The roles of human MTH1, MTH2 and MTH3 proteins in maintaining genome stability under oxidative stress

The hydrolysis of nucleotides containing 8-oxo-7,8-dihydroguanine (8-oxoG) is important in the maintenance of genome stability. Human cells possess three types of proteins, MTH1 (NUDT1), MTH2 (NUDT15) and MTH3 (NUDT18), which have the potential to hydrolyze deoxyribonucleoside di- and triphosphates containing 8-oxoG to the monophosphate, the form of which is unusable for DNA synthesis. To elucidate the physiological roles of these enzymes, we constructed single knockout (KO) cell lines for each of the MTH1, MTH2 and MTH3 genes and MTH1 and MTH2-double KO cell lines from the human HeLa S3 line using CRISPR/Cas9. With the exception of MTH3-KO, all of the KO cell lines showed similar proliferation rates to the parental line, HeLa S3, indicating that the MTH1 and MTH2 functions are dispensable for cell growth. On the other hand, the MTH3-KO cells showed a significantly slower growth rate, suggesting that MTH3 has a definite role in cell growth in addition to the cleavage of 8-oxoG-containing deoxyribonucleotide. MTH1-KO, MTH2-KO and MTH1- MTH2-KO cells exhibited increased sensitivity to hydrogen peroxide, whereas MTH3-KO did not. MTH1-KO cells showed only a slight increase in mutant frequency in comparison to the parental HeLa S3 line. The overproduction of MTH1 and MTH2 suppressed the mutator phenotype of mutT-deficient E. coli cells, whereas the overproduction of MTH3 did not show such a suppressive effect. Our findings suggest that both MTH1 and MTH2 are involved in the maintaining genome stability in human cells against oxidative stress, while MTH3 may play some other role(s).