Tooth and nail syndrome: A genetic disorder characterized by the absence of several teeth at birth and abnormalities of the nails. The disorder is also known as hypodontia and nail dysgenesis or Witkop syndrome (TNS). It was first described by Dr. Carl Witkop in 1965.
The tooth and nail defects in the syndrome are highly variable. The number and type of congenitally missing permanent and/or primary teeth vary. The nails are thin, slow-growing, brittle and spoon-shaped (koilonychia). Toenails are usually more severely affected than the fingernails. In rare cases, the nails spontaneously separate from the nail beds or are absent at birth.
The gene responsible for TNS was identified in 2001 and is termed MSX1. A nonsense mutation in this gene in TNS appears to encode a protein that is completely nonfunctional. Another nonsense mutation in MSX1 has been associated with oral clefting in addition to tooth agenesis (absence of teeth).