Twin studies provide us with a unique opportunity to determine the relative genetic and environmental contributions to a particular disease or feature.
We have been particularly interested in finding the genetic factors responsible for the features involved in glaucoma (intraocular pressure, corneal thickness, and optic disc size and shape). By comparing the similarity of these structures within identical, or monozygotic (MZ) twin pairs to that within non-identical, or dizygotic (DZ) pairs, we can determine how genetic or heritable each structure is. The more genetically based the structure is, the more similar they will be in MZ twin pairs (who share all of their genes), when compared to DZ twin pairs (who only share on average half their genes).
We are well on the way to establishing the heritability of the structures and features implicated in glaucoma. The next step will be to find and identify the genes that are responsible for these structures. These genes may be glaucoma genes.
Although the structures we are researching vary between individuals and in the normal population, there will most likely be certain genes that play a major role in controlling intraocular pressure and establishing the size and thickness of the cornea, and the size and shape of the optic disc. It is the variation of these structures between the non-identical twin pairs that will become important in the search for genes.
As part of this research, all twin participants have volunteered a DNA sample. This was either in the form of a cheek swab or as a blood sample. This DNA sample was first used to establish whether twin pairs are identical or non-identical when in doubt. The DNA in the non-identical twins will be subjected to a Genome Wide Scan. This involves tagging parts of the DNA with special markers. Although similar to one another, there will be subtle differences in the DNA from one non-identical twin to another. Using sophisticated analysis techniques we will try and link the differences (and similarities) between intraocular pressure, corneal thickness, and optic disc size in DZ twins with differences (and similarities) in their entire DNA marker pattern. In this way we hope to find regions of the DNA (gene regions) that explain the variation in eye features.
As we are studying normal twins, with normal eyes, the genes we identify will be responsible for the normal structure and function within the eye. But in the same way that researchers in Queensland identified a gene that causes albinism by investigating the genes for normal eye colour, we hope to find genes for glaucoma by first studying the genes for normal function.
An additional strength of a twin study is that can continue to be used for genetic analysis of many other unrelated phenotypes. Thereby rendering sample very useful for numerous other disease. Although this study is principally investigating the genetics of glaucoma and refractive error, the genetics of any biometric trait could be investigated in the future without requiring further laboratory expenditure.
