Evolution at Its Worst: Cancer SHP-Neurobiology of Development and Disease
Introduction to Cancer Cancer is currently the second leading cause of death in the US (22.8%) behind heart disease. Yearly 0.5% of the population is diagnosed with cancer. Although we have learned a lot about cancer over the years, many cancers still do not enjoy a decrease in mortality/morbidity (including most brain cancers). Cancer is particularly difficult of a disease to treat because each case and each type have a different underlying profile of mutations. Also, being derived from human cells, they express all the same proteins as normal cells (though usually in aberrant amounts) which complicates the search for drug targets.
Cancer is a Disease of Corrupted Evolution: Normal cells proliferate until they touch one another. During wound healing, they can reenter the cell cycle and grow until they seal the wound. Adult stem cells are proliferative cells with multipotent potential that usually have a very highly controlled balance between proliferation and differentiation. In tumor cells, the regulation of these processes are damaged, therefore cancer is a disease of inappropriate self-renewal. Once initiated, the transformed cell(s) will undergo rapid mutation and subsequent selection against conditions in the body and those cells that attain enhanced proliferation/survival, nutrient procreation, colonization capacity or drug resistance will survive to multiply their numbers.
Conditions of Natural Selection If the organism can reproduce If offspring inherit traits from their progenitors If there is a variability of traits (usually a result of natural mutation) If environment cannot support all individuals of a population Those individuals with the most adaptive traits for that environmental state will out compete others to preferentially pass on their genes to the next generation
Mechanism of Oncogensis Overexpression of an Oncogene Loss of Tumor Suppressor Genes Become immortalized Overexpression or Autocrine Production of Growth Factors Loss of DNA repair and checkpoint control Inappropriate resistance to apoptosis
Mechanisms of Cancer Progression Invasion Metastasis http://www.joycesurgical.com/cancerinfo.htm
Invasion Initial tumor is produced from a clone of neoplastic cells that overgrow (hyperplasia) and ignore their tissue boundaries. Larger tumors produce angiogenic factors (ie VEGF) to cause blood vessels to grow in an provide vascular support to the tumor. Very invasive cancers can grow throughout other tissues causing increased pressure, intense pain, and hemorrhage. http://www.ma.hw.ac.uk/~jas/researchinterests/cancerinvasion.html
Metastasis
Step-wise Mutation Allow Cancers to Overcome Barriers Mutations happen throughout the progression of tumerogenesis and give rise to clonal lines with enhanced aggressiveness over their progenitors. Often a cell requires 6 or 7 mutations before it is transformed to a metastatic cancer
Syndrome Li-Fraumeni Syndrome Cloned Gene Chromosomal Location Tumor Types cell cycle regulation, apoptosis 17p13 brain tumors, sarcomas, leukemia, breast cancer cell cycle regulation 13q14 retinoblastoma, osteogenic sarcoma transcriptional regulation 11p13 pediatric kidney cancer catalysis of RAS inactivation 17q11.2 neurofibromas, sarcomas, gliomas linkage of cell membrane to cytoskeleton 22q12.2 Schwann cell tumors, astrocytomas, meningiomas, ependynomas signaling through adhesion molecules to nucleus 5q21 colon cancer 9q34 facial angiofibromas GTPase activation 16 benign growths (hamartomas) in many tissues, astrocytomas, rhabdomyosarcomas regulation of TGFβ/BMP signal transduction 18q21.1 pancreatic carcinoma, colon cancer transmembrane receptor involved in axonal guidance via netrins 18q21.3 colorectal cancer repair of double strand breaks by association with Rad51 protein 17q21 breast and ovarian cancer similar to BRCA1? 13q12.3 breast and ovarian cancer P53 tumor suppressor OMIM data Familial Retinoblastoma Function RB1 tumor suppressor OMIM data Wilms Tumor WT1 tumor suppressor OMIM data Neurofibromatosis Type 1 NF1 protein=neurofibromin 1 Syndromes Associated With Predisposition To cancer And their related genes OMIM data Neurofibromatosis Type 2 tumor suppressor NF2 protein = merlin or OMIM data neurofibromin 2 tumor suppressor Familial Adenomatous Polyposis APC tumor suppressor OMIM data Tuberous sclerosis 1 TSC1 OMIM data tumor suppressor Tuberous sclerosis 2 TSC2 protein = hamartin protein = tuberin OMIM data Deleted in Pancreatic Carcinoma 4 tumor suppressor DPC4 also known as Smad4 OMIM data tumor suppressor Deleted in Colorectal Carcinoma DCC tumor suppressor OMIM data Familial Breast Cancer BRCA1 tumor suppressor OMIM data Familial Breast Cancer BRCA2 tumor suppressor OMIM data http://web.indstate.ed u/thcme/mwking/onc ogene.html Peutz-Jeghers Syndrome STK11 tumor suppressor OMIM data protein = serine-threonine kinase 11 potential regulation of vascular endothelial growth factor (VEGF) pathway 19p13.3 hyperpigmentation, multiple hamartomatous polyps, colorectal, breast and ovarian cancers
Circuit Showing Many Signaling Pathways Controlling Tumerogenesis
Representation of Mutation Sequence Giving Rise to a Metastasis
Rb1 is a Classic Tumor Repressor Gene Rb1 is a tumor suppressor gene that is deleted in the pediatric cancer retinoblastoma Rb1 normally binds and inhibits E2F transcription factors and prevent entry into S-phase of the cell cycle. Rb1 is inactivated when hyperphosphorylated by cyclin dependent kinases (CDKs). When Rb is lost, there is a loss of checkpoint control and often precocious entry into S-phase
How Gene Can be Lost and Produce the Disease in Heterozygous Individuals
Telomeres Telomeres are long CG-rich repeats that occur at the ends of chromosomes. They prime the synthesis of chromosomal ends Repeats shorten as the life of the cell progresses and is thought to be central in senescence. Reactivation of telomerase in humans is thought to be a major step in immortalization of a cell line.
