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

Abstract

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.

Figure 1

Front and lateral view of the patient at the age of 6 years. (a, b) Note triangular face, severe prognathism, hypomaxillia, anterior crossbite, large low-set ears, low forehead. (c) Note rickets, bilateral and symmetrical widening of the metaphyseal regions, curvature of both the femoral, and the tibial diaphyses.

Figure 2

(a) DNA sequencing at the junction between exon 6 and intron 6. The box indicates the nucleotide substitution A>G at the third nucleotide of the donor splice-site consensus sequences (c.388 +3A>G) found in the patient and in his heterozygous mother. (b) cDNA sequencing shows the absence of exon 6 in the patient, but not in his mother. (c) XCI in peripheral leukocytes. Analysis of methylation of HpaII site at the human AR locus. −HpaII, undigested DNA; +HpaII, digested DNA. The same results were obtained using HhaI (data not shown).