For patients with a duplication of a single exon e.g. exon 45 (176 nucleotides), skipping either the original or the duplicated exon will restore the wild type transcript, resulting in normal dystrophin (Figure 3). This has indeed been achieved in cultured cells from a patient with a duplication of exon 45.
Figure 3. Single exon skipping for duplications
Unfortunately, skipping one of the duplicated exons appeared less straightforward for a duplication of exon 44, where skipping was so efficient that both exons 44 were deleted. This resulted in a deletion of exon 44, which contains 148 nucleotides – not divisible by 3 – and thus disrupts the open reading frame. However, using a combination of AONs targeting exon 43 and AONs targeting exon 44, multiexon skipping of exon 43-44-44 could be achieved, thus restoring the open reading frame (Figure 4).
Figure 4. Triple exon skipping for duplications
For patients with larger duplications, the situation is more complex as the AON will target both the original exons (the skipping of which is detrimental for the reading frame even when the duplicated exons are skipped) and the duplicated exons (the skipping of which is beneficial for the reading frame). So far, we attempted to restore the reading frame for only a single patient with a large, atypical duplication (exon 52-62 duplicated between original exons 63 and 64). This was unsuccessful, but it is possible that reading frame restoration is feasible for less complex multiple exon duplications.