A novel chronic childhood sensory predominant neuropathy

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.

The audiovestibular profile of Brown-Vialetto-Van Laere syndrome

BACKGROUND

Brown-Vialetto-Van Laere syndrome, a rare disorder associated with motor, sensory and cranial nerve neuropathy, is caused by mutations in riboflavin transporter genes SLC52A2 and SLC52A3. Hearing loss is a characteristic feature of Brown-Vialetto-Van Laere syndrome and has been shown in recent studies to be characterised by auditory neuropathy spectrum disorder.

METHOD

This study reports the detailed audiovestibular profiles of four cases of Brown-Vialetto-Van Laere syndrome with SLC52A2 and SLC52A3 mutations. All of these patients had auditory neuropathy spectrum disorder.

RESULTS

There was significant heterogeneity in vestibular function and in the benefit gained from cochlear implantation. The audiological response to riboflavin therapy was also variable, in contrast to generalised improvement in motor function.

CONCLUSION

We suggest that comprehensive testing of vestibular function should be conducted in Brown-Vialetto-Van Laere syndrome, in addition to serial behavioural audiometry as part of the systematic examination of the effects of riboflavin.

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.

Charcot-Marie-Tooth disease: a clinico-genetic confrontation

Charcot-Marie-Tooth disease (CMT) is the most common neuromuscular disorder. It represents a group of clinically and genetically heterogeneous inherited neuropathies. Here, we review the results of molecular genetic investigations and the clinical and neurophysiological features of the different CMT subtypes. The products of genes associated with CMT phenotypes are important for the neuronal structure maintenance, axonal transport, nerve signal transduction and functions related to the cellular integrity. Identifying the molecular basis of CMT and studying the relevant genes and their functions is important to understand the pathophysiological mechanisms of these neurodegenerative disorders, and the processes involved in the normal development and function of the peripheral nervous system. The results of molecular genetic investigations have impact on the appropriate diagnosis, genetic counselling and possible new therapeutic options for CMT patients.

Polyneuropathies in teenagers: A clinicopathological study of 45 cases

The aim of the present study was to investigate the causes of polyneuropathy in teenagers and to describe some characteristic clinical, laboratory, electrophysiological and pathological features. Forty-five patients with peripheral nervous disorders aged 13-19 were studied. Hereditary polyneuropathy of different types was diagnosed in 28 patients (62%); nine showed chronic inflammatory demyelinating polyneuropathy (CIDP) and two showed vasculitic neuropathy. In two more cases polyneuropathy was attributed to toxic agents, while among the rest, one was diagnosed as metachromatic leucodystrophy (juvenile type), one as adrenoleucodystrophy, one as porphyric neuropathy and one as Fabry disease. The high incidence of hereditary neuropathies in teenagers differs from that in adults, but is similar to that encountered in children. In our study, CIDP appears to be a frequent cause of neuropathy in teenagers, while the other causes are broadly similar to those found in studies concerning children rather than adults.