Cancer Genetics

Cancer

• Disease characterized by uncontrolled cell growth– This causes tumors

• Tumors may be benign or malignant• Benign tumors grow in size but do not invade and destroy

surrounding tissue– These may still be deadly– Laryngeal papillomas for example can occlude the airway and cause

asphyxiation

• Cancer usually means malignant– Malignancy means a cell is invasive– Invasion destroys

• The internal cellular order of a tissue• Membranes and barriers that define organs

Malignancy

• Invasion is prerequisite to malignancy– Malignancy defined as the tendency to become worse,

or to progress in virulence– Cells which are invasive tend to spread

• This is metastasis• However an invasive tumor is not automatically metastatic

– Metastatic cells may migrate to other sites within the same organ

– Cells or groups of cells may travel through the vasculature or reticuloendothelial system to distant sites

The steps in invasion

• Initial mutation (initiation) alters genes resulting in growth

• Progressive growth (influenced by tumor promoters) creates more cells, each with a certain probability of mutating to more virulent state– Rapidly growing cells more prone to

mutation than quiescent cells– Mutations may occur in DNA repair

enzymes making other mutations more likely

– Mutations may also lead to genomic instability, fragmenting the genome• Each break means another mutation

• Mutant cells arise within the population of growing cells that are able to break through into surrounding tissues

Effects of mutations

• Up regulation of positive signals for growth• Down regulation of negative signals that suppress

growth• Elimination of signals that might stimulate

immunological attack or apoptosis (programmed cell death)

• Interference with the normal process of cell-cell binding

• Production of proteins that result in degradation of surrounding tissues

Tumor cell properties which may change during tumor progression

• antigenicity• growth rate• response to hormones• response to cytotoxic drugs• karyotypic abnormalities• capacity for invasion and metastasis

Germline versus somatic mutations that result in cancer

• Germline – Mutations in certain genes

• The mutation must not be so severe that it results in fetal death– Heritable

• Variable penetrance depending on mutation

• Somatic– Wider array of target genes

• Mutations that wouldn’t be tolerated during development can occur in somatic cells

– Sporadic appearance– Non-heritable

Environmental factors

• Experimentation and epidemiology indicate that cancer is caused by mutagens

• Most mutagens are carcinogens and most carcinogens are mutagens– Exceptions include tumor promoters such as TPA– This stimulates cell division without mutagenizing

• Identification of any particular agent is difficult

Unidentified environmental influences

• Japanese colon cancer rates rise and stomach cancer rates fall when Japanese move to America– Stomach cancer

• Helicobacter pylori?• Stress?

– Colon cancer?• Lifestyle – sedentary in US• Environmental exposures?

• Overall risk dependent on both exposure and genetic predisposition

Carcinogens• Mutagens = carcinogens generally• Works against specific genetic background• Two step experimental induction of cancer– Initiation = mutagenesis– Promotion = mitogenesis

Carcinogens(and UV, ionizing radiation, …)

• Common carcinogens act as both initiators and promoters– Polycyclic aromatic hydrocarbons –

cigarette smoke; soot; tobacco– Alkylating agents – mustard gas– Industrial agents – benzene; vinyl

chloride; asbestos– Metals – arsenic – Drugs – cyclophosophamide; DES– Food additives – Nitrosamines– Natural substances – Aflatoxin B1

• Promoters– Are not mutagens but provoke cell

division by activating positive growth signals

– TPA (PMA)

Human tumor viruses• These are an uncommon

cause of cancer– HTLV I and II – STD’s

• Retroviruses found in patients leukemia– HTLV I Adult T-Cell

leukemia/lymphoma– HTLV II Hairy Cell leukemia

• Infection only rarely results in disease

• Rare forms of leukemia • These are integrative viruses

– It’s genome inserts into chromosomal DNA as a necessary step in viral replication

– The integration site is therefore mutated

– Promoters on the virus can up-regulate transcription from proximal host genes

Viruses II

• Hepatitis B virus – may be STD’s– Chronic form may result in

hepatocellular carcinoma• Herpes virus – may be

STD’s– EBV provokes unregulated

growth in people chronically infected with Plasmodium• Burkitt’s lymphoma

– Kaposi’s sarcoma results from KSHV

Human papillomavirus

• HPV 16 and 18 – STD’s– Cervical cancer

• Unregulated expression of oncogenes E6 and E7 provoke tumors

• De-regulation is a result of integration of the virus such that molecules that regulate expression of E6 and E7 are destroyed

– Other HPV• Eg HPV-1 common warts;

HPV-11 laryngeal warts• Exist as episomes – not

integrated• Benign tumors (warts) that

may regress or be recurrent• Almost never progress to

malignancy

Types of Cancer• Epithelial tissue– Carcinoma

• Most common form of solid tumor in humans• Stem or basal cells are mutated• Not as common among animals

• Connective tissue– Sarcoma

• These are more common in animals and are often virally caused in them

• Others named for the cell type of origin– Glial cells – glioma– Melanocytes – melanoma– Neuroblastoma (from sympathetic nervous tissue

– most common in children), etc.

