Contribution of the latent transforming growth factor-beta binding protein 2 gene to etiology of primary open angle glaucoma and pseudoexfoliation syndrome

• Introduction
• Methods
• Results
• Discussion

Abstract

Purpose: To assess for the first time the possible contribution of latent transforming growth factor (TGF)-beta binding protein 2 (LTBP2), an extracellular matrix (ECM) protein that associates with fibrillin-1-containing microfibrils, to the etiology of primary open angle glaucoma (POAG) and pseudoexfoliation (PEX) syndrome. Mutations in LTBP2 have previously been shown to be the cause of primary congenital glaucoma (PCG) and other disorders that often manifest as secondary glaucoma.

Methods: All exons of LTBP2 were sequenced in the DNA of 42 unrelated patients with POAG and 48 unrelated patients with PEX syndrome. Contribution of candidate variations to disease was assessed by screening in control individuals and use of biochemical, bioinformatics, and evolutionary criteria, and in one case by segregation analysis within the family of a proband with POAG. Microscopy was performed on the skin of a patient with PEX syndrome whose condition developed into PEX glaucoma during the course of the study and on the skin of her son previously identified with PCG who harbored the same LTBP2 mutation.

Results: Among the 30 sequence variations observed in LTBP2, five found in five patients with POAG and two found in two patients with PEX glaucoma syndrome may contribute to their diseases. One of the mutations was observed in a patient with POAG and in a patient with PEX glaucoma syndrome. Light, fluorescent, and electron microscopy showed that a mutation present in one of the individuals affected with PEX glaucoma syndrome and in her son affected with PCG causes disruptions in the ECM.

Conclusions: Some LTBP2 sequence variations can contribute to the etiology of POAG and PEX glaucoma syndrome. It is not expected that in these diseases LTBP2 mutations behave in a strictly Mendelian fashion with complete penetrance. In conjunction with recent findings, the results suggest that anomalies in the ECM are among the factors that can contribute to POAG and PEX glaucoma syndrome. LTBP2 and other related ECM protein coding genes should be screened in larger cohorts with these diseases, which are common disorders and important to the public health.

LTBP2 (OMIM 602091), the gene encoding latent transforming growth factor (TGF)-beta binding protein 2, was identified in 2009 as a gene causing primary congenital glaucoma (PCG; OMIM 231300) [1,2]. More recently, it was shown that LTBP2 mutations can also cause megalocornea [3,4], microspherophakia [5], and Weill-Marchesani syndrome (WMS; OMIM 277600) and promote Marfan syndrome (MFS; OMIM 154700) features [6]. The possible contribution of LTBP2 to primary open angle glaucoma (POAG; OMIM 137760) and pseudoexfoliation syndrome (PEX; OMIM 177650) is addressed in this study. LTBP2 on chromosome 14 encodes an extracellular matrix (ECM) protein that is a member of a superfamily composed of multiple fibrillin and LTBP proteins [7,8]. LTBP2 is expressed in elastic tissues and associates with fibrillin-1-containing microfibrils [9]. It is believed that LTBP2 has functions related to those of microfibrils and elastic fibers. In addition to structural roles, it may affect TGF-β activities.

Glaucoma, the second leading cause of blindness, is a heterogeneous group of optic neuropathies that manifest by optic nerve head cupping or degeneration of the optic nerve, resulting in a specific pattern of visual field loss [10]. Increased intraocular pressure (IOP) is often associated with the condition. The disease is sub-grouped based on etiology, anatomy of the anterior chamber, and age at onset [10]. POAG accounts for 70% of glaucoma cases in Caucasian populations and usually affects individuals past the age of 40 [11]. In this form of glaucoma, the anterior chamber angle appears normal; this form is associated with variable severity and phenotypic expressivity [12,13]. Although POAG in some families demonstrates Mendelian inheritance, the disease usually presents as a complex disease. After identifying LTBP2 as a PCG causative gene, the authors considered the gene might also cause POAG [1,2]. A POAG locus (LOD scores of 3.99) on chromosome 14q has been reported in a linkage study [14]. Cytogenetic studies have also suggested a tentative link between 14q and glaucoma [15,16]. Furthermore, a quantitative trait locus for high IOP, a prominent feature of glaucoma, has been reported on 14q [17]. In addition, mutations in the PCG causative gene CYP1B1 have been observed in patients with POAG, and it was thought that other genes including LTBP2 may also have roles in the etiology of both diseases [18-20]. Finally, all the disorders so far identified that can be caused by mutations in LTBP2 often manifest secondary glaucoma.

