Adenomatous Polyposis Coli Allele Research Articles

Biology of the adenomatous polyposis coli tumor suppressor.

The adenomatous polyposis coli (APC) gene was first identified as the gene mutated in an inherited syndrome of colon cancer predisposition known as familial adenomatous polyposis coli (FAP). Mutation of APC is also found in 80% of all colorectal adenomas and carcinomas and is one of the earliest mutations in colon cancer progression. Similar to other tumor suppressor genes, both APC alleles are inactivated by mutation in colon tumors, resulting in the loss of full-length protein in tumor cells. The functional significance of altering APC is the dysregulation of several physiologic processes that govern colonic epithelial cell homeostasis, which include cell cycle progression, migration, differentiation, and apoptosis. Roles for APC in some of these processes are in large part attributable to its ability to regulate cytosolic levels of the signaling molecule beta-catenin and to affect the transcriptional profile in cells. This article summarizes numerous genetic, biochemical, and cell biologic studies on the mechanisms of APC-mediated tumor suppression. Mouse models of FAP, in which the APC gene has been genetically inactivated, have been particularly useful in testing therapeutic and chemopreventive strategies. These data have significant implications for colorectal cancer treatment approaches as well as for understanding other disease genes and cancers of other tissue types.

Molecular biology of colorectal cancer

Colorectal cancer is a significant cause of morbidity and mortality in Western populations. This cancer develops as a result of the pathologic transformation of normal colonic epithelium to an adenomatous polyp and ultimately an invasive cancer. The multistep progression requires years and possibly decades and is accompanied by a number of recently characterized genetic alterations. Mutations in two classes of genes, tumor-suppressor genes and proto-oncogenes, are thought to impart a proliferative advantage to cells and contribute to development of the malignant phenotype. Inactivating mutations of both copies (alleles) of the adenomatous polyposis coli (APC) gene—a tumor-suppressor gene on chromosome 5q—mark one of the earliest events in colorectal carcinogenesis. Germline mutation of the APC gene and subsequent somatic mutation of the second APC allele cause the inherited familial adenomatous polyposis syndrome. This syndrome is characterized by the presence of hundreds to thousands of colonic adenomatous polyps. If these polyps are left untreated, colorectal cancer develops. Mutation leading to dysregulation of the K- ras protooncogene is also thought to be an early event in colon cancer formation. Conversely, loss of heterozygosity on the long arm of chromosome 18 (18q) occurs later in the sequence of development from adenoma to carcinoma, and this mutation may predict poor prognosis. Loss of the 18q region is thought to contribute to inactivation of the DCC tumor-suppressor gene. More recent evidence suggests that other tumor-suppressor genes—DPC4 and MADR2 of the transforming growth factor β (TGF-β) pathway—also may be inactivated by allelic loss on chromosome 18q. In addition, mutation of the tumor-suppressor gene p53 on chromosome 17p appears to be a late phenomenon in colorectal carcinogenesis. This mutation may allow the growing tumor with multiple genetic alterations to evade cell cycle arrest and apoptosis. Neoplastic progression is probably accompanied by additional, undiscovered genetic events, which are indicated by allelic loss on chromosomes 1q, 4p, 6p, 8p, 9q, and 22q in 25% to 50% of colorectal cancers. Recently, a third class of genes, DNA repair genes, has been implicated in tumorigenesis of colorectal cancer. Study findings suggest that DNA mismatch repair deficiency, due to germline mutation of the hMSH2, hMLH1, hPMS1, or hPMS2 genes, contributes to development of hereditary nonpolyposis colorectal cancer. The majority of tumors in patients with this disease and 10% to 15% of sporadic colon cancers display microsatellite instability, also know as the replication error positive (RER+) phenotype. This molecular marker of DNA mismatch repair deficiency may predict improved patient survival. Mismatch repair deficiency is thought to lead to mutation and inactivation of the genes for type II TGF-β receptor and insulin-like growth-factor II receptor: Individuals from families at high risk for colorectal cancer (hereditary nonpolyposis colorectal cancer or familial adenomatous polyposis) should be offered genetic counseling, predictive molecular testing, and when indicated, endoscopic surveillance at appropriate intervals. Recent studies have examined colorectal carcinogenesis in the light of other genetic processes. Telomerase activity is present in almost all cancers, including colorectal cancer, but rarely in benign lesions such as adenomatous polyps or normal tissues. Furthermore, genetic alterations that allow transformed colorectal epithelial cells to escape cell cycle arrest or apoptosis also have been recognized. In addition, hypomethylation or hypermethylation of DNA sequences may alter gene expression without nucleic acid mutation.

