TSC1/Hamartin and Facial Angiofibromas

Biology 169Ann Hau

Overview

• Tuberous Sclerosis Complex (TSC) & facial angiofibromas

• Gene: TSC1 • Protein: Hamartin• TSC1 mutation and cancer

TSC1 Hamartin

Mutation

X Truncated hamartin

TUMOR GROWTH

Tuberous Sclerosis Complex (TSC)

• Bourneville’s Disease• Development of multiple

hamartomas• Tumor growth in skin, kidneys,

brain, heart, eyes, lungs, teeth• Seizures, autism, mental

retardation

Tuberous Sclerosis Complex (TSC)

• Variable phenotypic manifestations• 1 in 6,000 live births; 1 million

people worldwide; 50,000 Americans

Facial Angiofibromas

• Manifestation of TSC in skin • Benign tumors of the face• Treat with laser treatment or

dermabrasion

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Mild facial angiofibroma

Severe facial angiofibroma

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Genetics of TSC

• Autosomal dominant disorder• Mutation via LOH in one of two known

tumor suppressor genes– TSC2 on chromosome 16– TSC1 on chromosome 9

• Transmittance– Genetic inheritance (1/3 of cases)– Spontaneous genetic mutation (2/3 of

cases)

TSC1 and RB: Similar LOH at Locus

Hereditary TSC: 1/3 of cases

Sporadic TSC: 2/3 of cases

Lodish, et al. Molecular Cell Biology, 4th ed.

TSC1 Gene

• Identification by positional cloning

• Location at 9q34• 23 exons • Encodes hamartin• Homolog of TSC1

transcript in yeast S. pombe

van Slegtenhorst, M., et al. Identification of the tuberous sclerosis gene TSC1 on chromosome 9q34. Science 277: 805-808, 1997.

Hamartin Protein

• 1,164 amino acids; 130 kD• Hydrophilic • Transmembrane domain• Coiled-coil region

TSC1/Hamartin: Normal function

• Tumor suppressor• Forms a functional complex with

TSC2/tuberin in multiple signaling cascades

• Regulates growth, proliferation, migration, and differentiation

TSC1/Hamartin: Normal function

• Hamartin-tubulin interaction– Hamartin binds to tuberin via coiled-

coil domain, inhibits tuberin ubiquitination (Benvenuto et al., 2000)

– Hamartin protein expression and hamartin-tubulin interaction detected throughout entire cell cycle (Miloloza, et al. 2000)

– Phosphorylation may be key regulatory mechanism controlling TSC1-TSC2 function (Aicher, et al. 2001)

Tee, et al. 2002. Tuberin-hamartin complex and mTOR pathway

TSC:GENES GONE WILD!

Mutations of TSC1

• 32 distinct mutations identified– 30 truncating, 1 missense, 1 splice

site mutation– Small deletions, insertions, point

mutations– No genomic deletions or genomic

rearrangements• Mutated in sporadic bladder tumors

(Hornigold, et al. 1999)

TSC Lesions

• Shared characteristics– Abnormalities in cell size, number,

morphology, and location– Suggests role of TSC genes in

proliferation, growth, migration, differentiation

EXPERIMENTAL EVIDENCE

What happens when there is no TSC1?

• Mutants of TSC1 homolog in S. pombe– Defective uptake of nutrients from

environment (van Slegtenhorst, et al. 2004)

• Mutants of TSC1 homolog in Drosophila– “Gigas” phenotype: increased cell and

organ size; relatively normal differentiation and morphology (Ito, et al. 1999)

Ito, et al. gigas, a Drosophila Homolog of Tuberous Sclerosis Gene Product-2, Regulates the Cell Cycle Cell. Vol 96, 529-539, 19 February 1999

Mouse Models

• TSC1 knockout mice via gene targeting (Kobayashi et al. 2001)

– TSC1 -/- mutants died around embryonic days 10.5-11.5; growth failure, anaemia

– TSC1 +/- mutants developed renal and extrarenal tumors that have a loss of wildtype TSC1 allele

Kobayashi, et al. A germ-line Tsc1 mutation causes tumor development and embryonic lethality that are similar, but not identical to, those caused by Tsc2 mutation in mice. Proc. Nat. Acad. Sci. 98: 8762-8767, 2001.

Mouse Models

• TSC1 and TSC2 knockouts– Similar phenotypes suggest that a

common pathway for TSC1/2 products exists

– Heterozygous TSC1 and TSC2 mice have increased astrocyte proliferation: hamartin and tuberin are important growth regulators for astrocytes

Uhlmann, E., et al. Heterozygosity for the tuberous sclerosis complex (TSC) gene products results in increased astrocyte numbers and decreased p27-Kip1 expression in TSC2 +/- cells. Oncogene 21: 4050-4059, 2002.

Summary• TSC pathology implies role of TSC1 in

regulating growth, proliferation, differentiation, and migration

• TSC1 -/- mice phenotype indicates role in embryonic development of brain

• TSC1-TSC2 complex acts as a tumor suppression in the mTOR-S6K pathway

Summary• TSC1 is a negative regulator of

growth• TSC1 mutation causes cells to grow

faster: increased translation through constitutively active mTOR pathway

• Overgrowth of cells leads to growths of TSC; cells are growing when they shouldn’t be!