Question 1

What does PTEN do?

Accepted Answer

PTEN is a lipid phosphatase that dephosphorylates PIP3 back to PIP2, directly opposing PI3K kinase activity and suppressing the PI3K/AKT/mTOR signalling axis. This positions PTEN as the primary brake on the most commonly activated oncogenic pathway in human cancer.

Question 2

In which cancers is PTEN lost?

Accepted Answer

PTEN is lost or mutated in approximately 30% of glioblastomas and prostate cancers, and frequently in endometrial, breast, and thyroid cancers. Germline PTEN mutations cause Cowden syndrome, predisposing to breast, thyroid, and endometrial cancers through a two-hit mechanism.

Question 3

Is PTEN loss targetable with drugs?

Accepted Answer

Loss of PTEN results in constitutive AKT activation, creating selective dependency on the PI3K/mTOR pathway. PI3Kα inhibitors (alpelisib) and AKT inhibitors (capivasertib) are approved or in late trials for PTEN-loss tumours, though compensatory reactivation through receptor tyrosine kinases is a common resistance mechanism.

Question 4

What is the relationship between PTEN and AKT?

Accepted Answer

PTEN and AKT are in direct functional opposition: PTEN removes PIP3, the lipid second messenger that recruits AKT to the plasma membrane for activation. PTEN loss therefore equals AKT hyperactivation, driving downstream mTORC1 activity that controls protein synthesis, metabolism, and cell survival.

Question 5

How does PTEN loss differ from PIK3CA mutation in terms of PI3K pathway activation and druggability?

Accepted Answer

Both PTEN loss and PIK3CA activating mutations result in elevated PIP3 and constitutive AKT/mTOR signalling, but the therapeutic implications differ significantly. PIK3CA mutations create a hyperactive enzyme with an altered kinase domain that can be selectively inhibited by alpelisib (PI3Kα-selective). PTEN loss removes a phosphatase entirely — all PI3K isoforms (p110α, p110β, p110δ) now contribute unchecked PIP3 accumulation without a dominant druggable target. This means PI3Kα-selective inhibitors like alpelisib have reduced efficacy in PTEN-null tumours compared to PIK3CA-mutant tumours; pan-PI3K or AKT inhibitors (capivasertib) are more appropriate for PTEN-deficient disease.

Question 6

What is Cowden syndrome and what cancer risks does PTEN germline mutation confer?

Accepted Answer

Cowden syndrome (PTEN hamartoma tumour syndrome) is an autosomal dominant disorder caused by germline PTEN mutations, characterised by multiple hamartomas (benign overgrowths) and dramatically elevated cancer risks: ~85% lifetime breast cancer risk, ~35% endometrial cancer risk, ~35% thyroid cancer risk, and elevated colorectal and renal cancer risks. The facial trichilemmomas, oral papillomatosis, and macrocephaly are pathognomonic physical findings that prompt genetic testing. Surveillance recommendations include annual mammography from age 25, annual thyroid ultrasound, and annual endometrial sampling from age 30–35.

Question 7

Why is PTEN harder to target therapeutically than PIK3CA mutations?

Accepted Answer

The fundamental challenge is that PTEN is a tumour suppressor whose loss is the problem — you cannot inhibit an absent protein. The therapeutic approach must therefore target the downstream consequences of PTEN loss rather than PTEN itself. Strategies include: (1) AKT inhibitors (capivasertib) targeting the constitutive AKT activation downstream; (2) mTOR inhibitors (everolimus) for mTORC1-dependent growth; (3) pan-PI3K inhibitors that suppress all p110 isoforms contributing to PIP3 accumulation. All of these approaches face the feedback problem — mTORC1 inhibition relieves IRS-1 negative feedback and reactivates PI3K, creating compensatory AKT activation that limits efficacy.

PTEN Gene Function

Overview

More Tumor Suppressor Genes