UQ eSpace

Abstract

The current model for colorectal tumourigenesis defines four mutations which accumulate in a colonic epithelial cell as it progresses towards the cancer phenotype. It is the accumulation of these mutations rather than the order in which they occur that leads to malignant change, however, there is, in most cases, a preferred order. These mutations include activation of a ras proto-oncogene and inactivation of the APC, p53 and DCC tumour suppressor genes on chromosomes 5, 17 and 18 respectively. Further mutations must be needed for progression to malignancy and subsequently to metastasis, because advanced adenomas have been observed in which all four of these mutations occur simultaneously. The aim of this study was to augment the current model with further mutations that might be significant in the development of colorectal tumours, to determine the temporal significance of each mutational event, and to examine interactions between the various mutations in tumours with different histological grades and clinicopathological staging. Cytogenetic studies have shown that genetic material is often lost from tumour cells despite an overall increase in DNA content. This has been confirmed by molecular analysis and is a low level random phenomenon in many solid tumours. However, it has been shown that above background, non-random loss in a specific region indicates that it may contain a suppressor gene, as loss of a wild-type allele is a common mechanism of inactivation of such genes. Hence, in order to determine what other events may be involved in tumourigenic progression, other chromosomes (1, 7, 8, 9, 14 and 22) were chosen for analysis of allele loss either because other investigators had found indications of non-random loss there, or because they contained genes which might be functionally significant in colorectal tumourigenesis, in particular, protease inhibitor genes. In addition, chromosomes which contain the four mutations of the current model were examined as a point of reference for the current study, and in order to relate these events to new ones that were identified. Neoplasms from 119 patients (108 adenocarcinomas and 46 adenomas) were examined for loss of heterozygosity (LOH) on nine chromosomes, and for point mutations in the 12th codon of the c-Ki-ras gene. From the proportion of mutations which occurred in each stage of tumour progression, it was possible to assign some temporal significance to the mutations. This analysis confirmed the occurrence of K-ras and chromosome 5 mutations as early events. These occurred independently of all other mutations, and were age related. Loss of sequences from chromosomes 22 and 18 occurred sparingly in adenomas and commonly in carcinomas, and were independent events suggesting that they may represent different transition pathways between adenomas and carcinomas. Loss of sequences from chromosome 17 was very common in carcinomas, very rare in adenomas and weakly related to clinicopathological staging whereas loss of chromosomes 14 and 8 was also rare or absent in adenomas, but unlike 17, occurred far more frequently in advanced carcinomas, suggesting that these events occur late in the progression.