Impact of natural mutations on the riboflavin transporter 2 and their relevance to human riboflavin transporter deficiency 2

New insights into the nutritional genomics of adult-onset riboflavin-responsive diseases

Riboflavin, or vitamin B2, is an essential nutrient that serves as a precursor to flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN). The binding of the FAD and/or FMN cofactors to flavoproteins is critical for regulating their assembly and activity. There are over 90 proteins in the human flavoproteome that regulate a diverse array of biochemical pathways including mitochondrial metabolism, riboflavin transport, ubiquinone and FAD synthesis, antioxidant signalling, one-carbon metabolism, nitric oxide signalling and peroxisome oxidative metabolism. The identification of patients with genetic variants in flavoprotein genes that lead to adult-onset pathologies remains a major diagnostic challenge. However, once identified, many patients with adult-onset inborn errors of metabolism demonstrate remarkable responses to riboflavin therapy. We review the structure:function relationships of mutant flavoproteins and propose new mechanistic insights into adult-onset riboflavin-responsive pathologies and metabolic dysregulations that apply to multiple biochemical pathways. We further address the vexing issue of how the inheritance of genetic variants in flavoprotein genes leads to an adult-onset disease with complex symptomologies and varying severities. We also propose a broad clinical framework that may not only improve the current diagnostic rates, but also facilitate a personalized approach to riboflavin therapy that is low cost, safe and lead to transformative outcomes in many patients.

An Overview of the Putative Structural and Functional Properties of the GHBh1 Receptor through a Bioinformatics Approach

The neurotransmitter γ-hydroxybutyric acid (GHB) is suggested to be involved in neuronal energy homeostasis processes, but the substance is also used as a recreational drug and as a prescription medication for narcolepsy. GHB has several high-affinity targets in the brain, commonly generalized as the GHB receptor. However, little is known about the structural and functional properties of GHB receptor subtypes. This opinion article discusses the literature on the putative structural and functional properties of the GHBh1 receptor subtype. GHBh1 contains 11 transmembrane helices and at least one intracellular intrinsically disordered region (IDR). Additionally, GHBh1 shows a 100% overlap in amino acid sequence with the Riboflavin (vitamin B2) transporter, which opens the possibility of a possible dual-function (transceptor) structure. Riboflavin and GHB also share specific neuroprotective properties. Further research into the GHBh1 receptor subtype may pave the way for future therapeutic possibilities for GHB.

In silico investigation on structure-function relationship of members belonging to the human SLC52 transporter family

Riboflavin is an essential water-soluble vitamin that needs to be provided through the diet because of the conversion into flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN), important cofactors in hundreds of flavoenzymes. The adsorption and distribution of riboflavin is mediated by transmembrane transporters of the SLC52 family, namely RFVT1-3, whose mutations are mainly associated with two diseases, MADD and the Brown-Vialetto-Van Laere syndrome. Interest in RFVTs as pharmacological targets has increased in the last few years due to their overexpression in several cancer cells, which can be exploited both by blocking the uptake of riboflavin into the cancerous cells, and by performing cancer targeted delivery of drugs with a high affinity for RFVTs. In this work, we propose three-dimensional structural models for all three human riboflavin transporters obtained by state-of-the-art artificial intelligence-based methods, which were then further refined with molecular dynamics simulations. Furthermore, two of the most notable mutations concerning RFVT2 and RFVT3 (W31S and N21S, respectively) were investigated studying the interactions between the wild-type and mutated transporters with riboflavin.

Two Rare Cases of Long Surviving Riboflavin Transporter Deficiency with Co-Existing Adenosine Monophosphate Deaminase (AMP) Deficiency

