Mutational spectrum of N-hydroxy-N-acetyl-4-aminobiphenyl at exon 3 of the HPRT gene

4-Aminobiphenyl is a human bladder carcinogen present in many environmental sources, including cigarette smoke. It can be metabolized in two steps to the mutagen N-hydroxy-N-acetyl-4-aminobiphenyl (N-OH-AABP). In this study the mutational spectrum of N-OH-AABP-exposed human lymphoblastoid cells (TK6) was determined using HPRT as the target gene. Three large, HAT (hypoxanthine-aminopterin-thymidine)-cleaned TK6 cultures were independently treated with 20 μm N-OH-AABP for 24 h, allowed to recover for 4 days, then continuously exposed to 40 μm 6-thioguanine to select for induced mutants. Contemporary control cultures received vehicle in place of N-OH-AABP. N-OH-AABP treatment gave an 11-fold increase in mutation frequency. Mutations were delineated in exon 3 of the HPRT gene directly from genomic DNA extracted from both treated and untreated cells using the polymerase chain reaction, denaturing gradient gel electrophoresis (DGGE) and dideoxy sequencing. DGGE analysis showed N-OH-AABP increased both the number and type of mutations as compared with controls. The major background mutation was a G(197) →A transition. The major N-OH-AABP-induced changes were G(209-211) →T transversions (30%), a seven-base repeat at position 185 (17%) and an A(215) →T transversion (2%). The shift in the control spectrum of a transition to that of transversions and insertions suggest that the electrophile N-OH-AABP forms bulky adducts at the same sites on exon 3 of HPRT as do many other bulky electrophiles, causes replication errors by similar mechanisms, but induces at least one potentially signature mutation.