Patho-physiology of renal dysfunction in Bardet-Biedl Syndrome

Bardet-Biedl Syndrome (BBS) is a rare autosomal recessive disorder with renal and extra-renal involvement. The wide spectrum of clinical manifestations is associated to the high genetic heterogeneity. To date 21 genes have been identified in humans and the majority of them encode proteins located on the basal body of the primary cilium. For this reason the disease is has been included among the 'ciliopathies'. The renal involvement is extremely heterogeneous in BBS and is considered the main cause of morbidity and mortality. Recent evidences have suggested that mutations in BBS6, 10 and 12 are associated with a more severe renal dysfunction. The most common renal dysfunction is the urine concentrating defect, even though the underlying mechanism is not completely known. Recently we have demonstrated that hyposthenuria in BBS patients has a renal origin, and depends on desmopessin resistance. The majority of hyposthenuric BBS patients have a combined defect to both concentrate and dilute the urine. The combined defect is associated with a blunted increased urine Aquaproine-2 (u-AQP2) excretion in antidiuresis. A ccordingly, in vitro BBS10 silencing prevented AQP2 trafficking to the apical plasma membrane. However, after long term water restriction hyposthenuric BBS patients showed the same u-AQP2 excretion compared with controls, suggesting that other mechanisms are implicated into the pathogenesis of hyposhtenuria. The complete molecular mechanism underlying hyposhtenuria remains largely unknown in BBS. Whether this defect may represent a predictor factor for poor renal outcome remains to be elucidated.