Ocular Biomarkers of Riboflavin Transporter Deficiency

To describe the clinical presentation with a focus on ocular manifestations and response to riboflavin supplementation of 3 patients with riboflavin transporter deficiency (RTD) caused by mutations in SLC52A2 (SLC52A2-RTD).

Original Contribution Section Editors: Clare Fraser, MD Susan Mollan, MD Ocular Biomarkers of Riboflavin Transporter Deficiency Sabrina Bulas, MS, Emma C. Bedoukian, MS, LCGC, Erin C. O’Neil, MD, Ian D. Krantz, MD, Sabrina W. Yum, MD, Grant T. Liu, MD, Tomas S. Aleman, MD Background: To describe the clinical presentation with a focus on ocular manifestations and response to riboflavin supplementation of 3 patients with riboflavin transporter deficiency (RTD) caused by mutations in SLC52A2 (SLC52A2-RTD). Methods: This is a retrospective review of records of 3 children (aged 18, n = 2 and age = 8, n = 1) with SLC52A2RTD. Patients underwent comprehensive ophthalmic evaluations including color vision testing, pattern visual-evoked potentials (pVEPs, 1 patient) and spectral domain optical coherence tomography (SD-OCT) imaging. Patients received riboflavin supplements from the time of the molecular diagnosis of RTD. Results: Two unrelated 18-year-old patients with SLC52A2RTD had a symptomatic onset with sensorineural hearing loss and auditory neuropathy/dys-synchrony since age 3 and 11, respectively. On examination 7 years after symptomatic onset, they showed subnormal visual acuities (20/30 and 20/60, both eyes, respectively), preserved color vision, and a thin but measurable retinal ganglion cell layer (GCL) and nerve fiber (RNFL). The inner and outer nuclear layers were normal. The asymptomatic SLC52A2positive brother of one of these patients started riboflavin supplementation right after the molecular diagnosis and had normal vision and SD-OCTs 7 years later. Onset of riboflavin supplementation in one of the 2 symptomatic cases resulted in acute improvement of the pattern visualevoked potential and vision. Conclusions: Retinal ganglion cells and their axons are uniquely susceptible to RTD compared with other highly energy-dependent retinal neurons, such as photoreceptors, raising the possibility for alternative mechanisms of disease or protection. Riboflavin supplementation results in acute Division of Ophthalmology (SB, ECB, ECO, GTL, TSA), The Roberts Individualized Medical Genetics Center (ECB, IDK), Division of Genetics (IDK), Division of Neurology (SWY), The Children’s Hospital of Philadelphia, Philadelphia, Pennsylvania; and Department of Ophthalmology (ECO, TSA), Center for Advanced Retinal and Ocular Therapeutics, Scheie Eye Institute, Perelman School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania. T. S. Aleman holds intellectual property through the University of Pennsylvania for the development of a virtual reality orientation and mobility test. Address correspondence to Tomas S. Aleman, MD, Perelman Center for Advanced Medicine, University of Pennsylvania, 3400 Civic Center Boulevard, Philadelphia, PA 19104; E-mail: aleman@ pennmedicine.upenn.edu 110 functional improvement of vision and long-term preservation of GCL and RNFL if initiated early. Journal of Neuro-Ophthalmology 2023;43:110–115 doi: 10.1097/WNO.0000000000001678 © 2022 by North American Neuro-Ophthalmology Society R iboflavin transporter deficiency (RTD) caused by mutations in the Solute Carrier Family 52 Member (SLC52A2 and SLC52A3) genes leads to an infrequent progressive and severe neurodegenerative disorder (1–3). Patients present during childhood with cranial, sensory, and motor neuropathies, resulting in sensorineural hearing loss (SNHL), vision loss, sensory ataxia, and muscle weakness. Severe cases may be fatal within the first decade of life. RTDs have been described with 2 phenotypically overlapping conditions: Brown–Vialetto–Van Laere syndrome (BVVLS) (sensorineural deafness, bulbar palsy, and respiratory compromise) and Fazio–Londe syndrome (same associations without hearing loss) (1,4,5). SLC52A2 (OMIM 607882) encodes a human riboflavin transporter (hRFVT-2), one of 3 such transporters described to date. hRFVT-2 is ubiquitously expressed with highest levels found in the small intestine and brain, where it is believed to be involved in the cellular uptake of