The N-terminal p.(Ser38Cys) TIMP3 mutation underlying Sorsby fundus dystrophy is a founder mutation disrupting an intramolecular disulfide bond

Disease-Causing TIMP3 Variants and Deep Phenotyping of Two Czech Families with Sorsby Fundus Dystrophy Associated with Novel p.(Tyr152Cys) Mutation

We aim to report the ocular phenotype and molecular genetic findings in two Czech families with Sorsby fundus dystrophy and to review all the reported TIMP3 pathogenic variants. Two probands with Sorsby fundus dystrophy and three first-degree relatives underwent ocular examination and retinal imaging, including optical coherence tomography angiography. The DNA of the first proband was screened using a targeted ocular gene panel, while, in the second proband, direct sequencing of the TIMP3 coding region was performed. Sanger sequencing was also used for segregation analysis within the families. All the previously reported TIMP3 variants were reviewed using the American College of Medical Genetics and the Association for Molecular Pathology interpretation framework. A novel heterozygous variant, c.455A>G p.(Tyr152Cys), in TIMP3 was identified in both families and potentially de novo in one. Optical coherence tomography angiography documented in one patient the development of a choroidal neovascular membrane at 54 years. Including this study, 23 heterozygous variants in TIMP3 have been reported as disease-causing. Application of gene-specific criteria denoted eleven variants as pathogenic, eleven as likely pathogenic, and one as a variant of unknown significance. Our study expands the spectrum of TIMP3 pathogenic variants and highlights the importance of optical coherence tomography angiography for early detection of choroidal neovascular membranes.

Review: Mechanisms of TIMP-3 accumulation and pathogenesis in Sorsby fundus dystrophy

Sorsby fundus dystrophy (SFD) is a rare, inherited form of macular degeneration caused by mutations in the gene encoding tissue inhibitor of metalloproteinases 3 (TIMP-3). There are 21 mutations currently associated with SFD, with some variants (e.g., Ser179Cys, Tyr191Cys, and Ser204Cys) having been studied much more than others. We review what is currently known about the identified SFD variants in terms of their dimerization, metalloproteinase inhibition, and impact on angiogenesis, with a focus on disparities between reports and areas requiring further study. We also explore the potential molecular mechanisms leading to the accumulation of extracellular TIMP-3 in SFD and consider how accumulated TIMP-3 causes macular damage. Recent reports have identified extraocular pathologies in a small number of SFD patients. We discuss these intriguing findings and consider the apparent discrepancy between the widespread expression of TIMP-3 and the primarily retinal manifestations of SFD. The potential benefits of novel experimental approaches (e.g., metabolomics and stem cell models) in terms of investigating SFD pathology are presented. The review thus highlights gaps in our current molecular understanding of SFD and suggests ways to support the development of novel therapies.

Macular neovascularization in inherited retinal diseases: A review

Inherited retinal diseases (IRDs) are the most common cause of blindness in working-age adults. Macular neovascularization (MNV) may be a presenting feature or occurs as a late-stage complication in several IRDs. We performed an extensive literature review on MNV associated with IRDs. MNV is a well-known complication of Sorsby fundus dystrophy and pseudoxanthoma elasticum. Those with late-onset Stargardt disease may masquerade as exudative age-related macular degeneration (AMD) when MNV is the presenting feature. Peripherinopathies may develop MNV that responds well to a short course of anti-vascular endothelial growth factor (anti-VEGF) therapy, while bestrophinopathies tend to develop MNV in the early stages of the disease without vision loss. Enhanced S-cone syndrome manifests type 3 MNV that typically regresses into a subfoveal fibrotic nodule. MNV is only a rare complication in choroideraemia and rod-cone dystrophies. Most IRD-related MNVs exhibit a favorable visual prognosis requiring less intensive regimens of anti-vascular endothelial growth factor therapy compared to age-related macular degeneration. We discuss the role of key imaging modalities in the diagnosis of MNV across a wide spectrum of IRDs and highlight the gaps in our knowledge with respect to the natural history and prognosis to pave the way for future directions of research.

Hidden Relationships between N-Glycosylation and Disulfide Bonds in Individual Proteins

N-Glycosylation (NG) and disulfide bonds (DBs) are two prevalent co/post-translational modifications (PTMs) that are often conserved and coexist in membrane and secreted proteins involved in a large number of diseases. Both in the past and in recent times, the enzymes and chaperones regulating these PTMs have been constantly discovered to directly interact with each other or colocalize in the ER. However, beyond a few model proteins, how such cooperation affects N-glycan modification and disulfide bonding at selective sites in individual proteins is largely unknown. Here, we reviewed the literature to discover the current status in understanding the relationships between NG and DBs in individual proteins. Our results showed that more than 2700 human proteins carry both PTMs, and fewer than 2% of them have been investigated in the associations between NG and DBs. We summarized both these proteins with the reported relationships in the two PTMs and the tools used to discover the relationships. We hope that, by exposing this largely understudied field, more investigations can be encouraged to unveil the hidden relationships of NG and DBs in the majority of membranes and secreted proteins for pathophysiological understanding and biotherapeutic development.

