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Lecture 19

Genetics of CancerA cancer is a type of neoplasm.What is a neoplasm?A neoplasm can be defined as a disorder of cell growth that is triggered by a series of acquired mutations affecting a single cell and its clonal progeny. the causative mutations give the neoplastic cells a survival and growth advantage, resulting in excessive proliferation that is independent of physiologic growth signals (autonomous).

Robbins and Cotran: Pathologic Basis of Disease 9th Edition, 2015Not all neoplasms/tumors are cancerous

Benign tumorsaren't cancerous. They can often be removed, and, in most cases, they do not come back. Cells in benign tumors do not spread to other parts of the body.Malignant tumorsare cancerous. Cells in these tumors can invade nearby tissues and spread to other parts of the body. The spread of cancer from one part of the body to another is called metastasis.

Some cancers do not form solid tumors. For example, leukemia is a cancer of the bone marrow and blood, that are often referred to as liquid tumors.

http://www.cancer.gov/cancertopics/cancerlibrary/what-is-cancerWhat is Cancer?

Tumor: abnormal accumulation of cells not serving a physiological functionBenign tumor: NOT cancer, do not invade locally or travel distantly

Malignant tumor: Cancer, invade locally, and may travel to distant sites (not always)Metastasis: Process whereby cells leave a primary tumor and travel through the bloodstream or lymphatic system to a new location (distant sites).

most cancer deaths are due to metastatic diseasecancers have a predilection for certain organ sitesseed & soil hypothesismetastatic cells can remain dormant, then re-emerge later

Most common sties of metastasis are liver, lung, and bone5Figure 23.2 Abnormal proliferation of cancer cells produces a tumor that crowds out normal cells. (a) Metastatic lung-tumor masses (white protrusions) growing on a human liver

Cancers are typically diagnosed through pathological analysisBiopsy is obtainedTissue is fixed (usually formaldehyde) processed to remove water, paraffin is infused, slices (sections) are cutHematoxylin (stains nuclei blue) and eosin (stains cytoplasm pink) are most commonly used to visualize cellular architecture6Figure 23.2 Abnormal proliferation of cancer cells produces a tumor that crowds out normal cells. (b) A light micrograph of the section in part a showing areas of small, dark tumor cells invadinga region of larger, lighter normal liver cells. [Courtesy of J. Braun.]What do pathologists look for when analyzing biopsies?In general, a loss of differentiationDifferences in arrangements of cells, i.e. single layered epithelium becomes multilayeredCell shape changes, nuclear to cytoplasm ratio changesMorphology of the nucleus and/or chromatin changes Increase in frequency of mitotic cellsGrading depends on extent of differences from normalBoundaries are important, have cells crossed a structure that they do not normally?

https://visualsonline.cancer.gov/details.cfm?imageid=25128Carcinoma- cancer that begins in epithelial cells (of the skin that line or cover internal organs). There are subtypes of carcinoma, includingadenocarcinoma, which is a carcinoma of glandular origin in epithelial tissue. Most breast, prostate and colon cancers are adenocarcinomas

Sarcoma- cancer that begins in bone, cartilage, fat, muscle, blood vessels, or other connective or supportive tissue.

Leukemia- cancer that starts in blood-forming tissue such as the bone marrow and causes large numbers of abnormal blood cells to be produced and enter the blood.

Lymphoma and myeloma- cancers that begin in the cells of theimmune system.

Central nervous system cancers- cancers that begin in the tissues of the brain and spinal cord.What is Cancer?: Major Cancer Categories/Types

http://www.cancer.gov/cancertopics/cancerlibrary/what-is-cancerCancer is a term used for diseases in which abnormal cells divide without control and are able to invade other tissues. Cancer cells can spread to other parts of the body through the blood andlymphsystems.Cancer is not just one disease but many diseases. There are more than 100 different types of cancer. Most cancers are named for the organ or type of cell in which they start - for example, cancer that begins in the colon is called colon cancer; cancer that begins inmelanocytesof the skin is called melanoma.

