Urinary cytidine as an adjunct biomarker to improve the diagnostic ratio for gastric cancer in Taiwanese patients
Running Title: Urinary cytidine as an adjunct biomarker in gastric cancer
Wan-Yu Lo1,2,3, Long-Bin Jeng4, Chien-Chen Lai5, Fuu-Jen Tsai1,6*, Chiung-Tsung Lin7, 8, , William Tzu-Liang Chen4,9*
1Department of Medical Research, China Medical University Hospital, Taichung, Taiwan 2Graduate Institute of Integrated Medicine, China Medical University, Taichung, Taiwan 3 Department of Life Science, National Chung Hsing University, Taichung, Taiwan 4 Department of Surgery, China Medical University Hospital, Taichung, Taiwan 5Institute of Molecular Biology, National Chung Hsing University, Taichung, Taiwan
6Graduate Institute of Chinese Medical Science, China Medical University, Taichung, Taiwan 7Department of Laboratory Medicine, China Medical University Hospital, Taichung, Taiwan 8Department of Biomedical informatics, Asia University, Taichung, Taiwan
9Division of Colorectal Surgery, Department of Surgery, China Medical University Hospital, Taichung, Taiwan
(*Equal contribution as corresponding author)
*Correspondence to: Fuu-Jen Tsai
Department of Medical Research, China Medical University Hospital, Taichung, Taiwan Tel: +886-4-22053366 ext.3513
E-mail: d0704 @mail.cmu h . org .tw
Acknowledgments This study was supported by the Research laboratory of pediatrics, Children’s
Hospital, China Medical University and funded by a grant from the China Medical University Hospital (DMR-CS-003-102)
Abstract:
Background: Gastric cancer is a major public health concern as the fourth most common cancer,
and it is of particular relevance as the second most common cause of cancer death worldwide. We caparisoned the urinary nucleoside levels between the gastric patients and healthy volunteers that try to evaluate the diagnostic value in the gastric cancer.
Method: Urinary nucleosides from 49 gastric patients and 40 healthy volunteers were evaluated by
high-performance liquid chromatography/electrospray ionization–tandem mass spectrometry (HPLC/ESI–MS/MS) under optimized conditions as determined in our previous study.
Results: The mean levels of 5 urinary nucleosides, cytidine, 3-methylcytidine (m3C),
1-methyladenosine (m1A), adenosine, and inosine, were found to be elevated in cancer patients, but only cytidine showed a significant elevation. Moreover, cytidine levels were significantly elevated by an average of 1.42-fold in patients with late stage (S3 + 4) disease. Combining the determined levels of preoperative serum alpha-fetoprotein (AFP, cutoff of 20 µg/L) or carbohydrate antigen 19-9 (CA119-9-19-9, cutoff of 37 U/mL) with the mean urinary cytidine level was shown to improve the diagnostic ratio (sensitivity) for gastric cancer from 16.3% (8/49 patients) to 38.8% (8 + 11/49 patients) or from 28.6% (14/49 patients) to 51.0% (14 + 11/49 patients), respectively.
Introduction
Gastric cancer is a major public health concern as the fourth most common cancer, and it is of particular relevance as the second most common cause of cancer death worldwide [1]. The highest incidence rates are found in East Asia, East Europe, and South America [2]. Unfortunately, most gastric cancer patients are diagnosed at an advanced stage of the disease at which point tumor resection may not be an option. A lot of patients with advanced or recurrent gastric cancer, it is clear that the discovery of biomarkers and their application to traditional diagnostic methods would be of value to prevention and treatment strategies.
Over the last decade, systems biology has developed into a new research platform, which currently occupies a prominent position in biomedical research. Other branches of systems biology, for example, transcriptomics, proteomics, and metabolomics have gained prominence as discovery tools since the completion of the genome sequencing project. The power of metabolomics has been applied to toxicological studies [3], the diagnosis of inborn metabolic errors [4], and to biomedicine for the diagnosis of amyotrophic lateral sclerosis [5]. Moreover, metabolomics is becoming an increasingly important tool for cancer diagnosis [6] and for prediction of cancer progression in response to particular therapeutic approach [7].
Nucleosides are the primary constituents of ribonucleic acids (RNAs). When RNAs are biotransformed, normal nucleosides can either be metabolized or reutilized to synthesize nucleic acids. However, in particular cases, some RNAs are transformed to modified nucleosides, which
can neither be further degraded nor reutilized. These nucleosides are excreted intact in urine as end products because of a lack of specific phosphorylases [8]. Modified nucleosides are regarded as indicators of the whole-body turnover of RNA. In cancer, which are characterized by unregulated cell proliferation, RNA metabolism increases dramatically and higher concentrations of urinary excreted modified nucleosides are observed. Consequently, the urinary levels of modified nucleosides can reflect RNA degradation in the organism; thus, they can be used as potential cancer biomarkers [9-11]. Nevertheless, to date, no specific pattern has been discovered. The search for specific biomarkers for specific cancers is crucial for early cancer diagnosis. In many cases, efficient separation and detection techniques are required to assess the levels of these biomarkers.
