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Home , Carcinogenesis, Oncogene, Tumor suppressor gene

Introduction • depends on the regulated division and self-elimination (programmed cell Biology death) of each of its constituent members except its stem cells • A tumor arises as a result of uncontrolled and failure for self-elimination Chapter 18 • Alterations in three groups of are responsible Eric J. Hall., Amato Giaccia, for the deregulated control mechanisms that are cells: proto-, tumor- Radiobiology for the Radiologist supressor genes, and DNA stability genes

Proto-oncogenes Tumor-suppressor genes

• Proto-oncogenes are components of signaling • Tumor-suppressor genes are also components of networks that act as positive growth the same signaling networks as proto-oncogenes, except that they act as negative growth regulators regulators in response to , cytokines, • They modulate proliferation and survival by and cell-to-cell contact antagonizing the biochemical functions of proto- • A gain-of-function in only one copy oncogenes or responding to unchecked growth signals of a protooncogene results in a dominantly • In contrast to oncogenes, inactivation of both acting that often fails to respond to copies of tumor-suppressor genes is required for extracellular signals loss of function in most cases

DNA stability genes Mechanisms of

• DNA stability genes form a class of genes • A single genetic alteration that leads to the involved in both monitoring and activation of an oncogene or loss of a tumor maintaining the integrity of DNA suppressor does not lead to the formation of a solid tumor or • Loss of these genes results in defective • Most tumors contain heterogeneous populations of sensing of DNA lesions as well as improper cells that differ in their ability to repopulate the repair of the damaged template tumor or form metastases • Loss of function results in accumulation of • The malignant progression from normal tissue to tumor to metastasis occurs in a number of discrete "steps" over a period of time

1 Oncogenes Mechanisms of carcinogenesis • First discovered from • Malignant studies of transformation results from mutations in three that cause cancer in groups of genes: (Rous, early – gain-of-function 1900s) mutations that activate – Cell-free extracts derived oncogenes from chicken Restriction – loss-of-function could cause in point mutations that inactivate healthy chickens tumor suppressor genes – loss of activity of DNA stability (e.g., repair) • Only a certain portion of the viral was identified as genes that increases the needed for (proto-oncogene) probability for genomic instability • Either gene mutation or over-expression

Mechanisms of oncogene activation Mechanisms of oncogene activation

• Oncogenes are mutant or over-expressed forms of normal cellular genes (proto-oncogenes); the alteration can be produced by various agents • It is a dominant gene, mutation in only one copy leads to its activation • At least four mechanisms exist: – Retroviral integration of proto-oncogene sequences in retroviral through recombination – DNA mutation of regulatory sites – Gene amplification – rearrangement

Chromosome translocation Oncogene activation • The first real breakthrough in identifying tumor- specific chromosome alterations occurred in the late 1950s when Dr. Peter Nowell found a consistent shortened version of chromosome 22 in individuals afflicted with chronic myelogenous (CML)

2 studies Tumor-suppressor genes

• Another class of genes, anti-oncogenes • Recessive gene, both copies of tumor-suppressor gene have to be inactivated in order to loose function of suppressing malignant transformation • First discovered through family history studies of patients with hereditary , such as retinoblastoma (Rb gene) or Li-Fraumeni syndrome ( gene)

Li-Fraumeni syndrome studies Tumor-suppressor genes

• Extremely rare syndrome, predisposes to development of multiple cancers by young adulthood • Led to discovery of p53 gene, its suppression results in a number of tumors

Tumor-suppressor genes The multi-step nature of cancer

• Carcinogenesis is a multi-step process: a number of distinct events that may be separated in time have to occur • Genetic analysis of cells from solid tumors suggests alterations, mutations, or deletions in multiple • Often tumor-suppressor gene is lost through somatic signaling genes, either oncogenes or suppressor homozygosity: one chromosome of a pair is lost, a genes. For example, 6 to 12 mutations have been occurs in the remaining chromosome; the suggested for the formation of a chromosome with the deletion replicates • The following stages can be identified in tumor • This process has been documented for a number of development: initiation, promotion, and progression tumors

3 The multi-step nature of cancer The multi-step nature of cancer

• A model proposed for correlates a series of chromosomal and molecular events with the changes in the histopathology of normal epithelium during the multistage formation of colorectal cancer and metastatic carcinoma

