Understanding the consequences of reduced expression of CNBP in myotonic dystrophy type 2 pathogenesis

Myotonic dystrophy type 2 (DM2) is an autosomal dominant multisystemic disorder that primarily affects skeletal muscle, leading to progressive muscle fiber degeneration and dysfunction. DM2 is caused by a CCTG repeat expansion within intron 1 of the CNBP (Cellular Nucleic acid-Binding Protein) gene, although the precise pathogenic mechanisms remain incompletely understood. Specifically, the relationship between the dystrophic phenotype and CNBP loss of function induced by the DNA expansion remains unclear. This project aims to determine whether CNBP expression is reduced in the context of DM2 and to explore the functional consequences of CNBP depletion on muscle physiology and performance. To address these points, we employed complementary in vitro and in vivo models, including myoblasts cell line, DM2 patient-derived cells, Drosophila, and mice. We demonstrate that CCTG expansion correlates with reduced CNBP expression in both DM2 patient-derived cells and in a novel Drosophila DM2 model that we have generated. Constitutive ablation of CNBP in Drosophila and mice causes severe locomotor defects and muscle atrophy. Molecular analysis conducted in CNBP-silenced myoblast cell lines highlights the role of CNBP in regulating critical pathways of muscle cell activity, including myogenesis and metabolism. In particular, we show that CNBP depletion induces inactivation of the PI3K-Akt-mTOR pathway, with consequent aberrant activation of autophagy in all our models. Importantly, we demonstrate that genetic blockade of autophagic activity through interference with ATG7, a key factor in autophagosome formation, rescues the locomotor defects caused by CNBP depletion in Drosophila. Together, our data identify PI3K-Akt-mTOR deregulation and the consequent activation of autophagy as a novel contributing mechanism to DM2 pathogenesis, suggesting new potential therapeutic approaches based on autophagy inhibition.