Pseudoexfoliation (PEX) syndrome is a common disorder that usually affects elderly individuals [21]. It is often accompanied by glaucoma and is in fact one of the most common causes of glaucoma. PEX syndrome is considered a fibrotic matrix disorder, characterized by stress-induced elastosis and excessive production and abnormal cross-linking of elastic microfibrils [22]. Aggregates in the form of what is known as PEX material deposit mainly in the anterior segment of the eye. PEX material consists of various elastic and microfibril-related proteins, including fibrillin-1 and LTBP2. Given that PEX is a fibrotic disorder often associated with glaucoma (PEXG) and that LTBP2 is a component of PEX material, the possibility that LTBP2 mutations may be associated with this disease was also considered.

We performed mutation screening of LTBP2 in a cohort of 90 patients with POAG or PEX syndrome. To the best of our knowledge, this is the first time LTBP2 has been screened in patients with these disorders. Sequence variations were found in both types of patients, and the properties of the variations support the proposition that LTBP2 may have a role in the etiology of these diseases. Microscopic examination of skin fibroblasts of two patients with LTBP2 mutations showed notable disruption of the ECM. Our results emphasize that the ocular anomalies in some patients with POAG and PEX can be due to disruptions in the ECM.

This study was performed in accordance with the Declaration of Helsinki and with approval of the ethics board of the University of Tehran. All participants or their responsible guardians consented to participate after being informed of the nature of the research.

Thirty LTBP2 sequence variations were observed in the 90 individuals screened (Appendix 2). Nineteen are previously reported variations, and 11 are novel variations. The allele frequency of 18 of the non-novel variations as reported in the Single Nucleotide Polymorphism Database (dbSNP) ranged from 0.012 to 0.405. The dbSNP data incorporates data derived from the 1000 Genomes Project. Six of the novel variations were observed in controls, created synonymous codons, were not highly conserved during evolution, and/or were intronic variations predicted not to affect splicing. We opted for a conservative approach and considered that these 24 variations did not contribute to disease status; clearly, variations reported in databases and variations observed in controls may predispose individuals to disease, particularly common and late onset diseases such as POAG and PEX syndrome. A previously identified PCG causative mutation and five novel variations were considered reasonable candidates for being variations or risk factors contributing to POAG and/or PEX. All but one were observed in the heterozygous state. Considerations including position of the affected amino acid within the protein, the biochemical nature of the amino acid change, the bioinformatics prediction of the effect of the change, the extent of the evolutionary conservation, and absence in controls were taken into account in making this assessment (Figure 1 and Figure 2). Segregation with disease status and familial data were also considered for one variation (p.Arg495Gln). All the novel variations and data evidencing disease association or absence of disease association are presented in Table 1 and Appendix 3. LTBP2 protein sequence alignments that allow assessment of evolutionary conservation are presented in Appendix 4. The position of candidate disease-influencing variations and previously reported mutations within the LTBP2 protein are presented in Figure 3. Some features of the variations observed in this study are described further.

Five novel coding variations in LTBP2 assessed as causing or as risk factors for disease were observed in five of the 42 patients with POAG screened (Table 1). Clinical features of the five patients are presented in Appendix 5. Age at onset in the patients harboring these putative POAG-contributing mutations ranged from 22 to 70 years. P.Pro432Leu among these was also observed in a patient with PEXG. The variation affecting p.Arg495Gln was observed in two siblings with POAG (Figure 4A,B). A fundus image of an eye of the proband showing features that confirm a diagnosis of glaucomatous optic neuropathy and results of perimetry showing visual field loss are presented in Figure 4C and Figure 5, respectively. Age at onset of the two sisters was 20 and 22 years. Furthermore, the deceased father and grandfather had had glaucoma and had become blind in one or both eyes in the third decade of their lives. Genetic analysis showed that the mother carried wild-type alleles, and clinical examination showed that she was free of ocular anomalies. The nucleotide sequence variation that caused p.Arg495Gln was not observed in 400 unaffected elderly ethnically matched control individuals. Interestingly, both siblings and also another POAG patient (213R) with an LTBP2 mutation had the same cardiac anomalies that had previously been attributed to LTBP2 mutations in WMS and MFS pedigrees [6]. The variation affecting p.Gln1417Arg was the only putative POAG-contributing variation found that occurred in the homozygous state (Table 1).The parents, who are deceased, were first cousins. Of the five patients who harbored the novel coding variations, all except the patient with the variation that caused p.Arg495Gln were apparently sporadic as they reported they were the only known individual with POAG in the respective families.

