Heritability of Corneal Curvature and Astigmatism

Heritability and Genetic Factors of Astigmatism and Corneal Curvature: a Systematic Review and Meta-analysis

PURPOSE

To systematically review and meta-analyze all reported heritability studies of refractive astigmatism (RA), corneal astigmatism (CA) and corneal curvature (CC), and evaluate the existing genetic associations of RA, CA and CC.

DESIGN

Systematic review and meta-analysis (PROSPERO ID: CRD42023447370).

METHODS

Studies that reported the heritability and genetic associations of RA, CA and/or CC were identified from PubMed, Web of Science and EMBASE (from inception to October 1, 2023). Newcastle-Ottawa Scale criteria was used to assess the risk of bias. Meta-analyses of heritability were conducted using random-effects model for mean difference. All current genetic associations were catalogued according to level of statistical significance.

RESULTS

Pooled heritabilities were moderate for RA (h2 = 0.46, 95% CI: 0.27 - 0.65), CA (h2 = 0.48, 95% CI: 0.38 - 0.58) and CC (h2 = 0.64, 95% CI: 0.53 - 0.76). Subgroup analyses revealed significant differences between analysis methods (CA: P < 0.01; CC: P = 0.03) and populations (CA: P < 0.01; CC: P < 0.01) in both CA and CC, and between age groups in CA (P < 0.01). Totally 50 single-nucleotide polymorphisms (SNPs) in 10 genes have been reported with overlapping associations with RA, CA and/or CC, with BMP3, FMNL2, HERC2, PROX1-AS1 and ZC3H11B associated with RA and CA, FBN1, NHSL1 and PDGFRA with CA and CC, TRAF3IP1 with RA and CC; and CASC15 with RA, CA and CC.

CONCLUSIONS

This study confirms moderate heritabilities of RA, CA and CC. Through evaluating overlapping SNPs or genes between these three phenotypes, we prioritized 50 SNPs in 10 genes as candidate variants for further validation. These findings highlight the complex genetic architecture of astigmatism and indicate shared and distinct genetic markers for different astigmatism-related corneal parameters. Future studies in different populations and functional studies evaluating the roles of the involved genes in astigmatism are warranted.

Association of Maternal and Paternal Astigmatism With Child Astigmatism in the Hong Kong Children Eye Study

IMPORTANCE

Parental astigmatism is a factor associated with risk for development of child astigmatism; however, the magnitude of the association has not been determined.

OBJECTIVE

To determine the association between parental and child astigmatism.

DESIGN, SETTING, AND PARTICIPANTS

This population-based, cross-sectional study included participants from familial trios, each comprising a child aged 6 to 8 years and both parents, recruited from the Hong Kong Children Eye Study. No restriction criteria were set on the children in terms of refractive status. Data were analyzed from February to June 2022.

EXPOSURES

Cycloplegic autorefraction and autokeratometry were conducted on the children, whereas noncycloplegic autorefraction and autokeratometry were conducted on their parents. The children were categorized into 6 groups on the basis of the severity of astigmatism of both parents. Information on parental education, family income, and children's outdoor and near work time were obtained by questionnaires.

MAIN OUTCOMES AND MEASURES

The primary outcome was the odds of child astigmatism among the 6 categories of children. Associations of factors with child astigmatism were evaluated by logistic regression analyses.

RESULTS

A total of 17 124 participants from 5708 trios (2964 boys and 2754 girls) at a mean (SD) age of 7.32 (0.87) years, and 11 416 parents were examined. Astigmatism of 1.0 D or greater in both parents was associated with greater odds of refractive astigmatism (RA) (odds ratio [OR], 1.62; 95% CI, 1.15-2.26) and corneal astigmatism (CA) (OR, 1.94; 95% CI, 1.50-2.50) in the child. The respective ORs increased to 3.10 (95% CI, 1.34-7.21) and 4.31 (95% CI, 1.76-10.55) when both parents had astigmatism 2.0 D or greater. Higher parental astigmatism conferred higher risks for both RA and CA in children (P for trend <.001). Parental astigmatism was significantly associated with greater odds of corresponding child astigmatism (maternal RA: OR, 0.76; 95% CI, 0.68-0.84; paternal RA: OR, 0.82; 95% CI, 0.74-0.91; maternal CA: OR, 1.70; 95% CI, 1.51-1.93; paternal CA: OR, 1.33; 95% CI, 1.19-1.49).

