Disorders of left-right asymmetry: heterotaxy and situs inversus

Cilia function is critical to the development of proper organ laterality. Primary ciliary dyskinesia (PCD) causes randomization of situs. Heterotaxy, or situs ambiguus, is an abnormal arrangement of the thoracic and abdominal organs that results in congenital anomalies. Animal models and developmental biological approaches have defined pathways required during embryogenesis for proper left-right pattern formation. New candidates for genetic causes of human laterality disorders have emerged from recent studies on the assembly, transport, and signaling functions of cilia at the node as well as identification of cilia within the developing heart. There is evidence that deleterious genetic variants within one or more developmental pathways may disrupt signaling in a synergistic or combinatorial fashion to cause congenital anomalies. The molecular pathways underlying PCD and heterotaxy are being discovered at a rapid pace, and there is increasing recognition of the overlap between these two categories of laterality disorders and their relationship to isolated cardiovascular malformations. This review focuses on the clinical manifestations, molecular mechanisms, and human genetics of these disorders of laterality.