Lecture 25. Transformation and Oncogenesis. Flint et al, Chapter 18

• Cancer: a genetic disease• Results from growth of successive populations of

cells in which mutations have accumulated• Alteration of steps in regulatory pathways that

control cell communication and proliferation• Uncontrolled growth Cellular disorganization

Cancer• Approx. 20% of all human cancers are of viral origin.• Viruses are major causes of liver and cervical

cancers• Malignancy can result as a consequence of a side

effect of viral infection or of host response to virus.

Some cancer terms (Box 18.1)• Neoplasm: an abnormal new growth• Benign: a growth that does not infiltrate into surrounding tissues.• Malignant: Any disease of a progressive, fatal nature• Tumor: swelling; caused by abnormal cell growth not from

inflammation; can be benign or malignant.• Cancer: A malignant tumor; growth not encapsulated; infiltrates

into surrounding tissues; spread by lymphatic vessels to other parts of body; death caused by destruction of other organs, by extreme debility and anemia or by hemorrhage.

• Carcinogenesis: Complex multistage process by which cancer develops.

• Oncogenic: causing a tumor• Metastases: Secondary tumors derived from cells of primary

tumor that disseminated to other parts of the body.

Types of cancersCancers get their names based on the tissue of origin

•Adenoma: A cancer of hormone secreting cells. Many cancers of reproductive tracts.•Carcinoma: cancer of epithelioid tissue•Fibroblast: tissue derived from connective tissue•Fibropapilloma: Solid tumor of cells derived from connective tissue•Hepatocellular carcinoma: a cancer of liver epithelial cells•Endothelioma: any tumor, particularly a benign one, arising from the endothelial lining of blood vessels•Leukemia: A cancer of white blood cells•Lymphoma: a cancer of lymphoid tissue•Retinoblastoma: Cancer of retinal cells•Sarcoma: A cancer of fibroblasts

Transformed cells (Table 18.1)

Much of what we know about cancer is derived from studies of Transformed Cells

These have abnormal growth parameters and behaviors:• Immortality: can grow indefinitely• Reduced requirement for serum growth factors• Loss of capacity for growth arrest upon nutrient deprivation• High saturation densities• Loss of contact inhibition• Anchorage independent (can grow in soft agar)• Altered morphology (rounded and refractile)• Tumorogenic: can cause tumors when transplanted into

animals

Sensing the environment: (Fig. 18.3)

• Cells must sense what is going on around them

• Cell surface receptors interact with ligands

• Signal transduction cascades

• Second messengers• Activation and

repression of genes

The cell cycle (Fig. 18.4)

• Cell growth regulated by an internal timer: cell cycle

• Divided into 4 phases• G1: cell growth,

restriction point• S: DNA synthesis• G2: preparation for cell

division• M: Mitosis

Cell cycle control (Fig. 18.5)

• Rb protein: phosphorylation status of Rb used to control cell cycle– Rb phosphorylation: allows

passage of G1 restriction point, entry into S-phase

– Rb dephosphorylation: signals end of M phase.

•Cell cycle is controlled by the cyclin-Cdk machinery•Different cyclins and cyclin dependent kinases expressed at different stages of the cell cycle.

Oncogenic Viruses

• Cause cancer by inducing changes that affect cell proliferation

• Approx 20% of all human cancers causes by one of 5 viruses”– 1. Epstein-Barr virus– 2. Hepatitis B– 3. Hepatitis C,– 4. HTLV I– 5. Hum. Papillomaviruses

Oncogenic Viruses: a Genetic Paradigm for Cancer (Fig. 18.6)

• Study of viral transformation of cells laid the foundations for our current understanding of cancer.

• Enabled identification of Oncogenes and Tumor Suppressor genes

• Foundation for the genetic paradigm of cancer

History

• 1908: Ellerman and Bang show that avain leukemia could be transmitted through filtered extracts or serum from infected birds.

