Role of DNA polymerase η in the UV mutation spectrum in human cells

In humans, inactivation of the DNA polymerase η gene (pol η) results in sunlight sensitivity and causes the cancer-prone xeroderma pigmentosum variant syndrome (XP-V). Cells from XP-V individuals have a reduced capacity to replicate UV-damaged DNA and show hypermutability after UV exposure. Biochemical assays have demonstrated the ability of pol η to bypass cis-syn-cyclobutane thymine dimers, the most common lesion generated in DNA by UV. In most cases, this bypass is error-free. To determine the actual requirement of pol η in vivo, XP-V cells (XP30RO) were complemented by the wild type pol η gene. We have used two pol η-corrected clones to study the in vivo characteristics of mutations produced by DNA polymerases during DNA synthesis of UV-irradiated shuttle vectors transfected into human host cells, which had or had not been exposed previously to UV radiation. The functional complementation of XP-V cells by pol η reduced the mutation frequencies both at CG and TA base pairs and restored UV mutagenesis to a normal level. UV irradiation of host cells prior to transfection strongly increased the mutation frequency in undamaged vectors and, in addition, especially in the pol η-deficient XP30RO cells at 5′-TT sites in UV-irradiated plasmids. These results clearly show the protective role of pol η against UV-induced lesions and the activation by UV of pol η-independent mutagenic processes.