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

Genotype-phenotype profile of global ASPH-associated ectopia lentis and clinical findings from a Chinese cohort

BACKGROUND

Traboulsi syndrome is an under-recognized syndromic form of ectopia lentis (EL) caused by the aspartate beta-Hydroxylase (ASPH) variant. The genotype-phenotype profile of ASPH-associated disease is poorly understood due to the rarity of the condition.

METHODS

We conducted targeted next-generation sequencing and bioinformatics analysis to identify potentially pathogenic ASPH variants in the cohort. Furthermore, we characterized the expression pattern of ASPH and major components of the zonules using single-cell RNA-sequencing (scRNA-seq) and evaluated the genotype-phenotype correlations by combining our data and those from the literature.

RESULTS

We identified a novel missense variant c.2075G > A (p.G692D) and a recurrent nonsense variant c.1126C > G (p.R376*) of ASPH in two pedigrees from a Chinese cohort of EL. Both probands were 5-year-old boys with canonical facial dysmorphisms and bilateral anteriorly-dislocated lenses. Other ocular comorbidities included microspherophakia, shallow anterior chamber, and narrow chamber angel. No cardiac involvements or filtering blebs were identified. The single-cell expression atlas of ciliary epithelium demonstrated the coexpression of ASPH with FBN1, FBN2, and LTBP2 in the non-pigmented ciliary epithelium cells. Furthermore, molecular modeling simulation of p.G692D revealed increased affinity to the cb EGF-like domain and a subsequent destabilized calcium-binding motif. The genotype-phenotype analysis demonstrated that patients with cardiac involvements all harbored biallelic truncation variants.

CONCLUSIONS

The data from this study provide new insights into the genotype-phenotype profile of ASPH-associated disease and implicate the potential role of ASPH in the pathogenesis of EL.

Mutations in ADAMTSL4 cause a unique form of autosomal-recessive congenital ectopia lentis

Several unique mutations of ADAMTSL4 leading to congenital ectopia lentis (CEL) have been previously reported by our team. The purpose of this study is to find out the possible mechanism of a recurrent novel intronic variant in ADAMTSL4 led to CEL. Twelve novel ADAMTSL4 mutations with a unique form congenital ectopic lentis were detected previously by panel-based NGS. Genetic analysis verified a novel heterozygous ADAMTSL4 variation c.2177+4A>G on Intron 11 in two unrelated patients with iris and lens abnormalities. MINI-Gene assay showed two splicing modes of mRNA that process two different bands A and B, and mutant-type shows replacement with the splicing mode of Exon 11 skipping. Construction of wild-type and mutant ADAMTSL4 vector showed the appearance of premature termination codons (PTC). In vitro expression detection showed significant down-regulated expression level of ADAMTSL4 mRNAs and proteins in cells transfected with mutant vectors compared with in wild-type group. On the contrary, translation inhibitor CHX and small interfering RNA of UPF1 (si-UPF1) significantly increased mRNA or protein expression of ADAMTSL4 in cells transfected with the mutant vectors. 12 novel mutations in ADAMTSL4 gene have been previously reported by our team in 6 CEL patients with a unique series of ocular abnormalities. The recurrent novel ADAMTSL4 mutation c.2177+4A>G triggering the splicing mode of Exon 11 skipping and NMD would cause the decrease of ADAMTSL4 proteins that participate in biosynthesis and assembly of microfibers, which might lead to CEL, and suggest that sequencing of certain intronic splicing varition might be a vital tool for genetic counseling and prenatal diagnoses.

Genotype-phenotype Correlations of Ocular Posterior Segment Abnormalities in Marfan Syndrome

Purpose

Marfan syndrome (MFS) is a connective tissue disorder caused by mutations in the fibrillin-1 ( (FBN1). In addition to typical phenotypes such as ectopia lentis (EL) and aortic dilation, patients with MFS are prone to ocular posterior segment abnormalities, including retinal detachment (RD), maculopathy, and posterior staphyloma (PS). This study aims to investigate the correlations between FBN1 genotype and posterior segment abnormalities within a Chinese cohort of MFS.

