Instability of microsatellites is a hallmark of the DNA replication error phenotype (RER+) due to the inactivation of mismatch repair genes. In humans, microsatellite instability has first been described in colorectal tumors developing in either hereditary nonpolyposis colorectal cancer or sporadic patients. Colorectal tumorigenesis in RER+ and RER- tumors is probably due to distinct mechanisms, and RER+ tumors have a better prognosis than RER- tumors. The study of the RER status of a tumor may thus be important in the future to determine biological prognosis factors and investigate therapeutical strategies. The RER status of 134 primary tumors and 26 cell lines derived from colorectal cancers was established by PCR amplification and analysis of a minimum of 32 microsatellite loci. This characterization allowed us to unambiguously classify 35 primary tumors and 7 cell lines as RER+. Typing of a single poly(A) tract, BAT-26, was sufficient to confirm the RER status of 159 of these 160 tumors and cell lines. Moreover, in DNA from unaffected individuals, normal tissues of a subset of the RER+ patients, and all RER- tumors or cell lines, BAT-26 was quasi-monomorphic, showing only minor size variations. BAT-26 alleles showing shortening from 4 to 15 bp were observed in all but 1 of the RER+ tumors and cell lines. The size difference between the range of normal large alleles and unstable small alleles was sufficient to be detected by electrophoresis on conventional polyacrylamide gels stained with ethidium bromide. We thus propose a simple, low-cost, and rapid method to screen for the RER status of colorectal cancer primary tumors and cell lines, even in the absence of matching normal DNA and, in most cases, without the need for radioactivity. This method could easily be set up in routine laboratories.
This work was supported by grants from the Association pour la Recherche contre le Cancer and the Association Française contre les Myopathies. J-M. Hoang was supported by a grant from the Ligue Nationale contre le Cancer.