Developmental abnormalities in the cornea of a mouse model for Marfan syndrome

Corneal Biomechanics Are Associated With FBN1 Mutations in Patients With Marfan Syndrome and Ectopia Lentis

Background

We investigated the corneal biomechanical properties and their genotype-phenotype correlation correlations in patients with Marfan syndrome (MFS) and ectopia lentis (EL).

Methods

Patients with MFS with EL underwent panel-based next-generation sequencing in this retrospective cohort study. The FBN1 genotypes were categorized into the dominant-negative (DN) group and the haploinsufficiency (HI) group. The DN variants were further subclassified based on the affected residues and their locations. Corneal biomechanical parameters were measured using dynamic Scheimpflug-based biomechanical analysis (CorVis ST). The correlations between corneal biomechanical properties and FBN1 genotype or nongenetic factors were analyzed. The differences between patients with MFS and normal control were also evaluated after matching confounding factors.

Results

One hundred one consecutive MFS probands participated in this study, with a median age of 6 years. Patients with HI and DN variants affecting critical residues, namely the DN (-Cys + CaB) variants, exhibited significantly higher deformation amplitude ratios (P = 0.029) and lower stress-strain index values (P = 0.007) compared with those in the DN (others) group, indicating lower corneal stiffness in the former group. DN variants in the FUN-EGF3 region were associated with lower deformation amplitude ratios (P = 0.011) and higher stress-strain index values (P = 0.002), whereas those in the DN-CD region exhibited the opposite pattern. Compromised corneal stiffness was significantly associated with HI and DN (-Cys + CaB) variants (b = -0.184; P = 0.01) and variants located outside the FUN-EGF3 region (b = 0.256; P = 0.001), after adjusting for confounding factors. Compared with matched controls, patients with MFS demonstrated significantly higher deformation amplitude ratios (P = 0.023), further confirming decreased corneal stiffness in this population.

Conclusions

The FBN1 genotype impacts the corneal biomechanical properties of patients with MFS and EL. Corneal biomechanics provide a novel platform to study the genotype-phenotype correlation of MFS.

Novel compound heterozygous variants in LTBP2 associated with relative anterior microphthalmos

PURPOSE

Relative anterior microphthalmos (RAM) is a rare congenital defect associated with severe vision impairment that is primarily caused by genetic alterations. The purpose of this study was to identify the causative genetic variants in two Chinese families with RAM with an autosomal recessive inheritance pattern.

METHODS

DNA samples were obtained from two probands and their family members. Targeted next-generation sequencing (NGS) was used to screen 425 genes associated with inherited eye diseases to identify possible disease-causing variants in the two patients. Sanger sequencing was subsequently used to validate the results in both families.

RESULTS

The targeted NGS panel identified potentially causative novel variants of the latent transforming growth factor beta binding protein 2 (LTBP2) gene in the two RAM families: a missense variant (c.2771C > T; p.Ala924Val) and an intronic variant (c.4582 + 9A > G) in Family A and a different missense variant (c.5239C > A; p.Arg1747Ser) and a synonymous variant (c.951G > A; p.Pro317Pro) in Family B. These four novel variants all cosegregated with the disease phenotype.

CONCLUSION

To our knowledge, this is the first study to report novel LTBP2 gene variants related to RAM. Considering the importance of LTBP2 in ocular development, we provide initial insights into the potential pathogenic mechanisms of LTBP2 in RAM.

In Search of Mouse Models for Exfoliation Syndrome

PURPOSE

Exfoliation syndrome (XFS) is a systemic connective tissue disorder with elusive pathophysiology. We hypothesize that a mouse model with elastic fiber defects caused by lack of lysyl oxidase like 1 (LOXL1 encoded by Loxl1), combined with microfibril deficiency due to Fbn1 mutation (encoding fibrillin-1, Fbn1C1041G/+) will display ocular and systemic phenotypes of XFS.

METHODS

Loxl1-/- was crossed with Fbn1C1041G/+ to create double mutant (dbm) mice. Intraocular pressure (IOP), visual acuity (VA), electroretinogram (ERG), and biometry were characterized in 4 genotypes (wt, Fbn1C1041G/+, Loxl1-/-, dbm) at 16 weeks of age. Optic nerve (ON) area was measured by ImageJ, and axon counting was achieved by AxonJ. Deep whole-body phenotyping was performed in wt and dbm mice. Two-tailed Student t test was used for statistical analysis.