Design

Retrospective study.

Participants

One hundred twenty-one eyes of 121 patients with confirmed FBN1 mutations between January 2015 and May 2023 were included.

Methods

Comprehensive ophthalmic examination findings were reviewed, and the incidence of RD, atrophic, tractional, and neovascular maculopathy (ATN classification system), and PS was analyzed between different genotype groups. Only the more severely affected eye from each patient was included.

Main Outcome Measures

Clinical features and risk factors.

Results

Of 121 patients, 60 eyes (49.59%) exhibited posterior segment abnormalities, including RD (4, 3.31%), maculopathy (47, 38.84%), and PS (54, 44.63%). The mean age was 11.53 ± 11.66 years, with 79.34% of patients <20 years old. The location and region of mutations were found to be associated with the incidence of maculopathy (P = 0.013, P = 0.033) and PS (P = 0.043, P = 0.036). Mutations in the middle region had a lower incidence of maculopathy and PS (P = 0.028 and P = 0.006, respectively) than those in C-terminal region. Mutations in the transforming growth factor-β (TGF-β) regulating sequence exhibited a higher incidence of maculopathy and PS (P = 0.020, P = 0.040). Importantly, the location and region of mutations were also associated with the incidence of atrophic maculopathy (P = 0.013 and P = 0.033, respectively). Mutations in the middle region had a significantly lower probability of atrophic maculopathy (P = 0.006), while mutations in the TGF-β regulating region had a higher incidence of atrophic maculopathy (P = 0.020).

Conclusions

Maculopathy and PS were associated with the location and region of FBN1 mutations. Patients with mutations in the TGF-β regulating region faced an increased risk of developing retinopathy.

Financial Disclosures

Proprietary or commercial disclosure may be found in the Footnotes and Disclosures at the end of this article.

Genotype Associated With Visual Prognosis in Patients With Congenital Ectopia Lentis Following Lens Surgery: A Prospective Cohort Study

PURPOSE

To investigate the relationship between visual prognosis and genotype in patients undergoing lens surgery for congenital ectopia lentis (EL).

DESIGN

Prospective clinical cohort study.

METHODS

Patients with congenital EL who underwent lens removal and intraocular lens implantation received panel-based next-generation sequencing. Patients were grouped into children and adolescents/adults based on the age at surgery. The visual prognosis, including best-corrected visual acuity (BCVA) and amblyopia, was stratified into short-term and medium to long-term.

RESULTS

This study included 329 probands with congenital EL, with a median age at lens surgery of 7.00 years (interquartile range [IQR] = 5.00, 12.50 years). Children with the non-FBN1 mutation exhibited inferior medium to long-term postoperative BCVA (0.26 [IQR: 0.14, 0.33] vs 0.15 [IQR: 0.10, 0.22], P = .034) and a higher prevalence of amblyopia (44.4% vs 16.8%, P = .012) compared to those with FBN1 mutation. Multivariable analysis showed that genotype (FBN1 vs non-FBN1 mutation) was significantly associated with medium to long-term postoperative BCVA (b = -0.128, 95% CI -0.214 to -0.042, P = .004) and amblyopia (OR = 0.20, 95% CI 0.05-0.78, P = .020) in children. Further classification of FBN1 genotype did not yield significant correlations with visual prognosis. However, no significant correlation was observed between genotype and short-term visual prognosis in the children. Children with less severe EL (OR = 0.13, 95% CI 0.02-0.85, P = .033) had lower risks of amblyopia in the short-term follow-up. For adolescent and adult patients with congenital EL, those with poor preoperative BCVA and long axial length should be informed of suboptimal visual prognosis.

