A case report of riboflavin transporter deficiency: A novel heterozygous pathogenic variant in the SLC52A3 gene

Riboflavin transporter deficiency (RTD) is a neurodegenerative disorder that presents from infancy to adulthood with a progressive axonal neuropathy characterized by a variety of neurologic symptoms including hearing loss, weakness, bulbar palsy, and respiratory insufficiency. Pathogenic variants in SLC52A2 and SLC52A3 are implicated in the pathogenesis of RTD type 2 and 3, respectively. Early identification of this disorder is critical, as it is treatable with riboflavin supplementation. We describe a 16-year-old female with a phenotype consistent with RTD3 found to have a novel heterozygous SLC52A3 variant. Though RTD is typically considered an autosomal recessive condition, her heterozygous variant was thought to be disease causing after further genetic analysis and given her improvement in response to riboflavin supplementation. This case highlights the importance of reinterpretation of genetic testing, particularly when there is a high clinical suspicion for disease.

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.

Recent advances in riboflavin transporter RFVT and its genetic disease

Riboflavin (vitamin B2) is essential for cellular growth and function. It is enzymatically converted to flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), which participate in the metabolic oxidation-reduction reactions of carbohydrates, amino acids, and lipids. Human riboflavin transporters RFVT1, RFVT2, and RFVT3 have been identified and characterized since 2008. They are highly specific transporters of riboflavin. RFVT3 has functional characteristics different from those of RFVT1 and RFVT2. RFVT3 contributes to absorption in the small intestine, reabsorption in the kidney, and transport to the fetus in the placenta, while RFVT2 mediates the tissue distribution of riboflavin from the blood. Several mutations in the SLC52A2 gene encoding RFVT2 and the SLC52A3 gene encoding RFVT3 were found in patients with a rare neurological disorder known as Brown-Vialetto-Van Laere syndrome. These patients commonly present with bulbar palsy, hearing loss, muscle weakness, and respiratory symptoms in infancy or later in childhood. A decrease in plasma riboflavin levels has been observed in several cases. Recent studies on knockout mice and patient-derived cells have advanced the understanding of these mechanisms. Here, we summarize novel findings on RFVT1-3 and their genetic diseases and discuss their potential as therapeutic drugs.

Three cases of adult-onset Brown-Vialetto-Van Laere syndrome: Novel variants in SLC52A3 gene and MRI abnormalities

Brown-Vialetto-Van Laere syndrome is a rare, autosomal, recessive neurological condition caused by variants in the riboflavin transporter genes SLC52A2 and SLC52A3. Here, we report on three cases. Case 1 was a 35-year-old woman from a consanguineous family who presented with progressive deafness, subacute multiple cranial nerve impairments (III, VII, IX, XII), and MRI abnormalities (including as hypersignal from the cranial nerves). The patient was homozygous for a novel SLC52A3variant. Case 2 was the woman's brother, who presented similar symptoms. Case 3 was an 18-year-old woman experiencing progressive hearing loss, bilateral steppage gait and a cranial nerves impairment (VII and XII). MRI revealed hypersignal in the root nerves and cauda equina. A novel heterozygous variant in SLC52A3 was identified. A subacute history of polyradiculoneuropathy along with progressive deafness, cranial nerve impairment, and MRI abnormalities should raise suspicion for Brown-Vialetto-Van Laere syndrome.

A case report of sudden-onset auditory neuropathy spectrum disorder associated with Brown-Vialetto-Van Laere syndrome (riboflavin transporter deficiency)

OBJECTIVE

The purpose of this paper is to describe a child with auditory neuropathy spectrum disorder (ANSD) associated with Brown-Vialetto-Van Laere (BVVL) syndrome, which is a rare, inherited, neurodegenerative disorder that is caused by defects in riboflavin transporter genes.

DESIGN

We report the audiological and clinical profile of a child who presented with a complaint of sudden loss of speech understanding associated with an atypical form of ANSD. He was later diagnosed with BVVL.

STUDY SAMPLE

An 11-year-old boy with ANSD associated with BVVL.

RESULTS

The patient's severe neurological symptoms improved within a year of supplementation with high doses of riboflavin. His fluctuating hearing loss and 0% WDS remained unchanged. The patient was able to use hearing aids without any discomfort after treatment initiation, but he stopped using them again due to a lack of benefit in speech understanding. Although cochlear implantation was recommended, the patient and his family decided not to consider it for another year since they still had hope for complete recovery.

