Cell Cycle Regulation and Cancer Cancer Second leading

Cell Cycle Regulation and Cancer

Cancer • Second leading cause of disease in Western Countries • 1 million new cases per year in U. S. – 500, 000 per year die • War “declared” on cancer approximately 30 years ago • Slowly treatments are changing/improving based upon better genetic understanding of the varieties

Cancer Rates in US

Cancer is a Genetic Disease • Genome alterations – One nucleotide to large-scale chromosome rearrangements, amplifications and deletions – Mostly in somatic cells (unless associated with inherited risk—about 1% of total) – Alter cellular functions • DNA repair, cell division , apoptosis, cellular differentiation and cell-cell contact/communication

Normal and Cancer Karyotypes • Chromosome painting • (a) is a normal cell, (b) is a “very messed up” cancer cell

What is Cancer? • Large number of complex diseases • Behave differently depending upon cell type from which originate – Age on onset, invasiveness, response to treatment • Common general properties – Abnormal cell growth/division (cell proliferation) • If only this is a benign tumor • When grow in culture without contact inhibition are referred to as transformed – Spread to other regions of body (metastasis) • Malignant tumors

Clonal Origin of Tumors • Tumor arises from a single cell • Burkitt’s lymphoma – Translocation involving chromosome 8 (myc) and either chromosomes 2, 14, or 22 (near an immunoglobulin gene – All cells from a patient have breakpoints at exactly the same points as shown by DNA sequence analysis – Cancer cells in tumors of females all use same X chromosome (same one in Barr body)

Multistep Process • Cancer requires mutation of multiple genes • Age relationship with cancer consistent with this – If one mutation caused cancer then rate would be constant independent of age • It increases dramatically with age… • Delay between carcinogen exposure and onset – 5 -8 year delay between carcinogen exposure (Hiroshima and Nagasaki) and onset of leukemia – 15 year delay between tuberculosis X-ray treatment and onset of breast cancer

Age and Cancer • Note log scale for incidence rate

Multistep Process…Continued • Cancers often develop in progressive steps – From mildly aberrant cells to malignant – See figure 18 -3 – Process called tumorigenesis

Tumorigenesis of Cervical Cancer

Properties of Cancer Cells • Genetic instability – Mutator phenotype – Duplicating, losing and translocating chromosomes or portions of them common • Chronic myelogenous leukemia (CML) – Chromosome 9/chromosome 22 translocation – BCR gene fused to ABL (protein kinase) – Mutant signal transduction protein stimulates cells constantly to proliferate

Genome Instability • Double minutes (DMs) – Miniature chromosomes giving many copies of rgion • Homogeneous staining regions (HSRs) – Tandem gene duplications

Chromosomal Translocation in CML

Xeroderma Pigmentosum • Failure to remove pyrimidine dimers from DNA – Excision repair defect • Patients often develop skin cancer and must stay out of sunlight

HFNPCC • Hereditary nonpolyposis colorectal cancer • Higher than normal rates of colon (first noted) but also elevated rates of ovary, uterine and kidney cancers • 1/200 persons, autosomal dominant • Eight genes associated and four involve mismatch repair systems

HNPCC Pedigree • Colon, Stomach endometiral, pancreatic, bladder • Orange also other cancers, multiple slashes unknown cause of death

Defects in Cell Cycle Regulation • Cell cycle • G 1, S, G 2, M phases • Progression through cycle is regulated and specific blocks or checkpoints exist • Nondividing cell (quiescent) is in an extended G 1 phase called G 0 – Cancer cells never enter G 0

Cell Cycle

Cell Cycle Checkpoints • G 1/S – Monitors cell size and for DNA damage • G 2/M – Replication complete, DNA damage? • M – Spindle fibers connected, etc. ? • G 0 – Does body require more of my type of cell?

Regulators of Cell Cycle • Cyclins and cyclin-dependent kinases (CDKs) • Cyclins synthesized and destroyed in a precise pattern – A cyclin bind to a specific CDKs, activating it • Other proteins phosphorylated/activated • CDK 4/cyclin. D activate transcription factors for genes such as DNA polymerase delta and DNA ligase • CDK 1/cyclin. B trigger events of early mitosis (chromosome condensation, nuclear membrane breakdown, etc. )

Cyclin Levels

Activation of CDKs

Apoptosis • Programmed cell death, cell suicide • Pathway should be activated if “something goes wrong” – Especially involving DNA/chromosome damage • Involves proteases called caspases • Regulated by Bcl 2 and BAX – BAX homodimer promotes apoptosis, Bcl 2 homodimer blocks apoptosis – Some cancer cells overproduce Bcl 2 and are resistant to some chemotherapies and radiation treatment • Proteins involved in cell cycle checkpoints regulate pathway

Control of Apoptosis

Functions of Cancer Causing Genes/Alleles • Many disrupt control of cell cycle • Oncogenes – Proto-oncogenes • Normal genes that if mutated may act to make a cell cancerous • Recessive, cancer causing forms active and stimulates cell division • C-oncogenes and v-oncogenes • Tumor suppressors – Genes whose products act to regulate cell cycle – Loss of gene product function contributes to cancer process – Recessive, commonly involved with inherited risk • About 200 proto-oncogenes and tumor suppressor genes

