DiGeorge syndrome: the use of model organisms to dissect complex genetics.

The research interest in DiGeorge syndrome (DGS) is partly due to its clinical importance. However, fundamental questions of genetics and developmental biology related to DGS are inspiring investigators to experiment with model systems. Most DGS cases are caused by a heterozygous chromosomal deletion del22q11, and the search for haploinsufficient genes has been successful in mice and led to the discovery of Tbx1 as a major player in the development of the pharyngeal arches and pouches. Whether TBX1 is haploinsufficient in humans, as several other T-box genes are, is yet to be proven. The puzzling clinical variability in patients with del22q11 is also being addressed in model organisms. Consistent with clinical data, experiments in mice indicate that genetics can only explain part of the phenotypic variability. The recent identification of phenotypic modifiers further underscores the complex genetics of this syndrome.