APC expression in normal human tissues

The tumour suppressor gene APC codes for a 2843-amino acid protein whose precise functions are still poorly understood. This paper describes the development of two new antisera to APC (to amino- and carboxy-terminal epitopes) which permit localization of the protein by immunohistochemistry in archival paraffin sections. The protein is expressed in a wide variety of normal epithelial tissues. Its distribution frequently coincides with the location of post-replicative cells within tissues. Staining patterns demonstrate that the APC protein, although often diffusely cytoplasmic in distribution, may also accumulate in the apical and immediately subapical regions, or along the lateral margins of certain cells. These results indicate that APC is significant in many tissues in addition to the colorectal epithelium. They are compatible with a function related to signalling at the adherens junction and possibly with other more complex roles in cells committed to terminal differentiation.

Absence of p53 permits propagation of mutant cells following genotoxic damage

Much evidence has been gathered in support of a critical role for p53 in the cellular response to DNA damage. p53 dysfunction is associated with progression and poor prognosis of many human cancers and with a high incidence of tumours in p53 knockout mice. The absence of a p53-dependent G1 arrest that facilitates DNA repair or apoptosis might impact critically on clinical cancer in two ways. First, by abrogating the impact on therapy that operates via genotoxic damage and apoptosis; and second, by encouraging progression either by inducing genomic instability and DNA mis-repair or by permitting survival of mutants. However, experiments examining the relationship between p53 deficiency and mutation frequency have so far failed to confirm these predictions. The precise role played by p53 is therefore unclear. We now report use of a short term in vitro approach to assess the influence of p53 on radiation-induced mutations at the hprt locus in murine B cell precursors that are normally radiation ultrasensitive. We find a high number of hprt mutants among X-irradiated p53 null cells, which results from preferential survival as clonogenic mutants rather than from a p53-dependent increase in mutation rate. This result has important implications for genotoxic cancer therapy.

Colorectal carcinoma in Hong Kong: epidemiology and genetic mutations

The incidence of colorectal carcinoma is rising at an alarming pace in Asian urban societies such as Hong Kong. Detailed examination of the epidemiological pattern and genetic mutation of colorectal cancer in the Hong Kong Chinese population is overdue. We compared the reported age incidence of colorectal carcinoma in Hong Kong with that of Scotland and other countries. Hong Kong showed a much higher incidence of colorectal carcinoma among the young age groups. By comparison with other countries, this raised incidence among the young appeared to be related to southern Chinese societies. The recent dramatic rise in colorectal cancer in Hong Kong was largely attributable to an increase in the over 50 years age group, while the young incidence remained unchanged. We also defined the mutation spectrum of p53 and Ki-ras in 67 unselected cases by direct DNA sequencing. Interestingly, insertion/deletion mutations in p53 from colorectal carcinoma in Hong Kong showed a significantly higher frequency (17.2%) than the Scottish data (0%) and the world database (6.6%), although the overall frequency of p53 mutation (43%) in Hong Kong was similar to others. The high incidence of colorectal carcinoma in young people and the raised proportion of frameshift mutations in p53 encourage further search for a genetic basis for susceptibility to this disease in the Hong Kong Chinese population.

Cancer risk associated with germline DNA mismatch repair gene mutations

The autosomal dominant syndrome of Hereditary Nonpolyposis Colorectal Cancer (HNPCC) is due to germline DNA mismatch repair gene mutations in most cases. However, the penetrance of such mutations outwith classical HNPCC kindreds is unknown because families studied to date have been specifically selected for research purposes. Using a population-based strategy, we have calculated the lifetime cancer risk associated with germline DNA mismatch repair gene mutations, irrespective of their family history. We identified 67 gene carriers whose risk to age 70 for all cancers was 91% for males and 69% for females. The risk of developing colorectal cancer was significantly greater for males than for females (74% versus 30%, P= 0.006). The risk of uterine cancer (42%) exceeded that for colorectal cancer in females, emphasising the need for uterine screening. Our findings give further insight into the biological effect of defective DNA mismatch repair. We have demonstrated a systematic approach to identifying individuals at high risk of cancer but who may not be part of classical HNPCC families. The risk estimates derived from these analyses provide a rational basis on which to guide genetic counselling and to tailor clinical surveillance.