CONCLUSIONS

Sudden-onset ANSD can be the earliest sign of undetected BVVL syndrome. Early detection of BVVL is crucial since all symptoms can be reversible with an early intervention of high doses of riboflavin supplementation.

Brown Vialetto Van Laere syndrome: presenting with left ventricular non-compaction and mimicking mitochondrial disorders

BACKGROUND

Brown-Vialetto-Van Laere syndrome (BVVLS) is a rare, treatable neurodegenerative disorder with a variable clinical presentation, caused by mutations in three different riboflavin transporter genes.

CASE

An 11-year-old-boy presented with respiratory insufficiency and a rapidly progressive muscle weakness. He was the fifth child of a consanguineous marriage with a medical history of hearing loss. He was peripherally week with a reduced muscle tone. Upper extremity muscles were effected more than lower limbs. He deteriorated rapidly and became quadriplegic. Brain magnetic resonance imaging and magnetic resonance spectroscopy were normal. Echocardiography revealed left ventricular non-compaction. A homozygous c.1088C > T (p.363L) missense mutation was identified in SLC52A2 gene. Significant clinical improvement was seen with high dose riboflavin.

CONCLUSION

This is the first reported BVVLS case presented with left ventricle-non compaction which may be caused by a secondary respiratory chain deficiency. Riboflavin transporter deficiencies should be considered in the differential diagnosis of mitochondrial disorders and secondary respiratory chain deficiencies should be thought during the follow-up of BVVLS.

A case of adult onset Sandhoff disease that mimics Brown-Vialetto-Van Laere syndrome

Sandhoff disease is a rare fatal infantile neurologic disorder. Adult onset Sandhoff is even rarer. Variability of clinical features in adult onset Sandhoff patients and overlaps between these and features of other neurologic diseases have sometimes led to mis-diagnosis. We describe an adult onset Sandhoff disease affected individual whose clinical presentation were also consistent with the Brown-Vialetto-Van Laere syndrome (BVVL) diagnosis. Screening of BVVL-causing genes, SLC52A3 and SLC52A2, did not identify candidate disease-causing mutations, but exome sequencing revealed compound heterozygous mutations in the known Sandhoff disease-causing gene, HEXB. Decreased blood hexosaminidase activity and evidence of cerebellar atrophy confirmed Sandhoff disease diagnosis. To the best of our knowledge, this is the first report of a Sandhoff disease case that mimics BVVL and that presents with prominent cranial nerve involvement. For differential diagnosis, measurement of hexosaminidase activity and MRI should quickly be performed. Genetic analysis can be done for confirmation of diagnosis.

Hematologic presentation and the role of untargeted metabolomics analysis in monitoring treatment for riboflavin transporter deficiency

Riboflavin transporter deficiency (RTD) (MIM #614707) is a neurogenetic disorder with its most common manifestations including sensorineural hearing loss, peripheral neuropathy, respiratory insufficiency, and bulbar palsy. Here, we present a 2-year-old boy whose initial presentation was severe macrocytic anemia necessitating multiple blood transfusions and intermittent neutropenia; he subsequently developed ataxia and dysarthria. Trio-exome sequencing detected compound heterozygous variants in SLC52A2 that were classified as pathogenic and a variant of uncertain significance. Bone marrow evaluation demonstrated megaloblastic changes. Notably, his anemia and neutropenia resolved after treatment with oral riboflavin, thus expanding the clinical phenotype of this disorder. We reiterate the importance of starting riboflavin supplementation in a young child who presents with macrocytic anemia and neurological features while awaiting biochemical and genetic work up. We detected multiple biochemical abnormalities with the help of untargeted metabolomics analysis associated with abnormal flavin adenine nucleotide function which normalized after treatment, emphasizing the reversible pathomechanisms involved in this disorder. The utility of untargeted metabolomics analysis to monitor the effects of riboflavin supplementation in RTD has not been previously reported.

