Cancer genetics

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  • Harrison's Internal Medicine Chapter 79. Cancer Genetics Cancer Is a Genetic Disease Cancer arises through a series of somatic alterations in DNA that result in unrestrained cellular proliferation. Most of these alterations involve actual sequence changes in DNA (i.e., mutations). They may arise as a consequence of random replication errors, exposure to carcinogens (e.g., radiation), or faulty DNA repair processes. While most cancers arise sporadically, familial clustering of cancers occurs in certain families that carry a germline mutation in a cancer gene.

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  • Oncogenes in Human Cancer Oncogenes of the kind found in human cancers were initially discovered through their presence in the genome of retroviruses capable of causing cancers in chickens, mice, and rats. The cellular homologues of these viral genes are often targets of mutation or aberrant regulation in human cancer. Whereas many oncogenes were discovered because of their presence in retroviruses, other oncogenes, particularly those involved in translocations characteristic of particular leukemias and lymphomas, were isolated through genomic approaches.

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  • General Classes of Cancer Genes There are two major classes of cancer genes. The first class comprises genes that directly affect cell growth either positively (oncogenes) or negatively (tumor-suppressor genes). These genes exert their effects on tumor growth through their ability to control cell division (cell birth) or cell death (apoptosis). Oncogenes are tightly regulated in normal cells. In cancer cells, oncogenes acquire mutations that relieve this control and lead to increased activity of the gene product.

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  • While most autosomal dominant inherited cancer syndromes are due to mutations in tumor-suppressor genes (Table 79-1), there are a few interesting exceptions. Multiple endocrine neoplasia type II, a dominant disorder characterized by pituitary adenomas, medullary carcinoma of the thyroid, and (in some pedigrees) pheochromocytoma, is due to gain-of-function mutations in the protooncogene RET on chromosome 10. Similarly, gain-of-function mutations in the tyrosine kinase domain of the MET oncogene lead to hereditary papillary renal carcinoma.

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  • Familial Cancer Syndromes A small fraction of cancers occur in patients with a genetic predisposition. In these families, the affected individuals have a predisposing loss-of-function mutation in one allele of a tumor-suppressor gene or caretaker gene. The tumors in these patients show a loss of the remaining normal allele as a result of somatic events (point mutations or deletions), in agreement with the Knudson hypothesis (Fig. 79-3).

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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).

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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.

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  • Chromosomal Instability in Solid Tumors Solid tumors are generally highly aneuploid, containing an abnormal number of chromosomes; these chromosomes also exhibit structural alterations such as translocations, deletions, and amplifications. Again, colon cancer has proven to be a particularly useful model for the study of chromosomal instability (CIN). As described above, some familial cases are characterized by the presence of MIN.

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  • Figure 79-8 A cDNA array experiment. RNA is prepared from cells, reverse transcribed to cDNA, and labeled with fluorescent dyes (typically green for normal cells and red for cancer cells). The fluorescent probes are mixed and hybridized to the cDNA array. Each spot on the array is a cDNA fragment that represents a different gene. The image is then captured with a fluorescence camera; red spots indicate higher expression in tumor compared with reference while green spots represent the opposite. Yellow signals indicate equal expression levels in normal and tumor specimens.

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  • Lung cancer is the leading cause of cancer death among men and women in the United States with 170,000 deaths per year. This exceeds the sum of the next three leading causes of death due to cancer: breast, colon, and prostate. There are over 1 million deaths worldwide due to lung cancer, making it truly an epidemic. Fewer than 15% achieve a 5-yr survival. The vast majority (85%) present with advanced disease, although stage I patients may have a 5-yr survival approaching 70% (1).

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  • Harrison's Internal Medicine Chapter 64. The Practice of Genetics in Clinical Medicine Implications of Molecular Genetics for Internal Medicine The field of medical genetics has traditionally focused on chromosomal abnormalities (Chap. 63) and Mendelian disorders (Chap. 62). However, there is genetic susceptibility to many common adult-onset diseases, including atherosclerosis, cardiac disorders, asthma, hypertension, autoimmune diseases, diabetes mellitus, macular degeneration, Alzheimer's disease, psychiatric disorders, and many forms of cancer.

