Novel TMEM98, MFRP, PRSS56 variants in a large United States high hyperopia and nanophthalmos cohort

Comprehensive genetic analysis uncovers the mutational spectrum of MFRP and its genotype-phenotype correlation in a large cohort of Chinese microphthalmia patients

PURPOSE

Microphthalmia is a severe congenital ocular disease featured by abnormal ocular development. The aim of this study was to detail the genetic and clinical characteristics of a large cohort of Chinese patients with microphthalmia related to MFRP variants, focusing on uncovering genotype-phenotype correlations.

METHODS

Fifty microphthalmia patients from 44 unrelated Chinese families were recruited. Whole-exome sequencing (WES) was conducted to analyze the coding regions and adjacent intronic regions of MFRP. Axial lengths (AL) were measured for all probands and available family members. Protein structures of mutations with high frequency in our cohort were predicted. The genotype-phenotype correlations were explored by statistical analysis.

RESULTS

Sixteen MFRP variants were detected in 17 families, accounting for 38.64 % of all microphthalmia families. There were 9 novel mutations (c.427+1G>C, c.428-2A>C, c.561_575del:p.A188_E192del, c.836G>A:p.C279Y, c.1010_1021del:p.H337_E340del:p.Y479*, c.1516_1517del:p.S506Pfs*66, c.1561T>G:p.C521G, c.1616G>A:p.R539H, and c.1735C>T:p.P579S) and six previously reported variants in MFRP, with p.E496K and p.H337_E340del being highly frequent, found in eight (47.06 %) and two families (11.76 %), respectively. Seven variants (43.75 %) were located in the C-terminal cysteine-rich frizzled-related domain (CRD) (7/16, 43.75 %). Protein prediction implicated p.E496K and p.H337_E340del mutations might lead to a destabilization of the MFRP protein. The average AL of all 42 eyes was 16.02 ± 1.05 mm, and 78.36 % of eyes with AL < 16 mm harbored p.E496K variant. Twenty-six eyes with variant variant had shorter AL than that of the other 16 eyes without this variant (p = 0.006), highlighting a novel genotype-phenotype correlation.

CONCLUSIONS

In this largest cohort of Chinese patients with microphthalmia, the 9 novel variants, high frequency of p.E496W, and mutation hotspots in CRD reveals unique insights into the MFRP mutation spectrum among Chinese patients, indicating ethnic variability. A new genotype-phenotype correlation that p.E496K variant associated with a shorter AL is unveiled. Our findings enhance the current knowledge of MFRP-associated microphthalmia and provide valuable information for prenatal diagnosis as well as future therapy.

MYRF: A unique transmembrane transcription factor- from proteolytic self-processing to its multifaceted roles in animal development

The Myelin Regulator Factor (MYRF) is a master regulator governing myelin formation and maintenance in the central nervous system. The conservation of MYRF across metazoans and its broad tissue expression suggest it has functions extending beyond the well-established role in myelination. Loss of MYRF results in developmental lethality in both invertebrates and vertebrates, and MYRF haploinsufficiency in humans causes MYRF-related Cardiac Urogenital Syndrome, underscoring its importance in animal development; however, these mechanisms are largely unexplored. MYRF, an unconventional transcription factor, begins embedded in the membrane and undergoes intramolecular chaperone mediated trimerization, which triggers self-cleavage, allowing its N-terminal segment with an Ig-fold DNA-binding domain to enter the nucleus for transcriptional regulation. Recent research suggests developmental regulation of cleavage, yet the mechanisms remain enigmatic. While some parts of MYRF's structure have been elucidated, others remain obscure, leaving questions about how these motifs are linked to its intricate processing and function.

Pathogenic variants of MFRP and PRSS56 genes are major causes of nanophthalmos in Japanese patients

BACKGROUND

Nanophthalmos (NNO) is a rare condition with significantly shorter axial length than normal. Several genes are known to cause NNO, among them the MFRP and PRSS56 genes have been reported to cause majority of NNOs. The purpose of this study was to determine the genetic basis of Japanese patients with NNO.

MATERIALS AND METHODS

We studied seven patients with NNO. Whole exome sequencing (WES) and Sanger sequencing were performed to determine the variants causing the NNO. We also reviewed the medical charts of the patients to determine the phenotype of these seven patients.

