Genetics of Cancer


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Genetics of Cancer: An Overview

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Genetics of Cancer

  1. 1. CENETICS OF CANCER Salwa Hassan Teama M.D. Clinical Pathology and Oncology National Cancer Institute/ Cairo/ Egypt
  2. 2. Cancer is one of the most common and severe problems of clinical medicine. Statistics sow that cancer in some form strikes more than one third of the population, account for more than 20% of all deaths, and, in developed countries, is responsible for more than 10% of the total cost of medical care. Cancer is a complex disease that result from the same basic process of uncontrolled growth. Cell proliferation results in a mass that invades neighboring tissues and may metastasize to more distant sites. The growth is autonomous, increasingly malignant, if untreated, invariably fatal. Some cancers, however, such as blood cancers, do not form tumors. Tumor formation is a multistep process involving many of genetic changes in the evolving tumor cell population. Tumors are classified by site, tissue type and degree of malignancy. Most cancer are disorders of later life, but some are affect childhood. CANCER
  3. 3. A tumor is composed of a parenchyma of proliferating cells, with a stroma of connective tissue and blood vessels. Three main form:  Sarcoma: Tumor arise from mesenchymal tissue  Carcinoma: Tumor originate from epithelial tissue  Hematopoietic and lymphoid malignancies: leukemia and lymphoma. Different Types of Cancer
  4. 4. Nearly all cancers are caused by abnormalities in the genetic material of the transformed cell. In order for a normal cell to transform into a cancer cell, genes which regulate cell growth and differentiation must be altered. When normal regulation is altered, uncontrolled growth is initiated and a malignant tumor develop. Genetic changes can occur at many levels, from gain or loss of entire chromosomes to a mutation affecting a single DNA nucleotide. New aspects of the genetics of cancer pathogenesis, such as DNA methylation and microRNAs are increasingly recognized. The Genetic Nature of Cancer
  5. 5.  Cancer is a genetic disorder in which the normal control of cell growth is lost. The basic mechanism in all cancer is mutation. Carcinogenic agents are involved through causing mutation.  The mutation affects genes responsible for cell proliferation, cell development and other cellular activities.  Initiation of cancer; cells undergoing a series of genetic mutation or alteration which result in their instability to respond normally to intracellular/extracellular signals that control proliferation, differentiation and death.
  6. 6. Germline mutations are responsible for 5% to 10% of cancer cases. This is also called familial cancer. These mutations are present in every cell of the body and are passed from parent to child. Sporadic cancer or somatic mutation are caused by tobacco, over-exposure to UV radiation, and other toxins and chemicals. These mutations are not in every cell of the body and are not passed from parent to child.
  7. 7. Randomly acquired through errors in DNA replication. Inherited and thus present in all cells from birth. The heritability of cancers are usually affected by complex interactions between carcinogens and the host's genome. Carcinogens, such as tobacco smoke, radiation, chemicals, or infectious agents (Viruses are involved in cancers). Causes of Cancer
  8. 8. There is strong evidence that a tumor is a clone of cells derived from a single ancestral cell in which the initiating events has taken place. However the clonal origin of tumor does not imply homogeneity among cancer cells, since further genetic change and microenviromental influences occur during tumor growth lead to heterogeneity in a wide variety of the properties of the individual tumor growth. The Clonal Nature of Cancer
  9. 9. Many aspects of cell function are controlled by a balance of positive and negative signals from inside and outside the cell. In normal tissues, there is a balance between cell proliferation and cell death. In tumor, this balance is lost because of genetic defect that lead to faulty cell signaling or to imbalance of molecules that stimulate (cyclin and cyclin dependent kinases) or inhibit (cyclin dependent kinases inhibitors) the movement of cells around the cell cycle.
