In this paper we describe the analysis of the MSI status in a cohort of colorectal cancer patients from Macedonia. The study was conducted with two different methods, nonradioactive PCR-PAGE and fluorescent multiplex PCR/capillary electrophoresis assay. Similar results were obtained with both methods, making the fluorescent multi­plex PCR the method of choice for a routine MSI analysis due to its simplicity, speed and lesser manipulation requirements. From the five microsatellite markers used in this study, the BAT26 exhibited 100% sensitivity and 100% specificity in the detection of MSI-H phenotype in tumors. The sensitivity and specificity for D17S250, D18S61, D18S58 and DCC markers were 81.8 and 100%, 72.7 and 98.9%, 63.6 and 98.9%, and 45.5 and 98.9%, respectively. These data support the suggestion of Yamamoto et al. [20] that BAT26 can be used as a single marker for the detection of MSI status in colorectal tumors.

The frequency of MSI in our group of patients was 10.3% which is within the reported range in several exten­sive studies [11,21]. None of the tumors with the MSI-H phenotype had nodal involvement, and 10/11 of these were right sided (caecum, colon ascendens and transversum). These tumors were also associated with lower Dukes’ stage at diagnosis and primarily mucinous histotype. These data are in agreement with the general opinion about the histopathological characteristics and MSI status of colo­rectal tumors [21]. However, MSI status was not associated with age in our group of patients, contrary to several studies published recently [13,21]. Bearing in mind that HNPCC is generally associated with a younger age at diagnosis, in comparison with cases with the sporadic form of the disease, one of the possible explanations of this discrepancy is that there is a lower frequency of HNPCC carriers in our group of patients. Since this was a retrospective study, we did not have sufficient information on the family history of each patient, or enough material to conduct mutational analyses of MMR genes to clarify this issue. A prospective study is underway which should resolve this question in the near future.

Instability at only one locus was detected in three patients. In a recent study it was concluded that this type of instability can be detected in most, if not all, colorectal carcinomas, if a sufficient number of markers is used [17]. Other authors have presented evidence that the MSH6 gene inactivation is associated with MSI-L phenotype [22]. Since no detailed family history and no material for MSH6 gene analysis was available, it is difficult to discriminate between these two possibilities, and this issue will also be addressed in the ongoing prospective study.