Brown-Vialetto-Van Laere syndrome and Fazio-Londe syndrome: A novel mutation and in silico analyses

Brown-Vialetto-Van Laere syndrome, a rare neurological disorder is due to SLC52A3 mutations. Here, the SLC52A3 protein and its mutations are in silico structurally and functionally analyzed among all the reported patients and a novel mutation is also reported. After clinical evaluations, SLC52A3 gene was sequenced and segregation analysis of the mutations was also checked. A comprehensive search was performed on the reported mutations of SLC52A3 gene. In silico structural and functional analyses of the mutations and interactome analyses of the protein were done using available software tools. Mutations of 37 affected individuals were identified. Thirty three mutations were determined. c.502A > C was a novel variant that it was segregated within the family. One mutation (c.639C > G) was responsible for 12% of the mutations. Segregation analysis, secondary structure, functional prediction achieved for the novel mutation showed pathogenicity of this variant. BVVL is a very rare disorder; SLC52A3 mutations are distributed among different populations and there might be one frequent mutation in this gene. BVVL should be more considered in Iran. In addition to segregation analysis, computational analyses could accelerate understanding the extent of pathogenicity of the novel variants.

A Case with Brown-Vialetto-Van Laere Syndrome: A Sudden Onset Auditory Neuropathy Spectrum Disorder

The Brown-Vialetto-Van Laere syndrome (BVVLS) is a rare neurological disorder that may present at all ages with sensorineural hearing loss, bulbar palsy and respiratory compromise. We describe a 6-year-old male patient who suffered bilateral sudden onset severe hearing loss for two years. Audiological investigations revealed sudden onset auditory neuropathy spectrum disorder bilaterally. He also had neurological complaints. During riboflavin therapy an improvement in hearing loss and the benefit of hearing aids were observed. In BVVLS, it is difficult to plan and apply auditory rehabilitation interventions and the results vary from patient to patient. In audiological evaluation, it should be borne in mind that subjective and objective tests are complemental. Early medical intervention and regular audiological follow-up are very important for effective hearing rehabilitation in the patients with BVVLS.

A Chinese pedigree with Brown-Vialetto-Van Laere syndrome due to two novel mutations of SLC52A2 gene: clinical course and response to riboflavin

Background

Brown-Vialetto-Van Laere Syndrome (BVVLS), a rare neurological disorder characterized by motor, sensory, and cranial neuronopathies, is mainly associated with defective riboflavin transporters encoded by SLC52A2 and SLC52A3 genes. Clinical outcomes have been shown to be improved significantly by high-dose riboflavin supplementation. The aim of this study was to identify genetic causes and further evaluate the clinical course and response to riboflavin in a Chinese pedigree with BVVLS.

Case presentation

We report the novel compound heterozygous variants c.1328G>A p.(Cys443Tyr) and c.1022_1023insC p. (Leu341Profs*103) of SLC52A2 gene in a female proband who presented in our out-patient clinic at the age of one-year-old with progressive mental and motor regression, breath holding, and brain stem dysfunction including facial weakness, hearing loss, dysphagia. Following high-dose riboflavin supplementation, the respiratory insufficiency and mental, motor, and bulbar function improved. However, sensorineural hearing loss was not improved. The missense variant site was highly conserved. Both variants were not found in the population database gnomAD. The two variants were inherited from her mother and father, respectively. Both variants were predicted to be deleterious by Polyphen2, Mutation taster, and SIFT and were classified as likely pathogenic according to the ACMG guideline.

Conclusions

Two novel pathogenic variations of SLC52A2 gene were firstly found from a Chinese pedigree with BVVLS. Clinical outcomes could be improved by early diagnosis and riboflavin supplementation.

Riboflavin transporter deficiency mimicking mitochondrial myopathy caused by complex II deficiency

