Microsatellite instability in colorectal and oesophageal cancer.
The development and progression of carcinogenesis is a major area of interest to many scientists. Numerous factors, including both environmental and genetic have been implicated in the causation of cancer. It is clear that both these factors and others contribute to neoplastic development and progression. Microsatellites are short tandem repeat sequences which are located in the intron segments of the genome. These noncoding sequences range from 2 to 6 base pairs. An increase or decrease in the number of repeat sequences is referred to as microsatellite instability, also referred to as genetic instability. It is thought that microsatellite instability arises as a result of defects in DNA repair process. During DNA synthesis, the DNA repair genes ensure that the correct nucleotide is incorporated into the newly synthesised DNA strand, so when a mismatch base is incorporated, this is promptly removed and replaced with the correct base. However, if the repair system is defective this would give rise to numerous genetic aberrations along that region of the genome. Recently, microsatellite instability and allelic imbalance/loss of heterozygosity have been shown to play an important role in the development of many cancers, especially colorectal cancer (CRC) associated with the hereditary nonpolyposis colorectal cancer (HNPCC) syndrome. This study was undertaken to investigate microsatellite instability and allelic imbalance in colorectal and oesophageal carcinomas in the KwaZulu Natal region of South Africa. The molecular analysis was correlated with clinicopathological data to establish a baseline level on which further studies could be performed. In addition, this study represents the first fluorescent based microsatellite analysis of these two common cancers in South Africa. Normal and tumour DNA was isolated from formalin fIxed paraffin embedded tissue. Fluorescent-based DNA technology using an automated DNA sequencer (Alf Express Automated DNA Sequencer) was employed. CY5 labelled primers for microsatellite markers (DCC, D18S34, D18S58, D3S659, D2S123 and D3S1255) were used. The data was captured and analysed using the Fragment Manager Software. The informativity of the microsatellite markers used in this study ranged from 50% to 71.8%. LOH/AI in the region of the DCC gene in the under 35 years of age CRC was 39.1%, while MSI in this region occurred in 31.25% of cases. The DNA repair gene status in these young patients was as follows: LOH/AI: 31.3% and MSI: 40.4%. In the over 50 years of age CRC, LOH/AI in the 18q region was 28% and MSI was 38%. The DNA repair genes (hMSH2 and hMLH1) in this cohort showed LOH/AI in 24% and MSI also in 24%. As regards oesophageal cancer, LOH/AI in the 18q region was 20.5% and MSI 7.7%. The repair genes showed LOH/AI in 17.9% and MSI in 10.25% of cases. When the molecular events were correlated with clinicopathological features, no statistically significant pattern emerged. However, it must be remembered that relatively small numbers of cases (39) were analysed.In conclusion: • No statistical correlation was found between clinicopathological characteristics and the molecular analysis in either CRC and oesophageal cancer. • LOH/AI and MSI was higher in the under 35 age group. • LOH/AI and MSI in 18q, 2p and 3p in sporadic CRC were similar to other fluorescent-based studies in patients over 50 years of age. • LOH/AI and MSI in 18q, 2p and 3p in oesophageal cancer was similar to studies from other geographical areas. • Finally, fluorescent-based microsatellite PCR and analysis was found to be an objective and efficient technique.