Challenges facing genetic diagnostics of Huntington's disease: an update

Since 1993, when a polymorphic CAG trinucleotide repeat was established as the cause of Huntington's disease (HD), fragment analysis has been the first-tier test in genetic diagnosis of HD. However, in recent years it has become increasingly clear that, in addition to the repeat length itself, sequence variations within the HTT repeat and elsewhere in the genome also play a crucial role in the onset and clinical presentation of the disease.

Sequence interruptions or non-canonical repeat motifs have historically been difficult to identify, as PCR cannot distinguish an interrupted repeat tract when assessed by capillary electrophoresis. However, these non-canonical interruptions may directly affect the phenotype or result in an underestimation of the uninterrupted CAG repeat length. While next-generation sequencing (NGS) with short read lengths has revolutionized genetic diagnostics, it faces limitations in diagnosing repeat expansion diseases. The literature search methodology involved a review of relevant studies and clinical data.

In contrast, long-read sequencing (LRS) generates read lengths of over 10 kb and can thus overcome the limitations associated with short reads. This enables detailed investigation of repeat sequences and the identification of complex patterns of alternating sequence motifs, potentially providing additional clinically relevant information.