CONCLUSIONS AND RELEVANCE

The findings of this cross-sectional study suggest that parental astigmatism may confer an independent and dose-dependent association with child astigmatism. Children with parents with astigmatism should have early eye examinations for timely detection of astigmatism to facilitate age-appropriate vision correction and visual development.

Risk factors for astigmatic components and internal compensation: the Nanjing Eye Study

Purpose

To determine the risk factors for total astigmatism (TA), anterior corneal astigmatism (ACA), and internal compensation in Chinese preschool children.

Methods

In the population-based Nanjing Eye Study, children were measured for noncycloplegic refractive error and for biometric parameters. Data from questionnaires and measures from right eyes were analyzed for determining risk factors for TA, ACA, and internal compensation from multivariate logistic regression models.

Results

Of 1327 children (66.8 ± 3.4 months, 53.2% male), older age of the child (OR = 0.95 for per month increase; P = 0.03), older paternal age at child birth (OR = 1.04 for per year increase; P = 0.03), paternal astigmatism (OR = 1.89; P = 0.003), maternal astigmatism (OR = 1.73, P = 0.008), and second-hand smoke exposure during pregnancy (OR = 1.64; P = 0.03) were associated with higher risk of TA, while partial breastfeeding (OR = 0.49, P = 0.006) or formula feeding (OR = 0.46, P = 0.003) were associated with lower risk of TA. Larger ratio of axial length to corneal radius (OR = 16.16 for per unit increase; P = 0.001), maternal working during pregnancy (OR = 1.27; P = 0.04), and cesarean delivery (OR = 1.68, P = 0.04) were associated with higher risk of ACA, while formula feeding was associated with lower risk of ACA (OR = 0.57, P = 0.01). Paternal astigmatism (OR = 0.50, P = 0.01) and assisted reproduction (OR = 0.56, P = 0.03) were associated with lower risk of horizontal or vertical internal compensation. More outdoor activity time (OR = 1.15 for per hour increase, P = 0.01) was associated with higher risk of oblique internal compensation while more nighttime sleep on weekends (OR = 0.83 for per hour increase, P = 0.01) was associated with lower risk of oblique internal compensation.

Conclusions

Our study confirmed some previously reported risk factors and identified some novel risk factors for astigmatism including formula feeding for lower risk of both ACA and TA, and older paternal age at child birth for higher risk of TA.

Genome-wide association meta-analysis of corneal curvature identifies novel loci and shared genetic influences across axial length and refractive error

Corneal curvature, a highly heritable trait, is a key clinical endophenotype for myopia - a major cause of visual impairment and blindness in the world. Here we present a trans-ethnic meta-analysis of corneal curvature GWAS in 44,042 individuals of Caucasian and Asian with replication in 88,218 UK Biobank data. We identified 47 loci (of which 26 are novel), with population-specific signals as well as shared signals across ethnicities. Some identified variants showed precise scaling in corneal curvature and eye elongation (i.e. axial length) to maintain eyes in emmetropia (i.e. HDAC11/FBLN2 rs2630445, RBP3 rs11204213); others exhibited association with myopia with little pleiotropic effects on eye elongation. Implicated genes are involved in extracellular matrix organization, developmental process for body and eye, connective tissue cartilage and glycosylation protein activities. Our study provides insights into population-specific novel genes for corneal curvature, and their pleiotropic effect in regulating eye size or conferring susceptibility to myopia.

CMPK1 and RBP3 are associated with corneal curvature in Asian populations

Corneal curvature (CC) measures the steepness of the cornea and is an important parameter for clinically diseases such as astigmatism and myopia. Despite the high heritability of CC, only two associated genes have been discovered to date. We performed a three-stage genome-wide association study meta-analysis in 12 660 Asian individuals. Our Stage 1 was done in multiethnic cohorts comprising 7440 individuals, followed by a Stage 2 replication in 2473 Chinese and Stage 3 in 2747 Japanese. The SNP array genotype data were imputed up to the 1000 Genomes Project Phase 1 cosmopolitan panel. The SNP association with the radii of CC was investigated in the linear regression model with the adjustment of age, gender and principal components. In addition to the known genes, MTOR (also known as FRAP1) and PDGFRA, we discovered two novel genes associated with CC: CMPK1 (rs17103186, P = 3.3 × 10(-12)) and RBP3 (rs11204213 [Val884Met], P = 1.1 × 10(-13)). The missense RBP3 SNP, rs11204213, was also associated with axial length (AL) (P = 4.2 × 10(-6)) and had larger effects on both CC and AL compared with other SNPs. The index SNPs at the four indicated loci explained 1.9% of CC variance across the Stages 1 and 2 cohorts, while 33.8% of CC variance was explained by the genome-wide imputation data. We identified two novel genes influencing CC, which are related to either corneal shape or eye size. This study provides additional insights into genetic architecture of corneal shape.

