Genetics of Cancer Lecture 7 Alterations in the Cell Cycle and Gene Mutations that Cause Cancer

How do we define cancer? Cancer is a group of disorders that causes cells to escape normal controls on cell division -cancer cells divide more frequently -cancer cells are not inhibited by contact with other cells and can form tumors -cancer cells can invade other tissues, a process called metastasis

Cancer cells grow into tumors. Non-cancerous cells form sheets. Cancer cells can invade other tissues.

Invasion and Metastasis Cancers spread through invasion and metastasis. Invasion is the direct migration and penetration of cancer cells into tissues. Metastasis refers to the ability of cancer cells to enter the vasculature and migrate to distant sites.

Control of the Cell Cycle Mechanisms for controlling progress through the cell cycle: Checkpoints Length of Telomeres Chemical Signals from within and outside the cell

Cell Cycle Checkpoints G1 S G2 cytoplasm doubles chromosomes replicate assembly of components for division cytokinesis P M A T Mitosis DNA Damage Checkpoints Apoptosis Checkpoint Spindle Assembly

Length of Telomeres telomeres Telomeres are structures at the ends of chromosomes that shorten with each cell division. After 50 divisions, the shortened length of telomeres causes mitosis to stop.

Failure to Stop at Cell Cycle Checkpoints Mutation in a gene that usually slows the cell cycle Rate of cell division is accelerated. Failure to pause for DNA repair Faulty DNA leads to unregulated cell growth. Loss of control over telomere length Cancer cells have telomerase, an enzyme that elongates telomeres. Cells continue to divide after 50 mitoses.

Chemical Signals that Control the Cell Cycle Cyclin and Kinase -proteins that initiate mitosis -requires buildup of cyclin to pair with kinase Hormones -chemical signals from specialized glands that stimulate mitosis Growth Factors -produced locally that stimulate mitosis

Apoptosis: Cell Death Caspase enzymes carry out cell destruction Signal arrives at “death” receptor on cell White blood cells destroy cell fragments

Genetic Pathways to Cancer Microsatellite Instability (MSI-H) Deficiency in DNA mismatch repair (MMR) HNPCC (MSH2, MLH1, MSH6, PMS2) 15% sporadic (hypermethylation of MLH1 promoter) Chromosomal Instability (CIN) APC, TP53, KRAS, Loss of Heterozygosity, aneuploid DNA content CpG Island Methylator Phenotype (CIMP) Methylation of CpG islands in promoter regions of tumour suppressor genes Sporadic CRCs demonstrating MSI are a subset

Genetic Mutations That Can Cause Cancer Oncogenes Formed when proto-oncogenes that promote cell division are improperly activated. May lead to increased expression of the gene in a new location production of fusion proteins with new functions

Oncogene Tumor Suppressor Gene A series of mutations is responsible for the development of FAP colon cancer.

The c-ras Gene The c-H-ras oncogene was identified by the transfection test (homologue to the Harvey strain of the rat sarcoma virus) The mutant c-H-ras protein has a mutation that impairs its ability to hydrolyze GTP. This keeps the mutant protein in an active signaling mode and causes it to stimulate cell division. Mutant versions of c-ras have been found in many types of tumors.

In cancer cells, the RAS gene product is locked into its GTP-binding shape and does not require a signal at the receptor in order to stimulate cell division Ras Proto-Oncogene In response to growth factor binding at receptor, the Ras gene product combines with GTP to promote cell division

The Philadelphia Chromosome found in patients with Chronic Myeloid Leukemia causes a fusion protein to be made from a combination of genes on chromosomes 9 and 22.

Chromosomal Rearrangements: Burkitt’s Lymphoma Burkitt’s lymphoma is associated with reciprocal translocations involving chromosome 8 and a chromosome carrying an immunoglobulin gene (2, 14, or 22). The translocations juxtapose c-myc to the genes for the immunoglobulin genes, causing overexpression of c-myc in B cells. The c-myc gene encodes a transcription factor that activates genes for cell division.

