Refraction, like most complex traits, is probably influenced by multiple genetic factors. This supposition is supported by the fact that many regions of genetic linkage have been reported for both quantitative refraction and myopia. Given the inherent difficulties in replicating linkage findings when there is locus heterogeneity, it is not surprising that many of these reported linkage regions have not yet been replicated. However, in our current linkage analysis of refraction and refractive errors we were able to replicate several previously reported linkage peaks.
There is considerable debate regarding what constitutes evidence of replication in linkage studies. While statistical criteria, P < 0.01, have been established, it is less clear how differences in phenotype definition and chromosomal location of maximum LOD scores across studies should be evaluated. For example, can evidence of linkage in a population-based family study of quantitative refraction replicate a linkage peak identified in families with high myopia? It is well established that refractive errors are etiologically complex. This complexity may arise, in part, from the different mutations within the same gene that lead to differences in the severity of refractive errors and/or interactions between genes and environmental factors that influence the severity of the phenotype. It is possible that genes involved in the more severe forms of myopia also play a role in the more modest forms of refractive errors. Therefore, we feel that replication of linkage peaks can occur when different phenotype definitions are used. Furthermore, linkage signals are often quite broad (>10 Mb) and the location of the causal gene varies with respect to the maximum LOD score. Therefore, it is unclear how far apart a replication linkage peak can be from the initial linkage signal; however, overlap of the linkage regions is supportive of replication.
Our data support linkage to refractive errors, particularly hyperopia on 3q26. Genomewide significant linkage for refraction to this region was first reported in 506 dizygotic twins enrolled in the Twin Eye study. A maximum LOD of 3.7 was observed at marker D3S1614
with markers D3S1279
(168.1 Mb) and D3S1565
(175.3 Mb) flanking the linkage regions.23
To examine whether this linkage peak was due to genetic sharing among myopes versus hyperopes, Hammond et al.23
also conducted qualitative analysis refractive errors in these same families. Although there was little evidence to support linkage to this region in myopes (LOD<1), there was support for linkage of this region to hyperopia (LOD ~2.5). This finding is consistent with ours of genome-wide evidence suggestive of linkage to this region for hyperopia. Our minimum empiric P
= of 5.34 × 10−4
was observed at marker D3S1763
, with our linkage region spanning marker rs937478 (159.8 Mb) to rs1920122 (169.5 Mb), which overlaps that of Hammond et al. However, there was only slight evidence of linkage in the region to quantitative refraction, with a minimum empiric P
= of 0.036 near marker rs920417. Hammond et al. conducted follow-up linkage and association analysis of the 3q26 locus and found evidence of association for three genes, MFN1
(179.0 Mb), SOX2OT
(181.3Mb) and PSARL
These genes are located downstream of our linkage region. Follow-up studies are needed to determine whether genetic variation in the genes explains the linkage in these regions or additional genes are responsible for the linkage signal on 3q26.
While we had previously reported linkage on 22q to refraction,27
here we demonstrate linkage in this region to myopia as well as refraction, but little evidence of linkage to hyperopia. This finding is consistent with the initial report by Stambolian et al.19
of genome-wide significant evidence of linkage in this region to myopia, with an LOD (HLOD) of 3.54, at marker D22S685
(physical position 34.4 Mb) and a linkage region spanning D22S689
(28.7 Mb) and D22S445
(37.4 Mb). However, our linkage peak is located adjacent to that reported by them, with a minimum P
= 4.43 × 10−3
at rs737923 (19.1 Mb) with the region spanning rs2097596 (17.9 Mb) to rs374225 (20.0 Mb) for myopia and from rs3747026 (18.2 Mb) to rs1476445 (19.6 Mb) for adjusted refraction.19
There is additional evidence of linkage to refraction in our data at rs5762174 (27.9 Mb, empiric P
= 1.31 × 10−3
). Given that linkage signals can extend for several megabases and the location of the peak around a causal locus can vary markedly due to genetic heterogeneity and marker information content, these results could represent the same locus or they could be due to multiple loci on chromosome 22. In addition, the families studied by Stambolian et al. were highly ascertained for multiple myopes in each family, whereas the BDES families were population based. However the consistent linkage signals for myopia and refraction to this region of chromosome 22 strongly indicate that at least one major locus resides in this region.
Evidence suggestive of linkage to high myopia (> −6 D) on 7q36 was first reported by Naiglin et al.21
In their analysis of 21 French and 2 Algerian families, they reported a maximum LOD of 2.81 at D7S550
, with the region of linkage extending from D7S798
(152.7 Mb) to D7S2423
(157.4 Mb). Our linkage region for refraction ranged from rs1547958 (150.6 Mb) to rs1389240 (156.0 Mb), overlapping the region that they reported. However, there was no evidence supporting linkage to myopia in this region for our families.
In addition to the region on 7q36, we also had evidence of linkage to 7p15-21. Our previous linkage analysis of refraction in the BDES using microsatellite markers alone provided some evidence of linkage to this region, with the minimum multipoint P
= 2.2 × 10−3
at marker D7S3051
This linkage region spanned markers rs957960 (18.8 Mb) to rs1725074 (27.1 Mb). Ciner et al.24
have reported linkage to refraction adjacent to this region among 96 African American families. In their analysis they observed a maximum LOD of 5.87 with an associated P
= 5 × 10−5
with the linkage peak ranging from markers D7S1808
(27.9 Mb) to D7S2846
(38.0 Mb), respectively, which is adjacent to our region.
In addition to the linkage loci identified for refraction, myopia, and hyperopia, several genomewide association studies for myopia have recently been conducted. Linkage studies are better suited to detecting rare alleles associated with a strong effect (high odds ratio or high penetrance) of disease and are robust to allelic heterogeneity (multiple mutations in the same gene which result in the same phenotype). Association studies are better suited to detecting lower-penetrance common variation associated with disease. While variation on 5p15, 11q24, 15q14, and 15q25 have been associated with myopia it is not surprising that we did not detect linkage in these regions, given the relative strengths of linkage versus association methods (Verhoeven VJM, et al. IOVS
2010;51:ARVO E-Abstract 2972).39–41
Work is ongoing to refine these linkage peaks and to identify the genes underlying these linkage signals, with particular emphasis on the interesting regions on 3q36, 22q11, 7p21, and 7q36.