Biallelic likely pathogenic variants in SLC52A2 and SLC52A3 cause riboflavin transporter deficiency. It is characterized by muscle weakness, ataxia, progressive ponto-bulbar palsy, amyotrophy, and sensorineural hearing loss. Oral riboflavin halts disease progression and may reverse symptoms. We report two new patients whose clinical and biochemical features were mimicking mitochondrial myopathy. Patient 1 is an 8-year-old male with global developmental delay, axial and appendicular hypotonia, ataxia, and sensorineural hearing loss. His muscle biopsy showed complex II deficiency and ragged red fibers consistent with mitochondrial myopathy. Whole exome sequencing revealed a homozygous likely pathogenic variant in SLC52A2 (c.917G>A; p.Gly306Glu). Patient 2 is a 14-month-old boy with global developmental delay, respiratory insufficiency requiring ventilator support within the first year of life. His muscle biopsy revealed combined complex II + III deficiency and ragged red fibers consistent with mitochondrial myopathy. Whole exome sequencing identified a homozygous likely pathogenic variant in SCL52A3 (c.1223G>A; p.Gly408Asp). We report two new patients with riboflavin transporter deficiency, caused by mutations in two different riboflavin transporter genes. Both patients presented with complex II deficiency. This treatable neurometabolic disorder can mimic mitochondrial myopathy. In patients with complex II deficiency, riboflavin transporter deficiency should be included in the differential diagnosis to allow early treatment and improve neurodevelopmental outcome.

Clinical, pathological and functional characterization of riboflavin-responsive neuropathy

Brown-Vialetto-Van Laere syndrome represents a phenotypic spectrum of motor, sensory, and cranial nerve neuropathy, often with ataxia, optic atrophy and respiratory problems leading to ventilator-dependence. Loss-of-function mutations in two riboflavin transporter genes, SLC52A2 and SLC52A3, have recently been linked to Brown-Vialetto-Van Laere syndrome. However, the genetic frequency, neuropathology and downstream consequences of riboflavin transporter mutations are unclear. By screening a large cohort of 132 patients with early-onset severe sensory, motor and cranial nerve neuropathy we confirmed the strong genetic link between riboflavin transporter mutations and Brown-Vialetto-Van Laere syndrome, identifying 22 pathogenic mutations in SLC52A2 and SLC52A3, 14 of which were novel. Brain and spinal cord neuropathological examination of two cases with SLC52A3 mutations showed classical symmetrical brainstem lesions resembling pathology seen in mitochondrial disease, including severe neuronal loss in the lower cranial nerve nuclei, anterior horns and corresponding nerves, atrophy of the spinothalamic and spinocerebellar tracts and posterior column–medial lemniscus pathways. Mitochondrial dysfunction has previously been implicated in an array of neurodegenerative disorders. Since riboflavin metabolites are critical components of the mitochondrial electron transport chain, we hypothesized that reduced riboflavin transport would result in impaired mitochondrial activity, and confirmed this using in vitro and in vivo models. Electron transport chain complex I and complex II activity were decreased in SLC52A2 patient fibroblasts, while global knockdown of the single Drosophila melanogaster riboflavin transporter homologue revealed reduced levels of riboflavin, downstream metabolites, and electron transport chain complex I activity. This in turn led to abnormal mitochondrial membrane potential, respiratory chain activity and morphology. Riboflavin transporter knockdown in Drosophila also resulted in severely impaired locomotor activity and reduced lifespan, mirroring patient pathology, and these phenotypes could be partially rescued using a novel esterified derivative of riboflavin. Our findings expand the genetic, clinical and neuropathological features of Brown-Vialetto-Van Laere syndrome, implicate mitochondrial dysfunction as a downstream consequence of riboflavin transporter gene defects, and validate riboflavin esters as a potential therapeutic strategy.

A Case of Brown-Vialetto-Van Laere Syndrome Due To a Novel Mutation in SLC52A3 Gene: Clinical Course and Response to Riboflavin

Brown-Vialetto-Van Laere syndrome is a rare disorder characterized by motor, sensory, and cranial neuronopathies, associated with mutations in SLC52A2 and SLC52A3 genes that code for human riboflavin transporters RFVT2 and RFVT3, respectively. The authors describe the clinical course of a 6-year-old girl with Brown-Vialetto-Van Laere syndrome and a novel homozygous mutation c.1156T>C in the SLC52A3 gene, who presented at the age of 2.5 years with progressive brain stem dysfunction including ptosis, facial weakness, hearing loss, dysphagia, anarthria with bilateral vocal cord paralysis, and ataxic gait. She subsequently developed respiratory failure requiring tracheostomy and worsening dysphagia necessitating a gastrostomy. Following riboflavin supplementation, resolution of facial diplegia and ataxia, improvements in ptosis, and bulbar function including vocalization and respiration were noted. However, her sensorineural hearing loss remained unchanged. Similar to other cases of Brown-Vialetto-Van Laere syndrome, our patient responded favorably to early riboflavin supplementation with significant but not complete neurologic recovery.

