High-Throughput Screening of the Saccharomyces cerevisiae Genome for 2-Amino-3-Methylimidazo Quinoline Resistance Identifies Colon Cancer-Associated Genes

Abstract

Heterocyclic aromatic amines (HAAs) are potent carcinogenic agents found in charred meats and cigarette smoke. However, few eukaryotic resistance genes have been identified. We used Saccharomyces cerevisiae (budding yeast) to identify genes that confer resistance to 2-amino-3-methylimidazo[4,5-f]quinoline (IQ). CYP1A2 and NAT2 activate IQ to become a mutagenic nitrenium compound. We introduced an expression vector that contains human CYP1A2 and NAT2 genes into selected mutant strains and the diploid yeast deletion collection. The deletion libraries expressing CYP1A2 and NAT2 or no human genes were exposed to either 400 or 800 M IQ for five or ten generations. DNA barcodes were sequenced using the Illumina HiSeq 2500 platform and statistical significance was determined for exactly matched barcodes. Four screens for IQ resistance in the "humanized" collection identified 1160 unique ORFs, of which 337 were validated or duplicated in at least two screens. Two screens of the original yeast library identified 101 genes that overlap with the 337 previously identified. Selected genes were validated by growth curves, competitive growth assays, or trypan blue assays. Prominent among both sets are ribosomal protein genes, while nitrogen metabolism, cell wall synthesis, and phosphatase genes were identified among the "humanized" library. Protein complexes identified included the casein kinase 2 (CK2) and histone chaperone (HIR) complex. DNA repair genes included NTG1, RAD18, RAD9, PSY2 and UBC13. Polymorphisms in human NTHL1, the NTG1 ortholog, and RAD18 are risk factors for colon cancer. These studies thus provoke questions of whether genetic risk factors for colon cancer confer more HAA-associated toxicity.