UV-INDUCED MUTAGENESIS IN THE ENDOGENOUS HPRT GENE AND IN HPRT CDNA GENES INTEGRATED AT DIFFERENT POSITIONS OF THE HUMAN GENOME

The influence of the genomic position of a gene on UV-induced mutation s was studied in the endogenous hprt gene in human lymphoblastoid TK6 cells and in cell lines derived from TK6 each containing a single copy of a hamster hprt cDNA gene integrated on a retroviral vector in diff erent positions of the human genome. Previous studies showed that the genomic sequences surrounding the integration site influence spontaneo us mutagenesis, resulting in a 10-fold difference in mutation rates am ong the hprt cDNA genes. Here we demonstrate that the genomic position s of three integrated hprt cDNA genes do not influence UV-induced muta genesis. The mutability by UV irradiation in these cell lines is appro ximately the same (16.0 x 10(-6) per J/m(2)). The nature of the UV-ind uced mutations determined in two of the cell lines containing the inte grated hprt cDNA gene (approximately 30 mutants each) was also found n ot to be different. The endogenous hprt gene in the parental TK6 cells exhibits a significantly lower mutability (2.1 x 10(-6) per J/m(2)) t han the cDNA genes, but the spectrum is very similar. The spectrum in TK6 shows no influence of strand-specific repair and resembles most cl osely the spectrum obtained by McGregor et al. after irradiation of hu man cells synchronized in S-phase. This suggests that mutations arisin g in cells that are in S-phase at the time of irradiation constitute t he majority of the mutants in an asynchronous TK6 cell population. We hypothesize that repair in the endogenous hprt gene in TK6 cells is ve ry efficient, removing virtually all lesions before replication takes place except in cells that were in S-phase at the lime of irradiation when there is not enough time for repair. Furthermore we suggest that the higher mutability of the integrated hprt cDNA genes compared with the endogenous gene is caused by a less efficient repair in the cDNA g enes.