People suffering from TTD (trichothiodystrophy), a rare inherited disease, have brittle hair and nails and flaky skin. Some of them suddenly lose all their hair when they run a high fever, although it grows back later. Researchers from the Netherlands and Denmark have now found a molecular explanation for this strange phenomenon.

TTD is caused by a defect in a protein-encoding gene, XPD, that has a dual function: it is part of the machinery that transcribes DNA into RNA (which then makes protein), and it helps to repair DNA that has been damaged by ultraviolet (UV) light. The first function is especially impaired in TTD patients. As a consequence, they do not make enough of certain proteins that link keratin, the main component of hair, nails and skin, which then become brittle.

In this months Nature Genetics the researchers show that two patients who lost all their hair during a fever had the same mutation in the XPD gene. This mutation not only reduces the amount of XPD protein in their cells, but it further reduces transcription when the temperature is raised. Scientists call this a "temperature- sensitive mutant" because the mutation shows its full effect only above or below a certain temperature. For the patients the mutation means the following: When they get a fever, transcription works even less effectively than it normally does, leading to a further reduction in cross-linking proteins. Their hair becomes very fragile near the roots and breaks off.