Sequestration of MBNL1 Protein by Mutant ZNF9 mRNA inLymphocytes of Patients with Myotonic Dystrophy Type 2

Myotonic dystrophy type 2 (DM2) results from the (CCTG)n expansion in the ZNF9 gene located on the long arm of chromosome 3. The pathogenesis of DM2 includes sequestration of muscleblind-like (MBNL) proteins by pathological CCUG(exp) ZNF9 mRNA transcripts, leading to abnormal splicing of target pre-mRNAs and, consequently, to the development of the pathological phenotype. In this report, we demonstrate expression of the ZNF9 transcript, ZNF9 protein as well as MBNL1 protein in lymphocytes of non-DM controls and DM2 patients. In DM2 patients lymphocytes, MBNL1 protein is co-localized and partly sequestered in CCUG(exp) ZNF9 mRNA intranuclear foci. We suppose that non-muscle tissues expressing ZNF9 transcript in DM2 patients might be affected by a similar molecular mechanism as the skeletal muscle. The presence of the expanded ZNF9 transcript in peripheral blood lymphocytes may be useful for rapid diagnosis of DM2 from blood smears of suspected patients, or for a follow-up of patients treated with molecular therapy.