Research progress of age-related macular degeneration related gene polymorphism at high altitude

Xin Yan , Ling Li , Rui-Juan Guan

1Qinghai University, China

2Department of Ophthalmology, Qinghai Provincial People's Hospital, Xining, China

Keywords:Age related macular degeneration Gene polymorphism High altitude area Research progress

ABSTRACT Age-related macular degeneration (AMD) is a kind of progressive eye disease that seriously damages vision, and it is one of the important causes of blindness. In recent years, a large number of studies have found that there is a significant correlation between genetic factors and the occurrence and development of AMD. The study of gene polymorphism provides new ideas and directions for clinical diagnosis and treatment. In this paper, we will make a brief review of the research progress related to complement factor H (CFH), serine protease (HtrA1), age-related macular degeneration susceptibility factor 2 (ARMS2) and vascular endothelial growth factor (VEGF) gene single nucleotide polymorphisms (SNP).

Age-related macular degeneration is one of the most common irreversible blinding eye diseases, which will inevitably seriously affect the quality of life of patients.The disease affects tens of millions of people around the world, with hundreds of thousands of newly diagnosed patients each year.Clinically, age-related macular degeneration is divided into dry (Arophic) AMD (about 85% of all AMD cases), and late stage is named as neovascular (Exudative), namely wet AMD (10% - 15% of all AMD cases)[1].At present, there are more than 40 million AMD patients in China,With the aging process of society, the number of AMD patients will also rise, which has become one of the main risk factors of visual impairment.The occurrence and development of AMD are related to many factors, such as age, gender, race, habits, obesity, sun exposure, etc, but the specific cause of AMD is not clear yet[2].In recent years, a large number of studies have found that the occurrence and development of AMD are mostly related to heredity. The study shows that the heredity rate of AMD can reach 71%, which indicates that genetic factors are closely related to the occurrence of this disease[3].With the progress of science and technology, some related gene loci of AMD have been found one after another. The following is a brief review of several genes that are most closely related to the occurrence and development of AMD.

1. Complement factor H (CFH) gene and AMD

Complement factor H is also known as complement regulatory factor, located in 1q25-31, it is a plasma protein encoded as 155kda, called factor H (FH). It is the main inhibitor of complement alternative pathway. It interferes with the formation and activity of C3 invertase (c3bb), and also reduces the formation of c5b9 membrane attack complex (MAC) and two kinds of anaphylactic toxins C3a and C5a[4].However, the CFH Y402H variant reduced the ability of CFH to neutralize lipid oxide, enhanced their toxic and inflammatory effects, and proved the relationship between CFH Y402H and oxidative stress[5].Oxidative stress is recognized as a multifactorial and complex pathogenic factor. Previous studies have shown that CFH can protect retinal pigment epithelial (RPE) cells from hydrogen peroxide[6], but the specific damage mechanism is not clear. Céline Borras et al. [7]Found that only full-length CFH could protect human ARPE-19 cell lines and irpe from oxidative stress-induced cell death induced by 4-hydroxynonenal (4-HNE) exposure. Exposure to 4-HNE increased MAC deposition on RPE cells, while full-length CFH reduced MAC concentration, and only full-length CFH could prevent oxidative stress-induced apoptosis, which indicated that this effect was independent of MAC formation.At present, it has been confirmed that CFH itself can protect RPE cells from oxidation and block the activation of alternative complement pathway, but the specific protective mechanism is still unclear, which needs further study. However, the polymorphism of CFH Y402H is highly related to the occurrence of AMD. The occurrence of inflammation mediated by complement activation caused by its variation not only loses its protective effect, but also enhances the toxicity of MAC. If the activity of CFH is not restored, it may not be enough to play a full preventive and therapeutic role in AMD.

2. High Temperature Requirement Factor (HtrA1), age-related macular degeneration susceptibility factor 2 gene (ARMS2) and AMD

