2.1.Expression of m1A related proteins and its correlation with pathology stage in glioma
Based on GEPIA, a total of 163 glioma and 207 normal samples were included in TCGA data and the GTEx projects. The expression patterns of m1A related proteins in glioma were explored. As shown in Fig.1A, among the nine m1A regulators composed of writer (TRMT6, TRMT61A, TRMT10C), eraser (ALKBH1, ALKBH3), and reader (YTHDF1-3, YTHDC1), TRMT10C, ALKBH1, YTHDF1, YTHDF2, and YTHDF3 were significantly upregulated (P<0.05) in glioma compared with normal samples (Fig. 1A).
Next, the correlation between the expression levels of m1A regulators and clinical-pathological characteristic was also investigated with data from CGGA (the Chinese Glioma Genome Atlas) and TCGA. We found that, in TCGA database, the m1A regulators were significantly upregulated in WHO IV glioma compared to WHO grade II/III glioma except TRMT61A (Fig 1B, p < 0.05). Similarly, the TRMT6, TRMT10C, ALKBH1, YTHDF1, YTHDC1 in CGGA dataset were also significantly upregulated in WHO grade IV glioma (p<0.05), though no significant difference was observed in the expression levels of TRMT61A, ALKBH3, YTHDF3 in each stage group. Moreover, in both CGGA and TCGA database, YTHDF2, was significantly upregulated in WHO grade IV glioma compared to WHO grade III/ II glioma (Fig. 1B, p < 0.001). These results suggested that m1A regulators, especially YTHDF2, were remarkably upregulated in glioma and were positively correlated with pathology stage.
2.2. Survival analysis of m1A regulators in glioma
Kaplan-Meier analysis and a log-rank test were performed. Data samples were classified into high-expression group and low-expression group according to the optimal cut-off value of m1A regulators gene expression levels in CGGA and TCGA, respectively. As shown in Fig. 2, in data from CGGA database, high expression of YTHDF1 (Fig. 2A, p=0.047) and YTHDF2 (Fig. 2A, p=0.013) was associated with shorter overall survival rate (OS) of glioma. Consistently, in TCGA database, high-expression of ALKBH1 (Fig. 2B, p=0.031), ALKBH3 (Fig. 2B, p=0.027, YTHDF1 (Fig. 2B, p=0.039) and YTHDF2 (Fig. 2B, p=0.017) was also significantly correlated with shorter overall survival rate. However, no significant correlation was observed between the OS and the expression level of TRMT6, TRMT61A, TRMT10C, YTHDF3 and YTHDC1 in both CGGA and TCGA database (Fig. 2).
2.3. YTHDF1 and YTHDF2 were dysregulated in IDH mutated or 1p19q co-deleted glioma
IDH mutation was found to be a common genetic mutation promotor in glioma, while chromosome 1p/19q codeletion is increasingly being recognized as the crucial genetic marker for glioma patients and have been included in WHO classification of glioma in 2016[13, 14]. Hence, the expression level of YTHDF1 and YTHDF2 in IDH mutated or 1p19q co-deleted glioma were also explored. As shown in Fig. 3A, compared with wild type glioma patients, the YTHDF1 and YTHDF2 expression level were downregulated in IDH mutated patients both in CGTA and TCGA dataset. Consistently, the YTHDF2 expression level in 1p19q codeletion group was also significantly downregulated compared with 1p19q intact group (Fig. 3A). There was a general m1A regulator gene alteration in glioma, though no regular routine was observed (Fig. 3B). YTHDF1 and YTHDF2 had the highest alteration frequency of 4% and 3%, respectively. Followed by TRMT6 and TRMT61A, which presented an alteration frequency of 2.4%. ALKBH1 and YTHDF3 had an alteration frequency of 2%, while TRMT10C and ALKBH3 altered in 1.7% of all samples. Meanwhile, there was a rare alteration of YTHDC1 in glioma with a lowest alteration frequency at only 1.2% (Fig. 3B).
2.4. YTHDF2 co-expressed gene analysis in glioma
As shown in Fig. 4A, the YTHDF2 co-expressed gene in glioma were analyzed via LinkedOmics. In the TCGA and CGGA dataset, genes whose correlation coefficient against YTHDF2 expression bigger than 0.5 were selected. Next, the intersection of YTHDF2-related genes in TCGA and CGGA dataset were turned to GO enrichment and KEGG pathway analysis. The co-expressed genes were divided into three function groups: cellular component group, molecular function group, and biological process group. As depicted in Fig. 4B, YTHDF2 co-expressed genes were mainly involved in function related to ‘chromatin binding’, ‘catalytic activity, acting on RNA or DNA’, ‘histone binding’, ‘helicase activity’, ‘ATPase activity’ and ‘structural constituent of nuclear pore’. Furthermore, the co-expressed genes were mainly enriched in several bioprocesses, including mRNA processing or mRNA splicing, RNA splicing, chromosome segregation and mitotic nuclear division (Fig. 4B). Additionally, in the molecular function group, the genes were most likely the component of chromosome region, chromatin, nuclear speck, spindle, spliceosomal complex or kinetochore of cells. The KEGG analysis showed that these genes mainly enriched in pathways related to cell cycle, spliceosome, RNA transport, or mRNA surveillance pathway.
2.5. gene set enrichment analysis (GSEA) and Protein interaction analysis
Gene enrichment analysis indicated that the following gene sets including: GCNP_SHH_UP_LATE.V1_UP, E2F1_UP.V1_UP, VEGF_A_UP. V1. DN, BIOCARTA, IGF1MTOR_pathway, ERBB2_up.V1_DN, MYC_UP.V1_UP, BIOCARTA_G2_pathway and BIOCARTA_G1_pathway were enriched in YTHDF2 high expression group (Fig5A). The downstream candidate genes of YTHDF2 in glioma were analyzed by PPI network based on STRING database and Cytoscape. As shown in Fig. 5B, a total of 15 genes including HMGB2, ZNF644, DUSP12, UBAP2L, SRSF10, SFPQ, ELAVL1, NCBP1, SKIV2L2, CNOT6, CNOT1, PTBP1, HNRNPA2B1 and YTHDC1 were found to interact with YTHDF2. Additionally, the protein expression level of YHTDF2 was also found to be upregulated in glioma according to the immunohistochemical data from the Oncomine and HPA databases(Fig 5C).