Nanopore sequencing combined with adaptive sampling and NanoExpansion enables accurate characterization of repeat expansion disorders.

Expansion repeat disorders are caused by abnormal expansions of short tandem repeats (STRs). We evaluated targeted long-read nanopore sequencing for comprehensive characterization of STR expansions in 132 participants, including patients with myotonic dystrophy type 1 (DM1, n = 51) and DM2 (n = 3) already characterized by Southern blot, C9orf72-associated amyotrophic lateral sclerosis (ALS n = 11) evaluated by repeat primed polymerase chain reaction (RP-PCR; validation group), suspected spinocerebellar ataxia (SCA, n = 46), cerebellar ataxia with neuropathy and vestibular areflexia syndrome (CANVAS; n = 13), Huntington's disease (1), Kennedy disease (1), Friedreich ataxia (FRDA, 2), and 4 healthy controls (test group). Native DNA was sequenced using adaptive sampling (AS) on Oxford Nanopore GridION or PromethION-2Solo platforms. Long-PCR sequencing was performed for comparison. We also developed NanoExpansion, a Python-based tool for sizing expanded alleles and detecting sequence interruptions. We correctly characterized all DM1, DM2, ALS, FRDA, Huntington's disease, and Kennedy disease cases, and identified eight SCA27B, two SCA37, three SCA31, one SCA3, and nine CANVAS cases. Compared with the AS approach, long-PCR underestimated alleles exceeding 800 repeats. Repeat interruptions were detected in 27/51 (52.9%) DM1 cases and 1/10 (10%) ALS, but not in CANVAS, FRDA, Huntington's disease, Kennedy disease, or SCA cases. This integrated sequencing analysis pipeline provides accurate, comprehensive characterization of repeat expansion disorders and has the potential to improve routine genetic diagnosis.