Genetic studies have found two susceptibility genes: serine protease (HtrA1) and age-related macular degeneration susceptibility factor 2 (ARMS2), both of which are found in the retina[8].Recently, four kinds of DNA mutations with strong amd risk have been found in GWAS. Three of them are located in the closely related HtrA1 and ARMS2 genes. The proximity of the physical genome means that they can never be analyzed independently in the genetic process. Zhigang Lu et al. [9]Studied the relationship between oxidative stress and common polymorphism in HtrA1 / ARMS2 region of chromosome 10q26. They injected ox LDL (oxidized low density lipoprotein) into the retina of C57BL / 6 mice, and observed a characteristic similar to choroidal neovascularization (CNV), including leakage, increased VEGF expression and neovascularization.However, when ARPE-19 cells were incubated with ox-LDL, the expression of inflammatory cytokines and chemoattractants in monocytes increased, but the expression of VEGF did not increase significantly. They speculated that this was due to the special difference in the response to oxidative stress in vivo and in vitro.In order to verify this hypothesis, they cultured J774 macrophages with ox LDL and applied the regulatory medium to ARPE-19 cells. It was found that this intervention greatly enhanced the expression of VEGF in ARPE-19, indicating the necessity of inducing the increase of VEGF expression in RPE by the secretion of megaphage.In order to explore the synergistic effect of HtrA1 and ox LDL on the expression of VEGF, they compared the expression of VEGF in the pure ox LDL medium, HtrA1 medium and the co existing medium. Compared with the incubation with ox LDL alone, the expression of IL-6, IL-8, CCR2 and VEGF in ARPE-19 cells in HtrA1 + ox LDL medium increased significantly, which confirmed that HtrA1 + ox LDL had synergistic effect on gene expression. In this experiment, they also sought to inhibit the activity of HtrA1

with antibodies. By adding anti HtrA1 monoclonal antibodies to the culture medium, they reversed the increase of these gene expression, especially the expression of VEGF, which proved the effectiveness of the antibody in neutralizing the inflammatory effect of HtrA1 on cultured ARPE-19, and the neutralization ability of HtrA1 monoclonal antibody was successfully replicated in vivo. The pathological area of CNV in C57BL / 6 mice injected with oxLDL and HtrA1 under retina after laser-induced CNV was significantly larger than that in the group injected with oxLDL or HtrA1 alone, while the area of CNV in these mice decreased significantly in the presence of anti HtrA1 monoclonal antibody. This study confirmed that HtrA1 affects the inflammatory response induced by oxidative stress and choroidal neovascularization, which ultimately leads to the occurrence of AMD. However, the mechanism of this effect is still unclear, which needs further study. The synergistic effect of HtrA1 + ox LDL has been confirmed, but the specific mechanism is not clear and needs further exploration. However, this study provides evidence that HtrA1 will be a potential target for the treatment of AMD.

AREDS formula is a combination of high-dose antioxidants and high-dose zinc, which can reduce the risk of AMD from moderate to advanced stage by 25% within 5 years[10]. Awh et al. [11]Analyzed the response of CFH and ARMS2 genetic risks to 989 AREDS subjects to AREDS nutritional supplements. They found that the benefits of AREDS preparation had been eliminated for subjects with high expression of CFH allele. They speculated that this result was caused by the high dose of zinc in AREDS formula. With regard to CFH, the reverse effect of ARMS2 polymorphism was also determined. The authors speculate that if zinc therapy is used (alone or as part of AREDS preparations), the progression of AMD in subjects with high CFH and low ARMS2 risk alleles increases, while that in subjects with low CFH and high ARMS2 risk alleles decreases. Seddon et al.[12] analyzed the progress of mid late AMD and finally developed into two subtypes of neovascularization (NV) and geographic atrophy (GA) in late AMD.They found that for subjects with low CFH and high ARMS2 genetic risk, the decrease in the proportion of developing advanced amd was due to a decrease in the progression to NV, while there was no significant effect on the progression of central GA. The authors concluded that "the effectiveness of antioxidants and zinc supplements appears to vary by genotype". Demetrios g et al. [13]found that compared with placebo, those with high CFH and no risk allele of ARMS2 had an increased risk of developing NV. For those with low CFH and high risk allele of ARMS2, they found that treatment with AREDS had substantial therapeutic effect. These conclusions are consistent with those of awh and Seddon. AREDS preparation can effectively delay the progress to neovascularized AMD, but it has no obvious effect on map atrophic AMD, and there will be different reactions for individuals of different genotypes, and even for individuals of some genotypes, it will increase the risk of developing into advanced AMD. More research is needed to optimize AREDS preparation to meet the needs of different individuals.

