Exclusively Macular Phenotype of Non-Syndromic MFSD8-Related Disease: A Case Report

Abstract

Introduction

The purpose of this report was to highlight the clinical phenotype and imaging findings in a patient with an exclusively macular phenotype of non-syndromic MFSD8-related disease and to provide clinical evidence for pathogenicity reclassification of a variant of uncertain significance MFSD8 c.291G>C (p.Trp97Cys).

Case Presentation

A 47-year-old male with progressive vision loss exhibited symptoms indicative of maculopathy. These included decreased central vision, visual distortions, photophobia, poor depth perception, glare, impaired dark/light adaptation, difficulty reading, depressed multifocal ERG responses, and central ellipsoid dropout on SD-OCT. Evaluation included genetic testing, segregation analysis, and a complete ophthalmic examination, including slit-lamp exam, dilated fundus exam, FAF, SD-OCT, ERG, and Humphrey 24-2 visual fields. A 351 gene retinal dystrophy panel revealed two variants in MFSD8, including one pathogenic variant (c.1006G>C, p.Glu336Gln) and one likely pathogenic variant (c.291G>C, p.Trp97Cys), confirmed to be in trans via segregation testing.

Conclusion

This case underscores the importance of genetic testing in confirming variant inheritance and describes the clinical phenotype associated with MFSD8 c.291G>C (p.Trp97Cys). The variant contributes to a pathological non-syndromic phenotype when in trans with a pathogenic variant. Given the syndromic variants of MFSD8, patients with this specific variant in the homozygous or compound heterozygous state should be closely monitored for clinical manifestations associated with this condition. Genetic counseling should be recommended for affected individuals and their close relatives to provide informed guidance regarding prognosis, reproductive risks, and available support resources.

Introduction

Inherited retinal dystrophies (IRDs) are a diverse group of genetically and phenotypically variable disorders that lead to progressive vision loss due to retinal photoreceptor degeneration or dysfunction [1, 2]. With prevalence of 1 in 3,000 to 1 in 4,000 individuals globally, IRDs represent a significant cause of visual impairment and blindness, affecting over 2 million people worldwide [3–5]. The cost of visual impairment is also significant and not exclusively limited to direct medical expenses, as many young individuals with IRDs face challenges including integrating into the workforce, in addition to a significant burden on caretakers and family members [6, 7]. Patients with IRDs exhibit variable symptoms such as decreased central and/or peripheral vision, nyctalopia, photophobia, glare sensitivity, color vision defects, and visual distortions [1, 2]. Hereditary maculopathies, a subset of IRDs, exhibit some common features. Fundus examination can vary from normal to showing bull’s eye maculopathy or complete macular atrophy, with varying degrees of optic disk pallor and peripheral retinal changes [8]. The visual field is typically affected first by central scotomas, followed by patchy peripheral vision loss [9]. Optical coherence tomography (OCT) usually reflects the extent of damage to the photoreceptors [10]. Electroretinography (ERG) shows reduced photopic amplitude responses and increased cone thresholds on dark adaptometry [2]. More than 280 pathogenic genetic variants have been identified as causing IRDs, and this number continues to increase [11]. These genes encode proteins vital for photoreceptor outer segment development, metabolic stability, retinal pigment epithelium (RPE) function, and the phototransduction cascade. Inheritance patterns can be variable, and there is significant phenotypic overlap both between and within genes [12].

The Major Facilitator Superfamily Domain containing 8 gene (MFSD8), located on chromosome 4q28.1-q28.2, encodes a transmembrane lysosomal protein involved in substrate transport that is essential for cell homeostasis and function [13]. Although MFSD8 shows ubiquitously low-level expression, it is predominantly expressed in the CNS and its ocular extensions, the optic nerve, and the retina [13]. Homozygous loss-of-function variants in MFSD8 are commonly associated with variant-late infantile neuronal ceroid lipofuscinosis, a progressive neurological disorder characterized by myoclonus, seizures, ataxia, mental and motor coordination decline, and visual impairment [14]. However, an increasing number of pathogenic variants in MFSD8 have been reported to cause macular dystrophy with limited extraocular involvement. These variants have been shown to result in variable ocular presentations, ranging from cone-rod dystrophy to central cone involvement only [15–20]. Given the phenotypic diversity observed with MFSD8 mutations [15–20], it is crucial to document clinical phenotype and associated genotype. In this report, we highlight the clinical and imaging findings in a patient with a macular non-syndromic MFSD8-related phenotype and provide evidence to support the pathogenicity of the variant MFSD8 c.291G>C (p.Trp97Cys). The CARE Checklist has been completed by the authors for this case report and is provided as online supplementary material (for all online suppl. material, see https://doi.org/10.1159/000546220).