  10. 10.  The risk of cancer shows significant variation among different population in different environment. The risk must depend on exposure to carcinogens in the environment. e.g. Industrial cancer which result from prolonged exposure to carcinogenic chemicals as cancer of the skin in tar workers, cancer of the bladder in aniline dye workers,….. Cancer and the Environment
  11. 11. Source: Wikipedia Source: National Cancer Institu Cancer Genes
  12. 12. Cancer develops when several genes in a cell become mutated in a way that overrides the checks and balances of the cell. However, many cancers cannot be tied to a specific gene, and some genes may interact in unpredictable ways with other genes or factors in the environment to cause cancer. Source: Wikipedia
  13. 13. Oncogenes are known by three letter abbreviation which reflect their origin or the type of tumor with which they are associated. Cancer-promoting oncogenes: If oncogene is altered or overexpressed, either as a result of a mutation in the gene itself or by altered external control, the cell in which the change occurred can undergo uncontrolled growth, eventually malignant. Most oncogenes are mutated forms of normal genes, called proto-oncogenes. Oncogenes
  14. 14.  Oncogenes were identified initially in cancer causing viruses (DNA/RNA tumor virus).  The first confirmed oncogene was discovered in 1970 and was termed src. Src was in fact first discovered as an oncogene in a chicken retrovirus. Experiments performed by Dr. G. Steve Martin of the University of California, Berkeley demonstrated that the Src was indeed the oncogene of the virus. The first nucleotide sequence of v-src was sequenced in 1980 by A.P. Czernilofsky et al. Many of the viral oncogenes (v-onc) have homologous cellular counterparts (c-onc) known as proto-oncogenes.  The study of retroviral oncogene functions has provided insights into the role of cellular proto-oncogenes. Source:Wikipedia Oncogenes
  15. 15. Proto-oncogene is a normal gene that can become an oncogene due to mutations or increased expression. Proto-oncogenes code for proteins that help to regulate cell growth and differentiation. Proto-oncogenes are often involved in signal transduction and execution of mitogenic signals, usually through its protein product. Types of Proto-oncogenes Cellular oncogenes (c- oncogenes): proto-oncogene which have been to mutate in any individual. Normal oncogene (n- oncogene): proto-oncogenes that have not been found to mutate. Proto-oncogene
  16. 16.  These are typical cellular genes with typical control sequences. As eukaryotic genes:  Most have introns.  They are always at same place in genome.  No LTR sequences.  They show normal Mendelian inheritance because they are normal genes, essential to the functions of the cell  Cellular oncogenes are expressed by the cell at some period in the life of the cell, often when the cell is growing, replicating and differentiating normally. They are usually proteins that are involved in growth control  Cellular oncogenes are highly conserved. Characteristic of Cellular Proto-oncogene
  17. 17. Point mutations  Deletions, or insertions that lead to hyperactive gene product.  Deletions, or insertions in the promoter region of a proto-oncogene that lead to increased transcription. Gene amplification events leading to extra chromosomal copies of a proto- oncogene lead to normal protein greatly overproduced. Amplified segments of DNA are often detected as two types of cytogenetic change, double minute and homogeneously staining regions. Chromosomal translocation  Relocation of a proto-oncogene to a new chromosomal site that leads to higher expression  Fusion between a proto-oncogene and a second gene, which produces a fusion protein with oncogenic activity. Activation of Proto-oncogene
  18. 18. Source:
  19. 19. Oncogene can be classified according to their cellular location and function of their encoded oncoproteins in the signal transduction pathway:  Growth factors  Growth factor receptors  GTP binding proteins  Post receptor tyrosine kinase  Cytoplasmic oncogenes  Nuclear oncogenes  Apoptotic oncogenes Types of Oncogenes
  20. 20. Most oncogenes are dominant mutations; a single copy of this gene is sufficient for expression of the growth trait. This is also a "gain of function" mutation because the cells with the mutant form of the protein have gained a new function not present in cells with the normal gene.
  21. 21. Tumor suppressor gene or anti oncogene: Normal genes implicated in the control of cell cycle, repair DNA mistakes, and tell cells when to die (apoptosis or programmed cell death). The product of tumor suppressor genes normally block abnormal growth and malignant transformation and lead to malignancy when the function of both alleles is lost. When tumor suppressor genes don’t work properly, cells can grow out of control, which can lead to cancer. In contrast to mutations in proto-oncogene, which are dominant in their action, most mutation in tumor suppressor genes are recessive. Tumor Suppressor Gene
  22. 22.  Genes that control cell division Some tumor suppressor genes help control cell growth and reproduction. e.g. retinoblastoma gene (RB1). Abnormalities of the RB1 gene can lead to a type of eye cancer (retinoblastoma) in infants, as well as to other cancers  DNA repair genes: These are genes that fix any mistakes made when DNA is replicated (copied). Mistakes that aren't fixed become mutations, which may eventually lead cancer. e.g. Genes responsible for HNPCC (hereditary nonpolyposis colon cancer). When these genes do not repair the errors in DNA, HNPCC can result.  Cell "suicide" genes e.g. p53.