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  • Lung cancer is the most frequent cause of cancer deaths in both men and women in the U.S. (1). Although tobacco smoking is accepted as the number one cause of this devastating disease, our understanding of the acquired genetic changes leading to lung cancer is still rudimentary. Lung cancer is classifi ed into two major clinic-pathological groups, small cell lung carcinoma (SCLC) and non-small cell lung carcinoma (NSCLC) (2). Squamous cell carcinoma, adenocarcinoma, and large cell carcinoma are the major histologic types of NSCLC....

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  • The committee is grateful for the contribution of participants at its three public meetings. The presentations and discussions at these meeting were valuable in informing the committee about relevant research findings, issues of interest in the research community, the perspectives of advocacy organizations, and the concerns of individuals with breast cancer and their families. The agendas for these meetings appear in Appendix A.

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  • Table 79-3 Representative Oncogenes at Chromosomal Translocations Gene (Chromosome) Translocation Malignancy ABL BCR (22q11) (9q34.1)– (9;22)(q34;q11) Chronic myelogenous leukemia ATF1 EWS (22q12) (12q13)– (12;22)(q13;q12) Malignant melanoma of soft parts (MMSP) BCL1 IgH (14q32) (11q13.3)– (11;14)(q13;q32) Mantle lymphoma cell BCL2 IgH (14q32) (18q21.

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  • The second requirement is a clear and well defined starting point. For popu- lation-based cancer registries, the starting date (from which the survival is calculated) is the incidence date (see Section 17.3.1). The third requirement is a clear and well defined outcome. Death is gener- ally the outcome of interest, but some registries collect enough data to allow them to conduct analyses using recurrence of tumour, or first recurrence of a particular complication, as the outcome of interest.

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  • Harrison's Internal Medicine Chapter 80. Cancer Cell Biology and Angiogenesis Cancer Cell Biology and Angiogenesis: Introduction Two characteristic features define a cancer: unregulated cell growth and tissue invasion/metastasis. Unregulated cell growth without invasion is a feature of benign neoplasms, or new growths. Cancer is a synonym for malignant neoplasm. Cancers of epithelial tissues are called carcinomas; cancers of nonepithelial (mesenchymal) tissues are called sarcomas. Cancers arising from hematopoietic or lymphoid cells are called leukemias or lymphomas.

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  • In spite of the massive efforts being made worldwide to understand molecular genetics and epigenetic factors responsible for the initiation and progression of cancer, the statistics on this malignancy have remained enormously negative; the following data testify to this unfortunate human condition. There are more than 100 types of cancers that can inflict any part of the body. In 2005, 7.6 million people died of cancer, which makes up 13% of the 58 million deaths worldwide. Approximately 1.

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  • Many disorders exhibit the feature of locus heterogeneity, which refers to the fact that mutations in different genes can cause phenotypically similar disorders. For example, osteogenesis imperfecta (Chap. 357), long QT syndrome (Chap. 226), muscular dystrophy (Chap. 382), homocystinuria (Chap. 358), retinitis pigmentosa (Chap. 29), and hereditary predisposition to colon cancer (Chap. 87) or breast cancer (Chap. 86) can each be caused by mutations in distinct genes.

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  • Neither randomized clinical trials nor meta- analysis are available and evidence is based on a number of retrospective studies with multivariate for mortality risk factors or data from national cancer registries (Gilliland et al., 1997; Hundahl et al., 1998). Unfortunately, very remarkable differences in patient’s selection, staging systems, and clinical management affect the available studies. In particular, radioiodine treatment is not routinely carried out in a standard manner and outcome results of different studies are thus not comparable (Sciuto et al., 2009).

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  • There have been a significant number of advances in the field of cancer research since the first edition of Cancer Biology, which was published in 1981. These include advances in defining the genetic and phenotypic changes in cancer cells, the genetic susceptibility to cancer, molecular imaging to detect smaller and smaller tumors, the regulation of gene expression, and the ‘‘-omics’’ techniquesofgenomics, proteomics,and metabolomics, among others.

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