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Approximately 50% of patients with diabetes (DM) will eventually develop neuropathy. Large-scale studies have demonstrated that the regulation of blood glucose levels aids in the prevention and progression of diabetic peripheral neuropathy (DPN), but glycemic control does not impact established neuropathy or its severity (1). Causative factors also include microvascular insufficiency, defective neurotrophism, abnormal lipid metabolism, and oxidative and nitrosative stress (2). A key component of nitrosative stress, nitrotyrosine (NT), has multiple cytotoxic effects. Accumulation of NT has been identified in several neural structures in both rodents and humans (3), reflected by raised NT concentrations in the plasma of subjects with DM (4).
The aim of this study was to evaluate the relationship between NT concentrations with the presence and severity of possible/probable (PP-DM-DPN) and established (DM-DPN) neuropathy in subjects with DM compared with healthy control subjects (HCs). The study population consisted of 49 patients. Patient demographics are presented in Table 1. Subjects with DM were further stratified based on the presence of neuropathy using the current Toronto Expert Panel guidelines (5). Individuals were assessed using clinical neuropathy scores and nerve conduction studies (NCS) to measure nerve conduction velocities (CV), latencies, and amplitudes of specific nerves.
NT was found in the serum of all patients with significant mean differences in concentrations between all DM groups when compared with HCs; specifically, DM-DPN (P = 0.006), PP-DM-DPN (P = 0.028), and diabetic patients without DPN (DM–Non-DPN) (P = 0.046). Total neuropathy scores (TNS) were significantly higher in the neuropathy groups, as shown in Table 1. Significant differences were found for all three subcategories of the TNS (motor, sensory, and symptom scores) (Table 1). A positive correlation was found between NT concentrations and TNS (r = 0.676, P = 0.022) and symptom score (r = 0.673, P = 0.026) in the PP-DM-DPN.
Significant correlations also were found between NT concentrations and peroneal nerve ankle amplitude (r = −0.408, P = 0.018), above fibula amplitude (r = −0.375, P = 0.035), and above fibula CV (r = −0.486, P = 0.005) for the DM groups. Moreover, NT levels correlated significantly with NCS in the DM-DPN for peroneal nerve CV (r = −0.656, P = 0.0005) and peroneal nerve amplitude (r = −0.491, P = 0.01).
The findings in this study show that the circulating levels of NT are significantly raised in patients with DM when compared with HCs, with the strongest correlations seen with clinical and objective measures of neuropathy severity in patients with PP-DM-DPN and DM-DPN. This suggests that NT and nitrosative stress could play a role in the development of DPN and could provide a biomarker for DPN progression. These findings concur with previous animal and clinical studies demonstrating that nitrosative stress seems to have the largest impact in patients during the earlier stages of metabolic disease (2,4,6) and seems to be a potential target for intervention. This is a small, cross-sectional observational study, and larger scale trials are needed to determine longitudinal predictive value and the impact of the reduction of NT in progression and severity of neuropathy.
Duality of Interest. No potential conflicts of interest relevant to this article were reported.
Author Contributions. All authors contributed significantly to this study and are in agreement with the content of the article. J.F.E. and C.M.C. contributed to the conduction of the study, analyzed and interpreted the data, and wrote the manuscript. H.K.P. and A.I.V. contributed to the design and conduction of the study and wrote the manuscript. I.G.O. contributed to the design and conduction of the study. M.Y. contributed to the design and conduction of the study and reviewed the manuscript. A.I.V. is the guarantor of this work and, as such, had full access to all the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.
Prior Presentation. Preliminary versions of this study were presented as posters at the 74th Scientific Sessions of the American Diabetes Association, San Francisco, CA, 13–17 June 2014, as well as the 24th Annual Scientific Meeting of NEURODIAB, Sopron, Hungary, 12–14 September 2014.