Background
Periodontitis is a chronic inflammatory disease involving interactions between complex microbial biofilms, many cell populations and inflammatory mediators, leading to the destruction of the tooth-supporting structures like the periodontal ligament and the alveolar bone [
1]. Besides being a common cause of tooth loss, severe periodontitis (about 8.5 % of patients) can detrimentally affect systemic health, as it can increase the patients’ risk for diabetes, atherosclerosis, rheumatoid arthritis, and adverse pregnancy outcomes [
2‐
4]. Two major clinical entities of periodontitis are currently recognized: chronic periodontitis, which is more common, and aggressive periodontitis, a clinically challenging entity featured by an early onset and a rapid progression [
5]. The underlying etiology of both the two forms has not been fully elucidated. Therefore, gaining further insights into the molecular mechanisms of periodontitis will be of great significance for the treatment of periodontitis.
Previous studies have demonstrated that factors that may determine the presence and rate of progression of periodontitis are complex, which can be defined as the interplay of numerous parameters acting simultaneously and unpredictably [
1]. For instance, the tooth-associated microbial biofilm or dental plaque is essential but not sufficient to induce periodontitis. The host inflammatory response to the microbial challenge can finally cause the destruction of the periodontium [
6]. Inflammation and bone loss are hallmarks of periodontal disease [
7] and accumulated evidence demonstrates that a number of mediators are involved in these processes [
8]. Cochran et al. had reported that the reduction of inflammation and attenuation of the host’s immune reaction to the microbial plaque could lead to a decrease in the ratio of nuclear factor-kappa B ligand (RANKL)/osteoprotegerin (OPG) and a decrease in associated bone loss [
7]. Besides, a study has reviewed that cytokines such as interleukin-1 (IL-1) and tumor necrosis factor (TNF) are a significant and integral component of the host reaction to periodontal infection [
8]. In addition, secreted IL-8 induced by multiple stimuli like live bacteria and proinflammatory cytokines is associated with the inflammation and invasiveness of periodontitis [
9]. Despite of numerous investigations on periodontitis, the mechanism still remains largely unknown.
Using the same gene expression profile, Stoecklin et al. identified specific miRNAs (has-miR-210 and hsa-miR-185) and their target genes in gingival tissues [
10]. In addition, Kebschull et al. found that small differences in gene expression and the highly variable classifier performance suggested limited dissimilarities between established chronic periodontitis and aggressive periodontitis lesions [
11]. We sought to have an expression profiling comparison between periodontitis (chronic periodontitis and aggressive periodontitis collectively) and normal control, identifying more candidate genes involved in both chronic and aggressive periodontitis and to gain more insights into the molecular mechanisms of periodontitis progression.
Discussion
In this study, we used the microarray data to select genes associated with periodontitis. Totally, 762 DEGs in the periodontitis samples were identified compared with the control samples. The up-regulated genes were mainly enriched in the GO terms like cell activation and activation of immune response, as well as the pathways such as staphylococcus aureus infection and cytokine-cytokine receptor interaction. The down-regulated genes were mainly linked to tissue development and metabolism pathways. CTSS, PLEK, LCK, and PTGS2 were identified to be hub proteins in the PPI network or in the selected module. Besides, 9 TFs and 10 TFs were selected from the up-regulated genes and down-regulated genes respectively, for example, IRF4, IRF8, and FOSB.
Our results showed that 20,303 genes were mapped to the probes. Compared with the healthy samples, a total of 762 DEGs were identified in the periodontitis samples, including 507 up-regulated genes (FDR< 0.05 and log
2 FC ≥ 0.58) and 255 down-regulated genes (FDR< 0.05 and log
2 FC < −0.58). While, Kebschull et al. identified a total of 248 differentially regulated probes at an absolute fold change of ≥1.19 [
12]. They reported 30 overexpressed and only one under-expressed probe by an absolute change of >1.5 fold in aggressive periodontitis lesions compared with chronic periodontitis lesions. Besides, they found that 9258 probes were differentially expressed when compared the ‘diseased’ tissues with ‘healthy’ gingival tissues. Collectively, the results showed that we identified distinct genetic features in periodontitis samples using different screening methods with different thresholds.
