By: Asal Fotouhi, Molekylär Biovetenskap, WGI
Host: Siamak Haghdoost

Mechanisms of genomic instability induced by UV radiation in human cells

Genomic instability is the increased tendency of alterations within the genome. Ultraviolet radiation (UVR) is known to be highly mutagenic. What type of DNA damage is induced by which type of UVR wavelength is still a matter of debate. UVR induce genomic instability by formation of photoproducts and induction of reactive oxygen species (ROS). ROS can give rise to mutations via oxidation of nitrogen bases in the DNA or in the nucleotide pool (dNTP). Oxidized bases in the DNA can be repaired by different DNA repair pathways, mainly base excision repair. Oxidized dNTP can be incorporated into the DNA during replication leading to mutations. hMTH1 dephosphorylates oxidized dNTP, in particular 8-oxo-dGTP and 2-OH-dATP, and inhibits their incorporation into the DNA. The aim of the present study is to investigate the role of hMTH1 in genomic instability induced by UVR in human lymphoblastoid cells with low level of hMTH1 (stably transfected TK6 cells). Clonogenic survival and mutant frequency are measured (and study of micronucleus induction is ongoing) after exposure of the cells to UVA, B and C. As a biomarker for oxidative DNA base damage the level of extracellular and intracellular 8-oxo-dGTP are measured in cells exposed to UVA. The UVA-induced mutational spectra at the endogenous Thymidine kinase locus in hMTH1-transfected and non-transfected TK6 cells were investigated. Our results show that hMTH1 protects cells against UVA and UVB mutagenesis but not against UVC mutagenesis. The mutational spectra shows that GC>AT transition, which can be formed by incorporation of 2-OH-dATP from nucleotide pool into the DNA, is the dominant mutation type in cells exposed to UVA. We conclude that UVA induces mutations (at least partly) by induction of dNTP lesions, particularly 2-OH-dATP rather than 8-oxo-dGTP. The main finding of present project is that dNTP is an important target for UVA and UVB induced mutagenesis.