Case

A 47-year-old male was referred to our department by a community retina practice in September 2022 for evaluation of progressive central vision loss. The patient reported experiencing decreased central vision, visual distortions, photophobia, poor depth perception, glare, difficulty reading, and impaired dark and light adaptation over the past year. He denied experiencing flashes, floaters, and nyctalopia. His past medical history included allergic rhinitis, obesity (body mass index 40.0–44.9), hypertriglyceridemia, hypercholesterolemia, essential hypertension, nonalcoholic fatty liver disease, gout, sleep apnea, asthma, and vitamin B12 deficiency. His past ocular and surgical histories were unremarkable. He reported a nephew with history of obesity, intellectual disability, polydactyly, and retinitis pigmentosa, suggestive of a syndromic ciliopathy; however, no reported diagnosis was provided by the patient and no records were available for review. Otherwise, family history was noncontributory. Known consanguinity was denied. The patient denied smoking, illicit drug use, trauma, sun gazing, or laser pointer injury. His medications included albuterol, fluticasone, allopurinol, amlodipine, lisinopril, vitamin D3 (1,000 U), vitamin B12, and fexofenadine.

On presentation, his best-corrected Snellen visual acuity (BCVA) was 20/40 in both eyes. Intraocular pressures, pupillary examination, confrontation visual fields, and extraocular motility were normal. On Ishihara color plate test, the patient correctly identified 8 out of 12 with the right eye and 6 out of 12 plates with the left eye. The slit-lamp examination of the anterior segment was notable for a persistent tunica vasculosa lentis in the right eye. Dilated fundus examination was notable for RPE changes at the macula bilaterally and temporal lattice degeneration in the left eye (macular fundus photos, Fig. 1a, c). SD-OCT revealed bilateral vitreous syneresis and subfoveal ellipsoid zone loss with outer nuclear layer atrophy. Macular thickness was 167 μm in the right and 178 μm in the left eyes (Fig. 2a, b). Fundus with autofluorescence demonstrated an enlarged central hypo-autofluorescence with a partial circumferential hyper-autofluorescent ring bilaterally (Fig. 1b, d). Notably, as the referring practice performed angiography studies excluding neovascularization and other vascular abnormalities (both fluorescein angiography [FA] and OCT angiography [OCTA]), these were thus not repeated in our center. While the FA and OCTA images were not made available to us for review, a detailed report was provided at our request. The FA findings demonstrated staining and transmission defects consistent with a perifoveal RPE disruption, with normal arteriovenous transit, choroidal filling, and no filling delay. No evidence of optic nerve staining, macular edema, neovascularization of the disk or elsewhere, retinal hemorrhage, vascular leakage, or hypofluorescence indicative of capillary non-perfusion was noted. Similarly, the OCTA findings were consistent with “bull’s-eye” perifoveal RPE disruption, without evidence of macular edema or vascular pathology in either eye. Humphrey 24-2 visual field was significant for bilateral central scotomata. While the full-field ERG (ffERG) exhibited normal rod and cone responses in both eyes, the multifocal ERG (mfERG) revealed markedly reduced responses in ring 1, with a tendency toward response normalization when moving to the outer rings. The responses were less affected in rings 2 and 3, while those in rings 4 and 5 appeared normal (shown in Fig. 3a, b, upper panel). These electrophysiology results were consistent with macular dysfunction affecting the foveae of both eyes. Based on these findings, the patient was referred for genetic testing.

