Candidate high myopia loci on chromosomes 18p and 12q do not play a major role in susceptibility to common myopia

Myopia in African Americans Is Significantly Linked to Chromosome 7p15.2-14.2

Purpose

The purpose of this study was to perform genetic linkage analysis and association analysis on exome genotyping from highly aggregated African American families with nonpathogenic myopia. African Americans are a particularly understudied population with respect to myopia.

Methods

One hundred six African American families from the Philadelphia area with a family history of myopia were genotyped using an Illumina ExomePlus array and merged with previous microsatellite data. Myopia was initially measured in mean spherical equivalent (MSE) and converted to a binary phenotype where individuals were identified as affected, unaffected, or unknown. Parametric linkage analysis was performed on both individual variants (single-nucleotide polymorphisms [SNPs] and microsatellites) as well as gene-based markers. Family-based association analysis and transmission disequilibrium test (TDT) analysis modified for rare variants was also performed.

Results

Genetic linkage analysis identified 2 genomewide significant variants at 7p15.2 and 7p14.2 (in the intergenic region between MIR148A and NFE2L3 and in the noncoding RNA LOC401324) and 2 genomewide significant genes (CRHR2 and AVL9) both at 7p14.3. No genomewide results were found in the association analyses.

Conclusions

This study identified a significant linkage peak in African American families for myopia at 7p15.2 to 7p14.2, the first potential risk locus for myopia in African Americans. Interesting candidate genes are located in the region, including PDE1C, which is highly expressed in the eyes, and known to be involved in retinal development. Further identification of the causal variants at this linkage peak will help elucidate the genetics of myopia in this understudied population.

A novel G26A variation in 5' half of TGIF1 gene associates with high myopia in ethnic Kashmiri population from India

This study aims to look at novel variations in TGIF1 gene and explores their potential association with high myopia in an ethnic population from Kashmir (India). Genomic DNA was genotyped for polymorphic variations, and allele frequencies were tested for the Hardy–Weinberg disequilibrium in 240 ethnic Kashmiri cases with high myopia with a spherical equivalent of >−6 diopters (D) and compared with emmetropic controls with spherical equivalent within −0.5D in one or both eyes represented by a sample size of 228. In this study, we found a novel sequence variation G26A (GAT to AAT) in 5′ half of TGIF1 gene (p. aspartic acid >asparagine) at a frequency of 62% (148/240, P ≤ 0.0001). Variation appears to associate with high myopia significantly (P ≤ 0.001) as it happens to be present only in high myopia affected individuals. Further, it shows statistical significance for its association with gender and the degree of myopia (P ≤ 0.05). In addition, in silico predictions show that variation likely has an impact on the structure and functional properties of the protein. The assessment of the I-TASSER protein structure showed higher energy for a wild-type protein (−5820.186 kJ/mol) as compared to mutant protein (−6595.593 kJ/mol).

Genome-wide scans of myopia in Pennsylvania Amish families reveal significant linkage to 12q15, 8q21.3 and 5p15.33

Myopia is one of the most common ocular disorders in the world, yet the genetic etiology of the disease remains poorly understood. Specialized founder populations, such as the Pennsylvania Amish, provide the opportunity to utilize exclusive genomic architecture, like unique haplotypes, to better understand the genetic causes of myopia. We perform genetic linkage analysis on Pennsylvania Amish families that have a strong familial history of myopia to map any potential causal variants and genes for the disease. 293 individuals from 25 extended families were genotyped on the Illumina ExomePlus array and merged with previous microsatellite data. We coded myopia affection as a binary phenotype; myopia was defined as having a mean spherical equivalent (MSE) of less than or equal to - 1 D (diopters). Two-point and multipoint parametric linkage analyses were performed under an autosomal dominant model. When allowing for locus heterogeneity, we identified two novel genome-wide significantly linked variants at 12q15 (heterogeneity LOD, HLOD = 3.77) in PTPRB and at 8q21.3 (HLOD = 3.35) in CNGB3. We identified further three genome-wide significant variants within a single family. These three variants were located in exons of SLC6A18 at 5p15.33 (LODs ranged from 3.51 to 3.37). Multipoint analysis confirmed the significant signal at 5p15.33 with six genome-wide significant variants (LODs ranged from 3.6 to 3.3). Further suggestive evidence of linkage was observed in several other regions of the genome. All three novel linked regions contain strong candidate genes, especially CNGB3 on 8q21.3, which has been shown to affect photoreceptors and cause complete color blindness. Whole genome sequencing on these regions is planned to conclusively elucidate the causal variants.

