Diabetic macular edema : genetics and treatment response

Diabetic macular edema (DME), the most common cause of vision loss in people with diabetes, is a complex disease affecting the central part of the retina. The exact pathogenesis of DME is unclear, and studies suggest a strong genetic basis. However, there are only a handful of studies on the genetics of DME, and most have used a candidate gene approach and/or have small cohorts. Thus, there is a significant gap in our understanding of the genetic basis of DME.
The gold standard treatment for DME is intravitreal injection of anti-vascular endothelial growth factor (anti-VEGF) agents at monthly intervals. While the majority of patients show good outcomes, a large proportion report limited effectiveness. Studies on clinical predictors explain only a small proportion of the overall variation in treatment response, suggesting that genetic factors may be an important determinant of response to anti-VEGF agents.
The overarching aim of this project was to identify genetic risk factors for DME susceptibility and treatment response, divided into three specific aims: 1) identify genetic determinants of DME risk; 2) identify clinical and genetic determinants of anti-VEGF treatment response in DME; and 3) assess the difference in anti-VEGF treatment outcomes in DME compared to neovascular age-related macular degeneration (nAMD) and retinal vein occlusion (RVO) in a real-world clinical setting.
The first aim, to identify genetic risk factors for DME, used genome-wide association analyses of DME-related phenotypes in a combined cohort of Type 1 and Type 2 diabetes with 500 cases (Diabetes with DME) and 599 controls (Diabetes without DME) from Australia (Discovery cohort). The study identified several interesting suggestive risk loci on chromosome 6 (lead SNP, rs2817108, p = 1.55 × 10-6 ) and chromosome 4 (lead SNP, rs76040981, p = 4.89 × 10^-6 ). Replication in a larger independent European cohort from the United Kingdom (Cases = 1951, Controls = 6541) did not demonstrate strong replication; however, the associations were nominally significant, p < 0.05. Meta-analyses of the two cohorts revealed several additional suggestive loci approaching genome-wide significance. A sub-analysis of severe DME cases in the Australian cohort (Cases = 143, Controls = 599) revealed five suggestive loci, while an analysis restricted to participants with data available on vision used as a quantitative trait (N = 474) identified a novel locus on chromosome 10 (rs59697531) with promising p value p = 6.59 × 10^-8 ). Despite a relatively large cohort, no significant loci were identified; this highlights the genetic complexities of DME and the unmet need for further larger studies.
The second aim of this thesis was to investigate the clinical and genetic determinants of anti-VEGF treatment response in DME. Treatment responses broadly classified as functional (vision-related) and anatomical (central macular thickness related) were analyzed as quantitative and categorical traits using linear and logistic regression models, adjusting for appropriate covariates. Overall, there was a significant improvement in best-corrected visual acuity (BCVA) and reduction in central macular thickness (CMT) at the end of 12 months, p < 0.05. However, a substantial proportion of individuals lost vision (20.96%) or showed suboptimal reduction in CMT (40.72%). Baseline BCVA and CMT, an early response at four months, duration of DR, and anti-VEGF injection sub-type (aflibercept compared to other injection type) were significant predictors across several definitions of response. The clinical predictors explained a maximum of 23.09% of functional response and 52.33% of anatomical response; thus, a large proportion of variation in response remained unaccounted for.
For the genetic determinants associated with anti-VEGF treatment response, we undertook a genome-wide association analysis on 220 DME patients (European ancestry) treated with anti-VEGF therapy using genome-wide SNP array data. Two loci reached genome-wide significance (p < 5 × 10^-8 ) for association with change in CMT post-treatment: a single SNP on chromosome 6 near cancer susceptibility candidate 15 (CASC15) gene (rs78466540, p = 1.16 × 10^-9 ) and a locus on chromosome 12 near RP11-116D17.1 (lead SNP rs11614480, p = 2.69 × 10-8 ). Four loci were significantly associated with change in BCVA: two loci on chromosome 11, downstream of the neurotrimin (NTM) gene (lead SNP rs148980760, p = 5.30 × 10^-9 ) and intronic in RP11-744N12.3 (lead SNP rs57801753, p = 1.71 × 10^-8 ); one near phosphoglycerate mutase 1 pseudogene 1 (PGAM1P1) on chromosome 5 (rs187876551, p = 1.52 × 10^-8 ); and one near TBC1 domain family member 32 (TBC1D32) on chromosome 6 (rs118074968, p = 4.94 × 10^-8 ). In silico investigations of each locus identified multiple expression quantitative trait loci (eQTL) and potentially relevant candidate genes warranting further analysis. Given the need for longitudinal data with 12 months follow-up, achieving a sufficient sample size for these types of analyses is challenging. Therefore, through collaboration, this study aims to further explore and replicate these findings. The third and final aim was to investigate anti-VEGF treatment outcomes in other macular disorders (nAMD and RVO) compared to DME. At 12 months, changes in BCVA and CMT were significantly lower for DME (median change = 2 ETDRS letters and -54 microns, respectively). Baseline BCVA was a negative predictor, and baseline CMT a positive predictor of outcome under both univariable and multivariable model, p < 0.05. After adjusting for baseline BCVA and CMT, diagnosis of nAMD or RVO was a predictor of visual improvement compared with DME. These findings highlight the necessity of more intensive treatment strategies and better monitoring of DME patients. In conclusion, this dissertation comprehensively investigated genetic variation in DME susceptibility and clinical and genetic predictors of treatment response in a cohort of Australian patients. As expected, this thesis has further highlighted the complexities behind DME pathogenesis and treatment. Although genetic testing is gaining attention in clinical settings, there are few genetic studies on DME. Studies like the one described here aim to directly address the importance of genetic testing in the clinical setting. In addition, the genetic study of anti-VEGF response is the first step towards personalizing and optimizing treatment for DME patients based on pharmacogenetics.