Tumor Biology

Molecular Biology of Cancer

1. Cellular Homeostasis
• to achieve homeostasis in tissues, renewable cell populations must perform 4 related functions:


• must proliferate with proper timing and fidelity of DNA repair
• must differentiate in a pattern consistent with normal function of the tissue
• proliferation and involution rates must be balanced
• must repair any damage resulting from exposure to mutagens such as radiation, chemicals, or viruses


• a defect in any of these functions can result in tumor formation


2. Cancer Phenotype
• tumors are clonal in origin
• loss of contact inhibition
• greater autonomy from growth factors
• nuclear and cellular polymorphism
• loss of cellular polarity
• variation in DNA content from cell to cell (aneuploidy)
• development of invasive properties
• neovascularization (angiogenesis)
• resistance to apoptosis
• cell cycle differences


3. Genetic Alterations
1. Multistep Hypothesis
• cancer is fundamentally an alteration in genes that control cellular function
• development of the malignant phenotype is multifactorial
• some abnormal genes can be inherited as germline defects (cancer susceptibility genes)
• other genetic changes (somatic mutations) are acquired through interaction with agents that alter the cellular genome (radiation, mutagenic chemicals, viruses)
• carcinogenesis requires the successive accumulation of genetic defects (multi-hit hypothesis) that result in the altered cellular growth and differentiation characteristic of the malignant phenotype
• each mutation confers a growth advantage to the cell and enhances the rate at which subsequent mutations occur
• the genetic changes necessary to develop cancer are best understood for colon cancer and melanoma


1. Adenoma-Carcinoma Sequence in Colorectal Cancer
• most colorectal cancers arise from adenomatous polyps that become dysplastic and then malignant (invasive)
• mutations in the adenomatous polyposis coli (APC) gene occur early, kras and DCC gene mutations are intermediate steps, and p53 gene mutations occur late



2. Nevus-Melanoma Sequence
• nevi without atypia may progress to dysplastic nevi
• some dysplastic nevi may progress to the radial growth phase of melanoma
• some superficial melanomas may develop a clone of fast growing cells that expand in the vertical direction
• some of these melanomas may then develop the ability to metastasize



2. Oncogenes
• genes that promote the transformation of normal cells into tumor cells
• oncogenes are homologs of normal cellular genes that participate in cell growth and cell cycle regulation
• normal host genes are termed protooncogenes
• a mutational change in an oncogene leads to constitutive activation of the gene, which then results in uncontrolled cellular proliferation
• oncogenes are gain in function mutations



1. Growth Factors
• transformation of normal cells to malignant cells may result from increased production of stimulatory growth factors or decreased production of inhibitory growth factors (TGF-β)
• growth factors function by binding to a growth factor receptor
• activation of the growth factor receptor leads to activation of the intrinsic receptor enzymatic activity and signal transduction follows
• sis oncogene displays significant homology to the β-chain of PDGF
• overproduction of epidermal growth factor (EGF) will result in cellular transformation


2. Growth Factor Receptor Tyrosine Kinases
• prototypical example is the EGFR group (HER1 through HER4)
• constitutive activation of a growth factor receptor can also lead to tumor formation
• mutation of the erb protooncogene results in a defective EGFR that can be activated without EGF, leading to activation of tyrosine kinase and downstream signaling activation
• HER2 gene is amplified in 30% of breast and ovarian cancers and is associated with a poorer prognosis
• HER2 is a target for therapy by a recombinant antibody called trastuzumab (Herceptin)


3. Ras GTPases
• G proteins function as mediators between G-protein-linked receptors and membrane-associated enzymes
• after ligand binding, the G protein releases GDP and binds GTP
• G protein then dissociates and activates specific enzymes (cAMP, phospholipase C, protein kinase C)
• ras oncogenes have significant homology to G proteins
• because of a mutation at the GTP binding site, the ras protein does not hydrolyze GTP and so is continuously activated
• ras oncogenes are the second most commonly expressed oncogenes in human malignancies


4. Nuclear Transcription Factors
• directly alter gene expression by binding to DNA to effect gene transcription
• mutated oncogenes may upregulate or act as negative inhibitors by blocking the genes necessary for normal growth and differentiation
• c-myc is a direct regulator of the cell cycle and stimulates the cell to progress from G1 into S phase


3. Tumor Suppressor Genes and Cancer Predisposition Syndromes
• normal effect is to keep cellular growth in check
• loss of function leads to tumor formation
• most of the inherited cancer predisposition syndromes involve inheritance of one mutant allele and one normal allele of a tumor suppressor gene
• additionally, expression of the malignant phenotype requires loss of the normal allele through somatic mutation (‘two-hit’ hypothesis)
• in keeping with the multistep hypothesis, additional oncogenic mutations are required to develop the malignant phenotype (ras, c-myc, etc)


1. Hereditary Retinoblastoma and the Rb Gene
• usually presents in the first year of life
• Rb is a nuclear protein that regulates the entry of cells into S-phase
• Rb inhibits the ability of E2F transcription factor to bind DNA and initiate DNA synthesis
• in late G1, Rb is phosphorylated by CDKs and releases E2F
• E2F is a transcription factor responsible for leading the cell from G1 into S phase
• a mutation in Rb leads to unregulated cell growth through increased E2F activity


2. p53
• most frequently mutated gene in human cancer
• p53 is a DNA-binding protein responsible for protecting the integrity of the cell’s genome
• p53 is able to activate genes that halt the cell cycle in response to damage, allowing adequate time for DNA repair prior to replication
• in cases of severe damage, p53 is capable of triggering apoptosis, eliminating damaged cells before replication can take place
• intact p53 function is crucial for tumor prevention
• inactivation of p53 is one of the most detectable genetic defects in sporadic tumors
• inherited mutations of p53 (Li-Fraumeni syndrome) result in tumors of the breast, brain, adrenal, sarcoma, and leukemia