RESULTS

WES revealed that four patients from three families carried homozygous frameshift variants of the PRSS56 gene (c.1066dupC). Two novel variants of the MFRP gene were detected in the other two patients: one proband had a homozygous missense variant (c.1486 G>A) and the other had a compound heterozygous variant (c.1486 G>A and c.662_663insT). The axial length of the eight eyes with the PRSS56 variant was 15.69 ± 0.48 mm (mean ± SD) and that for the 4 eyes with the MFRP variant was 15.63 ± 0.69 mm. Three of the six cases with the PRSS56 or MFRP variant had the uveal effusion syndrome.

CONCLUSIONS

NNOs in Japanese patients are caused by variants of the PRSS56 and MFRP genes as in other ethnic populations. In addition, two new variants of the MFRP gene were found in our cohort. The phenotypes and anomalies in Japanese patients with NNO were similar to those reported for other ethnic populations.

The Genetic Determinants of Axial Length: From Microphthalmia to High Myopia in Childhood

The axial length of the eye is critical for normal visual function by enabling light to precisely focus on the retina. The mean axial length of the adult human eye is 23.5 mm, but the molecular mechanisms regulating ocular axial length remain poorly understood. Underdevelopment can lead to microphthalmia (defined as a small eye with an axial length of less than 19 mm at 1 year of age or less than 21 mm in adulthood) within the first trimester of pregnancy. However, continued overgrowth can lead to axial high myopia (an enlarged eye with an axial length of 26.5 mm or more) at any age. Both conditions show high genetic and phenotypic heterogeneity associated with significant visual morbidity worldwide. More than 90 genes can contribute to microphthalmia, and several hundred genes are associated with myopia, yet diagnostic yields are low. Crucially, the genetic pathways underpinning the specification of eye size are only now being discovered, with evidence suggesting that shared molecular pathways regulate under- or overgrowth of the eye. Improving our mechanistic understanding of axial length determination will help better inform us of genotype-phenotype correlations in both microphthalmia and myopia, dissect gene-environment interactions in myopia, and develop postnatal therapies that may influence overall eye growth.

Phenotypic consequences of a nanophthalmos-associated TMEM98 variant in human and mouse

Nanophthalmos is characterised by shorter posterior and anterior segments of the eye, with a predisposition towards high hyperopia and primary angle-closure glaucoma. Variants in TMEM98 have been associated with autosomal dominant nanophthalmos in multiple kindreds, but definitive evidence for causation has been limited. Here we used CRISPR/Cas9 mutagenesis to recreate the human nanophthalmos-associated TMEM98 p.(Ala193Pro) variant in mice. The p.(Ala193Pro) variant was associated with ocular phenotypes in both mice and humans, with dominant inheritance in humans and recessive inheritance in mice. Unlike their human counterparts, p.(Ala193Pro) homozygous mutant mice had normal axial length, normal intraocular pressure, and structurally normal scleral collagen. However, in both homozygous mice and heterozygous humans, the p.(Ala193Pro) variant was associated with discrete white spots throughout the retinal fundus, with corresponding retinal folds on histology. This direct comparison of a TMEM98 variant in mouse and human suggests that certain nanophthalmos-associated phenotypes are not only a consequence of a smaller eye, but that TMEM98 may itself play a primary role in retinal and scleral structure and integrity.

Zfp503/Nlz2 Is Required for RPE Differentiation and Optic Fissure Closure

Purpose

Uveal coloboma is a congenital eye malformation caused by failure of the optic fissure to close in early human development. Despite significant progress in identifying genes whose regulation is important for executing this closure, mutations are detected in a minority of cases using known gene panels, implying additional genetic complexity. We have previously shown knockdown of znf503 (the ortholog of mouse Zfp503) in zebrafish causes coloboma. Here we characterize Zfp503 knockout (KO) mice and evaluate transcriptomic profiling of mutant versus wild-type (WT) retinal pigment epithelium (RPE)/choroid.

Methods

Zfp503 KO mice were generated by gene targeting using homologous recombination. Embryos were characterized grossly and histologically. Patterns and level of developmentally relevant proteins/genes were examined with immunostaining/in situ hybridization. The transcriptomic profile of E11.5 KO RPE/choroid was compared to that of WT.