Fig. 1.

Fundus photograph of the right (a) and left (c) eye demonstrated macular RPE changes bilaterally, with temporal lattice degeneration in the left eye. FAF of the right (b) and left (d) eye demonstrated an enlarged central hypo-AF area with a partial circumference of a hyper-AF ring. FAF, fundus with autofluorescence; hypo-AF, hypo-autofluorescence; hyper-AF, hyper-autofluorescent.

Fig. 2.

SD-OCT revealed bilateral vitreous syneresis and subfoveal ellipsoid zone loss with ONL atrophy. Macular thickness was measured 167 μm in the right (a) and 178 μm in the left (b). ONL, outer nuclear layer.

Fig. 3.

MfERG of the right eye (a) and left eye (b) revealed markedly reduced responses in ring 1, with a tendency toward normalization in the outer rings. Rings 2 and 3 showed less reduction, while rings 4 and 5 appeared normal. These findings are consistent with macular dysfunction affecting the foveae of each eye.

A retinal dystrophy panel inclusive of 314 nuclear and 37 mitochondrial genes was performed using next-generation sequencing technology. Initial testing revealed two variants in MFDS8, including c.1006G>C (p.Glu336Gln), which was classified as pathogenic, and c.291G>C (p.Trp97Cys), which was classified as a variant of uncertain significance (VUS). Genetic testing also identified the following heterozygous variants in genes associated with recessive disease: a pathogenic variant in BBS1 (c.1169T>G [p.Met390Arg]), a pathogenic variant in TYR (c.1217C>T [p.Pro406Leu]), VUS in ABCA4 (c.769-784C>T), and a VUS in GPR143 (c.4G>A [p.Ala2Thr]). Segregation testing for the MFSD8 variants in family members confirmed that the patient is compound heterozygous for these variants and resulted in reclassification of the c.291G>C (p.Trp97Cys) variant to likely pathogenic. This, combined with the absence of other conclusive genetic findings, suggests that his ocular phenotype is likely due to compound heterozygosity of MFSD8 variants. Additionally, he is a carrier of disease related to BBS1 and TYR. The patient had a previous MRI following accidental trauma, which revealed a pineal gland cyst. Given the potential systemic manifestations of disease related to MFSD8, an evaluation with neurology was recommended; however, the patient declined this evaluation due to his age and absence of seizures, ataxia, myoclonus, and mental and motor regression. Nevertheless, we have emphasized the need for continuous follow-up for potential systemic manifestations of MFSD8-related disease to the patient. The patient was recommended to take omega-3 and lutein supplements and to avoid UV light exposure and smoking. A repeat ocular evaluation after 5 months revealed stable BCVA and SD-OCT findings.

Discussion

Identification and interpretation of the role of VUSs remain a significant challenge in genetic testing, as VUSs constitute a large proportion of reported genetic variants and become more frequent with the expansion of genetic testing panels [21]. This case of a 47-year-old male with progressive central vision loss highlights the clinical implications of compound heterozygous variants in MFSD8, specifically c.291G>C (p.Trp97Cys) and c.1006G>C (p.Glu336Gln).

The pathogenic variant, MFSD8 c.1006G>C (p.Glu336Gln), is a missense mutation associated with autosomal recessive MFSD8-related conditions [15]. The variant was found at a total frequency of 0.002515 within the European, African American, Admixed American, and Ashkenazi Jewish subpopulations [22]. Although there is a small physicochemical difference between glutamic acid and glutamine (Grantham score 29), three of five in silico tools – CADD, PolyPhen-2, and MutTaster predict a deleterious, damaging, and disease-causing effect on the protein’s structure and function, respectively [23]. This variant has been reported in individuals with maculopathies and cone dystrophies and is characterized as a hypomorphic variant that causes cone dysfunction when present in trans with a MFSD8 pathogenic variant [15, 16]. Therefore, this pathogenic variant alone is insufficient to clarify the patient’s clinical diagnosis.

