Unit III Flashcards
5 properties of a malignant cancer cell
- unresponsive to signals for proliferation control
- de-differentiated
- Invasive
- Metastatic
- Clonal in origin
Describe the multi-step process of carcinogenesis
Cancer cells are the accumulation of mutations in the DNA over an individuals lifetime.
E.g. - somatic mutations early in life (such as from UV light damage) that mutate genes involved in the DNA repair pathway may lead to cells with accumulated DNA damage over time (genetic instability).
- Cancer cells are “selected for” in that, of the cells with damaged DNA, the cells that have mutations related to carcinogenesis survive and proliferate much more effectively than surrounding tissue.
- Cancer is not often considered to be inherited in a Mendelian fashion, but rather the susceptibility to cancer is inherited
Types of genes that are often mutated in tumor initation
Oncogene: drive cellular proliferation
Anti-oncogenes (tumor suppressors): inhibit cellular proliferation
Cytogenetic abnormalities associated with malignancy
- translocation/deletions that activate oncogenes or inactivate tumor suppresors (ex. CML)
- Loss of heterozygosity —> inactivation of tumor suppressors
Describe the inheritance pattern of retinoblastoma
While the disease is considered to have an autosomal dominant inheritance, the mechanism of expression of the disease is actually autosomal recessive.
- One mutated copy of the gene may through the rare event of mitotic recombination become expressed twice in the same cell (LOH) - this cell proliferates - is selected for - and leads to tumorogensis
- Sporadic cases are extremely rare! these events require two independent mutation events.
Rb gene (location in genome)
13q14
Mechanisms for loss of heterozygosity
Rare events!
- Mitotic recombination
- errant recombination during DNA repair (homologous recombination pathway)
- chromosome loss and subsequent duplication
- point mutation on wild-type allele
Biochemical properties of Rb protein
- Rb is hypophosphorylated in non-proliferating cells (G0 or G1 phase)
- Rb is hyperphosphorylated in rapidly proliferating cells (S or G2 phase)
- hypophosphorylated form inhibits entry of cells to S phase, phosphorylation by CDK2/cycE “inhibits the inhibitor” allowing cell to continue into S phase
Rb protein and cell cycle
Rb protein inhibits entry of cell into S phase - and is controlled by phosphorylation by CDKs. Mutation or loss of Rb protein lead to uninhibited growth (in absence of growth-factors, DNA checkpoint inhibition etc.)
Why does the Rb mutation affect the eye over other systems?
For cells in the eye, the Rb pathway is the only inhibition pathway for its cell cycle, whereas many other cells have backup mechanisms with genes in the Rb family (p107, p130, p53) - Fun fact - in mice, Rb deletion leads to 100% penetrant pituitary tumors!
What is the associated cancer of the APC gene?
Familial Adenomatous Polyposis
Describe the APC tumor suppression mechamism
Inhibitory protein of the Wnt signaling pathway - APC protein binds and signals for the degradation of the beta-catenin protein - a transcription factor that is activated in the Wnt pathway
- when beta-catenin is present in the nucleus it upregulates the oncogene c-myc
Therefore loss of APC protein –> higher levels of beta-catenin in the cell –> transcription of the oncogene c-myc —> unchecked proliferation of cells
-mutations of the APC gene tend to be clustered at the 5’ and 3’ ends of the gene.
BRCA1 and BRCA2 - tumor suppression mechanism
BRCA1 and BRCA2 are involved in the DNA repair pathway and in regulate checkpoint during DNA replication (damaged cells cannot proliferate until repaired) - mutations lead to accumulative damage that increases risk of developing cancer - particularly in breast cancers
-most BRCA1 mutations are frame shift resulting in a truncated BRCA1 protein. Mutational events near teh 5’ end are asociated with breast and less likely, ovarian cancer.
typical pattern of cancer inheritance mechanism - dominant vs recessive
- Dominant conditions (heterozygotes develop cancer) are typically gain-of function mutations in oncogenes
- Recessive conditions (homozygotes develop cancer) are typically loss-of-function mutations in tumor suppressor genes
Why p53 was originally incorrectly thought to be an oncogene
heterozygotes had the cancer phenotype! this pattern is seen because p53 forms a tetramer of 4 homologous subunits, and if 1 of these subunits is mutated then the whole protein will cease to function - in fact - mutant form of p53 is often more stable, exacerbating the effect
Oncogenic viruses - examples
Adenovirus, HPV
Ongogenic viruses, mechanism
These viruses have oncogenes that inactivate p53, and can also inactivate Rb protein. This is done in order to drive the cell to proliferate and produce viral proteins - and bypass of cells normal inhibitions is necessary for this process for some viruses
Oncogene discovery
Oncogenes were discovered looking at oncogenic retroviruses in animals
- virus inserts cDNA into host genome via reverse transcriptase in proximity to a proto-oncogene (example c-src)
- even if not necessary for viral reproduction, c-src may be transcribed and added to the viral RNA genome (now v-src)
- if mutation in this process occurs, v-src will become oncogenic
Many of these genes were tested in the laboratory by using the property of cancer cells called anchorage independence - aka cells with oncogenic properties can grow regardless of the medium of the culture, and in much higher density
Describe the mechanisms/effects of viral oncogenes
Viral oncogenes interfere with the pathways of cell growth in response to environmental stimulation via growth factors. The viral oncogenes can interfere in any part of this pathway. e.g.
- increase receptors that respond to GF
- increase effect of cytoplasmic signal transduction in response to GF
- create new receptors that bind a different ligand or bind ligand differently
Can either cause quantitative changes (increased number of proteins) or qualitative changes (altered function)
What does the v-src gene do?
v-src gene codes for a protein kinase that phosphorylates tyrosine –> which affects gene expression
What does the v-erb-B gene do?
v-erb-B gene codes for protein that mimics cell surface receptors for epidermal growth factor (EGFR)
Specifics: V-ABL
v-abl codes for protein kinase, similar in function to ABL gene seen in BCR-ABL CML.
Oncogenes as molecular markers for prognosis
Gene amplification of certain c-onc genes can be detected in some cancers, with increased amount of gene expression indicating a poorer prognosis. Underlines a quantitative mechanism of carcinogenesis. Cytogenetic analysis indicating translocation of c-onc genes to regions that increase expression also indicate poor prognosis (aka BCR-ABL translocation)
oncogenetic molecular markers example: N-myc
FISH analysis of the N-myc gene shows amplification in cases of neuroblastoma. Those with less than 10 copies of N-myc have a much better prognosis.