RESULTS

There was no difference in IOP between the 4 genotypes. VA was significantly reduced only in dbm mice. The majority of biometric parameters showed significant differences in all 3 mutant genotypes compared with wt, and dbm had exacerbated anomalies compared with single mutants. Dbm mice showed reduced retinal function and significantly enlarged ON area compared with wt. Dbm mice exhibited severe systemic phenotypes related to abnormal elastic fibers, such as pelvic organ prolapse and cardiovascular and pulmonary abnormalities.

CONCLUSIONS

Ocular and systemic findings in dbm mice support functional overlap between fibrillin-1 and LOXL1, 2 prominent components of exfoliation material. Although no elevated IOP or reduction of axon numbers was detected in dbm mice at 16 weeks of age, their reduced retinal function and enlarged ON area indicate early retinal ganglion cell dysfunction. Dbm mice also provide insight on the link between XFS and systemic diseases in humans. NOTE: Publication of this article is sponsored by the American Ophthalmological Society.

Lack of evidence for GWAS signals of exfoliation glaucoma working via monogenic loss-of-function mutation in the nearest gene

PURPOSE

Exfoliation syndrome (XFS) is a systemic disease of elastin-rich tissues involving a deposition of fibrillar exfoliative material (XFM) in the anterior chamber of the eye, which can promote glaucoma. The purpose of this study was to create mice with CRISPR/Cas9-induced variations in candidate genes identified from human genome-wide association studies (GWAS) and screen them for indices of XFS.

METHODS

Variants predicted to be deleterious were sought in the Agpat1, Cacna1a, Loxl1, Pomp, Rbms3, Sema6a, and Tlcd5 genes of C57BL/6J mice using CRISPR/Cas9-based gene editing. Strains were phenotyped by slit-lamp, SD-OCT imaging, and fundus exams at 1-5 mos of age. Smaller cohorts of 12-mos-old mice were also studied.

RESULTS

Deleterious variants were identified in six targets; Pomp was recalcitrant to targeting. Multiple alleles of some targets were isolated, yielding 12 strains. Across all genotypes and ages, 277 mice were assessed by 902 slit-lamp exams, 928 SD-OCT exams, and 358 fundus exams. Homozygosity for Agpat1 or Cacna1a mutations led to early lethality; homozygosity for Loxl1 mutations led to pelvic organ prolapse, preventing aging. Loxl1 homozygotes exhibited a conjunctival phenotype of potential relevance to XFS. Multiple other genotype-specific phenotypes were variously identified. XFM was not observed in any mice.

CONCLUSIONS

This study did not detect XFM in any of the strains. This may have been due to species-specific differences, background dependence, or insufficient aging. Alternatively, it is possible that the current candidates, selected based on proximity to GWAS signals, are not effectors acting via monogenic loss-of-function mechanisms.

Genetic mutation and aqueous humor metabolites alterations in a family with Marfan syndrome

This study used four generations of a Chinese family to reveal the genetic etiology and ocular manifestation pathogenesis of Marfan syndrome (MFS) through whole genome sequencing (WGS) and metabolomics analysis. In the study, we explored the pathogenic gene variant and aqueous humor (AH) metabolites alterations of MFS. Using WGS, a novel heterozygous variant (NM_000138: c.G4192A, p.D1398 N) in the fibrilin-1 (FBN1) gene was identified. This variant was co-segregated with the phenotype and considered "deleterious" and highly conserved during evolution. The p.D1398 N variant is located in a cbEGF-like domain and predicted to lead to a new splice site, which might result in structural and functional changes to the FBN1 protein. FBN1 is highly expressed in the mouse cornea, conjunctiva and lens capsule, which highlights the important role of FBN1 in eyeball development. AH metabolomics analysis identified eight differentially expressed metabolites, including 3-hydroxyphenylacetic acid, 4-pyridoxic acid, aminoadipic acid, azelaic acid, chlordiazepoxide, niacinamide, ribose, 1,5-bisphosphate and se-methylselenocysteine, associated with relevant metabolic pathways likely involved in the pathogenesis of ocular symptoms in MFS. Our analysis extends the existing spectrum of disease-causing mutations and reveals metabolites information related to the ophthalmic features of MFS. This may provide a new sight and a basis for the diagnosis and mechanism of MFS.

