Objective: This study was undertaken to investigate microsatellite instability and allelic imbalance in a cohort of young patients (<35 yr) and older patients (>50 yr) with sporadic colorectal carcinomas in the Kwa-Zulu Natal region of South Africa.
Methods: Normal and tumor DNA was isolated from formalin-fixed, paraffin-embedded tissue from 32 patients <35 yr and from 50 patients >50 yr of age. Tumors were staged using the modified Astler-Coller classification. Fluorescent-based DNA technology using an automated DNA sequencer (Alf Express Automated DNA Sequencer) was employed. CY5 labeled primers for microsatellite markers in chromosomes 18, 3, and 2 (DCC, D18S34, D18S58, D3S659, D3S1255, and D2S123) were used. The data were captured and analyzed using the Fragment Manager Software.
Results: The informativity of the microsatellite markers ranged from 50% to 71.8%. Microsatellite instability was seen in 44 of 82 cases (53.7%) for at least one of the six markers. Low frequency MSI (MSI-L) was seen in 28 of 82 cases (34.2%) and high frequency MSI (MSI-H) in 16 of 82 cases (19.5%). In the <35-yr age group, MSI-L was seen in six cases (18.8%) and MSI-H in 10 cases (31.3%). In the >50 yr age group, MSI-L was seen in 22 cases (44%) and MSI-H in six cases (12%). Twelve cases showed AI for DCC, seven showed AI for D18S34, and four showed AI for D18S58. MSI was found in 13, 10, and 16 cases for each of these markers, respectively. Allelic imbalance for the D3S659, D2S123, and D3S1255 loci was 3 of 82 cases (3.7%), 10 of 82 cases (12.2%), and 13 of 80 cases (16.3%), respectively. MSI was 14.6% for both D3S659 and D2S123 and was 6.3% for the D3S1255 marker.
Conclusions: Loss of heterozygosity in the region of the DCC locus ranged from 9.3% to 26.7%, and MSI ranged from 12.2% to 19.5% of cases. Allelic imbalance in the region of the repair genes ranged from 6.8% to 27% in the informative cases. MSI, however, ranged from 5% to 12%. These figures are similar to those of other studies done in other parts of the world. Further, no correlation was found between the genetic results and clinicopathological parameters: i.e., tumor stage, grade and clinical parameters of age and gender. However, genetic abnormalities were more common in the younger cohort of patients, and this may translate into the earlier age of presentation. This opens the potential for genetic screening.