Incidental Finding: Urine Alzheimer-Associated Neuronal Thread Protein (AD7c-NTP) and Serum Vitamin D25

INTRODUCTION

Alzheimer’s disease (AD) is one of the most devastating conditions affecting elderly in both Western and Eastern World [1, 2]. It is a chronic progressive neurodegenerative disease for which there is no effective treatment. National and international experts have been attempting to find diagnostic tools for the early identification of symptoms [3] to facilitate early identification and intervention of the disease [4]. Both urine Alzheimer-associated neuronal thread protein (AD7c-NTP) and serum 25-hydroxyvitamin D (25(OH)D) are factors in the evaluation of AD [5–7]. However, no correlation study between the two indicators has been reported in China or abroad. In our study to assess the value of urine AD7C-NTP and serum 25(OH)D for the diagnosis of AD, a correlation between the two indicators was found. This study investigated the correlation between urine AD7C-NTP and serum 25(OH)D and their diagnostic value for the diagnosis of AD.

METHODS

The diagnosis of AD was based on the Mini-Mental State Exam (MMSE) and Montreal Cognitive Assessment (MoCA) questionnaires. Subjects over 50 years old who were willing to undergo the MMSE and MoCA surveys and had complete physical examination data in our hospital were enrolled in the present study.

The study was approved by Ethics Committee of the Taihu Sanatorium of Jiangsu Province (No. YXLL16001; No. YXLL17001). Written informed consent was obtained from all participants.

The exclusion criteria were: 1) subjects on a large dose of antipsychotic medication long-term and a history of severe head trauma; 2) patients with medical diseases involving the functioning of the central nervous system; 3) patients with severe mental illness; 4) patients who failed to cooperate with the cognitive function assessment.

The detection method of AD7C-NTP was as follows: A volume of 5 ml of midstream urine was collected in the morning and assayed by enzyme-linked immunoassay (ELISA). A Swiss Tecan automatic enzyme immunoassay system was used, and the reagents were purchased from Shenzhen Anqun Biological Engineering Co.

The 25(OH)D assay was as follows: A volume of 5 ml of venous blood was drawn from the upper arm under a fasting condition in the early morning. The electrochemiluminescence (ECL) method was used for the assay with a Roche E601 automatic immunoassay analyzer with ECL technology made in the USA. The reagents were purchased from Roche Diagnostics GmbH in Germany.

Correlation of urine AD7C-NTP with serum 25(OH)D was analyzed by applying Spearman’s rank correlation test and Multiple stepwise regression analysis. A p-value of <0.05 was considered as significant correlation. Descriptive statistics were obtained for the variables where applicable, using SPSS version 16.0.

RESULTS

Among the subjects, one group had normal MMSE and MoCA scores and no clinical evidence of AD-related issues. The subjects were aged 51–91 years. Of the 50 subjects enrolled, 35 were male and 15 were female. It was unexpectedly found by Pearson correlation analysis that a negative correlation existed between urine AD7C-NTP and serum 25(OH)D.

Spearman’s rank correlation test

The correlation coefficient r was –0.460, with p = 0.001 (refer to Fig. 1 for details). Table 1 summarized the baseline characteristics of the subjects.

OPEN IN VIEWER

Fig. 1

The correlation between urine AD7C-NTP and Serum 25-OH-D.

OPEN IN VIEWER

Table 1

Baseline characteristics of the subjects

Variable
Age, mean (SD), y	66.06±7.79
Male, n	35
Female, n	15
AD7C-NTP, mean (SD), ng/mL	0.71±0.33
serum 25(OH)D, mean (SD), ng/mL	21.07±8.46

DISCUSSION

The results of the present study were encouraging. Levy et al. [8] found that ELISA for the detection of urine AD7c-NTP had a sensitivity of 90% and a specificity of 91% for the diagnosis of AD. Several studies have also suggested that urine AD7c-NTP may vary in different age and gender groups and the combination with other markers can improve the diagnostic accuracy for AD.

The transmembrane phosphoprotein AD7c-NTP has a molecular weight of 41 kD and is expressed only in neurons. It is abundantly present in neuronal fiber tangles in the brains of patients with AD, but not in other organs [9, 10]. The expression levels of AD7c-NTP were elevated only in the brain tissue, cerebrospinal fluid, and urine of patients with AD, and were positively correlated with the severity of AD [11]. We questioned why urinary AD7c-NTP would be negatively correlated with serum 25(OH)D, and what exactly the connection between them could be.

Serum 25(OH)D has diverse and complex physiological functions. For example, vitamin D may lower blood lipids by activating the vitamin D receptor and inhibiting the adipose storage effect of fat. Vitamin D may also promote intestinal calcium absorption and reduce blood lipid levels by combining fatty acids with calcium ions to form fatty acid calcium in the body [12]. Vitamin D also inhibits the formation and secretion of triglyceride in the liver [13] and regulates insulin secretion and reduces insulin resistance [14]. Increased insulin levels are important for regulating lipids.

Currently, few studies on the correlation between the urine AD7c-NTP levels and chronic diseases have not found a correlation between urine AD7c-NTP levels and the above chronic diseases or metabolic states. The above metabolic indicators do not serve as a connection for the correlation between the two indicators. However, as far as the evidence suggests, vitamin D may also play an effective role in protecting neurons by inhibiting pro-inflammatory factors involved in acetylcholine synthesis. The reduced level of 25(OH)D and inability for its role to be fully effective affects cognitive functioning. It was speculated that cognitive functioning might be the only bridge between the two indicators so far.

Zhao [15] reported that serum 25(OH)D3 levels were reduced in patients with AD, and monitoring the levels of serum 25(OH)D3 might have clinical significance in predicting the inflammatory stress response and oxidative stress status in patients with AD. However, an intervention study on the cognitive function and peripheral inflammatory factors in patients with AD by Jia [16] found that vitamin D3 supplementation (800 U/d) for 12 months improved the subjects’ cognitive functions and reduced peripheral blood inflammatory factor levels. Presumably, although the exact mechanism of these interventions remains unclear, the results of such interventions at least offer hope to patients with AD, provided, of course, that AD could be detected and diagnosed at an early stage.

The small sample size and cross-sectional survey-based design meant that a causal relationship between the two indicators could not necessarily be indicated. However, we hoped that the present result could be acknowledged by more researchers, and it is expected that a larger sample size and better-designed study would replicate the results of the present study as well as provide further investigation of the potential value.