CONCLUSIONS

Genotype significantly influences the medium to long-term visual prognosis in children with congenital EL. Genotype, along with preoperative BCVA, may assist in establishing reasonable expectations for patients regarding their visual outcomes after the lens surgery.

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.

Uncovering the Hidden World of Aqueous Humor Proteins for Discovery of Biomarkers for Marfan Syndrome

Ectopia lentis is a hallmark of Marfan syndrome (MFS), a genetic connective tissue disorder affecting 1/5000 to 1/10 000 individuals worldwide. Early detection in ophthalmology clinics and timely intervention of cardiovascular complications can be lifesaving. In this study, a modified proteomics workflow with liquid chromatography-tandem mass spectrometry (LC-MS/MS)-based data-independent acquisition (DIA) and field asymmetric ion mobility spectrometry (FAIMS) to profile the proteomes of aqueous humor (AH) and lens tissue from MFS children with ectopia lentis is utilized. Over 2300 and 2938 comparable proteins are identified in AH and the lens capsule, respectively. Functional enrichment analyses uncovered dysregulation of complement and coagulation-related pathways, collagen binding, and cell adhesion in MFS. Through weighted correlation network analysis (WGCNA) and machine learning, distinct modules associated with clinical traits are constructed and a unique biomarker panel (Q14376, Q99972, P02760, Q07507; gene names: GALE, MYOC, AMBP, DPT) is defined. These biomarkers are further validated using advanced parallel reaction monitoring (PRM) in an independent patient cohort. The results provide novel insights into the proteome characterization of ectopia lentis and offer a promising approach for developing a valuable biomarker panel to aid in the early diagnosis of Marfan syndrome via AH proteome.

Clinical and genetic risk factors underlying severe consequence identified in 75 families with unilateral high myopia

BACKGROUNDS

Unilateral high myopia (uHM), commonly observed in patients with retinal diseases or only with high myopia, is frequently associated with amblyopia with poor prognosis. This study aims to reveal the clinical and genetic spectrum of uHM in a large Chinese cohort.

METHODS

A total of 75 probands with simplex uHM were included in our Pediatric and Genetic Eye Clinic. Patients with significant posterior anomalies other than myopic fundus changes were excluded. Variants were detected by exome sequencing and then analyzed through multiple-step bioinformatic and co-segregation analysis and finally confirmed by Sanger sequencing. Genetic findings were correlated with associated clinical data for analysis.

RESULTS

Among the 75 probands with a mean age of 6.21 ± 4.70 years at the presentation, myopic fundus of C1 and C2 was observed in 73 (97.3%) probands. Surprisingly, specific peripheral changes were identified in 63 eyes involving 36 (48.0%) probands after extensive examination, including peripheral retinal avascular zone (74.6%, 47/63 eyes), neovascularization (54.0%), fluorescein leakage (31.7%), peripheral pigmentary changes (31.7%), and others. Exome sequencing identified 21 potential pathogenic variants of 13 genes in 20 of 75 (26.7%) probands, including genes for Stickler syndrome (COL11A1 and COL2A1; 6/20), FEVR (FZD4, LRP5, and TSPAN12; 5/20), and others (FBN1, GPR179, ZEB2, PAX6, GPR143, OPN1LW, FRMD7, and CACNA1F; 9/20). For the peripheral retinal changes in the 20 probands, variants in Stickler syndrome-related genes were predominantly associated with retinal pigmentary changes, lattice degeneration, and retinal avascular region, while variants in genes related to FEVR were mainly associated with the avascular zone, neovascularization, and fluorescein leakage.

CONCLUSIONS

Genetic defects were identified in about one-fourth of simplex uHM patients in which significant consequences may be hidden under a classic myopic fundus in up to half. To our knowledge, this is the first systematic genetic study on simplex uHM to date. In addition to routine care of strabismus and amblyopia, careful examination of the peripheral retina and genetic screening is warranted for patients with uHM in order to identify signs of risk for retinal detachment and other complications and provide meaningful genetic counseling.

