Cody Malone Paper

Cody Malone
Genetics
Dr. Robson
03/13/09

Article Review on Genetic Factors and Myopia
Myopia, also known as nearsightedness, is an ocular condition where the point of focus is in front of the retina, which results in difficult visual of distant object. Doctors measure myopia by diopters, which is a unit of lens curvature. Low myopia generally requires -2 to -3 diopters, medium myopia -3 to -6 diopters, and high myopia >-6 diopters. Myopia affects about 25% of people in the United States [4, 6,7]. In severe situations (high-grade myopia) the person affected can experience “floaters”, retinal detachment, macular degeneration and glaucoma; in the United States, less than 5% have high-grade [3,4,6]. Multiple studies have been performed to find the cause of myopia, with little results. Most agree myopia had two main causal factors: environment and genes.
Environmental factors can contribute to almost any defect in any spectrum. In cases of low myopia (~20% of the United States), strain to the eye such as schoolwork, computers and age seem to be the chief contributor [2,3,4]. The excessive squinting tends to narrow vision and in turn push the eye out, molding it to a more ovular shape which changes the focal point of vision, in turn creating nearsightedness. This seems to be more prevalent in older adults, since they have been performing these environmental factors longer, and putting strain on the eye longer [6].
The majority of research on myopia deals with high-grade myopia and any genetic links to the disability. There seems to be a recessive trait on the X chromosome, in turn making males in a myopic family more susceptible. Davenport’s research followed a family five generations in which the father was myopic. After five generations of children, the males were six times as likely to be myopic than females. Nallasamy, et al. also found a link on the Xq28 chromosome, with possibly two genes: protocadherin 15 (PCDH15) and ZW10 interacter (ZWINT). These two genes appear to be relevant only in high myopic men. Mutations in genes may also be a cause for high myopia [7], particularly in the NYX gene, though the mutation is not accompanied with night blindness (even though most high-grade myopic people also have night blindness).
Other evidence for genetic factors and myopia include the studies done in remote cultures such as Asia and Amish communities. A study of Taiwan students found 84% of high school students to have myopia, 21% of whom were high-grade myopic [6]. This study is comparable to the U.S., but about four times higher rates. Ibay, et al. performed a similar study among Amish and orthodox Jewish families, and found no conclusive data in the Jewish families, but one Amish family in particular in which high myopia seemed to be inheritable. The chromosomes 12q21-q23 and 18p11.2-p11.32 were researched for this study, and genes were silenced in the families without myopia, only the Amish family had an expression.
Interesting results stem from myopia, such as higher IQ’s and as mentioned above, night blindness [7]. Many are skeptical to the “result” of a higher IQ due to myopia, since low-grade myopia may be caused from tedious schoolwork and excessive reading, which would naturally raise some children’s IQ’s.
Personally, I find the evidence for inheritable factors almost unnecessary, because of the family history and myopia. In my family alone, I have found evidence for the inheritable trait, every father with myopia has had sons with myopia. This situation arises in many other families I have observed, there are few families who have only one member with myopia.
Myopia (also known as nearsightedness) affects anywhere from 25-84% of a given population, with 5-21% of whom having high-grade myopia. High myopia is characterized by the necessary use of a lens with >-6 diopters, and in some dramatic situations retinal detachment, macular degeneration and glaucoma accompany high myopia. Multiple factors contribute to the cause of myopia, high and low grade, such as environmental and genetic. The environmental factors found to contribute to myopia were schoolwork, computers and lifelong strain to the eye, though these generally produced low myopia instead of high myopia. Studies found high-grade myopia to affected more by genetic factors, and found chromosomes Xq28, 12q21-q23, and 18p11.2-p11.32 to have possible genes that may be susceptible to mutation and in turn high myopia. In conclusion, as Ibay, et al. stated in their document, “to understand myopia, it is necessary to apply the equation Genes + Environment = myopia”.

1. Crewther, Sheila G.; Lianga, Helena; Junghans, Barbara M.; Crewther, David P. Ionic Control of Ocular Growth and Refractive Change. Proceedings of the National Academy of Sciences of the United States of America. 2006; Vol. 103, No. 42: 15663-15668. http://www.jstor.org/stable/30050902?&Search=yes&term=Myopia&list=hide&searchUri=%2Faction%2FdoBasicSearch%3FQuery%3DMyopia%26x%3D0%26y%3D0%26wc%3Don&item=5&ttl=7185&returnArticleService=showArticle

2. Davenport, C.B. Sex Linkage in Man. 1966; 15: 401-432. http://www.genetics.org/cgi/reprint/15/5/401?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&fullt
ext=Myopia&andorexactfulltext=and&searchid=1&FIRSTINDEX=0&sortspec=relevance&resourcetype=H
WCIT

3. Goldschmidt, Ernst. The Mystery of Myopia. 2003; Vol 81, 5: 431-436. http://www3.interscience.wiley.com/cgi-bin/fulltext/118852601/

4. Ibay, Grace; Doan, Betty; Reider, Lauren; Dana, Debra; Schlifka, Melissa; Hu, Heping; Holmes, Taura; O'Neill, Jennifer; Owens, Robert; Ciner, Elise; Bailey–Wilson, Joan E.; Stambolian, Dwight. Candidate High Myopia Loci on Chromosomes 18p and 12q Do Not Play a Major Role in Susceptibility to Common Myopia. BMC Med Genet. 2004; 5: 20.
http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=512288&tool=pmcentrez

5. McKusick, V.A.; Kelly, Jane; O’Neill, Marla J.F. Myopia and Gene Map Locus 18p11.31. Johns Hopkins University. 2007. http://www.ncbi.nlm.nih.gov/entrez/dispomim.cgi?cmd=entry&id=160700

6. Nallasamy, Sudha; Paluru, Prasuna C.; Devoto, Marcella; Wasserman, Nora F.; Zhou, Jie ; Young, Terri L. Genetic Linkage Study of High-grade Myopia in a Hutterite Population from South Dakota. Molecular Vision 2007; 13:229-36. http://www.molvis.org/molvis/v13/a26/

7. Zhang, Qingjiong; Xiao, Xueshan; Li, Shiqiang; Jia, Xiaoyun; Yang, Zhikuan; Huang, Shizhou; Caruso, Rafael C.; Guan, Tianqin; Sergeev, Yuri; Guo, Xiangming; Hejtmancik, J. Fielding. Mutations in NYX of Individuals with High Myopia, but without Night Blindness. Mol Vis. 2007; 13: 330–336. http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=2642916&tool=pmcentrez

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