Understanding how errors accumulate in dividing cells matters enormously for cancer biology, aging, and the stability of the genome across a lifetime. A fundamental gap has persisted in this field for nearly nine decades: some mitotic crossover events — genetic recombinations occurring during ordinary cell division rather than reproduction — leave no detectable trace using conventional sequencing methods, effectively hiding evidence of genomic instability from researchers.
Published in PNAS, this work uses long-read DNA sequencing technology to expose these so-called 'hidden' mitotic crossovers that standard short-read platforms routinely miss. The key mechanism exploited is heterozygosity: when two homologous chromosomes carry different single-nucleotide polymorphisms (SNPs) at corresponding positions, a crossover event can sometimes produce daughter cells where both copies of a chromosomal segment become identical — a state called loss of heterozygosity (LOH). Long-read sequencing, which spans much greater physical distances along the chromosome in a single read, can phase SNPs in a way that reveals these cryptic LOH events that are otherwise indistinguishable from background noise in short-read data.
This finding has meaningful implications beyond pure genetics. Mitotic crossovers are not merely a curiosity — they are a recognized early driver of cancer when they cause tumor suppressor genes to shift from a heterozygous to a homozygous loss-of-function state. The frequency and distribution of these events in normal somatic tissue has been systematically underestimated, which means current models of somatic mutation accumulation — central to understanding both cancer risk and biological aging — may need revision. The study's primary limitation is that long-read sequencing remains expensive and computationally demanding at scale, so population-level surveys are not yet feasible. This is nonetheless a methodologically important advance: a new observational window, not yet a clinical tool, but one that could fundamentally reshape how researchers map genomic instability in aging tissues.