The Lowe OculoCerebroRenal syndrome of (LOCR) is a rare X-linked disorder of congenital cataracts, mental retardation, behavioral abnormalities, and renal tubulopathy. The disease is caused by loss-of-function mutations in the OCRL gene, which encodes Ocrl, a phosphatidylinositol(4,5)P2 5-phosphatase located in the trans-Golgi network, endosomes, plasma membrane ruffles, and clathrin-coated pits. The renal tubular abnormalities of LOCR, particularly low molecular weight (LMW) proteinuria and aminoaciduria, overlap with those of Dent disease, a disorder that affects only the kidney. Mutations in OCRL account for ~20% of Dent disease patients. It has been proposed that derangements in phosphatidylinositol(4,5)P2 metabolism in LOCR causes defective receptor endocytosis, such as apical trafficking or recycling of megalin, a protein responsible for reabsorbing LMW proteins in the kidney proximal tubule. The CNS abnormalities could arise by analogous defects in recycling of certain CNS receptors. Testing these hypotheses is hampered by the lack of a mouse model because mice with a knock-out of Ocrl, the urine homolog of OCRL, have no detectable phenotype. We have shown that the lack of renal phenotype in Ocrl-knockout mice is due to complete compensation by another phosphatidylinositol(4,5)P2 5-phosphatase, Inpp5b, encoded by murine Inpp5b and mouse INPP5B. Mice with a mutation in Inpp5b have only a mild phenotype but mutation of both Ocrl and Inpp5b causes early embryonic lethality, indicating functional overlap. INPP5B and Inpp5b have clear differences in transcription and splicing which could explain why INPP5B only partially compensates for loss of human OCRL while mouse Inpp5b completely compensates. We created transgenic mice deleted for both murine Ocrl and Inpp5b and hemizygous for a bacterial artificial chromosome (BAC) containing INPP5B, and found they have delayed growth and the proximal renal tubular defects seen in LOCR and Dent disease. Mice deleted for both murine Ocrl and Inpp5b and homozygous for the INPP5B-BAC insertion grow more normally and excrete much less LMW protein. We conclude that there is both a qualitative and quantitative difference in the ability of INPP5B and Inpp5b to compensate for loss of Ocrl in mice. In this current application, we propose to culture proximal tubular cells from these strains of mice and correlate INPP5B enzyme activity with subcelular levels of phosphatidylinositol(4,5)P2 and altered renal transport of low molecular weight proteins and amino acids. We will also use recombination in mouse embryonic stem cells to dissect and identify the differences between INPP5B and Inpp5b responsible for the difference in phenotype between Ocrl-deficient humans and mice. Finally, after completing a 10-generation backcross onto C57Bl/6, we will examine the mice for abnormalities in central nervous system by testing motor function, anxiety, learning, memory and by examining the brains neuropathologically.
Lowe syndrome is a rare disorder characterized by congenital lens cataracts, mental retardation, behavioral abnormalities, and kidney dysfunction caused by mutations in a gene, OCRL, which encodes an enzyme called Ocrl. Ocrl is part of a very complex metabolic pathway and is involved in the breakdown of phosphatidylinositol phosphate. We have shown that mice with mutations in the mouse version of OCRL have no detectable disease because of complete compensation by another mouse gene, caled Inpp5b, while the human version of this same gene, INPP5B, can only partially compensate for deficiency Ocrl in humans, thus allowing the renal disease to occur. We have made mice lacking both mouse Ocrl and mouse Inpp5b but expressing human INPP5B and have found they mimic the renal disease seen in Lowe syndrome. We are proposing to characterize kidney and brain abnormalities in these mice as well as determine why INPP5B and Inpp5b are different in how completely they compensate for loss of Ocrl in humans versus mice. We expect to gain insight into the metabolic pathways served by Ocrl that are important in causing the renal tubular defects and CNS abnormalities seen in Lowe syndrome.
|Green, Robert C; Berg, Jonathan S; Grody, Wayne W et al. (2013) ACMG recommendations for reporting of incidental findings in clinical exome and genome sequencing. Genet Med 15:565-74|