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Abstract
A human Na+/myo-inositol cotransporter (SLC5A3) gene was cloned; sequencing revealed
a single intron-free open reading frame of 2157 nucleotides. Containing 718 amino
acid residues, the predicted protein is highly homologous to the product of the canine
osmoregulatory SLC5A3 gene. The SLC5A3 protein is number 3 of the solute carrier family
5 and was previously designated SMIT. Using fluorescence in situ hybridization, the
human SLC5A3 gene was localized to band q22 on chromosome 21. Many tissues including
brain demonstrate gene expression. The inability of a trisomic 21 cell to downregulate
expression of three copies of this osmoregulatory gene could result in increased flux
of both myo-inositol and Na+ across the plasma membrane. The potential consequences
include perturbations in the cell membrane potential and tissue osmolyte levels. The
SLC5A3 gene may play a role in the pathogenesis of Down syndrome.