Germline APC mutation (Gln1317) in a cancer-prone family that does not result in familial adenomatous polyposis

Germline mutations of the adenomatous polyposis coli gene are associated with the dominantly inherited syndrome of familial adenomatous polyposis. Somatic mutations in this gene are an early event in sporadic colorectal tumorigenesis. Here we report a family with genetic characteristics that do not conform exactly to either of these situations. The index case and three siblings presented with colorectal cancer, and another sibling had lung cancer. There was no evidence of colorectal cancer susceptibility in previous generations, although one case of gastric cancer was observed. Using restriction fragment length polymorphism, single-strand conformational polymorphism, and sequencing analysis, we screened each living family member for alterations in the mutation cluster region of exon 15 of the APC gene. A constitutional single base pair substitution at codon 1317 was observed in two of the siblings with colorectal cancer, but neither exhibited any colonic features typical of FAP nor an early onset of cancer. This constitutional change is a missense mutation and therefore does not result in the truncation of the APC protein, the most commonly observed result of mutation in this gene. We present evidence that this change is not a polymorphism and may be capable of conferring a growth advantage. This particular germline APC mutation does not completely cosegregate with cancer in this family; therefore, we conclude that another gene locus may be responsible for the increased cancer risk observed.

Ki-ras mutations in adenomas: a characteristic of cancer-bearing colorectal mucosa

Activating mutations in the Ki-ras2 oncogene are frequently observed in sporadic colorectal adenomas and their incidence is reported to rise in large and tubulovillous adenomas to values close to those in carcinomas. This study shows that this property is a feature of adenomas growing in large bowel that has already demonstrated its propensity to engender malignant tumours: i.e., bowel in which there is a synchronous carcinoma. Adenomas from cancer-free bowel do not share this high incidence of Ki-ras mutations. This difference in mutation incidence between adenomas from cancer-free and cancer-bearing patients does not appear to derive from sampling bias relative to adenoma size, site, or patient age, nor is it found in another gene (APC) known to be of importance in adenoma formation. Large, dysplastic adenomas from cancer-bearing bowel, however, are particularly liable to carry Ki-ras mutations when they arise in patients over 70 years old. The observations suggest that the role of Ki-ras mutations may be more subtle than merely enhancing adenoma growth. Adenoma cells of cancer-prone individuals may suffer more mutational events than those in persons selected as cancer-free.

Extracolonic features of familial adenomatous polyposis in patients with sporadic colorectal cancer

We have investigated the occurrence of attenuated extracolonic manifestations (AEMs) of familial adenomatous polyposis (FAP) in patients with non-polyposis colorectal cancer. In a prospective case-control study, we observed that significantly more colorectal cancer patients exhibited AEM than did age and sex-matched controls (19.5% vs 7.5%, P < 0.004). However patients with AEMs do not have occult FAP, as we found no heterozygous adenomatous polyposis coli (APC) gene mutations despite extensive analysis of constitutional DNA. Genome-wide DNA replication errors (RERs) occur in a proportion of colorectal cancers, particularly right-sided lesions and in almost all tumours from hereditary non-polyposis colorectal cancer (HNPCC) patients. As AEMs have been reported in familial colon cancer cases, we investigated the relationship of AEMs to tumour RER phenotype. There was indeed an excess of AEMs in patients with right-sided tumours (30.2% of 53 patients vs 14.7% of 116 patients, P < 0.03) and in those with RER tumours (3 out of 12 patients with RER tumours vs none out of 21 patients with non-RER tumours, P < 0.05). Two patients with AEM were from HNPCC families compared with none of those without AEM (P < 0.05). The association of AEMs with colorectal cancer is intriguing, and we speculate that it may be a manifestation of mutational mosaicism of the APC gene, perhaps associated with a constitutional defect in DNA mismatch pair.

