Carcinogenesis

This implies that diet acts as a tumour promoter and not a tumour initiator; these conclusions agree with the results on bile acids described above.

Although almost all epidemiological studies of populations show a strong correlation between the disease and dietary fat or fibre, the correlations are very weak in case-control studies. Similarly there is a strong correlation between FBA concentration and LBC risk in populations but not in case-control studies. These observations are explicable in the context of the adenoma-carcinoma sequence; in case-control studies there are confounding genetic factors as well as the environmental factors causing the expression of adenoma- proneness that are naturally controlled for in population studies.

It is necessary now to carry out studies of the causation of colorectal adenomas, and an excellent case-control study of the role of diet in adenoma formation is in progress, organised by the European Research Group for Cancer Prevention (ECP). The results of such studies will not only increase our understanding of colorectal carcinogenesis but will also provide more options for cancer prevention.

Genetic aspects of carcinogenesis

Denise Sheer

Imperial Cancer Research Fund, Lincoln’s Inn Fields, London WC2A 3PX, UK

Malignant transformation is a multistage process in which both environmental and genetic factors play a role. Epidemiological studies, particularly of migrant populations, show that environmental factors are of overwhelming importance in determining the incidence of cancer. However, within a given population systemic factors such as immune status and metabolism of chemical carcinogens which have a genetic contribution undoubtedly influence the development of the disease. There are many, individually rare, clear-cut examples of inherited cancers, either through an autosomal dominant gene as in familial polyposis coli or Wilms’ tumour, or through an autosomal recessive gene as in the chromosome instability syndrome, Bloom’s syndrome. At the cellular level, cancer is believed to arise through a series of changes in the genome. Some of these genetic changes have now been defined and will be reviewed briefly, together with implica- tions for colorectal cancer.

Chromosome aberrations and oncogenes in cancer cells Most tumours have structural and/or numerical chromosome aberrations, some of which are consistently associated with particular types of tumour’. These non-random chromosome aberrations are believed to-confer a proliferative advantage to cells carrying them and to be involved in the pathogenesis of tumours. Leukaemias and lymphomas generally have chromosomes in the diploid range with few structural aberrations. Usually these are translocations where segments of two chromosomes become interchanged. Solid tumours generally have higher chromosome numbers with many structural abnormalities.

Although there have been several cytogenetic studies of colorectal tumours, consistent or specific chromosome aberrations have not yet been detected. However, chromo- somes 7 and 8 are often trisomic (i.e. there is a gain of one or both of these chromosomes’), and chromosome 12 is often rearranged3. The significance of these aberrations is not known. Representative karyotypes of two colorectal tumour cell lines, LS174T4 and H T 5 9 are shown in Figures I and 2 .

References 1. Hill MJ. In: Sherlock P, Morson BC, Barbara L, Veronesi U,

eds. Precancerous Lesions of the Gastrointestinal Tract. New York: Raven Press, 1983: 1-22. Morson BC. The polyp-cancer sequence in the large bowel. Proc R SOC Med 1974; 67: 451-7. Konishi F, Morson BC. Pathology of colorectal adenomas: a colonoscopic survey. J Clin Parhol 1982; 35: 830-41. Morson BC, Bussey HJR, Day DW, Hill MJ. Adenomas of the large bowel. Cancer Surveys 1983; 2: 451-77. Hill MJ, Morson BC, Thompson MH. The role of faecal bile acids in large bowel carcinogenesis. Er J Cancer 1983; 48: 143. Owen RW, Dodo M, Thompson MH, Hill MJ. The faecal ratio of lithocholic acid to deoxycholic acid may be an important aetiological factor in colorectal cancer. Eiochern SOC Trans 1984; 12: 861. Gregor 0, Toman R, Prusova F. Gastrointestinal cancer and nutrition. Gut 1968; 10: 1031-4.

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Studies of consistent chromosome aberrations in tumours have recently converged with advances in gene mapping and recombinant DNA technology. These have enabled the identification of a set of vertebrate cellular genes, called cellular oncogenes, which have been transduced into the genomes of acutely transforming retroviruses where they are responsible for the oncogenic properties of the viruses. Evidence is accumulating that several cellular oncogenes participate in the formation of naturally occurring tumours of non-viral aetiology when activated by quantitative or qualitative alterations’. Several mechanisms for activating cellular oncogenes have been demonstrated in tumours, including chromosome translocations, point mutations, and gene amplification.

Approximately 90 per cent of Burkitt’s lymphoma have a translocation between chromosomes 8 and 14, with the breakpoint in chromosome 8 at band 8q24 where the oncogene c-myc is normally located. Two variant transloca- tions are present in the remaining 10 per cent of Burkitt’s lymphomas, between chromosome 8 at band 8q24 and chromosome 2 or 22. Expression of the oncogene c-myc is enhanced in all three translocations, as a result of its juxtaposition with the immunoglobulin heavy chain genes (IgH) on chromosome 14, or with the K or h light chain genes on chromosomes 2 and 22, respectively’. It is of particular interest that the immunoglobulin genes function specifically in B cells, and that they are directly involved in chromosome translocations in Burkitt’s lymphoma which is a B cell malignancy. It will be important to determine whether tissue-specific genes are involved in the genesis of other tumours.

Approximately 15 per cent of solid tumours have activated cellular oncogenes of the ras gene family which are detected by their ability to transform the pre-neoplastic mouse cell line NIH 3T3 in DNA transfection studies. These are N-ras on chromosome 1, c-Ha-ras on chromosome 11 and c-Ki-ras on chromosome 12. Sequence analyses comparing the normal and activated forms of the c-ras genes have demonstrated amino acid substitutions at position 12 or 61 as a result of single point mutations6. The ras genes encode nucleotide- binding proteins of approximately 21 kd. It is not clear how these mutations alter the functions of the ras genes.

