Background
Prostate cancer ranks as the second most commonly diagnosed malignancy in males [
1]. In terms of treatment, challenges still exist, especially for castration-resistant prostate cancer (CRPC). CRPC with metastasis is reported to have a poor prognosis, with a median survival time of less than 2 years [
2]. In recent years, immune checkpoint inhibitors blocking programmed cell death 1 and its ligand (PD-1/PD-L1) and cytotoxic T-lymphocyte antigen-4 (CTLA-4) have shown promising preliminary results in various kinds of tumours. Two clinical trials of ipilimumab in metastatic CRPC have shown improved progression-free survival [
3,
4]. Nevertheless, the utilization of immunotherapy is still limited by low efficacy. Some response predictive biomarkers are under investigation, including tumour mutational burden (TMB).
TMB involves the number of non-synonymous somatic mutations per megabase pair (Mbp) of sequenced DNA. Mutations of tumours affect the mutational load, which in turn determines the chance of presenting immunogenically relevant neoantigens [
5]. High-TMB tumours tend to harbour more neoantigens than low-TMB tumours, which make the high-TMB tumours more immunogenic, resulting in an improved T cell response and subsequent enhancement of antitumour immunity. Given the function of TMB in immunity, clinical studies focused on melanoma and non-small-cell lung cancer have demonstrated that the TMB is associated with the immunotherapy treatment response [
6,
7]. Therefore, TMB is believed to be one of the candidates for predicting the efficacy of immunotherapy.
Next-generation sequencing (NGS) profiling of patients has enabled advancements, and The Cancer Genome Atlas (TCGA) offers convenient access to relevant information. In this study, the gene expression and mutation profiles of prostate cancer samples were extracted from TCGA, and the data were used to investigate the clinical significance of TMB and its related differentially expressed genes (DEGs) and immune cell infiltration signatures.
Discussion
Immunotherapies have shown preliminary results in prostate cancer. TMB is considered an emerging biomarker for response evaluation. Therefore, it is meaningful to investigate the relationship between TMB and prostate cancer. This study provides an overview of mutations, the clinical significance of TMB, and DEGs and infiltrating immune cells related to TMB in prostate cancer.
Among the top 10 mutated genes, TP53, SPOP, FOXA1 and ATM showed pivotal functions in the initiation and development of malignant prostate cancer. As a tumour suppressor, TP53 has a high mutation frequency among various kinds of tumours, and the mutant form is equipped with antiproliferative functions and is related to the metastasis and progression of prostate cancer [
13,
14]. Mutations in SPOP are considered the most common recurrent point mutations in prostate cancer [
15]. SPOP is crucial for the preservation of nuclear genome stability and is essential for the degradation of multiple proteins [
16,
17]. FOXA1 is necessary for androgen receptor-mediated activation of prostate genes [
18]. Furthermore, the expression of FOXA1 is related to tumorigenesis and the progression of prostate cancer [
19]. ATM is considered one of the DNA damage repair genes, and its activation can be seen in the earlier stages of prostate tumorigenesis [
20].
The clinical significance of TMB in prostate cancer was assessed. Similar to a previous prostate and renal cancer study, our study found that the group of patients with prostate cancer with high TMB had lower OS than the group of patients with prostate cancer with low TMB
[1][
21]. Furthermore, we observed higher TMB levels in patients older than 61 years than in those who were 61 years or younger and in the higher T stage and N1 stage groups. According to reports from the TCGA database, the prognostic role of TMB is unclear. In a study of bladder cancer, the high TMB group exhibited increased OS compared with the low TMB group [
22]. Two reasons probably account for the results of our research. One reason is that not all patients with a high TMB have an increased treatment response, as not every generated neoantigen has immunogenicity [
23]. That is, high TMB does not always initiate an antitumour response. Another reason is that the high TMB group in this study was older and had a more advanced stage than the low TMB group.
GO and KEGG analyses revealed that the DEGs between the group with high TMB and the group with low TMB were related to the terms spindle, chromosomal region, kinetochore, nuclear division, chromosome segregation, cell cycle, oocyte meiosis, receptor-ligand activity, growth factor activity and ECM-receptor interaction, all of which are associated with DNA mutation and cell proliferation. Furthermore, the results of GSEA also supported this observation, showing enrichment of pathways related to pyrimidine metabolism, DNA replication, DNA degradation and aminoacyl tRNA biosynthesis. None of the analyses showed associations with pathways related to immune mediation or response. This phenomenon is likely because the immunogenicity of prostate cancer cases with low TMB is poorer than that of lung cancer and melanoma [
24]. Because of this poor immunogenicity, the numbers of neoantigens generated by prostate cancer patients may be less than those generated by high-TMB cancer patients. The subsequent immune response in patients with low TMB is probably also weaker than that of high-TMB cancer patients.
Through CytoHubba analysis of the PPI network, we identified six hub genes: PLK1, KIF2C, MELK, EXO1, CEP55 and CDK1. All the genes were correlated with DFS, and CEP55 and CDK1 were associated with OS. PLK1 and MELK have been suggested to be potential targets in prostate cancer. As PLK1 plays a critical role in the proliferation of cells, centrosome abnormalities, mediation of the cell cycle and apoptosis, it is considered a potential treatment target in prostate cancer [
25]. It has been reported that targeting PLK1 can enhance the response to androgen signalling inhibitors or olaparib in CRPC [
26,
27]. MELK is upregulated in prostate cancer and related to aggressiveness. Furthermore, in vitro silencing of MELK can weaken the proliferation of prostate cancer cells, and in vivo tests also proved that an inhibitor of MELK could repress the growth of prostate cancer.
Different kinds of immune cells play a pivotal role in the tumour microenvironment and are also determinants of immunotherapy efficacy. Therefore, our study explored differences in the levels of immune cells between the group with high TMB and the group with low TMB. The analysis indicated that the group with high TMB had a significantly higher proportion of CD8 T cells and activated CD4 memory cells than the group with low TMB. These findings were similar to those in previous reports of bladder cancer [
22]. Generally, a high TMB can produce more neoantigens, eliciting a subsequent immune response. CD8 T cells are one of the determinants of antigen-specific responses. Therefore, the high TMB group with higher levels of CD8 T cells is likely to experience superior immunotherapy efficacy. However, a recent study demonstrated that metastatic CRPC with low TMB but a high density of CD8 T cells could also benefit from immune checkpoint inhibitors [
28]. Our findings suggesting that TMB has an interaction with immune infiltration are only preliminary, and further validation in clinical cohorts and investigations to explore the underlying mechanisms of the correlations are needed.
The study has some limitations that should be considered. Firstly, all the data were retrospectively collected which had bias. Secondly, the results of our study were preliminary exploration and should be further tested through in vitro or in vivo experiments. Thirdly, our study lacked sub-group analysis including CRPC patients and non-CRPC patients.
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