Neurological, Psychiatric, and Biochemical Aspects of Thiamine Deficiency in Children and Adults

Abstract

Thiamine (vitamin B1) is an essential nutrient that serves as a cofactor for a number of enzymes, mostly with mitochondrial localization. Some thiamine-dependent enzymes are involved in energy metabolism and biosynthesis of nucleic acids whereas others are part of the antioxidant machinery. The brain is highly vulnerable to thiamine deficiency due to its heavy reliance on mitochondrial ATP production. This is more evident during rapid growth (i.e., perinatal periods and children) in which thiamine deficiency is commonly associated with either malnutrition or genetic defects. Thiamine deficiency contributes to a number of conditions spanning from mild neurological and psychiatric symptoms (confusion, reduced memory, and sleep disturbances) to severe encephalopathy, ataxia, congestive heart failure, muscle atrophy, and even death. This review discusses the current knowledge on thiamine deficiency and associated morbidity of neurological and psychiatric disorders, with special emphasis on the pediatric population, as well as the putative beneficial effect of thiamine supplementation in autism spectrum disorder (ASD) and other neurological conditions.

Keywords: Krebs cycle; autism spectrum disorders; brain; depressive disorders; encephalomyopathies; pentose phosphate pathway; thiamine transporter.

Figure 1

Thiamine metabolism. (A) Chemical structure of thiamine and phosphorylated derivatives. (B) Absorption and cellular uptake of thiamine. Dietary thiamine is mainly in the form of phosphate derivatives and, before absorption, these are converted to free thiamine by intestinal phosphatases. Mammalians utilize THTR1 and THTR2 to transport thiamine. The THTR1 appears to function in the micromolar range, whereas the THTR-2 appears to function in the nanomolar range. Both transporters are expressed in the human small and large intestine, and the expression of THTR1 is higher than that of THTR2, with THTR1 found at both the apical and basolateral membranes of enterocytes (somewhat higher expression at the latter compared to the former domain). Expression of the THTR2 protein was found to be restricted to the apical membrane domain only [see (5) and references therein]. Free thiamine is converted to TPP by the action of TPK1 in the cytosol and dephosphorylation of TPP to TMP and thiamine by ACPP. At higher concentrations of thiamine, simple passive diffusion takes place [see (6) and references therein]. From blood, thiamine is taken up by either transporter (THTR1 or THTR2). Both carriers are widely distributed in the body, but the levels differ in different tissues. Once inside the target cells, thiamine is phosphorylated to TPP by TPK1 and to TTP by TDPK. Both TPP and TTP can be dephosphorylated by ACPP and THTPA, respectively. TPP in the cytosol is utilized as a cofactor for TKT, but it is also required as a cofactor in the mitochondrial compartment; hence, the mitochondrial SLC25A19 exists at the inner mitochondrial membrane. TPP must be bound to HACL1 and HACL2 for its transport to the peroxisomes. Abbreviations: ACPP, prostatic acid phosphatase; ADP, adenosine diphosphate; AMP, adenosine monophosphate; ATP, adenosine triphosphate; T, thiamine; TDPK, thiamine diphosphokinase; THTPA, thiamine triphosphatase; THTR, thiamine transporter; TMP, thiamine monophosphate; TPP, thiamine pyrophosphate. (C) Pathways involving TPP-dependent enzymes. In the cytosol, TPP is the cofactor of transketolase (TKT), an enzyme involved in the pentose phosphate pathway (PPP). The PPP produces nicotinamide adenine dinucleotide phosphate (NADPH) and ribose 5-phosphate (R5P), essential for antioxidant defense and fatty acid synthesis, and precursor in the biosynthesis of nucleotides, respectively. In mitochondria, TPP acts as a cofactor for branched-chain α-ketoacid dehydrogenase (BCKDH) complex, pyruvate dehydrogenase (PDH) complex, and α-ketoglutarate dehydrogenase. In peroxisomes, TPP is a cofactor for 2-hydroxyacyl-CoA lyase 1 (HACL1) as part of the fatty acid α-oxidation pathway.