Hyponatremia, a relative excess of total body water, is the most frequently encountered electrolyte abnormality. Although some instances are readily attributed to heart, liver, or kidney failure, many are unexplained. Water excess causes confusion, lethargy, seizures, and death. Even mild hyponatremia causes reversible deficits in coordination and cognition. The applicant's preliminary data show that the plasma sodium concentration is highly individual (i.e., relatively constant in any one individual) and is heritable. In a meta-genome-wide association study for common gene variants that influence water balance, the applicant and co-workers identified variants in a gene not previously suspected of playing a role in whole-body water balance, but exhibiting extremely high biological plausibility. The lead variant affects an intronic enhancer within the gene. Remarkably, a second association locus codes for the transcription factor predicted to bind this enhancer. Therefore, the overarching objective of this proposal is to demonstrate the centrality of this enhancer region, this gene variant, and this gene to systemic osmoregulation using a combination of in vitro and whole- animal models. In Aim 1, the functional significance (i.e., osmotic responsiveness) of the enhancer region and allele- specific effects will be probed through reporter gene and DNA binding assays. In Aim 2, the effect of downregulating this gene upon osmotic phenotype will be tested in cultured cells natively expressing the protein. In Aim 3, transgenic mouse models will be used to test the importance of this gene to systemic water balance under basal conditions, and in response to physiological maneuvers designed to perturb water balance.

Disorders of water balance are among the most frequently encountered and morbid of the blood chemistry problems, causing confusion, lethargy, seizures, coma, and death. Clinical factors predisposing to disordered water balance are present in a high percentage of patients served by the Department of Veterans Affairs. We previously showed that water balance is in large measure inherited. In examining the genetic basis for disordered water balance, we identified human gene changes (or variants) that, in a substantial fraction of the population, predispose to improper water handling in the body. We aim to understand how these genetic changes increases the risk of water imbalance through a series of investigations in the test-tube, in animal cells in culture, and in genetically-altered mice engineered to carry the human gene change.