Functions of oncogenes and tumor-suppressor genes Deregulated proliferation • Several categories of cell functions are • Normal cells rely on extracellural growth signals; perturbed by mutations in oncogenes and typically one cell secretes a mitogenic signal to tumor-suppressor genes stimulate the proliferation of another cell type • Signal is initiated at the (receptors) • Mostly these are functions related to and is transduced to the nucleus via a cascade of regulation of proliferation, growth-restriction affecting regulatory functions and signals • In contrast to untransformed cells, transformed cells • Combination in deregulations of these become autonomous in regulating their growth by functions lead to tumor initiation, responding to the mitogenic signals they themselves and metastasis produce

Deregulated proliferation Failure to respond to growth- restrictive signals • The oncogenic activation stimulates the cell into the , where it duplicates its genetic material before cell division • Inactive tumor suppressor genes fail to enforce the restriction point in , allowing cells to escape extracellular anti- proliferative signals

4 Failure to commit suicide (apoptosis) Oncogene

• Two major pathways that mediate cell death originate • myc codes for a that either from the cell membrane or from the mitochondria binds to the DNA of other • The signals transmitted by each pathway results in the genes and is therefore a activation of intracellular proteins, termed caspases, that factor • When a gene like myc is cleave a diverse number of proteins at specific sites altered to cause cancer, the • Cell lines deficient in Caspases 3 and 9 exhibit cancerous version of the substantially reduced levels of apoptosis during gene is called an oncogene development and in response to stress-inducing stimuli • The healthy version of the • Tumor-suppressor gene p53 in an important modulator gene that it is derived from of oncogene-induced apoptosis is called a proto-oncogene

Escaping A telomere is a region of DNA (repeat telomere sequence of TTAGGG) at each end of a • Angiogenesis, the recruitment of new blood vessels to chromatid, protecting it from deterioration and fusion with other regions of chronically low blood supply, is essential for the chromatids progression of solid tumors to malignancy • Each time a normal divides, the terminal end of • A number of proangiogenic growth factors have been the telomere is lost; successive divisions lead to progressive identified, VEGF was the first isolated that shortening, and after 40 to 60 divisions, vital DNA sequences could stimulate proliferation and migration of blood vessel are lost. At this point, the cell cannot divide further and cell lining undergoes senescence • Studies have shown that blocking the binding of VEGF to • Cancer cells avoid this process of aging by activating the its inhibits tumor angiogenesis and tumor growth. , which offsets the degradation of telomeres These findings have led to the development of new antibody at successive cell divisions; thus becoming immortal approaches for antiangiogenesis therapy for clinical use • Mutation in tumor-suppressor gene p53 is involved

Invasion and metastasis Gatekeepers and caretakers • The genetic circuits that regulate • Most tumor-suppressor genes can be broadly divided into (the entry of a two classes that have been called "gatekeepers" and foreign substance metastasis remain into a blood or "caretakers" lymph vessel) mainly undiscovered • Gatekeepers are genes that directly regulate the growth of • Decreased expression tumors by inhibiting cell division or promoting cell death, or impaired function rate limiting for tumor growth. Both (maternal and of adhesion molecules, paternal) must be lost or inactivated for a tumor to develop. and regulatory The identity of gatekeepers varies with each tissue alterations in chemical • Inactivation of caretaker genes does not directly promote the agents for digestion of growth of tumors, but leads instead to genomic instability extracellular matrix that only indirectly promotes growth by causing an increase (ECM) have been in mutation rate. The targets of the accelerated mutation rate implicated that occurs in cells with defective caretakers are the gatekeeper tumor-suppressor genes, oncogenes, or both

5 Heritable syndromes that affect Mismatch repair genes radiosensitivity • Mismatch repair (MR) genes are responsible for • Ataxia Telangiectasia (lack of voluntary coordination of correction of errors of DNA replication and muscle movements, overdeveloped blood vessels in ocular recombination that result in mispaired (but area, immune deficiency, high incidence of cancers) is associated with a hypersensitive skin reaction to ionizing undamaged) radiation and DNA breaking agents but not to light • Their primary function is to scan the genome as it • AT-like disorder (same clinical features, milder symptoms) replicates and spot errors of mismatch • Nijmegen Breakage syndrome (by microcephaly, a distinct • Mutations in MR genes were found responsible for facial appearance, short stature, immune deficiency, and a the mutator associated with a strong predisposition to lymphoid malignancy) - very high predisposition for hereditary nonpolyposis colon sensitivity to ionizing radiation cancer (HNPCC) and possibly other familial cancers • All three are autosomal recessive diseases, lack DNA-damage checkpoints, but different gene mutations are responsible

