CarcSeq detection of lorcaserin-induced clonal expansion of Pik3ca H1047R mutants in rat mammary tissue

Lorcaserin is a 5-hydroxytryptamine 2C (serotonin) receptor agonist and a nongenotoxic rat carcinogen, which induced mammary tumors in male and female rats in a 2-yr bioassay. Female Sprague Dawley rats were treated by gavage daily with 0, 30, or 100 mg/kg lorcaserin, replicating bioassay dosing but for shorter duration, 12 or 24 wk. To characterize exposure and eliminate possible confounding by a potentially genotoxic degradation product, lorcaserin and N-nitroso-lorcaserin were quantified in dosing solutions, terminal plasma, mammary, and liver samples using ultra-high-performance liquid chromatography-electrospray tandem mass spectrometry. N-nitroso-lorcaserin was not detected, supporting lorcaserin classification as nongenotoxic carcinogen. Mammary DNA samples (n = 6/dose/timepoint) were used to synthesize PCR products from gene segments encompassing hotspot cancer driver mutations, namely regions of Apc, Braf, Egfr, Hras, Kras, Nfe2l2, Pik3ca, Setbp1, Stk11, and Tp53. Mutant fractions (MFs) in the amplicons were quantified by CarcSeq, an error-corrected next-generation sequencing approach. Considering all recovered mutants, no significant differences between lorcaserin dose groups were observed. However, significant dose-responsive increases in Pik3ca H1047R mutation were observed at both timepoints (ANOVA, P < 0.05), with greater numbers of mutants and mutants with greater MFs observed at 24 wk as compared with 12 wk. These observations suggest lorcaserin promotes outgrowth of spontaneously occurring Pik3ca H1047R mutant clones leading to mammary carcinogenesis. Importantly, this work reports approaches to analyze clonal expansion and demonstrates CarcSeq detection of the carcinogenic impact (selective Pik3ca H0147R mutant expansion) of a nongenotoxic carcinogen using a treatment duration as short as 3 months.