Results of a study analysing cytokine levels in the aqueous humour and serum of patients with nonproliferative diabetic retinopathy with and without diabetic macular oedema support further research investigating transforming growth factor-β-induced Gene Human Clone 30 (BIGH3) as a potential biomarker for DMO.
BIGH3 (transforming growth factor-β-induced Gene Human Clone 30) is a promising serum biomarker for diabetic macular oedema (DMO) in patients with nonproliferative diabetic retinopathy (NPDR), according to Dr Andric Perez-Ortiz, who presented research at the Association for Research in Vision and Ophthalmology 2020 virtual annual conference. Dr Perez-Ortiz and colleagues conducted a case-control study analysing levels of BIGH3, transforming growth factor-β1 (TGF-β1) and transforming growth factor-β2 (TGF-β2) in the aqueous humour and serum of patients with DMO and unaffected controls.
The results showed that BIGH3 was the only cytokine that was present in significantly different concentrations in both fluids in DMO patients compared with controls, and the only cytokine with a positive significant correlation between its level in aqueous humour and serum. None of the three cytokines, however, correlated with macular thickness or optical coherence tomography (OCT) phenotype among patients with DMO.
According to Dr Perez-Ortiz, studies investigating expression and levels of these cytokines have all been performed in vitro. “The aim of our study was to measure the concentrations of TGF-β1, TGF-β2 and BIGH3 in aqueous humour of patients with NPDR—with and without DMO—and to assess their clinical applicability as serum markers by testing their correlation in aqueous humour and serum, and association with macular thickness and DMO phenotype,” said Dr Perez-Ortiz, an associate professor in the Department of Public Health at Universidad Panamericana in México City, Mexico and research fellow, Department of Surgery, Massachusetts General Hospital, Boston, Massachusetts, United States.
“The predictive capacity of BIGH3 certainly warrants further study,” he said. “Especially its sensitivity and specificity and/or predictive use as an early biomarker for DMO development.”
Interest in exploring the potential for TGF-β1, TGF-β2 and BIGH3 to serve as biomarkers for DMO is based on understanding of their role in the pathogenesis of DR and DMO. Dr Perez-Ortiz explained that pericyte dropout and increased inflammation and activation of proinflammatory transcription factors that induce monocyte and macrophage infiltration occur early in the pathogenesis of these diabetic eye diseases. Macrophages predominantly secrete TGF-β1 and TGF-β2 that induce apoptosis of retinal pigment epithelium (RPE) and upregulation of the gene encoding for BIGH3.
BIGH3 is an extracellular matrix protein that can act in an autocrine or paracrine manner to induce apoptosis of RPO cells, pericytes and retinal endothelial cells in a time- and dose-dependent fashion. The presence of BIGH3 has been observed in the histopathologic retinal section of a postmortem globe with documented NPDR deposited predominantly around retinal vessels and occasionally in choroidal vessels. “Hence it has been hypothesised that monocyte/macrophage-induced BIGH3 expression is an important progressive step in the pathophysiology of pericyte loss,” Dr Perez-Ortiz said.
The study investigating levels of the TGF-β cytokines and BIGH3 in patients with NPDR included adults aged 18 years and older with type 2 diabetes mellitus who were undergoing a surgical procedure (initiation of intravitreal injection for DMO patients and cataract surgery for controls). It included 26 patients with DMO and 27 patients without DMO.
Patients were excluded if they had systemic inflammatory disease or a history of any vitreous procedure. The aqueous humour sample of approximately 1 µL was collected in all eyes prior to any surgical intervention, and the cytokine levels in serum and aqueous humour were quantified with cytometric-bead assays.
The two groups were similar in mean age (~ 63 years) and sex distribution, but the average time since diabetes diagnosis was significantly longer in the DMO patients than in the controls. As would be expected, maximum central macular thickness (CMT) was also significantly greater in the cases compared with controls. Among patients with DMO, cystic changes and spongiform changes were the most common phenotypes identified on OCT.
The cytokine analyses showed that the aqueous humour concentration of TGF-β1 was significantly greater in the DMO eyes compared with the controls, whereas the reverse relationship was observed in serum. TGF-β2 also was present in a significantly higher concentration in the aqueous humour of the cases compared with controls, but its concentration in serum did not differ significantly between the two groups.
Further analyses showed that the ratio of TGF-β1 in aqueous humour serum was disproportionately elevated in the DMO cases compared with controls. In addition, a significant positive correlation was found in the DMO group between aqueous humour concentration and maximal CMT. The correlation between serum concentration and maximal central macular thickness was negative for TGF-β1 and positive for TGF-β2.
The concentration of BIGH3 was significantly higher in both serum and aqueous humour in the DMO group compared with controls. The ratio of BIGH3 concentration in aqueous humour to serum was not significantly different comparing cases and controls, and it did not correlate with CMT or OCT subtypes in the eyes with DMO.
Andric C. Perez-Ortiz, MD, MPH
Dr Perez-Ortiz has no relevant financial interests to disclose.