Oncogenes and tumor suppressor genes

The discovery of oncogenes

1) Transformation in tissue culture and the search for oncogenes (Fig. 19.23b)

-          mouse fibroblasts 3T3 cells

-          take DNA from human cancer cells

-          add

-          about one in a million incorporates a bit of human DNA that transforms 3T3 cells

o   no contact inhibition

o   reduced requirements for growth factors

o   loss of attachment

-          what human gene was added to the 3T3 cells?

o   Mutant version of the Ras gene – cancers have a mutant version

o   Turns out that these 3T3 cells have a mutation in p16 (an INK4, inhibits cdk4/6-cyclinD, G1 cyclin-cdks)

o   Primary cultures won’t be transformed by ras

2) Src and Rous Sarcoma Virus (Fig. 19.23a)

-          1911 Peyton Rous realized that a virus can cause cancer in chickens

-          In the 70’s, people found that it was because of a specific gene in the viral genome

o   Called it v-src (because it causes a sarcoma)

-          In the late 70’s, Varmus and Bishop showed that v-src is a mutated version of a cellular gene

o   Called the cellular gene c-src

-          This was the first oncogene discovered, and this is where that term was coined

-          What is c-src?

o   Tyrosine kinase in the cytoplasm

o   This kinase is regulated, sometimes on and sometimes off

o   V-src has 18 amino acids deleted at the C-terminus, so constitutively active

-          v-src is called an oncogene

-          c- src is called a proto-oncogene

o   note that a proto-oncogene is just a normal human gene, but one that if mutated in the right way can push the cell one step into transformation into a cancer cell

-          Are there more cancer causing viruses out there, particularly for humans?

-          Lots of excitement about this, but only two human cancers known to be caused in this way, by introduction of oncogene

o   HTLV-1

§  Causes a T cell leukemia in adults

§  Oncogene is tax

·         Activator

·         Stimulates IL2 and IL2 receptor

§  Pretty much confined to Japan and Caribbean

o   HPV (human papilloma virus)

§  A sexually transmitted virus

§  Feb 2007 report says about 26% of US women have an active HPV infection

§  Can infect the lining of the cervix

·         Can cause cervical cancer

·         All cases of cervical cancer are caused by this virus

§  In the viral genome are two viral genes, E6 and E7

·         Bind p53 and RB proteins and inactivate them

§  These E6 and E7 genes are completely unrelated to any human gene, so they are true viral oncogenes

·         Probably evolved because the HPV replicates better in human cells that are actively going through mitosis and replicating cellular DNA

·         So if virus can stimulate replication and mitosis, better for the virus

Converting proto-oncogenes into oncogenes (gain-of-function)

-          What kind of mutations result in this conversion?

1.      point mutation – like Ras mutations that prevent GTP hydrolysis

2.      translocation that results in fusion of two proteins (bcr-abl Philadelphia chromosome)

3.      translocation that results in gene being brought under control of a different promoter

§  c-myc in Burkitt’s lymphoma, c-myc gene is brought under control of the heavy-chain antibody genes

§  in avians, avian leuksis virus (ALV) retrovirus inserts next to c-myc, activating it

4.      amplification of proto-oncogene

§  N-myc is amplified in neuroblastoma (and a number of other tumors) as much as 100 copies/cell

·         myc is a transcription factor

·         this is used clinically as a way to predict prognosis of tumors – amplification is bad

·         myc induces cyclin D and E and CDK4

·         other things too, but we are still trying to figure out what role, if any, these play in cancer

All act as dominant mutations

Tumor suppressors

Discovery

-          idea comes from cell fusion experiments

-          sometimes can fuse a cancer cell to a noncancer cell

o   the fused cell is noncancerous

o   idea is that there is a tumor suppressor that suppresses cancer in the cancer cell

Tumor suppressor examples

-          proteins that act as “brakes” on the cell cycle

o   RB protein

o   Cdk inhibitors like p16

o   p53

o   DNA repair proteins

§  HNPPC

·         Hereditary colon cancer

·         Mutations in proteins for mismatch repair cause this

·         Idea is that repair of mismatches formed during DNA replication doesn’t happen in these cells

·         So mutation rate is much higher

·         Makes it much more likely that you’ll produce mutations in other oncogenes or tumor suppressor genes

·         So mutations in DNA repair genes/proteins just makes it much more likely that you’ll get mutations in other proteins that directly control cell cycle

-          Most have been identified by positional cloning from tumor-prone families

Problems I liked….

14, 15, 17, ,21, 25, 27