Selenium (Se) is an essential trace element needed for the biosynthesis of selenoproteins. Biosynthesis of such proteins depends on the selenocysteine incorporation sequence binding protein 2 (SBP2), which represents a key trans-acting factor during the translation process. We recently described children with mutations in SBP2 gene who displayed an abnormal thyroid function test(s) and reduced selenoprotein concentrations. Now we aimed to improve selenoprotein biosynthesis and thyroid hormone metabolism in SBP2 deficient subjects by supplementing an organic and inorganic Se form.
Three affected and two unaffected siblings received daily doses of 100, 200 or 400 μg selenomethionine-rich yeast and 400 μg sodium selenite for one month each. Serum was drawn at baseline and after supplementations. Thyroid function tests, extracellular glutathione peroxidase activity, Se and selenoprotein P concentrations were determined. Selenomethionine-rich yeast increased serum Se concentrations in all subjects irrespective of genotype. Sodium selenite was effective in increasing the selenoprotein P concentration in normal and to a lesser degree in affected subjects. Both forms failed to increase the glutathione peroxidase activity or to correct the thyroid function abnormalities in the SBP2 deficient individuals indicating that impaired deiodinase expression was not corrected. No adverse side-effects were observed.
We conclude that total serum Se concentrations in SBP2 deficient subjects respond to selenomethionine supplementation but this effect is not indicative for improved selenoprotein synthesis. Se is obviously not a limiting factor in the SBP2 deficient individuals when regular daily Se intake is provided. The phenotype does not depend on the daily Se intake and therefore would likely have a similar presentation in the different geographical areas of the world. These findings might help to identify and diagnose more individuals with selenoprotein biosynthesis defects who might present at young age with characteristic thyroid function test abnormalities, growth retardation, and reduced Se and selenoprotein concentrations.