The novel (TCTG)n motif in CNBP expanded alleles: composition, dynamics and genotype–phenotype correlation in Myotonic dystrophy type 2 (DM2)

Introduction. Myotonic dystrophy type 2 (DM2) is an autosomal dominant disorder caused by (CCTG)n repeat expansions in intron 1 of the CNBP gene. Recent evidence from long-read sequencing suggests these expansions may be more complex than previously recognized. Aim. To comprehensively characterize the composition, intergenerational dynamics, and clinical impact of novel (TCTG)n motifs within the CNBP expanded alleles in a large DM2 cohort. Methods. We analyzed 100 genetically confirmed DM2 individuals (45 sporadic, 55 familial). The presence of (TCTG)n blocks was detected using an optimized quadruplet-repeat primed PCR (QP-PCR) assay coupled with Sanger sequencing. In a subset of nine patients, Cas9-mediated enrichment followed by Nanopore Long-Read Sequencing (LRS) provided nucleotide-level resolution of the expanded alleles. Haplotype analysis was performed using STR markers. Results. We identified (TCTG)n blocks at the 3′ end of the expansion in 88% of patients. This refined assay corrected nine initial false-negative diagnoses from standard testing. LRS analysis confirmed the composition and revealed the dynamics of the (TCTG)n tract in familial transmission, showing a tendency for contraction and, in one case, complete loss. Genotype–phenotype correlation analysis indicated that the presence of the (TCTG)n motif acts as a disease modifier, significantly influencing the age of onset. Conclusion. The detailed characterization of the CNBP expansion reveals the novel (TCTG)n component that is integral to the DM2 genotype. Understanding its composition and dynamics enhances diagnostic accuracy and provides a new framework for genetic counselling, prognostic stratification and future personalized therapies.