Myopia is the most common disorder of the eye also known as nearsightedness. The objects at a certain distance appear blurred when the light that enter the eye is not focused accurately into the eye. The eye with myopia and people with eyesight of <= -6.0 diopters are found to be susceptible to many diseases of the eye. A person suffers from high-grade myopia when there is an anterior-posterior eye enlargement, frequent separation of the retina, change in the scleral collagen fibrils diameter and scleral thinning.
Adults develop myopia as a result of certain health conditions like diabetes or due to optical stress. Myopia affected 25 percent of people above 40 years of age in the United States and Western Europe, indicating that this condition is quite common in the west. The population based studies carried out recently reveals that the predominance of myopia is increasing especially in Asian inhabitants. Several studies have proved that development of myopia is seen mostly in people having higher education. Several other studies have shown a positive association between myopia and greater intelligent quotient.
Genetic and environmental factors influencing myopia
The progression and development of myopia are believed to be affected by the environmental as well as genetic factors. The environmental features that can lead to the development of myopia are exposure to light, diet, near work, lack of physical activity, urbanization and higher education level. People involved in occupations like carpet weavers, clinical microscopists and visual display workers will develop myopia faster and myopia is observed to be highly prevalent in these people.
In another study of Finland children, those who were reading for more time could observe fast advancement of myopia in them. Some of the studies have revealed that myopia is more prevalent in East Asia, which might be due to increase in the use of modern electronic devices since 3 decades. There are some recent studies showing that children spending more time in outdoor sports had reduced risk of myopia. The indirect association between outdoor activity and myopia development was found to be restricted to children who were genetically predisposed to family inheritance of myopia. The inverse relation of outdoor activity with myopia was observed in children having both parents as myopic than in those who have no myopic parents or have one myopic parent. The children having myopic parents were found to have long eyes and might develop myopia during childhood or at slightly higher age.
When behavior and environment are vital in the refractive changes, it is also understood from several studies that hereditary factors are also involved in optical refraction. The risk of occurrence of common form of refractive errors ranging from 2 to 5.61 was found to be recurring in the siblings as per the familial aggregation studies.
The refractive error that is distributed across the varied populations has the basis of the familial influence of refraction phenotypes. The environmental factors resulting in the ethnic and regional variation in refractive distribution will show variation within the population due to the genetic or hereditary factors.
It is shown in some of the research studies that high myopia can be a hereditary condition in most of the cases and the genetic influence would be looked as a dominant factor in its development. The genetic loci that represent the non-syndromic myopia were mapped with 12 loci that are also found to be linked to high myopia. These loci are MYP1 on chromosome Xq28, MYP2 on 18p11.31, MYP3 on 12q21-q23, MYP4 on 7q36, MYP5 on 17q21-q22, MYP11 on 4q22-q27, MYP12 on 2q37.1, MYP13 on Xq23-q25, MYP15 on 10q21.1, MYP16 on 5p15.33-p15.2; MYP18 on 14q22.1-q24.2, and MYP19 on chromosome 5p15.1-p13.3.
The studies on candidate gene association have identified certain high myopia liable genes which include type I collagen, alpha1 (COLIA1), beta1 (TGFB1), transforming growth factor, lumican (LUM), transforming growth beta-induced factor (TGIF), myocilin (MYOC), hepatocyte growth factor (HGF), uromodulin-like1 (UMODL1), and paired box6 (PAX6).
The genetic relationship between familial myopia and 3 single nucleotide polymorphisms (rs6214, rs10860860, rs2946834) existing in the large myopia group of Caucasian origin indicates that insulin growth factor-1 or IGF-1 could be the candidate gene for high myopia. The above SNPs are found to be present in the MYP3 locus in the region of q21-q23 on chromosome 12. This locus was found to be related to autosomal dominant high myopia. The single nucleotide polymorphism rs6214 is observed to be present in the 3'-untranslated region of IGF-1 while rs10860860 and rs2946834 SNPs are present in the non-coding sequence lying closer to IGF-1.
It is shown in another study that IGF-1 was found to be associated with certain ocular diseases like age related macular degeneration, retinopathy of prematurity and diabetic retinopathy. Another polymorphism rs12423791 was positively correlated to extreme myopia in the Chinese population. Two more gene loci were identified in a quantitative study conducted in 225 Caucasian families that were related to myopia. They are chromosomes 6q13-16.1 and 5q35.1-35.2.
In conclusion, genes supporting the environmental factors may be required vitally to govern the risks of high myopia.
Malgorzata Mrugacz (2013). Role of the Genetic Factors in the Development of Myopia, Genetic Disorders, Prof. Maria Puiu (Ed.,), ISBN: 978-953-51-0886-3, InTech, DOI: 10.5772/52543. Available from: http://www.intechopen.com/books/genetic-disorders/role-of-the-genetic-factors-in-the-development-of-myopia
Image source: http://www.nei.nih.gov/healthyeyes/eye_images/Myopia.gif
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