BACKGROUND The pro-mutagenic lesionO ⁶-methyldeoxyguanosine (O ⁶-MedG), a marker of exposure to many N-nitroso compounds (NOC), can be detected in normal and tumour DNA isolated from colorectal tissue. The biological significance of this exposure is, as yet, unknown but in situ NOC formation is bacterially catalysed suggesting that NOC formation and potentially DNA alkylation will vary throughout the large bowel.

AIMS To determine ifO ⁶-MedG levels in colorectal DNA vary within the large bowel.

PATIENTS We studied 62 men and women undergoing surgery for colorectal tumours in the north west of England.

RESULTS O ⁶-MedG was detected in 27 out of a total of 62 (43%) normal DNA samples and in 30 of 58 (52%) tumour DNA samples: it was present at concentrations of <0.01–0.94 and <0.01–0.151 μmolO ⁶-MedG/mol deoxyguanosine for normal and tumour DNA, respectively. Levels ofO ⁶-MedG in normal, but not tumour, DNA from the proximal colon were lower than those found in DNA from either the sigmoid colon (p=0.03) or rectum (p=0.05). When the analysis was restricted to samples that containedO ⁶-MedG, similar results were obtained in that O ⁶-MedG levels in normal DNA were lower in the proximal colon than in the sigmoid colon (p=0.04) or rectum (p=0.03).

CONCLUSIONS DNA alkylation varied within the large bowel possibly due to in situ NOC formation and was highest in areas of the colon and rectum where the highest incidence of large bowel tumours occurs, suggesting that DNA alkylation may play a role in the aetiology of colorectal cancer.