c-Ki-ras mutations in colorectal adenocarcinomas from a country with a rapidly changing colorectal cancer incidence

We have examined the incidence of mutation of the c-Ki-ras proto-oncogene i n colorectal adenocarcinomas from two different time periods, namely 1962-1 966 and 1994-1996. The first cohort of samples consisted of formalin-fixed, archival paraffin block and represent the oldest colorectal cancer samples for which ras mutation has been examined, white the second cohort of tumou rs were fresh, flash-frozen samples representative of genetic events occurr ing in contemporary times. Analysis of mutation status was undertaken by a mismatch-specific oligonucleotide hybridization analysis of exon 1 of the c -Ki-ras proto-oncogene after amplification by the polymerase chain reaction . Mutations in codon 12 or 13 of c-ki-ras were detected in 28% (14/50) of c ontemporary samples, a figure consistent with locally established mutation rates. In contrast no mutation was detected in any of the 18 samples from t he earlier period, a result that is statistically significant (P = 0.007). Age-standardized rates of colorectal cancer in Singapore have seen a marked increase over the last 30 years, and for the first time we have shown that such an increase in colorectal cancer is associated, at least in part with an increase in incidence of a specific mutagenic change. (C) 1999 Cancer R esearch Campaign.