In this study, we found that DEGs in periodontitis samples were mainly enriched in different GO terms and pathways, such as cell activation, activation of immune response, staphylococcus aureus infection and cytokine-cytokine receptor interaction, using KEGG database which were not used by Kebschull et al. [
12]. In their investigations, gene set enrichment analysis was performed and gene sets linked to apoptosis, immune response were enriched in aggressive periodontitis lesions, while genes sets linked to cellular metabolism and epithelial integrity were enriched in chronic periodontitis lesions [
12]. In a susceptible host, persistence of bacteria pathogens such as
Porphyromonas gingivalis results in aberrant and extended inflammation and subsequent destruction of the tooth-supporting structures [
24]. The immune cells such as antigen presenting cells (APC) initially responding to the challenge by bacteria pathogens, including
Porphyromonas gingivalis, poised strategically along portals of entry [
25]. After recognition of pathogen associated molecular patterns (PAMPs) via pattern recognition receptors (eg, toll like receptors [TLRs]), innate immune cells start responses aiming to clear the inciting agent [
26]. Moutsopoulos et al. had showed that
Porphyromonas gingivalis could promote T helper cell 17 (Th17) inducing pathways in chronic periodontitis [
24]. Thus, the enrichment results identified in our study was in accordance with the previous studies.
CTSS is a lysosomal cysteine proteinase that may participate in the degradation of antigenic proteins to peptides for presentation on MHC class II molecules [
27]. Deficiency of CTSS induces a high bone turnover and then leading to the less dense bone [
28]. Mogi et al. demonstrated that the expression level of the key bone degradation enzyme cathepsin K (another member of family proteins) in gingival crevicular fluid tissues of periodontitis patients was higher than that in normal tissues [
29]. Besides, IRF8 can specifically bind to the upstream regulatory region of type I interferon (IFN). Zhao et al. had demonstrated that IRF-8 was a regulator for osteoclastogenesis in bone metabolism [
30]. Soft tissue destruction and bone degradation were often found in periodontitis [
31]. Moreover, a study revealed that CTSS had the binding site for transcription factor IRF1, and combination of IRF8 and IRF1 could promote the CTSS expression [
32]. In the present study, CTSS was a hub protein in the PPI network and could be regulate by IRF8 in the regulatory network. In the context, we suggested that
CTSS might play an essential role in bone loss involved in periodontitis progression by interacting with
IRF8.
On the other hand, PLEK is a major substrate of protein kinase C in platelets and leukocytes and appears to play an important role in exocytosis through a currently unknown mechanism [
33]. Ding et al. proved that the phosphorylated PLEK increased the secretion of proinflammatory cytokine in mononuclear phagocytes [
34]. Besides, Ueki et al. had showed that the secreted monocytes activated by bacterial in gingival crevicular fluid was associated with periodontitis [
35]. On the other hand, IRF8 can specifically bind to the upstream regulatory region of IFN [
36]. Additionally, Bar-Or et al. showed that B cells could exhibit abnormal proinflammatory cytokine responses (such as exaggerated production of TNF) when activated in the context of the Th1 cytokine IFN [
37]. In this study, the results showed that
PLEK was a hub protein in the PPI network and could be regulated by IRF8 in the regulatory network. Therefore, we speculated that
PLEK might contribute to the periodontitis progression via interacting with
IRF-8.
PTGS2 is an isozyme of PTGS which is the key enzyme in prostaglandin biosynthesis, and acts both as a dioxygenase and as a peroxidase [
38]. The study of Zhang et al. had demonstrated that there was a hypermethylation pattern of the promoter in connection with a lower level of PTGS2 transcription in the inflamed tissues in chronic periodontitis [
39]. On the other hand, FOSB is one member of the Fos gene family which encodes leucine zipper proteins that can dimerize with proteins of the JUN family [
40]. T cell receptor (TCR)-driven early gene expression is controlled by numerous key transcription factors such as FOSB [
41]. Additionally, Sreeramkumar et al. had reported that PTGS2 was transcriptionally up-regulated in T cells during TCR/CD3 triggering and that it behaved as an early inducible gene in the T cell activation process [
42]. Moreover, Chen et al. had demonstrated that costimulatory double signals from CD28 and TCR were required for optimal expression of receptor activator of nuclear factor-κB ligand (RANKL) in periodontal tissues [
43]. In the present study, the results showed that
PTGS2 was involved in module 3 and could be regulated by FOSB in the regulatory network. Thus, we suggested that
PTGS2 might play a critical role in periodontitis progression involving in TCR signaling pathway via interacting with
FOSB.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
LS conceived of this study, participated in the design, and he performed the statistical analysis. JY carried out the study, together with ZH, collected important background information, and drafted the manuscript. BX participated in the design and helped to draft the manuscript. All authors read and approved the final manuscript.