189 Twins and Myopia
Twins, Myopia and Heritability Studies
Heritability studies for myopia using twins
Heritability studies ultimately aim to quantify the proportion of phenotypic variance that is due to genetic effects. In the case of myopia, heritability studies using twins as the model have defined myopia in terms of the quantitative measure of refraction. Heritability estimates for myopia have ranged from 0.77 in a cohort of British twins to 0.91 for a cohort of Danish twins.16–18 For the British twins, the heritability study was extended in a genetic linkage study whereby analysis was undertaken in order to determine where the gene or genes contributing to myopia are located in the human genome. This linkage analysis identified four regions in the genome on chromosomes 11p13, 3q26, 8p23, and 4q12 that may contain myopia genes.19 These results have been partly confirmed in two independent studies. The first by the same group confirmed linkage to the chromosome 3q26 region as harboring a myopia gene.20 The second study by Stambolian et al. (2005) in a family-based cohort independently confirmed the locus on chromosome 8p23.21 However, our own study using an Australian twin cohort did not find evidence to suggest that any of these chromosomal regions were linked to myopia.22 Further analysis is clearly warranted in order to identify the precise genetic change in these regions.
A heritability study in a cohort of Australian twins suggested there may be a gender effect with heritability estimates of 0.88 reported for male twins and 0.75 for females.23 However, the true extent of this gender effect in other cohorts remains to be further investigated. Furthermore, these heritability studies have also suggested that the environmental component to myopia is due predominately to unique environment effects rather than environmental factors that are shared between a twin and its co-twin. From these heritability estimates it is clear that the major influence on refraction, and hence on myopia, is likely to come from genes.
Furthermore, heritability studies have suggested that the gene effects on myopia are likely to be additive, meaning that these are the result of multiple genes, each with small effects that come together to cause myopia.17,23 In conjunction with this, a myopia heritability study by Dirani et al. (2006) in an Australian twin cohort suggested that the genetic effects are even more complex in that there are not only additive genetic effects, but also dominant effects whereby there is one predominant gene that causes
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myopia.23 In other words, there are likely to be multiple genes with varied and complex gene-gene interactions that are contributing to the development of myopia.
Although there is a significant amount of evidence from twin studies to suggest that myopia is predominately a genetic trait, not all studies are in agreement. A study by Angi et al. (1993) in an Italian twin cohort suggested that the genetic contribution to refraction was between 8–14%, indicating that the majority of the phenotype is not influenced by genetic effects.24 This appears to be the only heritability study in twins to argue against the involvement of genes in the development of myopia. It is of course possible that this result is true and there is something curiously different about this population in Italy, although this is unlikely to be the case as other studies have been performed on twin cohorts of similar ethnicity. The study by Angi et al. (1993) used a small sample of twins comprising of 29 pairs in total, which differs from the other heritability studies that used between 114 and 2301 twin pairs. Additionally, the Angi study utilized twins that were under the age of 7 years, in which refraction measurements may not yet have stabilized. Other studies have predominately used adult twins, which have more stable and reliable refraction measures provided that age-related changes in refraction (presbyopia) are accounted for. The small sample size and limited age ranges makes the Angi study results likely to be flawed, and until there is another study replicating this finding it must be treated with caution.
As we have discussed earlier in this chapter (see section “definitions”), the phenotypic definition of myopia is complicated and extends beyond simple refraction measures. It is influenced by the sub-phenotypes of ocular axial length, anterior chamber depth, and corneal curvature. With these sub-phenotypes in mind, heritability studies for myopia have been extended to ask the question of what is the genetic influence on axial length, corneal curvature, and anterior chamber depth. We will highlight three key twin studies that have undertaken a heritability analysis on these traits.
The first study by Lyhne et al. (2001) from Denmark analyzed 53 monozygotic and 61 dizygotic twin pairs using the classical twin model.17 This study reported heritability estimates of 0.94, 0.88–0.94, and 0.90–0.92 for axial length, anterior chamber depth, and corneal curvature respectively. Furthermore, axial length and corneal curvature measurements could largely be explained by additive genetic effects, whereas anterior chamber depth was explained mainly by dominant effects.
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The next study by Dirani et al. (2006) from Australia supported the results from Lyhne et al. (2001) in that it showed that these traits were predominately genetic in origin but there were some differences in the findings.23 For example, Dirani et al. (2006) suggested that axial length was influenced by dominant genetic effects rather than additive effects. Additionally, corneal curvature in the Dirani et al. (2006) study was also suggested as being influenced by dominant genetic effects as opposed to additive effects reported for the Lyhne et al. (2001) study. It is unclear why such a difference existed in the results for axial length, but those for corneal curvature could mostly be explained by discrepancies in the measures used. The Lyhne et al. (2001) study obtained keratometry measures using an autokeratometer and then averaged the values from the two principal meridians to get the final corneal curvature output. On the other hand, the Dirani et al. (2006) study calculated the difference between the two keratometry readings as the final corneal curvature output.
