Abstract
We investigated whether the c.47T > C polymorphism (SNP rs4880) in the manganese superoxide dismutase (SOD2) gene is a risk factor for primary angle closure glaucoma (PACG) in the Saudi population. Among cases (n = 139), the prevalence of various genotypes were 25.9%, 46.8% and 27.3% for T/T, C/T and C/C genotypes respectively. This trend was similar in the controls (n = 403); 22.6%, 50.1% and 27.3% for T/T, C/T and C/C respectively. The differences in genotype distribution were not statistically significant (p = 0.391 and 0.682 respectively). The minor allele frequency was 50.7% in cases and 52.4% in controls; this difference was not statistically significant (p = 0.676). Investigating the potential association between this SOD2 polymorphism and different clinical indices, there was a statistically significant difference among different genotype groups in terms of three important clinical indices for PACG; Mean age at onset, duration of onset to and the mean LogMAR visual acuity (p = 0.041, 0.018 and 0.033 respectively). The three markers are highly associated prognostic factors to diseases severity. If our results are proven in larger cohort and in various populations, then this SNP may have potentiality to be used as an indicator for PACG severity.
Primary angle-closure glaucoma (PACG) is a type of glaucoma characterized by a narrow iridocorneal angle resulting in a blockage of the aqueous outflow structures. It seems that there is an anatomical and physiological predisposition to PACG. Eastern Asian ascendance, hyperopia and female gender significantly predispose to this disease. It has been estimated that in Saudi Arabia, 40% of glaucoma patients belong to the PACG type.Citation1 Although a hereditary component for PACG exists, causative genes had not been identified until recently where a significant association at three new loci: rs11024102 in PLEKHA7 [P = 5.33 × 10(−12)], rs3753841 in COL11A1 [P = 9.22 × 10(−10)] and rs1015213 located between PCMTD1 and ST18 on chromosome 8q [P = 3.29 × 10(−9)] were reported.Citation2 The three SNPs may explain in part some aspects of the PACG pathogenesis, but not all. Oxidative stress has been implicated to cause increased IOP by triggering TM degeneration and thus contributing to alterations in the aqueous outflow pathway. Indeed, treatment with hydrogen peroxide (H2O2) impairs TM cell adhesion to the extracellular matrix and causes rearrangement of cytoskeletal structures. In humans, in vivo experiments have demonstrated that oxidative DNA damage is significantly more abundant in the TM cells of glaucoma patients. Additionally, both increased IOP and visual field damage were significantly related to the amount of oxidative DNA damage affecting TM cells. Superoxide dismutase (SOD) is a major antioxidant enzyme, which plays a vital role in clearance of reactive oxygen species (ROS). Among its isoforms is the manganese superoxide dismutase (SOD2) which plays an important role as a primary mitochondrial antioxidant enzyme.Citation3 Previous studies had shown that a polymorphism at c.47T > C (p.16 Val>Ala) in the SOD2 gene (Gene Reference Sequence NM_00636.2; SNP rs4880) was a risk factor for a range of diseases and conditions such as sepsis, sensorineural hearing loss, medulloblastoma, obstructive pulmonary disease, and pancreatic cancer. Because of: (1) the pivotal role of mitochondria in retinal ganglion cell survival in diseases like glaucomaCitation4; (2) SOD2 is found in human mitochondria and plays an important role in preventing oxidative stress status; (3) accumulated evidence of oxidative stress involvement in glaucoma, and (4) our recent study which showed the association of this polymorphism with various clinical indices important for POAG,Citation5 we decided to investigate whether this polymorphism is a risk factor for PACG in the Saudi population and investigate if there is a link between this polymorphism and various clinical indices important for PACG.
We recruited 139 Saudi PACG patients (cases) who satisfied strict clinical criteria for PACG which includes at least three of the following: (1) clinical documentation of angle closure, defined as the presence of appositional or synechial closure of the anterior chamber angle involving at least 270° by gonioscopy in either eye; (2) intraocular pressure elevated to a level ≥21 mmHg as measured by Goldmann applanation tonometry; (3) evidence of characteristic glaucomatous optic disk damage with excavation of the disc causing a cup-to-disk ratio (c/d) vertically of at least 0.70 in at least one eye; and (4) characteristic peripheral visual field loss including nerve fiber bundle defects (nasal step, arcuate scotoma, paracentral scotoma) or advanced visual field loss (central and/or temporal island of vision) as tested by Humphrey Field Analyzer in those patients with vision better than 20/200 or Goldmann Manual Perimetry in those with worse vision. Exclusion criteria included: (1) secondary angle closure glaucoma; (2) presence of pseudoexfoliation syndrome even if coexistent with angle closure; (3) another cause of optic nerve injury affecting either eye; (4) significant visual loss in both eyes not associated with glaucoma; (5) inability to visualize the fundus for optic disk assessment; or (6) refusal to participate.
Patients were recruited from the glaucoma clinic at King Abdulaziz University Hospital (KAUH) after signing an informed consent form approved by the institutional review board (proposal number # 08-657).
