Expression levels of the DNA repair enzyme HAP1 do not correlate with the radiosensitivities of human or HAP1-transfected rat cell lines

Apurinic/apyrimidinic (AP) sites in DNA are potentially lethal and mutageni c. They can arise spontaneously or following DNA damage from reactive oxyge n species or alkylating agents, and they constitute a significant product o f DNA damage following cellular exposure to ionizing radiation. The major A P endonuclease responsible for initiating the repair of these and other DNA lesions in human cells is HAP1, which also possesses a redox function, We have determined the cellular levels of this enzyme in 11 human tumour and f ibroblast cell lines in relation to clonogenic survival following ionizing radiation. Cellular HAP1 levels and surviving fraction at 2 Gy (SF2) varied five- and tenfold respectively, However, no correlation was found between these two parameters following exposure to gamma-irradiation at low (1.1 cG y per min) or high (108 cGy per min) dose rates. To examine this further, w ild-type and mutant versions of HAP1 were overexpressed, using an inducible HAP1 cDNA expression vector system, in the rat C6 glioma cell line which h as low endogenous AP endonuclease activity. Induction of wild-type HAP1 exp ression caused a > fivefold increase in the capacity of cellular extracts t o cleave an oligonucleotide substrate containing a single abasic site, but increased expression did not confer increased resistance to gamma-irradiati on at high- or low-dose rates, or to the methylating agent methyl methanesu lphonate (MMS), Expression in C6 cell lines of mutant forms of HAP1 deleted for either the redox activator or DNA repair functions displayed no appare nt titrational or dominant negative effects. These studies suggest that the levels of endogenous AP endonuclease activities in the various cell lines examined are not limiting for efficient repair in cells following exposure to ionizing radiation or MMS. This contrasts with the correlation we have f ound between HAP1 levels and radiosensitivity in cervix carcinomas (Herring et al (1998) Br J Cancer 78: 1128-1133), indicating that HAP1 levels in th is case assume a critical survival role and hence that established cell lin es might not be a suitable model for such studies.