  23. 23. p53 located human chromosome 17, gene with tumor suppressor activities.  p53 protein contains 393 amino acids and a single amino acid substitution can lead to loss of function of the gene. Mutations at amino acids 175, 248, and 273 can lead to loss of function and changes at 273 (13%) are the most common.  About 50% of human cancers can be associated with a p53 mutation including cancers of the bladder, breast, cervix, colon, lung, liver, prostate, and skin  p53 related cancers are also more aggressive and have a higher degree of fatalities  These all act as recessive mutations.
  24. 24. Inherited Abnormalities of Tumor Suppressor Genes have been found in several cancers that tend to run in families.  Mutations in p53, RB1, and the genes involved in HNPCC, .  A defective APC gene causes familial polyposis a condition in which people develop hundreds or thousands of colon polyps, some of which may eventually acquire several sporadic mutations and turn into colon cancer.  Abnormalities of the BRCA genes account for 5% to 10% of breast cancers. Non-inherited mutations of tumor suppressor genes  Acquired mutations of the p53 gene appear to be involved in a wide range of cancers, including lung, colorectal, and breast cancer, as well as many others.  Acquired changes in many other tumor suppressor genes also contribute to the development of sporadic (not inherited) cancers
  25. 25.  Several close (first or second degree) relatives with a common cancer.  Several close relatives with related cancers. e.g. breast and ovary or bowel and endometrial cancer.  Two family members with the same rare cancer  An unusually early age of onset  Bilateral tumors in paired organs  Synchronous or successive tumors  Tumors in two different organ systems in one individual Feature Suggestive of an Inherited Cancer Susceptibility Syndrome in Families
  26. 26.  The heritability of cancers are usually affected by complex interactions between carcinogens and the host's genome  The level of risk for persons with a family history of one of the common cancers such as bowel or breast cancer depends on a number of factors. These include the number of person at risk to the affected individuals and the age at which the affected family members develop cancers,…  The presence of an oncogene in a germ line cell (egg or sperm) results in an inherited predisposition for tumors in the offspring. However, a single oncogene is not usually sufficient to cause cancer, so inheritance of an oncogene does not necessarily result in cancer.  Persons at risk of an inherited cancer susceptibility can be screened for associated features of a familial cancer predisposing syndrome or for particular cancer. Inherited Susceptibility for the Common Cancers
  27. 27. Cancer is Multistep Process Cancer is multistep process
  28. 28.  The gene defective in the chromosome instability syndrome may be viewed as cancer genes. The enzymes defective in these syndromes must be intimately involved in DNA repair and maintenance of chromosome integrity.  Chromosome instability syndromes are a group of inherited conditions associated with chromosomal instability and breakage. They often lead to an increased tendency to develop certain types of malignancies. The following chromosome instability syndromes are known:  Ataxia telangiectasia  Ataxia telangiectasia-like disorder  Bloom syndrome  Fanconi anaemia  Nijmegen breakage syndrome Chromosome Instability Syndrome
  29. 29.  Many viruses infect humans but only a few viruses are known to promote human cancer. These include both DNA viruses and retroviruses, a type of RNA virus. Viruses associated with cancer include human papillomavirus (genital carcinomas), hepatitis B (liver carcinoma), Epstein-Barr virus (Burkitt's lymphoma and nasopharyngeal carcinoma), human T-cell leukemia virus (T- cell lymphoma); and, probably, a herpes virus called KSHV (Kaposi's sarcoma and some B cell lymphomas). Virus and Cancer
  30. 30. Viruses can also contribute to cancer by inserting their DNA into a chromosome in a host cell.  Insertion of the virus DNA directly into a proto-oncogene may mutate the gene into an oncogene, resulting in a tumor cell.  Insertion of the virus DNA near a gene in the chromosome that regulates cell growth and division can increase transcription of that gene, also resulting in a tumor cell.  Human papillomavirus makes proteins that bind to two tumor suppressors, p53 protein and RB protein, transforming these cells into tumor cells. DNA Tumor Viruses
  31. 31. Source: Wikipedia
  32. 32. CONCLUSION Cancer is a genetic disorder in which the normal control of cell growth is lost. The basic mechanism in all cancer is mutation, either in the germ line or much more frequently, in somatic cells. Cancer is multi-factorial diseases, much remains to be learned about the genetic processes of carcinogenesis and about he environmental factors that alter DNA and thus lead to malignancy.