Exome genotyping and linkage analysis identifies two novel linked regions and replicates two others for myopia in Ashkenazi Jewish families

BACKGROUND

Myopia is one of most common eye diseases in the world and affects 1 in 4 Americans. It is a complex disease caused by both environmental and genetics effects; the genetics effects are still not well understood. In this study, we performed genetic linkage analyses on Ashkenazi Jewish families with a strong familial history of myopia to elucidate any potential causal genes.

METHODS

Sixty-four extended Ashkenazi Jewish families were previously collected from New Jersey. Genotypes from the Illumina ExomePlus array were merged with prior microsatellite linkage data from these families. Additional custom markers were added for candidate regions reported in literature for myopia or refractive error. Myopia was defined as mean spherical equivalent (MSE) of -1D or worse and parametric two-point linkage analyses (using TwoPointLods) and multi-point linkage analyses (using SimWalk2) were performed as well as collapsed haplotype pattern (CHP) analysis in SEQLinkage and association analyses performed with FBAT and rv-TDT.

RESULTS

Strongest evidence of linkage was on 1p36(two-point LOD = 4.47) a region previously linked to refractive error (MYP14) but not myopia. Another genome-wide significant locus was found on 8q24.22 with a maximum two-point LOD score of 3.75. CHP analysis also detected the signal on 1p36, localized to the LINC00339 gene with a maximum HLOD of 3.47, as well as genome-wide significant signals on 7q36.1 and 11p15, which overlaps with the MYP7 locus.

CONCLUSIONS

We identified 2 novel linkage peaks for myopia on chromosomes 7 and 8 in these Ashkenazi Jewish families and replicated 2 more loci on chromosomes 1 and 11, one previously reported in refractive error but not myopia in these families and the other locus previously reported in the literature. Strong candidate genes have been identified within these linkage peaks in our families. Targeted sequencing in these regions will be necessary to definitively identify causal variants under these linkage peaks.

Myopia in Chinese families shows linkage to 10q26.13

Purpose

To determine genetic linkage between myopia and Han Chinese patients with a family history of the disease.

Methods

One hundred seventy-six Han Chinese patients from 34 extended families were given eye examinations, and mean spherical equivalent (MSE) in diopters (D) was calculated by adding the spherical component of the refraction to one-half the cylindrical component and taking the average of both eyes. The MSE was converted to a binary phenotype, where all patients with an MSE of -1.00 D or less were coded as affected. Unaffected individuals had an MSE greater than 0.00 D (ages 21 years and up), +1.50 (ages 11-20), or +2.00 D (ages 6-10 years). Individuals between the given upper threshold and -1.00 were coded as unknown. Patients were genotyped on an exome chip. Three types of linkage analyses were performed: single-variant two-point, multipoint, and collapsed haplotype pattern (CHP) variant two-point.

Results

The CHP variant two-point results identified a significant peak (heterogeneity logarithm of the odds [HLOD] = 3.73) at 10q26.13 in TACC2. The single-variant two-point and multipoint analyses showed highly suggestive linkage to the same region. The single-variant two-point results identified 25 suggestive variants at HTRA1, also at 10q26.13.

Conclusions

We report a significant genetic linkage between myopia and Han Chinese patients at 10q26.13. 10q26.13 contains several good candidate genes, such as TACC2 and the known age-related macular degeneration gene HTRA1. Targeted sequencing of the region is planned to identify the causal variant(s).

Insight into high myopia and the macula

The incidence of myopia is constantly on the rise. Patients of high myopia and pathological myopia are young and can lose vision due to a number of degenerative changes occurring at the macula. With the emergence of new technologies such as swept-source optical coherence tomography (OCT) and OCT angiography, our understanding of macular pathology in myopia has improved significantly. New conditions such as myopic traction maculopathy have been defined. Early, noninvasive detection of myopic choroidal neovascularization and its differentiation from lacquer cracks is possible with a greater degree of certainty. We discuss the impact of these new exciting and promising technologies and management of macular pathology in myopia. Incorporation of OCT in the microscope has also improved macular surgery. New concepts such as fovea-sparing internal limiting membrane peeling have emerged. A review of literature and our experience in managing all these conditions are discussed.