Results

Zfp503 is dynamically expressed in developing mouse eyes, and loss of its expression results in uveal coloboma. KO embryos exhibit altered mRNA levels and expression patterns of several key transcription factors involved in eye development, including Otx2, Mitf, Pax6, Pax2, Vax1, and Vax2, resulting in a failure to maintain the presumptive RPE, as evidenced by reduced melanin pigmentation and its differentiation into a neural retina-like lineage. Comparison of RNA sequencing data from WT and KO E11.5 embryos demonstrated reduced expression of melanin-related genes and significant overlap with genes known to be dynamically regulated at the optic fissure.

Conclusions

These results demonstrate a critical role of Zfp503 in maintaining RPE fate and optic fissure closure.

Non-vasogenic cystoid maculopathies

Besides cystoid macular edema due to a blood-retinal barrier breakdown, another type of macular cystoid spaces referred to as non-vasogenic cystoid maculopathies (NVCM) may be detected on optical coherence tomography but not on fluorescein angiography. Various causes may disrupt retinal cell cohesion or impair retinal pigment epithelium (RPE) and Müller cell functions in the maintenance of retinal dehydration, resulting in cystoid spaces formation. Tractional causes include vitreomacular traction, epiretinal membranes and myopic foveoschisis. Surgical treatment does not always allow cystoid space resorption. In inherited retinal dystrophies, cystoid spaces may be part of the disease as in X-linked retinoschisis or enhanced S-cone syndrome, or occur occasionally as in bestrophinopathies, retinitis pigmentosa and allied diseases, congenital microphthalmia, choroideremia, gyrate atrophy and Bietti crystalline dystrophy. In macular telangiectasia type 2, cystoid spaces and cavitations do not depend on the fluid leakage from telangiectasia. Various causes affecting RPE function may result in NVCM such as chronic central serous chorioretinopathy and paraneoplastic syndromes. Non-exudative age macular degeneration may also be complicated by intraretinal cystoid spaces in the absence of fluorescein leakage. In these diseases, cystoid spaces occur in a context of retinal cell loss. Various causes of optic atrophy, including open-angle glaucoma, result in microcystoid spaces in the inner nuclear layer due to a retrograde transsynaptic degeneration. Lastly, drug toxicity may also induce cystoid maculopathy. Identifying NVCM on multimodal imaging, including fluorescein angiography if needed, allows guiding the diagnosis of the causative disease and choosing adequate treatment when available.

Treatment of Nanophthalmos Cataracts: Surgery and Complications

PURPOSE

Cataract surgery in patients with nanophthalmos is complicated for ophthalmologists to perform. Due to the unique ocular anatomy, there is a high incidence of complex complications such as angle-closure glaucoma, fluid misdirection syndrome, and uveal effusion syndrome (UES) in the perioperative period of cataract surgery. This article will discuss the management options for cataract surgery in nanophthalmic eyes and complications.

METHODS

This review is searched through PubMed, focusing on articles published in the past 20 years. Articles were reviewed on the anatomical structure of nanophthalmic cataracts, the pathogenesis of complications, the selection of intraocular lenses, and surgical methods.

CONCLUSION

There is a strong correlation between abnormal ocular anatomy and complications in patients with nanophthalmos. Clinicians must not only select the appropriate intraocular lens formula based on the depth of the anterior chamber but also formulate personalized surgical methods based on its unique anatomical structure to avoid complications.

De novo frameshift mutation in YAP1 associated with bilateral uveal coloboma and microphthalmia

BACKGROUND

Uveal colobomata are eye defects that result from failure of the optic fissure of the neuroectoderm-derived optic cup to close between weeks 5-7 of fetal life. Mutations in YAP1 have previously been linked to uveal coloboma. We present the clinical features and genetic basis of a one-year-old male with bilateral uveal colobomata.

MATERIALS AND METHODS

Clinical features were gathered from an age-appropriate evaluation and retrospectively from clinical records. DNA samples were collected from the proband, his uncle (who also had coloboma), both parents, and one sibling. Whole-genome sequencing of the coding regions and intron-exon boundaries confirmed a mutation in the proband. These genetic findings were verified using the Sanger method of DNA sequencing.

RESULTS

The proband is a male with congenital bilateral colobomata (iris/retina/nerve), reduced vision, nystagmus with null point, bilateral microcornea, right microphthalmia, possible mild right hemifacial microsomia, a tubular nose, possible spina bifida occulta, and astigmatism. Whole-genome sequencing confirmed a heterozygous YAP1 frameshift mutation NM_001130145.3:c.178dupG p.(Asp60GlyfsTer52) in the proband. This mutation was absent in all other tested family members.

