Brown-Vialetto-Van Laere and Fazio-Londe syndromes: SLC52A3 mutations with puzzling phenotypes and inheritance.

BACKGROUND: Brown-Vialetto-Van Laere syndrome (BVVLS) and Fazio-Londe disease (FLD) are rare neurological disorders presenting with pontobulbar palsy, muscle weakness and respiratory insufficiency. Mutations in SLC52A2 (hRFVT-2) or SLC52A3 (hRFVT-3) genes can be responsible for these disorders with an autosomal recessive pattern of inheritance. The aim of this study was to screen for mutations in SLC52A2 and SLC52A3 among Indian families diagnosed with BVVLS and FLD. METHODS: SLC52A2 and SLC52A3 were screened in one FLD and three BVVLS patients by exon-specific amplification using PCR and sequencing. In silico predictions using bioinformatics tools and confocal imaging using HEK-293 cells were performed to determine the functional impact of identified mutations. RESULTS: Genetic analysis of a mother and son with BVVLS was identified with a novel homozygous mutation c.710C>T (p.Ala237Val) in SLC52A3. This variant was found to have an autosomal pseudodominant pattern of inheritance, which was neither listed in the Exome Variant Server or in the 1000 Genomes Project database. In silico analysis and confocal imaging of the p.Ala237Val variant showed higher degree of disorderness in hRFVT-3 that could affect riboflavin transport. Furthermore, a common homozygous mutation c.62A>G (p.Asn21Ser) was identified in other BVVLS and FLD patients. Despite having different clinical phenotypes, both BVVLS and FLD can be attributed to this mutation. CONCLUSION: A rare and peculiar pattern of autosomal pseudodominant inheritance is observed for the first time in two genetically related BVVLS cases with Indian origin and a common mutation c.62A>G (p.Asn21Ser) in SLC52A3 can be responsible for both BVVLS and FLD with variable phenotypes.

Adaptive regulation of riboflavin transport in heart: effect of dietary riboflavin deficiency in cardiovascular pathogenesis.

Deficiency or defective transport of riboflavin (RF) is known to cause neurological disorders, cataract, cardiovascular anomalies, and various cancers by altering the biochemical pathways. Mechanisms and regulation of RF uptake process is well characterized in the cells of intestine, liver, kidney, and brain origin, while very little is known in the heart. Hence, we aimed to understand the expression and regulation of RF transporters (rRFVT-1 and rRFVT-2) in cardiomyocytes during RF deficiency and also investigated the role of RF in ischemic cardiomyopathy and mitochondrial dysfunction in vivo. Riboflavin uptake assay revealed that RF transport in H9C2 is (1) significantly higher at pH 7.5, (2) independent of Na+ and (3) saturable with a Km of 3.746 µM. For in vivo studies, male Wistar rats (110-130 g) were provided riboflavin deficient food containing 0.3 ± 0.05 mg/kg riboflavin for 7 weeks, which resulted in over expression of both RFVTs in mRNA and protein level. RF deprivation resulted in the accumulation of cardiac biomarkers, histopathological abnormalities, and reduced mitochondrial membrane potential which evidenced the key role of RF in the development of cardiovascular pathogenesis. Besides, adaptive regulation of RF transporters upon RF deficiency signifies that RFVTs can be considered as an effective delivery system for drugs against cardiac diseases.

Fazio-Londe syndrome in siblings from India with different phenotypes.

BACKGROUND: Fazio-Londe syndrome also called progressive bulbar palsy of childhood is a very rare motor neuron disease of pediatric age group characterized by progressive paralysis of lower cranial nerves. OBJECTIVE: To describe Fazio-Londe syndrome in sibling with different phenotype. METHODS: A 6 years old female child presented with inability to close eyes, difficulty in swallowing, respiratory muscle weakness and voice change since 5 yr of age. Examination showed lower motor neuron facial nerve palsy, absent gag reflex, tongue atrophy, fasciculation, limb wasting and exaggerated deep tendon reflexes. An 11 year old boy, elder sibling of the above child presented with similar complaints at 10 years of age, other than later onset and lack of respiratory problem. Genetic testing in both cases confirmed the diagnosis of Fazio-Londe Syndrome. CONCLUSION: In any child who presents with progressive bulbar palsy with lower motor neuron facial palsy a diagnosis of Fazio-Londe Syndrome should be considered and family members should also be screened.

Riboflavin Responsive Mitochondrial Dysfunction in Neurodegenerative Diseases.

Mitochondria are the repository for various metabolites involved in diverse energy-generating processes, like the TCA cycle, oxidative phosphorylation, and metabolism of amino acids, fatty acids, and nucleotides, which rely significantly on flavoenzymes, such as oxidases, reductases, and dehydrogenases. Flavoenzymes are functionally dependent on biologically active flavin adenine dinucleotide (FAD) or flavin mononucleotide (FMN), which are derived from the dietary component riboflavin, a water soluble vitamin. Riboflavin regulates the structure and function of flavoenzymes through its cofactors FMN and FAD and, thus, protects the cells from oxidative stress and apoptosis. Hence, it is not surprising that any disturbance in riboflavin metabolism and absorption of this vitamin may have consequences on cellular FAD and FMN levels, resulting in mitochondrial dysfunction by reduced energy levels, leading to riboflavin associated disorders, like cataracts, neurodegenerative and cardiovascular diseases, etc. Furthermore, mutations in either nuclear or mitochondrial DNA encoding for flavoenzymes and flavin transporters significantly contribute to the development of various neurological disorders. Moreover, recent studies have evidenced that riboflavin supplementation remarkably improved the clinical symptoms, as well as the biochemical abnormalities, in patients with neuronopathies, like Brown-Vialetto-Van-Laere syndrome (BVVLS) and Fazio-Londe disease. This review presents an updated outlook on the cellular and molecular mechanisms of neurodegenerative disorders in which riboflavin deficiency leads to dysfunction in mitochondrial energy metabolism, and also highlights the significance of riboflavin supplementation in aforementioned disease conditions. Thus, the outcome of this critical assessment may exemplify a new avenue to enhance the understanding of possible mechanisms in the progression of neurodegenerative diseases and may provide new rational approaches of disease surveillance and treatment.

SLC52A2 [p.P141T] and SLC52A3 [p.N21S] causing Brown-Vialetto-Van Laere Syndrome in an Indian patient: First genetically proven case with mutations in two riboflavin transporters.

BACKGROUND: Brown-Vialetto-Van Laere Syndrome (BVVLS), a rare neurological disorder characterized by bulbar palsies and sensorineural deafness, is mainly associated with defective riboflavin transporters encoded by the SLC52A2 and SLC52A3 genes. METHODS: Here we present a 16-year-old BVVLS patient belonging to a five generation consanguineous family from Indian ethnicity with two homozygous missense mutations viz., c.421C>A [p.P141T] in SLC52A2 and c.62A>G [p.N21S] in SLC52A3. RESULTS: Functional characterization based on 3H-riboflavin uptake assay and live-cell confocal imaging revealed that the effect of mutation c.421C>A [p.P141T] identified in SLC52A2 had a slight reduction in riboflavin uptake; on the other hand, the c.62A>G [p.N21S] identified in SLC52A3 showed a drastic reduction in riboflavin uptake, which appeared to be due to impaired trafficking and membrane targeting of the hRFVT-3 protein. CONCLUSIONS: This is the first report presenting mutations in both riboflavin transporters hRFVT-2 and hRFVT-3 in the same BVVLS patient. Also, c.62A>G [p.N21S] in SLC52A3 appears to contribute more to the disease phenotype in this patient than c.421C>A [p.P141T] in SLC52A2.