Impact of cylinder axis on the treatment for astigmatic amblyopia

PURPOSE

To compare the effects of oblique astigmatism on refractive amblyopia in children aged 3-7 years with those having orthogonal astigmatism.

DESIGN

A retrospective review of medical records.

METHODS

The medical records of patients attending Tri-Service General Hospital in Taiwan from January 2003 to December 2010 were reviewed and summarized. Seventy-two children with oblique astigmatism-related refractive amblyopia (Group 1) and 82 children with orthogonal astigmatism (Group 2) were chosen. Characteristics such as baseline visual acuity (VA), the time course of VA improvement, refractive error, and family history were assessed.

RESULTS

Group 1 showed a worse baseline mean VA (±SD) of 0.61 (0.13) vs 0.52 (0.16) logMAR (P = .01), a slower rate of amblyopia improvement, and higher prevalence of parental oblique astigmatism (29% vs 5.5%; P < .01) than did Group 2. The cylinder power of astigmatism (in D) causing amblyopia in Group 1 of 2.48 (0.82) was lower than that in Group 2: 2.93 (0.71) (P = .006). However, Group 1 achieved a noninferior resolution of amblyopia (mean final VA 0.18 vs 0.16 logMAR) after longer treatment of 6.45 (2.44) vs 5.86 (2.92) months (P = .039).

CONCLUSIONS

A smaller degree of initial oblique astigmatism caused amblyopia than did orthogonal astigmatism. Although the children with oblique astigmatism achieved equal resolution rates after treatment, this took longer. Therefore, we should pay more attention to children with mild oblique astigmatism, as they are more likely to develop oblique astigmatism-related amblyopia. Moreover, early diagnosis and prompt treatment might help visual improvement.

Analysis of familial aggregation in total, against-the-rule, with-the-rule, and oblique astigmatism by conditional and marginal models in the Tehran eye study

Purpose:

The purpose was to determine the familial aggregation of the total, against-the-rule (ATR), with-the-rule (WTR), and oblique astigmatism by conditional and marginal models in the Tehran Eye Study.

Materials and Methods:

Total, ATR, WTR, and oblique astigmatism were studied in 3806 participants older than 5 years from August 2002 to December 2002 in the Tehran Eye Study. Astigmatism was defined as a cylinder worse than or equal to −0.5 D. WTR astigmatism was defined as 0 ± 19°, ATR astigmatism was defined as 90 ± 19°, and oblique when the axes were 20–70° and 110–160°. The familial aggregation was investigated with a conditional model (quadratic exponential) and marginal model (alternating logistic regression) after controlling for confounders.

Results:

Using the conditional model, the conditional familial aggregation odds ratios (OR) (95% confidence interval) for the total, WTR, ATRs, and oblique astigmatism were 1.49 (1.43–1.72), 1.91 (1.65–2.20), 2.00 (1.70–2.30), and 1.86 (1.37–2.54), respectively. In the marginal model, the marginal OR of the parent-offspring and sib-sib in the total astigmatism were 1.35 (1.13–1.63) and 1.54 (1.13–2.11), respectively; WTR 1.53 (1.06–2.20) and 1.94 (1.21–3.13) and; ATR 2.13 (1.01–4.50) and 2.23 (1.52-3.30). The model was statistically significant in sib-sib relationship only for oblique astigmatism with OR of 3.00 (1.25–7.20).

Conclusion:

The results indicate familial aggregation of astigmatism in the population in Tehran adjusted for age, gender, cataract, duration of education, and body mass index, so that the addition of a new family member affected with astigmatism, as well as having a sibling or parents with astigmatism, significantly increases the odds of exposure to the disease for all four phenotypes. This aggregation can be due to genetic and/or environmental factors. Dividing astigmatism into three phenotypes increased the odds ratios.