Although the levels of these compounds have been studied over a number of decades, it is only recently that mass spectrometric means have been employed for the diagnosis of diseases [12]. During the last decade, a number of analytical methods for measuring and monitoring nucleosides in biological fluid have been reported. Some of these methods include the following: enzyme-linked immunoassay [13], capillary electrophoresis (CE) [14], cathodic stripping [15], voltammetry, gas chromatography–mass spectrometry (GC–MS) [16-18], and high-performance liquid chromatography–mass spectrometry (HPLC–MS) [17, 19, 20]. Although the sensitivity and specificity of these methods are high, they involve complex preparation processes for extraction, hydrolysis, and derivatization. Thus, to simplify the process, liquid chromatography–tandem mass spectrometry (LC–MS/MS) was developed as a method of directly determining urinary nucleosides.
The co-eluted nucleosides from HPLC were detected by MS, and the selective reaction monitoring (SRM) mode used in this study improved the specificity and sensitivity for quantitation.
In this study, we used HPLC/electrospray ionization–MS/MS (HPLC/ESI–MS/MS) to detect the levels of 5 urinary nucleosides [cytidine, 3-methylcytidine (m3C), 1-methyladenosine (m1A), adenosine, and inosine] in urine samples from patients with gastric cancer and from healthy control subjects. The variable urinary nucleoside levels were determined and evaluated for diagnosis of primary gastric cancer.
Materials and Methods
Patient details
From January to December 2008, 49 patients with primary gastric cancer, which had been treated by resection at the Department of Surgery, China Medical University Hospital, were evaluated in this study. None of the patients had undergone treatment with medication or radiotherapy prior to this study. The control group (40 healthy volunteers) had undergone a routine annual health examination and was recruited from our Health Examination Center. Patients and healthy volunteers were asked to provide single, early-morning urine samples (preoperative samples). The samples were immediately sent to the laboratory and stored at −80 °C until analysis. Use of the urine samples for research purposes complied with the regulations set by the Institutional Review Board (DMR-IRB 97-029). The study was approved by the Ethical Committee of the China Medical University Hospital. The main characteristics of the cancer and control groups are reported in Supplemental data 1. There were no significant differences in body weight, body mass index
(BMI), smoking, age, or sex between the patients and control subjects. Urine samples and purification
Each urine sample was acidified by the addition of 2 mol/L HCl (adjusted to 0.01 mol/L HCl). The acidified urine was centrifuged, one milliliter of the supernatant was added to 100 μLof the internal standard (ISTD; tubercidin 2 μg/mL), and purified using an Oasis® MCX column (Waters, Milford, MA, USA) that had been conditioned and equilibrated with 1 mL methanol and water. The sample was directly loaded onto the MCX columns, washed with H2O (0.1% formic acid in H2O), eluted with 2.8% NH4OH in methanol, dried under a nitrogen stream, and dissolved in the mobile phase (100 μL).
Chemicals
The nucleosides under analysis in this study were obtained from Sigma–Aldrich (St. Louis, MO, USA): adenosine, cytidine, inosine, m3C, m1A, and tubercidin. Each nucleoside stock solution was prepared at a concentration of 100–1000 µg/L in a mixture of methanol and H2O according to solubility. Standard solutions of these five nucleosides were prepared as a mixed solution for the calibration, and the ISTD solution was prepared at a concentration of 2 µg/L. All stock solutions were stored in the dark at −20 °C until required. Water from a Milli-Q water system (Millipore, Molsheim, France) was used.
