Charcot-Marie-Tooth Disease Associated With a Novel Mutation in MFN2 Presenting With Subacute Vision Loss

A 40-year-old man with a medical history of hyperlipidemia and Type 2 diabetes mellitus presented for evaluation of vision loss that occurred 7 months before presentation. He experienced painless, blurred vision in the right eye, and similar symptoms in the left eye 1-2 weeks later with minimal progression. He also noticed that colors became dull. He denied any ocular pain, redness, diplopia, or headaches. He complained of bilateral lower extremity joint pain and muscle weakness, accompanied by loss of muscle tone over the past 10 years. Once an avid basketball player, he could no longer play the sport.

Clinical Correspondence Section Editors: Robert Avery, DO Karl C. Golnik, MD Caroline Froment, MD, PhD An-Guor Wang, MD Charcot–Marie–Tooth Disease Associated With a Novel Mutation in MFN2 Presenting With Subacute Vision Loss Yafeng Li, MD, PhD, Tomas S. Aleman, MD, Colin C. Quinn, MD, MS, Tian Xia, MD, Charles G. Miller, MD, PhD, Benjamin J. Kim, MD, Madhura A. Tamhankar, MD A 40-year-old man with a medical history of hyperlipidemia and Type 2 diabetes mellitus presented for evaluation of vision loss that occurred 7 months before presentation. He experienced painless, blurred vision in the right eye, and similar symptoms in the left eye 1–2 weeks later with minimal progression. He also noticed that colors became dull. He denied any ocular pain, redness, diplopia, or headaches. He complained of bilateral lower extremity joint pain and muscle weakness, accompanied by loss of muscle tone over the past 10 years. Once an avid basketball player, he could no longer play the sport. He consumed meat, and he denied any history of alcohol abuse or recreational drug use. His current medications were atorvastatin and glipizide. His lower extremity joint pain was managed with tramadol prescribed by his primary care physician for several years. Examination revealed best-corrected visual acuity of 20/150 in both eyes, with mild hyperopic refractive error. On Ishihara color plate testing, he could only discern the test plate in each eye. His pupils were equally reactive with no relative afferent pupillary defect. He was orthophoric on alignment testing, and ocular motilities were full. External, anterior segment examination, and intraocular pressures were normal. Fundus examination showed no apparent swelling or pallor of the optic nerves, with cup-to-disc ratio of 0.5 bilaterally and normal maculae (Fig. 1A, B). Optical coherence tomography imaging demonstrated grossly normal retinal nerve fiber layer (RNFL) thickness (Fig. 1C), but thinned ganglion cell layer + inner plexiform layer (GCL + IPL) (Fig. 1D). Goldmann perimetry showed enlarged blind spots and a large relative central scotoma in the right eye and a smaller central scotoma Department of Ophthalmology (YL, TSA, TX, CGM, BLK, MAT), Scheie Eye Institute, Philadelphia, Pennsylvania; and the Department of Neurology (CQ), Hospital of the University of Pennsylvania, Perelman School of Medicine of the University of Pennsylvania, Philadelphia, Pennsylvania. The authors report no conflicts of interest. Supplemental digital content is available for this article. Direct URL citations appear in the printed text and are provided in the HTML and PDF versions of this article on the journal’s Web site (www. jneuro-ophthalmology.com). Address correspondence to Madhura A. Tamhankar, MD, Scheie Eye Institute, 51 North 39th Street, Philadelphia, PA 19104; E-mail: [e-mail] Li et al: J Neuro-Ophthalmol 2022; 42: e381-e384 in the left eye, with an otherwise full peripheral extent (Fig. 1E). Humphrey visual field at 4 months after initial presentation showed a pattern of bilateral dense inferotemporal scotomas involving fixation (Fig. 1F). MRI of the brain and orbits was normal. Given the thinning of the GCL complex, the patient was referred to rule out any retinal etiology for vision loss. Full-field electroretinogram (ff-ERG) and multifocal electroretinograms (m-ERG) were within normal limits in both eyes, indicating that functional abnormalities likely reside superficial to the inner nuclear layer (INL; Fig. 2A, B). Laboratory studies, including genetic testing for the mitochondrial disease Leber hereditary optic neuropathy, serum folate and vitamin B12, as well as rapid plasma reagin, QuantiFERON TB-gold, angiotensin-converting enzyme, lysozyme, antinuclear antibodies, antineutrophil cytoplasmic antibody panel, and HIV, were all normal. Neurological work-up of his associated neuromuscular symptoms demonstrated elevated serum creatine kinase (738 U/L) and a length-dependent, axonal, motorpredominant polyneuropathy on nerve conduction studies and needle electromyography. Genetic testing using the Comprehensive Neuropathies Panel (Test 03200; Invitae, San Francisco, CA) revealed a novel heterozygous point mutation in