Brown-Vialetto-Van Laere Syndrome as a Mimic of Neuroimmune Disorders: 3 Cases From the Clinic and Review of the Literature

We present 3 patients identified at 2 different institutions with Brown-Vialetto-Van Laere syndrome. Each patient was initially diagnosed with a neuroimmune disorder for a period of a few weeks to a few months. In each case, genetic analysis revealed mutations in one of the riboflavin transporters, confirming Brown-Vialetto-Van Laere syndrome. It is likely that Brown-Vialetto-Van Laere syndrome is more common than previously reported, and because it mimics neuroimmune disorders, it may be misdiagnosed as such. It shares many features with diseases such as chronic inflammatory demyelinating neuropathy, may present with positive cerebrospinal fluid antibody titers, and may transiently respond to intravenous immunoglobulin. We review the literature on Brown-Vialetto-Van Laere syndrome and Fazio-Londe syndrome, 2 riboflavin transporter disorders, looking for clinical presentations that may lead to confusion with neuroimmune disorders. We emphasize the importance of correctly diagnosing the disease, as its treatment is relatively benign and will stop progression of the disease and may even reverse it.

Mutations in riboflavin transporter present with severe sensory loss and deafness in childhood

INTRODUCTION

We have identified a large consanguineous Lebanese family with 5 individuals with severe childhood-onset recessive sensory loss associated with deafness and variable optic atrophy.

METHODS

Autozygosity mapping was performed in all affected individuals, followed by whole-exome sequencing (WES) in 2 individuals.

RESULTS

WES identified a homozygous missense mutation (c.916G>A, p.G306R) in the cerebral riboflavin transporter SLC52A2, recently shown to cause Brown-Vialetto-Van-Laere syndrome (BVVLS), which is considered primarily a motor neuronopathy. Our patients have a phenotype distinct from BVVLS, characterized by severe progressive sensory loss mainly affecting vibration and proprioception that evolves to include sensorineural hearing loss in childhood, variable degrees of optic atrophy, and marked upper extremity weakness and atrophy. Treatment of 3 patients with 400 mg/day riboflavin over 3 months produced definite clinical improvement.

CONCLUSIONS

Mutations in SLC52A2 result in a recognizable phenotype distinct from BVVLS. Early recognition of this disorder is critical, given its potential treatability.

Treatable childhood neuronopathy caused by mutations in riboflavin transporter RFVT2

Childhood onset motor neuron diseases or neuronopathies are a clinically heterogeneous group of disorders. A particularly severe subgroup first described in 1894, and subsequently called Brown-Vialetto-Van Laere syndrome, is characterized by progressive pontobulbar palsy, sensorineural hearing loss and respiratory insufficiency. There has been no treatment for this progressive neurodegenerative disorder, which leads to respiratory failure and usually death during childhood. We recently reported the identification of SLC52A2, encoding riboflavin transporter RFVT2, as a new causative gene for Brown-Vialetto-Van Laere syndrome. We used both exome and Sanger sequencing to identify SLC52A2 mutations in patients presenting with cranial neuropathies and sensorimotor neuropathy with or without respiratory insufficiency. We undertook clinical, neurophysiological and biochemical characterization of patients with mutations in SLC52A2, functionally analysed the most prevalent mutations and initiated a regimen of high-dose oral riboflavin. We identified 18 patients from 13 families with compound heterozygous or homozygous mutations in SLC52A2. Affected individuals share a core phenotype of rapidly progressive axonal sensorimotor neuropathy (manifesting with sensory ataxia, severe weakness of the upper limbs and axial muscles with distinctly preserved strength of the lower limbs), hearing loss, optic atrophy and respiratory insufficiency. We demonstrate that SLC52A2 mutations cause reduced riboflavin uptake and reduced riboflavin transporter protein expression, and we report the response to high-dose oral riboflavin therapy in patients with SLC52A2 mutations, including significant and sustained clinical and biochemical improvements in two patients and preliminary clinical response data in 13 patients with associated biochemical improvements in 10 patients. The clinical and biochemical responses of this SLC52A2-specific cohort suggest that riboflavin supplementation can ameliorate the progression of this neurodegenerative condition, particularly when initiated soon after the onset of symptoms.