Leukemia• Abnormal proliferation and development of

hematopoietic cells in bone marrow and blood – This results in the expansion of cells that circulate in the

vasculature– Often in the form of blast cells, or undifferentiated cells of

hematopoietic origin– Classification

• Acute or chronic• Lymphogenous or myelogenous

– Suffixes -cytic verus -blastic refers to the maturity of the proliferating cell– A blast is a more immature form of a cell

• Leukemic or aleukemic– Aleukemic means there is no abnormal increase of cells in the blood

Lymphoma

• Cancer of the lymphoid tissue– Usually malignant– Named according to the predominant cell type

within the tumor or the discoverer• Burkitt’s lymphoma

– A B-cell proliferation confined to the lymphoid tissues– Also properly described as a "B-Cell lymphoma"

Types of cancer genes

Cancer Gene type

Transmission, effect How discovered

Cell types involved

Oncogenes Dominant, positive acting (gain of function)

Retroviral origins, transfection

Somatic, sometimes germline

Tumor Suppressor genes

Dominant or recessive, negative acting (loss of function)

Pedigrees mainly

Somatic and germline

Cancer is a genetic disease• Cell division accomplishes replacement of dead or sloughed

cells– Most cells are programmed to die as a result of normal terminal

differentiation• Apoptosis• Sloughing

• Genes affecting growth control may either turn cell division off or turn it on– Germline mutations may result in predisposition

• Often are Tumor suppressor genes– These are negative controls on cell growth

» Restrict cell division» Provoke apoptosis

– Somatic mutations may result in uncontrolled growth• Oncogenes

– These are positive signals to grow or inhibitory signals against apoptosis– Viral oncogenes interact with cellular cancer genes

Cell cycle• In G0 a cell imposes

control on cell growth• Restriction points on

DNA synthesis also exist in G1, S and G2 phases

• In order to enter S, a positive signal to divide must be received

• Negative effects on growth are thereby relieved

Families• How positive signals changed?– Hormone – autocrine loop

• Cell overproduces a signal that tells itself to grow– Sis – Platelet Derived Growth Factor beta subunit – Stomach

cancer – HST – FGF – Glioma

– Growth Factor Receptors• RET – Rearranged during Transfection – Multiple Endocrine

Neoplasia (MEN)• Erb B – (First identified in avian erythroblastosis virus)

– ERBB1 – EGFR homolog – glioblastoma – but viral oncogene– ERBB2 – (NEU) EGFR family – responsiveness to chemotherapy

in breast cancer – amplified in 20-30% breast cancers - over expression confers taxol resistance in breast cancer

– B3/B4 – no cancer connections

Positive acting Cancer Genes – Growth factor receptors

• Cells grow and differentiate under control of external signals

• Why would a cell divide?– It receives an external signal– It responds to that signal with division

• The signal is a growth factor– E.g. Epidermal Growth Factor

• Must have a specific receptor to respond– Epidermal Growth Factor Receptor

Nuclear• N-myc – related to v-myc and c-myc

• Avian myelocytomatosis virus • DNA binding protein

– amplification in neuroblastomas • Myb – related to v-myb

• Avian myeloblastosis viral oncogene • DNA binding protein

– malignant melanoma, lymphoma, leukemia• Fos – related to v-fos

• Finkel murine osteosarcoma• transcription factor of AP-1 complex

– Human osteosarcoma

DNA repair enzymes

• These are not oncogenes per se• They do not participate in cellular proliferative

signals• Instead they act to enhance the mutability of

the genome• This in turn increases the chances for

mutagenesis of cancer genes

Family History Flags of Hereditary Cancer

• Cancer in 2 or more relatives• Early age of diagnosis• Multiple primary tumors• Bilateral or multiple rare tumors• Existing mutation predisposes to cancer

Multistep evolution in cancer

Proto-oncogenes

• Induce telomerase activity• Block apoptosis• Stimulate proliferation• Increase blood supply

Tumor Suppresor Genes

• Gatekeepers: regulation of cell cycle; growth inhibition by cell-to-cell contact

• Caretakers: Repair DNA damage• Maintain genome integrity

Oncogene-caused cancer exemples

- Amplification of the same gene as a cause of oncogene-caused cancer (can be diagnosed by FISH)

- Chromosomal translocations as a cause of oncogene-caused cancer (Brc/Abl novel fusion gene that confers growth and survival advantage to Chronic Multiple Myeloma is a result of translocation)

- Common idea – overexpression or gain-of-function of oncogenes in cancer

Hereditary Syndromes of Oncogenes

• MEN2 – predisposition to thyroid cancer, GOF germline mutations

• Different mutations in RET gene (cell surface receptor tyrosine kinase)

• Different outcomes so genetic testing is important

Loss of second allele in cancer

Knudson hypothesis

Hereditary Syndromes of Tumor Suppresor Genes

• Retinoblastoma – Rb1 (gate-keeper gene)• Familial Adenomatous Polyposis (FAP)• Hereditary Nonpolyposis Colorectal Cancer

Syndrome• Hereditary Breast/Ovarian Cancer Syndrome

Knudsen two hit model for Retinoblastoma

• Model for heritable cancer• Rb-1 travels in families• Single hit in germline

predisposes for Retinoblastoma– results in heterozygosity for

all cells• Second hit in Rb necessary

to lose cell cycle control – This results in a Loss of

Heterozygosity– Specific mechanisms for the

second hit

Knudsen two hit model• Rb travels in families• Leukocoria is initial presentation

– Usual red reflection in an eye on photography is white

• Disease can be treated in bilateral form by removal of eyes– Radiation and chemotherapy can

eliminate tumors without loss of eyesight

• Cancer will reemerge in other tissues later in life– Notably as osteosarcoma

Normal Colon and FAP colon

Normal function of APC protein

Specific FAP phenotypes associated with specific APC mutations

• FAP is caused by germline LOF mutations in APC gene

Colorectal tumor DNA sample gel electrophoresis with microsatellite instability

Cell signaling in TGFbeta (defective in some cancers)

Hereditary Breast/Ovarian Cancer

• BRCA 1 / 2 genes• Associated with defective DNA repair complex

that repairs breaks and cross-links in DNA