Transcript TSC2 GENE ENCODES FOR TUBERIN

TSC2 GENE ENCODES
FOR TUBERIN
By May Doan
Content
Disease: Tuberous Sclerosis
 TSC2 Gene Discovered
 General Roles within the Organism
 Molecular Roles within the Cell
 Mutations and Cancer
 Possible Treatments
 Summary
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Disease: Tuberous Sclerosis
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Autosomal dominant
Rare, multisystem disorder
Loci assigned at chromosomes
9 and 16
Characterized by:
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Cortical lesions called tubers
Benign tumors called
hamartomas arising from the
brain, heart, lungs, and
kidneys
Intractable epilepsy
Mental retardation
Autism
What causes tuberous sclerosis?
TSC2 Gene Discovered
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PFGE: 5 TSC-related deletions
were IDed at Chromosome 16.
Positional Cloning: These
deletions were mapped to a
120kb “candidate region” in
which four genes were isolated.
One gene, now TSC2, was
interrupted in all five deletions.
Northen blot analysis identified
either shortened transcript or
reduced expression in TSC
patients.
This confirmed that TSC2 (5.5 kb)
is the chromosome 16 TSC gene.
So what is TSC2?
TSC2 is a Tumor Supressor Gene
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Follows Knudson’s “two-hit” hypothesis
Loss of Heterozygosity through various mutations:
 Germline
insertions
 De novo deletions
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TSC2 is NOT TSC1
 Similarities
 Differences
What does TSC2 do within the organism?
General Roles within the Organism
Abnormalities Observed
in TSC pathology
Implied Role of TSC2
Cell size
Cell growth
Cell number
Cell proliferation
Morphology
Differentiation
Location
Migration
Picture
How does TSC-2 function molecularly? What are the mechanisms in which it executes
all these roles?
TSC2 is a Kinase and Molecular Switch
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Regulate many biological processes=Participates in
many signaling pathways
 mTOR
 WNT/B-catenin
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TSC2tuberin (kinase domain and GAP domain)
mTOR Pathway
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TSC2
a Rheb-GTPase that suppresses
mTOR signaling
suspends the phosphorylation of
p70S6K and 4E-BP1
thereby regulates cell proliferation
TSC2 MutationsCancer
Mutations on
TSC2 gene
Tuberin lossof-function
Constitutively
Active Rheb
Hamartomas
(as in Tuberous
Sclerosis)
Uncontrolled
Cell
Proliferation
Activates
mTOR
Cascade
Possible Treatment: Rapamycin
Mutations on
TSC2 gene
Tuberin lossof-function
Constitutively
Active Rheb
Harmatomas
(as in Tuberous
Sclerosis)
Uncontrolled
Cell
Proliferation
Activates
mTOR
Cascade
WEEK 2: extensive
tumor cell death
DAY 4: reduction in
tumor size
Additional Comments
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Have not documented a consistent complete
pathologic response to long-term rapamycin
treatment.
In fact, after two months, there was evidence of
active kidney disease.
Only short-term clinical efficacy with minimal
toxocity.
Still issues to be worked out
 optimal
dosing schedule
 duration of responses
 potential resistance
WNT/B-Catenin Pathway
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Observations/Correlations
CyclinD gene promoter is responsive to the effects of
WNT/B-catenin stimulation.
 CyclinD mRNA is elevated in cortical tumors like
hamartomas.
 inhibition of TSC2 expression by anti-sense oligos was
accompanied by increased cyclin D1 protein
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Conclusion
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May be a link between the TSC2 and the B-catenin
signaling pathway as a possible mechanism of cyclin D1 upregulation in TSC pathology.
WNT/Beta-Catenin Pathway
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Hypothesis: The level of B-catenin is correspondingly up-regulated by increased
protein stability
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Known
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This places the function of the tuberin at the level of the B-catenin degradation complex.
Experiment
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TSC1/TSC2 act downstream of DSH and upstream of B-catenin.
Conclusion
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Aka: if B-catenin is not degraded, it is overly stable, producing more than normal CyclinD
Co-immunoprecipitation analyses showed interaction between TSC2 and GSK3/Akin
amount of endogenous GSK3 associated with tuberin in 293 cells was reduced by
Wnt stimulation.
Conclusion
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TSC2 binds the GSK3/Axin complex and promotes B-catenin degradation. Upon Wnt
stimulation, DSH destabilizes the degradation complex and increases the free B-catenin
pool.
Summary
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Tuberous Sclerosis is an autosomal dominant disease
characterized by skin lesions and hamartomas.
TSC2 was discovered in 1993 through positional cloning.
TSC2 regulates many biological processes.
TSC2’s protein product, tuberin, is a kinase and molecular
switch that participates in many pathways
Pathway related to cell proliferation: mTOR
Rapamycin has had positive clinical results to treat TSC in
rats.
Pathway related to cell fate determination and migration:
WNT/B-catenin