2.1 TME Classification and Mutation Mapping of MSI-H Colorectal Cancer Patients
Bagaev et al. identified four tumour microenvironment (TME) subtypes. The four TME subtypes predict response to immunotherapy in multiple cancers, including CRC. A visual tool revealing the TME subtypes integrated with targetable genomic alterations provides a planetary view of each tumour that can aid in oncology clinical decision-making [10]. The MSI-H CRC transcriptome data were downloaded from TCGA based on a visual algorithm model of machine learning integrating transcriptomics and genomics, and the TME model classified the TME of MSI-H CRC patients into four clusters (Figure 1. A), Cluster1, immune-depleted (D), Cluster2, fibrotic (F), Cluster3, immune-enriched, fibrotic (IE/F), and Cluster4, immune-enriched, non-fibrotic (IE).
Among these, a significant enrichment of angiogenic fibroblasts was predominant in type F. Enrichment of tumour immune infiltrate cells were largely absent in type D, whereas type IE was dominated by significant enrichment of anti-tumour immune infiltrate (ATI) cells. Type IE/F was dominated by significant enrichment of both angiogenic fibroblasts and ATI cells.
Based on the signal distribution associated with the four staging types of CRC patients, principal component analysis (PCA), the most commonly used method for reducing data dimensionality, was performed on the four immunophenotypes (Figure 1. B). PCA analysis showed that type IE was an independent group compared to other types, only the IE and IE/F subtypes had partial overlap, whereas IE and other subtypes had less overlap and were relatively discrete.
2.2 Screening Confirmation of Specific Genes in Patients with IE Type
Based on the results of TME classification (IE, IE/F, F, and D types) of MSI-H CRC patients, a differential expression profiling was performed for IE and IE/F types, IE and F types, and IE and D types for screening relevant differentially expressed candidate genes. Among these, 62, 44, and 51 associated differentially expressed genes (DEGs) were found in IE vs IE/F, IE vs F, and IE vs D, respectively. Concurrently, the screened DEGs were overlapped, and one common DEG (APOC1) was screened out (Figure 1. C).
The correlation between APOC1 expression and tumour mutational burden (TMB) scores and each cancer type was analysed, and APOC1 expression and TMB were found to be strongly correlated with CRC (p = 0.00) (Figure 2. A). MSI is one of the biomarkers for immunotherapy in CRC [11], therefore, we first performed a correlation analysis between APOC1 expression and MSI and found that APOC1 expression in CRC was positively correlated with MSI (p = 7.3e-08) (Figure 2. B).
The correlation between APOC1 expression and MMR-related genes mutL homolog 1 (MLH1), mutS homolog 2 (MSH2), mutS homolog 6 (MSH6), PMS1 homolog 2, mismatch repair system component (PMS2), and epithelial cell adhesion molecule (EPCAM) in various cancers was also analysed (Figure 2. C). APOC1 expression in CRC was found to be significantly negatively correlated with MMR-related genes. MSI results from defective MMR protein function that leads to a highly mutated genome phenotype [12], therefore, APOC1 expression in different MMR-related genes further regulates and influences MSI status.
2.3 APOC1 is Positively Correlated with Signalling Pathways such as Myeloid Cells and Cytokines and Immune Scores
Gene Set Enrichment Analysis (GSEA) of the Kyoto Encyclopedia of Genes and Genomes (KEGG) for APOC1 expression revealed cell adhesion molecule (CAMs), haematopoietic cell lineage, and cytokine-cytokine receptor interaction pathways. (Figure 3. A)
Functional enrichment analysis of the hallmarks of cancer was performed at different APOC1 expression levels (Figure 3. C). The results showed that among the many cancer characteristic factors, complement, allograft rejection and interferon-gamma response factors were significantly positively correlated with APOC1 expression. From the GSEA of KEGG and hallmarks conducted, it was evident that APOC1 expression was associated with numerous immune-related signalling pathways, further corroborating the positive correlation between APOC1 expression and immunity.
It was revealed that APOC1 expression and immune signalling pathways were closely related through the analysis of signalling pathways. The ESTIMATE algorithm calculates the correlation between APOC1 expression and immune and stromal scores [13-16]. In CRC, the immune score is a reliable and valid clinical method for predicting the risk of recurrence in patients with colon cancer, and its prognosis assessment is superior to other prognostic factors, including tumour, nodes, and metastases (TNM) staging and MSI [17, 18]. We performed the ESTIMATE algorithm correlation coefficient analysis of the relationship between APOC1 expression and immunity in CRC (Figure 3. B) and found that APOC1 expression was positively correlated with immune scores (Spearman's rank correlation coefficient, R = 0.599, P ≈ 0).
We analysed the correlation between APOC1 expression and stromal scores using the ESTIMATE algorithm correlation analysis method (Figure 3. D). APOC1 expression was positively correlated with stromal scores in CRC patients (Spearman's rank correlation coefficient, R = 0.663, P≈ 0), indicating that high APOC1 expression had a strong positive correlation with stromal cells. The immune and stromal scores of APOC1 in CRC further demonstrated a strong correlation between APOC1 and CRC immunotherapy.