Caucasian Families Exhibit Significant Linkage of Myopia to Chromosome 11p

Purpose

Myopia is a common visual disorder caused by eye overgrowth, resulting in blurry vision. It affects one in four Americans, and its prevalence is increasing. The genetic mechanisms that underpin myopia are not completely understood. Here, we use genotype data and linkage analyses to identify high-risk genetic loci that are significantly linked to myopia.

Methods

Individuals from 56 Caucasian families with a history of myopia were genotyped on an exome-based array, and the single nucleotide polymorphism (SNP) data were merged with microsatellite genotype data. Refractive error measures on the samples were converted into binary phenotypes consisting of affected, unaffected, or unknown myopia status. Parametric linkage analyses assuming an autosomal dominant model with 90% penetrance and 10% phenocopy rate were performed.

Results

Single variant two-point analyses yielded three significantly linked SNPs at 11p14.1 and 11p11.2; a further 45 SNPs at 11p were found to be suggestive. No other chromosome had any significant SNPs or more than seven suggestive linkages. Two of the significant SNPs were located in BBOX1-AS1 and one in the intergenic region between ORA47 and TRIM49B. Collapsed haplotype pattern two-point analysis and multipoint analyses also yielded multiple suggestively linked genes at 11p. Multipoint analysis also identified suggestive evidence of linkage on 20q13.

Conclusions

We identified three genome-wide significant linked variants on 11p for myopia in Caucasians. Although the novel specific signals still need to be replicated, 11p is a promising region that has been identified by other linkage studies with a number of potentially interesting candidate genes. We hope that the identification of these regions on 11p as potential causal regions for myopia will lead to more focus on these regions and maybe possible replication of our specific linkage peaks in other studies. We further plan targeted sequencing on 11p for our most highly linked families to more clearly understand the source of the linkage in this region.

Contribution of TGFβ1 codon 10 polymorphism to high myopia in an ethnic Kashmiri population from India

This study looks at novel variants of the TGFβ1 gene and their potential association with high myopia in an ethnic population from Kashmir, India. Allele frequencies of 247 Kashmiri subjects (from India) with high myopia and 176 ethnically matched healthy controls were tested for Hardy-Weinberg disequilibrium. The genotype and allele frequencies were evaluated using chi-square or Fisher's exact tests. One of the three SNPs in codon 10 showed a significant difference between patients and control subjects (rs1982073: p genotype = 0.003, p allele = 0.001). There were no statistically significant differences between patients and control subjects for the other two SNPs, rs1800471 at codon 25 and a novel variant at codon 52. SNP rs1982073, substituting proline with leucine, appeared to be significantly associated with high myopia (p < 0.05). In silico predictions show that substitutions are likely to have an impact on the structure and functional properties of the protein, making it imperative to understand their functional consequences in relation to high myopia.

Association of vitamin D receptor gene start codon (Fok1) polymorphism with high myopia

Background:

High myopia caused primarily due to abnormal emmetropization and excessive axial ocular elongation is associated with sight-threatening ocular pathology. Muscular dysfunction of ocular ciliary muscles due to altered intracellular calcium levels can result in defective mechanotransduction of the eye and retinal defocus. The vitamin D3 receptor (VDR; a intracellular hormone receptor) is known to mediate calcium homeostasis, influencing the development of myopia.

Materials and Methods:

In the present study, a total of 206 high myopia, 98 low myopia and 250 control samples were analyzed for VDR gene Fok1 (exon 2 start codon) polymorphism using polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) technique.

Results:

High myopia patients revealed decrease in the frequency of ff homozygotes (8.3%) as compared to control group (14.0%), with a corresponding increase in frequency of FF homozygotes (68.9% in high myopia vs. 62.8% in controls). The frequency of f allele carriers (Ff and ff) was increased in females of high myopia (35.6%) and low myopia cases (45.4%). Elevated frequency of f allele was found only in early age at onset cases of high myopia (0.227) and later age at onset (10–20 years) cases of low myopia (0.273) as well as in low myopia cases with parental consanguinity (0.458) (P 0.035; χ² = 6.692*).

Conclusion:

The results suggest that VDR gene might not be playing a direct role in the development of myopia, but might contribute indirectly to the risk conferred by mechanical stress factors or growth/development related factors through its role in calcium homeostasis and regulation of ciliary muscle function.