3. Familial Adenomatous Polyposis and the APC Gene
• autosomal dominant
• characterized by the development of hundreds of colon polyps and the nearly 100 percent progression to colorectal cancer in untreated individuals
• phenotype may also include duodenal adenomas and carcinomas, desmoid tumors, mandibular osteomas
• results from a germline mutation in the APC gene
• mutations in the APC gene have been found in all types of colon polyps and in over 80% of sporadic colon cancers
• APC regulates β-catenin, which is a nuclear transcription factor
• loss of APC function results in nuclear accumulation of β-catenin, leading to cellular proliferation


4. Familial Breast and Ovarian Cancer
• cancer susceptibility genes include BRCA-1 and BRCA-2
• lifetime risk of breast cancer is 80%
• BRCA-1 has a 60% risk of ovarian cancer; BRCA-2 has a 27% risk
• genes are involved in cell cycle control, DNA damage repair


5. Hereditary Nonpolyposis Colorectal Cancer (Lynch Syndrome)
• accounts for 2% of all colon cancers
• autosomal dominant with high penetrance
• caused by mutations in DNA mismatch repair genes
• cancers have a right-sided predominance that appear at an earlier age (median age of 45 years)
• extracolonic malignancies occur, especially of the ovary and endometrium


4. Regulation of Cell Death
• cell proliferation and involution rates must be balanced
• defects resulting in loss of normal apoptosis are associated with tumor formation
• induction of apoptosis may result from activation of death receptor (Fas) pathways or death receptor independent pathways (hypoxia, DNA damage, etc)
• apoptotic pathways involve the release of cytochrome c from mitochondria, which then activates various caspases (proteases)
• most tumors have defects in cell-death signaling pathways and are resistant to cell death
• bcl-2 oncogene is a major repressor of cell death
• p53 may induce growth arrest or apoptosis depending upon the cellular circumstances


5. Immortality
• except for stem cells, germline cells, and activated lymphocytes, normal cells have a limited replicative potential
• for a population of transformed cells to develop into a macroscopic tumor, the program that limits cell doublings must be turned off
• the number of doublings is controlled by telomeres, which prevent end to end chromosomal fusion
• each DNA replication results in telomeric shortening
• eventually, the progressive shortening of telomeres causes them to lose their ability to prevent chromosomal fusion
• chromosomal fusion inevitably leads to cell death
• telomerase, normally inactive in most cells, elongates telomeric DNA
• many tumors have an elevated telomerase activity
• if the telomeres are maintained above a certain critical level, the cell will have unlimited replicative potential and be immortal


6. Angiogenesis
• angiogenesis is a highly regulated process
• endogenous angiogenesis inhibitor factors prevent tumors from expanding beyond microscopic size
• angiogenic inhibitors counteract angiogenic signals from the tumor cells
• angiogenic activity is induced by growth factors such as VEGF, FGF, PDGF
• at least 26 different angiogenic inhibitors have been identified
• tumor growth occurs when angiogenesis stimulators overwhelm the angiogenesis inhibitors
• Avastin (bevacizumab), an anti-VEGF antibody, prolongs survival is patients with advanced colon cancer


7. Tissue Invasion and Metastasis
• distant metastasis is the cause of 90% of cancer deaths
• entire process is not well understood, but many steps must occur:
• tumor cells must detach from the primary tumor and infiltrate into the bloodstream or lymphatics (intravasation)
• tumor cells must survive immune destruction as well as other cellular defenses
• after arriving in the recipient organ capillary bed, the cells must extravasate into the organ parenchyma
• tumor cells must then develop a blood supply and evade host defenses



1. Tissue Invasion
• characterized by changes in the extracellular matrix (ECM) and its interaction with tumor cells
• cell-cell and cell-matrix interactions are mediated through cell adhesion molecules (CAMs)
• E-cadherin is responsible for cell polarity and organization of epithelium
• E-cadherin activity is lost in most epithelial tumors, and may be a prerequisite for tumor invasion and metastasis
• activation of extracellular proteases (collagenases, MMPs) are also critical to invasiveness and metastatic potential


2. Metastasis
• metastasis is not a random event: certain tumor cells (seed) have a propensity for a certain organ (soil)
• tumor cells may only grow in environments where favorable growth factors exist
• another theory proposes that endothelial cells in certain organs express adhesion molecules that bind tumor cells and permit extravasation
• chemokines expressed by target organs may selectively attract tumor cells that express receptors for the chemokines


8. Evasion of Immune Destruction
• much experimental and observational evidence demonstrates that the immune system can recognize and eliminate primary tumors
• however, the immune system fails to eliminate tumor cells with reduced immunogenicity, thereby selecting for tumor variants that have acquired immune evasion mechanisms


1. Elimination
• innate immune system is the first line of defense against transformed cells (macrophages, NK cells)
• activated innate immune cells produce IFN-γ, which has direct antitumor effects
• tumor lysis makes antigen available to trigger an adaptive immune response by T and B lymphocytes


2. Equilibrium
• innate and adaptive immune systems do not always completely eliminate the tumor
• some tumor cells in the tumor mass may have reduced immunogenicity and escape destruction
• equilibrium phase is characterized by a balance between tumor growth and tumor elimination


3. Escape
• over time, tumor variants may be selected that are able to escape immune detection and destruction
• host-related factors include treatment related immunosuppression, acquired or inherited immunodeficiency, and aging
• tumor-related escape mechanisms include loss of MHC alleles, reduced antigen processing and/or presentation, secretion of immunosuppressive factors (TGF-β, IL-10), stimulation of suppressor cells