Nucleoside determination
Chromatography was performed using a FinniganTM SurveyorTM HPLC system. HPLC analysis was performed on an Atlantis® dC18 column (2.1 × 100 mm, 3 µm) (Waters, Milford, MA,
USA). A guard column (Waters, Milford, MA, USA) was used to prolong the life of the HPLC column. The mobile phases used were (A) 2 mmol/L ammonium acetate (pH 5.0) in H2O and (B) 2 mmol/L ammonium acetate in 50% MeOH at a flow rate of 0.2 mL/min. The gradient conditions were as follows: isocratic elution (95% A) for 5 min, followed by a 2-min gradient to 20% B, then a 3-min gradient to 30% B, and a final 10-min gradient to 40% B. A Finnigan LCQ DECA XPPLUS quadrupole ion trap mass spectrometer (Thermo Finnigan, San Jose, CA) equipped with an electrospray ionization source was used. The mass spectrometer was operated in the positive ion mode by applying a voltage of 3.5 K to the ESI needle. The temperature of the heated capillary in the ESI source was set at 295 °C. The flow rate of the sheath gas (nitrogen) was set at 30 (arbitrary units). Selected-reaction mode (SRM) was used during the quantification experiment: the protonated ion was chosen as the precursor ion and isolated in the ion trap. The collision energy, represented as a percentage of a maximum possible energy sufficient to fragment the precursor ion, was used to produce product ion spectra (Table 1). We used SRM transitions for the individual quantification. The oven program and analyses were performed using the software package Xcalibur (Finnigan Corp., San Jose, CA).
Creatinine analysis
Urinary creatinine concentration was determined by the Synchron LX System (Synchron LX®, Beckman Coulter, Fullerton, California, USA). Creatinine from the urine sample combined with the reagent to produce a red complex. Absorbance readings were taken at 520 nm. The
absorbance has been shown to be a direct measure of the concentration in urine samples. Quantification of urinary nucleosides
To compensate for variations in urine concentration, all nucleoside concentrations were indexed against creatinine and expressed as μmol nucleoside/mmol creatinine [7, 21]. Urinary creatinine levels were determined by a modified Jaffe method and according our previous study [22, 29].
Statistical analysis
Associations between groups and clinical characteristics were evaluated by the Chi-square test and Mann–Whitney U test. The Student’s t-test was used to measure differences in levels of urinary nucleosides between the cancer and control group. P-values of less than 0.05 were considered to be significant. Statistical analyses and graphics were performed with the SPSS 16.0 statistical package.
Results
Method development and analysis
The separated nucleosides were monitored using positive ionization tandem mass spectrometry in SRM mode during MS/MS analysis. The protonated precursor ion [M + H]+ was the most abundant ion of all nucleosides and the protonated base ion [BH2]+ was the most abundant ion after collision-induced dissociation (CID). Previous studies have shown that the glycosidic bond that connects the base moiety and the ribose moiety tends to breakdown during the CID process [ 12, 23
] . In this study, the [M + H]+ ions were the most abundant in the full-scan mode and were selected as the parent ions in MS/MS analysis for all nucleosides. Figure 1 shows the MS/MS spectra of the nucleosides adenosine, cytidine, inosine, m1A, m3C, and ISTD as obtained by infusion in the positive ion mode.
A simple and well-established solid-phase extraction (SPE) method was used instead of liquid-phase extraction, which required more time and solvent. Salts and interfering substances were removed after SPE purification to minimize the matrix effect from the urine sample during the ESI process. The urine samples were reconstituted in mobile phases that were compatible with the optimal HPLC conditions (Supplement data S1). Following our previous study, tubercidin was used as the ISTD for the quantification of urinary nucleosides [24]. The linearity of the calibration curve was evaluated by the R2 regression coefficient of determination (values > 0.995) (Table 2). The accuracy of the method was measured by determining the mean concentration at various
concentrations of analyst and was calculated as percentage error of theoretical versus measured concentrations. Precision was estimated as the percent coefficient of variation (% CV) of the analyses. The inter-assay and intra-assay CVs were < 15%. Accuracy varied with concentration but was generally < 10% in accordance with the previous study (Supplement data S2).
Analysis of urinary nucleosides
The distribution patterns of all nucleosides in the cancer and control groups are summarized by the column formats (Figure 2). The mean levels of all five urinary nucleosides were elevated in the cancer group, but only the level of urinary cytidine was significantly elevated; approximately 1.95-fold higher in the cancer group (mean level = 2.38 μmol/mmol creatinine) than that in the control group (mean level = 1.22 μmol/mmol creatinine) (P = 0.019). Moreover, the mean urinary cytidine level was revealed to be significantly elevated in patients with late stage (S3 + 4) disease (mean level = 2.78 μmol/mmol creatinine) compared with patients with early stage (S1 + 2) disease (mean level = 1.96 μmol/mmol creatinine) (P = 0.039). It is noteworthy that the mean urinary cytidine level increased approximately 1.42-fold in the late stage (S3 + 4) patients (Figure
3).
