TUMOR SUPPRESSOR GENES: Retinoblastoma Gene, Knudson's Two Hit Hypothesis

1. TUMOR SUPRESSOR GENES
Insensitivity to Growth Inhibition

2. Learning outcomes
• Tumor suppression genes- general aspects
• Retinoblastoma gene
• Knudson’s two Hit Hypothesis
• Role of RB in regulation of cell cycle
• Mechanism of inactivation of RB
• Tumors associated with RB gene mutations

3. Tumor suppressor genes(TSG)
•Tumor suppressor proteins( products of TSG)
•Regulate cell growth by applying brakes to cell
proliferation ( Growth Inhibition)
•Failure of growth inhibition is seen in carcinogenesis
•Loss of function of these genes is a key event in
carcinogenesis

4. Gene Location Assoc. Human tumors
1. RB Nucleus(13q14) Retinoblastoma
Osteosarcoma
2. TP53 Nucleus(17p13) Ca lung,colon. head, neck &
breast
3. APC Cytosol Ca colon
4. Others
-WT1&2
-NF1&2
-BRCA1&2
Nucleus
Plasma membrane
Nucleus
Wilm’s tumor
Neurofibromatosis 1 & 2
Ca breast, ovary

5. Loss of Heterozygosity
Both Normal alleles of
Tumor Suppressor gene(TSG)
One normal allele(active)
another abnormal
allele(inactive) of TSG
Both abnormal alleles
( inactive ) of TSG
heterozygous
Loss of heterozygosity
Normal Function of
Tumor Suppressor Gene ( TSG)
Normal Function of
Tumor Suppressor Gene ( TSG)
Loss of Function of
Tumor Suppressor Gene ( TSG)
TUMOR
Deletion
Mutation

6. Retinoblastoma Gene ( RB Gene)
• First discovered Tumor suppressor gene
• Chromosome 13q14
• Retinoblastoma: is a childhood tumor with inactivation of this gene.

7. Knudson’s two-hit hypothesis
• Two mutations ( hits) involving both the alleles of TSG ( here RB
gene) is a basic requisite for the development of tumor
• Retinoblastoma can occur as hereditary or sporadic form
• Explanation for the same is given by Knudson’s two-hit hypothesis

8. Hereditary
Retinoblastoma
Sporadic
Retinoblastoma
1st Hit
Inherited
Mutation
2nd Hit
Acquired
Mutation
1st Hit
Acquired
Mutation
2nd Hit
Acquired
Mutation
Knudson’s two-hit hypothesis

9. RB Gene
• RB protein- product of RB gene
• Key role in regulation of the cell cycle
• “Governor” of the cell cycle

10. M
S
G1G2
Gap 1
Gap 2
Cells crossing this
will have to
complete mitosis
RB acts here !

11. E2F
Transcriptional Activation
G1 – S phase

12. Hypophosphorylated
RB

14. Histone Methyl
Transferase
Histone
Deacetylase
Chromatin Remodeling factors
Transcription
No Progression
to S phase
Hypophosphorylated
RB

15. E2F
Hyperphosphorylated
RB
Cyclin D- CDK4
Cyclin D- CDK 6
Cyclin E-CDK2

16. Transcriptional Activation
G1 – S phase
E2F

17. Hyperphosphorylated
RB

19. So, How does RB gets inactivated ?
Or
How does the Antiproliferative activity of RB is nullified?
1. Germline & somatic
Loss of function Mutations of RB Gene
2. Gene Amplifications of CDK 4 and
Cyclin D genes

20. 3. Functional inactivation of RB
• HPV 16
HPV E7 protein ( Viral Oncoprotein)
Binds to same RB pocket that sequesters E2F
Functional inactivation of RB
Release of E2F
Multiplication of cells
Cervical Carcinoma

21. Tumors associated with RB Gene mutations
1. Retinoblastoma
2. Osteosarcoma
3. Glioblastomas
4. Small cell carcinomas of lung
5. Breast cancers &
6. Bladder cancers

22. Summary
• Tumor suppression genes
• Retinoblastoma gene
• Knudson’s two Hit Hypothesis
• Role of RB in regulation of cell cycle
• Mechanism of inactivation of RB
• Tumors associated with RB gene mutations

23. http://ilovepathology.com/
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