Childhood hepatocellular adenoma in familial adenomatous polyposis: Mutations in adenomatous polyposis coli gene and p53

BACKGROUND & AIMS: Infantile and childhood liver tumors have been found in 0.42% of individuals with a germline mutation in the adenomatous polyposis coli (APC) gene. This study analyzed a hepatocellular adenoma of a 2-year-old child at risk for familial adenomatous polyposis to identify genetic alterations in hepatic tumors initiated by APC germline mutations.METHODS: Mutation screening was performed for the APC gene (protein truncation test and DNA sequence analysis), p53 gene (complementary DNA cloning and sequencing), and members of the Ras gene family (complementary DNA sequence analysis).RESULTS: Both the mother and child had a germinal CGA-->TGA transition at codon 1451 leading to an Arg1451Ter stop mutation in the APC gene. Loss of the wild-type APC allele as a second hit revealed hemizygosity of the inherited mutation in the tumor. Furthermore, a CGC-->CAC transition in the p53 gene of the adenoma resulted in an Arg-->His missense mutation in codon 175. No loss of heterozygosity was detected at the p53 locus. Ras gene mutations were not found.CONCLUSIONS: Biallelic inactivation of APC gene and p53 mutation are early events in hepatocellular tumorigenesis. Additional reports will confirm whether inherited APC gene mutations between codon 1444 and 1578 increase the risk for hepatic tumors.(Gastroenterology 1997 Mar;112(3):919-22)

Apoptosis and APC in colorectal tumorigenesis.

Tumors result from disruptions in the homeostatic mechanisms that regulate cell birth and cell death. In colon cancer, one of the earliest manifestation of this imbalance is the formation of polyps, caused by somatic and inherited mutations of the adenomatous polyposis coli (APC) tumor suppressor gene in both humans and mice. While the importance of APC in tumorigenesis is well documented, how it functions to prevent tumors remains a mystery. Using a novel inducible expression system, we show that expression of APC in human colorectal cancer cells containing endogenous inactive APC alleles results in a substantial diminution of cell growth. Further evaluation demonstrated that this was due to the induction of cell death through apoptosis. These results suggest that apoptosis plays a role not only in advanced tumors but also at the very earliest stages of neoplasia.

Loss of heterozygosity involving the APC gene in oral squamous cell carcinomas

The tumor suppressor gene adenomatous polyposis coli has been shown to be altered in colon and esophageal cancers. Because of similar causes of oral and esophageal cancers, we investigated allelic deletion of the adenomatous polyposis coli gene in oral cancers by examining tumor cells of persons normally heterozygous at a polymorphic restriction site in adenomatous polyposis coli. Deoxyribonucleic acid was extracted from 20 formalin-fixed microdissected sections and one fresh specimen of oral squamous cell carcinomas and amplified with the use of the polymerase chain reaction. The amplified deoxyribonucleic acid was digested with Rsa I, subjected to polyacrylamide gel electrophoresis, and examined for loss of heterozygosity in adenomatous polyposis coli alleles. Samples from nine persons were homozygous for the adenomatous polyposis coli restriction site in both tumor and normal tissues and thus were uninformative. Three of the 12 samples from heterozygous persons showed loss of one adenomatous polyposis coli allele in tumor tissues. The loss of an adenomatous polyposis coli gene allele in 25% of the carcinomas examined suggests that inactivation of adenomatous polyposis coli or another neighboring gene on chromosome 5q may be involved in carcinogenesis in the oral cavity.

Mutation in a splice-donor site of the APC gene in a family with polyposis and late age of colonic cancer death

Adenomatous polyposis coli (APC) is an autosomal dominant disease characterized by the development of hundreds of colorectal adenomatous polyps during the first decades of life. The expression of the disease varies, as the age of onset of colonic cancer and the severity of extracolonic manifestations often differ between affected families. An attenuated form of APC has also been described in which a small number of polyps and a later age of onset of colonic cancer is observed. Cloning of the APC gene has allowed disease-causing mutations in APC families to be identified. Here, we report a novel splice site mutation (a G to T transversion at position +5 of the splice donor site in intron 9) in the APC gene of affected individuals in an Italian family. Characterization of the transcription products from this mutant APC allele revealed that normal splicing was disrupted: a shorter mRNA was expressed in which exon 8 was connected directly to exon 10. This created a shift in the reading frame and the introduction of a stop codon at position 1358. In addition, some normal APC transcript was produced from the mutant allele in lymphoblastoid cells. A comparison of the clinical features of affected members of this family with four unrelated Italian APC kindreds, in which the same AAAAG deletion at position 3926 has been found, showed a significant difference in the onset of disease symptoms and in the age of death attributable to colorectal cancer. Inefficient exon skipping may be, at least in part, responsible for the delay in the development of the disease in the reported family.