We have identified three SNPs within VDR
(rs2239182, rs3819545, and rs2853559) that differed significantly between white adult cases with myopia between −0.75 and −4.00D and white nonmyopic controls. Associations between myopia and variations within VDR
and perhaps an additional SNP within GC
(rs7041) seemed most applicable to this group. Only one SNP, rs2853559, was significant for white subjects when the range of myopia was unrestricted. None were significant when either the sample as a whole including all ethnicities or when white subjects with myopia worse than −4.00 D were considered. Odds ratios ranged from 1.64 to 2.34 with a substantial increase in the model R2
, from 0.17 with age alone to 0.29 with the four SNPs adjusted for age. The size of these effects is somewhat larger than recent genome-wide association studies of myopia with a much larger sample size. Nakanishi et al.34
found odds ratios of 1.24 to 1.50 for SNPS within 11q24.1 in a genome-wide study of 830 cases with pathologic myopia and 1911 controls. More recently Hysi et al.35
and Solouki et al.36
reported associations between SNPs and myopia assessed as part of very large-scale, genome-wide scans: rs8027411 at 15q2535
(odds ratio for homozygous for risk allele = 1.16) and rs634990 at 15q14 (odds ratio for homozygous for risk allele = 1.83).36
To our knowledge, no genome-wide scans for genes related to myopia have identified VDR
as significantly related to myopia. That they have not is somewhat surprising, given the very large sample size, replication, and SNP density in recent reports.35–36
One possibility is that the present study results are false positives, a concern that can only be addressed through replication. We estimated the false-positive rate for SNPs with the allele frequencies observed in the study to be 3.8% through performing over 1000 simulations, making the probability of finding three significant SNPs by chance alone equal to 5.4 × 10−5
. An important difference is that the present study is more a candidate gene approach using a multivariate analysis of several, mostly independent, SNPs showing weak to no linkage disequilibrium. The effects shown in are from multivariate odds ratios. In agreement with recent reports,35,36
none of the SNPs from the present study showed genome-wide significance as singletons. The multiple SNP analysis we used might be better suited to this candidate gene approach, given the prohibitively high number of multiple SNP combinations in a genome-wide study.
is in the vicinity of the MYP9
locus at 4q12 but has not been named as a candidate gene.37
G was the risk allele for rs7041 in the present study. This result was unexpected, given that being homozygous for the G allele for rs7041 has been associated with higher plasma levels of 25(OH) vitamin D,24
assumed to be beneficial given the protective effects of time spent outdoors.7
The regions recently identified as associated with myopia that are closest to VDR
have been at 12q21.2-24.12 (MYP3
and SNPs within COL2A1
; however, linkage between COL2A1
seems unlikely and none of the SNPs within VDR
used in the present study was part of a panel used previously (Linkage Panel IVb; Illumina, San Diego, CA).38
The recent, detailed investigation of SNPs within and near COL2A1
did not include SNPs within VDR
One group has reported that the Bsm
I polymorphism within VDR
(rs1544410, not evaluated in the present study) is associated with myopia.39
In addition, SNPs within LRP5
(low-density lipoprotein receptor-related protein 5) located on chromosome 11, region q13.4, have recently been associated with refractive error (Simpson CL, et al. IOVS.
2009;50:ARVO E-Abstract 2817). This finding is relevant, because the LRP5
gene has been shown to interact with the vitamin D receptor and vitamin D, modifying osteoblast function in the mouse.40
Further study is needed to determine whether an interaction between VDR
is important in human refractive error.
Replication studies are needed to confirm the importance of these polymorphisms to myopia; pending replication, these results should be interpreted with caution. The sample size was relatively small, and a case–control design may be subject to false-positive findings if differences in ancestry between cases and controls create differences in allele frequencies that are more related to ancestry than to myopia. In addition, self-reported ethnicity and the limited number of ethnic categories used may have resulted in misclassification. With this caution in mind, our results were consistent across attempts to control for ancestry by adjusting for ethnicity in the multivariate analysis of the entire sample and by conducting the analysis in the white subsample. Obviously, such adjustments are not perfect, nor do they constitute a complete set of potential confounding variables. Unmeasured factors related to myopia such as time spent outdoors,7
may interact with or be more directly associated with these polymorphisms. A more complete study of a role for VDR
and vitamin D in myopia would compare myopes to nonmyopes, not only for genetic polymorphisms but also for differences in dietary intake of vitamin D, circulating blood levels of vitamin D, IQ, and visual activity profiles at various times of year to untangle the specific contributions of each factor and the interactions between them.
With one exception, the SNPs within VDR
are located in intron 7 of the gene in a region without strong linkage disequilibrium and that lies between the two major haplotype blocks that encompass the coding sequence of VDR
. While there are multiple adjacent regions to the associated SNPs of strong sequence conservation within 1000 bp or fewer that are suggestive of regulatory elements, none of the three SNPs is itself resident in a conserved element. One SNP, rs10735810 (rs228570), is outside of intron 7 and has a C-to-T nucleotide change that creates a 424-amino-acid VDR protein compared with a longer 427-amino-acid protein made in the presence of the C allele. Studies have shown the 424-amino-acid protein results in more transactivation of the vitamin D response element than for the longer protein,31,32
yet this SNP showed no association with myopia in any analyses, leaving the mechanism of action unclear.
Additional studies are needed to determine whether these polymorphisms have any biological significance in vitamin D receptor function or vitamin D metabolism relevant to the eye. Vitamin D is known to be a powerful regulator of cellular differentiation with strong anticancer and antiproliferative effects.44
As a member of the nuclear receptor family, the vitamin D receptor regulates gene transcription, forming heterodimeric partnerships with retinoid X receptors (RXR).45
The presence of 1,25-dihydroxyvitamin D3
initiates the formation of the vitamin D receptor/RXR heterodimer. Thus, an abnormality of the vitamin D receptor or a lack of vitamin D in the diet might affect transcription, and perhaps ocular growth, through these abnormalities. Retinoic acid receptors also form heterodimers with RXR, with retinoic acid and 1,25-dihydroxyvitamin D3
engaging in some crosstalk in signaling and cell-cycle regulation through overlapping binding specificities.45
Retinoic acid is noteworthy because it has been shown to be a bidirectional regulator of eye growth.46,47
In addition, feeding experimental animals large amounts of retinoic acid leads to increased eye growth.48,49
Levels of retinoic acid receptor-α mRNA have also been reported to increase in form deprivation myopia and recovery from myopia in the chick.50
Perhaps this crosstalk between vitamin D and retinoic acid exists in signaling pathways affecting eye growth. While speculative at this stage, these potential mechanisms suggest several experimental avenues to pursue.
In summary, we found that polymorphisms within the vitamin D receptor gene VDR were associated with myopia, particularly in white subjects with low myopia, accounting for a large proportion of model variance (12%) over age alone. The repeatability of this finding and the biological significance of these variations with respect to myopia are yet to be determined.