The missense variant, MFSD8 c.291G>C (p.Trp97Cys), is a nonconservative amino acid substitution that has not been reported in the literature of individuals affected with MFSD8-related conditions [24]. The variant was found at a total frequency of 0.00001921 within the non-Finnish European and Admixed American subpopulations [22]. There is a large physicochemical difference between tryptophan and cysteine (Grantham score 215), and in silico analyses using SIFT, PolyPhen-2, and MutTaster predict the alteration to be deleterious, possibly damaging, and disease causing, respectively [24]. Since this variant has not been published as a mutation or reported as a benign polymorphism, data available prior to this publication were insufficient to determine the role of this variant in disease, and it was reclassified from VUS to likely pathogenic at the time of writing this report.

The patient’s symptoms – decreased central vision, visual distortions, photophobia, and impaired light and dark adaptation – are consistent with the clinical features of MFSD8-related maculopathies [15–20]. On retinal imaging, the patient exhibited subfoveal ellipsoid zone loss with outer nuclear layer atrophy, as well as an enlarged foveal hypo-autofluorescence and a partial outer foveal hyper-autofluorescent ring, similar to previously reported findings [15, 16]. Notably, Humphrey 24-2 visual field testing identified bilateral central scotomata, and mfERG demonstrated markedly reduced responses in the central macula (ring 1) with relative preservation in the peripheral rings, while ffERG responses remained normal. For suspected IRDs, ERGs help distinguish between rod-primary, cone-primary, mixed rod-cone, or post-photoreceptor dysfunction, with pattern ERG and mfERG playing a role in identifying macular dysfunction [25]. In our patient, the electrophysiology results align with previous reports of an exclusively macular dysfunction, as evident by the changes on mfERG and the preserved ffERG responses [15, 16]. The BCVA and structural OCT findings over a 5-month follow-up period, combined with the notes available from the referring practice, suggest that the disease has a slowly progressive course. This case contributes to the limited data on non-syndromic MFSD8 presentations, highlighting novel imaging features and clinical characteristics that may aid in refining clinical and imaging criteria.

While syndromic cases of MFSD8 mutations are often associated with systemic features like seizures, myoclonus, and cognitive or motor regression, this patient’s presentation was purely ocular. The patient did not exhibit systemic symptoms of variant-late infantile neuronal ceroid lipofuscinosis, such as seizures or cognitive decline, highlighting the variability in MFSD8 phenotypes. Although the patient has a relatively complex medical history for his young age, including allergic rhinitis, obesity, hypertriglyceridemia, hypercholesterolemia, essential hypertension, nonalcoholic fatty liver disease, gout, sleep apnea, asthma, and vitamin B12 deficiency, these conditions are more consistent with common metabolic and lifestyle-related factors rather than a syndromic lysosomal storage disorder. Notably, our patient underwent a liver biopsy with histological analysis due to suspicion of hepatocellular carcinoma on a routine ultrasound exam. Histological analysis of the biopsied tissue ruled out hepatocellular carcinoma and established the diagnosis of nonalcoholic fatty liver disease. The biopsy also ruled out lipofuscinosis, as no findings consistent with ceroid lipofuscin and pigment deposition were reported [26, 27]. The above findings suggest a milder or atypical presentation, primarily affecting the retina without additional neurological findings, as seen in other cases [15–20]. A threshold model explains this phenotypic variability, suggesting that missense mutations lead to isolated retinal symptoms due to high MFSD8 expression in the neuroretina, while other tissues, including the brain, remain unaffected due to sufficient residual function [15, 17]. The threshold model is not exclusive to MFSD8. Diseases such as Bardet-Biedl syndrome, Joubert syndrome, Batten disease, and amyotrophic lateral sclerosis have variants that cause a primarily ocular phenotype without systemic symptoms [28–31].

