Biosensors for Detecting Trinucleotide Repeat Expansions

Short tandem repeat expansions cause a heterogeneous group of severe neurological and neuromuscular disorders, including Huntingtonʼs disease, myotonic dystrophy, fragile X syndrome, and Friedreichʼs ataxia. The clinical management of these conditions critically depends on accurate characterization of repeat length, somatic mosaicism, internal sequence interruptions, and epigenetic modifications. However, conventional diagnostic approaches face fundamental limitations in resolving large, GC-rich, or structurally complex alleles. This review examines the evolving landscape of STR diagnostics, focusing on the emerging field of biosensor platforms. We critically evaluate the technical and biological challenges posed by polymerase slippage, secondary structure formation, somatic mosaicism, and allele dropout. We also assess biosensor platforms – including electrochemical impedance sensors, CRISPR-Cas systems (notably the REPLICA platform employing Cas3-mediated collateral activity) – have achieved femtomolar to attomolar limits of detection, enabling rapid point-of-care triage. We discuss the analytical principles, performance characteristics, and clinical applicability of these technologies. Finally, we propose a future diagnostic paradigm integrating biosensor-based rapid screening with targeted long-read sequencing for definitive characterization. This synergistic two-tiered strategy could to combine the accessibility and speed of point-of-care diagnostics with the comprehensive analytical power of third-generation sequencing, paving the way for precision medicine in repeat expansion disorders.