Carcinogenesis Dr. Mamlook Elmagraby

Objectives of the lecture Upon completion of this lecture, students should be able to: Understand the concepts of dominant oncogenes and proto- oncogenes Be familiar with the different classes of oncogenes: growth factors, growth factor receptors, signal-transducing molecules, and nuclear regulatory factors Compare and contrast the different mechanisms of oncogene activation (point mutations, overexpression, gene amplification) Describe the concept of a tumor suppressor gene and list some examples

Objectives of the lecture Understand the relationship between DNA repair and mutation rates Know which inherited condition is associated with defects in DNA repair Discuss how dysregulation of apoptosis contribute to neoplasia Clarify the role of telomerase in cancer

Genetic regulators of normal and abnormal mitosis In normal cell growth, regulatory genes control mitosis   In normal cell growth, there are four regulatory genes: Proto-oncogenes are growth-promoting genes i.e. they encode for cell proliferation pathway Anti-oncogenes are growth-inhibiting or growth suppressor genes Apoptosis regulatory genes control the programmed cell death DNA repair genes are those normal genes which regulate the repair of DNA damage that has occurred during mitosis In cancer, the transformed cells are produced by abnormal cell growth due to genetic damage to these normal controlling genes

Genetic regulators of normal and abnormal mitosis Abnormalities in these four cell regulatory genes: Activation of growth-promoting oncogenes causing transformation of cell Inactivation of cancer-suppressor genes (i.e. inactivation of anti-oncogenes) permitting the cellular proliferation of transformed cells Abnormal apoptosis regulatory genes Failure of DNA repair genes and thus inability to repair the DNA damage resulting in mutations

The Major Genetic Properties of Cancer

Excessive and autonomous growth Growth promoting oncogenes Proto-oncogenes: normal cellular genes whose products promote cell proliferation Oncogenes: mutant or overexpressed versions of protooncogenes that function autonomously without a requirement for normal growth-promoting signals Mutated form of normal protooncogenes in cancer is called oncogenes   Overactivity of oncogenes enhances cell proliferation and promotes development of human cancer

Activation mechanisms of proto-oncogenes Promoters are DNA sequences whose purpose is not to encode information about the organism itself, but rather they serve as a kind of "On" switch to initiate the biological process of transcription for the genes which follow the promoter DNA sequence. The enzyme, RNA polymerase, which performs the transcription process, binds to the promoter sequence and then beings to work its way down the DNA segment, constructing RNA to match the DNA nucleotides over which the enzyme passes

Functions of Cellular Proto-Oncogenes 2. Growth Factor Receptors 1. Secreted Growth Factors 2. Growth Factor Receptors 4. Nuclear Proteins: Transcription Factors 3. Cytoplasmic Signal Transduction Proteins 5. Cell-Cycle Control Proteins

Growth promoting oncogenes Growth factors (GFs). Most soluble growth factors are made by one cell type and act on a neighboring cell to stimulate proliferation Normally, cells that produce the growth factor do not express the cognate receptor, preventing the formation of positive feedback loops within the same cell This “rule” may be broken by cancer cells Many cancer cells synthesize the growth factor and its receptor (glioblastomas, sarcomas) Hepatocyte growth factor and its receptor are both overexpressed in thyroid carcinomas

Growth promoting oncogenes Growth Factor Receptors Many of the growth factor receptors function as oncoproteins when they are mutated or if they overexpressed The mutated receptors deliver mitogenic signals to cells even in the absence of growth factors   These mutated receptors are common in leukemias, lymphomas EGF receptor, is overexpressed in lung carcinomas, glioblastomas The gene encoding HER2 is amplified in breast cancers The tumors having overexpressed receptor are sensitive to the mitogenic effects of small amounts of growth factors Epidermal growth factor (EGF)

Growth promoting oncogenes Cytoplasmic signal transduction proteins. Two important oncoproteins in the category of signaling molecules are RAS and ABL RAS 30% of all human tumors contain mutated RAS genes, and the frequency is even higher in some specific cancers (pancreatic adenocarcinoma)   Mutations of RAS gene interfere with breakdown of GTP, which is essential to inactivate RAS. RAS is thus trapped in its activated, GTP-bound form Signal transduction is the process by which a chemical or physical signal is transmitted through a cell as a series of molecular events, most commonly protein phosphorylation catalyzed by protein kinases, which ultimately results in a cellular response.  such responses include changes in the transcription or translation of genes, and post-translational and conformational changes in proteins, as well as changes in their location Proteins responsible for detecting stimuli are generally termed receptors

