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

Long-term outcomes in children with riboflavin transporter deficiency and surveillance recommendations

The aim of this longitudinal case series was to describe long-term functional outcome in a group of individuals with riboflavin transporter deficiency (RTD) treated with high-dose oral riboflavin. Data were collected between 2012 to 2022. Eleven individuals with RTD were assessed at 12-month intervals for monitoring of disease progression. Patients had commenced high-dose oral riboflavin from the time of genetic diagnosis. Individuals for whom riboflavin supplementation was initiated early after disease onset had better outcomes compared to those in whom diagnosis was delayed. Despite ongoing riboflavin supplementation, the Charcot-Marie-Tooth disease Pediatric Scale (CMTPedS) total score and the subitems of balance and the 6-Minute Walk Test distance as well as respiratory function worsened, while grip strength improved. There was evidence of improvement in hearing loss and optic atrophy limited to the first 12 months of treatment. While treatment with riboflavin slowed disease progression, patients were left with residual disability. To track disease progression and response to riboflavin supplementation over time, we recommend using the RTD Pediatric Scale and provide a list of clinical measures for regular surveillance of children with RTD.

Atypical presentations in an RTD patient and report of novel SLC52A3 and SLC52A2 mutations

INTRODUCTION

Riboflavin Transporter Deficiency (RTD) is a rare neurological disorder characterized by pontobulbar palsy, hearing loss, and motor cranial nerve involvement. SLC52A3 and SLC52A2 mutations are causes of RTD. SLC52A2 mutations are usually found in childhood onset cases. Fifteen Iranian RTD diagnosed patients without SLC52A2 mutations have been previously described. We aimed to identify causative mutations in two childhood cases.

METHODS

We recruited patients with diagnosis of BVVL. Comprehensive clinical evaluations were performed on the patients. SLC52A3 and SLC52A2 genes were PCR-amplified and Sanger sequenced. Candidate disease causing variations were screened for segregation with disease status in the respective families and control individuals.

RESULTS

A novel homozygous SLC52A3 mutation (p.Met1Val) and a heterozygous SLC52A2 mutation (p.Ala288Val) were both observed in one proband with typical RTD presentations. The aggregate of presentations in the early stages of disease in the second patient that included weakness in the lower extremities, absence of bulbar or hearing defects, prominent sensory polyneuropathy as evidenced in electrodiagnostic studies, and absence of sensory symptoms including sensory ataxia did not prompt immediate RTD diagnosis. Dysarthria and decreased hearing manifested later in the disease course. A novel homozygous SLC52A2 (p.Val314Met) mutation was identified.

CONCLUSION

A literature search found recent reports of other atypical RTD presentations. These include MRI findings, speech understanding difficulties accompanied by normal hearing, anemia, and left ventricular non-compaction. Knowledge of unusual presentations lessens the chance of misdiagnosis or delayed RTD diagnosis which, in light of favorable effects of riboflavin supplementation, is of immense importance.

NADH improves AIF dimerization and inhibits apoptosis in iPSCs-derived neurons from patients with auditory neuropathy spectrum disorder

Auditory neuropathy spectrum disorder (ANSD) is a hearing impairment involving disruptions to inner hair cells (IHCs), ribbon synapses, spiral ganglion neurons (SGNs), and/or the auditory nerve itself. The outcomes of cochlear implants (CI) for ANSD are variable and dependent on the location of lesion sites. Discovering a potential therapeutic agent for ANSD remains an urgent requirement. Here, 293T stable transfection cell lines and patient induced pluripotent stem cells (iPSCs)-derived auditory neurons carrying the apoptosis inducing factor (AIF) p.R422Q variant were used to pursue a therapeutic regent for ANSD. Nicotinamide adenine dinucleotide (NADH) is a main electron donor in the electron transport chain (ETC). In 293T stable transfection cells with the p.R422Q variant, NADH treatment improved AIF dimerization, rescued mitochondrial dysfunctions, and decreased cell apoptosis. The effects of NADH were further confirmed in patient iPSCs-derived neurons. The relative level of AIF dimers was increased to 150.7 % (P = 0.026) from 59.2 % in patient-neurons upon NADH treatment. Such increased AIF dimerization promoted the mitochondrial import of coiled-coil-helix-coiled-coil-helix domain-containing protein 4 (CHCHD4), which further restored mitochondrial functions. Similarly, the content of mitochondrial calcium (mCa2+) was downregulated from 136.7 % to 102.3 % (P = 0.0024) in patient-neurons upon NADH treatment. Such decreased mCa2+ levels inhibited calpain activity, ultimately reducing the percentage of apoptotic cells from 30.5 % to 21.1 % (P = 0.021). We also compared the therapeutic effects of gene correction and NADH treatment on hereditary ANSD. NADH treatment had comparable restorative effects on functions of ANSD patient-specific cells to that of gene correction. Our findings offer evidence of the molecular mechanisms of ANSD and introduce NADH as a potential therapeutic agent for ANSD therapy.

