Rare genetic causes of hypotrichosis
More than 300 genetic conditions exist that have hair abnormalities as one component. The hypohidroticectodermal dysplasias are a group of rare inheriteddisorders characterized by sparse hair, scanty sweat glands, and poor development of the nails and teeth. (Figs 13.13 and 13.14). Heat stroke may follow inad-equate sweat production. One type is inherited as an X-linked recessive. The responsible gene for this type (on chromosome Xq12) has recently been shown to encode for a protein (ectodysplasin) involved in the regulation of ectodermal appendage formation. The genes responsible for the dominant/recessive types encode for the ectodysplasin receptor.
In other inherited disorders the hair may be beaded and brittle (monilethrix); flattened and twisted (pilitorti); kinky (Menkes’ syndrome caused by muta-tions in a gene encoding for a copper transport-ing membrane protein); like bamboo (Netherton’ssyndrome, caused by a gene on chromosome 5q32encoding a serine protease inhibitor); partly broken in many places (trichorrhexis nodosa); ‘woolly’ or ‘uncombable’.