Microsatellite instability (MSI) has been well documented in tumor DNA from hereditary non-polyposis colon cancer (HNPCC) patients known to carry germline mutations in major mismatch repair (MMR) genes. It has been hypothesized that such germline mutations might also result in lower, yet detectable, clinically significant levels of MSI in constitutive tissues, (Parsons, et al. 1995. Science 268: 738). To that end, we used quantitative Small-Pool PCR (SP-PCR) to examine PBL and tumor DNA from five sporadic colorectal cancer (CRC) patients and six microsatellite instability-high (MSI-H) HNPCC patients –3 with MLH1 and 3 with MSH2 mutations (splice, missense, deletion and stop mutations were represented), age 36-71 yr. PBLs from age matched normal individuals were used as controls in every experiment. Each sample was studied at D2S123, D5S346, and D17S518, previously shown to be informative for quantitative MSI analysis, (Coolbaugh-Murphy, et al. 2005. Mech. Aging and Dev. 126:1051). PBL DNA from all six HNPCC patients demonstrated highly statistically significant increased frequency of mutant microsatellite fragments compared to the levels seen in the PBLs of the sporadic CRC patients or the age matched controls. The results from the sporadics are consistent with results from a prior study where a 70 y/o sporadic CRC patient, with an epigenetic MLH1 promoter methylation, had no higher MSI in PBL DNA than seen in age-matched normal control PBL DNA –even when 6 loci were scrutinized (Coolbaugh-Murphy, et al. 2004. Genomics 84: 419). These observations support the hypothesis that individuals carrying germline mutations predis posing them to HNPCC exhibit significantly increased levels of MSI in their constitutive tissues, which can be tested, quantified, and monitored. The data suggest the possibility that regardless of the MMR gene affected or the type of mutation leading to MSI, SP-PCR of PBL DNA might be used as a functional assay for genomic instability of clinical significance.

[Proc Amer Assoc Cancer Res, Volume 47, 2006]