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 mutations 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 different positions of the human genome. Previous studies showed that the genomic sequences surrounding the integration site influence spontaneous mutagenesis, resulting in a 10-fold difference in mutation rates among the hprt cDNA genes. Here we demonstrate that the genomic positions of three integrated hprt cDNA genes do not influence UV-induced mutagenesis. The mutability by UV irradiation in these cell lines is approximately the same (16.0 x 10(-6) per J/m2). The nature of the UV-induced mutations determined in two of the cell lines containing the integrated hprt cDNA gene (approximately 30 mutants each) was also found not to be different. The endogenous hprt gene in the parental TK6 cells exhibits a significantly lower mutability (2.1 x 10(-6) per J/m2) than the cDNA genes, but the spectrum is very similar. The spectrum in TK6 shows no influence of strand-specific repair and resembles most closely the spectrum obtained by McGregor et al. after irradiation of human cells synchronized in S-phase. This suggests that mutations arising in cells that are in S-phase at the time of irradiation constitute the majority of the mutants in an asynchronous TK6 cell population. We hypothesize that repair in the endogenous hprt gene in TK6 cells is very efficient, removing virtually all lesions before replication takes place except in cells that were in S-phase at the time 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 genes.