CAT25 is a mononucleotide marker to identify HNPCC patients.

Mismatch repair mutations are the cause of generalized genomic instability and are particularly evident at microsatellite loci, which is known as microsatellite instability (MSI). MSI is present in 85% to 90% of colorectal cancers and occurs in hereditary non-polyposis colorectal cancer (HNPCC). The National Cancer Institute recommends the "Bethesda panel" for MSI screening. Recently, a novel T(25) mononucleotide marker was described, termed CAT25. This microsatellite marker displays a quasi-monomorphic pattern in normal tissues. The aim of our study was to evaluate the performance of CAT25 in HNPCC patients and to compare its reliability with the results of the Bethesda panel. We tested 55 tumor tissues from HNPCC patients using both the Bethesda panel and the CAT25 mononucleotide marker. One hundred healthy blood donors were used as controls. The CAT25 microsatellite was found to be altered in all 13 colorectal cancers classified as MSI-H using the standard Bethesda panel. Colorectal tumors that showed a stable Bethesda pattern did not show altered CAT25 repeats. Additionally, CAT25 showed a monomorphic allele pattern in all tissue samples. In our series, the concordance between the Bethesda panel and CAT25 in identifying colorectal cancers with high MSI reached 100%. Our results suggest that the CAT25 microsatellite represents a sensitive and specific marker for MSI and could be, at least, included in the panel of markers for the identification of HNPCC patients.