Among the 15 patients with PEX and 33 patients with PEXG, four novel coding variations were observed that were not found in controls (Table 1). The variations affecting p.Ile667Leu and p.Met1567Val, though novel and rare (not observed in 200 controls), are probably not associated with disease (Table 1). The variations that caused p.Pro432Leu and p.Tyr1792fsX55 likely contribute to the disease status of the patients with PEX syndrome. P.Pro432Leu is the variation also observed in a patient with POAG. The difference in the frequency of the causative variation in the patients (2/180 chromosomes) and the controls (0/800 chromosomes) is statistically highly significant (p=0.003). The patient with PEX syndrome who carried the p.Tyr1792fsX55-causing mutation in the heterozygous state was the mother of the proband (individual 21) of the PCG pedigree in which LTBP2 was identified as a PCG causative gene [2]. Observation of PEX syndrome in a heterozygous carrier and PCG in a homozygous carrier of the same mutation was intriguing. Histology was performed to get some insight into the effects of the mutation and the differential effects in the two individuals. Skin tissue was used because access to ocular tissue was not feasible. Light, electron, and fluorescent microscopy shown abnormalities in the ECM of both patients (Figure 6, Figure 7, and Figure 8). In the light microscope images, the elastic fibers were sparser and fragmented in each patient compared to the respective control (Figure 6A–D). In general, disruption of the elastic fibers appeared more severe in the patient with PCG (Figure 6A) compared to his mother with PEX syndrome (Figure 6B), who was 55 years old and 25 years older than her son. Collagen fibers were also sparser in each patient compared to the respective control (Figure 6E–H). Finally, electron micrographs also clearly showed that the ECM of each patient was sparser than that of the controls (Figure 7A compared to Figure 7C, and Figure 7B compared to Figure 7D), and that the ECM of the patient with PCG was more severely disrupted than the ECM of the patient with PEX syndrome (Figure 7A compared to Figure 7B). Antibodies against LTBP2 and fibrillin-1-stained fibers that were longer and thicker in the ECM of the control individuals (Figure 8 and Figure 9). The fibers were sparse in the patient with PCG, but dense and convoluted in the ECM of the patient with PEX syndrome. This appearance is characteristic of microfibrils in patients with PEX syndrome [25]. The patterns described were consistently observed in multiple sections from the individuals studied. When the mother was examined in early 2011, she was diagnosed with PEX syndrome without glaucoma. Upon reexamination in 2012, her presentation had evolved into PEXG syndrome. Images of the eyes of the mother with PEXG syndrome and the son with PCG are shown in Figure 10. Depositions of PEX material and glaucomatous damage are evident in the mother’s eye.

Mutations in LTBP2 have previously been shown to cause several disorders, all of which have ocular manifestations, and one of the ocular manifestations is often glaucoma [1-5]. Contrary to other known genes causing PCG (CYP1B1) or POAG (MYOC, OPTN, and WDR36), one can easily consider a plausible cellular and molecular basis for association between LTBP2 and the glaucoma phenotype. LTBP2 is an extracellular matrix microfibril protein, and defects in the ECM of the trabecular meshwork may affect facility of aqueous fluid outflow resulting in increased IOP [26]. This notwithstanding, the consequences of LTBP2 mutations for regulating TGF-β signaling may also be relevant to the etiology of glaucoma. Five putative disease-contributing or risk factor mutations in LTBP2 were observed among 42 patients with POAG without CYP1B1 and MYOC mutations. Although the size of the cohort screened was small, the frequency of the observed LTBP2 mutations compares well with that of other known genes causing POAG [18,27]. Relevant to this finding, ADAMTS10 was recently reported as a POAG causative gene in a canine model of the disease [28]. ADAMTS10 is a member of a disintegrin and metalloproteinase with thrombospondin motifs family of secreted proteases that similarly to LTBP2 is involved in the formation of the ECM [29]. Both proteins are expressed in the trabecular meshwork [2,28]. POAG rarely exhibits Mendelian inheritance, and many genetic and possibly non-genetic factors contribute to its pathogenesis. Taken together, the data presented here support the proposal that structural and/or functional defects of the ECM and/or microfibrils are among the factors that contribute to POAG and that mutations in genes coding for various components of the ECM and microfibrils including LTBP2 are implicated in the disease’s etiology. ECM abnormalities have also previously been suggested to affect IOP elevation in glaucoma [30-33].

