Hepatocyte nuclear factor-1B (HNF-1B) is a transcription factor involved in embryonic development and tissue-specific gene expression in several organs, including the kidney. Recently heterozygous mutations in the HNF1B gene have been identified in patients with hypomagnesemia due to renal Mg2+ wasting. Interestingly. ChIP-chip data revealed HNF-1B binding sites in the FXYD2 gene, encoding the gamma-subunit of the Na+/K+-ATPase. The gamma-subunit has been described as one of the molecular players in the renal Mg2+ reabsorption in the distal convoluted tubule (DCT). Of note, the FXYD2 gene can be alternatively transcribed into two main variants, namely gamma a and gamma b. In the present study, we demonstrated via two different reporter gene assays that HNF-1B specifically acts as an activator of the gamma a-subunit, whereas the gamma b-subunit expression was not affected. Moreover, the HNF-1B mutations H69fsdelAC, H324S325fsdelCA, Y352finsA and K156E, previously identified in patients with hypomagnesemia, prevented transcription activation of gamma a-subunit via a dominant negative effect on wild type HNF1-B. By immunohistochemistry, it was shown that the gamma a- and gamma b-subunits colocalize at the basolateral membrane of the DCT segment of mouse kidney. On the basis of these data, we suggest that abnormalities involving the HNF-1B gene may impair the relative abundance of gamma a and gamma b, thus affecting the transcellular Mg2+ reabsorption in the DCT. (C) 2010 Elsevier Inc. All rights reserved.