3.4 Studies of the Genetics of Retinal Vein Occlusion

Because familial clustering of RVO is rare, only certain types of clinical genetics studies are used. Linkage analysis studies are notably missing in clinical genetics research regarding RVO.

3.4Studies of the Genetics of Retinal Vein Occlusion

The number of studies exploring possible genetic susceptibility to or protection from RVO is steadily growing. Our goal in this chapter is to expose the clinician to an important and expanding Þeld of inquiry. A prominent theme that emerges from reviewing the many genetic studies is the inconsistency of results. Inconsistency may result from varying genotype distributions across different populations, small sample sizes with insufÞcient statistical power to accomplish the goals of the studies, and varying standards for control groups.33,84 For example, in studies on associations of RVO and the factor V Leiden mutation, whether the control group is drawn from patients with coronary artery disease or from patients with deep venous thrombosis can change the interpretation of the results.33 Some studies have not used internal controls, but rather published data from other research groups, a practice considered by many to be unsound.32,77

Genome association studies with RVO can be based either on testing of particular polymorphisms, or agnostic genome-wide searching. The former has a higher probability of association with the disease than agnostic searches of polymorphisms, which can number in the thousands.74 The latter requires much larger sample sizes and is less efÞcient, but is less likely to miss associations.

Based on the evidence that atherosclerotic cardiovascular disease is associated with RVO, and the fact that atherosclerosis is associated with a cytokine proÞle, a number of cytokine genes have been identiÞed for investigation. Proinßammatory cytokines such as interleukin (IL)-6, IL-18, and monocyte chemoattractant protein (MCP)-1 are

associated with atherosclerosis and therefore have been suspected of association with RVO.54,74 Anti-inßammatory cytokines such as IL-10, which are associated with relative protection against atherosclerosis, have been investigated, since they could be associated with protection against RVOs.54,74

IL-1b, IL-6, tumor necrosis factor (TNF)-a, and MCP-1 upregulate the expression of tissue factor, thus theoretically promoting thrombosis. IL-1b and TNF-a downregulate the expression of plasminogen activator, thus inhibiting Þbrinolysis. Therefore, gene mutations for all of these cytokines have been tested for association with RVO.54,74 Similarly, IL-10 downregulates expression of tissue factor, and IL-4 upregulates plasminogen activator, so both have been tested for association with protection against RVO.54

In some studies, RVOs of all types are pooled based on the assumption that genetic risk factors are common across subtypes of RVO. In other studies, patients are divided into CRVO and BRVO groups. It is better to study RVOs by type, because their etiologies may differ in important ways, but assembling large enough samples of a particular type is harder. The approach of the studies discussed below Ð whether all RVOs were pooled or types were studied separately Ð is noted in Tables 3.2, 3.3, 3.4, 3.5, 3.6, and 3.7.

3.4.1Platelet Glycoprotein Receptor Genes

GPIIb/IIIa is a platelet glycoprotein receptor involved in platelet adhesion to the subendothelial matrix and in platelet-to-platelet adhesion.84 A polymorphism of the gene coding for the GPIIIa subunit (ITGB3) is characterized by a T-to-C substitution at nucleotide 176. It has been associated with increased platelet aggregability and hence a procoagulant effect. Studies have looked for an association of this polymorphism in patients with CRVO and BRVO, and no associations have been found (Table 3.2).49,84

The glycoprotein Ia/IIa complex (GPIa/IIa), also called integrin a2b2, is a collagen receptor involved in the adhesion of platelets to the vascular endothelium. The density of GPIa/IIa varies among individuals. The 807C/T polymorphism of the GPIa gene is linked to the density of col-

lagen receptors on the platelets with the −807T associated with high receptor density and the −807C allele associated with low receptor density. High density of collagen receptors on platelets has been proposed as a risk factor for thrombosis.18,25,46 The GPIa −807T allele has been

shown to be overrepresented and an independent risk factor for stroke in patients of 50 years or older.10 Studies in pooled RVOs and in BRVOs have not shown an association of this mutation with the affected eyes.25,84