riboflavin (6). Riboflavin, the water-soluble vitamin B2, acts as a precursor for flavin mononucleotide and flavin adenine dinucleotide, key cofactors in numerous metabolic pathways critical for normal cellular development and survival, including mitochondrial function through participation in the Krebs cycle and electron transport chain (ETC) (2). Accordingly, fibroblasts from patients affected by SLC52A2-RTD and knockout models show a marked decrease in the ETC activity (7). Ocular manifestations of SLC52A2-RTD include nystagmus and optic nerve atrophy (2,7–11). High-dose riboflavin supplementation has been lifesaving and improves neurologic symptoms, including vision (1,2,8,9,11,12). Identification of RFVT-2 deficiency on routine metabolic screen is difficult as plasma riboflavin metabolite levels may be normal. Identification of mutations in SLC52A2 is increasingly possible through gene screening, which should enable prompt Bulas et al: J Neuro-Ophthalmol 2023; 43: 110-115 Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited. Original Contribution treatment to deter neurologic deterioration or even improve symptoms, including vision (8). Here, we present a detailed retinal structural and functional phenotype of 3 patients from 2 families with biallelic mutations in SLC52A2 as well as the impact of riboflavin supplementation. METHODS This is a retrospective review of clinical records. The clinical visits adhered to the Health Insurance Portability and Accountability Act of 1996 (HIPAA) regulations; all personal identifiers have been removed. Patients underwent a complete ophthalmic examination. Imaging was performed with spectral domain optical coherence tomography (SD-OCT) (Heidelberg Engineering GmbH, Heidelberg, Germany). Segmentation of the SD-OCT was performed using the instrument’s segmentation algorithm supervised by examining the longitudinal profiles (LRPs) using a publicly available imaging analysis software (https://imagej.nih.gov/ij/download.html) (13). Fullfield electroretinograms (ERGs) and pattern visual-evoked potentials (pVEPs) were recorded using a computer-based system (Espion, Diagnosys LLC, Littleton, MA) adhering to the standards of the International Society for Clinical Electrophysiology of Vision (ISCEV). RESULTS Case 1 An 18-year-old woman presented with blurred vision. She had normal prenatal and perinatal history, including normal newborn hearing and metabolic screening. She had a speech delay, attention deficit hyperactivity disorder, headache, vertigo, and hearing loss first noted at 11 years of age. At age 12, an auditory brainstem response (ABR) confirmed bilateral asymmetric hearing loss. The right ear showed a severe sensorineural hearing loss (SNHL) from 250 to 1,500 Hz, milder above 2,000 Hz; the left ear showed a moderate hearing loss between 250 and 2,000 Hz, normal above 3,000 Hz. Distortion product otoacoustic emissions (DPOAEs) were consistent with auditory neuropathy/dys-synchrony in both ears, a form of hearing loss where cochlear hair cell function is preserved but afferent neural conduction along the auditory pathway is impaired (14). Temporal bone MRIs were unrevealing. Microarray revealed a 2.43 Mb microduplication within chromosome 1q21.1q21.2, involving over 30 genes and classified as a variant of unknown significance. Distal 1q21.2 microduplications are relatively frequent and have been reported in individuals with variable phenotypes including autism and developmental delay but are also observed in healthy individuals (15). A hearing loss gene panel (87 genes) identified 2 heterozygous variants of unknown significance in ATP6V1B1 (c.1061-13AG) and TRIOBP (c.5705A.C) that could not explain her SNHL. Mutations in these genes have only been associated with biallelic inheritance and autosomal Bulas et al: J Neuro-Ophthalmol 2023; 43: 110-115 recessive SNHL and are not known to be associated with ocular or mitochondrial disease. Hearing aids did not help, and she was fitted with cochlear implants (CIs). She was diagnosed with myopia at age 10 with best-corrected visual acuities (BCVA) of 20/30 in each eye at age 13, which was considered subnormal and was not investigated. At the time, there were no concerns of systemic involvement. BCVA remained subnormal over the next 5 