Improvement in diagnosis of gastric cancer
According to the clinical guidelines for gastrointestinal surgery, 16.3% (8/49 patients) and 28.6% (14/49 patients) were identified as high risk with preoperative serum AFP and CA19-9 levels of more than 20 µg/L and 37 U/mL, respectively. Furthermore, 6 of the 49 patients had elevated
levels of both AFP and CA19-9. The mean urinary cytidine level in the cancer group (2.38 μmol/mmol) was 1.95-fold greater than that in the control group; thus, this value was set as the cutoff value as in our previous study [24]. Eleven patients in the cancer group had higher urinary cytidine levels than the mean level (> 2.38 μmol/mmol); however, they had lower serum AFP (< 20 µg/L) and CA19-9 (< 37 U/mL) levels. By combining the preoperative serum AFP (cutoff of 20 µg/L) or CA19-9 levels (cutoff of 37 U/mL) with the mean urinary cytidine level, the diagnostic ratio (sensitivity) for gastric cancer improved from 16.3% (8/49 patients) to 38.8% (8 + 11/49 patients) or 28.6% (14/49 patients) to 51.0% (14 + 11/49 patients) (Figure 4 A, B). It is also noteworthy that the diagnostic sensitivities of the individual serum biomarkers were increased by more than 20% when combined with urinary cytidine.
Discussion
The first article concerning identification of nucleosides was published in 1978 [2 5 ] and described a method for identifying six nucleosides from urine samples from healthy volunteers and cancer patients. Since then, nucleosides have been successfully identified using chromatographic and electrophoretic techniques with different types of detection methods, which have been reviewed in [2 6 ] . To date, there are numerous publications concerned with the analysis of nucleosides by MS. Some of these are focused on all cis–diol metabolites with particular attention concentrated on their fragmentation pattern and identification. As these procedures are predominantly focused on qualitative analysis, there has been limited statistical exploration targeted at the evaluation of possible biomarkers for discriminatory study [27]. In contrast to the former approach, we focused on measuring the concentrations of 5 nucleosides identified in urine samples in order to evaluate differences between their levels in cancer patients and a healthy volunteer control group. We have developed a simple, rapid, and efficient HPLC/ESI–MS/MS method to analyze urinary nucleosides in hepatocellular carcinoma (HCC) [26], lung cancer [28], and breast cancer [29]. In a previous study, higher urinary cytidine levels were demonstrated in gastric cancer patients by normal HPLC analysis [30]. Furthermore, overexpression of urinary cytidine has been demonstrated in MCF-7 breast cancer cells [31]. This is the first study to identify urinary cytidine as a non-invasive biomarker for the diagnosis of gastric cancer in Taiwanese patients,
and to show a significantly higher level during disease stages S3 + 4 than during stages S1 + 2.
Some serum tumor markers, including AFP and CA19-9, have been reported to be elevated in a subpopulation of patients with gastric cancer [32, 33]. AFP, discovered approximately half a century ago by Abelev et al., is a sensitive marker for the diagnosis of HCC [34]. The serum AFP level is also increased in AFP-producing gastric cancer [35]. The threshold level for AFP is usually taken as 20 µg/L with cutoff line for the suggestion or diagnosis of HCC varying among different studies [36]. Different diagnostic cutoff points affect the calculation of sensitivity and specificity of the test. CA19-9, first described by Koprowski et al. in 1979 as a marker for colorectal cancer, became the most important tumor marker for pancreatic adenocarcinoma [37]. The critical level for CA19-9 is usually taken as 37 U/mL with cutoff points for gastrointestinal cancer diagnosis. At the present time, the value of these tumor markers in various stages of gastric cancer is still under debate, which was responsible for more than 40% of gastric cancer and likely to present abnormal serum level of tumor markers. Results from our previous study revealed that urinary nucleosides could act as adjunct markers to improve the diagnostic ratio for HCC [24]. Thus, we considered a combination of urinary cytidine and serum AFP or CA19-9 levels to determine the diagnostic ratio (sensitivity) for gastric cancer.
Our results showed that 8 patients had serum AFP levels that were higher than 20 µg/L, and 14 patients had serum CA19-9 levels that were higher than 37 U/mL. Among the 49 patients studied, 6 patients had both higher levels of AFP (>20 µg/L) and CA19-9 (>37 U/mL). We found that by combining the determined levels of serum AFP or CA19-9 with those of urinary cytidine, the for gastric cancer could be improved to 38.8% (8 + 11/49) or 51% (14 + 11/49) (Figure 4). These results suggest that urinary cytidine is an adjunct diagnostic marker that could improve a higher sensitivity for identifying individuals with increased risk of gastric cancer whose serum AFP or CA19-9 levels are only moderately elevated. Furthermore, the levels of urinary nucleosides could be a useful tool for diagnosing and monitoring gastric cancer for cancer research in Taiwan.
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