Study of Theories about Myopia Progression (STAMP) design and baseline data

PURPOSE

The Study of Theories about Myopia Progression (STAMP) is a 2-year, double-masked, randomized clinical trial of myopic children 6 to 11 years of age. STAMP will evaluate the 1-year effect of progressive addition lenses (PALs) compared with single vision lenses (SVLs) on central refraction, peripheral refraction in four quadrants, and accommodative response and convergence. STAMP will also evaluate any changes 1 year after discontinuing PALs. Baseline characteristics of enrolled children are reported.

METHODS

Eligible children had a high accommodative lag and either low myopia (less myopic than -2.25 diopter (D) spherical equivalent) or high myopia (more myopic than -2.25 D spherical equivalent) and esophoria at near. Children were randomly assigned to wear either PALs or SVLs for 1 year to determine the difference in myopia progression in the PAL group relative to the SVL group. All children will then wear SVLs for the 2nd year to evaluate the permanence of any treatment effect. Complete ocular biometric data are collected at 6-month intervals.

RESULTS

Over 17 months, 192 children were screened, and 85 (44%) were eligible and enrolled. Of these 85 children, 44 (52%) were girls, and 54 (64%) were esophoric at near. The mean age (± SD) was 9.8 ± 1.3 years. The right eye mean cycloplegic spherical equivalent refractive error was -1.95 ± 0.78 D. Horizontal relative peripheral hyperopia (30° nasal retina + 0.56 ± 0.59 D; 30° temporal retina + 0.61 ± 0.77 D) and vertical relative peripheral myopia (30° superior retina -0.36 ± 0.92 D; 20° inferior retina -0.48 ± 0.83 D) were found.

CONCLUSIONS

The baseline data for STAMP are reported. Asymmetry between vertical and horizontal meridian relative peripheral refraction was found. STAMP will use the ocular biometric changes associated with the PAL treatment effect to attempt to elucidate the mechanism responsible for the treatment effect.

The GEnes in Myopia (GEM) study in understanding the aetiology of refractive errors

Refractive errors represent the leading cause of correctable vision impairment and blindness in the world with an estimated 2 billion people affected. Refractive error refers to a group of refractive conditions including hypermetropia, myopia, astigmatism and presbyopia but relatively little is known about their aetiology. In order to explore the potential role of genetic determinants in refractive error the "GEnes in Myopia (GEM) study" was established in 2004. The findings that have resulted from this study have not only provided greater insight into the role of genes and other factors involved in myopia but have also gone some way to uncovering the aetiology of other refractive errors. This review will describe some of the major findings of the GEM study and their relative contribution to the literature, illuminate where the deficiencies are in our understanding of the development of refractive errors and how we will advance this field in the future.

Methodology and Recruitment of Probands and Their Families for the Genes in Myopia (GEM) Study

PURPOSE

Myopia is considered to be a complex disease involving both environmental and genetic factors. The Genes in Myopia (GEM) Study aims to recruit probands with myopia and their family members to allow genetic analysis of myopia to be undertaken. The purpose of this paper is to describe the methodology and recruitment of probands and families for the GEM Study.

METHODS

In a sample-based prospective study, 2,095 probands with myopia of -0.50 DS or worse and a positive family history of myopia were contacted via the Melbourne Excimer Laser Group (MELG) database. Probands and family members recruited into the study undertook a detailed assessment including questionnaire, best-corrected visual acuity, objective and subjective refraction, axial length, anterior chamber depth, keratometry readings, slit-lamp examination, height, weight and head circumference measurements, and blood sample collection for DNA analysis.

RESULTS

280 probands with myopia have been recruited into the GEM Study. Probands had a mean age of 49.33 yrs. (SD +/- 11.64) with the average age of myopia onset being 12.58 years (SD +/- 6.71). The average spherical-component refractive error was: right eye -5.13 DS (SD +/- 3.06) and left eye -5.14 DS (SD +/- 3.16). Probands with extreme myopia (-10 DS or worse) showed the highest study participation rate of 56%, when compared to high (-5 DS < -10 DS) (20%), moderate (-3 DS < - 5 DS) (18%) and low myopia (-0.5 DS < -3 DS) (10%). A total of 279 out of 505 (55%) additional family members recruited were also found to be myopic.

CONCLUSIONS

The GEM study has used a targeted approach to identify an Australian cohort with a diverse spread of myopia, ranging from low to extreme. Recruitment of probands via the use of an excimer laser practice has proved to be an efficient and economic means of identifying probands with a family history of myopia. In addition, the participation rate in the study appears to vary reflecting a proband's perception of disease severity.

