The sanitizing systems for endogenous nucleotide lesions, and their roles on spontaneous mutagenesis

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1.We have isolated YLR151c as a candidate for a functional homologue of E.coli mutT, and characterized as a novel Nudix hydrolase for oxidative nucleotide lesions such as 8-oxodGTP and 2-OH-dATP in Saccharomyces cerevisiae. We additionally tried to isolate human homologue of YLR151c. According to the results of database search, we chose two candidates, AAH18644 and XP058753, and examined 1)if the candidates can suppress the mutator phenotype of E.coli mutT-deficient mutant, and 2)if the purified recombinant gene products have the enzymatic activity as pyrophosphatase for 8-oxodGTP and 2-OH-dATP. However, we could not conclude that both candidates are homologue of YLR151c, yeast Nudix hydrolase for oxidative nucleotide lesions.2.We have isolated a gene HAM1 and characterized the gene product as a pyrophosphatase for the deaminated nucleotide lesions such as dITP in Saccharomyces cerevisiae, and also examined its roles on genome stability in both haploid and diploid cells. In haploid cells, the deficiency in Ham1 did not affect spontaneous mutagenesis, whereas it caused hyper-induction of loss of heterozygosity (LOH), which is one of the indicators for genome instability in diploid cells, through hyper-induction of homologous recombination. This is the first finding indicating that endogenous nucleotide lesions cause not only mutagenesis in haploid cells but also genome instability such as hyper-recombination and LOH in diploid cells. Because LOH is the most commonly reported event linked with carcinogenesis, this finding may suggest the strong relationship of endogenous nucleotide lesions with cancer.