Genome-wide detection of ultralow-frequency substitution mutations in cultures of mouse lymphoma L5178Y cells and Caenorhabditis elegans worms by PacBio sequencing

Many conventional genetic toxicology assays require specialized cell cultures or animals and can only detect mutations that inactivate the function of a reporter gene. These limitations make such assays incompatible with many toxicological models but could be overcome by the development of techniques capable of directly detecting genome-wide somatic mutations through DNA sequencing. PacBio sequencing can generate almost error-free consensus reads by repeatedly inspecting both DNA strands from circularized molecules (a method known as PacBio HiFi). In this study, we show that PacBio HiFi can detect genome-wide ultralow-frequency substitution mutations in cultures of mouse lymphoma L5178Y cells and Caenorhabditis elegans worms. The mutation frequencies (MFs) of unexposed samples in both models were ~1 × 10⁻⁷ mutations per base pair. Compared to these controls, PacBio HiFi detected MF increases of 23-fold in cultures of L5178Y cells exposed to 5 mM ethyl methanosulfonate (EMS) for 4 h, and 5-, 12-, and 29-fold in cultures of C. elegans worms exposed to 12.5, 25, and 50 mM EMS for 4 h, respectively. In both models, the mutation spectra of controls were diverse, while those derived from EMS-exposed samples were dominated by C:G → T:A transitions. To validate these results, clone sequencing analyses were performed on the same cultures of L5178Y cells. The results obtained by clone sequencing and PacBio HiFi were almost identical. Our results suggest that PacBio sequencing could be used for the detection, quantitation, and characterization of mutations in any DNA-containing sample, including those that are not compatible with conventional mutation detection approaches.