Weill-Marchesani syndrome: natural history and genotype-phenotype correlations from 18 news cases and review of literature

BACKGROUND

Weill-Marchesani syndrome (WMS) belongs to the group of acromelic dysplasias, defined by short stature, brachydactyly and joint limitations. WMS is characterised by specific ophthalmological abnormalities, although cardiovascular defects have also been reported. Monoallelic variations in FBN1 are associated with a dominant form of WMS, while biallelic variations in ADAMTS10, ADAMTS17 and LTBP2 are responsible for a recessive form of WMS.

OBJECTIVE

Natural history description of WMS and genotype-phenotype correlation establishment.

MATERIALS AND METHODS

Retrospective multicentre study and literature review.

INCLUSION CRITERIA

clinical diagnosis of WMS with identified pathogenic variants.

RESULTS

61 patients were included: 18 individuals from our cohort and 43 patients from literature. 21 had variants in ADAMTS17, 19 in FBN1, 19 in ADAMTS10 and 2 in LTBP2. All individuals presented with eye anomalies, mainly spherophakia (42/61) and ectopia lentis (39/61). Short stature was present in 73% (from -2.2 to -5.5 SD), 10/61 individuals had valvulopathy. Regarding FBN1 variants, patients with a variant located in transforming growth factor (TGF)-β-binding protein-like domain 5 (TB5) domain were significantly smaller than patients with FBN1 variant outside TB5 domain (p=0.0040).

CONCLUSION

Apart from the ophthalmological findings, which are mandatory for the diagnosis, the phenotype of WMS seems to be more variable than initially described, partially explained by genotype-phenotype correlation.

Role of fibrilins in human cancer: A narrative review

Background

Fibrillin is one of the extracellular matrix glycoproteins and participates in forming microfibrils found in many connective tissues. The microfibrils enable the elasticity and stretching properties of the ligaments and support connective tissues. There are three isoforms of fibrillin molecules identified in mammals: fibrillin 1 (FBN1), fibrillin 2 (FBN2), and fibrillin 3.

Objective

Multiple studies have shown that mutations in these genes or changes in their expression levels can be related to various diseases, including cancers. In this study, we focus on reviewing the role of the fibrillin family in multiple cancers.

Methods and Results

We performed a comprehensive literature review to search PubMed and Google Scholar for studies published so far on fibrillin gene expression and its role in cancers. In this review, we have focused on the expression of FBN1 and FBN2 genes in cancers such as the lung, intestine, ovary, pancreatic ductal, esophagus, and thyroid.

Conclusion

Altogether various studies showed higher expression of fibrillins in different tumor tissues correlated with the patient's survival. However, there are controversial findings, as some other cancers showed hypermethylated FBN promoters with lower gene expression levels.

Predicting axial length in patients with Marfan syndrome and ectopia lentis after modified capsular tension ring and intraocular lens implantation

PURPOSE

To predict the growth of axial length (AL) in patients with Marfan syndrome (MFS) and ectopia lentis (EL).

SETTING

Eye and ENT Hospital of Fudan University, Shanghai, China.

DESIGN

Consecutive retrospective case series.

METHODS

Eyes were evaluated that had modified capsular tension ring and intraocular lens (IOL) implantation. The rate of AL growth (RALG) was calculated using AL divided by log10-transformed age. A multivariate linear regression model of RALG was developed after validation.

RESULTS

128 patients with MFS and EL were enrolled with a median follow-up duration of about 3 years. RALG was independent of age between 3 years and 15 years old ( P = .799) and decreased to 0 thereafter ( P = .878). Preoperative AL was associated with RALG in patients under 15 years old ( P = .003). Beta values for the final model of RALG were as below: intercept (-9.794) and preoperative AL (0.664). The postoperative AL was predicted as: postAL = preAL + RALG × log 10 ([postAge + 0.6]/[preAge + 0.6]). The mean prediction error was -0.003 (95% CI, -0.386 to 0.3791) mm and the mean absolute percentage error was 1.93% (95% CI, 0.73% to 3.14%). A Python-based calculator was developed to use the predicted AL in selecting IOL power and setting undercorrection.

