Chapter 079. Cancer Genetics (Part 7)

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Mechanisms of Oncogene Activation Mechanisms that upregulate (or activate) cellular oncogenes fall into three broad categories: point mutation, DNA amplification, and chromosomal rearrangement. Point Mutation Point mutation is a common mechanism of oncogene activation. For example, mutations in one of the RAS genes (HRAS, KRAS, or NRAS) are present in up to 85% of pancreatic cancers and 50% of colon cancers but are relatively uncommon in other cancer types. Remarkably—and in contrast to the diversity of mutations found in tumor-suppressor genes (Fig. 79-4)—most of the activated RAS genes contain point mutations in codons 12, 13, or 61 (which convey resistance to...

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Chapter 079. Cancer Genetics (Part 7) Mechanisms of Oncogene Activation Mechanisms that upregulate (or activate) cellular oncogenes fall into three broad categories: point mutation, DNA amplification, and chromosomal rearrangement. Point Mutation Point mutation is a common mechanism of oncogene activation. For example, mutations in one of the RAS genes (HRAS, KRAS, or NRAS) are present in up to 85% of pancreatic cancers and 50% of colon cancers but are relatively uncommon in other cancer types. Remarkably—and in contrast to the diversity of mutations found in tumor-suppressor genes (Fig. 79-4)—most of the activated RAS genes contain point mutations in codons 12, 13, or 61 (which convey resistance to GAP, a protein that interacts with RAS and inactivates it through
substitution of the GTP cofactor with GDP). The restricted pattern of mutation compared to tumor-suppressor genes reflects the fact that gain-of-function mutations of oncogenes are more difficult to attain than simple inactivation. Indeed, inactivation of a gene can be attained through the introduction of a stop codon anywhere in the coding sequence, whereas activations require precise substitutions at residues that normally downregulate the activity of the encoded protein. The specificity of oncogene mutations provides specific diagnostic opportunities, as it is much simpler to find mutations at specified positions than it is when mutations can be scattered throughout the gene (as in tumor-suppressor genes). DNA Amplification The second mechanism for activation of oncogenes is DNA sequence amplification, leading to overexpression of the gene product. This increase in DNA copy number may cause cytologically recognizable chromosome alterations referred to as homogeneous staining regions (HSRs), if integrated within chromosomes, or double minutes (dmins), if extrachromosomal in nature. The recognition of DNA amplification is accomplished through various cytogenetic techniques such as comparative genomic hybridization (CGH) and fluorescence in situ hybridization (FISH), which allow the visualization of chromosomal
aberrations using fluorescent dyes. With these techniques, the entire genome can be surveyed for gains and losses of DNA sequences, thus pinpointing chromosomal regions likely to contain genes important in the development or progression of cancer. Noncytogenetic, molecular techniques for identifying amplifications have more recently become available. Numerous genes have been reported to be amplified in cancer. Several genes, including NMYC and LMYC, were identified through their presence within the amplified DNA sequences of a tumor and had homology to known oncogenes. Because the region amplified often extends to hundreds of thousands of base pairs, more than one oncogene may be amplified in some cancers (particularly sarcomas). Genes simultaneously amplified in many cases include MDM2, GLI, CDK4, and SAS. Demonstration of amplification of a cellular gene is often a predictor of poor prognosis. For example, ERBB2/HER2 and NMYC are often amplified in aggressive breast cancers and neuroblastoma, respectively. Chromosomal Rearrangement Chromosomal alterations provide important clues to the genetic changes in cancer. The chromosomal alterations in human solid tumors such as carcinomas are heterogeneous and complex and likely reflect selection for the loss of tumor- suppressor genes on the involved chromosome. In contrast, the chromosome
alterations in myeloid and lymphoid tumors are often simple translocations, i.e., reciprocal transfers of chromosome arms from one chromosome to another. Consequently, many detailed and informative chromosome analyses have been performed on hematopoietic cancers. The breakpoints of recurring chromosome abnormalities usually occur at the site of cellular oncogenes. Table 79-3 lists representative examples of recurring chromosome alterations in malignancy and the associated gene(s) rearranged or deregulated by the chromosomal rearrangement. Translocations are particularly common in lymphoid tumors, probably because these cell types normally rearrange their DNA to generate antigen receptors. Indeed, antigen receptor genes are commonly involved in the translocations, implying that an imperfect regulation of receptor gene rearrangement may be involved in the pathogenesis. An example is Burkitt's lymphoma, a B cell tumor characterized by a reciprocal translocation between chromosomes 8 and 14. Molecular analysis of Burkitt's lymphomas demonstrated that the breakpoints occurred within or near the MYC locus on chromosome 8 and within the immunoglobulin heavy chain locus on chromosome 14, resulting in the transcriptional activation of MYC. Enhancer activation by translocation, although not universal, appears to play an important role in malignant progression. In addition to transcription factors and signal transduction molecules, translocation may result in the overexpression of cell cycle regulatory proteins such as cyclins and of proteins that regulate cell death such as bcl-2.