the mitochondrial fusion protein mitofusin-2 (MFN2) gene (c.401T.A; p.Leu134Gln; See Supplemental Digital Content, Table S1, http://links.lww.com/ WNO/A512). Subsequent testing of the OPA1 gene uncovered no disease-causing variants. The patient’s parents were deceased, and his living, asymptomatic half-siblings were not available for genetic testing. To further assess the functional consequences of this MFN2 variant on visual function, a pattern ERG (p-ERG; Fig. 2C) and pattern visual evoked potentials (p-VEP; Fig. 2D) were recorded, which displayed grossly reduced amplitudes with normal latencies of the N95 and P100 components of the p-ERG and p-VEP, respectively. These functional and structural abnormalities are consistent with reduced retinal ganglion cell densities with preserved conduction velocities. Although the fundus appearance in our patient was unremarkable, his decreased visual acuity combined with central and paracentral scotomata, and associated thinning of the macular GCL + IPL was indicative of an optic neuropathy (Fig. 1C, D). This was corroborated by p-ERG and e381 Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited. Clinical Correspondence FIG. 1. Fundoscopy, optical coherence tomography, and visual field testing in a patient with a novel MFN2 point mutation (c.401T.A; p.Leu134Gln). A and B. Fundus photographs of both eyes: (A) normal-appearing optic nerve, vessels, and macula in the right eye with magnified view of the optic nerve head and peripapillary region (inset) and (B) the left eye with inset. C. OCT RNFL results at presentation, displaying normal thickness parameters in all quadrants. D. OCT GCL + IPL analysis displaying severe thinning in all sectors within the macula, with average thickness of 60 mm in the right eye and 64 mm in the left eye, respectively. E. At presentation, Goldmann visual fields show enlarged blind spots and central scotomas in both eyes, with noted difficulty in finding the fixation target. F. Four months after initial presentation, Humphrey visual fields 30-2 exhibit scattered superior losses and dense, homonymous appearing, inferotemporal losses involving fixation in both eyes. GCL, ganglion cell layer; IPL, inner plexiform layer; OCT, optical coherence tomography; RNFL, retinal nerve fiber layer. p-VEP findings. In addition, the patient’s neuromuscular symptoms prompted a neurological evaluation, which pointed to a motor predominant polyneuropathy. The novel variant in MFN2 confirmed our suspicion of a mitochondrial-based disorder while providing the specific diagnosis of autosomal dominant Charcot–Marie–Tooth disease Type 2A (CMT2A). This MFN2 variant has not been reported in large normal population databases. Based on crystallographic studies of the protein structure, the MFN2 molecule harbors a GTPase domain ranging from residues 96 to 357 that is critical for normal protein function (1). The novel variant reported herein replaces leucine by glutamine at codon 134 (L134Q), within the critical GTPase domain, and is considered likely pathogenic. In physicochemical terms, the substitution of leucine, a nonpolar amino acid, to glutamine, an uncharged polar amino e382 acid, has a Grantham score of greater than 100 (2). According to the classification of amino acid replacement, this leucine-to-glutamine substitution is also predicted to be deleterious. Although the phenotype matches the possible genetic cause because this MFN2 variant in an evolutionarily conserved region is expected to impair protein function, we could not obtain familial segregation data in support of its pathogenicity. Following the guidelines of the American College of Medical Genetics, we conservatively denote this variant as likely pathogenic until the uncertainty is resolved in future reports, allowing reclassification as pathogenic. CMT2A, an autosomal dominant axonal neuropathy, can be caused by mutations in the MFN2 gene that encodes MFN2, an intrinsic membrane protein of the mitochondrial outer membrane (1). Although optic atrophy has been reported Li et al: J Neuro-Ophthalmol 2022; 42: e381-e384 Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited. Clinical Correspondence FIG. 2. Standard electrophysiologic testing in the patient elicited using a commercially available system (espion e3; Diagnosys LLC, Lowell, MA). Only one eye shown for clarity; responses are nearly identical in the contralateral eye. A and B. Outer and middle retinal function: (A) Standard full-field electroretinograms (ff-ERGs) in the patient compared with a representative normal subject (gray traces). B. Multifocal electroretinography (m-ERG) traces (top panel) in the patient (black traces) compared with average normal traces (red), and m-ERG amplitudes (bottom panel) plotted against a pseudocolor scale. N indicates the nasal retina, T, temporal retina. Ff-ERGs and m-ERGs are within normal limits in the patient. C and D. Ganglion cell