Background
Gastric cancer (GC) is a malignant tumor type that seriously threatens human health, whose mortality rate ranks second globally [
1]. Approximately 1,000,000 new cases [
2] of GC occur every year. The existing treatments of GC include surgery, radiotherapy, chemotherapy and other adjuvant therapies, however, the prognosis of patients with GC remains poor [
3,
4]. In comparison, the 5-year survival rate of patients with early GC following radical resection is up to 90% [
5]. Therefore, the present study aimed to determine an effective diagnostic and therapeutic target to improve the survival time of patients with GC.
The human olfactomedin-like 2B (OLFML2B) gene is located on chromosome 1q23.3 [
6], consists of at least 8 exons and spans 40.7 kb, and is mainly expressed in the adult retina [
7]. The encoded protein belongs to the olfactomedin domain-containing proteins family. The members of this family share a common feature in that the C-terminus of the proteins contains a 250 amino acid region, which is named as the olfactomedin region [
8‐
12]. In addition, OLFML2B protein contains an unique Ser/Thr-rich region which was absent in other family members [
13,
14] and is able to bind to chondroitin sulphate-E or heparin in the extracellular matrix [
14]. However, the functions of OLFML2B in GC remain unclear.
Until now, certain specific markers, including human epidermal growth factor receptor-2 [
15,
16], vascular endothelial growth factor receptor 2 [
17], excision repair cross-complementation group 1 [
18], B-cell lymphoma-2 and Ki-67 [
19] have been used for the diagnosis of patients with GC. However, the majority of them were not able to accurately reflect diagnostic value and therapeutic efficiency. Therefore, we evaluated the expression of OLFML2B both TCGA database and clinical samples to reveal its clinical significance in GC, whilst investigating the status of OLFML2B from the cBioPortal to further reveal the molecular mechanisms of OLFML2B in the pathogenesis of GC.
Discussion
GC is a type of gastrointestinal tumor that may occur in any section of the stomach [
38]. It has been confirmed that GC originates from the gastric mucosa epithelium, and pathological results confirm that the majority of GCs are a type of adenocarcinoma [
38]. Although gastroscopy has been used to detect early GC, the results were poor, therefore it is necessary to screen out novel diagnostic and prognostic targets for GC.
The olfactomedin domain-containing proteins family included 13 members in mammals, the functions of them remain unclear. The OLFML2B protein was originally identified in ganglion cells, inner nuclear layers, the inner segment of the photoreceptor layer and the retinal pigmented epithelium. It exerts an important function by specifically binding to chondroitin sulphate-E or heparin in the extracellular matrix [
14]. To date, no studies have elaborated on the diagnostic and prognostic value of OLFML2B for GC. The present research, to the best of our knowledge, is the first one to systematically survey the clinical significance of OLFML2B in GC.
As showed in Table
1, χ
2 tests was performed to assess the correlation between OLFML2B expression (low & high) and clinicopathological characteristics. The result demonstrated that high expression of OLFML2B predicted deeper tumor invasion and later clinical stage, but it was not correlated with lymph node metastasis and distant metastasis. The main reasons were considered as follow: 1) There were multiple factors involved in the occurrence and progress of GC, including oncogene activation, inactivation of tumor suppressor genes, H.pylori infection, and so on. When χ
2 tests was performed, it couldn’t exclude influence of other factors. 2) 376 samples from TCGA database were included in our study, it couldn’t too enough to explain the correlation between OLFML2B expression and clinicopathological characteristics, so more samples need to be collected to evaluate their correlation in the future.
Table 1
Clinical association between OLFML2B expression and clinicopathological variables in GC patients
Age | 371a | | | | 0.008 | 0.872 |
≥ 60 | 255 | 123 | 132 | 0.872 | | |
< 60 | 116 | 57 | 59 | | | |
Sex | | | | | 0.044 | 0.392 |
Male | 243 | 113 | 130 | 0.391 | | |
Female | 133 | 68 | 65 | | | |
Tumor stage | | | | | 0.175 | 0.001 |
T2–4 | 356 | 164 | 192 | 0.001 | | |
T1 | 20 | 17 | 3 | | | |
Lymph metastasis | | | | | 0.025 | 0.626 |
Yes | 258 | 122 | 136 | 0.625 | | |
No | 118 | 59 | 59 | | | |
Distant metastasis | | | | | 0.016 | 0.764 |
Yes | 35 | 16 | 19 | 0.763 | | |
No | 341 | 165 | 176 | | | |
Clinical stage | 372a | | | | 0.115 | 0.027 |
II-IV | 321 | 148 | 173 | 0.027 | 0 | |
I | 51 | 32 | 19 | | | |
In addition, the analysis from TCGA database indicated that OLFML2B had a significantly higher expression in GC compared with in NG tissues, and the same trend was observed in 13 pairs of clinical samples by qRT-PCR and IHC test. Thus, OLFML2B might serve as an oncogene in the development of GC. It was inferred that OLFML2B may accelerate the growth of GC by promoting the proliferation of GC cells. Further investigations in vivo and in vitro are required in order to assess the molecular mechanisms of OLFML2B in GC.