• 1911: Rous showed that solid tumors could be produced in chickens by using cell-free extracts from a transplantable tumor (Rouse Sarcoma Virus: the first discovered retrovirus)

• 1933: Shope isolates papillomavirus from warts• 1978: Bishop and Varmus define oncogene

Oncogenic viruses and cancer (Table 18.2)

Family Associated Cancer(s)

RNA viruses

Flaviriridae

Hepatitis C virus Hepatocellular carcinoma

Retroviridae Haemopoetic cancers, sarcomas, carcinomas

DNA viruses

Adenoviridae Various solid tumors

Hepadnaviridae Hepatocellular carcinoma

Herpesviridae Lymphomas, carcinomas, sarcomas

Papillomaviridae Papillomas and carcinomas

Polyomaviridae Various solid tumors

Poxviridae Myxomas and fibromas

Insertional mutagenesis • Integration of retroviral progenomes

mutates the genome of a cell.• Proviral promoters can activate

transcription of nearby genes.• Transformation can occur if the nearby

gene is an oncogene.– e.g. c-myc

• Transformation can also occur if insertion disrupts tumor suppressor genes.

Viral transforming genes

• 2 general strategies–Permanent activation of cellular

signal transduction cascades

–Disruption of cell cycle regulation

Viral transforming genesv-oncogenes (see Table 18.6, Figs. 18.7. 18.8) • Characteristic of transforming viruses• Cellular origin (Bishop and Varmus, Nobel Prize 1989)• Picked up by retroviruses• Typically fusions of viral + cellular genes• Viral sequences alter expression, regulation and

localization of gene products– e.g. overexpression of myc is sufficient to inducetransformation– e.g. v-erbB is a truncated form of the epithelial growth factor receptor. Expression stimulates growth of cells by mimicking the “on” state of the

receptor.

Viral proteins that alter cellular signaling pathways (Fig. 18.13, Table 18.8)

• Constitutively active viral receptors Of viral origin, do not resemble cellular proteins.

• Proteins specifically recruit and activate signal transduction pathways

• e.g. LMP-1 in Epstein Barr virus

(Fig. 18.13)

Viral adapter proteins that alter cellular signaling pathways

• See Fig 18.14, and Table 18.9

• Bind to cellular tyrosine kinases

• Permanently activates them

• Turns on cellular signal transduction pathways.

• e.g. mT protein of Polyomavirus activates c-Src tyrosine kinase

Fig. 18.14

C-SRC

Cell cycle Regulation by the Rb protein

Fig. 18.16

Transformation via cell cycle control pathways

• Inhibition of Rb function by viral proteins Many viruses actively inhibit Rb function

• Result: bypass of restriction point control• Passage from G1 S phase• e.g. SV40 LT, adenovirus E1A, HPV E7 proteins

•(Fig. 18.17)

Transformation via cell cycle control pathways

• Production of virus specific cyclins

• e.g. Human herpesvirus 8 v-cyclin

• Binds to and activates Cdk6 Rb phosphorylation

• Promotes G1 S transition

Inhibition of p53 functions (Fig. 18.20, 21)• p53 is a tumor suppressor gene• Determines response of cells to DNA damage and

hypoxia• p53 promotes either

– Cell cycle arrest (until problem is fixed)– Apoptosis (unfixable problem)

• Virus infection is a stress that turns on p53• Proteins from many viruses mislocalize or block p53• e.g. Adenoviruses, papillomaviruses, polyomaviruses

p53 regulation (Fig. 18.20)

Inhibition of p53 functions (Fig. 18.21)

Oncogenesis by hepatitis viruses (Fig. 18.22)

• Hepatitis B (Hepadnavirus), Hepatitis C (Flavivirus)

• Persistent infections• Sustained low level lever damage due to

immune system attack• Lots of cell proliferation/regeneration• Lots of cellular DNA replication + Lots of

oxidative stress• = Increased chance of mutation

Oncogenesis by hepatitis viruses (Fig. 18.22)