years. In addition, she struggled of differentiating colors and low-contrast images and complained of photophobia. On presentation to our clinic at age 18, BCVA were 20/40 (right eye) and 20/50 (left eye); color vision (Farnsworth–Munsell D-15 panel) was normal. Ocular motility was full without nystagmus. She had mild temporal pallor of the optic nerves (Fig. 1A). SD-OCT demonstrated retinal ganglion cell layer (GCL) thinning conferring the fovea a flattened contour from loss of the foveal ridge (Fig. 1B, asterisk) and a thin retinal nerve fiber layer (RNFL) (Fig. 1B, arrow). Quantitation of the SD-OCT cross-sections confirmed thinning of the GCL with a normal inner nuclear layer and outer nuclear layer (Fig. 1C). Full-field ERGs were within normal limits for both cone-mediated (using LA 3.0 protocol) and rod-mediated (DA 0.01 protocol) responses (not shown). pVEPs had latencies at the upper limit of normal when measured with 1° elements and were nondetectable above the noise level when measured with a smaller 259 pattern (Fig. 2A). A sweep VEP objectively predicted visual acuities at the 20/25 (0.9 decimal) level (Fig. 2C). At this point, given that the prior microarray and SNHL panel findings were unrevealing, singleton exome testing was performed, which identified a pathogenic variant (c.808C.T; p.Q270X) and a variant of unknown significance (c.617C.T; p.A206V, paternally inherited) in SCL25A2, providing a likely diagnosis. Her mother was unavailable for testing. Plasma acylcarnitine levels were normal. Her neurological examination was normal, without sensory impairment or weakness, and with normal reflexes. Nerve conduction studies and electromyography (NCS and EMG) revealed normal motor and sensory responses, but with evidence of mild, chronic denervation consistent with a mild chronic motor neuropathy. She was started on high-dose riboflavin (50 mg/kg/day divided 3 times a day). At 6 months follow-up, pVEPs showed faster latencies for the 1° pattern and detectable responses to a smaller 259 target; there were no short-term structural changes (Fig. 2A). Case 2 A 3-year-old girl initially presented with concerns for hearing loss, which was eventually confirmed at age 7 as mild, bilateral, lower frequency SNHL, that raised to normal hearing at higher frequencies. She had normal prenatal, birth, and developmental histories, including normal newborn hearing and metabolic screening. The family history was unrevealing. DPOAEs were consistent with auditory neuropathy/dys-synchrony. Temporal bone MRI was normal. By age 10, her SNHL had progressed and 111 Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited. Original Contribution FIG. 1. Retinal imaging in SLC52A2-associated disease. A. Fundus appearance. B. SD-OCT horizontal, 9 mm cross section through the fovea and into the temporal retina in the patient. Outer retinal sublaminae are numbered according to conventional nomenclature: 1. outer limiting membrane (OLM), 2. inner segment ellipsoid region (ISe or EZ), 3. the contact cylinder between the apical RPE microvilli and the photoreceptor outer segments tips or interdigitation zone (IZ), and 4. basal RPE and Bruch membrane (RPE/BrM). Arrow in the patient points to a thin RNFL; asterisk denotes a flattened foveal ridge caused by GCL thinning. C. Thickness of the GCL, INL, and ONL as a function of eccentricity in the patient compared with the normal range (gray band = normal mean ± 2 SD). Nuclear layers are labeled: GCL indicates ganglion cell layer; INL, inner nuclear layer; N, nasal; ONL, outer nuclear layer; RNFL, retinal nerve fiber layer; SD-OCT, spectral domain optical coherence tomography; T, temporal retina. CIs were implanted bilaterally. A hearing loss gene screening panel (66 genes) identified homozygous c.11G.A variants of uncertain significance in GJB2; microarray was normal. She had decreased vision at age 11. Fundus examination showed bilateral mild optic nerve pallor that was not present on a routine examination 2 years prior. There was peripapillary RNFL and a GCL thinning on SD-OCT (Fig. 2B); the middle and outer retina were normal (Fig. 2C). In addition to her worsening hearing loss and vision, she feared descending stairs because of “fear of falling” attributed to gait ataxia (see neurologic examination