Conformational analysis of the riboflavin-responsive ETF:QO-p.Pro456Leu variant associated with mild multiple acyl-CoA dehydrogenase deficiency

"Liver Failure in an Infant of Late-Onset Glutaric Aciduria Type II": Case Report

Glutaric aciduria type II, also known as Multiple acyl-CoA Dehydrogenase Deficiency, results from a defect in the mitochondrial electron transport chain resulting in an inability to break down fatty-acids and amino acids. There are three phenotypes- type 1 and 2 are of neonatal onset and severe form, with and without congenital anomalies, respectively, and presents with acidosis, severe hypotonia, cardiomyopathy, hepatomegaly, and non-ketotic hypoglycemia. Type 3 or late-onset Multiple acyl-CoA Dehydrogenase Deficiency usually presents in the adolescent or adult age group with phenotype ranging from mild forms of myopathy and exercise intolerance to severe forms of acute metabolic decompensation on its chronic course. Type 3 Multiple acyl-CoA Dehydrogenase Deficiency rarely presents in infancy and in liver failure. We present a five-month-old developmentally normal female child with acute encephalopathy, hypotonia, non-ketotic hypoglycemia, metabolic acidosis, and liver failure, with a history of sibling death of suspected inborn error of metabolism. The blood acyl-carnitine levels in Tandem Mass Spectrometry and urinary organic acid analysis through Gas Chromatography-Mass Spectrometry were unremarkable. The patient initially responded to riboflavin, CoQ, and supportive management but ultimately succumbed to sepsis with shock and multi-organ dysfunction. The clinical exome sequencing reported a homozygous missense variation in exon 11 of the ETFDH gene (chr4:g.158706270C > T) that resulted in the amino acid substitution of Leucine for Proline at codon 456 (p.Pro456Leu) suggestive of Glutaric aciduria type IIc (OMIM#231,680).

Recognizing and Managing a Metabolic Crisis

In some relatively common inborn errors of metabolism there can be the accumulation of toxic compounds including ammonia and organic acids such as lactate and ketoacids, as well as energy deficits at the cellular level. The clinical presentation is often referred to as a metabolic emergency or crisis. Fasting and illness can result in encephalopathy within hours, and without appropriate recognition and intervention, the outcome may be permanent disability or death. This review outlines easy and readily available means of recognizing and diagnosing a metabolic emergency as well as general guidelines for management. Disease-specific interventions focus on parenteral nutrition to reverse catabolism, toxin removal strategies, and vitamin/nutrition supplementation.

Host-Guest Encapsulation of RIBO with TSC4X: Synthesis, Characterization, and Its Application by Physicochemical and Computational Investigations

In our present work, we synthesized a new encapsulated complex denoted as RIBO-TSC4X, which was derived from an important vitamin riboflavin (RIBO) and p-sulfonatothiacalix[4]arene(TSC4X). The synthesized complex RIBO-TSC4X was then characterized by utilizing several spectroscopic techniques such as ¹H-NMR, FT-IR, PXRD, SEM, and TGA. Job's plot has been employed to show the encapsulation of RIBO (guest) with TSC4X (host) having a 1:1 molar ratio. The molecular association constant of the complex entity (RIBO-TSC4X) was found to be 3116.29 ± 0.17 M^-1, suggesting the formation of a stable complex. The augment in aqueous solubility of the RIBO-TSC4X complex compared to pure RIBO was investigated by UV-vis spectroscopy, and it was viewed that the newly synthesized complex has almost 30 times enhanced solubility over pure RIBO. The enhancement of thermal stability upto 440 °C for the RIBO-TSC4X complex was examined by TG analysis. This research also forecasts RIBO's release behavior in the presence of CT-DNA, and at the same time, BSA binding study was also carried out. The synthesized RIBO-TSC4X complex exhibited comparatively better free radical scavenging activity, thereby minimizing oxidative injury of the cell as evident from a series of antioxidant and anti-lipid peroxidation assay. Furthermore, the RIBO-TSC4X complex showed peroxidase-like biomimetic activity, which is very useful for several enzyme catalyst reactions.

The use of pharmacological chaperones in rare diseases caused by reduced protein stability

Rare diseases are most often caused by inherited genetic disorders that, after translation, will result in a protein with altered function. Decreased protein stability is the most frequent mechanism associated with a congenital pathogenic missense mutation and it implies the destabilization of the folded conformation in favour of unfolded or misfolded states. In the cellular context and when experimental data is available, a mutant protein with altered thermodynamic stability often also results in impaired homeostasis, with the deleterious accumulation of protein aggregates, metabolites and/or metabolic by-products. In the last decades, a significant effort has enabled the characterization of rare diseases associated to protein stability defects and triggered the development of innovative therapeutic intervention lines, say, the use of pharmacological chaperones to correct the intracellular impaired homeostasis. Here, we review the current knowledge on rare diseases caused by reduced protein stability, paying special attention to the thermodynamic aspects of the protein destabilization, also focusing on some examples where pharmacological chaperones are being tested.

A single evolutionarily divergent mutation determines the different FAD-binding affinities of human and rat NQO1 due to site-specific phosphorylation

The phosphomimetic mutation S82D in the cancer-associated, FAD-dependent human NADP(H):quinone oxidoreductase 1 (hNQO1) causes a decrease in flavin-adenine dinucleotide-binding affinity and intracellular stability. We test in this work whether the evolutionarily recent neutral mutation R80H in the vicinity of S82 may alter the strong functional effects of S82 phosphorylation through electrostatic interactions. We show using biophysical and bioinformatic analyses that the reverse mutation H80R prevents the effects of S82D phosphorylation on hNQO1 by modulating the local stability. Consistently, in rat NQO1 (rNQO1) which contains R80, the effects of phosphorylation were milder, resembling the behaviour found in hNQO1 when this residue was humanized in rNQO1 (by the R80H mutation). Thus, apparently neutral and evolutionarily divergent mutations may determine the functional response of mammalian orthologues towards phosphorylation.