Heritable syndromes that affect Radiation-induced signal genomic instability transduction • Seckel syndrome (microcephaly and abnormal • Ionizing radiation can regulate the expression of development) early-response genes, resulting in the stimulation • (hematological abnormalities, median age of pathways and activation of ~30) transcription factors • Syndroms associated with decrease in RecQ : • It may also enhance the response of the cell to – Bloom syndrome (dwarfism, high sensitivity to light) radiation in terms of repair and cell-cycle arrest; – (premature aging) and provide a mechanism for secondary – Rothmund-Thompson (growth deficiency, photosensitivity, early stimulation of various late-response genes graying and hair loss) • Understanding of these defense mechanisms can • Abnormal DNA is accumulated through S phase help exploiting them for treatment of cancer

Approaches to Gene therapy

• Gene therapy is the of genes into an • Genes are individual's cells and tissues to treat a disease: introduced into mutant is replaced with a functional one tumor cells using • There are al least 6 different approaches viral vectors: – Suicide-gene therapy , – Cytotoxic targeted to p53-deficient cells adenovirus, and – Molecular immunology (cancer vaccines) herpesvirus – Tumor-suppressor gene therapy – Radiation-inducible gene linked to a cytotoxic agent – Targeting signal transduction pathways

6 Suicide-gene therapy • Suicide-gene therapy is based on Targeted p-53 deficient cells transducing cells with a gene that converts a prodrug into a cytotoxic agent • A cytotoxic virus can be constructed that is • There is a substantial bystander engineered to replicate and kill only in cells with Inhibits effect; that is, more cells are killed DNA mutant p53 synthesis than transduced initially • This therapy has produced growth • To the extent that mutant p53 is a hallmark of delay and some cures in cancer, this treatment differentiates between models normal cells and cancer cells • Because of the limited efficiency of gene delivery, suicide-gene • Growth arrest has been observed in model therapy needs to be combined with conventional radiotherapy animal tumors and in early clinical trials by •Phase I/I I clinical trials have targeting mutant p53 shown promise

Molecular immunology Tumor-suppressor gene therapy (cancer vaccines) • Tumor-suppressor gene therapy is the replacement, • The approach is to provoke a cellular immune with a correct copy, of the mutated gene that initiates response against the cancer by injecting a vaccine or contributes significantly to the malignant phenotype genetically engineered to express immune • The gene p53 has received the most attention of any stimulatory molecules or tumor-specific antigens gene because it is so commonly mutated in cancers • Molecular immunology shows some promise in animal models but is generally only effective • Phase I/I I clinical trials show some promise in the treatment of non-small-cell against small tumor burdens • The therapy is limited by a lack of information on the • Developing strategy is to combine molecular target genes that are essential for maintaining the immunology with suicide-gene therapy malignant phenotype and the fact that multiple genetic changes are involved

Radiation-inducible gene Radiation-inducible gene linked to linked to a cytotoxic agent a cytotoxic agent

• There is the potential to use a more radiation-specific gene and a more effective toxic agent • Combination of the physics of radiation-targeting • There is the possibility of including a promoter that is specific for technology with molecular gene therapy a particular tumor, for example, prostate or (in some human cancers, advancing to phase II trials)

7 Targeting signal transduction pathways Summary A hallmark of the malignant cell is the dysregulation of • Development of molecular techniques, such as growth and signal transduction pathways that often result gene identification and manipulation tools in resistance to radiotherapy. Several potential targets greatly advanced identification of specific have been identified: genes and understanding of genetic pathways –The epidermal growth factor receptor (EGFR) mediates growth responsible for tumor proliferation regulation in a wide spectrum of human cancers, and tumors expressing high levels of EGFR appear to be radioresistant • There is a number of approaches to gene –Raf-1 is a kinase that plays an important role in cell therapy; the winning approach will be a proliferation, differentiation, survival, and angiogenesis and is synergistic combination of several treatment therefore a prime target for novel cancer therapies –NF/cB is a cellular that plays a central role modalities in the cellular stress response

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