Brigette Hales
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Transcript of Brigette Hales
![Page 1: Brigette Hales](https://reader036.fdocuments.net/reader036/viewer/2022062400/577cc5e21a28aba7119d5337/html5/thumbnails/1.jpg)
Roles of Oncogenes in Proliferation
Expression of c-Myc, c-Fos and c-Jun in Hepatocellular Carcinoma
Yuen, et al. 2001
Presented by: Brigette Hales
March 18th, 2002
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Cell Cycle and Proliferation• Controlled and regulated by several
factors• Has a checkpoint mechanism to halt
progress of cycle in case of:– failure to properly complete a stage– DNA or chromosomal damage
• Failure of surveillance system to detect abnormalities cell death or formation of cancer cell
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Cell Cycle Continued...
• Certain cell cycle inhibitors present for surveillance can also prevent uncontrolled growth of cancer cells.
• Mutations of these can give rise to tumours.
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Normal Cell Cycle
R. Huskey, 1999
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Cell Cycle Checkpoints
R. Huskey, 1999
p53 acts prior to DNA replication by detecting DNAdamage and delaying entry into S until the damage has been repaired.
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Tumour Suppressor Genes and Oncogenes
• TSGs encode proteins that restrain uncontrolled cell growth and prevent malignancy in cells– ex: p53
• Oncogenes encode proteins that promote loss of cell cycle control, inhibition of apoptosis and malignancy in cells– ex: c-myc, c-fos and c-jun
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Where do oncogenes come from?• Proto-oncogenes are genes in normal cells which
encode proteins that have normal function in cells• Proto-oncogenes Oncogenes via:
– mutations causing altered properties of the proto-oncogene product, inhibiting its normal activity
– mutation of regulatory sequences leading to overexpression of the proto-oncogene
– incorporation of foreign DNA causing altered expression or altered proto-oncogene product
– some may arise from chromosomal translocations
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Types of Oncogenes
• Four categories based on their gene products: – growth factors or their receptors – cytoplasmic protein kinases – nuclear transcription factors (ex: c-myc)– products that regulate apoptosis (block/induce)
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Role of p53• Encodes a transcription factor that activates
expression of genes involved in cell cycle control• Regulatory roles include DNA damage control,
cell cycle arrest (G1 checkpoint) and induction of apoptosis
• wild type p53 can be induced by c-myc • p53 activity can be repressed in cancer cells via:
– excessive methylation of the p53 promoter– inactivation of protein product by inhibitory protein
MDM2
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Requirements for Altered Cell Growth
• Oncogenes act dominantly, needing only one gene copy to generate the altered phenotype
• Loss of cell growth control therefore requires:– mutations in both copies of tumour-suppressor genes– mutation of one copy of the proto-oncogene
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Alberts, et al. 1998
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Expression of c-Myc, c-Fos and c-Jun in Hepatocellular Carcinoma
• Objectives:– To evaluate the expression of these oncogenes
in patients with Hepatocellular Carcinoma (HCC).
– To elucidate the mechanism of hepato-carcinogenesis with regard to the expression of these oncogenes.
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What are they?• c-myc, c-fos and c-jun:
– oncogenes which encode transcription factors. – myc encodes a DNA-binding protein– fos and jun each encode a component of the
transcription factor AP1 • c-fos required for quiescent cells to enter cell cycle• c-jun is also involved in apoptosis
– overexpression due to mutation leads to unregulated cell proliferation and cell malignancy
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Materials and Methods:• Immunohistochemical staining of tumour
and adjacent non-tumour tissues using monoclonal primary antibodies against gene products of c-myc, c-fos and c-jun.
• Bcl-2 also traced using immunofluorescent antibodies
• Similar antibodies used for detection of mutated p53
• also used antibodies for detection of other phenotypic markers
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Immunohistochemical staining of HCC tissue samples
A) Expression of c-myc
B) Expression of c-fos
C) Expression of c-jun
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Results
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Results
• Negative association between expression of c-myc and mutated (i.e.: marked) p53
• expression of c-myc in tumour tissue was inversely proportional to the grade of differentiation of the HCC samples
• no association between expression of c-myc and bcl-2
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Discussion of c-myc results• Higher levels of expression in adjacent non-
tumour cells indicates RELATIVELY decreased c-myc in tumour cells.
• c-myc may serve as a checkpoint in cellular proliferation (as is p53)
• down-regulation of c-myc may therefore cause dysfunction of the checkpoint
• decreased c-myc expression was accompanied by an increase in mutant p53 expression jeopardizing apoptosis of altered cells
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Discussion continued...
• Reduced expression of c-myc is secondary to the poor differentiation of tumour cells, rather than causing their poorly differentiated phenotype.
• No role is played by c-myc in triggering the AP1 pathway in tumour cell proliferation (i.e.: no correlation between c-myc and c-fos/c-jun)
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Results• No association found between the expression
of c-fos/c-jun and p53 or bcl-2 • c-fos expression is greater in HCC tissue
compared with non-tumour tissue• c-jun expression is high in both types of
tissue• possible coordinated expression of c-fos and
c-jun in HCC tissue.
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Discussion of c-fos and c-jun results
• Possibility of coordinated expression of these two oncogenes in tumour tissue may reflect coordinated tumour cell progression and cell proliferation
• may be responsible for rapid tumour growth: a characteristic of HCC cells
• exact significance of coordinated expression remains to be determined
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Future Directions
• Confirmation of coordinated versus uncoordinated expression of c-fos and c-jun
• determine what triggers expression of c-myc, c-fos and c-jun oncogenes
• determine functional role of their gene products in the progression of hepato-carcinogenesis