Alteration of nephrocystins and IFT-A proteins causes similar ciliary phenotypes leading to Nephronophthisis

Abstract

Nephronophtisis (NPH) is a kidney ciliopathy often associated with extra-renal defects and for which 12 genes (NPHP1-12) have been identified. NPHP1 and NPHP4 control the ciliary access at the transition zone and the velocity of some intraflagellar transport (IFT)/BBS proteins in C.elegans. Recently, in a collaborative effort, we have identified, in families with isolated NPH, mutations in TTC21B as well as in WDR19, which encode the retrograde IFT-A proteins IFT139 and IFT144, respectively. By ciliome sequencing of 1600 candidate genes from 14 NPH patients followed by Sanger sequencing of a cohort of 52 patients, we have found respectively 8 and 7 patients carrying pathogenic missense mutations in genes coding IFT-A proteins, including WDR35, TTC21B and IFT140, which could partially affect their function. Together, these results indicate that IFT-A are involved in nephronophtisis. Moreover, alteration of cilia length was observed in patient kidney, Nphp4-/- mice kidney tubules and NPHP1 or NPHP4 knockdown IMCD3 cell lines. In these cells, primary cilia present swellings at the distal region accompanied by an accumulation of IFT-B at the base and the tip, similar to what was observed in IFT-A mutants, suggesting a possible alteration of retrograde transport. Additionally, ARL13B, a small GTPase required for proper cilium shape and IFT stability, is absent along the axoneme of NPHP4-KD-IMCD cells. By controlling the entry of ciliary components at the transition zone, NPHP1 and NPHP4 may modulate IFT-A cargos thus participating in the same pathway (i.e. Wnt/PCP), alteration of which would lead to renal lesions observed in nephronophthisis.