ccRCC accounts for 2–3% of all cancers in humans, and the morbidity and mortality of ccRCC have increased significantly in the past 20 years [15]. It is estimated that more than 300,000 people are diagnosed with ccRCC each year, which is expected to increase by 22% in 2020 [16]. More than 100,000 ccRCC-related deaths are reported each year. ccRCC is a complex heterogeneous disease, and its pathogenesis is unclear. Although a large number of studies has been carried out using microarrays and RNA-Seq to identify new biomarkers and therapeutic targets for ccRCC, there are inconsistencies between the differentially expressed genes found in different studies. Therefore, it is necessary to find a reliable and relatively simple method for prognosis.
Analysis using the Oncomine, GEO, and TCGA databases revealed that the expression level of CASP4 mRNA in ccRCC tissues was significantly higher than that in normal tissues. To verify this result, we performed immunohistochemical staining of 30 cases of ccRCC and adjacent normal tissues at our center. The immunohistochemical results showed that the expression level of CASP4 protein in ccRCC tissues was significantly higher than that in adjacent normal tissues.
The prognostic significance of CASP4 overexpression in tumors is controversial. Wang reported that in patients with gastric cancer, higher expression of CASP4 was associated with better OS [17]. However, Terlizzi et al found that CASP4 overexpression is associated with poor prognosis in patients with non-small cell lung cancer [18]. The prognostic significance of CASP4 in ccRCC has not been reported. We found that the survival time of patients with ccRCC showing high CASP4 mRNA expression was shorter than that of patients with low expression levels, which was verified by GEO datasets and experimental data. We also combined the differential expression level of CASP4 with clinicopathological parameters and found that its expression was related to pathological grade and clinical TNM stage.
In contrast, to further explain the possible molecular mechanism of CASP4 in ccRCC, we used GSEA and GSVA to analyze the data obtained from public databases to identify important pathways. GSEA and GSVA analysis illustrated that CASP4 is involved in the most significant pathways including ANTIGEN_PROCESSING_AND_PRESENTATION, DNA_REPLICATION, CELL_CYCLE, and nervous system diseases, which were enriched in ccRCC samples. Genes co-expressed with CASP4 were also identified, which may help to further explore the interaction between these genes. These results indicate that CASP4 plays a key role in the occurrence and development of ccRCC.
Previous studies confirmed that CASP4 encodes a protein related to immunity and inflammation, which is involved in the coordination of cellular processes, including cell homeostasis, inflammation, and apoptosis [19]. Our study showed that CASP4 is related to the innate immune response and inflammation and is closely related to the occurrence and development of tumors. Abnormal expression of CASP4 may thus be involved in the occurrence, development, and metastasis of tumors. Studies in tumor cells will identify roles for CASP4 in addition to the traditional concepts of its role in inflammatory bodies in the immune system and reveal the interdependent or mutually restricted relationship between CASP4 and upstream regulatory factors and downstream effector molecules, as well as pyroptosis, or other pathways of cell death.
This study also had some limitations. First, this was a retrospective study, and selective bias is inevitable. Second, the sample size of the study was relatively small, and the population diversity was poor. Third, we only used immunohistochemistry (a semi-quantitative method) to detect the expression of CASP4 protein. Finally, we did not clarify the potential mechanism of CASP4 participation in ccRCC.