TBC1D8B Loss-of-Function Mutations Lead to X-Linked Nephrotic Syndrome via Defective Trafficking Pathways

Abstract

Steroid-resistant nephrotic syndrome (SRNS) is characterized by high-range proteinuria and most often focal and segmental glomerulosclerosis (FSGS). Identification of mutations in genes causing SRNS has improved our understanding of disease mechanisms and highlighted defects in the podocyte, a highly specialized glomerular epithelial cell, as major factors in disease pathogenesis. By exome sequencing, we identified missense mutations in TBC1D8B in two families with an X-linked early-onset SRNS with FSGS. TBC1D8B is an uncharacterized Rab-GTPase-activating protein likely involved in endocytic and recycling pathways. Immunofluorescence studies revealed TBC1D8B presence in human glomeruli, and affected individual podocytes displayed architectural changes associated with migration defects commonly found in FSGS. In zebrafish we demonstrated that both knockdown and knockout of the unique TBC1D8B ortholog-induced proteinuria and that this phenotype was rescued by human TBC1D8B mRNA injection, but not by either of the two mutated mRNAs. We also showed an interaction between TBC1D8B and Rab11b, a key protein in vesicular recycling in cells. Interestingly, both internalization and recycling processes were dramatically decreased in affected individuals' podocytes and fibroblasts, confirming the crucial role of TBC1D8B in the cellular recycling processes, probably as a Rab11b GTPase-activating protein. Altogether, these results confirmed that pathogenic variations in TBC1D8B are involved in X-linked podocytopathy and points to alterations in recycling processes as a mechanism of SRNS.Steroid-resistant nephrotic syndrome (SRNS) is a glomerular disease characterized by massive proteinuria, most often associated with focal and segmental glomerulosclerosis (FSGS). 1 SRNS is responsible for chronic kidney disease and accounts for 15% of end-stage kidney disease (ESKD) cases in individuals under 25 years of age. 2 In glomeruli, podocytes are terminally differentiated, highly specialized epithelial cells. Podocyte foot processes (FPs) wrap the outer surface of the glomerular capillaries and interdigitate with FP of neighboring podocytes forming specialized cell-cell junctions called slit-diaphragms (SD), a major constituent of the glomerular filtration barrier. Identification of monogenic causes of SRNS involving more than 40 genes has helped decipher podocyte physiology. 3,4 Many studies have reported a strong relationship between the SD and the actin cytoskeleton, 4 and the role of vesicular trafficking has been well established in the maintenance of SD complexes. 5,6 Nevertheless, intracellular transport defects have rarely been related to monogenic SRNS. 7 Now, through studies aiming at identifying new genes mutated in SRNS in two families, we identified a potential actor of endosomal trafficking. The first family (family A) was from Ecuador and presented with isolated congenital SRNS consistent with an X-linked inheritance. Pedigree and clinical features are described in Figure 1A and Table 1. Briefly, affected females (I-2 a...

Deficiency of the Tbc1d21 gene causes male infertility with morphological abnormalities of the sperm mitochondria and flagellum in mice

“…For example, (1) missense mutations of TBC1D8B have been identified in cases of an X-linked early-onset steroid-resistant nephrotic syndrome. In podocyte cell models, mutated TBC1D8B results in defective trafficking pathways [ 52 ]; (2) more than 50 mutations of TBC1D24 are now associated with a range of inherited neurological disorders, including myoclonic epilepsy, epileptic encephalopathy, and the DOORS syndrome (deafness, onychodystrophy, osteodystrophy, mental retardation, and seizures). Additionally, the cellular functions of TBC1D24 are involved in regulating the small GTP-binding protein, ARF6 [ 9 , 53 , 54 ]; (3) truncating mutations in TBC1D23 are responsible for a form of pontocerebellar hypoplasia, by affecting dense core vesicles and lysosomal trafficking dynamics [ 55 ]; and (4) mutated TBC1D20 has been identified in 77 families affected by the Warburg micro syndrome, which is characterized by eye, brain, and endocrine abnormalities [ 56 ].…”

TBC1D8B Mutations Implicate RAB11-Dependent Vesicular Trafficking in the Pathogenesis of Nephrotic Syndrome

“…Supporting the significance of our findings, mutations of TBC1D8B were also reported most recently in two families with SRNS and X-linked inheritance. 64 TBC1D8B Specifically Binds Active RAB11A and RAB11B and the Mutations from Patients with Nephrotic Syndrome Affect Interaction with Endogenous RAB11 TBC1D8B is a member of a family of more than 40 TBCproteins that share the eponymous Tre-2-Bub2-Cdc16 (TBC) domain. 65 This domain commonly confers a functional role as a GTPase-activating protein (GAP) for specific Rab-GTPases, the master regulators of vesicular trafficking including endocytosis.…”

Inhibiting calpain 1 and 2 in cyclin G associated kinase–knockout mice mitigates podocyte injury

“…We have previously demonstrated that proteins critical for clathrin-mediated endocytosis -dynamin 1 and 2, synaptojanin1, and endophilin -are also critical to maintaining the glomerular filtration barrier (1). Recently, endocytic pathways have been implicated in human proteinuric diseases through identification of genetic mutations in GAPVD1, ANKFY1, and TBC1D8B (2,3). To further investigate clathrin-mediated endocytosis in human disease, in this study we mined the Nephroseq v5 transcriptomic database.…”