A gene variant that increases the risk of developing the aggressive "wet" form of age-related macular degeneration (AMD), the most common cause of blindness in people over age 50, is reported in two recent articles in Science by researchers at Yale School of Medicine.
AMD causes light-sensitive cells in the retina to break down, resulting in progressive loss of central vision. Of the two forms of AMD, the "dry" is more common than the "wet" form. Wet macular degeneration can rapidly lead to blindness, while the dry AMD progresses more slowly.
Last year, Josephine Hoh, associate professor in the Departments of Epidemiology & Public Health and Ophthalmology at Yale and senior author on one of the two new studies, identified a gene for dry AMD and found that both wet and dry AMD are associated with a variant in the complement factor H (CFH) gene on chromosome 1.
Hoh now reports they have found a single nucleotide polymorphism (SNP)—a one-base change in the sequence—of the regulatory part of the HTRA1 gene on chromosome 10 that leads to greatly increased risk of developing the wet form of AMD.
According to Hoh, buildup of abnormal blood vessels in Caucasian patients is compounded by development of large waste deposits called drusen. Chinese patients, she said, develop little to no drusen and progress directly to wet AMD. This study demonstrates that these two major genes, CFH and HTRA1, in two different biological pathways, each affect the risk for a distinct component of the AMD phenotype: CFH influences the drusen of dry AMD, whereas HTRA1 influences blood vessel development, the hallmark of the wet disease type. When the two processes are combined, it leads to the composite characteristics that are seen in some cases of AMD.
Hoh, her collaborators in Hong Kong, and her colleagues at Yale including Michael Snyder and Colin Barnstable in the Departments of Molecular, Cellular and Developmental Biology and Molecular Biophysics and Biochemistry, and Ophthalmology, did trans-racial gene mapping by comparing genomes between precisely defined populations to find the incidence of SNP in a Chinese population—96 with AMD and 130 with normal vision.
"We found that patients with the HTRA1 SNP were 10 times more likely to have wet AMD than those without this gene variant," said Hoh. "While this is only preliminary work, it points to possible directions for future treatment of wet AMD."
Hoh also worked on a replication study led by Kang Zhang at the University of Utah School of Medicine that found a link between the same SNP and AMD. Zhang and his team studied 581 Caucasian patients with AMD and 309 with normal vision. These patients had wet AMD as well as a large amount of drusen.
To confirm the association, the Utah team also examined several donor eyes and measured the expression of the gene and the encoded protein. They found that the expressions were elevated in the eyes of patients who carry HTRA1.
"The marker we have identified is very much associated with AMD, but no one has ever pinpointed the clinical features of the gene. We need to conduct further analysis in order to understand the biological mechanisms," said Hoh.
In addition to Hoh, Snyder and Barnstable, authors on the first study included first author Andrew DeWan, Mugen Liu, Stephen Hartman, Samuel Shao-Min Zhang, David T.L. Liu, Connie Zhao, Pancy O.S. Tam, Wai Man Chan, Dennis S.C. Lam and Chi Pui Pang.
In addition to Zhang and Hoh, authors on the second study included Zhenglin Yang, Nicola J. Camp, Hui Sun, Zongzhong Tong, Daniel Gibbs, D. Joshua Cameron, Haoyu Chen, Yu Zhao, Erik Pearson, Xi Li, Jeremy Chien, Andrew DeWan, Jennifer Harmon, Paul S. Bernstein, Viji Shridhar, Norman A. Zabriskie and Kimberly Howes.
Citations: 1. Science, 314 (5801) 989 (November 10, 2006)
2. Science, 314 (5801) 992 (November 10, 2006)