The water-soluble
micronutrient thiamine is required for normal tissue growth and development in humans.
Thiamine is accumulated into cells through the activity of two cell surface
thiamine transporters (hTHTR1 and hTHTR2), which are differentially targeted in polarized tissues. Mutational dysfunction of hTHTR1 is associated with the clinical condition of
thiamine-responsive
megaloblastic anemia: the symptoms of which are alleviated by
thiamine supplementation. Recently, two hTHTR2 mutants (G23V, T422A) have been discovered in clinical kindreds manifesting
biotin-responsive basal ganglia disease (BBGD): the symptoms of which are alleviated by
biotin administration. Why then does mutation of a specific
thiamine transporter
isoform precipitate a disorder correctable by exogenous
biotin? To investigate the suggestion that hTHTR2 can physiologically function as a
biotin transporter, we examined 1) the cell
biological basis of hTHTR2 dysfunction associated with the G23V and T422A mutations and 2) the substrate specificity of hTHTR2 and these clinically relevant mutants. We show that the G23V and T422A mutants both abrogate
thiamine transport activity rather than targeting of hTHTR2 to the cell surface. Furthermore,
biotin accumulation was not detectable in cells overexpressing either the full length hTHTR2 or the clinically relevant hTHTR2 mutants, yet was demonstrable in the same assay using cells overexpressing the human
sodium-dependent multivitamin transporter, a known
biotin transporter. These results cast doubt on the most parsimonious explanation for the BBGD phenotype, namely that hTHTR2 is a physiological
biotin transporter.