For the first time in this field, this study identified the expression and roles of SYNJ2 in LUSC. Based on the analysis of 3018 samples, SYNJ2 was significantly upregulated in LUSC at both mRNA and protein levels, and using multicenter samples, overexpressed SYNJ2 predicted poor prognosis for LUSC patients. The cancer-promoting effect of SYNJ2 may be related to protein digestion and absorption and extracellular matrix-receptor interaction. SYNJ2 expression was closely related to immune cell infiltration, indicating its role in the immune response. Moreover, this study initially investigated the expression levels and clinical relevance of SYNJ2 in pan-cancer, demonstrating the novel and potential biomarker for the prediction and treatment of cancers.
SYNJ2 is a synaptic binding protein with a role in endocytosis. Its involvement in human cancer has not been well-studied [6, 31]. Previous studies have reported that SYNJ2 is responsible for tumor initiation and progression. With genetic changes, SYNJ2 triggers colorectal cancer , medulloblastoma , prostate cancer , and hepatocellular carcinoma . Overexpressed SYNJ2 boosts the invasion and migration of glioma  and breast cancer cells and prompts poor prognosis for cancer patients .
However, only one study by Ben-Chetrit et al.  has focused on the relationship between SYNJ2 and lung cancer. To verify the correlation between SYNJ2 and tumors, Ben-Chetrit et al. used an online informatics tool to show that high SYNJ2 transcript levels indicated a shorter lifespan for patients with lung cancer. Yet the study used only single-source data and limited samples, ignored genetic background, and simply evaluated the survival time. Moreover, distinct histological subtypes of lung cancer are characterized by specific morphological and molecular phenotypes. Thus SYNJ2 may play diverse roles in LUSC, lung adenocarcinoma, and small cell lung cancer, while the study of Ben-Chetrit et al. concentrated on all lung cancer rather than specific subtypes was not sufficient to understand the full spectrum of functions and mechanisms of action of SYNJ2 in LUSC. Therefore, the roles of SYNJ2 in LUSC remain largely unknown.
In this study, integrated analysis based on data covering more than 2824 samples revealed upregulated SYNJ2 mRNA expression in LUSC. Furthermore, in-house experiments verified a higher SYNJ2 expression in LUSC tissues than in non-tumorous lung tissues at mRNA and protein levels. In addition, this research not only demonstrated that SYNJ2 made it feasible to distinguish between LUSC and non-LUSC specimens but indicated that patients with high levels of SYNJ2 tend to live shorter lives. Thus, SYNJ2 was dramatically upregulated in LUSC and can predict both the cancer status and the poor prognosis for patients with LUSC, which has not been reported before, indicating the novelty of this study.
The expression patterns of SYNJ2 in different cancers were various, and its clinical relevance is conspicuous. Distinct SYNJ2 expression levels were identified in 14 types of cancer cell lines. Previous research showed differential expression levels of SYNJ2 between some cancers (e.g., BRCA  and LIHC ) and their control samples, and our study further comprehensively analyzed 20 cancers and revealed that increased and decreased SYNJ2 expression levels were defined in seven cancers (CHOL, COAD, LIHC, LUAD, LUSC, PRAD, and STAD) and four cancers (GBM, KICH, KIRC, and KIRP), respectively. SYNJ2 expression made it feasible to differentiate multiple types of cancers from their corresponding normal samples with at least moderate accuracy, suggesting its potential in identifying cancer status. In addition, elevated SYNJ2 expression was associated with the unfavorable prognosis (at least one of OS, DSS, DFI, and PFI) of cancers with ACC, BRCA, GBM, KICH, KIRP, LGG, LIHC, LUAD, LUSC, MESO, PCPG, and UVM, while it represented favorable OS and DSS of THYM patients. From these results, SYNJ2 generally served as a risk marker for the prognosis of cancer patients. Taken together, SYNJ2 may be a novel marker for predicting cancer status and prognosis of multiple cancers.
Few studies have focused on the molecular mechanistic understanding of SYNJ2 in human malignant tumors. According to previous studies, high expression of SYNJ2 was correlated with hepatocellular tumorigenesis via the CTCF/POLR2A-SYNJ2 axis  and with metastasis of glioma  and breast cancer through regulating the formation of lamellipodia and invadopodia . In addition to these, SYNJ2 negatively regulated the PI3K/Akt pathway and was necessary for vesicle transport, focal adhesion, and lamellipodia as well as invadopodia formation, which led to cell survival, proliferation, invasion, and migration [14, 31]. Indeed, a similar finding can also be found in our study on LUSC (the proteins encoded by Up-PCEGs of SYNJ2 gathered in the extracellular matrix-receptor interaction). Our study also revealed that SYNJ2 might affect protein digestion and absorption (SYNJ2 protein is known to interact with the corresponding substrate, resulting in the translocation of the encoded protein to the plasma membrane and thereby inhibiting the clathrin-mediated endocytosis ). These findings implied that the roles SYNJ2 played in LUSC may be related to extracellular matrix-receptor interaction and protein metabolism; however, it requires further experiment exploration and verification. In addition, SYNJ2 was found related to the “DRUG METABOLISM CYTOCHROME P450” KEGG signaling pathway in several cancers (at least KICH, OV, and PCPG), indicating its druggable potential, which has also been demonstrated in BRCA before .
Little research has focused on the relevance between SYNJ2 and immunity before. However, based on the results of our study, SYNJ2 likely participated in the immune response by affecting filtration levels of immune cells, the mechanisms of which were complex with the fact that: (1) The infiltration levels of several antigen presenting cells (e.g., activated dendritic cells and neutrophils [33,34,35,36,37]), were significantly increased in LUSC samples with higher expression levels of SYNJ2. (2) SYNJ2 was positively associated with both infiltration levels of innate and acquired immune cells and expression levels of immune checkpoints in some cancers such as KICH and LIHC. (3) TMB and MSI were known to contribute to the production of new immune antigens and induction and promotion of immune responses [38, 39], and SYNJ2 showed its relationship with TMB and MSI in some cancers, implying its trigger role in this field. (4) Drugs targeting immune checkpoint block are promising in the treatment of cancers [40, 41]; SYNJ2 expression was significantly positively related to plenty of immune checkpoints in several cancers, suggesting that it may have similar clinical potential as immune checkpoints. Taken together, SYNJ2 was closely associated with the immune microenvironment and may serve as a potential marker of immune target treatment.
There are some limitations to this study. First, the number of in-house cohorts was small and must be expanded to include more clinical samples. Second, most of the in-house cases included in our study were at an early stage, did not have distant and lymph node metastases, and lacked clinical follow-up information, which limited our capacity to gain a comprehensive understanding of the clinical significance of SYNJ2 in LUSC. Last, a series of in vivo and in vitro experiments are needed to further explore the molecular mechanisms of action of SYNJ2 in LUSC. More scientific experiments in the future are required to address these limitations.
In conclusion, this study validated that SYNJ2 was significantly upregulated in LUSC tissues. Overexpressed SYNJ2 identities cancer status and predicts a poor prognosis for individuals with one type of multiple cancers, including LUSC. This research also disclosed the close association between SYNJ2 expression and the immune environment and the potential of this gene as a novel and potential biomarker for the prediction and treatment of multiple cancers, including LUSC.