Heritability of corneal refraction and corneal astigmatism: a population-based twin study among 66- to 79-year-old female twins

PURPOSE

To examine the heritability of corneal refraction power (CR) and corneal astigmatism (AST) in older women.

METHODS

Corneal refraction and AST were measured by IOL master in 52 monozygotic (MZ) and 47 dizygotic (DZ) female twin pairs aged 66-79 years. The relative contribution of genetic and environmental factors to individual differences in CR was estimated by applying an independent pathway model to the twin data and AST by intraclass correlations (ICC).

RESULTS

For the right eye, mean CR was 44.58 dioptres (D) (standard deviation (SD) ±1.28) When comparing CR of the right and left eye between MZ and DZ, no significant difference was found. Mean AST was 0.77 D (SD ±0.44) with no differences observed either between the MZ and the DZ individuals, or between the left and the right eyes. ICCs between the sisters for CR were, for the right eye, 0.882 and 0.378 for MZ and DZ, respectively, and for the left eye 0.855 and 0.358. For AST of the right eye, the ICCs were 0.533 and 0.096 for the MZ and DZ pairs, respectively, and for the left eye, the MZ and DZ correlations were 0.396 and 0.299. Quantitative genetic modelling showed that 81% of the variance in CR could be explained by genetic factors, additive genetic factors explaining 62% (95% confidence interval [CI] 44% -86%) and dominant genetic effect 19% (95% CI 7-49%) of the variance in CR. Different models were constructed to explain the heredity of AST. None of these models gave meaningful results, although the ICC values for MZ were higher than those for DZ.

CONCLUSIONS

Most of the variance in CR among older Finnish women could be explained by genetic factors.

Genome-wide meta-analysis of five Asian cohorts identifies PDGFRA as a susceptibility locus for corneal astigmatism

Corneal astigmatism refers to refractive abnormalities and irregularities in the curvature of the cornea, and this interferes with light being accurately focused at a single point in the eye. This ametropic condition is highly prevalent, influences visual acuity, and is a highly heritable trait. There is currently a paucity of research in the genetic etiology of corneal astigmatism. Here we report the results from five genome-wide association studies of corneal astigmatism across three Asian populations, with an initial discovery set of 4,254 Chinese and Malay individuals consisting of 2,249 cases and 2,005 controls. Replication was obtained from three surveys comprising of 2,139 Indians, an additional 929 Chinese children, and an independent 397 Chinese family trios. Variants in PDGFRA on chromosome 4q12 (lead SNP: rs7677751, allelic odds ratio = 1.26 (95% CI: 1.16-1.36), P(meta) = 7.87×10(-9)) were identified to be significantly associated with corneal astigmatism, exhibiting consistent effect sizes across all five cohorts. This highlights the potential role of variants in PDGFRA in the genetic etiology of corneal astigmatism across diverse Asian populations.

The heritability of ocular traits

Heritability is the proportion of phenotypic variation in a population that is attributable to genetic variation among individuals. Many ophthalmic disorders and biometric traits are known to have a genetic basis and consequently much work has been published in the literature estimating the heritability of various ocular parameters. We collated and summarized the findings of heritability studies conducted in the field of ophthalmology. We grouped the various studies broadly by phenotype as follows: refraction, primary open-angle glaucoma, age-related macular degeneration (AMD), cataract, diabetic retinopathy, and others. A total of 82 articles were retrieved from the literature relating to estimation of heritability for an ocular disease or biometric trait; of these, 37 papers were concerned with glaucoma, 28 with refraction, 4 with AMD, 5 with diabetic retinopathy, and 4 with cataract. The highest reported heritability for an ophthalmic trait is 0.99 for the phenotype ≥ 20 small hard drusen, indicating that observed variation in this parameter is largely governed by genetic factors. Over 60% of the studies employed a twin study design and a similar percentage utilized variance components methods and structural equation modeling (SEM) to derive their heritability values. Using modern SEM techniques, heritability estimates derived from twin subjects were generally higher than those from family data. Many of the estimates are in the moderate to high range, but to date the majority of genetic variants accounting for these findings have not been uncovered, hence much work remains to be undertaken to elucidate fully their molecular etiology.