CONCLUSIONS

We report a de novo mutation in YAP1 that likely results in nonsense-mediated decay. Given the association with YAP1 haploinsufficiency and colobomatous microphthalmia, this novel variant provides a molecular diagnosis for the proband. Further insight into YAP1 mutations may have implications in the prevention/treatment of uveal coloboma and other syndromic disorders.

Zebrafish: An In Vivo Screening Model to Study Ocular Phenotypes

Purpose

To establish a set of assays that allow the in vivo screening of candidate genes for ocular diseases in zebrafish, with an emphasis on refractive error.

Methods

Our pipeline includes the most relevant ocular screening measurements to assess (1) ocular biometry using spectral domain optical coherence tomography, (2) refractive status using an eccentric photorefractor, (3) intraocular pressure by tonometry, and (4) optokinetic response to study visual capability in zebrafish. To validate our pipeline and to demonstrate the potential of zebrafish as a valid animal model, we chose two well-characterized genes with an ocular phenotype (PRSS56 and FBN1) and generated two mutant zebrafish lines (prss56 and fbn1). Mutant fish were assessed at 2, 4, and 6 months after fertilization.

Results

With the proposed phenotyping pipeline, we showed that ocular biometry, refractive status, intraocular pressure, and visual function can be studied in zebrafish. In the prss56 mutant, the pipeline revealed a dramatic decrease in axial length, mainly owing to a decreased vitreous chamber depth, whereas in the fbn1 mutant, ectopia lentis was the most distinctive ocular phenotype observed. Tonometry in both mutant lines showed an increase in intraocular pressure.

Conclusions

The proposed pipeline was applied successfully in zebrafish and can be used for future genetic screenings of candidate genes. While validating our pipeline, we found a close resemblance between the ocular manifestations in the zebrafish mutants and patients harboring mutations in PRSS56 and FBN1. Our results support the validity of our pipeline and highlight the potential of zebrafish as an animal model for in vivo screening of candidate genes for ocular diseases.

Genetic Interaction between Mfrp and Adipor1 Mutations Affect Retinal Disease Phenotypes

Adipor1tm1Dgen and Mfrprd6 mutant mice share similar eye disease characteristics. Previously, studies established a functional relationship of ADIPOR1 and MFRP proteins in maintaining retinal lipidome homeostasis and visual function. However, the independent and/or interactive contribution of both genes to similar disease phenotypes, including fundus spots, decreased axial length, and photoreceptor degeneration has yet to be examined. We performed a gene-interaction study where homozygous Adipor1tm1Dgen and Mfrprd6 mice were bred together and the resulting doubly heterozygous F1 offspring were intercrossed to produce 210 F2 progeny. Four-month-old mice from all nine genotypic combinations obtained in the F2 generation were assessed for white spots by fundus photo documentation, for axial length by caliper measurements, and for photoreceptor degeneration by histology. Two-way factorial ANOVA was performed to study individual as well as gene interaction effects on each phenotype. Here, we report the first observation of reduced axial length in Adipor1tmlDgen homozygotes. We show that while Adipor1 and Mfrp interact to affect spotting and degeneration, they act independently to control axial length, highlighting the complex functional association between these two genes. Further examination of the molecular basis of this interaction may help in uncovering mechanisms by which these genes perturb ocular homeostasis.

Glaucoma Syndromes: Insights into Glaucoma Genetics and Pathogenesis from Monogenic Syndromic Disorders

Monogenic syndromic disorders frequently feature ocular manifestations, one of which is glaucoma. In many cases, glaucoma in children may go undetected, especially in those that have other severe systemic conditions that affect other parts of the eye and the body. Similarly, glaucoma may be the first presenting sign of a systemic syndrome. Awareness of syndromes associated with glaucoma is thus critical both for medical geneticists and ophthalmologists. In this review, we highlight six categories of disorders that feature glaucoma and other ocular or systemic manifestations: anterior segment dysgenesis syndromes, aniridia, metabolic disorders, collagen/vascular disorders, immunogenetic disorders, and nanophthalmos. The genetics, ocular and systemic features, and current and future treatment strategies are discussed. Findings from rare diseases also uncover important genes and pathways that may be involved in more common forms of glaucoma, and potential novel therapeutic strategies to target these pathways.