Studies on inherited tumours Knudson in 1971 proposed that dominantly-inherited tumours arise from two mutational events, the first of which is

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Carcinogenesis

Figure 1 trisomy I , 13 or I5

G-banded karyotype of colonic adenocarcinoma cell line LSI 74T. Trisomy 7 is present in every cell. Some cells also have

Figure 2 G-banded karyotype of rectal adenocarcinoma cell line HT55. Not the large number of marker (abnormal) chromosomes

inherited through the germ line and is present in all cells of the individual. The second mutation occurs at the relevant allele on the homologous chromosome thus rendering the cell homozygous for that mutation. Sporadic forms of these tumours arise from somatic mutations in both alleles. Thus, although the genes for susceptibility to these tumours are inherited in an autosomal dominant fashion, expression of the tumour phenotype in both sporadic and inherited forms is recessive, requiring both copies of the gene to be mutated.

Knudson’s proposals have recently received overwhelming support from studies of the childhood malignancies Wilms’

tumour and retinoblastoma, both of which occur in both sporadic and dominantly inherited forms. The observation that several affected individuals with Wilms’ tumour or retinoblastoma had germ-line deletions of chromosome 11 or 13, respectively, focused attention on these regions of the genome. Molecular studies have now shown that where normal tissue from affected individuals (sporadic or inherited forms) is heterozygous at these sites, tumour tissue is either hemi- or homozygous7. It is important to note that the same genes, on chromosome 11 for Wilms’ tumour or chromosome 13 for retinoblastoma, appear to be mutated in both sporadic

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Carcinogenesis

In this brief review I have described some of the genetic changes in cancer cells that are believed to contribute to tumorigenesis. The full implications of these findings will become apparent when we have identified the genes involved in these alterations, and can determine their functions in normal and cancer cells.

and inherited forms of these diseases. Although the relevant genes themselves have not been identified, genetic mechan- isms allowing their expression have been defined using knowledge of the chromosomal locations of the genes.

Applications to colorectal cancer Studies described above provide a framework for considering genetic mechanisms operating in colorectal cancer. A small percentage of patients with colorectal cancer have inherited an autosomal dominant gene conferring susceptibility to cancer’. The major genetic diseases which fall into this category are adenomatous polyposis; Gardner’s syndrome and Cancer Family Syndrome where affected individuals develop cancer at multiple sites, frequently the colon without multiple adenomas, and the endometrium.

Knowledge of the chromosomal locations of the genes for these diseases would enable early identification of affected individuals. Family studies are being carried out to determine linkage of DNA probes to susceptibility for these diseases (Bodmer et d9). If the same genetic mechanism operates in these diseases as in Wilms’ tumour and retinoblastoma, the identification of consistent chromosome rearrangements in colorectal tumours might pinpoint the locations of these genes. As stated above, few consistent chromosome aberra- tions have been recognized so far in colorectal tumours. This may be due to technical difficulties in obtaining chromosome preparations of sufficiently high quality, and also to the finding of multiple structural chromosome aberrations in many colorectal tumours. Adenomatous polyposis may prove to be highly informative in this respect since analysis of material from different stages of the adenoma-carcinoma sequence may enable early chromosome rearrangements to be determined. Very few cytogenetic studies have been done on primary adenomas from adenomatous polyposis, mainly because of the low mitotic index. However, it might be significant that two colon carcinoma cell lines derived from patients with adenomatous polyposis have rearrangements involving chromosome 17102’ .

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Rowley JD. Consistent chromosomal aberrations and oncogenes in human tumours. Cancer Surveys 1984; 3:

Reichman A, Martin P, Levin B. Chromosomal banding patterns in human large bowel cancer. Znt J Cancer 1981; 28:

Becher R, Gibas Z , Sandberg AA. Involvement of chromo- somes 7 and 12 in large bowel cancer: trisomy 7 and 12q-. Cancer Genet Cytogenet 1983; 9: 329-32. Tom BH, Rutzky LP, Jakstys MM, Oyasu R, Kaye CI, Kahan BD. Human colonic adenocarcinoma cells. I. Establishment and description of a new line. In Vitro 1976; 12: 180-91. Watkins JF, Sanger C. Properties of a cell line from human adenocarcinoma of the rectum. Br J Cancer 1977; 35: 785-94. Capon DJ, Seeburg PH, McGrath JP et al. Activation of Ki-ras2 gene in human colon and lung carcinomas by two different point mutations. Nature 1983; 304: 507-13. Knudson AG. Hereditary cancer, oncogenes and anti- oncogenes. Canc Res 1985; 45: 1437-43. Harnden D, Morten J, Featherstone T. Dominant susceptibil- ity to cancer in man. Adv Canc Res 1984; 41: 185-255. Bodmer J, Kennedy K, Brennan J, Bodmer W. HLA and genetic marker studies in adenomatous polyposis. Br J Surg

Paraskeva C, Buckle B, Sheer D, Wigley CB. The isolation and characterization of colorectal epithelial cell lines at different stages in malignant transformation from familial polyposis coli patients. Znt J Cancer 1984; 34: 49-56. Namba M, Miyamoto K, Hyodoh F et al. Establishment and characterization of a human colon carcinoma cell line (KMS-4) from a patient with hereditary adenomatosis of the colon and rectum. Int J Caner 1983; 32: 697-702.

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