Background
Although the incidence of gastric cancer (GC) is declining in most developed countries, it remains one of the most common causes of cancer-related death worldwide [
1]-[
3]. Appropriate stratification of patients is a pivotal aspect of individualized treatment, leading to reducing mortality from this cancer [
4],[
5].
According to its epidemiology, pathology, and location in the body, GC is recognized as three distinct malignancies arising in the same organ [
6]-[
8]. Shah et al. [
9] proposed a convincing classification of GC according to histopathologic and anatomic criteria as follows: (1) proximal nondiffuse GC where the tumor is located mainly in the gastric cardia with evidence of precursor glandular dysplasia or in situ carcinoma in the presence of chronic inflammation, usually without atrophy; (2) diffuse GC, which may be located anywhere in the stomach with no apparent gastritis that exhibits an entirely diffuse pattern of infiltration of cells with a poorly differentiated phenotype; and (3) distal nondiffuse GC, which is located mainly in the distal stomach with evidence of chronic gastritis that is predominantly differentiated or exhibits an intestinal phenotype. In this study, they demonstrated that the three GC subtypes are distinguished by their gene expression profiles. Therefore, the genetic diversity of GC subtypes should be considered in studies of genetic and epigenetic alterations related to gastric carcinogenesis and progression.
Prenyl diphosphate synthase subunit 2 (
PDSS2) was identified in 2005 [
10], and evidence indicates that it acts as a tumor suppressor [
11],[
12].
PDSS2 is required for the synthesis of coenzyme Q10 (CoQ10) [
13],[
14], which is synthesized in the mitochondrial inner membrane and plays a vital role in the mitochondrial respiratory chain, pyrimidine nucleoside biosynthesis, and modulation of apoptosis [
15].
PDSS2 resides within the chromosomal locus 6q16.3-21, a site of frequent microsatellite DNA instability and loss of heterozygosity (LOH) in GC [
16],[
17], supporting its role as a tumor suppressor in gastric epithelial cells. Moreover,
PDSS2 may suppress the development of malignant melanomas and lung cancers [
11],[
12]. Moreover, Chen et al. reported that enforced overexpression of
PDSS2 leads to apoptosis in a GC cell line by causing cell cycle arrest in the G0/G1 phase [
18]. These reports led us to make a hypothesis that
PDSS2 is a potential GC-related gene and a candidate of novel clinically-relevant prognostic marker of GC.
In this study, expression and methylation status of PDSS2 in GC were determined to evaluate the clinical significance and regulatory mechanisms of PDSS2 expression in GC. Our results indicate that PDSS2 expression provides a potential clinical biomarker of the progression and recurrence of GC.
Discussion
PDSSs are heterotetrameric enzymes comprising subunits encoded by
PDSS1 (10p12.1) and
PDSS2[
10],[
12]. PDSS activity requires both subunits [
10],[
14],[
15]. The association of
PDSS2 with GC was considered because of its chromosomal location (6q21), and because of its inactivation or loss from certain malignancies [
16],[
26]. Here we show that
PDSS2 mRNA was heterogeneously expressed in GC cell lines, and its expression was inhibited in 73% and 32% of GC cell lines and tumor tissues, respectively. We detected hypermethylation of the
PDSS2 promoter in five (45%) of 11 GC cell lines with significantly decreased levels of
PDSS2 expression. Further,
PDSS2 transcription was reactivated after cells were treated with an inhibitor of DNA methylation. These findings are the first to our knowledge to show that promoter hypermethylation regulates
PDSS2 transcription. However,
PDSS2 expression was decreased in some GC cells without hypermethylation. Because chromosome 6q is a frequent site of LOH in GC [
17],[
26],[
27], LOH may regulate
PDSS2 expression as well.
There has been a report demonstrating that
PDSS2 was expressed at decreased or undetectable expression in a small number of biopsy GC samples [
18]. In the present study, we analyzed 238 surgical specimens of tumors and the corresponding uninvolved tissue to gain further insight into the clinical significance of
PDSS2 expression in GC. Consistent with analyses of malignant melanoma and lung cancer [
11],[
12], most patients with GC harbored a decreased level of
PDSS2 mRNA in GC tissues, and the mean
PDSS2 expression level was significantly decreased in GC compared with normal adjacent tissues. IHC was conducted to determine whether the mRNA level reflected
PDSS2 protein expression. Because the IHC results indicated that the mRNA data were consistent with the protein level, subsequent analyses were performed according to the mRNA data, which are more amenable to the quantitative analysis [
7],[
23].
Decreased
PDSS2 mRNA expression in GCs was significantly associated with elevated preoperative CA19-9 levels and lymph node metastasis and was identified as an independent prognostic factor. Moreover, patients with decreased
PDSS2 mRNA expression in GC tissue experienced significantly earlier recurrence after R0 resection. Recently, Chen et al. investigated the tumor-suppressing activity of
PDSS2 in lung cancer [
28]. They reported that the forced overexpression of
PDSS2 caused massive cell death through apoptotic pathways and significantly inhibited colony formation and there was an inverse correlation between
PDSS2 expression and gelsolin expression, which is known to inhibit apoptosis and enhance cell invasion and metastasis [
29], though PDSS2 did not influence the sensitivity of the cancer cells to chemotherapeutic drugs [
28]. This tumor suppressive mechanism of
PDSS2 might be applied to GC as well.
The expression pattern of
PDSS2 mRNA and its prognostic impact were similar among the three GC subtypes (proximal nondiffuse, diffuse, and distal nondiffuse), indicating that
PDSS2 expression influences the pathogenesis of all types of GC. Shah et al. reported that one-third of amplified genes, possibly including
PDSS2, showed equivalent expression pattern among the three GC subtypes [
9].
GC is one of the tumors with a high frequency of aberrant methylation, and it frequently exhibits the CpG island methylator phenotype [
30],[
31]. The expression of a large number of genes is suppressed by CpG island hypermethylation in GC cells, including those encoding tumor suppressors, cell cycle regulators, inducers and executioners of apoptosis, proteins that promote the invasive phenotype, and DNA mismatch repair enzymes [
32]. These epigenetic alterations can serve as biomarkers that illuminate an increased metastatic potential and aggressive tumor phenotype [
33] as well as therapeutic targets [
34]. Therefore, identification of other genes that are regulated by methylation in GC cells will likely improve the management of GC.
The tumor suppressive function of PDSS2 are supported by the present findings as follows: (1) decreased expression of PDSS2 was frequently detected in GC tissues, (2) the mean level of PDSS2 expression was significantly lower in GC tissues, and (3) decreased expression of PDSS2 was associated with early recurrence and subsequent poor prognosis. PDSS2 expression levels in biopsy tissue obtained using endoscopic surveillance samples or in surgical specimens may be useful for predicting early recurrence and poor prognosis, which will likely aid efforts to design more efficacious therapeutic strategies.
This study was limited by its lack of sufficient functional analysis of PDSS2, which tempers the conclusion that it acts as a tumor suppressor in GC. Further studies including pathway analysis in gastric carcinogenesis and functional analysis are expected to clarify the molecular mechanisms underlying the biological activities of PDSS2 in GC.
Authors’ contributions
MK, HO, FS, DS, HT, RH and KM performed experiments and data analysis. DK, CT, SY, TF, GN, HS, MK, MF and YK collected cases and clinical data. MK and SN conceived and designed the study, and prepared the initial manuscript. YK supervised the project. All authors contributed to the final manuscript. All authors read and approved the final manuscript.
Competing interests
The authors declare that they have no competing interests.