below). The addition of an optic neuropathy to her SNHL prompted a trio exome 112 sequencing which uncovered biallelic pathogenic variants in SLC52A2 (a maternally inherited c.149dupA; p.Y50X; a paternally inherited c.1258G.A; p.A420T). She was found to have an 8976T.C; p.L149P variant of unknown significance in MT-ATP6 on mitochondrial sequencing, which was not present in the maternal sample tested. Disease manifestations of MT-ATP6–associated disease only occur at high heteroplasmy (16). Given her 38% heteroplasmy and normal lactate and pyruvate, this variant is likely noncontributory to her disease phenotype. Plasma acylcarnitine levels were normal. Her neurologic examination showed increased conversational response time; normal sensation to Bulas et al: J Neuro-Ophthalmol 2023; 43: 110-115 Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited. Original Contribution FIG. 2. Impact of riboflavin supplementation on structural and functional parameters in SLC52A2-associated disease. A. Pattern visual-evoked potential (p-VEP) elicited with (1° and 25-minute black and white, 100% contrast pattern) using a commercially available system (Espion e3, Diagnosys LLC, Lowell, MA) before and after supplementation with riboflavin. Gray traces are individual responses; averaged responses are shown in black. Vertical dashed lines in left panels denote the implicit time of the main VEP component (P100) and the shift to faster timing (horizontal left arrow) after treatment with riboflavin. Only one eye shown for clarity. GCL indicates ganglion cell layer; RNFL, retinal nerve fiber layer; SD-OCT, spectral domain optical coherence tomography. B. Peripapillary RNFL thickness along a 1.7 mm radius radial scans centered on the optic nerve head and compared with an existing normative database available in a commercially available SD-OCT system (Spectralis SD-OCT, Heidelberg Engineering, Heidelberg, Germany). Thickness values are color coded as thinned, borderline, within normal limits or above normal limits. Right eye of each patient is on the left panels, left eyes to the right, per convention. C. 6-mm long SD-OCT cross-sections through the fovea demonstrating a thinned GCL in Case 2 compared with a normal GCL in Case 3 (yellow bars). light touch and temperature, but reduced position sensation at toes; and decreased vibration sensation up to the knees. She had an ataxic gait with a positive Romberg’s sign, was unable to tandem walk, and had difficulty descending stairs. There was mild dysmetria. Deep tendon reflexes were globally absent. She had no muscle weakness but had difficulty with hopping. The patient was started on riboflavin aiming for 50 mg/kg/day, although the up titration of the dose only reached 30 mg/kg/day because of family concern that the medication was inducing fingertip dryness or cracking. NCS and EMG 2 months after treatment initiation revealed absent sensory responses in upper and lower extremities and evidence of chronic denervation in distal leg muscles. Bulas et al: J Neuro-Ophthalmol 2023; 43: 110-115 Initially, she demonstrated mild examination improvements (resolved dysmetria, conversion of Romberg’s sign to negative, and improved tandem gait) on therapy, but after 1 year, she stopped taking riboflavin for several months, which resulted in worsened tandem gait and positive Romberg’s sign. After restarting therapy, she continued to struggle with adherence and showed variable but overall mild decline in her balance and gait ataxia, reflected by worsening of the CMTPedS score (19/44 at 11 years old and 22/44 at 18 years old) (17). Her VA in the last visit (age 18) was unchanged at 20/60 in each eye. Color vision (Ishihara isochromatic plates) was normal. Motility was full without nystagmus. Fundus examination showed a mildly pale optic nerve bilaterally. 