The differential diagnoses for the ocular phenotype in this case include non-exudative age-related macular degeneration, solar and laser retinopathy, poppers maculopathy, talc retinopathy, and drug-induced retinal toxicity such as hydroxychloroquine, nonsteroidal anti-inflammatory drugs, and chronic sildenafil use. Age-related macular degeneration is suspected in older patients and typically presents with drusen on OCT [32], while a thorough history can help rule out acquired maculopathies [33]. The patient denied any exposures that could result in other diagnoses [33, 34]. The finding of a pathogenic BBS1 variant and family history of retinitis pigmentosa suspected to be associated with a syndromic ciliopathy prompted consideration of retinitis pigmentosa as a differential diagnosis. However, given the incongruence of the patient’s ocular phenotype with disease caused by BBS1 and the absence of a second BBS1 variant, we suspect that the patient is simply a carrier of BBS1-related disease. Combined effects of the multigene variants identified in this case are not known, but cannot be eliminated.

Given all of the above, and in accordance with joint consensus criteria for classifying pathogenic variants [35, 36], we advocated for the MFSD8 c.291G>C (p.Trp97Cys) variant to be reclassified as likely pathogenic. This is supported by the variant meeting two moderate criteria: (a) its rarity in control populations and (b) its presence in trans with a known pathogenic variant. Additionally, the variant meets three supporting criteria: (a) it occurs in a gene with a low rate of benign missense variation, where missense variants often cause disease; (b) multiple computational tools predict it to have a deleterious effect on the gene product, and (c) the patient’s phenotype is highly specific to a disease caused by a single genetic etiology. The combination of two moderate and three supporting criteria provides substantial evidence for reclassification of MFSD8 c.291G>C (p.Trp97Cys) as likely pathogenic. Family segregation analysis remains valuable for refining variant classifications, which may be updated as new knowledge becomes available.

Estimating the risk of progressive visual impairment is challenging due to the rarity of the condition. However, the relatively limited foveal phenotype with preserved extrafoveal function and the absence of neurological symptoms in our patient likely suggest a milder form. The limitations of this study include the lack of significant longitudinal follow-up and additional subjects. Detailed phenotypic descriptions of additional cases are essential to enhance early recognition and clarify the range of clinical manifestations in patients with non-syndromic MFSD8-related macular phenotypes. Future research should validate MFSD8 variants through in vitro or in vivo studies. Additionally, broader genetic screenings in diverse populations may uncover other rare variants, enhancing our understanding of MFSD8-related disorders. The genetic overlap between lysosomal storage disorders and IRDs may offer new treatment possibilities, such as gene therapy, which could benefit patients with pathogenic MFSD8 variants.

Conclusion

This study reports a novel combination of compound heterozygous variants in MFSD8, resulting in a non-syndromic, bilateral central macular dystrophy. The c.1006G>C (p.Glu336Gln) variant has been associated with both systemic and isolated retinal dystrophy, depending on concurrent MFSD8 mutations. In this patient, who is a compound heterozygote with the c.291G>C (p.Trp97Cys) variant, the disease is presented with typical retinal features of MFSD8-related conditions. Although the primary manifestation is confined to the retina, systemic symptoms may still develop, underscoring the importance of monitoring for disease progression, as stressed to this patient. Genetic counseling is recommended for all affected individuals and their families to discuss prognosis, reproductive risks, and available support resources.

Statement of Ethics

This study was performed in accordance with the 1964 Helsinki Declaration and its later amendments and was approved by the University of Pittsburgh Institutional Review Board (IRB), (IRB protocol number 19030187). Written informed consent was obtained from the patient for publication of the details of their medical case and any accompanying images.

Conflict of Interest Statement

None of the authors has any conflicts of interest to declare.

Funding Sources

No funding or grant support was received.

Author Contributions

S.G.: drafting of the manuscript and critical review of the manuscript. R.Z. and R.H.: acquisition of data and critical review of the manuscript. M.B. and M.A.: acquisition of data and formal analysis. A.B.: validation and critical review of the manuscript. J.-A.S.: acquisition of data and validation. B.R.: supervision, validation, acquisition of data, and critical review of the manuscript. All the authors attested that they met the current ICMJE criteria for authorship and have reviewed and approved the final version of the manuscript for publication.

Funding Statement

No funding or grant support was received.

Data Availability Statement

All data generated or analyzed during this study are included in this article and its online supplementary material files. Further inquiries can be directed to the corresponding author.