Transcript TSC2 - The University of North Carolina at Chapel Hill

TSC2 and Tuberous
Sclerosis
Charles G. Sproule IV
April 4, 2006
Biology 169
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Tuberous Sclerosis
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A Rare Genetic Disorder affecting 1 million
people worldwide
Abbreviated TSC
Discovered in the
late 1800’s
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Tuberous Sclerosis
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Incidence: 1 in every 6,000 Births
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1/3 of All Cases are Inherited
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Inherited as Autosomal Dominant
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2/3 of All Cases are Sporadic
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With proper treatment, most victims live
a normal life
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Clinical Manifestations
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TSC is characterized in multiple ways:
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Hamartomas in multiple organ systems
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Behavioral Manifestations
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Autism, Mental Retardation, Learning Disabilities
Seizure Disorders
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Most often occur in the Brain, Heart, Kidneys, Skin, and
Lungs.
Many patients are epileptic
Diagnosed by the “Clinical Triad”
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Seizures, Behavioral Manifestations, and
Presence of Benign Tumors (Mainly Hamartomas)
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Hamartomas
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Defined as a Common Benign Tumor
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Result from:
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abnormal formation
of normal tissue
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excessive
cellular growth
Growths are
called “Tubers”
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The TSC2 Gene
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Tuberous Sclerosis caused by a mutation in either the
TSC1 or TSC2 Gene
TSC2 Gene
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Discovered in 1993 via positional cloning
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Maps to Chromosome 16 (16p13.3)
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Evolutionarily Highly-Conserved Throughout Flies,
Mice, and Humans
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Encodes for the Protein
“Tuberin”
Very Large Gene
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Spans 41 Kilobases
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Has 41 Exons
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Tuberin Protein
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Characteristics:
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1807 Amino Acids
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190 kilo-Daltons
Expressed in most embryonic and mammalian
cells
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Found in Cytoplasm of Cells
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Has two main functional domains:
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Hamartin Interaction Domain
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GAP Domain
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Tuberin: A Closer Look
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Hamartin Interaction Domain:
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Responsible for Interaction with TSC1 Protein
Hamartin
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Hamartin and Tuberin form a complex in a
functional pathway
GAP Domain (GTPase Activating Protein)
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Very important clue as to function of protein
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GTPase Activating Protein
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Tuberin Pathway
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Tuberin Pathway: Simplified
Growth
Factors
Akt/Erk1
Hamartin
Tuberin
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Rheb
Components Downstream of Hamartin/Tuberin
mTOR
Complex:
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Rheb: Rheb-GTP activates mTOR
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mTOR: Directly Phosphorylates Transcription Factors
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Transcription Factors Cause:
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Protein Synthesis
Cell Growth
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Mutations in TSC2
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Various Genomic Mutations can occur in TSC2
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2/3 of all mutations found have truncating effect
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Small amount (<10%) are large genomic deletions
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Misense, Nonsense, and Insertions/Deletions usually
found
Truncating Mutation would mean a loss of the GAP
Domain Function
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TSC2 Mutations and Tuberin
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Mutation in TSC2 can result in:
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Loss of Function of Tuberin Protein
Looking at Tuberin, we Know That:
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Tuberin is a Rheb-GTPase
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Healthy Tuberin suppresses Growth
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Mutation could cause excessive cell growth
Therefore we know that Tuberin is a:
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TUMOR SUPPRESSOR GENE
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Loss of Function
Growth
Factors
Akt/Erk1
Hamartin
Tuberin
Rheb
mTOR
Normal Pathway
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Loss of Function
Growth
Factors
Akt/Erk1
Hamartin
Tuberin
Rheb
mTOR
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Mutation in either TSC1 or TSC2
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Hamartin/Tuberin Complex is not functional
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Loss of Function
Rheb
mTOR
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Rheb is now constitutively Active
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Rheb excessively activates mTOR
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mTOR activation yields excessive Protein
Synthesis and Cell Growth
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TSC2 Mutations and Cancer
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Excessive Cell Growth = Tumorigenesis!
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TSC2 Mutations cause excessive cell growth
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Hamartomas are caused by excessive cell growth
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Thus, TSC2 mutations cause Hamartomas
Both TSC2 genes must be mutated
Follows Knudson’s 2-Hit Model for Tumor
Suppressor Genes
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Homozygous Mice Knockouts
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Embryonic Lethal at age 10.5 Days
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Tuberin is thus essential for embryologic
development
Embryos exhibited:
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Unclosed Neural Tubes
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Thickened Myocardium
Thickened Myocardium is indicative of Tuberin
function in suppressing cell growth
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Homozygous Mice Knockouts
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(Above) Neural Crest
Malformation in Embryos at
10.5 and 11.5 Days
(Side) Failure of Neural Crest
to Close at 11.5 Days
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Homozygous Mice Knockouts
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Embryologic Myocardial Defects at age 12.5 and age 11.5 Days
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Note increased cell density
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Figures show excessive cell growth early on in embryos
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Excessive cell growth indicative of TSC2 Mutation
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Heterozygous Mouse
Mutations
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Germ Line Heterozygous Mutations in TSC2
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Causes renal carcinomas in mice
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Carcinomas are 100% penetrant
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Unknown reason for phenotypic discrepancy
among mice and humans
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Additional cancers were also found in some mice
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Cancers of Liver
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Brain Tumors
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Heterozygous Mouse Mutations
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Kidneys from a 7-Month
Heterozygous Mutant
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Note renal carcinoma
formation (White Spots)
Bottom Figure subjected to
carcinogens
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This induced earlier LOH
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More severe renal carcinomas
were found
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TSC2 Mutations in Humans
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TSC2 LOF Mutations cause Tuberous Sclerosis in Humans
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Deficient cell-growth signaling pathway
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Leads to benign tumors, or hamartomas, in multiple
organ systems.
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TSC Prognosis/Treatment
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Severity of TSC varies on a case by case basis
TSC2 mutations may cause more severe cases of
TSC
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Most live a normal life with routine checkups
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< 2% of TSC tumors become malignant
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Treatment Includes
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Anti-seizure Medications
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Intervention and Pharmacology for Behavioral Problems
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Removal Of Tumors
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Rapamyacin
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mTOR is a proto-oncogene downstream of the
TSC complex
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Once active, it promotes protein synthesis and
thus cell growth
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mTOR – mammalian target of Rapamyacin
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Rapamyacin is an mTOR inhibitor
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Rapamyacin is already widely used in
preventing immune system dysfunction
during surgeries
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Rapamyacin
Rheb
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mTOR
Rapamyacin
Rapamyacin binds mTOR, deactivating it
All Downstream signaling is halted, and thus
cell growth is contained
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Currently in Phase II Trials
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So far Results are good
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What Lies Ahead
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Continued Pursuit of Rapamyacin/ Pharmacology
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Fully Understanding Tuberin Function
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Main Function is suppressing mTOR and cell growth
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Depending on Cell Type, Hamartin/Tuberin may play
roles in:
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mTOR Inhibition
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WNT/β-Catenin Signaling
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Intracellular Trafficking
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References
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Brendan D. Manning, M. Nicole Logsdon, Alex I. Lipovsky, Derek Abbott, David J.
Kwiatkowski, and Lewis C. Cantley. Feedback inhibition of Akt signaling limits the
growth of tumors lacking Tsc2. Genes & Dev., Aug 2005; 19: 1773 - 1778.
Kobayashi T, Hirayama Y, Kobayashi E, Kubo Y, Hino O. 1995. A germline insertion
in the tuberous sclerosis (Tsc2) gene gives rise to the Eker rat model of
dominantly inherited cancer [published erratum appears in Nat Genet 1995
Feb9(2):218]. Nat Genet 9:70 –74.
Kobayashi, T., Minowa, O., Kuno, J., Mitani, H., Hino, O., and Noda, T. (1999).
Renal carcinogenesis, hepatic hemangiomatosis, and embryonic lethality caused
by a germ-line Tsc2 mutation in mice. Cancer Res. 59, 1206–1211.
Manning BD, Tee AR, Logsdon MN, Blenis J, Cantley LC. 2002.Identification of the
tuberous sclerosis complex-2 tumor suppressor gene product tuberin as a target
of the phosphoinositide 3-kinase/akt pathway. Mol Cell 10:151–162.
Tuberous Sclerosis Alliance. (http://www.tsalliance.org) 2005.
Yeung RS: Multiple roles of the tuberous sclerosis complex genes. Genes
Chromosomes Cancer 38: 368-375, 2003
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