Growth promoting oncogenes ABL In chronic myeloid leukemia, a part of the ABL gene is translocated from chromosome 9 to chromosome 22, where it fuses with part of BCR gene   This fusion gene encodes as BCR-ABL hybrid protein that have a constitutive tyrosine kinase activity Constitutive continuously produced regardless of the needs of cells tyrosine kinase functions as an "on" or "off" switch in many cellular functions

Growth promoting oncogenes Nuclear Transcription Factors. Several oncogenes, function as transcription factors that regulate the expression of growth-promoting genes (CDKs)   Of these, MYC is involved most commonly in human tumors Dysregulation of MYC promotes the progression of cells through the cell cycle Dysregulation of MYC results from a (8;14) translocation in Burkitt lymphoma MYC also is amplified in breast, colon, lung cancers Transcription Factors work alone or with other proteins in a complex, by promoting (as an activator), or blocking (as a repressor) the recruitment of RNA polymerase (the enzyme that performs the transcription of genetic information from DNA to RNA) to specific genes

The chromosomal translocation and associated oncogenes in Burkitt lymphoma and chronic myelogenous leukemia.

Growth promoting oncogenes Cell cycle regulatory proteins. Complexes of cyclins with CDKs drive the cell cycle and normally are controlled by CDK inhibitors   Mutations in genes encoding cyclins, CDKs, and CDK inhibitors result in uncontrolled cell cycle progression Mutations in genes encoding cell cycle regulatory proteins are found in melanomas and brain, lung, pancreatic cancers

Insensitivity to Growth Inhibitory Signals: Tumor Suppressor Genes Tumor suppressor genes are regulatory genes preventing the abnormal division of cells Deletion or mutation of a tumor suppressor gene allows the cell to proliferate and is important for the initiation of malignant tumors Both copies of tumor suppressor genes must be dysfunctional for tumor development to occur  

RB: GOVERNOR OF THE CELL CYCLE In cases of familial retinoblastoma, one defective copy of the RB gene is present in the germ line, so that only one additional somatic mutation is needed to completely eliminate RB function   RB exerts anti-proliferative effects by controlling the G1-to-S transition of the cell cycle. In its active form, RB is hypophosphorylated and binds to E2F transcription factors. This interaction prevents transcription of genes like cyclin E that are needed for DNA replication, and so the cells are arrested in G1 Growth factor signaling leads to cyclin D expression, activation of cyclin D–CDK4/6 complexes, inactivation of RB by phosphorylation, and thus release of E2F Loss of cell cycle control is fundamental to malignant transformation. Almost all cancers have a disabled G1 checkpoint due to mutation of either RB or genes that affect RB function, such as cyclin D, CDK4, and CDKIs Many oncogenic DNA viruses, like HPV, encode proteins (E7) that bind RB and render it nonfunctional.

TP53: guardian of the genome TP53 encodes p53, the central monitor of stress in the cell, which can be activated by anoxia, inappropriate oncogene signaling, or DNA damage.   Activated p53 controls the expression and activity of genes involved in cell cycle arrest, DNA repair, cellular senescence, and apoptosis. Activated p53 drives transcription of p21, which prevents RB phosphorylation, thereby causing a G1-S block in the cell cycle. This pause allows the cells to repair DNA damage. If DNA damage cannot be repaired, p53 induces cellular senescence or apoptosis Of human tumors, 70% demonstrate mutations in TP53. Patients with the rare Li-Fraumeni syndrome inherit one defective copy of TP53 in the germ line, such that only one additional mutation is required to lose normal p53 function Li-Fraumeni syndrome patients are prone to develop a wide variety of tumors. As with RB, p53 can be incapacitated when bound by proteins encoded by oncogenic DNA viruses such as HPV