The third study by Zhu et al. (2008) focussed on axial length and added further support for a genetic involvement in this trait.25 This study used 131 monozygotic and 302 dizygotic twin pairs and reported a heritability of 0.81 for axial length, which was largely determined by additive genetic effects. This study went further in the analysis and performed a genetic linkage analysis in order to determine where the gene or genes contributing to axial length are located in the human genome. This work suggested that a region on human chromosome 5q spanning 98 cM is likely to contain a gene for axial length. Additionally, a number of other genomic regions on chromosomes 6, 10, and 14 also indicated the likely existence of other genes that might influence axial length. Replication studies are required to establish the consistency of these regions as well as further characterization of the identified genomic regions in order to establish the exact nature of the causative genes.
Further extension of the basic heritability analysis also needs to be undertaken in order to determine if there are common genes between myopia and its underlying determinants. To date, two such studies have been undertaken, which performed a statistical analysis to determine the extent to which genetic and environmental effects influencing axial length also influence refraction or anterior chamber depth.8,26 These studies were undertaken by fitting a bivariate Cholesky decomposition model to axial length and refraction or axial length and anterior chamber depth within the heritability analysis. For a more detailed description on this statistical
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methodology, we refer the reader to Loehlin et al.27 The first study by Dirani et al. (2008) suggested that not only were refraction and axial length highly heritable traits, but also that 50% of the genes influencing refraction are common to axial length. The second study by He et al. (2008) went on to suggest that 25% of the genes influencing axial length are also common to anterior chamber depth, suggesting that there are shared genetic influences on myopia and its determinants.
Although we have endeavored to convince the reader of the utility of using twins in heritability studies for myopia, a word of caution is required at this point. As we have hinted at earlier, the results from heritability studies may not necessarily always be reliable. Results need to be interpreted with scientific rigor and some scepticism. General flaws that are evident in the myopia twin literature are in the methodologies used in various studies. Some studies used less reliable measures of refraction, such as questionnaires or current eyeglass prescriptions. Given the high number of underor un-corrected myopic individuals in the community, this measure can in some cases provide an unreliable estimate of true refraction measures.28 Additionally, it is known that age (>50 years) as well as gender can affect refraction measures, so these also need to be taken into account. Caution should also be taken with studies that have small sample sizes, as highlighted above, or those with selection biases, as this may skew statistical calculations. Additionally, using cohorts of older twins poses problems as the effects of age-related hyperopic shift can skew true refraction measures. Having said all this, it should be noted that many of the myopia heritability studies have provided robust and reliable heritability estimates for a number of traits. These have convincingly demonstrated there is a clear and strong genetic involvement in myopia and its underlying sub-phenotypes, such as axial length.
Limitations of using twins in heritability studies
Heritability studies and the underlying principle of the classical twin model have proven to be a valuable tool for the study of myopia and its determinants. These tools can also be used to study other traits to quantitate the level of involvement of genes and environment in their etiology, but discussion of these studies are beyond the scope of this chapter. Heritability studies for myopia have also been undertaken using extended families rather than twins, which may provide some advantages given that
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the classical twin model relies on a number of assumptions that may skew heritability calculations.
One of the major assumptions of the classical twin model is the “equal environment assumption” (EEA). The EEA assumes that monozygotic twins and dizygotic twins are exposed to the same degree of environmental factors. Environmental factors can be considered to be anything that is non-genetic in origin that can influence the development of myopia. If the EEA is true, then any phenotypic differences between monozygotic and dizygotic twin will likely be due to genetic effects. However, it has been argued that monozygotic twins have a greater sharing of environment during their upbringing as they tend to be treated more alike than dizygotic twins.29 If this argument holds (and hence the EEA does not), then one would expect that monozygotic twins will exhibit a greater phenotypic similarly due to greater shared environment rather than due to shared genes. A natural test for the EEA is to observe twins that have mislabeled zygosities, or in other words, monozygotic twins that were thought to be dizygotic by their parents or vice versa. However, the number of mislabeled twins is small, and hence, there is limited data available to test the validity of the EEA with respect to myopia.
The classical twin model also makes the assumption that genetic and environmental effects on the trait, myopia in this case, are independent. The model does not account for the possibility of gene-environment interactions (see Chapter 1.3 for further detail). It has been suggested that there is likely to be an interplay between genes and the environment, such that genetically susceptible individuals may develop myopia if they are exposed to environmental risk factors such as excessive near work.17 The classical twin model does not account for the potential effect that environmental factors may have on the phenotypic expression of myopia during a person’s lifetime.
These fundamental assumptions of the classical twin model described above generally result in an overestimation of the genetic influences on myopia. This is exemplified by the fact that myopia studies using family designs have calculated lower heritability values of 0.5 and 0.61 for refraction compared to twin based studies.30,31 More recent advances in statistical methodologies for twin analysis have meant that these limitations have less of an impact on the heritability estimates, but nevertheless heritability estimates from twin studies should generally be considered as the upper limit.