1. Phosphatase and Tensin Homolog Deleted on Chromosome 10
PTEN
Phosphatase and Tensin Homolog Deleted on Chromosome 10
Angela Lee
March 6, 2008

2. PTEN is a tumor suppressor discovered in 1997
It is located on Chromosome 10, at an area that frequently experiences loss of heterozygosity.
Genetics Home Reference, U.S. National Library of Medicine

3. PTEN Structure Structure of PTEN
The structure of PTEN gives us clues about the protein’s function.
A phosphatase domain is shown here in red, indicating that PTEN is a phosphatase.
The C2 domain in blue helps PTEN localize to the plasma membrane which is where PTEN does its work.
When they first looked at the structure of PTEN, they thought it was a protein phosphatase, because of the similarity in sequence with other protein phosphatases.
However, they were wrong…
Chow and Baker, Cancer Letters, 1-13 (2006)

4. PTEN is a lipid phosphatase
The main function of PTEN is to remove a specific phosphate from the D3 position on the PIP3 lipid, changing it into PIP2.
Whereas before, PIP3 could bind with the PH domain of Akt, after it is dephosphorylated by PTEN it cannot.
There is no functional redundancy for PTEN. This is probably why PTEN mutations are found in so many tumors.
No Functional Redundancy for PTEN.
Figure 6.19a The Biology of Cancer (© Garland Science 2007)

5. PTEN inhibits PI3K/AKT signaling
PTEN fits into the larger PI3K/AKT signaling cascade as an inhibitor.
When PTEN is missing, the PI3K/Akt signaling cascade is constitutively active.
The receptor tyrosine kinase stimulates the lipid kinase PI3K.
PI3K the creates the lipid PIP3.
PIP3 binds with AKT and recruits it to the membrane.
AKT is phosphorylated by PDK1 and is activated. That eventually leads to cellular changes.
PTEN acts as a negative regulator on this signaling by preventing the binding of PIP3 with AKT.
Sigma Aldrich

6. PI3K/AKT signaling cascade
Downstream of Ras.
PTEN
The PI3K/AKT signaling cascade is downstream of Ras.
Figure The Biology of Cancer (© Garland Science 2007)

7. PI3K/AKT signaling cascade
PTEN
Akt is the main downstream target of PTEN. You can see that active Akt prevents apoptosis, stimulates cell growth, and stimulates cell proliferation .
Figure The Biology of Cancer (© Garland Science 2007)

8. PTEN loss is not sufficient for tumor development
PTEN loss is insufficient for tumor development.
The loss of PTEN is not enough to develop tumors. In studies where both copies of PTEN were conditionally inactivated in mouse cells, there was no initial tumor formation. Over a period of time, other genetic alterations occurred, which led to tumor formation.
Inactivation of PTEN is not sufficient for tumor development, but it provides a good environment for tumor growth.
PTEN conditionally inactivated in mouse cells: no initial tumor formation.
Figure The Biology of Cancer (© Garland Science 2007)

9. PTEN and knockout mice PTEN -/- : Lethal
PTEN +/- : Viable, but gets cancer.
PTEN & knockout mice
Let’s look at PTEN in model organisms.
- In embryonic mice, complete loss of PTEN is lethal. They die at 7.5 days. Shows that PTEN is essential for normal development.
- Heterozygous mouse embryos are viable, but they spontaneously develop tumors in the endometrium, liver, prostate, GI tract, thyroid, and thymus. This mouse has two lymphomas. In many of the tumors, the wild type allele is damaged or lost.
Podsypanina, K. et. al, Proc. Natl. Acad. Sci USA, Vol. 96, 1563–1568 (1999)

10. PTEN regulates cell growth
Drosophila PTEN -/- cells are larger than wildtype cells
PTEN
Since complete loss of PTEN is lethal in mice, they looked at the same mutation in flies. They were able to this by making a mosiac eye in the fly, with a patch of mutant cells in an otherwise wildtype eye. Each cluster is about 30 cells, the PTEN mutant cells here are much larger than the wild-type cells (upper right corner).
H. Huang et. al, Development,126, (1999)

11. PTEN regulates cell growth
Drosophila PTEN -/- cells are larger than wildtype cells
Drosophila: AktDN rescues PTEN -/- lethality.
PTEN
Another experiment with flies showed that dominant negative Akt can rescue the lethality caused by a homozygous mutant PTEN.
Both of these results support the idea that PTEN regulates cell growth, and the results makes sense with the pathway.
H. Huang et. al, Development,126, (1999)

12. PTEN appears to have an effect on the cytoskeleton and cell migration
Loss of PTEN in mouse embryo fibroblasts  increased cell motility.
PTEN, PI3K, and PIP3 create a gradient  cell movement.
PTEN appears to have an effect on the cytoskeleton and cell migration
It’s been shown that loss of PTEN in mouse embryo fibroblasts  increased cell motility.
They also think creating this gradient with PTEN, PI3K, and PIP3 help the cell direct movement. In resting cells, PTEN is distributed uniformly at the membrane and PI3K is in cytoplasm. After stimulation, PI3K is activated, localizes at leading edge of cell and produces PIP3, and a pseudopod is formed. PTEN localizes at the rear and the sides.
Sulis and Parsons, TRENDS in Cell Biology, Vol.13 No. 9, (2003)

13. PTEN is mutated in a lot of cancers
PTEN is mutated in a lot of cancers, explain diagram
- Mutations in PTEN occur along the entire gene, no type of mutation is more common than the other.
- Mutations are most likely to occur in the part that encodes the phosphatase domain. Mutations here would take away PTEN’s lipid phosphatase capability.
Table 6.4 The Biology of Cancer (© Garland Science 2007)

14. PTEN and Cowden Syndrome
- 85% with Cowden Syndrome have an inherited PTEN mutation.
PTEN and Cowden Syndrome
Cowden Syndrome affects 1 in 200,000 people. 85% of those with Cowden Syndrome have an inherited PTEN mutation.

15. PTEN and Cowden Syndrome
85% with Cowden Syndrome have an inherited PTEN mutation.
- Autosomal dominant.
Multiple hamartomas- benign growths.
Cowden Syndrome is inherited in an autosomal dominant fashion. Inheritance of one bad copy makes almost 100% likely that the second copy will get hit too.
People with Cowden Syndrome often have multiple hamartomas: non-cancerous tumor-like growths usually found on the skin or in mucous membranes. These hamartomas result from a loss of heterozygosity, when the 2nd copy of PTEN is hit.

16. PTEN and Cowden Syndrome
85% with Cowden Syndrome have an inherited PTEN mutation.
- Autosomal dominant.
Multiple hamartomas- benign growths.
Greater risk for breast, thyroid, endometrium cancers.
- Increased surveillance
Furthermore, people with Cowden Syndrome have an increased risk of developing certain cancers because of the high probability of LOH in the second PTEN allele. There is an especially high risk for breast, thyroid, and endometrium cancers. The current management tool is increased surveillance, to catch signs of cancer as soon as possible.

17. Cowden Syndrome and Clinical Trials
Solid Malignancies for Breast Cancer and Cowden Syndrome
- Phase I/II
- Inhibitors of PI3K
PTEN
Clinical Trials
There are currently two clinical trials for patients with Cowden Syndrome. Both trials are for solid malignancies for Breast Cancer and Cowden Syndrome. They’re both in Phase I/II and both drugs are PI3k inhibitors.
There are 27 clinical trials for PTEN.