A Reciprocal Translocation Involved in Burkitt’s Lymphoma

Genetic Mutations That Can Cause Cancer Tumor Suppressor Genes Genes that inhibit cell division are inactivated. Mutation in a gene that halts the cell cycle in G1 causes retinoblastoma. Mutation in p53, a gene that promotes apoptosis if a cell has damaged DNA, leads to a variety of cancers. Mutation in BRCA1, involved in tumor suppression and DNA repair, leads to inherited breast cancer.

of other genes, including gatekeeper genes. “Gatekeepers” are the genes that directly control cell birth and cell death. RB, p53, PTEN, APC, BRCA1 and BRCA2 “Caretakers” do not directly control cell birth or cell death but rather control the rate of mutations of other genes, including gatekeeper genes. XP-A, ATM, hMSH1, hMLH2, hPMS1,hPMS2, WRN-H

Tumor growth kinetics is different for gatekeepers and caretakers When a gatekeeper gene is altered through mutation, the rate of cell birth exceeds that of cell death, and a tumor is initiated. When a caretaker gene is altered, the cell accumulates mutations at a high rate and the process of tumorigenesis is accelerated.

In Normal Cells, the Rb Gene Product Controls the G1 S Transition Rb = product of Retinoblastoma gene, inhibits action of E2F until chemically modified E2F = transcription factor required to activate genes for DNA synthesis CDK-cyclin (intracellular signal) modifies Rb so the E2F can mediate the G1S transition People prone to retinoblastoma have one mutated copy of the Rb gene (Rb-) and one normal copy (Rb+). Conversion of the Rb+ copy to Rb- by mutation leads to uncontrolled growth of retinal cells.

Knudsons 2-hit mutation model for RB

Structure of RB1 protein DNA tumor virus oncoproteins: SV40 large T, HPV E7 and Ad E1A also bind the A/B pocket and block RB function in cell cycle regulation

In Normal Cells, the p53 Gene Product Acts at the G1 S Checkpoint Preventing Entry Into S Phase If DNA Is Damaged p53 = transcription factor that causes p21 to be produced p21 inhibits intracellular signals that would activate EF2 Cells with damaged DNA do not pass the G1S checkpoint In cancer cells the mutated p53 gene product no longer stimulates p21 production. Cells will pass the G1 S checkpoint even when chromosomal damage exists.

In Normal Cells, the p53 Gene Product Stimulates Apoptosis If DNA Damage Cannot Be Repaired p53 gives an internal signal for apoptosis In cancer cells, a mutated p53 gene product no longer initiates self-destruction. Cells with damaged DNA can divide and more DNA damage can be accumulated. p53 is the most frequently mutated of all known cancer-causing genes, contributing to many types of cancer.

Genetic Mutations That Can Cause Cancer DNA Repair Genes Genes that promote DNA repair are inactivated. BRCA1 is a tumor suppressor involved in DNA repair. Faulty copies of BRCA1 cause inherited breast cancer. The disease Xeroderma Pigmentosum results from a defect in excision repair.

Knudson’s Two-Hit Hypothesis When tumor suppressor genes are mutated, a predisposition to develop cancer often follows a dominant pattern of inheritance. The mutation is usually a loss-of-function mutation in the tumor suppressor gene. Cancer develops only if a second mutation in somatic cells knocks out the function of the wild-type allele.

p53 is a Transcription Factor Most mutations in that inactivate p53 are in the DNA-binding domain (DBD) and impair its ability to bind enhancer sequences in its target genes. Mutations in this domain are “lost-of-function.” OD: homo-oligomerazation domain. Mutations in this domain are “dominant negative.” TAD: transcriptional activation domain

APC controls proliferation and differentiation of cells pAPC mutations are associated with adenomatous polyposis coli, which often leads to colorectal cancer. pAPC regulates the renewal of cells in the epithelium of the large intestine. Loss of pAPC function results in the formation of polyps. pAPC binds to catenin, which binds to transcription factors. Cells with mutations in pAPC lose their ability to control catenin levels. Familial adenomatous polyposis (FAP):rare autosomal dominant dissease.

APC controls proliferation and differentiation of cells

phMSH2 regulate genome-wide instability The phMSH2 protein is a homologue of the bacterial and yeast MutS protein, which is involved in DNA repair. Mutations in the hMSH2 gene are associated with hereditary nonpolyposis colorectal cancer (HNPCC), a dominant autosomal condition. Cells in HNPCC tumors exhibit genetic instability.

BRCA1 and pBRCA2 regulate DNA repair. Mutations in the tumor suppressor genes BRCA1 (Ch17) and BRCA2 (Ch13) have been implicated in hereditary breast and ovarian cancer. Both genes encode proteins that are localized in the nucleus and have putative transcriptional activation domains. pBRCA1 and pBRCA2 may be involved in DNA repair in human cells.