113 Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited. Original Contribution Case 3 The patient is the asymptomatic younger brother of Case 2. At 4 years of age, targeted genetic testing by PCR amplification and capillary sequencing conducted after the diagnosis of his sister confirmed the same SLC52A2 mutations. His neurologic examination before starting riboflavin therapy was normal, except for absent deep tendon reflexes in his legs. NCS and EMG revealed a pure sensory neuropathy/neuronopathy with the absence of sural, median, ulnar, and radial sensory responses. He was started on riboflavin at 50 mg/kg/day divided 3 times daily at age 4. Repeat NCS 2 years later was stable. At age 8, he had a normal audiology evaluation. His BCVA was 20/20 in each eye. Color vision (Ishihara test) was normal. Fundus examination and his RNFL (Fig. 2B) and GCL (Fig. 2C) thicknesses were normal bilaterally. The preservation of the retinal structure and visual function in this patient who underwent supplementation with riboflavin at a presymptomatic stage contrasts with the functional and structural outcome of his sister (Case 2) and of Case 1, who were both diagnosed and treated after symptomatic onset. Although Case 1 and 2 showed stable subnormal central vision on riboflavin supplementation, they show obvious residual, presumed stable RNFL and GCL loss (Figs. 1, 2B). CONCLUSIONS Ocular findings in SLC52A2-RTD include variable but generally poor visual acuity, nystagmus, and optic nerve atrophy (2,7–11). Consistent with the reported variability in severity of the ocular presentations, two of our patients had subnormal or normal central vision, including preserved color vision and only mild optic nerve pallor. Despite the nearly subclinical presentation of Cases 1 and 2, SDOCT revealed overt GCL and RNFL thinning bilaterally. Relative preservation of VA and color vision in similar cases may relate to selective sparing of GCL cells and axonal groups that serve the most central regions. Nystagmus, which was not present in any of our cases, is a welldocumented, often presenting manifestation of RTD (2,7–11). The presence of nystagmus in RTD may be assumed to result from oculomotor instability as a result of the loss of axons serving the most central retina, especially during critical periods of maturation of the fixation mechanisms, or alternatively, from central abnormalities affecting the oculomotor system (18). The later presentation of visual symptoms in our patients is consistent with a mechanism where the onset of GC and/or axonal damage from RTD occurs after fixation mechanisms are established. Now that the molecular cause of RTD is known with the identification of mutations in SLC52A2 or SLC53A3, prompt treatment to deter neurologic deterioration or even improve symptoms, including vision, is possible (8). To avoid delays in the diagnosis, a high level of suspicion should be adopted in complex cases where subtle and inter114 mittent combinations of neurologic symptoms, such as ataxia, hearing, and vision loss, are present. Including RTD genes on hearing loss, neuro-ophthalmology (optic nerve atrophy/optic neuropathy), metabolic, ataxia, and even mitochondrial disease gene screening panels may lead to a prompt diagnosis. Early diagnosis and treatment, exemplified in Case 3, can lead to a successful functional and structural outcome. A detailed structural and functional evaluation as illustrated herein can guide the diagnostic work up and management whenever a systemic metabolic or neurologic disease with a possible ocular association is suspected. The mechanism of the optic neuropathy in RTD is not well elucidated. A process encompassing axonal neuropathy combined with reactive inflammatory demyelination has been previously posited (11). The retina and the visual system has one of the highest energy demands per tissue weight (19,20). Riboflavin cofactors, flavin adenine dinucleotide and flavin mononucleotide, are crucial to oxidative metabolism. The neurologic disease resulting from RTD should, in theory, relate directly to the energy demands from the tissue affected by the resulting mitochondrial dysfunction and, by extension, should primarily affect mitochondria-rich cellular elements (21). In all of our cases, however, the middle, outer retina and retinal pigment epithelium were spared, despite similarly high energetic needs, especially for mitochondrionrich, energy-dependent, photoreceptors (19). Interestingly, this pattern resembles the functional loss in the auditory pathway where nerve conduction impairment exists despite preserved function in cochlear hair cells, which are analogous to retinal photoreceptor cells, and contrasts with the rod and cone photoreceptor degeneration that