Comparative analysis of CRISPR/Cas9-targeted nanopore sequencing approaches in repeat expansion disorders

More than 50 repeat expansion disorders have been identified, with long-read sequencing marking a new milestone in the diagnosis of these disorders. Despite these major achievements, the comprehensive characterization of short tandem repeats in a pathological context remains challenging, primarily due to their inherent characteristics such as motif complexity, high GC content, and variable length. In this study, our aim was to thoroughly characterize repeat expansions in two neuromuscular diseases: myotonic dystrophy type 1 (DM1) and oculopharyngodistal myopathy (OPDM) using CRISPR/Cas9- targeted long-read sequencing (Oxford Nanopore Technologies, ONT). We conducted precise analyses of the DM1 and OPDM loci, determining repeat size, repeat length distribution, expansion architecture and DNA methylation, using three different basecalling strategies (MinKnow software, Dorado and Bonito). We demonstrated the importance of the basecalling strategy in repeat expansion characterization. We proposed guidelines to perform CRISPR-Cas9 targeted long-read sequencing (no longer supported by ONT), from library preparation to bioinformatical analyses. Finally, we showed, for the first time, somatic mosaicism, hypermethylation of LRP12 loci in OPDM symptomatic patients and changes in the repeat tract structure of these patients. We propose a strategy based on CRISPR/Cas9-enrichment long-read sequencing for repeat expansion diseases, which could be readily applicable in research but also in diagnostic settings.