p53 mutations and chromosome instability in basal cell carcinomas developed at an early or late age.

Tumor DNA from 45 primary basal cell carcinoma (BCC) biopsies was screened for p53 gene mutations, chromosome 9 allele loss, and microsatellite instability. p53 mutation frequency increased significantly as a function of the age at BCC onset ranging from 6% (1/16) in early BCC (before age 40 years) to 35% (10/29) in late BCC. All p53 mutations found implicated sunlight as the mutagen. Chromosome 9 instability (allele loss or microsatellite instability) was detected at high frequency (38%) independently of age at tumor onset. Allelic loss was confined to chromosome 9q, whereas microsatellite instability was observed prevalently on chromosome 9p often in association with a replication error (RER+) phenotype. Most of our late BCC patients reported occupational sun exposure, while early BCC patients recalled childhood (0-20 years) recreational sun exposure. These data suggest that chronic exposure to sunlight is responsible for accumulation of p53 mutations and thus for late BCC appearance, whereas acute UV exposure in childhood and adolescence leads to early skin cancer development in genetically susceptible individuals via a p53-independent pathway.

Molecular analysis of mutations induced by chemical carcinogens in mammalian cells. II. The use of recombinant DNA vectors.

One approach to molecular and mechanistic studies of mutagenesis in mammalian cells is to introduce a mutational target gene into the cells as part of a shuttle vector which is capable of replication in both mammalian cells and bacteria. Following mutagenesis in the mammalian cell host, the shuttle vector sequences are recovered from the mammalian cells and introduced into bacteria, where large amounts of the mutant gene can be produced for sequence analysis. The variety of shuttle vector systems which have been developed for this purpose will be described. Shuttle vectors have been widely used for the molecular analysis of mutations induced by physical and chemical agents and to investigate the factors which modulate mutation fixation. The data regarding chemically induced mutational spectra will be reviewed with particular emphasis on the studies aimed to dissect the complex process which lead from DNA lesion to mutation.

Strand bias of ultraviolet light-induced mutations in a transcriptionally active gene in human cells.

Ultraviolet (UV)-induced repair and mutational spectra were analyzed in an inducible marker gene, the metallothionein-l/guamine-xanthine phosphoribosyl transferase (gpt) fusion gene, carried by an Epstein-Barr virus-derived shuttle vector episomically maintained in human cells. The repair rate of UV photodimers from the shuttle-vector molecules was typical of transcriptionally active sequences, 70% of the dimers being removed within 8 h after irradiation. The spectrum obtained under basal gene transcription was compared with that obtained under induced transcription. In both cases, base substitutions at dipyrimidine sequences predominated. Multiple mutations and deletions probably due to recombinational events induced by UV damage were also observed. Most of the UV-mutated dipyrimidine sites were located in the transcribed strand and were independent of the transcriptional activity of the target gene. In contrast, the distribution of mutations throughout the coding region of the gpt gene was affected by transcription, with a preferential clustering of mutations occurring in the 3' half of the gene after transcription induction. The strand bias observed in the UV spectra most likely reflects selection for nonfunctional gpt protein.