In vitro mutational spectrum of cyclopenta[cd]pyrene in the human HPRT gene

Cyclopenta[cd]pyrene (CPP) is a widely distributed polycyclic aromatic hydrocarbon with potent mutagenic and carcinogenic activity. In order to acquire an understanding of the mutagenic pathways of CPP, we studied mutations induced by this chemical in human cells. Four independent cultures of a human cell line expressing cytochrome P450 CYP1A1 (cell line MCL-5) were treated with CPP, and mutants at the hypoxanthine phosphoribosyltransferase (HPRT) locus were selected en masse by 6-thioguanine (6TG) resistance. The kinds and positions of the mutations were analyzed using the combination of high-fidelity polymerase chain reaction (hifi-PCR) and denaturing gradient gel electrophoresis (DGGE). The third exon of the HPRT gene was amplified from the 6TG-resistant cells using the hifi-PCR and the amplified fragment was subsequently analyzed by DGGE to separate mutant sequences from the wild-type sequence. Mutant bands were excised from the gel, amplified using PCR and sequenced. Sixteen different mutations were identified and consisted mostly of the G to T and A to T transversions. Other mutations identified included G to A and A to G transitions, a G to C transversion, and a single G deletion. Of these mutations, six occurred within a run of six guanines. The predominance of transversions involving a guanine or an adenine observed with CPP is similar to the data previously reported for the racemic mixtures of benzo[a]pyrene (B[a]P), suggesting that the mechanisms of mutation induced by CPP may be similar to those induced by B[a]P.