Extracellular matrix and vascular dynamics in the kidney of a murine model for Marfan syndrome

Fibrillin-1 is a pivotal structural component of the kidney's glomerulus and peritubular tissue. Mutations in the fibrillin-1 gene result in Marfan syndrome (MFS), an autosomal dominant disease of the connective tissue. Although the kidney is not considered a classically affected organ in MFS, several case reports describe glomerular disease in patients. Therefore, this study aimed to characterize the kidney in the mgΔlpn-mouse model of MFS. Affected animals presented a significant reduction of glomerulus, glomerulus-capillary, and urinary space, and a significant reduction of fibrillin-1 and fibronectin in the glomerulus. Transmission electron microscopy and 3D-ultrastructure analysis revealed decreased amounts of microfibrils which also appeared fragmented in the MFS mice. Increased collagen fibers types I and III, MMP-9, and α-actin were also observed in affected animals, suggesting a tissue-remodeling process in the kidney. Video microscopy analysis showed an increase of microvessel distribution coupled with reduction of blood-flow velocity, while ultrasound flow analysis revealed significantly lower blood flow in the kidney artery and vein of the MFS mice. The structural and hemodynamic changes of the kidney indicate the presence of kidney remodeling and vascular resistance in this MFS model. Both processes are associated with hypertension which is expected to worsen the cardiovascular phenotype in MFS.

Elastic Fibre Proteins in Elastogenesis and Wound Healing

As essential components of our connective tissues, elastic fibres give tissues such as major blood vessels, skin and the lungs their elasticity. Their formation is complex and co-ordinately regulated by multiple factors. In this review, we describe key players in elastogenesis: fibrillin-1, tropoelastin, latent TGFβ binding protein-4, and fibulin-4 and -5. We summarise their roles in elastogenesis, discuss the effect of their mutations on relevant diseases, and describe their interactions involved in forming the elastic fibre network. Moreover, we look into their roles in wound repair for a better understanding of their potential application in tissue regeneration.

Ocular morbidity in Marfan syndrome: a nationwide epidemiological study

BACKGROUND

Ophthalmic complications are profound in Marfan syndrome (MFS). However, the overall burden is not well described. Our purpose was to evaluate the ocular morbidity in a nationwide perspective.

METHODS

We identified the ocular morbidity in patients with MFS (n=407) by use of Danish national healthcare registers, using number and timing of hospital contacts related to ophthalmic diagnoses, to ophthalmic surgery and to prescriptions for ophthalmic medication. An age-matched and gender-matched background population (n=40 700) was used as comparator.

RESULTS

Among MFS, 56% (226/407) of the patients had at least one registration of an ophthalmic diagnosis as inpatient or outpatient during the study period (HR of 8.0 (95% CI 7.0 to 9.2)). Seven out of 11 main groups of diagnoses were affected, including 'Lens', 'Choroid and retina', 'Ocular muscles, binocular movement, accommodation and refraction', 'Glaucoma', Visual disturbances and blindness', 'Vitreous body and globe', and 'Sclera, cornea, iris and ciliary body'. The number of surgical procedures as well as the use of ophthalmic medication in patients with MFS was significantly increased.

CONCLUSION

This nationwide epidemiological study of ocular morbidity in MFS demonstrates a profound burden and emphasises the need for thorough and experienced ophthalmological surveillance.

Collagen XII Regulates Corneal Stromal Structure by Modulating Transforming Growth Factor-β Activity

Collagen XII is a regulator of corneal stroma structure and function. The current study examined the role of collagen XII in regulating corneal stromal transforming growth factor (TGF)-β activation and latency. Specifically, with the use of conventional collagen XII null mouse model, the role of collagen XII in the regulation of TGF-β latency and activity in vivo was investigated. Functional quantification of latent TGF-β in stromal matrix was performed by using transformed mink lung reporter cells that produce luciferase as a function of active TGF-β. Col12a1 knockdown with shRNA was used to test the role of collagen XII in TGF-β activation. Col12a1^-/- hypertrophic stromata were observed with keratocyte hyperplasia. Increased collagen fibril forward signal was found by second harmonic generation microscopy in the absence of collagen XII. Collagen XII regulated mRNA synthesis of Serpine1, Col1a1, and Col5a1 and deposition of collagens in the extracellular matrix. A functional plasminogen activator inhibitor luciferase assay showed that collagen XII is necessary for latent TGF-β storage in the extracellular matrix and that collagen XII down-regulates active TGF-β. Collagen XII dictates stromal structure and function by regulating TGF-β activity. A hypertrophic phenotype in Col12a1^-/- corneal tissue can be explained by abnormal up-regulation of TGF-β activation and decreased latent storage.