results from the ablation of a riboflavin transporter in an animal model (14,22,23). The selective GCL abnormalities and optic nerve atrophy reported in patients with RTD, as well as in numerous primary mitochondrial diseases that result in optic neuropathies, suggests a particular vulnerability or an inability to maintain cellular function, not only at the neuronal cell body and dendritic contacts, but also along long axonal projections (3). The GCL and surrounding glia may depend on the availability of riboflavin and its cofactors for cellular processes beyond energetics (20). High-dose riboflavin supplementation can be life saving and has improved neurologic symptoms, including vision in these patients (1,2,8,9,11,12). Optic nerve function improved significantly and acutely after supplementation with riboflavin in one of our patients (Case 1). We speculate that riboflavin supplementation may have stabilized optic nerve disease and vision loss in one patient (Case 2) and may have prevented vision loss in another (Case 3). Unlike the obvious acute changes in visual function after supplementation documented in Case 1, long-term therapeutic effects on the natural history of the optic neuropathy associated with this disorder require confirmation in appropriately designed prospective studies. Nevertheless, the cases Bulas et al: J Neuro-Ophthalmol 2023; 43: 110-115 Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited. Original Contribution illustrate that detailed characterizations of the retinal structure and visual function in patients with RTD have the potential to provide biomarkers of the impact of the disease and its management on the health of the visual system and, by extension, of the overall neurologic health of patients suffering from these conditions. STATEMENT OF AUTHORSHIP Conception and design: S. Bulas, E. C. Bedoukian, E. C. O’Neil, T. S. Aleman; Acquisition of data: S. Bulas, E. C Bedoukian, E. C. O’Neil, I. D. Krantz, S. W. Yum, G. T. Liu, T. S. Aleman; Analysis and interpretation of data: S. Bulas, E. C. Bedoukian, E. C. O’Neil, I. D. Krantz, S. W. Yum, G. T. Liu, T. S. Aleman. Drafting the manuscript: S. Bulas, E. C. Bedoukian, E. C. O’Neil, I. D. Krantz, S. W. Yum, G. T. Liu, T. S. Aleman; Revising the manuscript for intellectual content: S. Bulas, E. C Bedoukian, E. C. O’Neil, I. D. Krantz, S. W. Yum, G. T. Liu, T. S. Aleman. Final approval of the completed manuscript: S. Bulas, E. C. Bedoukian, E. C. O’Neil, I. D. Krantz, S. W. Yum, G. T. Liu, T. S. Aleman. 7. 8. 9. 10. 11. ACKNOWLEDGMENTS 12. Dr. E. C. O’Neil is a recipient of the Diana Davis Spencer Clinical/Research Fellowship Award Program in Inherited Retinal Degenerations within FFB’s Alan Laties Career Development Program. Additional support is from grants from Hope for Vision, Macula Vision Research Foundation, and The Pennsylvania Lions Sight Conservation and Research Foundation. 13. REFERENCES 15. 1. Bosch AM, Stroek K, Abeling NG, Waterham HR, Ijlst L, Wanders RJ. The Brown-Vialetto-Van Laere and Fazio Londe syndrome revisited: natural history, genetics, treatment and future perspectives. Orphanet J Rare Dis. 2012;7:83. 2. Foley AR, Menezes MP, Pandraud A, Gonzalez MA, Al-Odaib A, Abrams AJ, Sugano K, Yonezawa A, Manzur AY, Burns J, Hughes I, McCullagh BG, Jungbluth H, Lim MJ, Lin JP, Megarbane A, Urtizberea JA, Shah AH, Antony J, Webster R, Broomfield A, Ng J, Mathew AA, O’Byrne JJ, Forman E, Scoto M, Prasad M, O’Brien K, Olpin S, Oppenheim M, Hargreaves I, Land JM, Wang MX, Carpenter K, Horvath R, Straub V, Lek M, Gold W, Farrell MO, Brandner S, Phadke R, Matsubara K, McGarvey ML, Scherer SS, Baxter PS, King MD, Clayton P, Rahman S, Reilly MM, Ouvrier RA, Christodoulou J, Züchner S, Muntoni F, Houlden H. Treatable childhood neuronopathy caused by mutations in riboflavin transporter RFVT2. Brain. 2014;137:44–56. 3. O’Callaghan B, Bosch AM, Houlden H. An update on the genetics, clinical presentation, and pathomechanisms of human riboflavin transporter deficiency. J Inherit Metab Dis. 2019;42:598–607. 4. Green P, Wiseman M, Crow YJ, Houlden H, Riphagen S, Lin JP, Raymond FL, Childs AM, Sheridan E, Edwards S, Josifova DJ. Brown-Vialetto-Van Laere syndrome, a Ponto-Bulbar palsy with deafness, is caused by mutations in C20orf54. Am J Hum Genet. 