Cancer

What is Cancer? uncontrolled cell growth (as opposed to steady-state replacement of cells) usually accompanied by de-differentiation of cells cancerous mass = tumor or neoplasm Natural 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

Genetic Phenomenon Cancer involves changes in DNA sequence i.e. it is genetic Cancer 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.

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.

Stages of Cancer Initiation: 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 vessels Invasiveness--ability to penetrate basal membranes. Tumors that can't do this are benign, those that can are malignant Metastasis--ability to go through the blood and colonize other tissues

Gene Transfer Experiments Gene Transfer experiments are an approach to identifying oncogenes Normal 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

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 monolayer However, mutagens, etc. will create foci (plural of focus) of piled up cells starting with a single transformed cell. Basis for oncogene assay.

GENES PLAYING ROLE IN CANCER DEVELOPMENT • Oncogenes • Tumor suppressor genes • DNA repair genes

- What are the genes responsible for tumorigenic cell growth? + ++ Normal + - Proto-oncogenes Cell growth and proliferation Tumor suppressor genes Cancer Mutated or “activated” oncogenes ++ Malignant transformation Loss or mutation of Tumor suppressor genes

ONCOGENES Oncogenes are mutated forms of cellular proto-oncogenes. Proto-oncogenes code for cellular proteins which regulate normal cell growth and differentiation.

Activating Oncogenes Normally, 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 protein make lots of the normal protein There are a variety of ways to accomplish these events.

Tumor suppressor genes Normal function - inhibit cell proliferation Absence/inactivation of inhibitor --> cancer Both gene copies must be defective

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.

What are oncogenes and tumor suppresors? A lot of fundamental cell processes have been investigated as part of understanding oncogenes. Several basic types: growth factors growth factor receptors at the cell surface signal transduction proteins transcription factors cell cycle regulatory proteins DNA damage detection and repair proteins

Cell Cycle Control Complex and not fully understood yet. Many overlapping control systems In 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 -> S G2 -> mitosis metaphase spindle attachment G1-S checkpoint. The main control point for cells with damaged DNA G2-M checkpoint. Cells must have completed DNA repair to pass this point Mitosis 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.

Genome Integrity Mutation 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.

DNA Damage Detection Several 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 damage Xeroderma 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 dominant When 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

Apoptosis: Programmed Cell Death After a certain number of generations (mitosis & cytokinesis) a normal cell exits the cell cycle and dies. Cancer cells escape apoptosis Instead of proliferating, differentiating, and functioning normally until they die, cancer cells proliferate rapidly and do not make the transition to apoptosis.

Cellular equilibrium Proliferation Death Differentiation Transit Renewing Transit Proliferating Exiting

Cancer: disruption of cellular equilibrium Proliferation Differentiation Death

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.