Genomewide linkage scans for ocular refraction and meta-analysis of four populations in the Myopia Family Study

PURPOSE

Genomewide linkage scans were performed in Caucasian (CAUC) and Old Order Amish (OOA) families to identify genomic regions containing genes responsible for refractive error control. We also performed a meta-analysis by combining these results with our previous linkage results from Ashkenazi Jewish (ASHK) and African American (AFRAM) families.

METHODS

Two hundred seventy-one CAUC and 411 OOA participants (36 and 61 families, respectively) were recruited to participate in the Myopia Family Study. Recruitment criteria were designed to enrich the sample for multiplex myopic families. Genomewide, model-free, multipoint linkage analyses were performed separately for each population by using >370 microsatellite markers. Empirical significance levels were determined via gene-dropping simulations. A meta-analysis was performed by combining linkage results from the CAUC, OOA, AFRAM, and ASHK samples, and results were compared to previously reported loci for myopia and refraction.

RESULTS

Suggestive evidence of linkage was found at 12q24 (LOD = 4.583, P = 0.00037) and 4q21 (LOD = 2.72, P = 0.0028) in the CAUC sample and at 5qter (LOD = 3.271, P = 0.0014) in the OOA. Meta-analysis linkage results were largely driven by population-specific signals from ASHK and AFRAM families. The meta-analysis showed suggestive evidence of linkage to 4q21-22 (meta-P = 0.00214) adjacent to the previously reported MYP9 and MYP11 loci.

CONCLUSIONS

The results showed suggestive evidence of linkage of ocular refraction to 12q24 and 4q21 in CAUC and to 5qter in OOA families. The meta-analysis supports the view that several genes play a role in refractive development across populations. In MFS families, four broad genomic regions (on 1p, 4q, 7p, and 12q) most likely contain genes that influence ocular refraction.

An international collaborative family-based whole-genome linkage scan for high-grade myopia

PURPOSE

Several nonsyndromic high-grade myopia loci have been mapped primarily by microsatellite markers and a limited number of pedigrees. In this study, whole-genome linkage scans were performed for high-grade myopia, using single nucleotide polymorphisms (SNPs) in 254 families from five independent sites.

METHODS

Genomic DNA samples from 1411 subjects were genotyped (Linkage Panel IVb; Illumina, San Diego, CA). Linkage analyses were performed on 1201 samples from 10 Asian, 12 African-American, and 221 Caucasian families, screening for 5744 SNPs after quality-control exclusions. Two disease states defined by sphere (SPH) and spherical equivalence (SE; sphere+cylinder/2) were analyzed. Parametric and nonparametric two-point and multipoint linkage analyses were performed using the FASTLINK, HOMOG, and MERLIN programs. Multiple stratified datasets were examined, including overall, center-specific, and race-specific. Linkage regions were declared suggestive if they had a peak LOD score >or= 1.5.

RESULTS

The MYP1, MYP3, MYP6, MYP11, MYP12, and MYP14 loci were replicated. The novel region q34.11 on chromosome 9 (max NPL= 2.07 at rs913275) was identified. Chromosome 12, region q21.2-24.12 (36.59 cM, MYP3 locus) showed significant linkage (peak HLOD = 3.48) at rs337663 in the overall dataset by SPH and was detected by the Duke, Asian, and Caucasian subsets as well. Potential shared interval was race dependent-a 9.4-cM region (rs163016-rs1520724) driven by the Asian subset and a 13.43-cM region (rs163016-rs1520724) driven by the Caucasian subset.

CONCLUSIONS

The present study is the largest linkage scan to date for familial high-grade myopia. The outcomes will facilitate the identification of genes implicated in myopic refractive error development and ocular growth.

Molecular genetics of human myopia: an update

Myopia, or nearsightedness, is the most common human eye disorder in the world, and is a significant global public health concern. Along with cataract, macular degeneration, infectious disease, and vitamin A deficiency, myopia is one of the most important causes of visual impairment worldwide. Severe or high-grade myopia is a leading cause of blindness because of its associated ocular morbidities of retinal detachment, macular choroidal degeneration, premature cataract, and glaucoma. Ample evidence documents the heritability of the non-syndromic forms of this condition, especially for high-grade myopia, commonly referred to as myopic spherical refractive power of 5 to 6 diopters or higher. Multiple high-grade myopia genetic loci have been identified, and confirmatory studies identifying high-grade and moderate myopia loci have also occurred. In general, myopia susceptibility genes are unknown with few association studies performed, and without confirmation in other research laboratories or testing of separate patient cohorts.

