Distribution of genetic variants in preneoplastic areas of colorectal tumours

Loss of heterozygosity (LOH) may vary almost randomly within a colorectal tumour due to the heterogeneous morphologic character of these tumours. Despite this, as a rule, single biopsies are the source of genetic material used in studies of markers important for prognosis, clinical behaviour of the disease, or susceptibility of specific tumours to different treatment modalities.

To evaluate the importance of intratumoural variation for the results of analysis of LOH and point mutations in colorectal cancer and to determine the frequency of genetic alterations in different types of pre-neoplastic areas of the tumours, 36 consecutively operated patients with colorectal cancer were studied. After fixation, specimens were mounted on large slides containing the whole tumour. The specimens were sub classified into different areas defined as normal tissue, normal tissue closely adjacent the tumour mass, adenoma, dysplasia and invasive cancer cells. These areas were dissected and subjected to DNA extraction.

The extracted genomic DNA was studied for LOH at chromosome 5q, 17p, and 18q and for k-ras mutations. Overall, a correlation between the intratumoural degree of neoplastic progression and the frequency of LOH and k-ras mutations was seen. These correlations were significant (p<0.008) except for dysplasia/adenomatous tissue versus invasive cancer. Microsatellite instability was found in 9% of the tumours, all except one in invasive parts of the tumours.

This study demonstrates a statistical correlation between intratumoural differences in neoplastic degree of dedifferentiation and genetic instability in terms of LOH and point mutations of the k-ras gene in colorectal carcinoma. The importance of a careful dissection in order to localise the region with the highest probability of genetic aberrations and multiple biopsing must not be neglected. The observation that the prevalence of k-ras mutations and LOH are correlated to the degree of dedifferentiation within a colorectal tumour is in line with the concept that selected cell clones are responsible for the neoplastic progression of the tumour.