Insulin receptor splicing alteration in myotonic dystrophy type 2

Abstract

Myotonic dystrophy (DM) is caused by either an untranslated CTG expansion in the 3' untranslated region of the DMPK gene on chromosome 19 (dystrophia myotonica type 1 [DM1]), or an untranslated CCTG tetranucleotide repeat expansion in intron 1 of the ZNF9 gene on chromosome 3 (dystrophia myotonica type 2 [DM2]). RNA-binding proteins adhere to transcripts of the repeat expansions that accumulate in the nucleus, and a trans-dominant dysregulation of pre-mRNA alternative splicing has been demonstrated for several genes. In muscle from patients with DM1, altered insulin-receptor splicing to the nonmuscle isoform corresponds to the insulin insensitivity and diabetes that are part of the DM phenotype; because of insulin-receptor species differences, this effect is not seen in mouse models of the disease. We now demonstrate that comparable splicing abnormalities occur in DM2 muscle prior to the development of muscle histopathology, thus demonstrating an early pathogenic effect of RNA expansions.

Figure 1

Measurement of IR isoforms: RT-PCR analyses of IR isoforms in RNA from DM1, DM2, and normal muscle biopsies. The upper band represents the insulin-sensitive IR-B isoform (containing exon 11), and the lower band shows the insulin-insensitive IR-A isoform (without exon 11). In DM1 and DM2 muscle, the IR-A isoform predominates, as evidenced by the low percentage of IR-B relative to that in unaffected control.

Figure 2

Histological and FISH features in DM2: histological sections of skeletal-muscle biopsies on the same patient, performed 8 years apart. The earlier biopsy from biceps brachii (A1 and B1) shows normal histology, with only one fiber in the entire section having a single central nucleus (arrowhead). The later biopsy from vastus lateralis (A2 and B2) shows characteristic histological features of DM, with angulated fibers, severely atrophic fibers with pyknotic nuclei (“nuclear bag fibers”) (indicated by an asterisk [*]), and proliferation of centrally located nuclei (arrowhead). RNA-FISH shows ribonuclear inclusions (red foci, small arrows) within blue DAPI-stained nuclei in both samples (B1 and B2), including within all nuclei of a severely atrophic fiber (inset).