Genetic isolates in ophthalmic diseases

In recent years, noteworthy gains have been made in unravelling the genetic contribution to some complex ocular diseases, principally age-related macular degeneration. Yet, a relatively poor understanding of the genetic aetiology for many other heritable blinding diseases, such as glaucoma, keratoconus and myopia, remains. Genetic isolates, populations with varying degrees of geographical or cultural seclusion, provide an effective means for investigating the molecular mechanisms involved in human diseases. This is particularly true for rare diseases in which founded alleles can be rapidly driven to a high frequency due to restriction of gene flow in the population. Recent success in complex gene mapping has resulted from the widened linkage disequilibrium (LD) in the genome of genetically isolated populations. An improved understanding of the predisposing genetic risk factors allows for enhanced screening modalities and paves the foundations for the translation of genomic technology into the clinic. This review focuses on the role population isolates have had in the investigation of genes underlying complex eye diseases and discusses their likely usefulness given the expansion of large-scale case-control association studies.

Genomewide scan of ocular refraction in African-American families shows significant linkage to chromosome 7p15

Refractive development is influenced by environmental and genetic factors. Genetic studies have identified several regions of linkage to ocular refraction, but none have been carried out in African-derived populations. We performed quantitative trait locus linkage analyses in African-American (AA) families to identify genomic regions responsible for refraction. We recruited 493 AA individuals in 96 families to participate in the Myopia Family Study. Genotyping of 387 microsatellite markers was performed on 398 participants. The mean refraction among genotyped individuals was -2.87 D (SD=3.58) and myopia of at least 1 D was present in 267 (68%) participants. Multipoint, regression-based, linkage analyses were carried out on a logarithmic transformation of ocular refraction using the statistical package MERLIN-REGRESS. Empirical significance levels were determined via 4,898 whole-genome gene-dropping simulations. Linkage analyses were repeated after clustering families into two subgroups based on admixture proportions as determined by the software package STRUCTURE. Genomewide significant linkage was seen at 47 cM on chromosome 7 (logarithm of the odds ratio (LOD)=5.87, P=0.00005). In addition, three regions on chromosomes 2p, 3p and 10p showed suggestive evidence of linkage (LOD>2, P<0.005) for ocular refraction. We mapped the first quantitative trait locus for ocular refraction in an AA population to chr.7p15. Two previous studies in European-derived families reported some evidence of linkage to a nearby region, suggesting that this region may contain polymorphisms that mediate refraction across populations. The genomic region under our linkage peak spans approximately 17 Mb and contains approximately 170 genes. Further refinement of this region will be pursued in future studies.

A review of current approaches to identifying human genes involved in myopia

The prevalence of myopia is high in many parts of the world, particularly among the Orientals such as Chinese and Japanese. Like other complex diseases such as diabetes and hypertension, myopia is likely to be caused by both genetic and environmental factors, and possibly their interactions. Owing to multiple genes with small effects, genetic heterogeneity and phenotypic complexity, the study of the genetics of myopia poses a complex challenge. This paper reviews the current approaches to the genetic analysis of complex diseases and how these can be applied to the identification of genes that predispose humans to myopia. These approaches include parametric linkage analysis, non-parametric linkage analysis like allele-sharing methods and genetic association studies. Basic concepts, advantages and disadvantages of these approaches are discussed and explained using examples from the literature on myopia. Microsatellites and single nucleotide polymorphisms are common genetic markers in the human genome and are indispensable tools for gene mapping. High throughput genotyping of millions of such markers has become feasible and efficient with recent technological advances. In turn, this makes the identification of myopia susceptibility genes a reality.

Candidate gene and locus analysis of myopia

PURPOSE

A previous study has reported evidence of a strong linkage, but no association, between paired box gene 6 (PAX6) and myopia. We attempted to replicate these findings and to conduct a candidate gene and locus evaluation of genetic involvement in common forms of myopia.

