High expression of CPNE5 and CPNE9 predicts positive prognosis in multiple myeloma

1. Introduction

Multiple myeloma (MM) is a common malignant tumor disease of blood system, which is characterized by abnormal proliferation of clonal plasma cells. It is estimated that there were 176,404 new MM patients in 2020, accounting for 0.9% of all cancer patients, and 117,077 deaths of MM patients, accounting for 1.2% of all cancer deaths [1]. Although the prognosis of MM patients has improved with the improvement of MM diagnosis and treatment, only 45% of patients have an overall survival (OS) of 6 years [2]. It is currently believed that genetic changes, bone marrow microenvironmental effects and epigenetic modifications play an important role in the occurrence and development of MM [3, 4, 5]. Although genetic abnormalities have made great progress in the assessment of MM progression and prognosis, there is still an urgent need to find more effective prognostic biomarkers, which are of great clinical significance for optimizing the personalized treatment plan and improving the clinical efficacy of MM patients. It is also a current research hotspot.

CPNEs are highly conserved intracellular calcium-dependent lipid-binding proteins originally identified by Creutz et al. [6] from the extracts of paramecium tetraurelia, which play a fundamental role in eukaryotic cell biology. The N-terminal of CPNEs contains two C2 domains and a domain similar to the A domain, in which the C2 domain plays a role in membrane transport and the A-like domain mediates the interaction between integrins and extracellular ligands. The HUGO Gene Nomenclature Committee has identified nine homologues of CPNEs in humans, namely CPNE1-9. Jiang et al. [7] found that the down-regulation of CPNE1 can inhibit the proliferation and metastasis potential of osteosarcoma cells and reduce the expression of a variety of tumor-related genes. In non-small cell lung cancer (NSCLC), inhibition of CPNE1 expression can effectively reduce the proliferation and movement of cancer cells [8]. In addition, CPNE3 is also highly expressed in a variety of cancer cells and can regulate ErbB2 signaling pathway-mediated movement and migration of cancer cells [9, 10, 11]. Genetic variants in CPNE5 have been found to be associated with alcohol dependence and obesity [12]. As a protein specifically expressed in the brain, CPNE6 has a high degree of homology with human CPNE1, and is closely related to synaptic plasticity and memory formation [13, 14]. A recent study found that Ca 2 + -dependent CPNE6 can interact with synaptobrevin2, thereby selectively inhibiting spontaneous neurotransmission [15]. CPNE7 is mainly involved in the differentiation and formation of odontoblasts [16, 17]. The invasive acute myeloid leukemia with t(12;21) translocation may be closely related to the fusion of CPNE8 and AML1/Runx1 [18].

The CPNEs have been deeply researched in a variety of diseases, especially for the pathogenesis, treatment and prognosis of malignant tumors. However, the prognostic value of CPNEs in MM is still unclear. This study explored the asscociation between the expression levels of CPNEs and survival of MM patients.

2. Subjects and methods

3. Results

4. Discussion

In our study, a total of 729 patients were enrolled to explore the prognosis role of CPNE1-9 in MM. High expression of CPNE5 and CPNE9 were proved to be positive prognosis indicators. CPNE5 was an independent favorable predictor for EFS and OS, and CPNE9 was an independent positive indicator for EFS. Combination of high expressions of CPNE5 and CPNE9 might be a better predictor for the survival of MM patients.

CPNE5 mainly exists in cytoplasm of human cells and can be detected in the heart, stomach, spleen, lymph nodes and testis [25]. In esophageal squamous cell carcinoma, the expression of CPNE5 in cancer tissues is lower than that in adjacent tissues, and patients with low CPNE5 expression have shorter OS. Moreover, low expression of CPNE5 may promote the local growth of esophageal cancer and increase the resistance of chemotherapy drugs [26]. These findings suggest that CPNE5 is closely related to tumorigenesis and may affect the prognosis of cancer patients. In our study, MM patients with high expression of CPNE5 had longer OS and EFS. And the clinical and molecular characteristics analysis found that LDH, ALB and the expressions of CDK4 and FGFR3 in the CPNE5 high group were lower than that in the CPNE5 low group. All these factors are related to MM progression or prognosis. For instance, LDH is an independent poor prognostic factor for MM, which promotes tumor growth, increases tumor burden and chemotherapy drug resistance [27]. ALB is an important indicator for evaluating the prognosis of MM in the International Staging System (ISS), when ALB ⩾ 3.5 g/dL and B2M 3.5 mg/L, the patients in stage I have the longest median survival time [28]. As a target gene in MM, inhibition of CDK4 can promote apoptosis of cells, thereby exerting an anti-cancer effect in MM cells [29]. FGFR3 is an oncogene and plays an essential role in the progression of MM [30]. Therefore, the positive prognosis effect of CPNE5 may be partly due to the association of CPNE5 with these prognostic factors. To this end, we further conducted a multivariate analysis and revealed that CPNE5 is an independent prognosis indicator for MM patients. Collectively, high expression of CPNE5 might be an independent positive indicator in MM.

A study by Liu et al. [31] found that CPNE9 is specifically expressed in pancreatic tumor tissues, which indicates that CPNE9 is related to cancer progression. Moreover, CPNE9 was the top five of the prognostic 14-gene signatures that constructed the prognostic model with a high predictive ability for glioblastoma [32]. This suggests that CPNE9 has important clinical significance for the prognosis assessment of glioblastoma patients. In our study, MM patients with high expression of CPNE9 had longer OS and EFS. At the same time, the CPNE9 high group had lower expressions of B2M, CRP, LDH and HK2 compared with the CPNE9 low group. In R-ISS, patients with B2M ⩾ 5.5 mg/L, high-risk chromosomal abnormalities and/or high LDH levels are in stage III and have the worst prognosis [33]. Elevated CRP can promote osteoclast differentiation and accelerate bone destruction mechanism, which also has a negative impact on the prognosis of MM [34, 35]. Meanwhile, the level of HK2 is negatively correlated with EFS and OS, and may become a potential therapeutic target for MM [36]. These clinical and molecular characteristics negatively correlated with CPNE9 expression are all poor prognostic factors for MM, which are consistent with our findings and further confirm that the high expression of CPNE9 is related to the positive prognosis of MM.

Multivariate Cox regression analysis showed that LDH and CPNE5 were independent prognostic factors for EFS and OS. ALB, B2M and CPNE2 were independent prognostic factors for OS, and CPNE9 was an independent prognostic factor for EFS. These results further confirmed that high expressions of CPNE5 and CPNE9 were positive prognostic factors in MM patients. The co-expression analysis of CPNE1-9 in MM showed that there was no obvious expression correlation between CPNE family members, indicating that there was no mutual regulation mechanism between CPNE5 and CPNE9. In addition, the survival analysis of the combined expression of CPNE5 and CPNE9 found that the survival probabilities and cumulative events of CPNE5 High CPNE9 High group were significantly higher than other groups. Therefore, the combination of CPNE5/CPNE9 may become a better positive prognostic factor for MM patients.

5. Conclusion

In summary, we have shown that high expressions of CPNE5 and CPNE9 are positive prognostic factors in MM patients, especially for the combination of CPNE5 and CPNE9, and they may be potential targets for MM therapy. But the specific molecular mechanisms of CPNE5 and CPNE9 in the development of MM are still unclear. Further studies with larger sample size are needed to validate our findings, and the pathogenesis of CPNE5 and CPNE9 in MM also needs additional exploration.