Oncogenes/Proto-oncogenes • Cyclin D 1 and Cyclin E are proto-oncogenes – Often amplified or over expressed due to other mutations (e. g. translocation) in many cancers • cyclin. D 1 allows for DNA replication (S phase) • Over expression seems to contribute to cell’s progression from G 0 phase and begin division

ras Proto-oncogenes • Involved in signal transduction pathway – As are many proto-oncogene products • ras family genes mutated in 40% of all cancers • Involved in signal transduction pathway from growth factor receptor to nucleus – G protein – Mutant form lacks GTPase activity and remains active • See figure 18 -11

Ras Pathway • Growth factor binds receptor • Receptor exchanges GTP for GDP on Ras – Ras activated • Ras Raf Mek Map Kinase transcription factors genes turned on

Mutant Ras Protein • Single amino acid changes create N-ras and K-ras variants

p 53 Tumor Suppressor Gene • Mutated (inactivated) in more than 50% of all cancers • p 53 regulates (activates or represses) transcription of more than 50 different genes • p 53 regulated by Mdm 2 (prevents the phosphorylations and acetylations that activate inactive p 53) • Activated p 53 levels rise rapidly if DNA is damaged or repair intermediates accumulate

P 53 Function • Activated p 53 acts as transcription factor to turn on genes that – arrest the cell cycle so DNA can be repaired • Initiates synthesis of p 21, which inhibits CDK 4/cyuclin. D 1 complex, blocking entry into S phase • Genes expressed which retard rate of DNA replication • Other products block G 2/M progression – Initiate apoptosis if DNA cannot be readily repaired • Turns on Bax gene, represses Bcl 2 gene • Bax homodimers activate process of cell destruction • Cancer cells lacking p 53 do not initiate pathway even if DNA/cellular damage is great

RB 1 Tumor Suppressor Gene • Retinoblastoma 1 gene • Involved in breast, bone, lung, bladder and retinal cancers (among others) • Inheriting one mutated (inactivated) copy of gene increases chances of retinoblastoma formation from 1/14, 000 -20, 000 to 85% (plus increases other cancer rates) – Loss of second copy in a cell eliminates function – Normal cells unlikely to lose both good copies

p. RB Function • Tumor suppressor protein that controls the G 1/S checkpoint • Found in nucleus and activity regulated by level of phosphorylation (by CDK 4/cyclin. D 1 complex) – Nonphosphorylated version binds to TFs such as E 2 F, inactivating them – Free E 2 F and the other regulators turn on >30 genes required for transition to S phase

Familial Retinoblastoma

Inherited Predisposition for Cancer • About 1 -2% of cancer has an inherited or familial component – 50 different forms known at present • Inherited in Mendelian fashion but most all genes/alleles are recessive – Second copy must be mutated in a somatic cell • Called loss of heterozygosity (and loss of function) • Loss of second copy in germ line lethal • RB 1 and APC (lost in FAP, familial adenomatous polyposis) are examples of such genes

Multistep Development of Colon Cancer • APC loss causes cells to partially escape cell cycle regulation, DCC seems to be involved in cell adhesion and differentiation

Transforming Viruses • Viruses discovered to cause cancer in animals – Acute transforming viruses • Commonly but not always retroviruses – Rous sarcoma virus (RSV) discovered by Francis Peyton Rous discovered in 1910 as a causative agent of chicken sarcomas (solid tumors of muscle, bone or fat) • Many years later shown to be retrovirus • Nobel Prize in 1966 (link of viruses to cancer)

Retroviruses • ss. RNA chromosome • Chromosome copied to DNA by reverse transcriptase upon entry into cell • DNA integrated into host cell chromosome – Provirus • Provirus has strong promoter elements in U 5 and U 3 terminal sequences – U 5 expresses gag, pol and env • Oncogenic when – Integrate near proto-oncogene and cause inappropriate or over expression – Bring v-onc as part of viral chromosome

Retroviruses • Many transforming retroviruses are defective in the sense that one or more of gal/pol/env have been deleted to make room for the v-onc

Viral Oncogenes • Most v-onc genes have normal cellular counterparts – If simply mutated to the oncogenic form and not in a virus are called c-onc

Human Cancer-Associated Viruses • To date no acute transforming retroviruses have been discovered in humans – Viruses can contribute to but not be the sole cause of human cancer – However, up to 15% of all cancers have a viral association • Papillomaviruses HPV 16 and 18, hepatitis B virus, Epstein. Barr virus, Human T-cell leukemia virus are examples of cancer-associated viruses

Human Viruses Associated With Cancer • Non-retroviral varieties • Many of these v-onc genes act to stimulate the cell cycle (viruses needs host replication apparatus to multiply

V-onc Gene Product Action • Some v-onc gene products have their transforming effect by binding and thereby “taking out” certain tumor suppressor gene products – Cell division required to provide replication apparatus for virus – Bad, but does open some interesting treatment possibilities…

Environmental Agents and Cancer • Natural and man-made carcinogens – Chemicals, radiation, chronic infections • 30% of cancer deaths associated with cigarettes – Seems to preferentially mutate proto-oncogene and tumor suppressor genes • Red meat consumption – How cooked? • Alcohol-based inflammation of the liver • Aflatoxin (mold on peanuts) • UV light or ionizing radiation – Radon gas (up to 50% of radiation exposure? ? ? )