In order to determine the extent and variation of allelic loss in colorectal cancer, Vogelstein et a l (1989) studied polymorphic DNA markers from every non- acrocentric autosomal arm in 56 paired colorectal carcinoma and normal mucosa specimens. Loss of heterozygosity (LOH) was found to be a common event, being detected with each marker tested. However of most significance was the finding that alleles from chromosomal arms 17p and 18q were lost in more than 75% of tumours (Vogelstein et a l, 1989). Other studies also reported a high frequency of LOH on chromosome 17p and 18q in colorectal carcinomas from both sporadic colorectal cancer patients and patients with the dominantly inherited colorectal cancer predisposition syndrome, familial adenomatous polyposis (FAP) (Law et al, 1988; Vogelstein et a l, 1988; Sasaki et a l, 1989; Fey et a\., 1989). In addition to studying colorectal carcinomas, Vogelstein et a l (1988) examined colorectal adenomas representing different stages of neoplastic development, from sporadic and FAP patients. Three classes of adenomas were described with size, grade of dysplasia, and villous component generally increasing from Class I to Class III. Class I adenomas were generally small tubular adenomas with low grade dysplasia. Class II adenomas consisted of lesions without associated foci of carcinoma, and Class III adenomas were more advanced adenomas that had given rise to areas of invasive
adenocarcinoma. On chromosome 18q, allele loss was detected frequently in carcinomas and in Class III adenomas but infrequently in the earlier stage Class I and II adenomas. On chromosome 17p, LOH was observed frequently in carcinomas but infrequently in all 3 classes of adenomas (Vogelstein e t a l , 1988).
Allelic loss on chromosome 5q also appeared to be significant in FAP and sporadic colorectal cancers. The first clue as to the importance of this chromosome came from cytogenetic analysis of an individual with both FAP and mental retardation which revealed a constitutional deletion in the long arm of chromosome 5 (del (5) (q l3 q l5 ) or (ql5q22) (Herrera e t a l , 1986). Following this observation linkage analyses in FAP families demonstrated tight linkage of the disease to polymorphic DNA markers on chromosome 5q21 (Bodmer et a l , 1987; Leppert et a\., 1987). Thus a chromosomal location for the gene responsible for FAP was suggested. Loss of heterozygosity studies were carried out using polymorphic markers from this region in FAP and sporadic colorectal tumours. In accordance with the 'recessive' hypothesis for tumour suppressor genes, in individuals with a germline mutation in the FAP gene there would be a requirement for inactivation of the remaining wild-type allele (for example by loss or intragenic mutation) at the predisposition locus for tumour formation, while in sporadic colorectal cancer cases both mutational events would occur somatically (Knudson 1971; 1985). Allelic loss of chromosome 5q was reported in 20-50% of colorectal carcinomas from patients with sporadic colorectal cancer and FAP (Solomon et a l, 1987; Vogelstein et al,
1988; Sasaki et a l, 1989; Vogelstein e t a l , 1989; Okomoio et al., 1990). Vogelstein
et al. (1988) also detected LOH in about 30% of sporadic colorectal adenomas, even Class II adenomas less than 1cm in size. However in adenomas from patients with FAP, allelic losses of chromosome 5q were rarely observed (Solomon et a l, 1987; Vogelstein et a l, 1988; Rees et a l, 1989). This finding contrasted with the pattern of allelic loss seen in other inherited tumour predisposition syndromes such as retinoblastoma(Cavenee e t a l , 1983; 1985).
Allelic losses on many other chromosomes have been observed in colorectal tumours. A high rate of loss on chromosome Ip, particularly in the region lp36, has been reported in sporadic colorectal carcinomas (42-84%) (Leister et a l, 1990; Praml
et al., 1995a) and sporadic colorectal adenomas (20%) (Lothe e t a l , 1995) leading to the suggestion that inactivation of a tumour suppressor gene in this chromosomal region is an early event in the adenoma-carcinoma sequence (Bardi et a l , 1993; Lothe et a l, 1995). LOH on chromosome 8p has been detected in approximately one-half of colorectal carcinomas (Emi e t a l , 1992; Fujiwara et al., 1993; Gustafson
suppressor genes on chromosome 8p for colorectal carcinoma (Fujiwara etal., 1993). The association of chromosome 8p LOH with advanced stage colorectal carcinomas (Fujiwara et al., 1993) and the low rate of 8p LOH in colorectal adenomas (Cunningham et a l, 1994; Gustafson et a l, 1996) suggested that inactivation of the putative chromosome 8p tumour suppressor genes is important in the later stages of colorectal tumourigenesis. Other chromosomes showing a high frequency of allele loss in colorectal cancer include chromosome 14 (30-53% colorectal carinomas) (Sasaki et a l, 1989; Young et a l, 1993a) and chromosome 22 (19-40% colorectal carcinomas) (Vogelstein et al., 1989; Sasaki et a l, 1989; Yana et al., 1995). LOH on these two chromosomes was associated with advanced Dukes' stages (Dukes' C l and C2) (Young et a l, 1993; Yana e t a l , 1995).
In their genetic model of colorectal tumourigenesis, Fearon and Vogelstein (1990) proposed that loss or inactivation of the familial adenomatous polyposis gene on chromosome 5q may be the initiating event, leading to the formation of precancerous adenomatous polyps. Allelic deletions on chromosomes 18q and 17p were proposed to respectively occur at later stages of tumourigenesis, loss of 17p being associated with the progression from advanced adenoma to carcinoma. Loss of tumour suppressor genes on other chromosomes also occurs and the accumulated loss correlates with the ability of the carcinomas to metastasize and cause death (Fearon and Vogelstein, 1990). Genes on chromosomes 5, 17, and 18 involved in colorectal cancer development have been identified and will be described in the following sections.