Methylation tolerance due to an O6-methylguanine DNA methyltransferase (MGMT) field defect in the colonic mucosa: an initiating step in the development of mismatch repair-deficient colorectal cancers.
OBJECTIVE
O(6)-Methylguanine-DNA methyltransferase (MGMT) removes methyl adducts from O(6)-guanine. Known as methylation tolerance, selection for mismatch repair (MMR)-deficient cells that are unable to initiate lethal processing of O(6)-methylguanine-induced mismatches in DNA is observed in vitro as a consequence of MGMT deficiency. It was therefore hypothesised that an MGMT field defect may constitute a preneoplastic event for the development of MMR-deficient tumours displaying microsatellite instability (MSI).


METHODS
MGMT expression was investigated by immunohistochemistry and the methylation status of the gene promoter by PCR in neoplastic, adjacent and distant mucosal tissues of patients with MSI or non-MSI (MSS) colorectal cancer (CRC). The cancers were familial (42 MSI, 13 MSS) or sporadic (40 MSI, 49 MSS) in origin, or arose in the context of inflammatory bowel disease (IBD; 13 MSI, 36 MSS). Colonic mucosa from patients with diverticulitis (n=20) or IBD (n=39 in 27 patients) without cancer served as controls.


RESULTS
Loss of MGMT expression was more frequent in MSI than MSS CRC (p=0.047). In comparison with MSS tumours, MSI CRC occurred more frequently adjacent to patches of mucosa that lacked MGMT expression (p=0.002). Overall, loss of MGMT expression was associated with MGMT gene promoter methylation (p=0.03).


CONCLUSIONS
MGMT field defects are more frequently associated with MSI than MSS CRC. These findings indicate that methylation tolerance may be a crucial initiating step prior to MMR deficiency in the development of MSI CRC in familial, sporadic and IBD settings.
Equipe Instabilité des Microsatellites et Cancers, Hôpital Saint-Antoine, Paris cedex, France.
Magali
M
Svrcek
Olivier
O
Buhard
Chrystelle
C
Colas
Florence
F
Coulet
Sylvie
S
Dumont
Illiasse
I
Massaoudi
Amel
A
Lamri
Richard
R
Hamelin
Jacques
J
Cosnes
Carla
C
Oliveira
Raquel
R
Seruca
Marie-Pierre
MP
Gaub
Michèle
M
Legrain
Ada
A
Collura
Olivier
O
Lascols
Emmanuel
E
Tiret
Jean-François
JF
Fléjou
Alex
A
Duval
D048868:Adaptor Proteins, Signal Transducing / Q000235:genetics; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D015179:Colorectal Neoplasms / Q000201:enzymology / Q000235:genetics* / Q000473:pathology; D019175:DNA Methylation; D053843:DNA Mismatch Repair; D004273:DNA, Neoplasm / Q000235:genetics*; D005260:Female; D006801:Humans; D007413:Intestinal Mucosa / Q000201:enzymology; D008297:Male; D053842:Microsatellite Instability; D008875:Middle Aged; D000070957:MutL Protein Homolog 1; D009154:Mutation; D009367:Neoplasm Staging; D009687:Nuclear Proteins / Q000235:genetics; D019853:O(6)-Methylguanine-DNA Methyltransferase / Q000235:genetics* / Q000378:metabolism; D011230:Precancerous Conditions / Q000235:genetics; D011401:Promoter Regions, Genetic / Q000235:genetics; D011518:Proto-Oncogene Proteins / Q000235:genetics; D048493:Proto-Oncogene Proteins B-raf / Q000235:genetics; D016283:Proto-Oncogene Proteins p21(ras); D055815:Young Adult; D018631:ras Proteins / Q000235:genetics
D048868:Adaptor Proteins, Signal Transducing; D004273:DNA, Neoplasm; C117307:KRAS protein, human; C501486:MLH1 protein, human; D009687:Nuclear Proteins; D011518:Proto-Oncogene Proteins; D019853:O(6)-Methylguanine-DNA Methyltransferase; C482119:BRAF protein, human; D048493:Proto-Oncogene Proteins B-raf; D000070957:MutL Protein Homolog 1; D016283:Proto-Oncogene Proteins p21(ras); D018631:ras Proteins