A substantial proportion of microsatellite-unstable colon tumors carry TP53 mutations while not showing chromosomal instability.

Chromosomal instability in colon tumors implies the presence of numerical and structural chromosome aberrations and is further characterized by the absence of microsatellite instability and the occurrence of KRAS and/or TP53 mutations. In a previous screening of 194 colon tumors for both microsatellite instability and TP53 mutation, we found 25 microsatellite-unstable tumors, in 9 (36%) of which, presumed to be chromosomally stable, there were TP53 mutations. This prompted us to investigate whether a TP53 mutation in these microsatellite-unstable tumors would be an indicator of chromosomal instability, that is, whether this would be a category of tumors showing both microsatellite and chromosomal instability. For chromosomal instability assessment, we performed array-comparative genomic hybridization analysis of tumor and control DNA extracted from formalin-fixed, paraffin-embedded stage III colon tumor specimens. The array consisted of 435 subtelomere-specific BACs. We compared all but one (whose DNA was of bad quality) of the microsatellite-unstable TP53 mutation-containing tumors (8) with a similarly sized group of microsatellite-unstable tumors without TP53 mutation (11). Microsatellite-unstable tumors with a TP53 mutation showed on average 0.9 aberrations (range 0-3) when assessed with this array system. Those without a TP53 mutation showed on average 0.7 aberrations (range 0-2). Thus, microsatellite-unstable tumors showed few chromosomal abnormalities regardless of TP53 mutation status. Because, in our study, the microsatellite-stable tumors had on average 3.4chromosomal abnormalities (range 0-7), a clear difference exists between microsatellite-unstable and -stable tumors. Because a substantial proportion of microsatellite-unstable colon tumors carry a TP53 mutation while showing relativelyfewchromosomal aberrations, a TP53 mutation in these tumors cannot be considered to be an indicator of chromosomal instability.