2010;86:485–489. 5. Dipti S, Childs AM, Livingston JH, Aggarwal AK, Miller M, Williams C, Crow YJ. Brown-Vialetto-Van Laere syndrome; variability in age at onset and disease progression highlighting the phenotypic overlap with Fazio-Londe disease. Brain Dev. 2005;27:443–446. 6. Yonezawa A, Inui K. Novel riboflavin transporter family RFVT/SLC52: identification, nomenclature, functional Bulas et al: J Neuro-Ophthalmol 2023; 43: 110-115 14. 16. 17. 18. 19. 20. 21. 22. 23. characterization and genetic diseases of RFVT/SLC52. Mol Aspects Med. 2013;34:693–701. Manole A, Jaunmuktane Z, Hargreaves I, Ludtmann MHR, Salpietro V, Bello OD, Pope S, Pandraud A, Horga A, Scalco RS, Li A, Ashokkumar B, Lourenço CM, Heales S, Horvath R, Chinnery PF, Toro C, Singleton AB, Jacques TS, Abramov AY, Muntoni F, Hanna MG, Reilly MM, Revesz T, Kullmann DM, Jepson JEC, Houlden H. Clinical, pathological and functional characterization of riboflavin-responsive neuropathy. Brain. 2017;140:2820–2837. Haack TB, Makowski C, Yao Y, Graf E, Hempel M, Wieland T, Tauer U, Ahting U, Mayr JA, Freisinger P, Yoshimatsu H, Inui K, Strom TM, Meitinger T, Yonezawa A, Prokisch H. Impaired riboflavin transport due to missense mutations in SLC52A2 causes Brown-Vialetto-Van Laere syndrome. J Inherit Metab Dis. 2012;35:943–948. Shashi V, Petrovski S, Schoch K, Crimian R, Case LE, Khalid R, El-Dairi MA, Jiang YH, Mikati MA, Goldstein DB. Sustained therapeutic response to riboflavin in a child with a progressive neurological condition, diagnosed by whole-exome sequencing. Cold Spring Harb Mol Case Stud. 2015;1:a000265. Jaeger B, Bosch AM. Clinical presentation and outcome of riboflavin transporter deficiency: mini review after five years of experience. J Inherit Metab Dis. 2016;39:559–564. Bamaga AK, Maamari RN, Culican SM, Shinawi M, Golumbek PT. Child neurology: Brown-Vialetto-Van Laere syndrome: dramatic visual recovery after delayed riboflavin therapy. Neurology. 2018;91:938–941. Anand G, Hasan N, Jayapal S, Huma Z, Ali T, Hull J, Blair E, McShane T, Jayawant S. Early use of high-dose riboflavin in a case of Brown-Vialetto-Van Laere syndrome. Dev Med Child Neurol. 2012;54:187–189. Aleman TS, Ventura CV, Cavalcanti MM, Serrano LW, Traband A, Nti AA, Gois AL, Bravo-Filho V, Martins TT, Nichols CW, Maia M, Belfort R. Quantitative assessment of microstructural changes of the retina in infants with congenital Zika syndrome. JAMA Ophthalmol. 2017;135:1069–1076. Rance G. Auditory neuropathy/dys-synchrony and its perceptual consequences. Trends Amplif. 2005;9:1–43. Dolcetti A, Silversides CK, Marshall CR, Lionel AC, Stavropoulos DJ, Scherer SW, Bassett AS. 1q21.1 microduplication expression in adults. Genet Med. 2013;15:282–289. Ganetzky RD, Stendel C, McCormick EM, Zolkipli-Cunningham Z, Goldstein AC, Klopstock T, Falk MJ. MT-ATP6 mitochondrial disease variants: phenotypic and biochemical features analysis in 218 published cases and cohort of 14 new cases. Hum Mutat. 2019;40:499–515. Burns J, Ouvrier R, Estilow T, Shy R, Laurá M, Pallant JF, Lek M, Muntoni F, Reilly MM, Pareyson D, Acsadi G, Shy ME, Finkel RS. Validation of the Charcot-Marie-Tooth disease pediatric scale as an outcome measure of disability. Ann Neurol. 2012;71:642–652. Mustari MJ, Ono S, Vitorello KC. How disturbed visual processing early in life leads to disorders of gaze-holding and smooth pursuit. Prog Brain Res. 2008;171:487–495. Wong-Riley M. Energy metabolism of the visual system. Eye Brain. 2010;2:99–116. Kubo Y, Miki S, Akanuma SI, Hosoya KI. Riboflavin transport mediated by riboflavin transporters (RFVTs/SLC52A) at the rat outer blood-retinal barrier. Drug Metab Pharmacokinet. 2019;34:380–386. Udhayabanu T, Manole A, Rajeshwari M, Varalakshmi P, Houlden H, Ashokkumar B. Riboflavin responsive mitochondrial dysfunction in neurodegenerative diseases. J Clin Med. 2017;6:52. Irinoda K, Sato S. Contribution to the ocular manifestation of riboflavin deficiency. Tohoku J Exp Med. 1954;61:93–104. Kelley RA, Al-Ubaidi MR, Sinha T, Genc AM, Makia MS, Ikelle L, Naash MI. Ablation of the riboflavin-binding protein retbindin reduces flavin levels and leads to progressive and dosedependent degeneration of rods and cones. J Biol Chem. 2017;292:21023–21034. 115 Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited.