Joubert syndrome: insights into brain development, cilium biology, and complex disease.

Joubert syndrome (JS) is a primarily autosomal recessive condition characterized by hypotonia, ataxia, abnormal eye movements, and intellectual disability with a distinctive mid-hindbrain malformation (the "molar tooth sign"). Variable features include retinal dystrophy, cystic kidney disease, liver fibrosis and polydactyly. Recently, substantial progress has been made in our understanding of the genetic basis of JS, including identification of seven causal genes (NPHP1, AHI1, CEP290, RPGRIP1L, TMEM67/MKS3, ARL13B and CC2D2A). Despite this progress, the known genes account for <50% of cases and few strong genotype-phenotype correlations exist in JS; however, genetic testing can be prioritized based on clinical features. While all seven JS genes have been implicated in the function of the primary cilium/basal body organelle (PC/BB), little is known about how the PC/BB is required for brain, kidney, retina and liver development/function, nor how disruption of PC/BB function leads to diseases of these organs. Recent work on the function of the PC/BB indicates that the organelle is required for multiple signaling pathways including sonic hedgehog, WNT and platelet derived growth factor. Due to shared clinical features and underlying molecular pathophysiology, JS is included in the rapidly expanding group of disorders called ciliopathies. The ciliopathies are emerging as models for more complex diseases, where sequence variants in multiple genes contribute to the phenotype expressed in any given patient.