In addition to the 807C/T polymorphism, an 873G/A polymorphism of the GPIa gene has been linked to the density of collagen receptors

on the platelets: the −873A allele is associated with high receptor density and the −873G with low receptor density. An association of the −807TT/−873AA genotype with RVO was not found.18 The authors suggested that their failure to Þnd this expected association resulted from the low frequency of this genotype in the population together with their small sample size.18

RAPC is more pronounced in persons homozygous for the FVLM than for those who are heterozygous.51

The FVLM is present in 2Ð15% of Caucasians. It is highly variable across different ethnic groups and is uncommon in patients with African, Asian, Australasian,andnativeAmericanancestry.11,33,53,65,81 Because the FVLM is rare in non-Caucasians, studies looking for an association between FVLM and RVO drawn from non-Caucasian populations will be less likely to Þnd any association.49

Testing for the FVLM involves PCR techniques that are relatively expensive, whereas APC testing is commonly available at any clinical laboratory and is relatively inexpensive. Therefore, the usual testing scenario is to screen patients by testing for RAPC and then to pursue only those positive for RAPC with PCR testing for the FVLM.44 In clinical practice, the Þrst step is to check the activated partial thromboplastin time (PTT) ratio. If it is less than some cutoff value speciÞc to the laboratory, commonly 1.9Ð2.0, then DNA analysis by PCR is performed to detect the R506Q mutation.33 Some investigators have stopped at the step of determining resistance to activated protein C.49,86 Because there are acquired bases for RAPC, these patients cannot be said to have FVLM.53 Acquired RAPC can arise from pregnancy, surgery, systemic lupus erythematosus, stroke, and use of oral contraceptives.31,81

There is agreement that the FVLM is associated strongly with systemic deep venous thrombosis. Heterozygosity increases the lifetime risk for systemic thrombosis 5Ð10 times and homozygosity by 50Ð100 times.44 When the FVLM is added to other risk factors such as oral contraceptive use or pregnancy, the risk posed by the FVLM is multiplied.81 In contrast, the FVLM is not associated with arterial vascular disease, and the association with RVO is inconsistent and for the most part negative.11,18,53 In a comprehensive review of nine studies looking for an association of the FVLM with pooled RVO, one found an association and eight did not.77 Similarly, of ten studies looking for an association of the FVLM with CRVO, six showed an association and four did not.77 Also, of two studies looking for an association of the

FVLM and BRVO, neither showed an association.77 A review of this extensive negative data is available in Tourville.77

Since TourvilleÕs review, several new studies have been published on this topic, all of them negative (Table 3.6). Some studies have looked at subgroup analyses that suggest an association, such as an association among patients without vascular risk factors, but the number of patients in such subgroup analyses is small, so conÞdence in the suggested association is low.66 A metaanalysis of 18 studies with 1,748 cases of pooled RVO and 2,716 controls showed a small association of the FVLM with pooled RVO with a combined odds ratio of 1.66 (95% conÞdence interval (CI) 1.19Ð2.32). This suggests that the multiple negative studies were negative because of their small size. Even if one accepts the conclusion of this meta-analysis, the clinical importance of the FVLM as a risk factor for RVO is small.67

Although the evidence suggests that the FVLM is not associated with any form of RVO, it has been associated with an increased risk of neovascularization of the iris after CRVO.37 In a retrospective study of 166 patients with CRVO in Sweden, 11 of 20 (55%) with the FVLM developed neovascular complications, whereas 45/146 (31%) without FVLM did so (P = 0.04, OR 2.7, 95% CI 1.1Ð7.1).37 Also, in the special situation of BehcetÕs disease, presence of the FVLM increases the risk of RVO.11

Although there are cases in which the FVLM and RVO have been found together, the frequency is too low to make a search for the FVLM routine.6,34 In patients with a family history of thrombosis, a test for RAPC should be performed, and if positive, a test for FVLM is warranted. Positive testing for the FVLM should lead to counseling about reduction in acquired risk factors for venous occlusions including RVO.31 Anticoagulant therapy in response to a discovery of the FVLM is recommended for heterozygous patients who have recurrent RVO or RVO with systemic venous thrombosis. It is recommended for all homozygotes, even if only a single episode of RVO has occurred. Consultation with a hematologist is recommended, and family members should be screened for the mutation.44