Serum apolipoprotein A1 and B are associated with 6-month persistent and incident diabetic macular oedema in type 2 diabetes

Introduction

Diabetic retinopathy (DR) is the most frequent diabetic microvascular complication causing irreversible visual loss among individuals with diabetes.1 2 Importantly, diabetic macular oedema (DMO) is a common feature of DR responsible for most of visual impairment in persons with diabetes.1–3 While DMO was often considered as presenting in more severe form of DR, recent studies have documented that DMO can also be present solely in less severe DR.4–7 Studies have suggested that DMO was present in nearly 10% of individuals with type 2 diabetes and may develop in 25% of individuals with type 2 diabetes within 10 years of follow-up.4 5

Serum lipids have been considered as one of the major risk factors for DMO, documented in multiple clinical studies.8 9 It has been suggested that in individuals with diabetes, extended dyslipidaemia, along with its complex interactions with other major diabetes risk factors such as long diabetes duration, poor glycaemic and blood pressure control, may exaggerate diabetes-related retinal microvascular injury and cause breakdown of the blood-retinal barrier (BRB).6 7 BRB breakdown may further cause extravascular accumulation of fluid and lipoprotein deposits particularly manifested at the macular region and clinically appeared as DMO.6–8

In addition to standard serum lipid profile, which normally consists of cholesterol, triglycerides, high-density (HDL) and low-density lipoproteins (LDL), there has been an emerging interest in serum apolipoprotein A1 (ApoA1) and B (ApoB). ApoA1 is the main component of HDL and ApoB is found in LDL, intermediate-density (IDL) and very low-density lipoprotein (VLDL).7 8 10 11 Previous studies have reported cross-sectional associations between serum ApoA1 and B with DR and DMO and further suggested that the roles of ApoA1 and ApoB in the mechanisms of DMO are different than standard lipid profile.10 11 ApoA1 is a transporter of lipid particles from peripheral tissue, including retina, to the liver and ApoB sends these particles from liver back to the tissue.10 We speculated that dysregulation of apolipoproteins A1 and B therefore may amplify the accumulation of lipid particles in the retinal vasculature, which escalate the magnitude of BRB breakdown and persevering DMO regardless of treatment. However, there is scarcity of prospective data regarding serum apolipoproteins and DMO.5 10–12

In this study, we investigated the associations of serum ApoA1 and ApoB levels measured at baseline and their changes with persistent and incident DMO after 6 months of follow-up in patients with type 2 diabetes.

Methods

This was a clinic-based prospective cohort study. We consecutively recruited 53 untreated patients with type 2 diabetes mellitus (DM) and DR, with or without DMO, aged >30 years between March and October 2021 from Jakarta Eye Center (JEC) Primasana Hospital, Jakarta, Indonesia. We excluded eyes with proliferative DR (PDR) and vitreous bleeding and/or tractional retinal detachment. We also excluded patients who has a history of liver disease, kidney failure, malignancy, myocardial infaction or stroke in the past 3 months; or had any infections in the past 6 weeks. Written informed consent was obtained from each participant.

Results

There were initially 53 eligible participants (89 eyes) recruited at baseline. However, 10 participants died, and 5 were severely ill due to COVID-19 during the study period, leaving 38 participants (62 eyes) included in the final analyses, 42 eyes (67.7%) with DR without DMO and 20 eyes (32.3%) with DMO at baseline (online supplemental table 1). There were six participants at baseline (15.8%) who had DMO in one eye and none in the other eye. Characteristics of dropped-out participants were presented in online supplemental table 2. Table 1 shows the baseline characteristics of participants/eyes included in the final analysis. Male gender was more common in the DMO group than in the non-DMO group (73.3% vs 39.5%, p=0.03). Furthermore, group with DMO had longer diabetes duration (8.2 years vs 5.6 years), worse visual acuity, higher total cholesterol level, lower total body fat and higher resting metabolic rate at baseline than non-DMO group. Other baseline lipid profiles and serum apolipoprotein levels were similar between DMO and non-DMO groups.

