The molecular basis of kidney diseases: cystinosis and hereditary nephrotic syndrome
The research projects of our team are focused on two major topics; cystinosis and hereditary nephrotic syndrome. These projects stem from the seminal work done by our group that identified the CTNS gene involved in cystinosis in 1998, and the NPHS2 gene encoding podocin in 2000, the first gene identified in steroid-resistant nephrotic syndrome (SRNS).
Cystinosis is an inherited lysosomal storage disorder characterized by a defective lysosomal efflux of cystine, a proximal tubulopathy being the main and earliest renal symptom of the disease. The causative gene, CTNS, encodes a glycosylated lysosomal membrane protein, cystinosin, which acts as a cystine-proton symporter. It is composed of seven transmembrane (TM) domains and contains two lysosomal targeting motifs: a tyrosine-based signal (GYDQL) in its C-terminal tail and a non-classical motif in its ﬁfth inter-TM loop. We recently showed that the tyrosine-based GYDQL motif of cystinosin is a ‘strong’ AP-3 interacting motif responsible for lysosomal targeting of cystinosin by a direct intracellular pathway. Our on-going projects are aimed at characterizing new functions of cystinosin beyond the lysosomal transport of cystine, mainly by studying new pathways (mTORC1 signaling and autophagy) that have been highlighted by the identification of protein interacting with cystinosin by proteomics, and at elucidating mechanisms underlying the proximal tubular dysfunction, using in vitro and in vivo models developed in the lab.
Nephrotic syndrome is a clinical entity characterized by massive proteinuria, hypoalbuminemia, hyperlipidemia and edema. The identification of genes involved in rare familial forms of steroid-resistant nephrotic syndrome (SRNS) has highlighted the crucial role played by the podocyte, a highly specialized glomerular cell, in the integrity of the glomerular filtration barrier. Almost 40 genes have been identified that, if mutated, are responsible for monogenic forms of SRNS (autosomal recessive and dominant). Most of these genes encode proteins expressed in the podocyte, particularly structural proteins and actin network regulators.
Recently, we have described the crucial role of podocin dimerization in the pathogenicity of NPHS2 mutations, leading to incomplete penetrance of some associations of mutations. Mutations in this gene, originally found by our group, are responsible for more than 40% of SRNS and these results have a direct impact on patients care, both clinically and for genetic counseling, and open new perspectives for evaluating the pathogenicity of mutations in autosomal recessive diseases. We have also identified a homozygous missense mutation in the ciliary gene TTC21B which unexpectedly causes familial focal and segmental glomerulosclerosis (with a dual phenotype of both primary tubulointerstitial and glomerular lesions). In addition, our group has identified the first gene (WDR73) involved in Galloway-Mowat syndrome (GMS), associating microcephaly and SRNS, as well as, very recently, five other genes, encoding proteins forming a highly conserved complex, also mutated in patients with GMS. Mutations in genes such as WDR73 affect both the kidney and the nervous system, in line with growing evidence that podocytes and neurons share a large set of common features. Moreover, our discovery of mutations in genes such as TTC21B and WDR73 has highlighted the emergent role of proteins involved in microtubule dynamic and organization in podocyte pathophysiology.
The main objective of our group is to unravel new actors and molecular networks driving podocyte differentiation and/or maintenance, and to identify new pathophysiological mechanisms leading to SRNS, especially those related to actin/microtubule networks and cell survival.
Our projects comprise:
1) the study of podocin biogenesis, trafficking and degradation, as well as screening of molecules modifying podocin folding and trafficking with the aim of testing new and innovative therapies,
2) the identification of new genes involved in SRNS by next-generation sequencing and the characterization of their gene products by using cellular models, as well as knock-in and knock-out animal models (fruit fly Drosophila melanogaster, zebrafish and mouse) generated with the CRISPR/Cas9 technology.
3) the modeling of hereditary nephrotic syndrome with human induced pluripotent stem cells (iPSCs) created from patient’s cells with isolated SRNS and/or Galloway-Mowat syndrome in order to generate new insights into the molecular pathways altered in these disorders.