Nephrolithiasis, kidney failure and bone disorders in Dent disease patients with and without CLCN5 mutations

open9noDent disease (DD) is a rare X-linked recessive renal tubulopathy characterised by low-molecular-weight proteinuria (LMWP), hypercalciuria, nephrocalcinosis and/or nephrolithiasis. DD is caused by mutations in both the CLCN5 and OCRL genes. CLCN5 encodes the electrogenic chloride/proton exchanger ClC-5 which is involved in the tubular reabsorption of albumin and LMW proteins, OCRL encodes the inositol polyphosphate 5-phosphatase, and was initially associated with Lowe syndrome. In approximately 25 % of patients, no CLCN5 and OCRL mutations were detected. The aim of our study was to evaluate whether calcium phosphate metabolism disorders and their clinical complications are differently distributed among DD patients with and without CLCN5 mutations. Sixty-four male subjects were studied and classified into three groups: Group I (with CLCN5 mutations), Group II (without CLCN5 mutations) and Group III (family members with the same CLCN5 mutation). LMWP, hypercalciuria and phosphaturic tubulopathy and the consequent clinical complications nephrocalcinosis, nephrolithiasis, bone disorders, and chronic kidney disease (CKD) were considered present or absent in each patient. We found that the distribution of nephrolithiasis, bone disorders and CKD differs among patients with and without CLCN5 mutations. Only in patients harbouring CLCN5 mutations was age-independent nephrolithiasis associated with hypercalciuria, suggesting that nephrolithiasis is linked to altered proximal tubular function caused by a loss of ClC-5 function, in agreement with ClC-5 KO animal models. Similarly, only in patients harbouring CLCN5 mutations was age-independent kidney failure associated with nephrocalcinosis, suggesting that kidney failure is the consequence of a ClC-5 dysfunction, as in ClC-5 KO animal models. Bone disorders are a relevant feature of DD phenotype, as patients were mainly young males and this complication occurred independently of age. The triad of symptoms, LMWP, hypercalciuria, and nephrocalcinosis, was present in almost all patients with CLCN5 mutations but not in those without CLCN5 mutations. This lack of homogeneity of clinical manifestations suggests that the difference in phenotypes between the two groups might reflect different pathophysiological mechanisms, probably depending on the diverse genes involved. Overall, our results might suggest that in patients without CLCN5 mutations several genes instead of the prospected third DD underpin patients' phenotypes.openAnglani, Franca; D’Angelo, Angela; Bertizzolo, Luisa Maria; Tosetto, Enrica; Ceol, Monica; Cremasco, Daniela; Bonfante, Luciana; Addis, Maria Antonietta; Del Prete, DorellaAnglani, Franca; D'Angelo, Angela; Bertizzolo, Luisa Maria; Tosetto, Enrica; Ceol, Monica; Cremasco, Daniela; Bonfante, Luciana; Addis, Maria Antonietta; DEL PRETE, Dorell

All known patient mutations in the ASH-RhoGAP domains of OCRL affect targeting and APPL1 binding

Mutations in the inositol 5-phosphatase OCRL are responsible for Lowe syndrome, an X-linked disorder characterized by bilateral cataracts, mental retardation, neonatal hypotonia, and renal Fanconi syndrome, and for Dent disease, another X-linked condition characterized by kidney reabsorption defects. We have previously described an interaction of OCRL with the endocytic adaptor APPL1 that links OCRL to protein networks involved in the disease phenotype. Here, we provide new evidence showing that among the interactions which target OCRL to membranes of the endocytic pathway, binding to APPL1 is the only one abolished by all known disease-causing missense mutations in the ASH-RhoGAP domains of the protein. Furthermore, we demonstrate that APPL1 and rab5 independently contribute to recruit OCRL to enlarged endosomes induced by the expression of constitutively active Rab5. Thus, binding to APPL1 helps localize OCRL at specific cellular sites, and disruption of this interaction may play a role in disease

An atypical Dent\u2019s disease phenotype caused by co-inheritance of mutations at CLCN5 and OCRL genes

Dent's disease is an X-linked renal tubulopathy caused by mutations mainly affecting the CLCN5 gene. Defects in the OCRL gene, which is usually mutated in patients with Lowe syndrome, have been shown to lead to a Dent-like phenotype called Dent disease 2. However, about 20% of patients with Dent's disease carry no CLCN5/OCRL mutations. The disease's genetic heterogeneity is accompanied by interfamilial and intrafamilial phenotypic heterogeneity. We report on a case of Dent's disease with a very unusual phenotype (dysmorphic features, ocular abnormalities, growth delay, rickets, mild mental retardation) in which a digenic inheritance was discovered. Two different, novel disease-causing mutations were detected, both inherited from the patient's healthy mother, that is a truncating mutation in the CLCN5 gene (A249fs*20) and a donor splice-site alteration in the OCRL gene (c.388+3A>G). The mRNA analysis of the patient's leukocytes revealed an aberrantly spliced OCRL mRNA caused by in-frame exon 6 skipping, leading to a shorter protein, but keeping intact the central inositol 5-phosphatase domain and the C-terminal side of the ASH-RhoGAP domain. Only wild-type mRNA was observed in the mother's leukocytes due to a completely skewed X inactivation. Our results are the first to reveal the effect of an epistatic second modifier in Dent's disease too, which can modulate its expressivity. We surmise that the severe Dent disease 2 phenotype of our patient might be due to an addictive interaction of the mutations at two different genes