Correlation between FBN1 mutations and ocular features with ectopia lentis in the setting of Marfan syndrome and related fibrillinopathies

Mutations of fibrillin-1 (FBN1) have been associated with Marfan syndrome and pleiotropic connective tissue disorders, collectively termed as "type I fibrillinopathy". However, few genotype-phenotype correlations are known in the ocular system. Patients with congenital ectopia lentis (EL) received panel-based next-generation sequencing, complemented with multiplex ligation-dependent probe amplification. In a total of 125 probands, the ocular phenotypes were compared for different types of FBN1 mutations. Premature termination codons were associated with less severe EL and a thinner central corneal thickness (CCT) than the inframe mutations. The eyes of patients with mutations in the C-terminal region had a higher incidence of posterior staphyloma than those in the middle and N-terminal regions. Mutations in the TGF-β-regulating sequence had larger horizontal corneal diameters (white-to-white [WTW]), higher incidence of posterior staphyloma, but less severe EL than those with mutations in other regions. Mutations in the neonatal region were associated with thinner CCT. Longer axial length (AL) was associated with mutations in the C-terminal region or TGF-β regulating sequence after adjusting for age, EL severity, and corneal curvature radius. FBN1 genotype-phenotype correlations were established for some ocular features, including EL severity, AL, WTW, CCT, and so forth, providing novel perspectives and directions for further mechanistic studies.

Contrast-Enhanced Tissue Processing of Fibrillin-Rich Elastic Fibres for 3D Visualization by Volume Scanning Electron Microscopy

Elastic fibres constitute an important component of the extracellular matrix and currently are the subject of intensive study in order to elucidate their assembly, function and involvement in cell-matrix interactions and disease. However, few studies to date have investigated the 3D architecture of the elastic fibre system in bulk tissue. We describe a protocol for preparation of tissue samples, including primary fixation and backscatter electron contrast-enhancement steps, through dehydration into stable resin-embedded blocks for volume electron microscopy. The use of low molecular weight tannic acid and alcoholic lead staining are critical stages in this procedure. Block preparation by ultramicrotomy and evaporative metal coating prior to microscopical examination are also described. We present images acquired from serial block face scanning electron microscopy of cornea and aorta showing target structures clearly differentiated from cells and other matrix components. The processing method imparts high contrast to fibrillin-containing elastic fibres, thus facilitating their segmentation and rendering into 3D reconstructions by image analysis software from large serial image datasets.

A Mouse Model of Brittle Cornea Syndrome caused by mutation in Zfp469

Brittle cornea syndrome (BCS) is a rare recessive condition characterised by extreme thinning of the cornea and sclera. BCS results from loss-of-function mutations in the poorly understood genes ZNF469 or PRDM5. In order to determine the function of ZNF469 and to elucidate pathogenic mechanisms, we used genome editing to recapitulate a human ZNF469 BCS mutation in the orthologous mouse gene Zfp469. Ophthalmic phenotyping showed that homozygous Zfp469 mutation causes significant central and peripheral corneal thinning arising from reduced stromal thickness. Expression of key components of the corneal stroma in primary keratocytes from Zfp469^BCS/BCS mice is affected, including decreased Col1a1 and Col1a2 expression. This alters the collagen type I/collagen type V ratio and results in collagen fibrils with smaller diameter and increased fibril density in homozygous mutant corneas, correlating with decreased biomechanical strength in the cornea. Cell-derived matrices generated by primary keratocytes show reduced deposition of collagen type I, offering an in vitro model for stromal dysfunction. Work remains to determine whether modulating ZNF469 activity will have therapeutic benefit in BCS or in conditions such as keratoconus in which the cornea thins progressively.

This article has an associated First Person interview with the first author of the paper.

Summary: A mouse model of brittle cornea syndrome was created to elucidate molecular mechanisms underlying the pathology of this rare connective tissue disorder in which extremely thin corneas rupture, causing irreversible blindness.

Scheimpflug-Based Analysis of the Reflectivity of the Cornea in Marfan Syndrome

Purpose

We sought to investigate corneal reflectivity in Marfan syndrome (MFS) on the basis of Scheimpflug light intensity distribution.

Methods

In a retrospective case-control analysis, the left eyes of 40 MFS patients and 40 age- and refraction-matched healthy controls were investigated. Patients with MFS meeting the Ghent II diagnostic criteria and with genetic confirmation of disease were included. Exclusion criteria were the following: coexisting corneal, conjunctival, or scleral pathology; use of medication known to affect corneal transparency; history of ocular surgery; and insufficient data. Scheimpflug tomography images were exported to analyze corneal transparency in different corneal layers and regions. Each corneal image was automatically segmented, after which the corresponding pixel intensities in the defined regions of interest were statistically modeled using a Weibull probability density function from which parameters α (transparency) and β (homogeneity) were derived.

Results

The cornea in MFS showed significantly higher light reflectivity (overall cornea, α = 71 ± 17 arbitrary units (a.u.)) than in the control group (overall cornea, α = 59 ± 15 a.u.) (t test, P = 0.003). The α parameter was significantly higher in MFS eyes in all examined layers and regions (P < 0.05), whereas the β parameter showed no statistical difference between MFS and controls (P > 0.05). The difference in α did not correlate with ocular biometric properties (corneal thickness and curvature) or ectopia lentis (P > 0.05).

