Renal cystic dysplasia, paucity of bile ducts, situs inversus, bowing of the femora in two siblings in the Reunion Island: a ciliopathy?

Novel NEK8 Mutations Cause Severe Syndromic Renal Cystic Dysplasia through YAP Dysregulation

Ciliopathies are a group of genetic multi-systemic disorders related to dysfunction of the primary cilium, a sensory organelle present at the cell surface that regulates key signaling pathways during development and tissue homeostasis. In order to identify novel genes whose mutations would cause severe developmental ciliopathies, >500 patients/fetuses were analyzed by a targeted high throughput sequencing approach allowing exome sequencing of >1200 ciliary genes. NEK8/NPHP9 mutations were identified in five cases with severe overlapping phenotypes including renal cystic dysplasia/hypodysplasia, situs inversus, cardiopathy with hypertrophic septum and bile duct paucity. These cases highlight a genotype-phenotype correlation, with missense and nonsense mutations associated with hypodysplasia and enlarged cystic organs, respectively. Functional analyses of NEK8 mutations in patient fibroblasts and mIMCD3 cells showed that these mutations differentially affect ciliogenesis, proliferation/apoptosis/DNA damage response, as well as epithelial morphogenesis. Notably, missense mutations exacerbated some of the defects due to NEK8 loss of function, highlighting their likely gain-of-function effect. We also showed that NEK8 missense and loss-of-function mutations differentially affect the regulation of the main Hippo signaling effector, YAP, as well as the expression of its target genes in patient fibroblasts and renal cells. YAP imbalance was also observed in enlarged spheroids of Nek8-invalidated renal epithelial cells grown in 3D culture, as well as in cystic kidneys of Jck mice. Moreover, co-injection of nek8 MO with WT or mutated NEK8-GFP RNA in zebrafish embryos led to shortened dorsally curved body axis, similar to embryos injected with human YAP RNA. Finally, treatment with Verteporfin, an inhibitor of YAP transcriptional activity, partially rescued the 3D spheroid defects of Nek8-invalidated cells and the abnormalities of NEK8-overexpressing zebrafish embryos. Altogether, our study demonstrates that NEK8 human mutations cause major organ developmental defects due to altered ciliogenesis and cell differentiation/proliferation through deregulation of the Hippo pathway.

Genes mutated in ciliopathies encode proteins with various localizations and functions at the primary cilium. Here we report novel NEK8 mutations in patients with renal cystic hypodysplasia and associated ciliopathy defects. NEK8 belongs to a protein complex defining the Inversin compartment of the cilium. It is also a negative regulator of the Hippo signaling pathway that controls organ growth. We report genotype-phenotype correlation in the patients. We functionally demonstrate that the two types of mutations (missense versus nonsense) differentially affect ciliogenesis, cell apoptosis and epithelialisation. We also show that all the mutations lead to dysregulation of the Hippo pathway through nuclear YAP imbalance but that the nature of this imbalance is different according to the type of mutation. We confirm alteration of the Hippo pathway associated with Nek8 mutation in vivo in Jck mice. Remarkably, we show that morphogenesis defects observed in Nek8 knockdown epithelial cells or zebrafish embryos are rescued by Verteporfin, a specific inhibitor of YAP transcriptional activity, demonstrating the causative role of YAP dysregulation in the occurrence of these defects. Altogether, this study links NEK8 mutations to dysregulation of the Hippo pathway and provide molecular clues to understand the variability of the multiorgan defects in the patients.