APC-β- Catenin Pathways The APC gene exerts anti-proliferative actions by regulating the destruction of the cytoplasmic protein β-catenin   With a loss of APC, β-catenin is not destroyed, and it translocates to the nucleus, where it acts as a growth-promoting transcription factor In familial adenomatous polyposis syndrome, inheritance of a germ line mutation in the APC gene and sporadic loss of the sole normal allele causes the development of hundreds of colonic polyps at a young age. One or more of these polyps evolves into a colonic cancer. Somatic loss of both alleles of the APC gene is seen in approximately 70% of sporadic colon cancers

Evasion of Apoptosis

Evasion (escape or avoid) of Apoptosis Evasion of cell death by cancers mainly involves acquired abnormalities that interfere with the intrinsic (mitochondrial) pathway of apoptosis The most common abnormalities involve loss of p53 function, either by way of TP53 mutations or overexpression of the p53 inhibitor MDM2   Other cancers evade cell death by overexpressing anti-apoptotic members of the BCL2 family, such as BCL2, BCL-XL, and MCL1, which protect cells from the action of BAX and BAK, the pro-apoptotic members of the BCL2 family In a large majority of follicular B-cell lymphomas, BCL2 levels are high because of a (14;18) translocation that fuses the BCL2 gene with regulatory elements of the immunoglobulin heavy chain gene

DNA Damage and Repair System in Cancer

DNA damage and repair system in cancer Normal cells during mitosis suffer from minor damage to the DNA which is detected and repaired before mitosis is completed Similarly, small mutational damage to the dividing cell by exogenous factors (radiation, chemical carcinogens) is also repaired P53 gene is responsible for detection and repair of DNA damage By this mechanism, integrity of the genome is maintained

DNA damage and repair system in cancer If the system of DNA repair is defective, the defect in unrepaired DNA is passed to the next progeny of cells and cancer results   The examples of DNA repair genes defect exist in the following inherited disorders associated with increased susceptibility to cancer: Hereditary non-polyposis colon cancer Xeroderma pigmentosum Hereditary breast cancer

Limitless Replicative Potential (Immortality)

Telomeres and telomerase in cancer After each mitosis there is progressive shortening of telomeres which are the terminal tips of chromosomes After repetitive mitosis, telomeres are lost in normal cells and the cells cease to undergo mitosis Telomerase helps in repair of such damage to DNA and maintains normal telomere length in successive cell divisions Cancer cells in most malignancies have markedly upregulated telomerase enzyme, and hence telomere length is maintained In cancer cells, mitosis does not slow down or stop, thereby immortalising the cancer cells

Oncogenesis Mechanism Action Example Growth Promotion Overexpression of growth factor receptors (epidermal growth factor) making cells more sensitive to growth stimuli HER2 Increased growth factor signal transduction by an oncogene that lacks the GTPase activity that limits GTP induction of cytoplasmic kinases that drive cell growth RAS Lack of normal gene regulation through translocation of a gene where it is controlled by surrounding genes to a place where it is no longer inhibited BCR-ABL Binding of oncogene product to the nucleus with DNA transcriptional activation to promote entry into the cell cycle C-MYC Oncogenesis, mechanisms

Loss of Tumor Suppressor Gene Function Mechanism Action Example Loss of Tumor Suppressor Gene Function Loss of normal growth inhibition BRCA-1 Lack of regulation of cell adhesion with loss of growth control through cell interaction APC Loss of down-regulation of growth promoting signal transduction NF-1 Loss of regulation of cell cycle activation through sequestation of transcriptional factors RB Loss of regulation of cell cycle activation through lack of inhibition of cell proliferation that allows DNA repair p53 Limitation of Apoptosis Overexpression of gene, activated by translocation, prevents apoptosis BCL-2 Oncogenesis, mechanisms

Breast and ovarian carcinomas Gene Associated Neoplasms HER2 Breast and ovarian carcinomas RAS Many carcinomas and leukemias BCR-ABL Chronic myelogenous leukemia, acute lymphocytic leukemia C-MYC Lymphomas BRCA-1 APC Colonic adenocarcinomas NF-1 Neurofibromas and neurofibrosarcomas Rb Retinoblastomas, osteosarcomas, small cell lung carcinomas p53 Many carcinomas BCL-2 Chronic lymphocytic leukemia, lymphomas Oncogenesis, examples