METHODS

Samples were collected from 517 individuals in 123 families with a myopic child participating in the Orinda Longitudinal Study of Myopia or the Contact Lens and Myopia Progression Study. Myopia in the proband children was defined as -0.75 D or more and as being present in both meridians on cycloplegic autorefraction (1% tropicamide). Affected status in parents and siblings was determined by survey. After DNA was extracted from buccal mucosal cells and genotyped using assays for microsatellite markers and single nucleotide polymorphisms (SNPs), DNA was analyzed for linkage disequilibrium. Markers on chromosomes 12 and 18 were selected as regions previously associated with pathological myopia. SNPs were also analyzed in genes where their expression pattern or their association with syndromes conveys myopia as part of the phenotype (FGF2, BDNF, COL2A1, COL18A1, and PAX6).

RESULTS

The SNP rs1635529 for COL2A1 on 12q13.11 showed highly significant over-transmission to affected individuals (p=0.00007). No SNP for FGF2, BDNF, COL18A1, or PAX6 showed significant over-transmission to affected individuals after correction for multiple comparisons. Markers on chromosome 12 and 18 previously associated with pathological myopia also showed no significant associations with the more common form of myopia in this study.

CONCLUSIONS

As reported previously by others, PAX6 showed no association with myopia. Associations in the current analysis are suggestive of involvement of COL2A1. Future studies should focus on replication in other samples and in genome-wide approaches.

Evaluation of Accuracy in Proband-Reported Family History and Its Determinants: The Genes in Myopia Family Study

PURPOSE

Proband-reported family histories are widely used in epidemiological and genetic studies. The accuracy of these reports may have significant effects on the intended outcome, particularly in genetic studies. This study aims to determine the accuracy of proband-reported family history of myopia and to assess whether demographic or clinical factors are predictive of an accurate history.

METHODS

In 2004 to 2005, the study recruited 120 myopic probands (< or = -0.50 D spherical equivalent in both eyes) aged 18 to 72 years and 358 nuclear family members residing within Victoria, Australia as part of the Genes in Myopia (GEM) family study. Data collection used an examiner-administered questionnaire with an ocular examination. Proband-reported family history of myopia was evaluated for agreement with ophthalmic examination results of family members.

RESULTS

The statistical measures of accuracy used in this report were sensitivity, specificity, positive predictive value, and negative predictive value. Sensitivity varied from 85 to 98%, specificity from 84 to 96%, positive predictive value from 83 to 97%, and negative predictive value from 84 to 97%. Following multivariate analysis, an evaluation of demographic and clinical factors indicated that the highest predictive accuracy was obtained from proband reporting of their children [odds ratio (OR), 0.38; 95% confidence interval (CI), 0.15 to 0.94] whereas the most inaccurate reporting of a proband was when there was less-severe maternal myopia (per 0.50 D less myopic) (OR, 1.23; 95% CI, 1.06 to 1.43) or for increase in total education of the proband (per 1 year increase) (OR, 1.22; 95% CI, 1.04 to 1.42).

CONCLUSIONS

Several variables influence the accuracy of obtaining a family history of myopia. A questionnaire-based approach alone will introduce some error into the study and this should be taken into account when designing and undertaking family-based epidemiological or genetic studies of myopia.

Genomewide scan in Ashkenazi Jewish families demonstrates evidence of linkage of ocular refraction to a QTL on chromosome 1p36

The development of refractive error is mediated by both environmental and genetic factors. We performed regression-based quantitative trait locus (QTL) linkage analysis on Ashkenazi Jewish families to identify regions in the genome responsible for ocular refraction. We measured refractive error on individuals in 49 multi-generational American families of Ashkenazi Jewish descent. The average family size was 11.1 individuals and was composed of 2.7 generations. Recruitment criteria specified that each family contain at least two myopic members. The mean spherical equivalent refractive error in the sample was -3.46D (SD=3.29) and 87% of individuals were myopic. Microsatellite genotyping with 387 markers was performed on 411 individuals. We performed multipoint regression-based linkage analysis for ocular refraction and a log transformation of the trait using the statistical package Merlin-Regress. Empirical genomewide significance levels were estimated through gene-dropping simulations by generating random genotypes at each of the 387 markers in 200 replicates of our pedigrees. Maximum LOD scores of 9.5 for ocular refraction and 8.7 for log-transformed refraction (LTR) were observed at 49.1 cM on chromosome 1p36 between markers D1S552 and D1S1622. The empirical genomewide significance levels were P=0.065 for ocular refraction and P<0.005 for LTR, providing strong evidence for linkage of refraction to this locus. The inter-marker region containing the peak spans 11 Mb and contains approximately 189 genes.

CONCLUSION

We found genomewide significant evidence for linkage of refractive error to a novel QTL on chromosome 1p36 in an Ashkenazi Jewish population.