Electron transfer flavoprotein and its role in mitochondrial energy metabolism in health and disease

Electron transfer flavoprotein (ETF) is an enzyme with orthologs from bacteria to humans. Human ETF is nuclear encoded by two separate genes, ETFA and ETFB, respectively. After translation, the two subunits are imported to the mitochondrial matrix space and assemble into a heterodimer containing one FAD and one AMP as cofactors. ETF functions as a hub taking up electrons from at least 14 flavoenzymes, feeding them into the respiratory chain. This represents a major source of reducing power for the electron transport chain from fatty acid oxidation and amino acid degradation. Transfer of electrons from the donor enzymes to ETF occurs by direct transfer between the enzyme bound flavins, a process that is tightly regulated by the polypeptide chain and by protein:protein interactions. ETF, in turn relays electrons to the iron sulfur cluster of the inner membrane protein ETF:QO, from where they travel via the FAD in ETF:QO to ubiquinone, entering the respiratory chain at the level of complex III. ETF recognizes its dehydrogenase partners via a recognition loop that anchors the protein on its partner followed by dynamic movements of the ETF flavin domain that bring redox cofactors in close proximity, thus promoting electron transfer. Genetic mutations in the ETFA or ETFB genes cause the Mendelian disorder multiple acyl-CoA dehydrogenase deficiency (MADD; OMIM #231680). We here review the knowledge on human ETF and investigations of the effects of disease-associated missense mutations in this protein that have promoted the understanding of the essential role that ETF plays in cellular metabolism and human disease.

Clinical characteristics and gene mutation analysis of an adult patient with ETFDH‑related multiple acyl‑CoA dehydrogenase deficiency

Multiple acyl‑CoA dehydrogenase deficiency (MADD) is a rare autosomal recessive disorder of fatty acid metabolism caused by defects in electron transfer flavoprotein (ETF) or electron transfer flavoprotein dehydrogenase (ETFDH). These defects are mainly classified into the neonatal and late‑onset types, based on their clinical manifestations. ETFDH gene mutations are generally considered to be associated with the late‑onset type. The present study reported an adult woman with late‑onset MADD accompanied with biochemical and muscle biopsy findings indicating metabolic disorders. Gene sequencing analysis showed that the c.1514T>C homozygous mutation in the region of the 12th exon of the ETFDH gene, which led to the amino acid substitution p.I505T (isoleucine > threonine), resulting in defective ETFDH protein function. The results of family verification revealed that the homozygous mutation originated from her parents. The female patient was treated with a large dose of vitamin B2, L‑carnitine and coenzyme Q10, and the symptoms were significantly relieved. The c.1514T>C mutation in the ETFDH gene, was considered as a novel pathogenic mutation that had not been previously reported. Therefore, it was hypothesized that this mutation was responsible for the clinical characteristics of the adult female patient. Overall, this novel mutation could expand the spectrum of the ETFDH gene mutation and provide the basis for the etiological and prenatal diagnosis of MADD.

Development of Novel Experimental Models to Study Flavoproteome Alterations in Human Neuromuscular Diseases: The Effect of Rf Therapy

Inborn errors of Riboflavin (Rf) transport and metabolism have been recently related to severe human neuromuscular disorders, as resulting in profound alteration of human flavoproteome and, therefore, of cellular bioenergetics. This explains why the interest in studying the “flavin world”, a topic which has not been intensively investigated before, has increased much over the last few years. This also prompts basic questions concerning how Rf transporters and FAD (flavin adenine dinucleotide) -forming enzymes work in humans, and how they can create a coordinated network ensuring the maintenance of intracellular flavoproteome. The concept of a coordinated cellular “flavin network”, introduced long ago studying humans suffering for Multiple Acyl-CoA Dehydrogenase Deficiency (MADD), has been, later on, addressed in model organisms and more recently in cell models. In the frame of the underlying relevance of a correct supply of Rf in humans and of a better understanding of the molecular rationale of Rf therapy in patients, this review wants to deal with theories and existing experimental models in the aim to potentiate possible therapeutic interventions in Rf-related neuromuscular diseases.

Riboflavin Deficiency-Implications for General Human Health and Inborn Errors of Metabolism

As an essential vitamin, the role of riboflavin in human diet and health is increasingly being highlighted. Insufficient dietary intake of riboflavin is often reported in nutritional surveys and population studies, even in non-developing countries with abundant sources of riboflavin-rich dietary products. A latent subclinical riboflavin deficiency can result in a significant clinical phenotype when combined with inborn genetic disturbances or environmental and physiological factors like infections, exercise, diet, aging and pregnancy. Riboflavin, and more importantly its derivatives, flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), play a crucial role in essential cellular processes including mitochondrial energy metabolism, stress responses, vitamin and cofactor biogenesis, where they function as cofactors to ensure the catalytic activity and folding/stability of flavoenzymes. Numerous inborn errors of flavin metabolism and flavoenzyme function have been described, and supplementation with riboflavin has in many cases been shown to be lifesaving or to mitigate symptoms. This review discusses the environmental, physiological and genetic factors that affect cellular riboflavin status. We describe the crucial role of riboflavin for general human health, and the clear benefits of riboflavin treatment in patients with inborn errors of metabolism.