Cas9-enriched nanopore sequencing enables comprehensive and multiplexed detection of repeat expansions

Short tandem repeat (STR) expansion is a major genetic mechanism underlying numerous neurogenetic disorders. However, traditional PCR amplification and short-read next-generation sequencing (NGS)-based methods often fail to detect complex and large expansions including methylation information. Here, we modified an amplification-free nanopore Cas9-targeted sequencing (nCATS) platform and developed a dedicated analysis algorithm, STRiker, for the simultaneous assessment of all currently defined STR loci (56 sites) using a single test with genomic DNA from patient-derived blood cells. We ultimately identified pathogenic repeat expansions in 12 of 37 patients (32.4%) with cerebellar ataxia who remained genetically undiagnosed despite extensive prior genetic testing, in FGF14 (n=4), ATXN8OS , NOP56 , RFC1 (n=2 each), and PRNP and NOTCH2NLC (n=1 each). Additionally, family-based cascade screening revealed six relatives with repeat expansions in five families. These results demonstrated a broader diversity of pathogenic repeat structures, particularly in FGF14 , and illustrated that CpG methylation can mitigate the pathogenic effects of repeat expansions. This approach offers a powerful strategy for improving the diagnosis of STR-related neurogenetic diseases, for cerebellar ataxia and other STR-related diseases in the future.