Conclusions

The cornea in MFS shows significantly higher reflectivity than healthy controls with similar levels of homogeneity.

Translational Relevance

The proposed methodology detects corneal reflectivity changes in MFS not available from regular slit-lamp examination.

The mgΔlpn mouse model for Marfan syndrome recapitulates the ocular phenotypes of the disease

PURPOSE

Fibrillin-1 and -2 are major components of tissue microfibrils that compose the ciliary zonule and cornea. While mutations in human fibrillin-1 lead to ectopia lentis, a major manifestation of Marfan syndrome (MFS), in mice fibrillin-2 can compensate for reduced/lack of fibrillin-1 and maintain the integrity of ocular structures. Here we examine the consequences of a heterozygous dominant-negative mutation in the Fbn1 gene in the ocular system of the mgΔ^lpn mouse model for MFS.

METHODS

Eyes from mgΔ^lpn and wild-type mice at 3 and 6 months of age were analyzed by histology. The ciliary zonule was analyzed by scanning electron microscopy (SEM) and immunofluorescence.

RESULTS

Mutant mice presented a significantly larger distance of the ciliary body to the lens at 3 and 6 months of age when compared to wild-type, and ectopia lentis. Immunofluorescence and SEM corroborated those findings in MFS mice, revealing a disorganized mesh of microfibrils on the floor of the ciliary body. Moreover, mutant mice also had a larger volume of the anterior chamber, possibly due to excess aqueous humor. Finally, losartan treatment had limited efficacy in improving ocular phenotypes.

CONCLUSIONS

In contrast with null or hypomorphic mutations, expression of a dominant-negative form of fibrillin-1 leads to disruption of microfibrils in the zonule of mice. This in turn causes lens dislocation and enlargement of the anterior chamber. Therefore, heterozygous mgΔ^lpn mice recapitulate the major ocular phenotypes of MFS and can be instrumental in understanding the development of the disease.

Composition, structure and function of the corneal stroma

No other tissue in the body depends more on the composition and organization of the extracellular matrix (ECM) for normal structure and function than the corneal stroma. The precise arrangement and orientation of collagen fibrils, lamellae and keratocytes that occurs during development and is needed in adults to maintain stromal function is dependent on the regulated interaction of multiple ECM components that contribute to attain the unique properties of the cornea: transparency, shape, mechanical strength, and avascularity. This review summarizes the contribution of different ECM components, their structure, regulation and function in modulating the properties of the corneal stroma. Fibril forming collagens (I, III, V), fibril associated collagens with interrupted triple helices (XII and XIV), network forming collagens (IV, VI and VIII) as well as small leucine-rich proteoglycans (SLRP) expressed in the stroma: decorin, biglycan, lumican, keratocan, and fibromodulin are some of the ECM components reviewed in this manuscript. There are spatial and temporal differences in the expression of these ECM components, as well as interactions among them that contribute to stromal function. Unique regions within the stroma like Bowman's layer and Descemet's layer are discussed. To define the complexity of corneal stroma composition and structure as well as the relationship to function is a daunting task. Our knowledge is expanding, and we expect that this review provides a comprehensive overview of current knowledge, definition of gaps and suggests future research directions.

Distribution and Function of Glycosaminoglycans and Proteoglycans in the Development, Homeostasis and Pathology of the Ocular Surface

The ocular surface, which forms the interface between the eye and the external environment, includes the cornea, corneoscleral limbus, the conjunctiva and the accessory glands that produce the tear film. Glycosaminoglycans (GAGs) and proteoglycans (PGs) have been shown to play important roles in the development, hemostasis and pathology of the ocular surface. Herein we review the current literature related to the distribution and function of GAGs and PGs within the ocular surface, with focus on the cornea. The unique organization of ECM components within the cornea is essential for the maintenance of corneal transparency and function. Many studies have described the importance of GAGs within the epithelial and stromal compartment, while very few studies have analyzed the ECM of the endothelial layer. Importantly, GAGs have been shown to be essential for maintaining corneal homeostasis, epithelial cell differentiation and wound healing, and, more recently, a role has been suggested for the ECM in regulating limbal stem cells, corneal innervation, corneal inflammation, corneal angiogenesis and lymphangiogenesis. Reports have also associated genetic defects of the ECM to corneal pathologies. Thus, we also highlight the role of different GAGs and PGs in ocular surface homeostasis, as well as in pathology.