Table 2 shows the baseline characteristics of participants who had persistent/incident DMO, regressive DMO and without DMO at 6 months of follow-up. Individuals with persistent/incident DMO had significantly higher baseline ApoB levels compared with those with regressed or no DMO at 6 months (165±26.9 mg/dL in persistent/incident DMO vs 128±23.6 mg/dL in regressed DMO vs 110±26.7 mg/dL in no DMO; p<0.01). Similar finding was also observed for ApoB/A1 (1.38±0.3 in persistent/incident DMO vs 0.91±0.1 in regressed DMO vs 0.70±0.2 in no DMO; p=0.01). No significance differences were observed for all the other parameters.

Table 3 shows the associations between lipids and apolipoproteins with persistent/incident DMO at 6 months. After adjusting for baseline age, gender, duration of diabetes and SBP, higher quartile of ApoA1 was associated with significantly lower risk of persistent/incident DMO at 6 months compared with highest quartile of ApoA1 (p value for trend 0.021), while higher ApoB/A1 was significantly associated with higher risk of persistent/incident DMO at 6 month (OR 1.99; 95% CI 1.03 to 3.87; p value 0.04). Other lipid parameters (total cholesterol, HDL, LDL and triglycerides) were not associated with persistent/incident DMO at 6 months.

Table 4 shows the associations between 6-month changes in apolipoprotein levels with persistent/incident DMO at the end of the study. Every 10 mg/dL increase in ApoA1 levels was related to lower risk of persistent/incident DMO (OR 0.69; 95% CI 0.49 to 0.92; p value 0.016), while every 0.2 increase in ApoB/A1 was significantly associated with higher risk of persistent/incident DMO (OR 1.4; 95% CI 1.1 to 1.9; p value 0.013) at the end of the study. These associations remained significant after controlling for baseline age, gender, duration of diabetes and SBP.

Discussion

This prospective study demonstrated that individuals with diabetes with higher ApoA1 at baseline had lower risk of persistent/incident DMO and those with higher ApoB/A1 had higher risk of persistent/incident DMO after 6 months of follow-up. Increasing ApoA1 level was also significantly associated with lower risk of persistent/incident DMO, while increasing ApoB/A1 ratio was associated with higher risk of persistent/incident DMO at 6 months despite any treatment, independent of baseline age, gender, duration of diabetes, SBP and HbA1c. Findings from this study suggest that serum apolipoproteins, particularly ApoA1 and ApoB/A1 levels, might be important risk factors for DMO and that changes in ApoA1 and ApoB/A1 might also be predictors of persistent/incident DMO despite any treatment.

There is scarcity of data regarding serum apolipoproteins and DMO.5 10–12 Previous studies have mostly reported only cross-sectional associations of serum apolipoproteins with presence of DR.5 10 11 18 19 ApoA1 was inversely associated with presence and severity of DR, while ApoB and ApoB/A1 have shown their positive relationships with both presence and severity of DR.10 20 21 In this study, we did not find significant associations between serum apolipoproteins and presence of DR at baseline because we only included patients with DR. Nevertheless, our study findings provide additional knowledge in the existing literature that serum apolipoproteins were prospectively associated with DMO.

Associations of ApoA1 and ApoB/A1 with persistent/incident DMO after 6 months fitted well with pathophysiological process of DMO. The central mechanism in the pathophysiology of DMO is breakdown of blood-retinal barrier (BRB).6 7 18 BRB is a biological unit mainly formed by tight junction of retinal vascular endothelial (RVE) cells, network of glial cells and retinal pigment epithelium, which preserve low permeability milieu at the retina.6 7 18 In diabetes, there is evidence of RVE cells loss resulted in the disruption of BRB, increased permeability and movement of fluids across BRB, and finally fluid accumulation in the inner retina.6 7

