2 What is Cancer?uncontrolled cell growth (as opposed to steady-state replacement of cells)usually accompanied by de-differentiation of cellscancerous mass = tumor or neoplasmNatural selection: cells which grow faster than others will take up more and more space.Our cells have multiple defenses against cells overgrowing their allotted locations.Cancer occurs when those defenses have been removed.starts with one transformed cell
3 Genetic PhenomenonCancer involves changes in DNA sequence i.e. it is geneticCancer is not epigenetic i.e. changes in patterns of gene expression without DNA changes.If cancer were epigenetic, it might be easier to reverse.Epigenetic changes, such as DNA methylation and histone modification, do occur in cancer, but they are rarely or never the underlying cause.
4 Cancer is a progressive disease Needs 5-6 mutations for full-blown cancer.Involves natural selection--in a slow-growing tumor, a faster growing mutant will take over.
5 Stages of CancerInitiation: A mutation that transforms the cell, leaving it capable of unrestrained growth.Promotion: No growth unless cell enters S phase (many cells are arrested and need a promoter, a mitogen, to get them started)Progression:Angiogenesis--invasion of tumor by blood vesselsInvasiveness--ability to penetrate basal membranes. Tumors that can't do this are benign, those that can are malignantMetastasis--ability to go through the blood and colonize other tissues
6 Gene Transfer Experiments Gene Transfer experiments are an approach to identifying oncogenesNormal fibroblasts will multiply in Petri dishes, but they have 2 specific properties of interest:contact inhibition: they stop growing when they touch, leading to a monolayer.finite number (50-60) of cell divisions before death
7 Partially Transformed Cells When transformed, cells lose contact inhibition (they pile up) and become immortal.NIH 3T3 mouse cells are partially transformed: immortal but still contact inhibited. That is, they grow in a monolayerHowever, mutagens, etc. will create foci (plural of focus) of piled up cells starting with a single transformed cell.Basis for oncogene assay.
8 GENES PLAYING ROLE IN CANCER DEVELOPMENT • Oncogenes• Tumor suppressor genes• DNA repair genes
9 - What are the genes responsible for tumorigenic cell growth? + ++ Normal+-Proto-oncogenesCell growthandproliferationTumor suppressor genesCancerMutated or “activated”oncogenes++MalignanttransformationLoss or mutation ofTumor suppressor genes
10 ONCOGENES Oncogenes are mutated forms of cellular proto-oncogenes. Proto-oncogenes code for cellular proteins which regulate normal cell growth and differentiation.
11 Activating OncogenesNormally, cellular oncogenes are proto-oncogenes: they have a regular cellular function and aren’t involved with cancer.Two basic ways of converting proto-oncogenes into oncogenes:mutate the proteinmake lots of the normal proteinThere are a variety of ways to accomplish these events.
12 Tumor suppressor genes Normal function - inhibit cell proliferationAbsence/inactivation of inhibitor --> cancerBoth gene copies must be defective
13 Tumor Suppressor genes A distinction:Oncogenes act in a dominant fashion: one mutant copy plus one normal copy gives a tumor.Tumor suppressor genes are recessive: one mutant and one normal is still wild type--need both copies mutant to give a tumor.Wild-type oncogenes (proto-oncogenes) promote cell proliferation; mutant versions enhance this property.On the other hand, tumor suppressors regulate and inhibit cell proliferation; mutant versions remove controls on proliferation.Tumor suppressor genes mostly found by cloning familial cancer genes and chromosome regions commonly deleted in tumor cells.
14 What are oncogenes and tumor suppresors? A lot of fundamental cell processes have been investigated as part of understanding oncogenes.Several basic types:growth factorsgrowth factor receptors at the cell surfacesignal transduction proteinstranscription factorscell cycle regulatory proteinsDNA damage detection and repair proteins
15 Cell Cycle ControlComplex and not fully understood yet. Many overlapping control systemsIn general a cell can: stay in interphase, divide, or undergo programmed cell death (apoptosis).Checkpoints: the cell cannot proceed past them until certain conditions are met.G1 -> SG2 -> mitosismetaphase spindle attachmentG1-S checkpoint. The main control point for cells with damaged DNAG2-M checkpoint. Cells must have completed DNA repair to pass this pointMitosis is initiated by the MPF (maturation promoting factor) protein complex, composed of cyclins and CDKs which have built up over the course of the cell cycle.
16 Genome IntegrityMutation is a constant problem. Many mechanisms prevent cells with seriously mutated DNA from dividing.Malignant cells usually undergo chromosomal rearrangements, leading to new fused genes and loss of heterozygosity.Spindle checkpoint. During mitosis, cells can only proceed into anaphase when all of the chromosomes are properly attached to the spindle. A protein complex on the kinetochore is displaced when the kinetochore is attached to the spindle.Telomerase. This enzyme prevents the loss of DNA at the ends of chromosomes, an inevitable consequence of replication. It is inactive in most cells, which results in them dying after 60 or so cell divisions. However, it is re-activated in 85% of successful tumor cells, resulting in cellular immortality. This is one of the most common markers of cancer.
17 DNA Damage DetectionSeveral DNA repair systems are tumor suppressor genes.BRCA1 and BRCA2: genes implicated in breast cancer. Also ATM, the ataxia telangiectasia protein. Part of a multi-protein BASC complex that scans the DNA for damageXeroderma pigmentosum: DNA damage caused by sunlight isn't repaired, leading to skin tumors.Several forms, involving nucleotide excision DNA repair enzymes.Hereditary non-polyposis colon cancer (doesn't form polyps).Autosomal dominantWhen the genomes of HNPCC patients were scanned for LOH, many micro-satellite loci had changes in repeat number, all over the genome.Related to E. coli mutator system MutHLS,Mutations in these genes increase mutation rate in E. coli up to 1000 x.Mismatch repair system: removes mismatched DNA bases on newly synthesized strand and re-synthesizes that stretch of DNA.Human analogue of MutS gene mapped to region of HNPCC gene, and turned out to be mutant in HNPCC patients.One human homologue of MutL is also responsible for much of HNPCC
18 Apoptosis: Programmed Cell Death After a certain number of generations (mitosis & cytokinesis) a normal cell exits the cell cycle and dies.Cancer cells escape apoptosisInstead of proliferating, differentiating, and functioning normally until they die, cancer cells proliferate rapidly and do not make the transition to apoptosis.
19 Cellular equilibrium Proliferation Death Differentiation Transit RenewingTransitProliferatingExiting
20 Cancer: disruption of cellular equilibrium ProliferationDifferentiationDeath
21 Cancer as Multi-step Process Progression of colorectal cancer (familial adenomatous cancer)Adenomas are polyps seen in the colon. They start as abnormal crypt cells, progress to benign polyps, and finally become cancerous.The following shows one way an FAP case may develop:Normal colon epithelium has mutation in APC tumor suppressor gene causing rapidly proliferating epithelium.Activation by mutation of KRAS (ras-K) leads to polyp formation.Loss of heterozygosity tumor suppressor gene in 18q (exact gene not clear) leads to late stage polyp.Mutation in p53 leads to carcinoma.