The Role of Vitamin A in Retinal Diseases

Vitamin A is an essential fat-soluble vitamin that occurs in various chemical forms. It is essential for several physiological processes. Either hyper- or hypovitaminosis can be harmful. One of the most important vitamin A functions is its involvement in visual phototransduction, where it serves as the crucial part of photopigment, the first molecule in the process of transforming photons of light into electrical signals. In this process, large quantities of vitamin A in the form of 11-cis-retinal are being isomerized to all-trans-retinal and then quickly recycled back to 11-cis-retinal. Complex machinery of transporters and enzymes is involved in this process (i.e., the visual cycle). Any fault in the machinery may not only reduce the efficiency of visual detection but also cause the accumulation of toxic chemicals in the retina. This review provides a comprehensive overview of diseases that are directly or indirectly connected with vitamin A pathways in the retina. It includes the pathophysiological background and clinical presentation of each disease and summarizes the already existing therapeutic and prospective interventions.

Longitudinal phenotypic study of late-onset retinal degeneration due to a founder variant c.562C>A p.(Pro188Thr) in the C1QTNF5 gene

Background: Late-onset retinal degeneration (L-ORD) is a rare autosomal dominant retinal dystrophy related to C1QTNF5 gene variants.Materials and methods: Twenty-six patients (21-81 years) with L-ORD due to c.562C>A p.(Pro188Thr) with a mean follow-up time of 8 years (range 1-37 years) underwent an extensive ophthalmic work-up.Results: Best-corrected visual acuity (BCVA) and visual fields were maintained up to 50 to 55 years (n = 8), with a gradual decline, but conservation of functional central vision between 55 to 65 years (n = 15), followed by a steep decrease in overall visual function beyond 65 years (n = 9). Classic anterior segment findings in L-ORD of abnormally long, anteriorly inserted lens zonules were absent in most patients (n = 24/26). In contrast, findings of iris transillumination and sphincter pupillae atrophy with poor dilation were novel. Patients presented with three completely different initial fundus phenotypes: adjoining pavingstone-like atrophic patches (type 1) (n = 6/20); tiny yellow-white subretinal dots (type 2) (n = 8/20); or larger yellow, thick, round sub-RPE drusenoid deposits (type 3) (n = 4/20). Two patients had a mixed phenotype. Although different in presentation phenotype, patients eventually all progressed to a common panretinal atrophy with diffuse intraretinal pigment migration beyond the age of 65. Progression pace, and thus visual prognosis, differed depending on presentation phenotype. Specifically, type 2 appears to have a more benign course.Conclusions: Phenotypic analysis showed three distinct presenting phenotypes with a considerable intrafamilial variability both in age of onset of clinical signs and in disease progression, with a fair visual potential (>20/40) until the seventh decade.Abbreviations: L-ORD: Late-onset retinal degeneration; C1QTNF5: complement 1Q tumor necrosis factor 5; OCT: Ocular coherence tomography; BCVA: Best-corrected visual acuity; RPE: Retinal pigment epithelium; ffERG: Full-field electroretinography; IRD: Inherited retinal dystrophy; CNV: Choroidal neovascularization; LAZ: Long anteriorly inserted zonules; AMPK: AMP-activated protein kinase; IOP: Intraocular pressure; cSLO: confocal scanning laser ophthalmoscopy; BAF: Blue light autofluorescence; NIR-AF: Near-infrared autofluorescence; NIR-R: Near-infrared reflectance; RF: Red-free; SD-OCT: Spectral domain ocular coherence tomography; HRR: Hardy-Rand-Rittler pseudo-isochromatic plates; AS: anterior segment; UBM: ultrasound biomicroscopy; PCR: Polymerase chain reaction; SNP: Single nucleotide polymorphism; VEGF: Vascular endothelial growth factor; IZ: Interdigitation zone; EZ: Ellipsoid zone; ELM: External limiting membrane; LP: Light perception; AMD: Age-related macular degeneration; SFD: Sorsby fundus dystrophy.

In vitro stem cell modelling demonstrates a proof-of-concept for excess functional mutant TIMP3 as the cause of Sorsby fundus dystrophy