CONCLUSIONS

The AL growth of patients with MFS followed a logarithmic pattern and ceased at about age 15. A prediction model of postoperative AL was established for individual MFS patients between 3 and 15 years old, which could potentially optimize the IOL power selection.

Morphometric assessment of the ciliary body in patients with Marfan syndrome and ectopia lentis: A quantitative study using ultrasound biomicroscopy: Ciliary body morphology in Marfan syndrome and ectopia lentis

PURPOSE

To explore the biometric characteristics of the ciliary body in patients with Marfan syndrome (MFS) and ectopia lentis (EL).

DESIGN

Cross-sectional study.

METHODS

Seventy-two consecutive MFS patients with EL and 72 non-disease controls were recruited. Ciliary body biometric parameters such as ciliary muscle cross-sectional area at 2000 μm from the scleral spur (CMA2000), ciliary muscle thickness at 1000 μm from the scleral spur (CMT1000), and maximum ciliary body thickness (CBTmax) were measured from multiple directions with ultrasound biomicroscopy (UBM). The relationship between ciliary body parameters and other ocular characteristics was also evaluated.

RESULTS

Average CMA2000, CMT1000, and CBTmax were 0.692 ± 0.015 mm², 0.405 ± 0.010 mm, and 0.855±0.023 mm in MFS eyes, respectively, and were significantly smaller than controls (all p < 0.001). The prevalence of ciliary body thinning was 22.2% in the MFS group versus 0 in controls (p < 0.001); eyes with more severe EL had smaller CMA2000 (p = 0.050), thinner CMT1000 (p = 0.022) and shorter CBTmax (p = 0.015). Patients with microspherophakia (MSP) had even smaller CMA2000 (p = 0.033) and CMT1000 (p = 0.044) than those without MSP. The most common subluxation direction was in the superonasal quadrant (25, 39.7%), which probably correlates with the thinnest CMT1000 in the inferotemporal quadrant (p = 0.005).

CONCLUSIONS

MFS patients with EL had thinner ciliary muscles, shorter ciliary processes, and a higher prevalence of ciliary body thinning, especially those with MSP. Both the extent and direction of subluxation were associated with ciliary body biometry.

What Should We Pay More Attention to Marfan Syndrome Expecting Ectopia Lentis: Incidence and Risk Factors of Retinal Manifestations

(1) Background: This paper investigates the incidence and risk factors of retinal manifestations in patients with Marfan syndrome (MFS) in a Chinese cohort. (2) Methods: This is a population-based cross-sectional study. In total, 344 eyes (172 MFS participants) were enrolled, each of whom underwent a detailed ocular examination. B-scan ultrasonography, ultra-wide-angle fundus images and optical coherence tomography images were conducted to assess posterior staphyloma, types of retinal damages and maculopathy. (3) Results: MFS patients have a high proportion (32.5%) of maculopathy, among which atrophy is the most common type (27.6%). Compared with participants without maculopathy, participants with maculopathy had a longer axial length (AL), higher incidence of posterior staphyloma, macular split and retinal detachment (RD) (p < 0.001, p < 0.001, p < 0.001 and p = 0.001). Moreover, the stage of RD has a significant correlation with longer AL and shallower anterior chamber depth (ACD) (p = 0.001 and p = 0.034, respectively). (4) Conclusions: A higher incidence and earlier onset of fundus lesions were found in MFS patients. Yearly systematic examination is recommended for MFS children with fundus manifestation until the cardiovascular and skeletal development is complete.