The mechanism of BRB breakdown is complex and highly influenced by long diabetes duration, poor glycaemic control and other systemic profiles including dyslipidaemia.6 7 Serum apolipoproteins, which are closely related to lipids circulation, are speculated to play important role in the retinal vascular damage.10 Apolipoproteins bind lipid particles to form lipoprotein and transport these lipoproteins in the blood and cerebrospinal fluid, and are key in lipid uptake and clearance in peripheral tissue.22 23 ApoA1, which is the main component of HDL, is important in transporting lipids from tissue to the liver for degradation process.24 25 Moreover, ApoA1 is also associated with decreased production of some important proinflammatory cytokines (ie, tumour necrosis factor, interleukin 1β and interleukin 6) and may downregulate vascular endothelial growth factor, hence showing vasoprotective effects in small vessels.10 24 25 Meanwhile, ApoB is the main apolipoprotein in chylomicrons, VLDL, LDL and IDL which transport cholesterol from the liver to tissues. ApoB is also known to be responsible for inflammatory and atherosclerotic process.26 These explain that higher ApoA1 level in some individuals may link to less atherogenic process at the retinal vasculature, less severe RVE cells loss and less BRB damage than those who had lower ApoA1, manifested as lower risk of persistent/incident DMO. At the same time, individuals with higher ApoB/A1 may have more dysregulated lipid circulation resulting in more lipoprotein deposition at the retinal vasculature, resulting in more severe BRB breakdown and thus higher risk of persistent/incident DMO.10 27

There are strong implications of this study findings. Our data showed no associations of standard lipid profile with DMO. This was in line with previous study showing stronger associations of serum apolipoproteins with DR than standard lipid profile, strengthen the notion that serum apolipoproteins were possibly more important measures than standard lipid profile in the context of DR/DMO. In addition, our study also demonstrated that changes in ApoA1 were associated with lower risk of DMO and changes in ApoB/A1 were associated with higher risk of DMO at the end of study, despite anti-VEGF treatment. Importantly, individuals with higher ApoA1 at baseline were likely to have their DMO regressed at the end of the study (online supplemental table 3). These suggest that serum apolipoprotein measures might be used for clinical marker to identify those who will likely have successful outcomes of anti-VEGF treatment and that improvement of serum apolipoprotein level may be necessary to yield good treatment outcome.

The strengths of this study include its prospective design, assessment of DMO using SD-OCT verified by retinal specialist and repeated measurement of serum apolipoproteins at baseline and at the end of study. However, some limitations were also present. First, the proportion of patient who did not return for follow-up was considerably high due to COVID-19 morbidity and mortality (28.4%). Our analyses showed that proportion of DMO in group with completed follow-up and dropped out was not significantly different. Individuals who failed to complete the follow-up had significantly higher SBP, lipid profiles and serum ApoA1 and ApoB levels compared with those who completed the follow-up (online supplemental table 2). While this may have induced bias, this difference is less likely to diminish the associations or change the direction of associations. In fact, because dropped-out participants had higher serum lipids and apolipoproteins, if they were included in the final analysis, the associations between serum apolipoproteins with DMO could have been stronger. We have attempted to contact the patient and offer home pick-up; however, COVID-19 restriction had become significant disadvantage for further effort. Second, due to small number of participants, we were unable to split individuals with DMO progression in the separate analysis and we were also unable to estimate DMO risk measures for those with better apolipoprotein profiles at the end of the study. Lastly, the overall follow-up time might be less sufficient to observe the long-term effect of apolipoprotein levels on clinical DMO. Also, there was some information bias because we did not know exactly the history of DMO before our baseline or how long has the DMO been present before the baseline examination. Chronic DMO may respond less optimally to treatment. Further cohort study with larger sample size and longer follow-up period is needed to eliminate this information bias as well as to conclude the predictive value of apolipoproteins in patients with DR/DMO.

In conclusion, findings from our study demonstrated that persons with diabetes with higher ApoA1 at baseline had lower risk of persistent/incident DMO than those with lower ApoA1. Meanwhile, individuals with diabetes with higher ApoB/A1 showed higher risk of persistent/incident DMO after 6 months of follow-up than those who had lower ApoB/A1. Increasing ApoA1 level was associated with lower risk of persistent/incident DMO, while increasing ApoB/A1 ratio was associated with higher risk of persistent/incident DMO at 6 months despite anti-VEGF. While larger prospective study is needed to confirm, our data provide initial evidence that serum ApoA1 and ApoB/A1 levels may be important risk factors for DMO and that changes in ApoA1 and ApoB/A1 might also be predictors of persistent/incident DMO despite anti-VEGF treatment.