Sorsby fundus dystrophy (SFD) is a rare autosomal dominant disease of the macula that leads to bilateral loss of central vision and is caused by mutations in the TIMP3 gene. However, the mechanisms by which TIMP3 mutations cause SFD are poorly understood. Here, we generated human induced pluripotent stem cell-derived retinal pigmented epithelial (hiPSC-RPE) cells from three SFD patients carrying TIMP3 p.(Ser204Cys) and three non-affected controls to study disease-related structural and functional differences in the RPE. SFD-hiPSC-RPE exhibited characteristic RPE structure and physiology but showed significantly reduced transepithelial electrical resistance associated with enriched expression of cytoskeletal remodelling proteins. SFD-hiPSC-RPE exhibited basolateral accumulation of TIMP3 monomers, despite no change in TIMP3 gene expression. TIMP3 dimers were observed in both SFD and control hiPSC-RPE, suggesting that mutant TIMP3 dimerisation does not drive SFD pathology. Furthermore, mutant TIMP3 retained matrix metalloproteinase activity. Proteomic profiling showed increased expression of ECM proteins, endothelial cell interactions and angiogenesis-related pathways in SFD-hiPSC-RPE. By contrast, there were no changes in VEGF secretion. However, SFD-hiPSC-RPE secreted higher levels of monocyte chemoattractant protein 1, PDGF and angiogenin. Our findings provide a proof-of-concept that SFD patient-derived hiPSC-RPE mimic mature RPE cells and support the hypothesis that excess accumulation of mutant TIMP3, rather than an absence or deficiency of functional TIMP3, drives ECM and angiogenesis-related changes in SFD. © 2020 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

A novel TIMP3 mutation associated with a retinitis pigmentosa-like phenotype

BACKGROUND

Sorsby Fundus Dystrophy is an inherited macular degeneration caused by pathogenic variants in the TIMP3 gene. Clinical exam findings typically drusen -like deposits beneath the RPE or reticular pseudo drusen deposits above the RPE with a majority of patients developing choroidal neovascularization.

MATERIALS AND METHODS

Case report of two members of a family that present with atypical clinical exam findings. Protein modeling of the novel Y137CTIMP3 variant was performed and compared with other known variants.

RESULTS

In this study we describe a father and son initially diagnosed with retinitis pigmentosa of unknown genetic origin. More recent genetic testing of the patients, identified a novel c.410A>G; p.Tyr137Cys variant of uncertain clinical significance in the Tissue Inhibitor of Metalloproteinase-3 (TIMP3) gene. The atypical clinical findings led us to compare the theoretical molecular effects of this variant on the TIMP3 protein structure and interactions with other proteins using homology modeling and machine learning predictions.

CONCLUSIONS

It is important to consider mutations in TIMP3 in atypical cases of Retinitis Pigmentosa particularly in the absence of known variants.

The Diverse Roles of TIMP-3: Insights into Degenerative Diseases of the Senescent Retina and Brain

Tissue inhibitor of metalloproteinase-3 (TIMP-3) is a component of the extracellular environment, where it mediates diverse processes including matrix regulation/turnover, inflammation and angiogenesis. Rare TIMP-3 risk alleles and mutations are directly linked with retinopathies such as age-related macular degeneration (AMD) and Sorsby fundus dystrophy, and potentially, through indirect mechanisms, with Alzheimer's disease. Insights into TIMP-3 activities may be gleaned from studying Sorsby-linked mutations. However, recent findings do not fully support the prevailing hypothesis that a gain of function through the dimerisation of mutated TIMP-3 is responsible for retinopathy. Findings from Alzheimer's patients suggest a hitherto poorly studied relationship between TIMP-3 and the Alzheimer's-linked amyloid-beta (A) proteins that warrant further scrutiny. This may also have implications for understanding AMD as aged/diseased retinae contain high levels of A. Findings from TIMP-3 knockout and mutant knock-in mice have not led to new treatments, particularly as the latter does not satisfactorily recapitulate the Sorsby phenotype. However, recent advances in stem cell and in vitro approaches offer novel insights into understanding TIMP-3 pathology in the retina-brain axis, which has so far not been collectively examined. We propose that TIMP-3 activities could extend beyond its hitherto supposed functions to cause age-related changes and disease in these organs.

The N-terminal p.(Ser38Cys) TIMP3 mutation underlying Sorsby fundus dystrophy is a founder mutation disrupting an intramolecular disulfide bond

Sorsby fundus dystrophy (SFD) is a macular degeneration caused by mutations in TIMP3, the majority of which introduce a novel cysteine. However, the exact molecular mechanisms underlying SFD remain unknown. We aimed to provide novel insights into the functional consequences of a distinct N-terminal mutation. Haplotype reconstruction in three SFD families revealed that the identified c.113C>G, p.(Ser38Cys) mutation is a founder in Belgian and northern French families with a late-onset SFD phenotype. Functional consequences of the p.(Ser38Cys) mutation were investigated by high-resolution Western blot analysis of wild type and mutant TIMP3 using patient fibroblasts and in vitro generated proteins, and by molecular modeling of TIMP3 and its interaction partners. We could not confirm a previous hypothesis on dimerization of mutant TIMP3 proteins. However, we identified aberrant intramolecular disulfide bonding. Our data provide evidence for disruption of the established Cys36-Cys143 disulfide bond and formation of a novel Cys36-Cys38 bond, possibly associated with increased glycosylation of the protein. In conclusion, we propose a novel pathogenetic mechanism underlying the p.(Ser38Cys) TIMP3 founder mutation involving intramolecular disulfide bonding. These results provide new insights into the pathogenesis of SFD and other retinopathies linked to mutations in TIMP3, such as age-related macular degeneration.