Ultrastructural characterization of the MBNL1- containing foci occurring in myoblast and myotube nuclei from patients affected by myotonic dystrophy type 2

Myotonic dystrophy type 2 (DM2) is an autosomal dominantly inherited disease due to the CCTG repeat expansion in intron 1 of the zinc finger protein 9 (ZNF9) gene, and exhibiting multi-systemic clinical features. The expanded-CCUG-containing transcripts are retained in the cell nucleus and accumulate as discrete RNA-containing foci which specifically sequester the Muscleblind-like 1 (MBNL1) protein, a RNA binding factor involved in the regulation of alternative splicing. The knowledge about the nature of such foci is still largely incomplete; in particular, no information has been reported so far about their ultrastructural features. In this study, the nuclear foci occurring in cultured myoblasts and myotubes from DM2 patients were characterised at transmission electron microscopy by conventional morphology and immunocytochemistry. Our results demonstrate that the MBNL1-containing nuclear foci appear as roundish domains (100-200 nm in diameter) showing a rather homogeneous structure, with fine fibrils spreading out at their periphery. Based on the resemblance to HERDS (i.e. the RNP-containing nuclear aggregates forming upon transcriptional arrest), we hypothesize that the MBNL1-containing domains may represent accumulation sites of RNA processing factors, thus contributing to a general alteration of mRNA expression.