HARVEY RAS (H-RAS) POINT MUTATIONS ARE INDUCED BY 4-NITROQUINOLINE-1-OXIDE IN MURINE ORAL SQUAMOUS EPITHELIA, WHILE SQUAMOUS-CELL CARCINOMAS AND LOSS OF HETEROZYGOSITY OCCUR WITHOUT ADDITIONAL EXPOSURE

Tumorigenesis is a multistep genetic process requiring several somatic mutations for neoplastic transformation. These mutations appear to he sequential, random, and independent events. However, we find linked, nonrandom ras mutations occurring during 4-nitroquinoline-1-oxide-indu ced tumorigenesis months after exposure to the carcinogen had ceased. The carcinogen had been topically applied to the oral cavity of CBA mi ce for 4 to 16 necks. Dysplasia developed after 24 weeks, and carcinom a in situ and squamous cell carcinoma developed after 28 weeks. H-ras mutations were detected in 13 of 25 tissue specimens (10 of 14 invasiv e carcinomas and 2 of 4 carcinoma in situ, 1 of 5 dysplastic tissue, a nd 0 of 2 normal tissues). Approximately one-half of the tumors had G to A point mutations at codon 12 of the cellular H-ras proto-oncogene on mouse chromosome 7. None had codon 11, 13, or 61 mutations. Loss of heterozygosity occurred in 5 of 14 invasive cancers. Larger invasive squamous cell carcinomas consistently lost the wild-type allele, where as preneoplastic lesions and small tumors were heterozygous for ras. T his suggests a causal relationship between carcinogen treatment, H-ras activation, and initiation of tumorigenesis. The wild-type allele in mouse chromosome 7 is lost with the progression of tumorigenesis long after exposure to the carcinogen. Thus, loss of heterozygosity of the ras gene appears to occur without multiple carcinogen-induced mutation s, i.e., as a result of a cascade of events induced by an earlier ras mutation.