Biallelic ADAMTSL4 variants in a Chinese cohort of congenital ectopia lentis: Implications for genotype-phenotype relationships

ADAMTSL4 variants are one of the common causes of congenital ectopia lentis (EL), reported ocular comorbidities of which include iris anomalies, cataract, and glaucoma. However, a genotype-phenotype correlation has not been established. Potentially pathogenic ADAMTSL4 variants were screened from a Chinese cohort of congenital EL using panel-based next-generation sequencing followed by multiple bioinformatics analyses. The genotype-phenotype correlation was assessed via a systematic review of ADAMTSL4 variants within our data and those from the literature. A total of 12 variants of ADAMTSL4, including seven frameshift variants, one nonsense variant, two splicing variants, and two missense variants, were found in nine probands. Combing genetic and clinical information from 72 probands in the literature revealed 37 ADAMTSL4 variants known to cause EL, and the ethnic difference was prominent. The lens was inclined to dislocate inferior temporally (22, 27.16%), while the pupil was always located oppositely (9, 81.82%). Several anterior segments anomalies were identified, including ectopia pupillae (15, 18.52%), persistent pupillary membrane (9, 11.10%), poor pupil dilation (4, 30.8%), cataract (13, 24.10%), and glaucoma (8, 13.33%). Genotype-phenotype analysis revealed that truncation variants had higher risks of combined iris anomalies, including either ectopia pupillae or a persistent pupillary membrane (p =  0.007). The data from this study not only extend our knowledge of the ADAMTSL4 variant spectrum but also suggest that deleterious variants of ADAMTSL4 might be associated with severe ocular phenotypes.

Mutation analysis of SUOX in isolated sulfite oxidase deficiency with ectopia lentis as the presenting feature: insights into genotype-phenotype correlation

Background

Isolated sulfite oxidase deficiency (ISOD) caused by sulfite oxidase gene (SUOX) mutations is a rare neurometabolic disease associated with ectopia lentis (EL). However, few genotype–phenotype correlations have been established yet.

Methods

Potentially pathogenic SUOX mutations were screened from a Chinese cohort of congenital EL using panel-based next-generation sequencing and analyzed with multiple bioinformatics tools. The genotype–phenotype correlations were evaluated via a systematic review of SUOX mutations within our data and from the literature.

Results

A novel paternal missense mutation, c.205G > C (p.A69P), and a recurrent maternal nonsense mutation, c.1200 C > G (p.Y400*), of SUOX were identified in a 4-year-old boy from 312 probands. The biochemical assays manifested elevated urine sulfite and S-sulfocysteine accompanied by decreased homocysteine in the blood. The patient had bilateral EL and normal fundus, yet minimal neurological involvement and normal brain structure. Molecular modeling simulation revealed the p.A69P mutant had an unstable structure but an unchanged affinity for sulfite, while the truncated p.Y400* mutant showed decreased binding capacity. Genotype–phenotype analysis demonstrated patients with biallelic missense mutations had milder symptoms (P = 0.023), later age of onset (P < 0.001), and a higher incidence of regression (P = 0.017) than other genotypes. No correlations were found regarding EL and other neurological symptoms.

Conclusion

The data from this study not only enrich the known mutation spectrum of SUOX but also suggest that missense mutations are associated with mild and atypical symptoms.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13023-022-02544-x.

Genotype-phenotype correlations of marfan syndrome and related fibrillinopathies: Phenomenon and molecular relevance

Marfan syndrome (MFS, OMIM: 154700) is a heritable multisystemic disease characterized by a wide range of clinical manifestations. The underlying molecular defect is caused by variants in the FBN1. Meanwhile, FBN1 variants are also detected in a spectrum of connective tissue disorders collectively termed as ‘type I fibrillinopathies’. A multitude of FBN1 variants is reported and most of them are unique in each pedigree. Although MFS is being considered a monogenic disorder, it is speculated that the allelic heterogeneity of FBN1 variants contributes to various manifestations, distinct prognoses, and differential responses to the therapies in affected patients. Significant progress in the genotype–phenotype correlations of MFS have emerged in the last 20 years, though, some of the associations were still in debate. This review aims to update the recent advances in the genotype-phenotype correlations of MFS and related fibrillinopathies. The molecular bases and pathological mechanisms are summarized for better support of the observed correlations. Other factors contributing to the phenotype heterogeneity and future research directions were also discussed. Dissecting the genotype-phenotype correlation of FBN1 variants and related disorders will provide valuable information in risk stratification, prognosis, and choice of therapy.

