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

1.我们分离到了大肠杆菌mutT功能同源物YLR 151 c，并鉴定其为一种新的营养素水解酶，可水解酿酒酵母中的8-oxodGTP和2-OH-dATP等氧化核苷酸损伤。我们还尝试分离YLR 151 c的人类同源物。根据数据库检索的结果，我们选择了两个候选物，AAH 18644和XP 058753，并检查1）候选物是否可以抑制大肠杆菌mutT缺陷型突变体的突变子表型，以及2）纯化的重组基因产物是否具有作为8-oxodGTP和2-OH-dATP的焦磷酸酶的酶活性。但是，我们不能得出结论，这两个候选人是YLR 151 c，酵母nutrient水解酶的同源物的氧化核苷酸损伤。2.我们已经分离了一个基因HAM 1，并确定了基因产物作为焦磷酸酶的脱氨基核苷酸损伤，如dITP在酿酒酵母中，并研究了它在基因组稳定性的作用在单倍体和二倍体细胞。在单倍体细胞中，Ham 1的缺陷并不影响自发突变，但它通过超诱导同源重组引起超诱导杂合性缺失（洛），这是二倍体细胞中基因组不稳定性的指标之一。这是第一个发现，表明内源性核苷酸损伤不仅引起单倍体细胞的诱变，而且引起二倍体细胞的基因组不稳定性，如超重组和洛缺失。由于洛缺失是最常报道的与癌症发生相关的事件，这一发现可能表明内源性核苷酸损伤与癌症的密切关系。

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.