Dent Disease with mutations in OCRL1

Abstract

Dent disease is an X-linked renal proximal tubulopathy associated with mutations in the chloride channel gene CLCN5. Lowe syndrome, a multisystem disease characterized by renal tubulopathy, congenital cataracts, and mental retardation, is associated with mutations in the gene OCRL1, which encodes a phosphatidylinositol 4,5-bisphosphate (PIP(2)) 5-phosphatase. Genetic heterogeneity has been suspected in Dent disease, but no other gene for Dent disease has been reported. We studied male probands in 13 families, all of whom met strict criteria for Dent disease but lacked mutations in CLCN5. Linkage analysis in the one large family localized the gene to a candidate region at Xq25-Xq27.1. Sequencing of candidate genes revealed a mutation in the OCRL1 gene. Of the 13 families studied, OCRL1 mutations were found in 5. PIP(2) 5-phosphatase activity was markedly reduced in skin fibroblasts cultured from the probands of these five families, and protein expression, measured by western blotting, was reduced or absent. Slit-lamp examinations performed in childhood or adulthood for all five probands showed normal results. Unlike patients with typical Lowe syndrome, none of these patients had metabolic acidosis. Three of the five probands had mild mental retardation, whereas two had no developmental delay or behavioral disturbance. These findings demonstrate that mutations in OCRL1 can occur with the isolated renal phenotype of Dent disease in patients lacking the cataracts, renal tubular acidosis, and neurological abnormalities that are characteristic of Lowe syndrome. This observation confirms genetic heterogeneity in Dent disease and demonstrates more-extensive phenotypic heterogeneity in Lowe syndrome than was previously appreciated. It establishes that the diagnostic criteria for disorders resulting from mutations in the Lowe syndrome gene OCRL1 need to be revised.

Figure 1

Segregation of the R301C mutation in family 24. The pedigree is shown, with affected subjects identified as blackened symbols. Digestion of the 298-bp exon 11 amplicons with the restriction endonuclease HhaI in normal subjects results in two products of 158 bp and 140 bp (subjects IV:1, II:3, II:4, and III:8). The mutated sequence produces a single detectable digest of 298 bp (subjects IV:2, II:5, III:4, III:5, III:6, and III:7). Heterozygous females display all three products (subjects III:2, III:3, II:7, and II:8). The lane containing PhiX174/HaeIII molecular weight markers is indicated by “M.”

Figure 2

Western blot analysis of OCRL1 in control and patient fibroblasts. Fibroblast supernatant (20 μg), which was prepared as described in the text, was loaded in each lane. Lane 1, a Lowe syndrome patient known to not express OCRL1; lane 2, an unaffected individual (control fibroblasts); lane 3, proband from family 25; lane 4, proband from family 20; lane 5, proband from family 29; lane 6, proband from family 26; and lanes 7 and 8, two affected brothers from family 24. The positions and sizes of broad-range protein markers (Bio-Rad) are shown on the left. The presence of adequate protein loading in all lanes is demonstrated in the lower panel by a western blot analysis of the same blot with the use of antibody against β-tubulin as a control.