The CTCs are invasive circulating metastatic cells migrating from primary or metastatic sites of tumors. These cells, which survive in circulating blood, function as metastasis precursors and may also colonize, forming secondary tumors and causing refractory and recurrent cancer[19–21]. Thus, any molecular predictors which demonstrate a high specificity to OS and a strong correlation to metastasis might be applied as CTC detection tools.
In this study, the differences in gene expression between OS cell lines or primary OS cells vs healthy donor cells identified significant upregulation of 8 novel OS-specific genes (COL1A2, GJA1, PLS3, COL5A2, COL3A1, CDR1, COL2A1, and EGR1). Most of them, including COL1A2, GJA1, COL5A2, EGR1 and COL3A1, have been previously reported as upregulated genes or the translated proteins of those genes associated with OS progression in OS tissues when compared to normal tissues[22–26]. In addition, high expression of COL2A1, CDR1 and PLS3 has also been found to be related to tumor progression in several types of tumors[27–31].
The previously reported OS markers Ezrin and VIM were also analyzed for gene expression in retrospective samples. Ezrin, a cross-linker protein, plays an essential role in many metastatic phenotypes of cancers including pediatric sarcomas, OS and rhabdomyosarcoma[32]. The expression level of Ezrin was high in OS circulating cells, especially in OS metastatic stage III in the Enneking staging system[11]. Vimentin, a mesenchymal marker, has recently been reported to be an indicator of epithelial-to-mesenchymal transition (EMT) associated with migration and metastasis in various cancers as well[33, 34]. Vimentin has also been reported to be highly expressed in human OS tumor tissue[35].
The CTC enrichment process is necessary to discard blood components which might [36]. There are several techniques to enrich CTCs from fresh whole blood including Ficoll-Hypaque density gradient centrifugation, filter-based methods, magnetic bead based CD45 negative and vimentin positive selection[10, 11, 37] which need fresh whole blood or an affinity column for isolating CTCs. The samples for gene expression analysis in our study were enriched by buffy coat preparation. The total RNA was extracted from the − 80oC frozen buffy coats without preservatives since the frozen cells would otherwise be lysed by ice crystallization after thawing leading to injury[38]. To avoid losing CTC-total RNAs and to reduce the number of enrichment steps, total RNA from all buffy coats was extracted immediately after thawing. Other genetic components which are not released by CTCs, especially leukocyte RNAs, were main interfering factors which were not able to be discarded in this study. Previous publications have demonstrated that leucocyte common antigen (CD45) expression level is related to a number of leukocytes[39, 40]. Accordingly, the gene expression level of each OS cell line spiked PBMC sample was presented as a relative quantification curve between OS-specific gene and CD45.
We expected a correlative relationship between the expression of OS-specific genes and the number of OS cells, i.e., an expression pattern of the relative quantification curve of the specific OS genes (n = 8) and the two evaluated metastatic genes. The results demonstrated that only seven genes (COL1A2, COL3A1, COL5A2, EGR1, PLS3, Ezrin and VIM) out of the 10 genes tested showed a completely or partially separated pattern for at least one of the 5 OS cell lines (Table S1 and Fig. S2-6). CDR1, COL2A1 and GJA1 were not highly expressed among the five OS cells (SaOS-2, MNNG, MG63, U2OS and 143B) resulting in interference when spiked OS cell lines had high numbers of PBMCs, causing the level of expression of those genes to be an unreliable indicator.
Measurement of candidate genes expression was also performed in clinical samples, including OS and healthy buffy coats. The expression of EGR1, PLS3 and VIM showed statistically significant differences between the two groups (P < 0.05) but not COL1A2, COL3A1, Ezrin and COL5A2 (Fig. 3A). Not surprisingly, OS exhibits a high heterogeneity and complexity of genomic and expression level between patients[41].
We further narrowed the samples to patients and donors age younger than 25, then reanalyzed the expression of seven candidate genes. The results indicated that for COL1A2, COL3A1, Ezrin and COL5A2, all gene expression differences between OS patients and donors were noticeably increased while the differences for COL3A1, Ezrin and COL5A2 bordered on statistical significance (P < 0.1).
The efficiency of each candidate gene in the prediction of OS was evaluated with binary logistic regression with the MFP algorithm using relative expression data. The model that included VIM, Ezrin, COL1A2 and PLS3 performed the best in terms of ability to discriminate OS samples from healthy donor samples. We identified the OS probability cut-off point at 0.2943. In further clinical studies, patients with a higher possibility of OS than the cut-off point will require confirmation of results with standard diagnostic tests. Patients who present with an OS probability higher than 0.8243 are suspect for metastasis occurrence; in those cases, follow-up with high sensitivity micro-metastasis tests such as bone scans or PET scans is appropriate. In this study, the model identified all OS patients as positive; there were two false positive samples from normal buffy coats at a probability cut-off value of 0.2943. The same model also exhibited the ability to predict OS metastasis at a probability cut-off value of 0.8243 with all positive results in metastatic OS (III) samples and some false positive results in non-metastatic OS and normal samples. We suggest that patients with positive results should be informed and that metastasis should be confirmed using other clinical tools.
The origin of OS is known to be mesenchymal stem cells. Most OS is malignancy developing from osteoblast cells with genetic and epigenetic mutation accumulation[42]. Due to the limited number of samples, it was not possible to isolate circulating OS cells from frozen buffy coat. To confirm whether candidate genes were specific to OS cells, the expression of COL1A2, EGR1, PLS3, Ezrin and VIM was evaluated in primary OS cells with qRT-PCR and compared to PBMCs from healthy donors. Among the four candidate genes, the expression of COL1A2, PLS3 and VIM in OS cells was significantly higher than PBMC (P < 0.05) but not Ezrin [Fig. S8].
In agreement with previous studies, our qRT-PCR results demonstrated that VIM normally expresses in OS during diagnosis and consistently expresses in both buffy coats and primary cells[35, 42, 43]. Expression of COL1A2 has been found in human osteoblast lineages[44] and has been shown to be related to migration, invasion, proliferation, and metastasis promotion in various cancers including OS, with significantly higher expression in buffy coats and primary cells[23, 45, 46].
The roles of PLS3 on actin (F-actin) formation in normal bone have been previously described. Oncology studies have demonstrated that PLS3 is a novel CTC marker for prognosis in breast and colorectal cancer[31, 47]. In our study, a significantly higher expression level of PLS3 was found in OS patients when compared to healthy donors. Thus, PLS3 could be a novel liquid biopsy marker for prognostic prediction in OS.
On the other hand, ezrin, which has been reported to be a typical EMT marker, was borderline significantly highly expressed in frozen OS buffy coat but not in primary OS cells. Due to the fact that there was no difference in Ezrin expression between OS primary cells and PBMCs, we suggest that the population of clusters of tumor cells with the potential to become CTCs in the tumor population was low. However, Ezrin expression in primary cells might not be an indication of CTC clusters. High expression of Ezrin, both in RNA and in protein levels in OS patients, was positively correlated with metastatic stage and OS recurrence[48, 49].
Comparative expression analysis in this study did not find significant differences in VIM or Ezrin between OS primary cells and PBMCs, although they have previously been reported as OS candidate genes[11, 50]. These gene expression unrepresented in whole tumor population, this might be definitive for OS cells in circulating blood. Comparative expression analysis of genes between single circulating tumor cells and other circulating blood cells should be further investigated. This could potentially improve the detection of OS circulating cells by identifying more precise predictors.