and optic nerve function: (C) Pattern ERG (p-ERG) to a 1° square pattern presented at 100% contrast in a CRT monitor using the same ERG instrumentation. The amplitudes of the standard p-ERG components are measured between peaks and troughs. The P50 amplitude is measured from the trough of N35 to the peak of P50. The N95 amplitude is measured from the peak of P50 to the trough of N95. This component is normally dominated by GCL responses. Amplitudes of the N95 component in the patient (black trace) are reduced compared with a representative normal subject (gray trace). The timing of the N95 component response is slightly faster in the patient (arrow) compared with the representative normal subject (vertical dashed line). D. Pattern visual-evoked potential (p-VEP) elicited with the same CRT monitor and pattern (1°, black and white, 100% contrast pattern) as used for the p-ERG. Responses in the patient (black traces) are markedly reduced in amplitude while the peak timing to the main component of the response (P100) is normal (vertical dashed line). CRT, cathode ray tube; GCL, ganglion cell layer. in some patients with CMT2A, the pathophysiology is not well understood, aside from it being related to mitochondrial dysfunction (3). Despite a mostly unremarkable fundus appearance, a comprehensive evaluation, including multimodal retinal imaging combined with electrophysiology and visual field testing, revealed macular GCL + IPL thinning, GCL dysfunction, and underlying central visual field defects in our patient. Li et al: J Neuro-Ophthalmol 2022; 42: e381-e384 These findings together led to the diagnosis of a neurologic disease unrecognized for years. Thinning of the GCL + IPL layer with an apparently discordant normal RNFL thickness in our patient suggests either a stage of the disease preceding axonal loss or axonal swelling because of defective intracellular transport or glial remodeling, thereby resulting in an overestimate of the peripapillary RNFL thickness as noted in other genetic optic e383 Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited. Clinical Correspondence neuropathies (4). Alternatively, and perhaps less likely in this axonal neuropathy, the findings may suggest a primary mechanism at the level of the soma of ganglion cells. Inner retinal abnormalities associated with some optic neuropathies, such as schisis and inner nuclear layer remodeling or dysfunction, were not detected in our case. The symmetric bitemporal, inferior visual field defects (Fig. 1F) corresponded to bilateral thinner nasal and superior parafoveal GCL + IPL (Fig. 1D), suggestive of a predilection for ganglion cells with projections within the papillomacular bundle and perhaps crossed fibers (5). Additional studies are necessary to further explore the mechanism of visual dysfunction in molecularly confirmed CMT2A patients, as well as to determine whether a visually presymptomatic stage may exist in some MNF2-associated optic neuropathies that may only be revealed with a comprehensive diagnostic approach as exemplified in this report. STATEMENT OF AUTHORSHIP Category 1: a. Conception and design: Y. Li , T. S. Aleman, T. Xia, C. G. Miller, B. J. Kim, and M. A. Tamhankar; b. Acquisition of data: Y. Li , T. S. Aleman, C. Quinn, T. Xia, C. G. Miller, B. J. Kim, and M. A. Tamhankar; c. Analysis and interpretation of data: Y. Li , T. S. Aleman, C. Quinn, B. J. Kim, and M. A. Tamhankar. Category 2: a. e384 Drafting the manuscript: Y. Li, T. S. Aleman, C. Quinn, T. Xia, C. G. Miller, B. J. Kim, and M. A. Tamhankar; b. Revising it for intellectual content: Y. Li, T. S. Aleman, C. Quinn, and M. A. Tamhankar. Category 3: a. Final approval of the completed manuscript: Y. Li, T. S. Aleman, B. J. Kim, and M. A. Tamhankar. REFERENCES 1. Li YJ, Cao YL, Feng JX, Qi Y, Meng S, Yang JF, Zhong YT, Kang S, Chen X, Lan L, Luo L, Yu B, Chen S, Chan DC, Hu J, Gao S. Structural insights of human mitofusin-2 into mitochondrial fusion and CMT2A onset. Nat Commun. 2019;10:4914. 2. Grantham R. Amino acid difference formula to help explain protein evolution. Science. 1974;185:862–864. 3. Hamedani AG, Wilson JA, Avery RA, Scherer SS. Optic neuropathy in charcot–marie–tooth disease. J Neuro-Ophthalmology. 2020 (epub ahead of print). 4. Barboni P, Savini G, Valentino ML, Montagna P, Cortelli P, De Negri AM, Sadun F, Bianchi S, Longanesi L, Zanini M, de Vivo A, Carelli V. Retinal nerve fiber layer evaluation by optical coherence tomography in Leber’s hereditary optic neuropathy. Ophthalmology. 2005;112:120–126. 5. Mallery RM, Poolman P, Thurtell MJ, Wang JK, Garvin MK, Ledolter J, Kardon RH. The pattern of visual fixation eccentricity and instability in optic neuropathy and its spatial relationship to retinal ganglion cell layer thickness. Invest Ophthalmol Vis Sci. 2016;57:OCT429–OCT437. Li et al: J Neuro-Ophthalmol 2022; 42: e381-e384 Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited.