SMAD4/DPC4:

A Tumor Suppressor

James Brooks

March 23rd, 2006

Found on chromosome 18 in humans, encodes 11 exons

A 552 amino acid protein found in the cytoplasm

SMAD Structure: MH1 and MH2 Domains

SMAD proteins exist as homo-oligomers

• The quaternary structure of SMADs consists of three monomers attached at their MH2 domains (homo-oligomers)

• MH2 domains are also responsible for hetero-oligomerization

SMAD4 is expressed at all stages of development

Above: Northern blot (RNA) of SMAD4 (3.8 kb) at various stages of development in mouse (L32 is a loading control)

SMAD4 mutant mouse embryos exhibit early embryonic lethality

• By day 6.5, SMAD4 mutant mouse embryos appear much smaller than wild type ones; the difference in size is attributed to reduced cell proliferation

• By day 7.5, SMAD4 mutant mouse embryos are lethal

SMAD4 is the convergence point of several TGF-β pathways

SMAD4 activity regulates cell proliferation in the TGF-β signaling pathway

Left: SMAD4 distribution in absence of TGF-β

Right: SMAD4 after translocation into nucleus following stimulation by TGF-β

SMAD4 activity regulates cell proliferation in the TGF-β signaling pathway

The Smad4 gene is often mutated in the MH2 domain

Familial Juvenile Polyposis (FJP)

• Clinical manifestation of germline SMAD4 mutation, phenotypically similar to that of familial adenomatous polyposis (FAP) from loss of APC function

• Autosomal Dominant inheritance pattern

• SMAD4 is mutated in approximately 50% of FJP cases

• Agrees with Knudson’s two-hit model

Above: A juvenile polyp

Smad4 mutation is often seen in late-stage malignancies that have metastasized

This makes sense!

To sum things up…

• SMAD4 is a tumor suppressor that regulates cell proliferation through interaction with transcription factors

• When inactive, it is located in the cytosol. It is translocated into the nucleus once activated

• Loss of SMAD4 is associated with familial juvenile polyposis, and is often absent in malignant pancreatic and colon carcinomas

References• Karyotype: http://www.amnh.org/learn/musings/FA01/ia/HW_01.jpg

• SMAD4 Crystal Structure: Shi et. al. A structural basis for the mutational inactivation of the tumour suppressor Smad4. 3 Jul 1997. Nature.

• SMAD MH1 and MH2 Domain Structure: http://ethesis.helsinki.fi/julkaisut/laa/haart/vk/roth/discussion.html

• Smad Mediated TGF-Beta Signaling Pathway: Miyaki et. al. Role of Smad4 (DPC4) inactivation in human cancer. 2003. Biochemical and Biophysical Research Communications.

• Smad4 Northern Blot: Sirard et. al. The tumor suppressor gene Smad4/DPC4 is required for gastrulation and later for anterior development of the mouse embryo. 1998. Genes and Development.

• Wild-type and Smad Mutant embryos: Sirard et. al. The tumor suppressor gene Smad4/DPC4 is required for gastrulation and later for anterior development of the mouse embryo. 1998. Genes and Development.

• TGF-Beta Superfamily Signaling Pathway: Waite and Eng. From developmental disorder to heritable cancer: it’s all in the BMP/TGF-β family. 2003. Nature.

• TGF-Beta Signaling Pathway: Lodish et. al. Molecular Cell Biology p. 957

• SMAD4 Localization Photos: http://biochemistry.utoronto.ca/research/signal_transduction_and_regulation.html

References, Continued• Mutations in SMAD4: Miyaki et. al. Role of Smad4 (DPC4) inactivation in human cancer. 2003.

Biochemical and Biophysical Research Communications.

• Juvenile Polyp: http://www.surgery.uiowa.edu/surgery/physpubs/familialjuvenilepolyposis.html

• SMAD4 in Cancer: Miyaki et. al. Role of Smad4 (DPC4) inactivation in human cancer. 2003. Biochemical and Biophysical Research Communications.