PEX syndrome is a fibrillinopathy that can present with numerous clinical complications [34]. Ocular tissues are most commonly affected. In addition to LTBP2 being a component of PEX material, there is increased TGF-β and LTBP2 in the aqueous fluid of affected individuals [25]. In addition, LTBP2 and FBN1 were identified as upregulated genes in anterior segment eye tissues of individuals with PEX [35]. The role of TGF-β1 in the pathogenesis of PEX has been emphasized, and it was suggested that LTBP2 via its effects on TGF-β activation may be involved in the disease process [25]. Variable incidence of PEX in different populations and increased risk in relatives of affected individuals indicate a genetic component in the disease’s etiopathogenesis, but knowledge of the genetics of PEX is rudimentary [22]. Association between some single nucleotide polymorphisms in LOXL1 encoding lysyl oxidase-like 1 and PEX syndrome has been reported, although the PEX-associated variants in LOXL1 exhibit low penetrance [36-38]. Lysyl oxidase-like 1 is a cross-linking enzyme with functions in elastic fiber formation and stabilization [39]. Here, we report the presence of two putative LTBP2 mutations in patients with PEX. One mutation affected p.Pro432Leu and was observed in a patient with PEXG. The fact that it was also observed in a patient with glaucoma argues in favor of its pathogenicity. The second mutation caused p.Tyr1792fsX55 and was observed in a patient with PEXG; the same mutation in the homozygous state caused PCG in other family members. It was absent in 400 control individuals over the age of 60 without eye disorders. Penetrance of the p.Tyr1792fsX55-causing mutation regarding PEX is apparently incomplete because the father of the family who harbored the same mutation was diagnosed normal; he is now deceased. Histological analysis showed that the p.Tyr1792fsX55-causing mutation disrupted the ECM and microfibril structures, and that the disruptions were more severe in homozygous carriers of the mutation. Histological analysis in a patient with WMS who harbored a different mutation in LTBP2 also showed disruption of the ECM and microfibril structures. These findings further support the contention that LTBP2 mutations may contribute to various disorders with ocular manifestations by affecting the extracellular matrix.

There is variability in phenotypic consequences of mutations in LTBP2. A notable observation is that the mutation affecting p.Pro432Leu was observed in individuals with POAG and PEXG. The data presented here suggest that the penetrance of most observed LTBP2 mutations is incomplete as disease in the patients usually appeared sporadically. This is consistent with the common classification of POAG and PEX syndrome as complex disorders. As both diseases are multifactorial, factors in addition to the LTBP2 sequence variations affected disease status in the individuals harboring the mutation. Phenotypic presentations in individuals of the Roma/Gypsy population who harbored homozygous mutations causing p.Arg299X in LTBP2 were reported to range from PCG with trabecular meshwork dysgenesis to Marfan syndrome-like zonular disease and late onset angle closure glaucoma [40]. In another family study, the same p.Arg299X mutation was reported to cause a recessive ocular syndrome exhibiting megalocornea, spherophakia, and secondary glaucoma [3]. Homozygous LTBP2 mutations are likely to result in a more severe phenotype; homozygous mutations were reported to cause PCG and WMS, which are early onset diseases with more severe clinical manifestations than POAG and PEX syndrome, which are late onset diseases [1,2]. Notably, POAG and PEX are not considered recessively inherited diseases. Phenotypic variability associated with FBN1 mutations is also evident. As stated, mutations in FBN1 can cause simple ectopia lentis, MFS, WMS as well as other related disorders [41]. The mutation affecting p.Cys1223Tyr has been observed in patients with MFS [42] and Shprintzen-Goldberg syndrome [43]. The variations in phenotypic manifestations of mutations in the two related genes LTBP2 and FBN1 may be due to environmental, epigenetic, or stochastic events. More interesting, it is also likely that there are epistatic relationships between genes coding various components of the ECM. An intriguing possibility is that some ECM-related proteins may be able to compensate one another’s functions to different degrees.

In conclusion, we report that LTBP2 mutations may cause or be risk factors for POAG and PEX. The findings emphasize the importance of the extracellular matrix to biologic functions affected in these disorders. POAG and PEX syndrome are common disorders important to public health issues of most populations. It is of paramount importance that larger patient cohorts with these diseases be screened for mutations in LTBP2 and other related ECM protein coding genes to better quantify their contributions to the diseases.