Case report: Biochemical and clinical phenotypes caused by cysteine substitutions in the epidermal growth factor-like domains of fibrillin-1

Marfan syndrome, an autosomal dominant disorder of connective tissue, is primarily caused by mutations in the fibrillin-1 (FBN1) gene, which encodes the protein fibrillin-1. The protein is composed of epidermal growth factor-like (EGF-like) domains, transforming growth factor beta-binding protein-like (TB) domains, and hybrid (Hyb) domains and is an important component of elastin-related microfibrils in elastic fiber tissue. In this study, we report a cysteine to tyrosine substitution in two different domains of fibrillin-1, both of which cause Marfan syndrome with ocular abnormalities, in two families. Using protease degradation and liquid chromatography-tandem mass spectrometry analyses, we explored the different effects of substitution of cysteine by tyrosine in an EGF-like and a calcium-binding (cb) EGF-like domain on protein stability. The results showed that cysteine mutations in the EGF domain are more likely to result in altered proteolytic sensitivity and thermostability than those in the cbEGF domain. Furthermore, cysteine mutations can lead to new enzymatic sites exposure or hidden canonical cleavage sites. These results indicate the differential clinical phenotypes and molecular pathogenesis of Marfan syndrome caused by cysteine mutations in different fibrillin-1 domains. These results strongly suggest that failure to form disulfide bonds and abnormal proteolysis of fibrillin-1 caused by cysteine mutations may be an important factor underlying the pathogenesis of diseases caused by fibrillin-1 mutations, such as Marfan syndrome.

Cysteine Substitution and Calcium-Binding Mutations in FBN1 cbEGF-Like Domains Are Associated With Severe Ocular Involvement in Patients With Congenital Ectopia Lentis

Purpose: To investigate the clinical manifestations of congenital ectopia lentis (CEL) in patients with fibrillin (FBN1) calcium-binding epidermal growth factor (cbEGF)-like mutations.

Design: Retrospective cohort study.

Methods: Consecutive 68 CEL probands with FBN1 cbEGF-like mutations were recruited, mostly comprising Marfan syndrome (MFS) patients. Patients were classified into the cysteine group (n = 43), calcium (Ca²⁺)-binding group (n = 13) or the others (n = 12) according to their genotypes. Ocular biometrics, morbidities and visual performance were compared among different mutation groups. Linear regression was used to evaluate the risk factors for axial length (AL) elongation.

Results: With age-adjustment, cysteine substitution and Ca²⁺-binding mutations positively contributed to AL elongation (standardized coefficient: 0.410 and 0.367, p = 0.008 and 0.017, respectively). In addition, cataract formation was more frequently detected in patients with Ca²⁺-binding mutations (observed n = 3, expected n = 1.0; p = 0.036). Patients with cysteine substitutions had the poorest preoperative visual acuity among the three groups (p = 0.012) and did not recover as well as other patients. More MFS diagnoses were made in patients with cysteine substitutions (observed n = 16, expected n = 12.6), while ectopia lentis syndrome was detected more often in patients with cbEGF-like mutations out of the functional regions (observed n = 6, expected n = 2.5; p = 0.023).

Conclusion: Compared with patients with cbEGF-like mutations out of functional regions, patients with cysteine substitutions or Ca²⁺-binding mutations had longer ALs with age adjustment, poorer ocular involvement, visual performance, and systematic manifestations.