2.4 APOC1 is Highly Correlated with Immune-Related Cell Enrichment and Immune Checkpoints in COAD Patients
To further clarify the subtype of tumour immune infiltrating cells, the TIMER algorithm was applied to estimate the association between various immune cell types and the APOC1 expression signature. As shown in the scatter plot, the upregulation of APOC1 expression correlated with the infiltration of dendritic cells (DCs) and anti-tumour lymphocyte subpopulations (Spearman's rank correlation coefficient, P≈ 0) (Figure 4. A). Lymphocyte subpopulation analysis is an important indicator for cellular and humoral immune function with an overall response to the current immune function, status, and balance level of the body, and the significance of observing the efficacy of treatment and detecting prognosis [16, 19]. The analysis showed that APOC1 expression was positively correlated with immune-related cells and most closely correlated with the signal (R = 0.566) of the degree of enrichment of DCs, which possess antigen-presenting properties, called antigen-presenting cells (APCs, an important part of the immune system). DCs may be associated with APOC1 expression. To prove this conjecture, we explored the correlation of antigen-related genes with the APOC1 expression signature using Pearson's correlation coefficient (Figure 4. B). The neoantigen load was positively correlated with the APOC1 expression signature (Spearman's rank correlation coefficient, P = 1.45e-06), representing tumours with high antigen load. The high APOC1 expression can benefit immunotherapy.
APOC1 expression in different immune checkpoint factors and each cancer species was analysed using Pearson's correlation linear analysis (Figure 4. C). APOC1 was highly expressed in different immune checkpoint factors in CRC. Concurrently, APOC1 expression in clear cell renal cell carcinoma (ccRCC), uroepithelial, breast, cervical, and endometrial cancers and many other cancer types showed a more significant correlation with immune checkpoints.
2.5 Analysis of Single Cell Data Set and Other Validation Set Results in Colorectal Cancer
The GSE146889 dataset of CRC with MSI-H was downloaded from the Gene Expression Omnibus (GEO) database, and the typing pattern of the TME in this dataset was found to be the same as that of the TCGA dataset after analysis using machine learning algorithms (Figure 5. A). All four immunophenotyping patterns, including IE, IE/F, F, and D types, were more obvious, and there were significant differences in APOC1 expression in the different immunophenotypes (Figure 5. B). APOC1 expression was highest in the IE type, followed by the IE/F type and lowest in the D type.
To further explore whether high APOC1 expression was derived from tumour cells or TME, the single-cell dataset GSE132465 with MSI-H CRC was downloaded from the GEO database (mainly SMC03-T/SMC03-N, SMC06-T/SMC06-N, SMC10-T/SMC10-N, SMC24-T). The cell clustering distribution of these seven samples was obtained by the descending clustering analysis (Figure 5. C). We performed an enrichment analysis based on known canonical cell marker genes (Figure 5. D) and detected six major cell types, which were classified into epithelial cells, immune cells (T cells, B lymphocytes, and myeloid cells), stromal cell types, and other cells (Figure 6. A).
The results of cell cluster analysis revealed that CRC distribution was mainly from the epithelial tissue. The clustering of epithelial cells in CRC was further analysed by performing a dimensionality reduction clustering of epithelial cells from the seven samples (three paired samples and one tumour sample) (Figure 6. B left panel). The tumour tissue contained normal and malignant tumour cells, and the seven samples were clustered according to malignant and normal cells (Figure 6. B right panel). The correlation between the expression amount of APOC1 in malignant and normal cells in the seven samples was also analysed (Figure 6. C), APOC1 was more highly expressed in malignant cells than in normal cells (p = 4.5e-23). We analysed APOC1 expression in normal and tumour tissues in the TCGA-COAD dataset (Figure 6.D). APOC1 expression in tumour tissues was significantly higher than that in normal tissues. The same conclusion that APOC1 is highly expressed in malignant tumour cells in CRC patients can be drawn on different detection techniques and platforms,
Since APOC1 was highly expressed in malignant tumour cells, APOC1 expression in SMC03-T, SMC06-T, SMC10-T, and SMC24-T was further analysed (Figure 6. E). Although they are all tumour tissues, APOC1 expression was significantly higher in SMC06-T than in other tumour tissues. Our analysis of the factors of the expression levels may be more than just the difference between normal and tumour tissues, therefore, we further analysed the distribution of cells in these four tumour tissues. (Figure 6. F) The degree of infiltration of immune T cells in SMC06-T was the highest, and the distribution trend of T cells in the four samples was the same as that of APOC1 expression. This further confirms the correlation between APOC1 expression and immunotherapy.
We downloaded the IMvigor210 cohort dataset, and a survival curve analysis was performed on the dataset. Patients with high APOC1 expression in the IMvigor210 cohort had a highly significant survival time advantage over those with low expression (Figure 7. A). The data of patients who had undergone PD-1/PD-L1 immunotherapy were categorised into groups, the clinical response group, according to complete response (CR) and partial response (PR), and the clinical non-response group, according to stable disease (SD) and progressive disease (PD). The potential significant difference in APOC1 expression between the CR/PR and SD/PD groups was analysed using analysis of variance (ANOVA) (Figure 7. B). The results revealed a significant difference in APOC1 expression between the two groups. High APOC1 expression was associated with more responders than low expression, suggesting a strong correlation between high APOC1 expression and response to immunotherapy.
We further validated the correlation between APOC1 expression and neoantigen in the validation dataset (Figure 7. C). Consistent with the previous findings, high APOC1 expression was positively correlated with neoantigen.
APOC1 expression in the classical immune desert, excluded, and infiltrated phenotypes were also analysed (Figure 7.D). APOC1 was significantly correlated with each immunophenotype and highly expressed in the immuno-infiltrative type (p = 2.9e-25). This immunophenotyping analysis further demonstrated that APOC1 is highly expressed in CRC patients with the immuno-infiltrative type, which has a better prognosis.
Thus, we conclude that APOC1 can be used as an immunotherapeutic prognostic marker.