Introduction
SPDL1 is also called coiled-coil domain containing-99 (CCDC99), which encodes Spindly. Spindly is a coiled-coil domain-containing protein that involves in cell division and chromosome segregation, including mitotic spindle formation and spindle checkpoint [
1,
2]. The spindle assembly checkpoint (SAC) monitors the attachment between microtubules and kinetochores during prometaphase. In response to unattached kinetochores, the SAC generates the mitotic checkpoint complex (MCC), a multimeric assembly that can delay chromosome segregation [
3,
4]. Spindly involves in the SAC silencing which causes the inhibition of MCC formation, subsequently changes the metaphase status to anaphase status [
5‐
7]. The inhibition of spindly leads to unstable interactions between centromeres and microtubules, leading to defects in chromosomal arrangement, prophase delay, and cell accumulation during mitosis [
8,
9]. Besides, Spindly can interact with components of the RZZ complex (Rod/ZW10/Zwilch) and the dynein-dynactin complex at kinetochores [
9‐
11]. Spindly colocalizes with dynein/dynactin at the leading edge of migrating cells and cells lacking Spindly migrate slower than wild type cells, which means Spindly may play an important role in the cell migration process [
12,
13].
In addition to its role in normal tissue cells, SPDL1 is also involved in the tumor growth and prognosis. To date, several studies have reported that the CCDC99 over-expressed in tumor tissues in different cancer types, including lung cancer [
14], oral squamous cell carcinoma (OSCC) [
15], pancreatic ductal adenocarcinoma (PDAC) [
16]. High expression of Spindly was an independent indicator for poor prognosis and associated with cellular proliferation in OSCC [
15]. However, the recent study showed that overexpression of SPDL1 in PDAC was associated with a better prognosis [
17]. SPDL1 was defined as a candidate tumor suppressor in colorectal cancer (CRC) to play an essential role in the downstream of Myocardin-related transcription factor B (MRTFB) that could regulate CRC growth and survival [
18]. Moreover, Spindly inhibition could enhance the cytotoxic activity of paclitaxel due to an increase in the duration of mitotic delay which provide a novel insight into circumventing paclitaxel resistance in cancer treatment [
19]. Interestingly, the variant of SPDL1 reduce the incidence of different cancer types due to lower chromosomal alterations accumulated over time [
20]. SPDL1 was identified as tumor infiltration CD8+ T cell-related genes in a prognostic model in LUAD, which indicate the potential relation of SPDL1 and immunity [
21].
Considering the heterogeneity of different tumor types, a pan-cancer analysis is needed to get a comprehensive understanding of SPDL1. In this study, we dissected the oncogenic role of SPDL1 across all cancer types of TCGA from various aspects, including gene expression, genetic alteration, methylation level, protein phosphorylation, immunology, survival prognosis, and gene enrichment analysis, which can further uncover the potential molecular mechanism of SPDL1 in cancers and its value in clinical prognosis of cancer patients.
Discussion
SPDL1 is considered to play an important role in the cell cycle, which is associated with the progression of many cancers. As far as we know, there is no information available about the analysis of SPDL1 in pan-cancers. Thus, in this study we have analyzed the molecular features of SPDL1, such as gene expression, genetic alteration, protein expression, methylation level, protein phosphorylation, immune infiltration, survival prognosis, and gene enrichment analysis in different cancer types.
Spindly is reported to be overexpressed in lung cancer cells [
19], and we confirmed this through a public online database. For lung cancer in TCGA, the relationship between survival prognosis and SPDL1 expression was detected, and we found that high SPDL1 expression was related with poor OS and DFS in LUAD (P = .0015, P = .03, respectively) but not in LUSC (P>.05) (Fig.
2) based on the GEPIA2 tool. Additionally, we conducted a survival analysis in 865 LUAD and 675 LUSC, and found that high SPDL1 expression was associated with poor OS, PPS in LUAD, and poor FP in LUSC using the Kaplan-Meier plotter tool (Additional file
3: Table S2). We then performed IHC in LUAD and found a statistical correlation between high SPDL1 expression and poor OS (P = .03841). In conclusion, our results revealed that the high SPDL1 expression in lung cancer was linked to poor OS. Tumor-infiltrating CD8+ T cells have a vital effect on the immune response in lung cancer [
38‐
40]. Hence, several immune-related prognostic models have been developed [
41,
42]. SPDL1 was regarded as one of the genes associated with CD8+ T cells infiltration in the early stage of LUAD, which may provide valuable predictions for the survival risk of patients [
43]. However, we did not find the significant correlation between SPDL1 expression and the immune infiltration level of CD8+ T cells in all stages of LUAD with different algorithms, but we found it to be statistically significant in LUSC (Additional file
2: Fig. S10). In LUAD, the expression of SPDL1 is significantly related to TMB, HRD and aneuploidy and may be involved in the tumorigenesis of LUAD.
