skipping area for unmet medical needs, supported by a strong IP position.