A second group (n = 403) of healthy Saudi Arabs controls (Control group) free from glaucoma by examination were recruited. Entry criteria for those subjects were age >40, normal IOP range (≥6 and ≤21 mmHg), open angles on gonioscopy, and normal optic nerves on examination.
DNA was extracted from blood samples from patients and controls and genotyped for SNP rs4880 of the SOD2 genes as described previously.Citation5
Among our 139 cases, only 12 (9.2%) had a family history of glaucoma. Meanwhile, around one third of them; 36 (27.7%) were diabetic and one fourth; 33 (25.4%) were hypertensive. On the other hand, among our control group, 138 (34.2%) were diabetic and 128 (31.8%) were hypertensive. Consequently, there were no statistically significant difference between cases and controls neither in terms of the prevalence of diabetes mellitus nor in hypertension [p = 0.069; 95% CI: (−0.165 – 0.007)] and 0.074; 95% CI: (−0.159 – 0.008) for diabetes mellitus and hypertension respectively (). However, only 17 (13.1%) of them were found to be aware of having glaucoma. Out of the 139 glaucoma cases, 136 (97.7%) were bilateral and three (2.3%) were unilateral.
TABLE 1. Cases and controls distribution by demographic and systemic co-morbidity.
Among cases, the prevalence of the wild type allele “T/T” was 36 (25.9%) while the heterozygous mutated genotype “C/T” was more dominant; 65 (46.8%) and the homozygous fully mutated genotype “C/C” was 38 (27.3%). This trend was almost similar to the genotype distribution in controls which was 91 (22.6%); 202 (50.1%) and 110 (27.3%) for “T/T”; “C/T” and “C/C” respectively. There was no statistically significant difference between cases and controls in terms of genotype distribution on both heterozygous mutants [OR: 0.81, (95% CI: 0.493–1.355), p = 0.391] and homozygous mutant [OR: 0.87, (95% CI: 0.495–1.544), p = 0.683] respectively compared to the wildtype allele distribution ().
TABLE 2. Distribution of genotypes and allele frequencies in cases and control.
Investigating potential association between SOD polymorphism and different glaucoma indices, our findings show that there was an overall difference in “age at onset” across groups (p = 0.041), however, there were no significant associations between age at onset and different types of polymorphism in the pairwise analysis; (p = 0.141 and 0.145) for “C/T” and “C/C” specifically. Nevertheless, looking at the mean duration from onset to presentation to the hospital, it was found that there is a clear decreasing trend between such duration and the genotype of the polymorphism where the mean (±SD) duration to presentation was 64.8 (±54.1), 60 (±42) and 16.2 (±18.8) for “T/T”, “C/T”, and “C/C” respectively. Moreover, this descending trend was statistically significant across groups (p = 0.018) and in comparing the homozygous variant “C/C” to the wildtype homozygous genotype “T/T”; p = 0.021 which indicates that the difference between these two groups is the source of overall variation.
With regards to the glaucoma clinical indices, the intraocular pressure (IOP) didn’t show any statistically significant differences either across groups (p = 0.848) or in pairwise comparisons (p = 0.703 and 0.866) for “C/T” and “C/C” respectively. For the cup/disc ratio, there was a recognized ascending trend in severity of cupping directly proportional to the variant genotype whereas the mean (±SD) was 24.3 (±36.5) in the “T/T” genotype group while it increased to 28.9 (±35.4) and 29.5 (±37.4) in both “C/T” and “C/C” groups respectively. However, these differences did not reach statistical significance either for all groups comparison (p = 0.744) or for the pairwise comparisons (p = 0.453, and 0.548) for both “C/T” and “C/C” groups compared to the “T/T” group respectively. As regards the mean number of medications, there was a similar ascending trend such as in the mean CDR, where it increased from 1.4 (±1.1) in the “T/T” group to 1.5 (±1.1) and 1.7 (±1.2) in both “C/T” and “C/C” groups respectively. However, this trend was not significant in the across groups or the between group’s analyses.
Additionally, looking at the mean value of LogMAR visual acuity, there was a significant difference across the different genotype groups (p = 0.033). Moreover, the source of this variation was due to the significant difference between the heterozygous mutant allele group and the wildtype allele group: 0.5 (±0.7) compared to 0.8 (±0.9); p = 0.009.
Overall, it was recognized that all clinical indicators show their highest average levels among the homozygous fully mutated group “C/C” compared to the overall mean values of the whole group of cases. Despite the fact that this increase was not always statistically significant, it may still be used as an indicator for PACG severity (). If these results are proven in a larger cohort and in various populations, then this SNP (c.47T > C) in the SOD2 gene has the potential to be used as a marker for PACG severity.
TABLE 3. Overall distribution of clinical indices at presentation by different genotypes.
Acknowledgements
The authors would like to thank the Glaucoma Research Chair at the Department of Ophthalmology, College of Medicine, King Saud University for funding this research. The authors would like to express deep thanks and appreciation to Ms Priscilla W. Gikandi at the Department of Ophthalmology for formatting this manuscript.
Declaration of interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.
References
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