Frequent expression of aberrant chimeric Cdkn2a transcripts in mouse models of muscular dystrophy

Muscular dystrophies (MDs) are rare hereditary disorders, characterized by progressive muscle weakness due to loss of functional muscle tissue and its replacement by fat and connective tissue. MD mouse models, such as mdx-mice harboring a nonsense variant in the murine dystrophin gene (Dmd), are prone to develop age-related skeletal muscle-derived sarcomas, which is in line with multiple lines of evidence that MD related genes like Dmd act as tumor suppressors in mice and men. Previously, we proposed that genetic instability and cancer-like mutations might arise early in dystrophic muscle. Here, we show that microscopic tumors in dystrophic muscle occur frequently in mice without clinically overt sarcomas. Representing sarcoma pre-stages, these "microsarcomas" share a subset of recurrent MD-specific genetic number alterations. The majority of MD sarcomas and microsarcomas harbor heterozygous Cdkn2a deletions, due to a deletion breakpoint hotspot within the downstream gene Gm12606, a long intergenic non-coding RNA (lincRNA). This lincRNA gene was also found to be involved in expression of aberrant chimeric RNA fusion transcripts, consisting of the first coding exon of the Cdkn2a gene (corresponding to either p19^Arf or p14^Ink4a) fused to an exon of Gm12606. Remarkably, expression of these fusion transcripts was highly specific to MD muscles and sarcomas and was never observed in muscle from healthy wild-type mice. In addition, muscles from MD double-mutants displayed an increased incidence of fusion transcript expression. Because readthrough transcription was associated with fusion transcript expression also in sarcomas lacking deletions at the Cdkn2a locus, expression of these chimeric RNA transcripts might be the result of dystrophy-induced stress on transcription. We propose that genetic alterations of the Cdkn2a locus, both at the DNA and RNA level, constitute hallmark events in the dystrophy-cancer continuum.