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#apaperaday: Genetic diagnosis of Duchenne and Becker muscular dystrophy through mRNA analysis: new splicing events

In today’s #apaperaday, Prof. Aartsma-Rus reads and comments on the paper titled: Genetic diagnosis of Duchenne and Becker muscular dystrophy through mRNA analysis: new splicing events

Today’s pick is from BMJ Open Journal of medical genetics by Segarra-Casas et al on mRNA analysis to detect splicing mutations for Duchenne and Becker patients.  DOI: 10.1136/jmg-2022-108828

Duchenne is caused by mutations in the dystrophin gene that disrupt the genetic code, while Becker is caused by mutations that maintain the genetic code and allow the production of a partially functional protein. This ‘reading frame rule’ applies to >90% of mutations.

To obtain a diagnosis of Duchenne or Becker it is crucial to find the mutation. This allows initiation of care, family planning and establishing whether mutation specific therapies apply to Duchenne cases.

Genetic diagnosis currently is done by checking for deletions and duplications of one or more exons (present in ~70% of patients). If these are not detected, each exon is analyzed for small mutations (present in ~25% of patients). For ~5% of patients also these are not found.

Authors here argue that for these remaining patients, mutations affecting splicing are likely (fully agree) and that for THESE patients doing a muscle biopsy to analyze mRNA is justified (fully agree again). This is what they did here for 7 patients where no mutation was found.

For 2 Duchenne patients they found cryptic splicing (part of intron aberrantly included in mRNA). This was found for intron 15 and intron 37. For 1 Becker patient they found cryptic splicing of intron 62, which was partial (explaining why patient had Becker and not Duchenne)

For one intermediate patient they found inclusion of a piece of chromosome 8 between exon 2 and exon 5. It turned out that the patient had an insertion of part of chromosome 8 in intron 2. For 3 Becker patients no clear missplicing was found.

These patients showed skipping of exon 9, 71 & 78, but this is also found in unaffected individuals. It is unclear why these patients have Becker. 2 might be clinically misdiagnosed. Dystrophin abundance was reduced with protein analysis, but this is also possible for LGMD. One patient shows absence of part of the rod domain, while other domains are present. This is clearly a sign of having a BMD protein. Really strange why the mRNA profile was normal for this patient. (Authors have no clarification, but neither do I).

Authors stress that the mRNA analysis was crucial to elucidate the cases they identified while whole gene sequencing alone would not have allowed this for 3 of the patients. The intronic mutations leading to cryptic splicing were not marked as pathogenic.

They were also not predicted to result in cryptic splicing. Very nice work and kudos to the authors for going the extra mile to find mutations for these patients!