Nanophthalmos and hemiretinal vein occlusion: A case report

MFRP variations cause nanophthalmos in five Chinese families with distinct phenotypic diversity

Purpose

Nanophthalmos is a congenital ocular structural anomaly that can cause significant visual loss in children. The early diagnosis and then taking appropriate clinical and surgical treatment remains a challenge for many ophthalmologists because of genetic and phenotypic heterogeneity. The objective of this study is to identify the genetic cause of nanophthalmos in the affected families and analyze the clinical phenotype of nanophthalmos with MFRP gene variation (Microphthalmia, isolated; OMIM#611040 and Nanophthalmos 2; OMIM#609549, respectively).

Methods

Comprehensive ophthalmic examinations were performed on participants to confirm the phenotype. The genotype was identified using whole exome sequencing, and further verified the results among other family members by Sanger sequencing. The normal protein structure was constructed using Alphafold. Mutant proteins were visualized using pymol software. Pathogenicity of identified variant was determined by in silico analysis and the guidelines of American College of Medical Genetics and Genomics (ACMG). The relationship between genetic variants and clinical features was analyzed.

Results

Five nanophthalmos families were autosomal recessive, of which four families carried homozygous variants and one family had compound heterozygous variants in the MFRP gene. Both family one and family three carried the homozygous missense variant c.1486G>A (p.Glu496Lys) in the MFRP gene (Clinvar:SCV005060845), which is a novel variant and evaluated as likely pathogenic according to the ACMG guidelines and in silico analysis. The proband of family one presented papilloedema in both eyes, irregular borders, thickened retinas at the posterior pole, tortuous and dilated retinal vessels, and indistinguishable arteries and veins, while the proband of family three presented uveal effusion syndrome-like changes in the right eye. In families one and 3, despite carrying the same gene variant, the probands had completely different clinical phenotypes. The homozygous nonsense variant c.271C>T (p.Gln91Ter) (Clinvar:SCV005060846) of the MFRP gene was detected in family 2, presenting shallow anterior chamber in both eyes, pigmentation of peripheral retina 360° from the equator to the serrated rim showing a clear demarcation from the normal retina in the form of strips. Family four proband carried the homozygous missense variant c.1411G>A (p.Val471Met) in the MFRP gene (Clinvar:SCV005060847), family five proband carried compound heterozygous missense variants c.1486G>A (p.Glu496Lys) and c.602G>T (p.Arg201Leu) in the MFRP gene (Clinvar:SCV005060848), which is a novel variant and evaluated as likely pathogenic according to the ACMG guidelines and in silico analysis, and they all presented clinically with binocular angle-closure glaucoma, family four also had retinal vein occlusion in the right eye during the follow-up.

Conclusion

In this study, pathogenic variants of the MFRP gene were detected in five nanophthalmos families, including two novel variants. It also revealed a distinct phenotypic diversity among five probands harboring variants in the MFRP gene. Our findings extend the phenotype associated with MFRP variants and is helpful for ophthalmologists in early diagnosis and making effective treatment and rehabilitation strategies.

Myelin regulatory factor deficiency is associated with the retinal photoreceptor defects in mice

Previously, we reported the myelin regulatory factor (MYRF) as a candidate gene for nanophthalmos. We have also produced Myrf knockdown (Myrf+/-) mouse strain to investigate the cellular and molecular phenotypes of reduced MYRF expression in the retina. Myrf+/- mouse strain was generated using the CRISPR/Cas9 system. Optomotor response system, electroretinogram (ERG), spectral-domain optical coherence tomography (SD-OCT), histology, and immunohistochemistry were performed to evaluate retinal spatial vision, electrophysiological function, retinal thickness, and pathological changes in cone or rod photoreceptors, respectively. RNA sequencing (RNA-seq) was performed to investigate the underlying molecular mechanism linking Myrf deficiency with photoreceptor defects. The genotype and phenotype of CRISPR/Cas9-induced Myrf+/- mice and their offspring were comprehensively investigated. Photoreceptor defects were detected in the retinas of Myrf+/- mice. Visual acuity and ERG responses were decreased in Myrf+/- mice compared with the control mice (Myrf+/+). The loss of cone and rod neurons was proportional to the decreased outer nuclear layer (ONL) thickness. Moreover, RNA-seq revealed that phototransduction and estrogen signaling pathways played important roles in the Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis. Myrf+/- mouse strain provides a good model to investigate the function of the MYRF gene. Photoreceptor defects with impaired functions of spatial vision and retinal electrophysiology indicate an important role played by MYRF in retinal development. Alterations in phototransduction and estrogen signaling pathways play important roles in linking Myrf deficiency with retinal photoreceptor defects.

Nanophthalmos: A Review of the Clinical Spectrum and Genetics

Nanophthalmos is a clinical spectrum of disorders with a phenotypically small but structurally normal eye. These disorders present significant clinical challenges to ophthalmologists due to a high rate of secondary angle-closure glaucoma, spontaneous choroidal effusions, and perioperative complications with cataract and retinal surgeries. Nanophthalmos may present as a sporadic or familial disorder, with autosomal-dominant or recessive inheritance. To date, five genes (i.e., MFRP, TMEM98, PRSS56, BEST1, and CRB1) and two loci have been implicated in familial forms of nanophthalmos. Here, we review the definition of nanophthalmos, the clinical and pathogenic features of the condition, and the genetics of this disorder.