Angiogenesis in Sickle Cell Retinopathy

Abstract

Sickle cell disease has the highest incidence for a population at risk of any genetically derived disease. Sickle cell disease is caused by a point mutation in the beta globin gene. The abnormal hemoglobin is less soluble and precipitates in the red blood cell (RBC or erythrocyte), distorting the cell shape. The elongated, multipointed, ``sickled'' erythrocytes are less pliable, causing an increase in blood viscosity, sluggish blood flow, and tissue hypoxia. The impairment of blood flow accounts for nearly all clinical manifestations of the sickling syndrome. Vaso-occlusion occurs in most organs. The vicious cycle of erythrostasis during sickle cell disease is most clearly observed in the eye. The initiating event, which occurs most often in peripheral retina, is vaso-occlusion, which we have observed in three children less than 2 years of age with sickle cell anemia. At the interface of nonperfused/perfused peripheral retina, arteriovenous (A/V) anastomoses and hairpin loops form, shunting blood from occluded arterioles to the nearest draining vessels. The initial angiogenic structures, buds or loop-like new vessels, form at hairpin loops and AN crossings but not at AN anastomoses as has been reported in clinical studies. Florid tufts of neovascularization (characterized by profuse fluorescein leakage), called sea fans, evolve later at these sites. Development of sea fans probably involves multiple angiogenic events in that most have more than one feeding arteriole and draining venule. The neovascularization (NV) grows peripherally, presumably in response to angiogenic growth factors produced in the ischemic nonperfused retina. We have observed high levels of both vascular endothelial cell growth factor (VEGF) and basic fibroblast growth factor (bFGF) associated with these neovascular formations. Elevated levels of pigment epithelial-derived factor (PEDF) are also present in viable vessels of sea fans, in the matrix of these neovascular membranes, and in feeder vessels for the NV. The PEDF/VEGF ratio in normal subjects and sickle cell subjects without proliferative retinopathy is greater than 2.0, whereas in sea fans it is 1.0 because of increased levels of VEGF. Immunoreactivity for PEDF was prominent in retinal vessel remnants in nonperfused peripheral retina and in atrophic sea fans, while VEGF immunoreactivity was weak or absent in these structures. We have examined a transgenic mouse line that produces 72.2% human beta(S) and 44.7% human alpha-globin. The mice Produce RBCs that sickle under hypoxic conditions, and they appear to experience organ damage similar to that observed in human sickle cell disease: splenomegaly and vaso-occlusions in lung and kidney. In these mice we have observed retinopathy that is similar to the human: vaso-occlusions occur in the retinal vasculature; adjacent to nonperfused areas, AN anastomoses form; both intra- and extra-retinal NV occurs. Choroidal NV and chorioretinal lesions, which resemble the human black sunburst lesion, were observed frequently. In summary, a unique form of angiogenesis called sea fans occurs in sickle cell retinopathy adjacent to peripheral areas of nonperfusion. High levels of VEGF, bFGF, and PEDF are associated with these formations. Animals expressing high levels of human sickle beta globin have a retinopathy similar to human subjects.