As shown in the KEGG pathway analysis, SPDL1 was associate with “Oocyte meiosis”. Knockdown of Spindly restored the asymmetric division of oocytes, which was a normal process in mammalian before fertilization [
44]. We next explored the role of SPDL1 in the reproductive system. Notably, we found that only the comparison of SPDL1 phosphorylation levels in S555 with OV showed the significant differences (Additional file
2: Fig. S9A). In contrast, there was no association between SPDL1 expression and genomic instability in OV. The effect of SPDL1 phosphorylation on OV tumorigenesis needs further investigation. We observed that high expression of SPDL1 was associated with poor OS, RFS, PPS in ovarian cancer, especially in subgroups, such as “grade 4”, “stage I, III, IV” (Additional file
3: Table S3). More experiments are needed to confirm the roles of SPDL1 in prognosis of ovarian cancer. Survival analysis showed a correlation between high SPDL1 expression and poor OS in UCEC. Compared to UCEC cases without SPDL1 alteration, DSS and PFS were better in cases with SPDL1 alteration, but DFS and OS were worse. Enhanced RNA tissue specificity was shown in the normal testis samples. Moreover, SPDL1 expression was not significantly correlated with genomic instability and the survival rate in TGCT. All in all, the SPDL1 gene may not be associated with the development of the TGCT.
A previous study reported that SPDL1 was a tumor suppressor gene for colorectal cancer (CRC), which acted in the downstream of MRTFB to regulate CRC growth [
18]. In our work, there was no significant correlation between survival and SPDL1 expression in COAD and READ, but it showed a trend that lower SPDL1 expression levels were associated with poor prognosis. The expression of SPDL1 was associated with TMB, MSI, HRD in COAD and READ. The hub gene set variation analysis (HGSVA) score of the gene set, which include the SPDL1, may reflect the pathological progression from liver cirrhosis to HCC, and SPDL1 expression was an independent prognostic factor for both OS and RFS [
45]. In our research, it showed a correlation between high SPDL1 expression and poor OS and DFS in LIHC. Based on the Kaplan-Meier plotter, a high SPDL1 expression level was associated with poor OS, DMFS, PPS, RFS in breast cancer. Interestingly, when we analyzed the patients with negative ER and positive PR of breast cancer, SPDL1 low expression is significantly correlated with poor OS. Hence, we should also focus on the other clinical characteristics. Notably, in contrast with most of other tumors, a low expression level was linked to poor OS, FP, PPS in gastric cancer (P < .05). This suggested that the SPDL1 gene worked differently in gastric cancer.
To analyze the role of SPDL1 in cancers, we performed an enrichment analysis of SPDL1-related genes and proteins. It was involved in “Cell cycle” and “Platinum drug resistance” based on KEGG analysis, and there was a publication used SPDL1 to detect cell cycle progression [
46]. This finding was also consistent with the previous analysis that inhibiting of Spindly was cytotoxic to oral squamous cell carcinoma (OSCC) cells and increased its chemical sensitivity to cisplatin [
15]. Paclitaxel, one of the Microtubule-targeting agents (MTAs), delays cells in mitosis which could lead to cell death in mitosis (DiM) through the accumulation of an apoptotic signal [
47,
48]. Insufficient expression of SPDL1 was observed during paclitaxel resistance, and spindle inhibition increased the efficacy of low-dose paclitaxel [
14,
19]. Above all, we need to study the potential therapeutic benefits of combining SPDL1 inhibition with platinum drug.
Although the correlations of SPDL1 expression with survival prognosis, genetic alteration, genomic instability, DNA methylation, protein phosphorylation and immune infiltration were computationally explored and analyzed, some limitations should be acknowledged. Firstly, the experiment was only conducted in LUAD tissues, but not in other tumors. The second limitation is the lack of both the molecular and cellular mechanism of the SPDL1 expression. In the future, more experiments and large-scale clinical trials are needed to further validate these findings and to explore the follow-ups of functional mechanisms of SPDL1 in cancers.
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