Influence of smoking and donor age on the spectrum of in vivo mutation at the HPRT-locus in T lymphocytes of healthy adults

Types and frequencies of in vivo mutation in the hypoxanthine-guanine phosp horibosyl-transferase (HPRT) gene was studied in 142 T cell mutants from 78 healthy nonsmoking and smoking adults with a mean of 65 years. The HPRT mu tant frequency in the nonsmokers was 18.7 +/- 12.0 X 10(-6), and in the smo kers 26.6 +/- 18.5 X 10(-6) (mean +/- S.D., P < 0.01). Among 107 single bas e pair substitutions (SBS) in the coding region of the HPRT gene, one new m utable site, one novel nonsense mutation and three not previously reported SBS were identified. Transitions accounted for 59% of the SBS and transvers ions for 41%. GC > AT transitions were the predominant type of mutation, wi th 50% of all SBS. The mutations showed a nonrandom distribution along the coding sequence, with three significant hotspots at positions 143, 197 and 617 (13, 14 and 7 mutations, respectively). There was no difference between smokers and nonsmokers with I regard to the distribution of mutations at t hese hotspot positions. However, 85% of the mutations at GC base pairs and 88% of the mutations at AT base pairs in smokers occurred at sites with gua nine or thymine, respectively, in the nontranscribed DNA strand. Moreover, smokers had a higher frequency of transversions and lower frequency of tran sitions than nonsmokers did. Particularly, GC > TA transversions were incre ased in smokers (11%) compared to nonsmokers (2%), which suggests that toba cco-smoke induced adducts at guanine bases in the nontranscribed DNA strand contributes to the increase of HPRT mutation in smokers. Overall, these re sults were very similar to the mutational spectra in two younger study popu lations reported previously [K.J. Burkhart-Schultz. C.L. Thompson, I.M. Jon es, Spectrum of somatic mutation at the hypoxanthine phosphoribosyltransfer ase (HPRT) gene of healthy people, Carcinogenesis 17 (1996) 1871-1883; A. P odlutsky, A.-ICI. Osterholm, S.-M. Hou, A. Hofmaier, B. Lambert, Spectrum o f point mutations in the coding region of the hypoxanthine-guanine phosphor ibosyltransferase, Carcinogenesis 19 (1998) 557-566], With the possible exc eption of an increase of mutations at hotspot position 143, and a decrease of 5-methylcytosine deamination mediated transitions at CpG-sites in the ol der individuals, there were no differences between the mutational spectra o f old and young adults. In conclusion, both smoking and ageing seem to have minor influences on the spectrum of HPRT mutation in T cells. (C) 1999 Els evier Science B.V. All rights reserved.