Assignment of the Aquaporin-8 water channel gene (AQP8) to human chromosome 16p12

Aquaporins represent water channel proteins widely spread throughout nature where they are responsible for the high water permeability characterizing numerous plasma membranes (Agre, 1998). A number of aquaporins have been described in mammals where their sites of expression predict roles in physiology and disease (Agre et al., 1998). Some of these functions have been already demonstrated (see Agre et al., 1998 for references). Interest is triggered by the recent discovery of the Aquaporin- 8 water channel (AQP8) in rat (Ishibashi et al., 1997; Koyama et al., 1997), mouse (Ma et al., 1997) and human (Koyama et al., 1998) tissues where its distribution predicts roles in secretion of pancreatic juice, bile and primary saliva, fecal dehydration, transport of fluid across placenta, and in the movement of water across the spermatozoa plasma membrane. An involvement of AQP8 in clinical disorders of unknown molecular origin is also expected. In agreement with phylogenetic analyses, the genomic organization of the AQP8 gene indicates a separate evolutionary pathway for AQP8 compared to that of the other mammalian aquaporins (Koyama et al., 1998; Calamita et al., submitted for publication). This distinction may be responsible for regulatory and functional features which could still be unknown at the present. Uncertain is the assumption that the mouse, rat and human AQP8 are orthologous to each other. Here we report the fine mapping of the AQP8 gene to human chromosome 16p12, a region which is syntenic to the region where the mouse AQP8 gene maps (Calamita et al., submitted for publication). This result demonstrates that the mouse and human AQP8 genes are orthologous and provides important insight into the phylogenesis of the aquaporins.