As demonstrated in Tables
2 and
3, univariate and multivariate analysis did not select OLFML2B expression as independent prognostic factor. The causes were explained as follow: 1) There were many factors that influenced the prognosis of GC, the common ones were listed in current study and other mixed facters couldn’t be excluded. 2)Analysis from Kaplan-Meier plotter and OncoLnc revealed that high OLFML2B expression was significantly associated with a shorter OS for all patients with GC. Considering these facts it can be deduced from our result OLFML2B expression have the clinical utility for predicting prognosis in patients with GC.
Table 2
Univariate analysis of prognostic factors of GC
Age(<60/≥60) | 1.557 | (1.034,2.346) | 0.034 |
Gender (Male/Female) | 1.468 | (0.984,2.189) | 0.060 |
Tumor size (T2–4/T1) | 3.503 | (0.865,14.188) | 0.079 |
Lymph metastasis (Yes/No) | 1.689 | (1.095,2.607) | 0.018 |
Distant metastasis (Yes/No) | 2.265 | (1.337,3.889) | 0.002 |
Clinical stage (II- IV/I) | 1.558 | (0.872,2.783) | 0.134 |
OLFML2B expression (High/Low) | 1.331 | (0.929,1.907) | 0.119 |
Table 3
Multivariate analysis of prognostic factors of GC
Age, years (≥60/< 60) | 1.669 | (1.095,2.542) | 0.017 |
Gender (Male/Female) | 1.313 | (0.875,1.970) | 0.189 |
Tumor size (T2–4/T1) | 2.485 | (0.547,11.298) | 0.239 |
Lymph metastasis (Yes/No) | 1.676 | (0.947,2.966) | 0.076 |
Distant metastasis (Yes/No) | 2.293 | (1.345,3.909) | 0.002 |
Clinical stage (II-IV/I) | 0.835 | (0.376,1.854) | 0.659 |
OLFML2B expression (High/Low) | 1.198 | (0.830,1.727) | 0.334 |
To date, multiple methods were used to diagnose GC, among which serum marker was the more convenient than others, the common markers in clinical included serum CA199 and CEA. However, the specificity of them for diagnosing GC was related to the depth of tumor invasion and individual differences. Our results demonstrated OLFML2B had a significant diagnostic value for GC (AUC = 0.867; P < 0.0001). There is currently no reports on the association of OLFML2B expression and these markers. Therefore, combined detection of multiple markers may improve the diagnostic ratio of GC in future.
However, whether OLFML2B is effective for clinical application as replacements for or in addition to the prognostic parameters currently in use is still unclear, and further investigation is called for to identify whether combined detection of OLFML2B together with some of these other molecules would be valuable in improving prognostic effectiveness.
The alterations to OLFML2B in GC were detected to identify potential mechanisms in GC. The result indicated that 5% of OLFML2B contained alterations, among which the missense mutation was the primary type of alteration, followed by amplification, mRNA upregulation and deep deletion. It was hypothesised that the missense mutation of OLFML2B was associated with the unique Ser/Thr-rich region, and the missense mutation may facilitate the transcription of OLFML2B. Futhermore, OLFML2B protein may specifically bind to chondroitin sulphate-E in the extracellular matrix, which may be associated with the size of the tumor in clinical pathology. Then, the coexpressed genes in stomach neoplasms were examined by Coexpedia, and a total of 40 genes were identified. Thus, it could be deduced that the functions of OLFML2B in GC were achieved by interacting with other genes, particularly MMP19, which indicated that OLFML2B might be involved in the epithelial mesenchymal transition process. In addition, a number of potential biological processes associated with OLFML2B, particularly cell growth and apoptosis, were predicted by FunRich software, which could be infered that these processes were unbalanced in GC, and further studies were required to verify the present results. However, although the bioinformatics analysis provided us a novel direction for future research, further studies need to be done to validate the functions of OLFML2B in GC.