Clinical and Histopathologic Correlates of Asymmetric Retinitis Pigmentosa

Importance

Asymmetric retinitis pigmentosa (RP) is a rare presentation of a normally symmetric condition. Histopathologic evidence should be examined to see if this asymmetry extends to the tissue and cellular levels.

Objective

To determine whether additional information can be obtained about asymmetric RP from studying clinical imaging and pathology correlates, including pathology samples from autopsied eyes.

Design, Setting, and Participants

In this case report, clinical and postmortem histopathological characteristics were compared in 2 eyes of a patient in her 50s with asymmetric RP. Individuals with rare mendelian diseases, such as RP, were studied using data from the curated National Eye Institute Eye Pathology collection.

Main Outcome and Measures

Results of clinical evaluation, multimodal retinal imaging, histopathology, and molecular genetic testing in a case of nonsyndromic asymmetric RP using resources from the ocular pathology collection.

Results

Eyes from a deceased patient in her 50s with nonsyndromic asymmetric RP found within the ocular pathology collection were studied. The patient was diagnosed with RP as an adolescent and presented in her 50s to the eye clinic with advanced RP, with the left eye affected much more severely than the right. The patient's phenotype was studied using in vivo imaging and postmortem histopathology to identify interocular differences in tissue degeneration. Extraction of blood-derived DNA and formalin-fixed paraffin-embedded DNA from autopsied eyes analyzed using next-generation sequencing did not yield a definitive molecular diagnosis nor significant tissue differences.

Conclusions and Relevance

This study demonstrates newly reported histopathological and molecular correlates in asymmetric RP. This report also highlights the relevance of studying previously seen patients and reevaluating their conditions using resources within the ocular pathology collection to gain further insight on their disease.

Clinical diagnosis of presumed SOX2 gonadosomatic mosaicism

Purpose:

To describe a family with presumed SOX2 gonadosomatic mosaicism diagnosed upon ophthalmic examination of the proband’s mother.

Methods:

The family underwent comprehensive ophthalmic and physical examination. Variant detection was performed using trio exome analysis on peripheral leukocyte DNA from blood and saliva samples. Variant segregation analysis was performed using a custom panel NGS sequencing. An identified variant in the SOX2 gene was confirmed in the proband by Sanger sequencing.

Results:

We report an individual with bilateral microphthalmia, developmental delay, hearing loss, and dysmorphic features. Her mother was found to have asymptomatic forme fruste uveal coloboma affecting her anterior segment. Her father, aunt, and sisters were unaffected. Trio exome sequence analysis showed an apparent de novo heterozygous deletion in the proband, NM_003106.3:c.70_89del, NP_003097.1:p. (Asn24Argfs*65), classified as pathogenic. Testing of the other family members’ peripheral blood and saliva was negative for this variant. The iris transillumination abnormalities in the proband’s mother supports a gonadosomatic mosaicism scenario.

Conclusions:

The results from this family underscore the importance of performing detailed evaluations of the parents of apparently sporadically affected individuals with heritable ophthalmic disorders. The identification of mildly affected individuals could substantially alter recurrence risks.

A novel proline substitution (Arg201Pro) in alpha helix 8 of TMEM98 causes autosomal dominant nanophthalmos-4, closed angle glaucoma and attenuated visual acuity

Nanophthalmos-4 is a rare autosomal dominant disorder caused by two known variations in TMEM98. An Austrian Caucasian pedigree was identified suffering from nanophthalmos and late onset angle-closure glaucoma and premature loss of visual acuity. Whole exome sequencing identified segregation of a c.602G > C transversion in TMEM98 (p.Arg201Pro) as potentially causative. A protein homology model generated showed a TMEM98 structure comprising α4, α5/6, α7 and α8 antiparallel helix bundles and two predicted transmembrane domains in α1 and α7 that have been confirmed in vitro. Both p.Arg201Pro and the two missense variations representing proline insertions identified previously to cause nanophthalmos-4 (p.Ala193Pro and p.His196Pro) are located in the charge polarized helix α8 (p.183-p210). Stability of the C-terminal alpha helical structure of TMEM98 is therefore essential to prevent the development of human nanophthalmos-4. Precise molecular diagnosis could lead to the development of tailored therapies for patients with orphan ocular disease.