Bardet–Biedl syndrome and other ciliopathies

Ciliopathies encompass a genotypically complex and phenotypically variable and overlapping series of disorders that makes the general term ‘ciliopathies’ very useful. The genes behind these conditions encode parts of the machinery of the primary cilium. This is also true of the major cystic kidney disorders autosomal dominant polycystic kidney disease and autosomal recessive polycystic kidney disease, but the ‘long tails’ of other ciliopathies are characterized by variable nephropathy (often without cyst formation), retinopathy, and effects on brain and skeletal development. Not all have substantial renal phenotypes. Bardet–Biedl syndrome (BBS) is an autosomal dominant condition characterized by obesity, retinopathy, nephropathy, and learning difficulty, but renal abnormalities are varied and end-stage renal failure occurs in only a minority. Many BBS genes have been described. Alström syndrome is a rare recessive disorder again associated with obesity and retinopathy, but also deafness and dilated cardiomyopathy. Renal failure is a common but later feature. Joubert syndrome is an autosomal dominant condition but can arise from mutations in at least 10 genes. It has a wide phenotypic variation with a common link being hypodysplasia of the cerebellar vermis and other abnormalities giving rise to the ‘molar tooth sign’ on cerebral magnetic resonance imaging scanning, associated with hypotonia in infancy, central ataxia, ocular apraxia, developmental delay, and varying degrees of cognitive impairment. Jeune syndrome is a recessive condition characterized by osteochondrodysplasia which can give rise to hypodevelopment of the chest wall known as suffocating thoracic dystrophy, in addition to other manifestations.