Low O6-alkylguanine DNA-alkyltransferase activity in normal colorectal tissue is associated with colorectal tumours containing a GC-->AT transition in the K-ras oncogene

Carcinogenesis. 1997 Jul;18(7):1299-302. doi: 10.1093/carcin/18.7.1299.


O6-alkylguanine DNA-alkyltransferase (ATase) provides protection against the toxic, mutagenic and carcinogenic effects of alkylating agents, principally by removing the promutagenic lesion O6-alkylguanine from DNA. Differences in ATase activity in human tissue may thus determine mutational susceptibility. As GC-->AT transitions, which can be induced by O6-alkylguanine in DNA, are commonly observed in the K-ras oncogene of alkylating agent induced animal tumours and in human colorectal tumours, we have examined whether differences in ATase activity may affect the risk of K-ras mutations in humans with colorectal tumours. NTase activity in normal tissue from individuals with a K-ras mutation in colorectal tissue and more specifically a GC-->AT transition (but not a transversion mutation) was significantly lower than that in individuals without a mutation (P < 0.01). Thus, individuals with low ATase activity in normal tissue (i.e. below the median) were at increased risk of having a transition (OR 10.1; 95% CI 1.9-99.0), but not a transversion mutation (OR 1.7; 95% CI 0.3-12.2). There were no significant differences in tumour ATase activity in individuals with or without a mutation. These results suggest that ATase can protect colorectal tissue against the mutagenic effects of alkylating agents and furthermore, that alkylating agent exposure plays a role in the aetiology of colorectal tumours containing a GC-->AT transition in the K-ras oncogene.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Colon / enzymology*
  • Colorectal Neoplasms / genetics*
  • Genes, ras*
  • Humans
  • Methyltransferases / metabolism*
  • Mutation*
  • O(6)-Methylguanine-DNA Methyltransferase
  • Rectum / enzymology*


  • Methyltransferases
  • O(6)-Methylguanine-DNA Methyltransferase