Loss of heterozygosity at 5q21 in non-small cell lung cancer: a frequent event but without evidence of apc mutation

Four genetic polymorphisms in the APC and MCC genes at chromosome 5q21 were analysed for loss of heterozygosity (LOH) in 97 primary squamous carcinomas and adenocarcinomas of the lung. LOH was identified in at least two polymorphic loci in 41 percent of informative cases. There was no significant difference in the frequency of LOH between squamous carcinomas and adenocarcinomas. Within the adenocarcinoma group, however, LOH appeared to be more common in tumours having a bronchial origin (5/9; 56 per cent) than in parenchymal adenocarcinoma (6/21; 29 per cent). All 32 tumours showing LOH at one or more polymorphic sites were examined for mutations in the mutation cluster region (MCR) of APC by single-strand conformational polymorphism (SSCP) analysis. Mutations were not detected in any of these cases. We therefore propose that it is likely that a tumour suppressor gene on 5q other than APC is involved in the pathogenesis of lung cancer.

APC mutations in colorectal tumors with mismatch repair deficiency

We have investigated the influence of genetic instability [replication error (RER) phenotype] on APC (adenomatous polyposis coli), a gene thought to initiate colorectal tumorigenesis. The prevalence of APC mutations was similar in RER and non-RER tumors, indicating that both tumor types share this step in neoplastic transformation. However, in a total of 101 sequenced mutations, we noted a substantial excess of APC frameshift mutations in the RER cases (70% in RER tumors versus 47% in non-RER tumors, P < 0.04). These frameshifts were characteristic of mutations arising in cells deficient in DNA mismatch repair, with a predilection for mononucleotide repeats in the RER tumors (P < 0.0002), particularly (A)n tracts (P < 0.00007). These findings suggest that the genetic instability that is reflected by the RER phenotype precedes, and is responsible for, APC mutation in RER large bowel tumors and have important implications for understanding the very earliest stages of neoplasia in patients with tumors deficient in mismatch repair.

Microsatellite instability and the role of hMSH2 in sporadic colorectalcancer

Microsatellite instability (MSI) occurs in most tumours from patients with hereditary non-polyposis colorectal cancer (HNPCC) and in around 17% of sporadic colorectal cancers. Germline defects in mismatch repair (MMR) genes are responsible for the majority of large HNPCC families, with hMSH2 accounting for at least 50%. MMR gene defects also occur in a small proportion of sporadic colorectal tumours with MSI. Here we report a systematic analysis of mismatch repair deficiency in 215 Scottish patients with sporadic colorectal tumours. We found that 16.4% of tumours exhibited MSI; survival analysis by Cox proportional hazards method showed a substantial survival advantage for patients with tumours showing MSI, independent of other prognostic factors. Tumours with MSI were screened for hMSH2 mutations and although 61% were found to have alterations, of these only 1/24 was exonic. The majority of these changes were reductions in length at intronic mononucleotide tracts and we postulate that these alterations are the result of a genetic defect elsewhere, although they may compromise hMSH2 function as a second step in tumourigenesis. Our findings indicate that instability confers an improved prognosis in colorectal cancer and, despite the fact that these two groups of tumours share similar biological characteristics, the genetic basis of HNPCC and sporadic colorectal cancer with MSI is different.

Detailed physical and deletion mapping of 8p with isolation of YAC clones from tumour suppressor loci involved in colorectal cancer

Loss of heterozygosity (LOH) of markers at chromosome 8p is frequently noted in many different tumour types, including colorectal cancer. Numerous investigations indicate the presence of more than tumour suppressor gene (TSG) located on 8p. In this study, we describe a detailed LOH map in colorectal cancer and relate this to physical mapping data from reduced radiation 8p hybrids, yeast artificial chromosome (YAC) co-localisation of markers and fluorescence in situ hybridisation data. These data indicate the presence of two regions harbouring putative TSG's between the polymorphic markers for the LPL gene-D8S298 (approximately 4 Mb) and the markers D8S136-D8S137 (approximately 8 Mb). Yeast Artificial Chromosomes (YAC) have been isolated from these regions of interest to aid the localisation of the putative TSG's.

