<i>TBC1D8B</i> Mutations Implicate RAB11-Dependent Vesicular Trafficking in the Pathogenesis of Nephrotic Syndrome.

Abstract

Mutations in about 50 genes have been identified as monogenic causes of nephrotic syndrome, a frequent cause of CKD. These genes delineated the pathogenetic pathways and rendered significant insight into podocyte biology. We used whole-exome sequencing to identify novel monogenic causes of steroid-resistant nephrotic syndrome (SRNS). We analyzed the functional significance of an SRNS-associated gene <i>in vitro</i> and in podocyte-like <i>Drosophila</i> nephrocytes. We identified hemizygous missense mutations in the gene <i>TBC1D8B</i> in five families with nephrotic syndrome. Coimmunoprecipitation assays indicated interactions between TBC1D8B and active forms of RAB11. Silencing <i>TBC1D8B</i> in HEK293T cells increased basal autophagy and exocytosis, two cellular functions that are independently regulated by RAB11. This suggests that TBC1D8B plays a regulatory role by inhibiting endogenous RAB11. Coimmunoprecipitation assays showed TBC1D8B also interacts with the slit diaphragm protein nephrin, and colocalizes with it in immortalized cell lines. Overexpressed murine <i>Tbc1d8b</i> with patient-derived mutations had lower affinity for endogenous RAB11 and nephrin compared with wild-type Tbc1d8b protein. Knockdown of <i>Tbc1d8b</i> in <i>Drosophila</i> impaired function of the podocyte-like nephrocytes, and caused mistrafficking of Sns, the <i>Drosophila</i> ortholog of nephrin. Expression of <i>Rab11</i> RNAi in nephrocytes entailed defective delivery of slit diaphragm protein to the membrane, whereas <i>RAB11</i> overexpression revealed a partial phenotypic overlap to <i>Tbc1d8b</i> loss of function. Novel mutations in <i>TBC1D8B</i> are monogenic causes of SRNS. This gene inhibits RAB11. Our findings suggest that RAB11-dependent vesicular nephrin trafficking plays a role in the pathogenesis of nephrotic syndrome.