9Estimated Cancer Deaths in the US in 2013

For many (perhaps majority) of cancers we do not know the etiological factors10Lung cancer is by far the most common cancer in men (28%), followed by prostate (10%), and colon & rectum (9%). In women, lung (26%), breast (14%), and colon & rectum (9%) are the leading sites of cancer death. . 580,350 deaths per year in the USA: 1450 747s down per year. That is 4 747s every day!

Cancer epidemiology: Role of environment: diet? infectious agents? Key attributes of cancer cellsThey grow (biosynthesize) when they should not, aided by a metabolism shifted from oxidative phosphorylation toward aerobic glycolysis.They go through the cell-division cycle when they should not.They escape from their home tissues (that is, they are invasive) and survive and proliferate in foreign sites (that is, they metastasize).They have abnormal stress responses, enabling them to survive and continue dividing in conditions of stress that would arrest or kill normal cells, and they are less prone than normal cells to commit suicide by apoptosis.They are genetically and epigenetically unstable.They escape replicative cell senescence, either by producing telomerase or by acquiring another way of stabilizing their telomeres.

How do cells acquire these attributes?Bold: amd12Cancer is a disease of DNA/chromatin alterations:both genetic and epigenetic changes occur that confer new attributes to cells and contribute to the evolution of a malignant phenotype

Cancer: failure to regulate cell number, involves increased cell division AND decreased cell death (and other changes!)Evidence that cancer is a genetic disease

DNA damaging agents increase risk for cancerRecurrent chromosomal abnormalities (some blood cancers are associated with chromosomal translocations)

Chronic myeloid leukemia: 90% of patients have reciprocal 9:22 translocationBCR-ABL is a tyrosine kinase that is the cause of CMLGleevec is a drug that inhibits this kinase and is curative in most casesTwo categories of cancers: hereditary vs. sporadic

Both are involve genetic changesIn hereditary, cancer risk is transmitted through germlineSporadic is not transmitted to offspringHereditary cases currently account for 5-10 of all casesInheritance of a familial cancer allele increases risk, does not always lead to disease Example: Breast-ovarian cancer syndrome 1 is associated with a mutant BRCA1 gene

Inheritance of mutant gene on Chromosome 13 greatly increases riskBilateral tumors often occur in patients with mutation Example: Retinoblastoma, rare childhood eye tumor

Knudsons two hit hypothesis for retinoblastoma; there must be a recessive gene that, in its wild type form, prevents retinal cancers.SporadicFamilial16Figure 23.3 Alfred Knudson proposed that retinoblastoma results from two separate genetic defects, both of which are necessary for cancer to develop.

Oncogenes and tumor suppressor genesOncogenes: dominant-acting stimulatory genes that promote cancer, usually by stimulating cell divisionProto-oncogenes: responsible for basic cellular functions in normal cells; when mutated, they become oncogenes.

Tumor-suppressor genes: recessive-acting inhibitory genes that generally slow or block cell division

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18Figure 23.5a Both oncogenes and tumor-suppressor genes contribute to cancer but differ in their modes of action and dominance.

19Figure 23.5b Both oncogenes and tumor-suppressor genes contribute to cancer but differ in their modes of action and dominance.

Knudsons two hit hypothesis for retinoblastoma; there must be a recessive gene that, in its wild type form, prevents retinal cancers.SporadicFamilial20Figure 23.3 Alfred Knudson proposed that retinoblastoma results from two separate genetic defects, both of which are necessary for cancer to develop.

How were oncogenes first discovered?Rous sacroma virus (RSV), early 20th century, retrovirus responsible for transmission of chicken sarcoma1970 Bishop & Varmus showed that mammalian cells have a src geneHow did RSV get a src gene?Retroviruses copy their RNA genome into DNA using reverse transcriptaseRSV genome integrates into host genome randomlyAt some point, an RSV genome lands next to the host src geneWhen the region is transcribed, the src open reading frame is accidently copied into the RSV genome

Discovering oncogenesExtract DNA from a tumorTransfect it into cells in culture in a petri dishNormal cells grow as a monolayerIdentify clumps of cells (foci) that can pile up on each otherGrow them up, characterize DNA that has been taken up to identify the oncogene

Proto-oncogenes: typically encode growth factors, growth factor receptors, GTP-binding proteins, & transcription factors23

Finding tumor suppressors: more difficult, often discovered as familial pre-disposition genes. Like Retinoblastoma gene (Rb)24

How Rb controls the G1-to-S transition25Figure 23.7 The RB protein helps control the progression through the G1/S checkpoint by binding transcription factor E2F.