Benefit of high-dose oral riboflavin therapy in riboflavin transporter deficiency

Riboflavin transporter deficiency (RTD) is a progressive inherited neuropathy of childhood onset, characterised by pontobulbar palsy, sensorineural deafness, sensory ataxia, muscle weakness, optic atrophy and respiratory failure. Riboflavin supplementation is beneficial in short-term reports, but the quantum of benefit in various clinical domains is not well understood. A PubMed search was conducted, which identified 94 genetically confirmed cases of RTD who received riboflavin supplementation and had follow-up assessments. Information on the clinical and functional status before and after riboflavin supplementation was collected and analysed. Seventy-six of the 94 patients (80.9%) showed an overall improvement after riboflavin supplementation, and the remaining (19.1%) were stable, though some patients had deteriorations in individual domains with no reported deaths. The domains that had the highest rates of response to riboflavin supplementation were gross motor function (93.3% improved), bulbar palsy (91.3%) and ataxia (90.0%). Improvements were also seen in limb muscle weakness, audiology, facial nerve palsy and respiratory function. Despite treatment, many patients required assistance to ambulate and had severe or profound hearing loss and some remained gastrostomy or tracheostomy dependent. Riboflavin supplementation is a lifesaving intervention for patients with RTD and results in a profound improvement in several functional domains, with early diagnosis and treatment further improving outcomes. Despite treatment, patients are left with residual disability. There is a need to accurately measure functional outcomes in children with RTD and develop additional disease-modifying therapies.

Normal Outcome With Prenatal Intervention for Riboflavin Transporter Defect

BACKGROUND

Riboflavin transporter deficiency is a rare but severe neurometabolic disorder.

METHODS

We report two siblings with pathogenic variants in SLC52A3 gene, resulting in riboflavin transporter 3 deficiency.

RESULTS

The first sibling was diagnosed at age 11 months with severe respiratory compromise and regression of developmental milestones. His symptoms significantly improved with riboflavin supplementation therapy. The younger sibling was diagnosed by antenatal genetic analysis; riboflavin supplementation was initiated in utero and continued from birth. Now at age two years, he remains clinically asymptomatic despite genetic confirmation of riboflavin transporter deficiency.

CONCLUSIONS

Antenatal riboflavin supplementation is a safe and effective treatment for the prevention of symptomatic manifestations of riboflavin transporter deficiency.

Role of Otolaryngologists in the Treatment of Patients With Riboflavin Transporter Deficiency: A Case Report

Riboflavin transporter deficiency (RTD) is a rare genetic disorder that can have detrimental effects on the nervous system, causing progressive neurodegeneration. Here, we report the second case of RTD in Saudi Arabia. An 18-month-old boy presented to the otolaryngology clinic with six weeks history of progressive noisy breathing associated with drooling, choking, and difficulty in swallowing. Progressive regression of the child's motor and communicative abilities was reported as well. Upon examination, the child had biphasic stridor, chest retractions, bilateral facial palsy, and hypotonia. The presence of an aerodigestive foreign body or congenital anomalies was excluded using bronchoscopy and esophagoscopy. Empirical high-dose riboflavin replacement therapy was initiated upon anticipation of diagnosis. Whole exome sequencing revealed a SLC52A3 gene mutation, which confirmed the diagnosis of RTD. After a period of intensive care unit (ICU) admission with endotracheal intubation, the child's general condition improved, and he was weaned off of respiratory support. Tracheostomy was avoided in this patient, as he responded to riboflavin replacement therapy. During the disease course, an audiological assessment revealed severe bilateral sensorineural hearing loss. He was discharged home on gastrostomy feeding owing to the risk of frequent aspiration, and he was regularly followed up by the swallowing team. The early initiation of high-dose riboflavin replacement appears to be of great value. The benefits of cochlear implants in RTD have been reported, but not fully established. This case report will increase awareness in the otolaryngology community about patients with this rare disease who might initially present to the clinic with an otolaryngology-related complaint.