Loss of Telomeres Results in Genome Instability and Gene Duplication/Loss
Tumors Can Form from Many Neural Cell Types
Neural Tumors Isolate Themselves to Different Age Groups Pediatric Neural Tumors: Retinoblastoma: arises in the retina. Caused by a mutation in Rb1. Neuroblastoma: most common extracranial solid tumor in children. Arise from neural crest sympathicoblasts (sympathetic neural stem cells) Medulloblastoma: arise in the cerebellum and often resemble proliferating pluripotent postnatal external granule cells. Adult Neural Tumors: Astrocytoma: neoplastic astrocyte Oligoastrocytoma Glioblastoma multiforme (GBM): highly proliferative and metastatic cancer usually associated with a poor prognosis. The most common of all primary brain tumor cases.
WHO Grading System for Astrocytomas Type I: pilocytic astrocytoma (panel A) Type II: diffuse astrocytoma (B and C) Type III: anaplastic astrocytoma (D) Type IV: glioblastoma multiforme (E and F)
Neural Tumors Preferentially Form in Proliferative Areas of the Brain
Neurosphere Assay Is a Method to Test SelfRenewal Reynolds and Rietze, 2005
Neurospheres Differentiate Upon Plating Suspended Attached Differentiated
Further Evidence that Cancer is a Disease of Stemness Transfection of oncogenes by virus to stem cells but not differentiated astrocytes produces gliomas in rat. Neurospheres can be isolated from gliomas and these are positive for the stem cell markers bmi-1, musashi-1, Sox2, and nestin. Differentiating these neurospheres reproduces the marker profile of the cells found in the tumor.
Maturation-Arrest Theory Progenitor cells that differentiate into a specific cell lineage can lose the capacity to do so and divide continually Only a percentage of a tumor are actually proliferative and contain stem cells that produce the cell types of that tumor.
CD133+ Cells Are Tumerogenic It has been shown that purified CD133+ tumor cells generate tumors in a transplant recipient rat. CD133+ cells can be extracted from these secondary tumors and transplanted into new hosts, producing more tumors CD133- cells did not grow into tumors
There are many signaling pathways that have been shown to be deregulated in tumor lines
Brain tumor progenitors can be purified by the cell surface antigen CD133 This CD133+ positive population can be passaged as a neurosphere culture to test self-renewal and differentiation
Good Twin vs Evil Twin: Using Stems Cells to Target Cancer Treatment Since stem cells have been shown to migrate to sites of injury in the body, scientists have asked whether they can target and migrate to tumors. Many tumors secrete factors, such as EGF, that either stimulate their own growth (autocrine loop) or recruit cells to themselves. Stem cells are though to follow these signals to the source (tumor or injury). The recent evidence linking tumorogenesis to stem cell fate imply that these stem cells might be following these tumor factors because the share the fate (and therefore receptors, signal responsiveness) with their pathogenic cousins.
Stem Cells Target and Migrate to Brain Tumors in vivo A cluster of either fibroblasts (top panel, A) or neural stem cells (NSC, top panel, B) were plated on adherent tumor monolayer. Both seeded cells are labelled in blue. After 5 days, NSCs but not fibroblasts have migrated over the entire surface of the tumor. These investigators injected cancer cells (labeled in green with GFP, lower panel) to initiate a tumor into the brain of a rat, and then injected in labeled neural stem cells at another location. The injected neurons, irregardless of where they are injected, migrate in the direction of the tumor and colonize it. They mostly stall around the boundary of the tumor and normal tissue. Tumor appears in green. The migrating NSCs show up in red or blue.
Using Stem Cells for Localized Chemotherapy It is well known that chemotherapy treatment for cancer is poorly tolerated, due to its high toxicity to normal cells. These NSCs can be engineered with retroviruses to express the enzyme cytosine deaminase (CD), which can convert the nontoxic prodrug 5fluorocytosine to the oncolytic drug 5-fluorouracil. These cells can then function as a guided missile that colonize tumors and then convert the prodrug at high levels locally to the chemotherapy agent, destroying the tumor but sparing most of the normal tissue.c
Stem cells Expressing IL-4 Can Prolong the Lifespan of a Rat with GBM
Treatment of Rats with Glioblastoma Tumor with IL-4 Expressing Cells Causes Tumor Regression. Before Treatment (whiter mass is tumor) After Treatment (tumor has largely disappeared)
Interferon-producing Stem Cells Can also be Used to Treat Cancer. Another protein, interferon, is known to diminish proliferation and sensitize surrounding cells for death. This protein is often secreted from cells after they have become infected with a pathogen as a signal to their neighbors, and systemic treatment in patients comes with toxicity. Mesenchymal stem cells engineered to express and secrete interferon can colonize lung tumors and suppress tumor expansion.