Chapter 079. Cancer Genetics (Part 4)

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Familial adenomatous polyposis (FAP) is a dominantly inherited colon cancer syndrome due to germline mutations in the adenomatous polyposis coli (APC) tumor-suppressor gene on chromosome 5. Patients with this syndrome develop hundreds to thousands of adenomas in the colon. Each of these adenomas has lost the normal remaining allele of APC but has not yet accumulated the required additional mutations to generate fully malignant cells (Fig. 79-2). However, out of these thousands of benign adenomas, several will invariably acquire further abnormalities and a subset will even develop into fully malignant cancers. APC is thus considered to be a gatekeeper...

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Nội dung Text: Chapter 079. Cancer Genetics (Part 4)

Chapter 079. Cancer Genetics (Part 4) Familial adenomatous polyposis (FAP) is a dominantly inherited colon cancer syndrome due to germline mutations in the adenomatous polyposis coli (APC) tumor-suppressor gene on chromosome 5. Patients with this syndrome develop hundreds to thousands of adenomas in the colon. Each of these adenomas has lost the normal remaining allele of APC but has not yet accumulated the required additional mutations to generate fully malignant cells (Fig. 79-2). However, out of these thousands of benign adenomas, several will invariably acquire further abnormalities and a subset will even develop into fully malignant cancers. APC is thus considered to be a gatekeeper for colon tumorigenesis; Fig. 79-4 shows germline and somatic mutations found in the APC gene. The function of the APC protein is still not completely understood but likely provides differentiation and apoptotic cues to colonic cells as they migrate up the crypts.
Defects in this process may lead to abnormal accumulation of cells that should normally undergo apoptosis and slough off. Figure 79-4 Germline and somatic mutations in the tumor-suppressor gene APC. APC encodes a 2843-amino-acid protein with 6 major domains: an oligomerization region (O), armadillo repeats (ARM), 15-amino-acid repeats (15 AA), 20-amino-acid repeats (20 AA), a basic region, and a domain involved in binding EB1 and the Drosophila discs large homologue (E/D). Shown are the positions within the APC gene of a total of 650 somatic and 826 germline mutations (from the APC database at http://p53.free.fr). The vast majority of these
mutations result in the truncation of the APC protein. Germline mutations are found to be relatively evenly distributed up to codon 1600 except for 2 mutation hotspots at amino acids 1061 and 1309, which together account for one-third of the mutations found in familial adenomatous polyposis (FAP) families. Somatic APC mutations in colon tumors cluster in an area of the gene known as the mutation cluster region (MCR). The location of the MCR suggests that the 20- amino-acid domain plays a crucial role in tumor suppression. Note that loss of the second functional APC allele in tumors from FAP families often occurs through loss of heterozygosity. In contrast to FAP, patients with hereditary nonpolyposis colon cancer (HNPCC, or Lynch syndrome) do not develop multiple polyposis but instead develop only one or a small number of adenomas that rapidly progress to cancer. HNPCC is commonly defined by family history, with at least three individuals over at least two generations developing colon or endometrial cancer, and with at least one individual diagnosed before the age of 50. Most HNPCC is due to mutations in one of four DNA mismatch repair genes (Table 79-1), which are components of a repair system that is normally responsible for correcting errors in freshly replicated DNA. Germline mutations in MSH2 and MLH1 account for >60% of HNPCC cases, while mutations in MSH6 and PMS2 are much less frequent. When a somatic mutation inactivates the remaining wild-type allele of a mismatch repair gene, the cell develops a hypermutable phenotype characterized
by profound genomic instability, especially for the short repeated sequences called microsatellites. This microsatellite instability (MIN) favors the development of cancer by increasing the rate of mutations in many genes, including oncogenes and tumor-suppressor genes (Fig. 79-2). These genes can thus be considered caretakers. Figure 79-5 shows an example of the instability in allele sizes for dinucleotide repeats in the cancers of HNPCC patients. Figure 79-5 Demonstration of microsatellite instability in normal and tumor tissue from hereditary nonpolyposis colon cancer (HNPCC) patients. In each case the lane marked T contains DNA from a tumor, and the lane marked N contains DNA from normal tissue of the same patient. The marker (D2S123, located on chromosome 2) is a microsatellite composed of a tandem repeat of the dinucleotide CA, which varies in length from chromosome to chromosome.
Normally, however, the length of the repeat is stable in somatic tissues. In this example, a polymerase chain reaction analysis has been applied to genomic DNA, and new alleles for the marker are apparent in tumors 1, 2, 5, and 7. Because the tumor tissue is defective in DNA mismatch repair, clonal abnormalities in copying of the CA repeat have arisen. Errors are also occurring in functional genes, eventually resulting in the malignant phenotype. (From LA Aaltonen et al, Clues to the pathogenesis of familial colorectal cancer. Science 260:812, 1993, with permission; Copyright 1993 AAAS.)