Abstract
Sodium-dependent multivitamin transporter (SMVT) deficiency is a recently described multivitamindependent inherited metabolic disorder (IMD). SMVT is encoded by the SLC5A6 gene located on chromosome 2p23.3. We describe a clinical case of SMVT deficiency in a child with developmental delay, microcephaly, persistent neurological symptoms, skin lesions and frequent upper respiratory tract infections. A tandem mass spectrometry study of the metabolic profile of amino acids and acylcarnitines, carried out twice, showed increased blood levels of methylmalonylcarnitine + 3-hydroxyisovalerylcarnitine (C4DC/C5OH). Urine analysis by gas chromatography-mass spectrometry showed persistently increased excretion of 3-OH-isovaleric acid. The combination of increased blood concentrations of C4DC/C5OH and increased urinary excretion of 3-OHisovaleric acid suggested a metabolic disorder associated with impaired biotin metabolism. Determination of the level of biotinidase in the blood did not reveal deviations from the reference values. Biotin was prescribed. As a result of clinical exome sequencing, complex heterozygous variants of the SLC5A6 gene encoding SMVT were identified. The patient was treated with increased doses of oral biotin, pantothenic acid, and lipoic acid, which resulted in significant clinical improvement.