OCRL-mutated fibroblasts from patients with Dent-2 disease exhibit INPP5B-independent phenotypic variability relatively to Lowe syndrome cells

Abstract

OCRL mutations are associated with both Lowe syndrome and Dent-2 disease, two rare X-linked conditions. Lowe syndrome is an oculo-cerebro-renal disorder, whereas Dent-2 patients mainly present renal proximal tubulopathy. Loss of OCRL-1, a phosphoinositide-5-phosphatase, leads in Lowe patients' fibroblasts to phosphatidylinositol-4,5-bisphosphate (PI(4,5)P2) accumulation, with defects in F-actin network, α-actinin distribution and ciliogenesis, whereas fibroblasts of Dent-2 patients are still uncharacterized. To search for mechanisms linked to clinical variability observed between these two OCRL mutation-associated pathologies, we compared dermal fibroblasts from independent patients, four affected by Dent-2 disease and six with Lowe syndrome. For the first time, we describe that Dent-2 fibroblasts with OCRL loss-of-function (LOF) mutations exhibit decrease in actin stress fibers, appearance of punctate α-actinin signals and alteration in primary cilia formation. Interestingly, we quantified these phenotypes as clearly intermediate between Lowe and control fibroblasts, thus suggesting that levels of these defects correlate with clinical variations observed between patients with OCRL mutations. In addition, we show that Lowe and Dent-2 fibroblasts display similar PI(4,5)P2 accumulation levels. Finally, we analyzed INPP5B, a paralogous gene already reported to exhibit functional redundancy with OCRL, and report neither differences in its expression at RNA or protein levels, nor specific allelic variations between fibroblasts of patients. Altogether, we describe here differential phenotypes between fibroblasts from Lowe and Dent-2 patients, both associated with OCRL LOF mutations, we exclude direct roles of PI(4,5)P2 and INPP5B in this phenotypic variability and we underline potential key alterations leading to ocular and neurological clinical features in Lowe syndrome.

The oculocerebrorenal syndrome of Lowe: an update

“…This clinical observation is supported by a recent study on fibroblasts from patients with Lowe syndrome and Dent-2 disease in which Montjean et al [86] demonstrated an intermediate phenotype of Dent-2 fibroblasts in terms of the F-actin network, alpha-actinin, and primary cilia. Of note, PtdIns(4,5)P 2 was elevated in cells from patients with Lowe syndrome and Dent-2 disease, and it did not differ between these groups.…”

“…Suchy et al reported prenatal diagnosis by measuring PtdIns(4,5)P 2 5-phosphatase activity in cultured amniocytes [85]. However, the lack of genotype–phenotype correlation and the fact that Dent-2 patients and patients with Lowe syndrome have comparable PtdIns(4,5)P 2 5-phosphatase activity [18, 86] limit prenatal diagnosis with respect to disease severity. Other parameters that can be used for prenatal screening are elevated maternal serum and amniotic fluid alpha-fetoprotein [87], or the presence of fetal cataract on ultrasonography images [11].…”

“…Indeed, double knock-out of inpp5b and ocrl resulted in embryonic lethality, while double knock-out with expression of human INPP5B in mice using a bacterial artificial chromosome created a phenotype resembling Lowe syndrome/Dent-2 disease (postnatal growth failure, LMW proteinuria, and aminoaciduria) [96]. It is questionable, however, whether this mechanism does explain the phenotypic differences between patients with Dent-2 disease and those with Lowe syndrome as Montjean et al observed identical expression not only of OCRL but also of INPP5B at the RNA and protein levels in fibroblasts from both Dent-2 and Lowe syndrome patients [86]. …”

Role of Ocrl1 and Inpp5E in primary cilia assembly and maintenance: a phosphatidylinositol phosphatase relay system?

“…However, Dent-2 disease fibroblasts exhibited milder phenotypes than those from LS patient cells. 79 Notably, the levels of PI(4,5)P 2 and Inpp5B were very similar in LS and Dent patient fibroblasts. 79 These results suggest that Inpp5B does not contribute to the milder phenotypes exhibited by the Dent-2 patient fibroblasts and that, in addition to the phosphatase activity, other functions contribute to the phenotypic manifestations seen in patients.…”

“…56 Most recently, fibroblasts of LS and Dent-2 patients were compared for phenotypic defect. 79 Fibroblasts from both disease groups showed the same category of defects such as lesser F-actin fibers, disorganized α-actinin staining, and ciliogenesis defects. However, Dent-2 disease fibroblasts exhibited milder phenotypes than those from LS patient cells.…”