Germline APC mutation (Gln1317) in a cancer-prone family that does not result in familial adenomatous polyposis

Germline mutations of the adenomatous polyposis coli gene are associated with the dominantly inherited syndrome of familial adenomatous polyposis. Somatic mutations in this gene are an early event in sporadic colorectal tumorigenesis. Here we report a family with genetic characteristics that do not conform exactly to either of these situations. The index case and three siblings presented with colorectal cancer, and another sibling had lung cancer. There was no evidence of colorectal cancer susceptibility in previous generations, although one case of gastric cancer was observed. Using restriction fragment length polymorphism, single-strand conformational polymorphism, and sequencing analysis, we screened each living family member for alterations in the mutation cluster region of exon 15 of the APC gene. A constitutional single base pair substitution at codon 1317 was observed in two of the siblings with colorectal cancer, but neither exhibited any colonic features typical of FAP nor an early onset of cancer. This constitutional change is a missense mutation and therefore does not result in the truncation of the APC protein, the most commonly observed result of mutation in this gene. We present evidence that this change is not a polymorphism and may be capable of conferring a growth advantage. This particular germline APC mutation does not completely cosegregate with cancer in this family; therefore, we conclude that another gene locus may be responsible for the increased cancer risk observed.

Expression of the human mismatch repair gene hMSH2 in normal and neoplastic tissues

Hereditary nonpolyposis colorectal cancer is caused by inherited mutations of mismatch repair genes. We developed monoclonal antibodies to the prototype human mismatch repair gene hMSH2 and used them to detect an immunoreactive protein of M(r) 100,000 in mismatch-proficient cell lines. In addition, a M(r) 150,000 protein coimmunoprecipitated with the hMSH2 gene product in cell lines expressing hMSH2. Immunohistochemistry demonstrated that the hMSH2 protein was exclusively nuclear. Whereas the hMSH2 protein was expressed in a variety of tissues, the most striking pattern was observed in esophageal and intestinal epithelia, where expression was limited to the replicating compartment. Neoplastic cells within benign and malignant mismatch repair-proficient tumors expressed the protein, but no hMSH2 immunoreactivity was observed in the colorectal tumors of patients with germline hMSH2 mutation. These results have implications for tumorigenic mechanisms and, potentially, for diagnosis.

p53-independent death and p53-induced protection against apoptosis in fibroblasts treated with chemotherapeutic drugs

Many recent studies have implicated p53 in the cellular response to injury and induction of cell death by apoptosis. In a rat embryonal fibroblast cell line transformed with c-Ha-ras and a mutant temperature-sensitive p53 (val135), cells were G1 arrested at the permissive temperature of 32 degrees C when overexpressed p53 was in wild-type conformation. In this state cells were resistant to apoptosis induced by etoposide (at up to 50 microM) or bleomycin (15 microU ml-1). Cells at 37 degrees C with overexpressed p53 in mutant conformation were freed from this growth arrest, continued proliferating and showed dose-dependent increases in apoptosis. This death is independent of wild-type p53 function. Control cells containing a non-temperature-sensitive mutant p53 (phe132) were sensitive to both etoposide and bleomycin after 24 h at 32 degrees C and 37 degrees C, indicating that the results are not simply due to temperature effects on pharmacokinetics or DNA damage. Our data show that induction of a stable p53-mediated growth arrest renders these cells much less likely to undergo apoptosis in response to certain anti-cancer drugs, and we conclude that the regulatory role of p53 in apoptosis is influenced by the particular cellular context in which this gene is expressed.

Human papillomavirus type 18 is associated with less apoptosis in fibroblast tumours than human papillomavirus type 16

In human cervical neoplasia human papillomavirus (HPV) type 18 has a higher cancer/cervical intraepithelial neoplasia (CIN) prevalence ratio than HPV 16. Fibrosarcomas derived from rat fibroblasts transfected with HPV 16 or 18 genomes showed increased apoptosis compared with controls. However, HPV 18 was associated with significantly less apoptosis than HPV 16, affording one possible explanation for the more rapidly progressive cervical neoplasia associated with HPV 18.

