PTEN

What does the name stand for?

Phosphatase and tensin homolog deleted on chromosome ten. The first word describes the type of enzyme, one that strips phosphate groups off other molecules. Tensin is a protein the sequence resembles. The rest records where the missing region was first mapped. The older name PTEN tyrosine phosphatase misleads, because although the enzyme can act on proteins in a test tube, its work in a living cell is done on a fat in the cell membrane, and that is where its tumor suppressing power lies.

How does it act as a brake?

Protein receptors at the cell surface respond to growth signals by having an enzyme called PI3K add a phosphate to a membrane fat. That modified fat recruits the machinery telling the cell to grow, divide and resist death. PTEN removes the same phosphate, undoing the step and returning the membrane to its resting state. The pathway is therefore held in balance by two opposing enzymes, and the amount of active signal reflects which one is winning.

What happens when it is lost?

The brake comes off and the growth pathway runs without a signal to justify it. Unlike many tumor suppressors, PTEN is sensitive to dose: losing one of the two copies, or simply making less of the protein, is enough to shift the balance, so complete loss is not required. Deletion, mutation and silencing of the gene are all seen, and the result turns up across a wide range of cancer types, prominently in the lining of the uterus, the prostate, the brain and the breast.

What happens when it is faulty from birth?

Inheriting one damaged copy produces a group of overlapping conditions known collectively as the PTEN hamartoma tumor syndromes, of which Cowden syndrome is the best known. They feature benign overgrowths in skin, gut and other tissues, an unusually large head, and a raised lifetime risk of cancers of the breast, thyroid, uterus and kidney. The distinction matters because the surveillance offered differs from that following a sporadic tumor.