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P53: a major tumor suppressorSignal-transduction pathways

External signal triggers a cascade of intracellular reactions producing a specific responseRas protein

Proto-oncogenes often encode component of signal transduction pathways 27

Ras: mutated in 20-30% of all cancers

Activating mutations leave Ras in on (GTP-bound) state

Transmembrane receptors:

Overrepexpression may allow signaling without growth factor binding

Mutation can make tyrosine kinase constitutively active28Figure 23.9 The Ras signal-transduction pathway conducts signals from growth factors and hormones to the nucleus and stimulates the cell cycle. Mutations in this pathway often contribute to cancer.Alterations in cell physiology that underlie malignant growth

BLC2 and the importance of death Follicular B-cell lymphomaFairly indolent diseaseTranslocation between Chr 18 and Chr 14Bcl2 gene comes under the control of an Immunoglobulin gene enhancerGet overproduction of Bcl2 protein

Alterations in cell physiology that underlie malignant growth: angiogenesis

Angiogenesis in an experimental model of subcutaneous cancer xenograftAngiogenesis33

Genentech angiogenesis cartoon34Angiogenesis FactorsVascular endothelial cell growth factorFibroblast growth factorMatrix metalloproteinasesSlide from Ed Gabrielson35The Immune System and CancerInflammation increases risk of carcinogenesisHepatitis and hepatocellular carcinomaHelicobacter pylori and gastric cancerAnti-cancer immune responseCytotoxic and memory T-cellsSlide from Ed GabrielsonHow Cancers are TreatedLocalized DiseaseSurgeryRadiationMedical therapiesTraditional cytotoxic chemotherapiesTargeted therapiesCombinationsMetastatic Surgery at timesRadiation oftenMedical therapiesImmunotherapies

DNA-damaging chemotherapies often increase risk for other cancers to develop later 38Why We're Losing The War On Cancer [And How To Win It] (FORTUNE Magazine, 2004)Clifton Leaf & Doris BurkeRobert Weinberg: "A fundamental problem which remains to be solved in the whole cancer research effort, in terms of therapies, is that the preclinical models of human cancer, in large part, stink." Mouse models of cancer: xenografts"99% of investigators in industry and in academia use xenografts." Why? "It is very convenient, easily manipulated "You can assess tumor size just by looking at it." V. Dixit, VP for Research in molecular oncology, Genentech, quoted in Fortune.

HUMAN TUMOR CELLS Shortfalls of xenograftsXenograft shrinkage is poor predictor of efficacy in humansCannot accurately replicate tumor microenvironmentMetastasis is exceedingly rareWhat are the alternatives?Genetically engineered mouse (GEM) modelsTechnologies to add or subtract genes from miceCancer molecular genetics and genome sequencingMake cognate mutations in mice Does it work? Rarely. Why not? Why is it hard to make good GEM models of prostate cancer ?Mouse geneticists, self-trained in rodent pathologyHuman pathologists, no formal training in rodent pathologyLack of essential genetic tools: tissue-specific regulatory elementsA different approachDevelop the necessary genetic toolsBring in human prostate pathology expertiseModel central oncogenic & tumor suppressor pathways most often altered in human prostate cancerMouse geneticists, self-trained in rodent prostate pathologyHuman pathologists, no formal training in rodent prostate pathologyAngelo De Marzo, MD, PhD

Hoxb13-Cre/PtenFl/Fl

Hoxb13/MYC

Hoxb13-MYC/ Hoxb13-Cre/PtenFL

Hoxb13/MYC|Hoxb13/Cre|PtenFl/FL

Hoxb13/Cre|PtenFl/FL

Hoxb13/MYC|Hoxb13/Cre|PtenFl/FL

FVB

CK18Poorly differentiated prostate adenocarcinoma