Genetic instability occurs in the majority of young patients with colorectal cancer

Replication errors (RER) associated with genetic instability have been found in cancers of several different types and particularly in the tumours of patients with hereditary non-polyposis colorectal cancer (HNPCC). We have here determined the prevalence of such instability in relation to age among patients without HNPCC. Colorectal cancers (CRCs) in the majority of patients 35 years of age or younger exhibited instability (58% of 31 patients), whereas CRCs from patients older than 35 uncommonly did (12% of 158, p < 0.0001). Twelve of the patients under 35 with instability were evaluated for alterations of mismatch repair genes, and five were found to harbour germline mutations. These data suggest that the mechanisms underlying tumour development in young CRC patients differ from those in most older patients, regardless of HNPCC status. The results have important implications for genetic testing and management of young CRC patients and their families.

17q allele loss is associated with lymph node metastasis in locally aggressive human colorectal cancer

A consecutive series of 87 colorectal tumours were studied for loss of a polymorphic probe on chromosome 17q and of the 64 informative cases, 13 (20 per cent) showed loss of heterozygosity (LOH). Examples of LOH were found in carcinomas of all stages and in a large non-invasive adenoma. There was no correlation between 17q LOH and patient age, sex, standard clinicopathological variables (differentiation and nature of tumour margin), DNA ploidy, or tumour site, nor was 17q LOH associated with 17p LOH defined at four loci adjacent to p53. However, comparison of Dukes' B and C carcinomas revealed that tumours which had metastasized to regional lymph nodes at the time of primary surgery were significantly more likely to have lost this 17q allele. Clinical follow-up of this cohort of patients showed no significant difference in survival between patients whose tumours had lost or retained 17q. Thus, we conclude that 17q allele loss is associated with lymph node metastasis in locally aggressive colorectal tumours but probably not with blood-borne metastasis.

Mutation of the p53 gene precedes aneuploid clonal divergence in colorectal carcinoma

To establish whether p53 mutation precedes or follows clonal divergence in human colorectal carcinomas, 17 tumours were analysed at multiple sites (2-5 each) for single-strand conformation polymorphisms (SSCP) within exons 5-8 of the p53 gene. A previous study had demonstrated subclones of differing DNA ploidy in these tumours, but all showed immunocytochemical evidence for p53 stabilisation, using the monoclonal antibody PAb 1801. Mutations within exons 5-8 of p53 were identified by the presence of an abnormally migrating band in 10 of the 17 carcinomas: five in exon 5, four in exon 7 and one in exon 8. In each of these positive cases, samples from different parts of the carcinoma showed identical gel migration patterns in SSCP analysis. Similarly, the remaining seven tumours were concordant for absence of band shift across all samples of each tumour. Six SSCP-positive cases contained multiple populations differing in DNA ploidy, while four were homogeneously diploid or aneuploid throughout. Very similar proportions were observed in the SSCP-negative cases. In four positive tumours the mutation was confirmed by sequencing or through alteration of nucleotide-specific restriction enzyme cleavage. Identical mutations appeared in every sample from the same tumour. The results provide unequivocal evidence that the same mutant allele of p53 is present throughout each tumour bearing a mutation, regardless of the clonal variation identified by analysis of DNA ploidy. We conclude that in colorectal tumorigenesis mutation of p53 occurs as a single event which precedes and may facilitate the aneuploid clonal divergence of carcinomas.

The genetic regulation of apoptosis

Dramatic advances, most of them within the past two years, have provided a picture of the genetic regulation of apoptosis in mammalian cells. Although much detail remains to be filled in, the general structure--concordant with programmed death in invertebrates--includes signalling systems, genetic determination of susceptibility, critical proteins capable of reversing or re-affirming the death sentence, and a common effector pathway driven by specific proteases.

Cell death in health and disease: the biology and regulation of apoptosis

Apoptosis is a morphologically stereotyped form of cell death, prevalent in multicellular organisms, by which single cells are deleted from the midst of living tissues. Recognition of the cellular corpses and their removal by phagocytosis occurs without disturbance to tissue architecture or function and without initiating inflammation. Apoptosis is regulable and is of fundamental importance to tissue development and homeostasis. Cellular susceptibility to apoptosis is determined by a variety of signals, of both extracellular and internal origin, including proliferative status. Dysregulated apoptosis is important in the pathogenesis of several important human diseases including neoplasia, and recognition of the defects involved is prompting development of new therapeutic strategies.