Patients
We retrospectively analyzed the gene expression profiles of frozen colorectal cancer tumor tissue samples from one of the largest individual data sets: CIT/GSE39582 CRC cohort. The data set was obtained directly in its processed format from GEO database through Bioconductor package ‘GEOquery’. Overall patients were included in this study. The batch effects were corrected using ‘combat’ algorithm implemented in R package ‘sva’ and z-scores for each gene were used for the following analyses. Both paper charts and electronic medical records were carefully reviewed when necessary.
Construction and analysis of protein-protein interaction network
To find potential therapeutic targets on colon CSCs, nine colon stem cell markers were selected from previous studies to construct a protein-protein interaction (PPI) network related to colon CSCs (Table 1). The protein interaction information of these markers were obtained using the BioGRID database (Version 3.5.168)[21, 22]. To investigate the association of chemotherapeutic sensitivity, 232 patients with chemotherapy and complete prognostic information in the CIT cohort (GSE39582) were used as the discovery data set.[23] To obtain genes related to prognosis of colorectal cancer and avoid the influence of sample distribution, the corresponding genes resulted from PPI analyses were further selected using the log-rank test with 1000 times randomization (80% portion of samples each time) to assess the association between each gene and patients’ disease-free survival in the discovery cohort. Genes with significant frequency more than 500 in repeated log-rank tests were identified as key genes.
Validation cohort
The CIT cohort, one of the largest individual data sets of colorectal cancer is used to validate the potential therapeutic value of TMEM17. The expression of TMEM17 were analysed in 17 cancer samples and its paired normal tissue, while other 566 patients’ data were used to conduct a prognostic analysis. The optimal cut-off point of TMEM17 expression was determined based on disease-free survival (DFS) information of these patients using the function 'surv_cutpoint' from R package 'survminer'.
Short interfering RNA
The used TMEM17 siRNA sequence: si-TMEM17 #1: GCAGCATTATGATGCTTCA; si-TMEM17 #3: GGTCATGTATAGAAGAGAT. The Lipofectamine RNAiMAX kit (Invitrogen) was used for siRNA transfection following the manufacturer’s instructions. Cells were transfected with 100 nM final concentration of siRNA duplexes at the optimal seeding density. After 24 hours, cells were digested and re-seeded for following experiments.
RT-quantitative PCR
Total RNA was isolated using TRIzol reagent (Life Technologies, Carlsbad, CA) and the RNeasy Mini Kit (Qiagen, Germany) and reverse transcribed into cDNA using the cDNA Synthesis Kit (Transgen Biotech, China). RT-PCR was performed using and the KAPA SYBR Fast qPCR kit (KAPA Biosystems, Wilmington, MA). For quantification of mRNA levels, 18S level was used as an internal control. The specific primers used for TMEM17 were: 5’- GTTCAGTGATTCCAATCGGACC- 3’; 3’- ACCACAGTGGGAA ATAGTAGGT- 5’.
Immunoblotting
Cells were collected and lysed in RIPA buffer supplemented with protease and phosphatase inhibitors for 30 mins. Equal amounts of protein extract were separated on SDS polyacrylamide gels and transferred to polyvinyl difluoride (PVDF) membranes. Membranes were blocked with 5% BSA for 2 hours at room temperature and then probed primary antibody overnight at 4 °C. The used antibodies were: anti-TMEM17 (Santa Cruz, CA; sc-514433), anti-EPCAM (Beyotime, China; AF0141), anti-LGR5 (Abcam, UK; ab75850), anti-MYC (Abcam, UK; ab32072), anti-Vimentin (Cell Signaling Technology, USA; #5741), anti-Snail (Cell Signaling Technology, USA; #3879) and anti-GAPDH (Cell Signaling Technology, USA; #5174).
Cell proliferation assay
For the cell proliferation assay, optimal cells were plated in triplicate in a 96-well format. After 24 hours, the medium was refreshed with optimal drug treatment. Cells were then lysed with CellTilter-Glo (CTG, Promega, Madison,WI), and the fluorescence signal was detected with a microplate reader on days 0, 2, 3, 4 and 5.
Colony formation assay and Tumorsphere formation assay
For the clonogenic assay, optimal cells were seeded in 6 well plates and refresh the medium every three days in 37℃. Colonies were formed after 8 to 10 days culture. The colonies were fixed with methanol and stained with crystal violet (0.5% crystal violet, 20% methanol).
For the tumorsphere assay, single-cell suspensions were plated (5000 cells/well) in a 12 well ultra-low attachment plates with Mammocult medium (Stem cell Technologies), supplemented with fresh hydrocortisone (0.5μg/ml) and heparin (1:500) and culture in 37℃. 0.5ml fresh medium was added every three days and tumorspheres were formed after 7-10 days culture. The spheres were stained with 2-(4-iodophenyl)-3-(4-nitrophenyl)-5-phenyl-2H-tetrazolium chloride (INT) (Sigma-Aldrich) and quantified.
Pathway Analysis
Enrichment analysis for differentially expressed genes between high/low TMEM17 expression using R package ‘gProfileR’ in the CIT data set. The cut-off point of high/low TMEM17 expression was based on the optimal cut-off point in disease-free survival (DFS) analysis in the CIT. For interested biological pathways, Gene Set Enrichment Analysis (GSEA) was further performed using Bioconductor package ‘HTSanalyzeR’.[24, 25]
Tissue microarray and immunohistochemistry staining
A total of 318 CRC patients with pTNM stage Ⅰ to Ⅲ from January 2002 to June 2006 were included in this study and the pathological specimens were constructed in a tissue microarray (TMA). Tumour staging was assessed according to the criteria of the Seventh Edition of the American Joint Committee on Cancer (AJCC) stage.[26] The clinicopathological data were collected from the CRC database of the follow-up office and approved by the Institutional Review Board of the The Sixth Affiliated Hospital, Sun Yat-sen University.
Paraffin-embedded tissue samples were cut into 5μm sections and antigen retrieval was performed with citrate buffer (Beyotime, China; P0081). After blocking with 10% goat serum, samples were incubation with primary antibody overnight at 4°C, followed by diaminobenzidine staining. IHC staining was evaluated in semi-quantitative method as described before.[27] Each TMA spot was marked with an intensity score and percentage of positive tumour cells score from 1 to 4. TMA scores were determined by the intensity score multiply proportion of area score. A final score was determined as the average of the duplex. The optimal cut-off point of TMEM17 expression was conducted based on X-tile software (X-tile 3.6.1).[28]
Statistical analysis
Graphs were expressed as mean ± SD from three independent experiments. Statistical difference between two groups was evaluated by two-tailed student’s t-test, and that of multiple groups was analysed by two-way ANOVA. Survival curves were evaluated by Log-rank (Mantel-Cox) test. P-values < 0.05 were considered as statistically significant.