Since exon skipping modulates the amounts of dystrophin RNA transcripts (i.e. the DNA copies that are translated into protein), we wanted to study this in more detail. What we see is that much more dystrophin RNA and protein is produced in heart than skeletal muscle, and slightly more dystrophin in diaphragm than skeletal muscle. This may in part explain why exon skipping works less in heart than other muscles: if there is more transcripts, skipping levels will be lower even when similar amounts of antisense oligonucleotide (the exon skipping compound) end up in heart and a limb muscle (example: 10 AONs vs 100 dystrophin RNAs in e.g. calf muscle = 10% skip, but 10 AONs vs 1000 dystrophin RNAs in heart = 1% skip) (note that another part of the explanation is that antisense oligonucleotides reach the heart with lower efficiency, so this is part of the explanation, not the full explanation).
The next thing we show is that the dystrophin transcripts are not all complete, so the amount of transcripts containing exon 1/the beginning is higher than the amount of transcripts containing later exons/the end (and in order for the transcripts to be translated into a functional dystrophin, they have to be complete from start to end). The cause of this phenomenon (them not being complete) is not known. In healthy mouse muscle, there is a slight imbalance, in mdx muscle (without dystrophin) there is a much bigger imbalance (so the percentage of transcripts being incomplete is higher). When the reading frame is restored in mdx muscle by exon skipping, the imbalance is not restored. Also, in Becker patient muscles we see this imbalance - the level of imbalance is between that found in healthy muscle and muscle without dystrophin and varies a lot between Becker patients. To put it very simple, we find in Becker patients that the amount of dystrophin they produce relates not to the amount of dystrophin transcripts but to the amount of COMPLETE transcripts (this is very logical, but the imbalance of dystrophin has not been well studied, so we did not know this before).