Family history of myopia is not related to the final amount of refractive error in low and moderate myopia

BACKGROUND

Previous research has found an association between family history and presence of myopia. It is possible that family history also could be related to the final amount of refractive error developed. If that were true, then family history of myopia could have predictive value for the amount of refractive error a child or young adult would develop after first lens prescription.

METHODS

Consecutive myopic adult outpatients were enrolled during the year 2003. They received a refractive examination and a questionnaire concerning age of onset of lens use, academic achievement, and parental history of myopia.

RESULTS

In the group of 271 mild and moderate myopes (myopia lower than -6 D) there were 157 subjects with at least one myopic parent and 114 subjects without family history. The presence or absence of a family history of myopia was not associated with either the final myopic refractive error (-3.2 +/- 1.5 D and -2.9 +/- 1.3 D, respectively, P = 0.08) or the age of onset of lens use (17.0 +/- 6.2 years vs 18.1 +/- 6.7 years, respectively, P = 0.15).

CONCLUSIONS

In the present retrospective study of a clinical sample of mild and moderate myopic subjects, family history of myopia did not show predictive value for either the age of first prescription, or the final refractive error developed in adulthood.

How genetic is school myopia?

Myopia is of diverse aetiology. A small proportion of myopia is clearly familial, generally early in onset and of high level, with defined chromosomal localisations and in some cases, causal genetic mutations. However, in economically developed societies, most myopia appears during childhood, particularly during the school years. The chromosomal localisations characterised so far for high familial myopia do not seem to be relevant to school myopia. Family correlations in refractive error and axial length are consistent with a genetic contribution to variations in school myopia, but potentially confound shared genes and shared environments. High heritability values are obtained from twin studies, but rest on contestable assumptions, and require further critical analysis, particularly in view of the low heritability values obtained from parent-offspring correlations where there has been rapid environmental change between generations. Since heritability is a population-specific parameter, the values obtained on twins cannot be extrapolated to define the genetic contribution to variation in the general population. In addition, high heritability sets no limit to the potential for environmentally induced change. There is in fact strong evidence for rapid, environmentally induced change in the prevalence of myopia, associated with increased education and urbanisation. These environmental impacts have been found in all major branches of the human family, defined in modern molecular terms, with the exception of the Pacific Islanders, where the evidence is too limited to draw conclusions. The idea that populations of East Asian origin have an intrinsically higher prevalence of myopia is not supported by the very low prevalence reported for them in rural areas, and by the high prevalence of myopia reported for Indians in Singapore. A propensity to develop myopia in "myopigenic" environments thus appears to be a common human characteristic. Overall, while there may be a small genetic contribution to school myopia, detectable under conditions of low environmental variation, environmental change appears to be the major factor increasing the prevalence of myopia around the world. There is, moreover, little evidence to support the idea that individuals or populations differ in their susceptibility to environmental risk factors.

Genomewide linkage scan for myopia susceptibility loci among Ashkenazi Jewish families shows evidence of linkage on chromosome 22q12

Mild/moderate (common) myopia is a very common disorder, with both genetic and environmental influences. The environmental factors are related to near work and can be measured. There are no known genetic loci for common myopia. Our goal is to find evidence for a myopia susceptibility gene causing common myopia. Cycloplegic and manifest refraction were performed on 44 large American families of Ashkenazi Jewish descent, each with at least two affected siblings. Individuals with at least -1.00 diopter or lower in each meridian of both eyes were classified as myopic. Microsatellite genotyping with 387 markers was performed by the Center for Inherited Disease Research. Linkage analyses were conducted with parametric and nonparametric methods by use of 12 different penetrance models. The family-based association test was used for an association scan. A maximum multipoint parametric heterogeneity LOD (HLOD) score of 3.54 was observed at marker D22S685, and nonparametric linkage analyses gave consistent results, with a P value of.0002 at this marker. The parametric multipoint HLOD scores exceeded 3.0 for a 4-cM interval, and significant evidence of genetic heterogeneity was observed. This genomewide scan is the first step toward identifying a gene on chromosome 22 with an influence on common myopia. At present, we are following up our linkage results on chromosome 22 with a dense map of >1,500 single-nucleotide-polymorphism markers for fine mapping and association analyses. Identification of a susceptibility locus in this region may eventually lead to a better understanding of gene-environment interactions in the causation of this complex trait.