3. VEGF gene and AMD

There are many subtypes of VEGF, ranging in length from 121 to 206 amino acids, of which VEGF165 is the main subtype of human[14].VEGF165 binds to cell surface receptor tyrosine kinase: VEGFR-1 and VEGFR-2[15].VEGFR-1 mediates vascular permeability and VEGFR-2 is involved in angiogenesis[16].Therefore, VEGF mediated changes are of great significance in the early stage (vascular permeability) and late stage (neovascularization) of AMD.At present, the treatment of wet AMD is mainly intravitreal injection of anti VEGF agent, which can inhibit the formation of retinal neovascularization and bleeding, and significantly improve vision[17 ].However, with the continuous growth of age, the intraocular level continues to decline, leakage often occurs repeatedly, and anti VEGF preparations need to be injected repeatedly[18].Several studies have suggested that multiple injections increase the risk of map atrophy in the macular region, which may offset the benefits for a long time[ 19].A large multicenter clinical trial, Comparison age-related macular degeneration treatment trial (CATT) comparison, compared four treatment groups of neovascular patients, once a month ranizumab, once a month bevacizumab, PRN ranizumab and PRN bevacizumab.Two years later, there was no difference between ranizumab and bevacizumab when the administration plan was the same, and the visual effect of the subjects who injected the two drugs every month was significantly better than that of PRN injection of the same drug, but at the same time, it was noted that many patients had new discoloration spots in the macula. Through fundus photography and fluorescein angiography, compared with PRN group, the occurrence of discoloration spots in the monthly injection group was found The rate is higher. The researchers call these spots GA, which means that photoreceptors and RPE cells in this area die. The incidence of GA in the treatment group is higher than that in the observation group, but there is no clear evidence[20].Luttun et al.[21] believe that VEGF is a survival factor of neovascular endothelial cells, but mature vascular endothelial cells receive other survival signals from extracellular matrix (ECM) and surrounding cells. Therefore, most mature blood vessels are not damaged by VEGF inhibition, but capillaries with membranous pores are more susceptible. Rosenfeld et al[22] proposed that the choroidal endothelial cells in patients with neovascular amd have membranous pores, but they no longer depend on VEGF for survival shortly after the formation. Therefore, the injection of anti VEGF into the eyes reduced the leakage of choroidal vessels and improved vision, but the average area of choroidal vessels measured by fluorescent angiography did not decrease within two years.This also shows the necessity of continuous injection in patients with NV, because it can not eliminate the choroidal NV, and with the decrease of the level of intraocular antagonists, the level of VEGF increases, which stimulates the recovery of leakage and growth of choroidal NV, the aggregation of other cells and the deposition of ECM. If the treatment is delayed, the aggregation of cells around choroidal vessels and the deposition of ECM will promote scar formation, photoreceptor damage and Permanent loss of vision.Long D et al. [23]Through the observation of the experimental mouse model, thought that VEGF did not help the survival and function of photoreceptors, because VEGFR2 inhibitor could well inhibit the choroidal NV without affecting the function or survival of photoreceptors, while VEGF did not slow down the loss or death of photoreceptor function in hereditary retinal degeneration mice, and the inhibition of VEGF did not aggravate the disease, and type 3 choroidal blood The retina of tubuloma mice was atrophic due to oxidative damage, and the inhibition of VEGF had no effect.At present, it has been confirmed that VEGF has a high correlation with AMD, which can change the permeability of blood vessels and promote the formation of new blood vessels to develop into AMD. VEGF inhibitors can effectively improve the visual function of patients, but they can not reduce the area of new blood vessels, often need to be injected repeatedly, but whether long-term injection of VEGF inhibitors will lead to map atrophic AMD , no clear evidence. There are special environmental factors in plateau area, such as strong ultraviolet, long time of illumination, lack of oxygen, difference of living habits and so on. Studies have shown that individuals exposed to long-term sunlight and more ultraviolet radiation have a significantly increased risk of AMD[24].In the hypoxia environment, VEGF expression will also increase, promote the formation of new blood vessels, and promote the development of AMD[25].Because the diet structure of people in high altitude areas is mainly high-fat, but research shows that highfat diet also increases the risk of AMD[26].According to one data, the incidence rate of AMD in the Tibetan population of Qinghai province was 58.70%, significantly higher than that in other areas with lower altitude[27].Because the incidence rate of AMD is high in plateau area, its treatment is particularly important.Han Xia et al.[28] Carried out an experiment in the plateau area, respectively set conbercept as the observation group and triamcinolone acetonide as the control group, once a month, after 3 months, the total effective rate of the observation group was significantly higher than that of the control group. The result of this study shows that it has good curative effect in plateau area with special environment, and it is cheap, safe and reliable.

With the further development of population aging, the incidence rate and number of AMD will increase simultaneously, and the burden of the whole society will be increasing. Therefore, amd has long been a common medical problem in the world.Because the pathogenesis of AMD is complex, there is no clear explanation.With the rapid progress of genomics and biotechnology, these emerging technologies have been used in the field of AMD, providing great help for the diagnosis and treatment of AMD.At present, many researchers are committed to finding new methods from the gene level, and many have made corresponding progress and achievements.At present, CFH, HtrA1, ARMS2, VEGF and other genes are recognized to be significantly related to AMD, and corresponding treatment methods are proposed to promote the diagnosis and treatment of AMD. With the deepening of the research on the disease, there are new breakthroughs in the diagnosis and treatment of age-related macular degeneration, accelerating the pace of precision medicine. However, due to the complexity of the etiology and pathogenesis of the disease, the research on the disease is still insufficient, so we need to continue to strengthen the research.