MUTATIONAL SPECTRA OF THE DIETARY CARCINOGEN 2-AMINO-1-METHYL-6-PHENYLIMIDAZO[4,5-B]PYRIDINE (PHIP) AT THE CHINESE-HAMSTER HPRT LOCUS

The mutagenic 'fingerprint' of the cooked food carcinogen 2-amino-1-me thyl-6-phenylimidazo[4,5-b]pyridine (PhIP) was determined in a Chinese hamster cell line genetically engineered to express human CYP1A2, (XE Mh1A2-MZ). The parental Chinese hamster V79 and XEMh1A2-MZ cells were exposed to PhIP at various concentrations for 24 h, There was a dose-d ependent increase in frequency of mutations at the hypoxanthine-guanin e phosphoribosyltransferase (hprt) locus only in the metabolically com petent XEMh1A2-MZ cells, The mutant frequency ranged from 25 to 90 x 1 0(-6) with final concentrations of 2.5 to 100 mu M PhIP compared to 8 x 10(-6) in the solvent controls and the V79MZ cells, The molecular na ture of PhIP-induced mutations in XEMh1A2-MZ cells was determined by e xamining DNA sequence modifications at the hprt locus in forty five 6- thioguanine resistant (6-TG(r)) mutant clones, Single base substitutio ns, predominantly GC-->TA transversions, were the major class of PhIP- induced mutation, However, a -1 frameshift 'hotspot' in a 5'-GGGA sequ ence was also observed, With the exception of a compound modification, all of the PhIP-induced mutations involved G.C base pairs, This is co nsistent with the previously observed PhIP-induced mutations in cultur ed mammalian cells and P-32-postlabelling experiments that show PhIP a dducts to the guanine base and that the major adduct is at the CS posi tion. Furthermore, nearly all of these mutations involved guanine base s on the non-transcribed strand which is possibly indicative of prefer ential repair of PhIP adducts from the transcribed strand, Nearest nei ghbour analysis of induced base substitutions indicates a preference f or 5' guanine and 3' adenine, These data effectively define a mutation 'fingerprint' for PhIP, which may provide the basis for definitive st udies on the role of PhIP in diet associated cancers such as tumours o f colon, It is, therefore, intriguing that in their recent report of m utation in tumours of the colon induced by PhIP in male rats Kakiuchi et al, (Proc. Natl Acad. Sci. USA, 92, 910-914) report that four out o f eight tumours had an identical mutation of the tumour suppressor gen e ape which comprised of a -1 G frameshift in a 5'-GGGA sequence.