ELEVATED FREQUENCIES OF BENZO(A)PYRENE-INDUCED HPRT MUTATIONS IN INTERNAL TISSUE OF XPA-DEFICIENT MICE

Xeroderma pigmentosum (XP) patients are hypersensitive to sunlight and have a high predisposition to developing cancer. At the cellular leve l, XP patients are defective in nucleotide excision repair (NER), Rece ntly, mice have been generated via gene targeting that are deficient i n the expression of the XPA gene [A. de Vries et al., Nature (Lond.), 377: 169-173, 1995], We have assessed the consequences of defective NE R for mutagenesis in normal and XPA mice exposed to benzo(a)pyrene and 2-acetylaminofluorene. To study mutagenesis, mature T lymphocytes wer e isolated from the spleen and stimulated to proliferate in vitro to s elect for mutants at the endogenous Hprt locus. Background mutant freq uencies in normal and XPA mice were very similar and not influenced by age, Single doses of benzo(a)pyrene administered i.p. resulted in a d ose-dependent increase of the Hprt mutant frequency in normal mice, In addition, after chronic exposure to benzo(a)pyrene, Hprt mutants were readily detectable in XPA mice at an early onset of treatment but onl y at a later stage in normal mice, In contrast, chronic treatment of e ither normal or XPA mice with 2-acetylaminofluorene did not increase H prt mutant frequency above the background frequency. This absence of s ignificant induction of Hprt mutants can be entirely attributed to the low frequency of 2-acetylaminofluorene-induced DNA adducts in lymphoi d tissue. These results provide the first direct evidence in mammals t hat deficient NER leads to enhanced mutagenesis in endogenous genes in internal tissue after exposure to relevant environmental mutagens, su ch as benzo(a)pyrene.