(1) Background: Riboflavin transporter deficiency (RTD), formerly known as Brown−Vialetto−Van Laere syndrome, is a rare condition that causes a progressive neurological syndrome in early life with features of auditory and optic neuropathy, weakness of bulbar muscles and the diaphragm and sensorimotor neuropathy. Pathologic mutations in the genes that code for riboflavin transporters have been identified as the genetic basis of RTD, and the majority of the genetically confirmed cases are caused by mutations of SLC52A3, a riboflavin transporter 2 coding gene or compound mutations in SLC52A2, encoding riboflavin transporter 3. Fatality in childhood is common if the condition is left untreated, but survival into adulthood has been reported in cases treated with high-dose oral riboflavin. (2) Case summary: We report two long-term survivors of RTD type 2 due to compound heterozygous 185T> G and 1258G>A mutations in gene SLC2A2. They are two brothers in a family in which two female siblings died in childhood from a similar neurological disorder. Brother one, the older RTD survivor, is aged 71, and brother two is aged 58. Both have significant visual impairment from optic nerve atrophy and sensory ataxia. Their muscle biopsies showed decreased muscle adenosine monophosphate (AMP) deaminase activity. No AMPD1 mutation was detected through whole-genome sequencing. (3) Conclusion: Co-existing riboflavin transporter deficiency (RTD) type 2 and muscle AMP deaminase deficiency has not been previously reported. Apart from the possibility that there is a milder phenotype associated with these mutations in SLC2A2, AMP deaminase deficiency might have contributed to a survival benefit by preserving muscle function through accumulating intracellular AMP.

Combined isobutyryl-CoA and multiple acyl-CoA dehydrogenase deficiency in a boy with altered riboflavin homeostasis

In this report, we describe the case of an 11-year-old boy, who came to our attention for myalgia and muscle weakness, associated with inappetence and vomiting. Hypertransaminasemia was also noted, with ultrasound evidence of hepatomegaly. Biochemical investigations revealed acylcarnitine and organic acid profiles resembling those seen in MADD, that is, multiple acyl-CoA dehydrogenase deficiencies (OMIM #231680) a rare inherited disorder of fatty acids, amino acids, and choline metabolism. The patient carried a single pathogenetic variant in the ETFDH gene (c.524G>A, p.Arg175His) and no pathogenetic variant in the riboflavin (Rf) homeostasis related genes (SLC52A1, SLC52A2, SLC52A3, SLC25A32, FLAD1). Instead, compound heterozygosity was found in the ACAD8 gene (c.512C>G, p.Ser171Cys; c.822C>A, p.Asn274Lys), coding for isobutyryl-CoA dehydrogenase (IBD), whose pathogenic variants are associated to IBD deficiency (OMIM #611283), a rare autosomal recessive disorder of valine catabolism. The c.822C>A was never previously described in a patient. Subsequent further analyses of Rf homeostasis showed reduced levels of flavins in plasma and altered FAD-dependent enzymatic activities in erythrocytes, as well as a significant reduction in the level of the plasma membrane Rf transporter 2 in erythrocytes. The observed Rf/flavin scarcity in this patient, possibly associated with a decreased ETF:QO efficiency might be responsible for the observed MADD-like phenotype. The patient's clinical picture improved after supplementation of Rf, l-carnitine, Coenzyme Q10, and also 3OH-butyrate. This report demonstrates that, even in the absence of genetic defects in genes involved in Rf homeostasis, further targeted molecular analysis may reveal secondary and possibly treatable biochemical alterations in this pattern.

Strategies for Successful Over-Expression of Human Membrane Transport Systems Using Bacterial Hosts: Future Perspectives

Ten percent of human genes encode for membrane transport systems, which are key components in maintaining cell homeostasis. They are involved in the transport of nutrients, catabolites, vitamins, and ions, allowing the absorption and distribution of these compounds to the various body regions. In addition, roughly 60% of FDA-approved drugs interact with membrane proteins, among which are transporters, often responsible for pharmacokinetics and side effects. Defects of membrane transport systems can cause diseases; however, knowledge of the structure/function relationships of transporters is still limited. Among the expression of hosts that produce human membrane transport systems, E. coli is one of the most favorable for its low cultivation costs, fast growth, handiness, and extensive knowledge of its genetics and molecular mechanisms. However, the expression in E. coli of human membrane proteins is often toxic due to the hydrophobicity of these proteins and the diversity in structure with respect to their bacterial counterparts. Moreover, differences in codon usage between humans and bacteria hamper translation. This review summarizes the many strategies exploited to achieve the expression of human transport systems in bacteria, providing a guide to help people who want to deal with this topic.