Enlibio,Your biological research partner
Xinqidi Biotech Co.,Ltd,Wuhan,China 2008-2022
R&D 14th year

The genomic landscape of cholangiocarcinoma reveals the disruption of post-transcriptional modifiers

Issuing time:2022-06-02 10:42

Abstract

Molecular variation between geographical populations and subtypes indicate potential genomic heterogeneity and novel genomic features within CCA. Here, we analyze exome-sequencing data of 87 perihilar cholangiocarcinoma (pCCA) and 261 intrahepatic cholangiocarcinoma (iCCA) cases from 3 Asian centers (including 43 pCCAs and 24 iCCAs from our center). iCCA tumours demonstrate a higher tumor mutation burden and copy number alteration burden (CNAB) than pCCA tumours, and high CNAB indicates a poorer pCCA prognosis. We identify 12 significantly mutated genes and 5 focal CNA regions, and demonstrate common mutations in post-transcriptional modification-related potential driver genes METTL14 and RBM10 in pCCA tumours. Finally we demonstrate the tumour-suppressive role of METTL14, a major RNA N6-adenosine methyltransferase (m6A), and illustrate that its loss-of-function mutation R298H may act through m6A modification on potential driver gene MACF1. Our results may be valuable for better understanding of how post-transcriptional modification can affect CCA development, and highlight both similarities and differences between pCCA and iCCA.

Introduction

Cholangiocarcinoma (CCA) is a malignant primary hepatobiliary disease originating from every point of the biliary tree, from the canals of Hering to the main bile duct1. In recent decades, the incidence and mortality of CCA have increased globally, especially in East Asia2,3. In China, the incidence of CCA reached 6–7.55 per 100,000 individuals4,5. The mortality of CCA is almost equal to its incidence due to resistance to common treatments and poor prognosis6. The overall 5-year survival rate is only 10%7. Even for the patients with CCA who are amenable to surgical resection, and the corresponding five-year survival rate is only 25–35%8, and high recurrence rates of 50 to 60% persist upon diagnosis, with no effective therapy.

CCA is regarded as a group of different diseases that are further divided into intrahepatic CCA (iCCA), perihilar CCA (pCCA), and distal CCA (dCCA) based on anatomical location9. pCCA is the most frequent subtype, accounting for 50–60% of all CCA tumors9. In general, the incidence and mortality rates of iCCA were reported rising worldwide in the past decade, whereas the incidences of pCCA seem to be stable or decreasing10. Besides the burden of HCV infection has been linked with the incidence of iCCA, infection with liver flukes and HBV-related liver diseases have shown a stronger association for iCCA in East Asia10,11. Primary sclerosing cholangitis (PSC) is considered as a predominant risk factor for CCA, especially for pCCA10,12. The pronounced geographical and etiology heterogeneity of iCCA and pCCA indicated the potential diverse cancer-initiating cell in these two subtypes of CCA13. In detail, iCCA is considered to originate two different cell of origin (mucin-secreting cholangiocyte or hepatic progenitor cell), due to the significant inter-tumor heterogeneity14. Unlike iCCA, pCCA only originated from mucin-secreting cholangiocyte13,15.

To date, previous studies of the genomic alterations in a variety of bile duct cancer, especially for iCCA, have demonstrated some known commonalities in the mutation of CCA. These studies identified a number of driver genes, but most of them represented the genomic features of iCCA, such as TP53, KRAS, SMAD4, and ARID1A16,17,18,19,20. Notably, the latest integrative genomic analysis of Caucasian extrahepatic CCA (eCCA) (including the pCCA and dCCA) revealed distinct molecular characterization of eCCA in West21. However, the epidemiological profile of CCA and its subtypes showed enormous geographical variation, indicating underlying genomic heterogeneity across different regions5. The prevalence of CCA is highest in East Asia and the subtype distribution is similar. Thus, integration analysis in sequencing data from Asian countries may find significant genomic features ignored by previous studies.

Here, we perform whole-exome sequencing on 43 pCCA and 24 iCCA, and analyze an additional 44 pCCA and 237 iCCA to investigate the genomic landscapes of pCCA and iCCA, respectively. Our study identifies significant mutated CCA driver genes and emphasizes the similar and different genomic characteristics between pCCA and iCCA. Notably, we highlight the effect of underlying post-transcriptional modification in CCA occurrence and development, and demonstrate that METTL14 and its loss-of-function mutation R298H play an important role in CCA process through N6-methyladenosine (m6A) mRNA methylation on the potential driver gene MACF1. Our results may be valuable for better understanding of post-transcriptional modification effects CCA development, and the similarities and differences between pCCA and iCCA.

Results

Mutational spectrum of pCCA and iCCA

We conducted integration analysis of exome-sequencing data from 87 pCCA cases (including 43 Nanjing Medical University (NMU) cases and 44 International Cancer Genome Consortium (ICGC) cases) and 261 iCCA cases (including 24 NMU cases, 135 ICGC cases, and 102 cases from Zou et al.’s study17). The general information and clinical features of patients from the NMU study are listed in Supplementary Table 1. The mean sequencing depth of the NMU study was 94.4× in tumor tissues and 91.8× in normal tissues dissected adjacent to the tumor. Detailed sequencing coverage and depth information are listed in Supplementary Table 2. We detected a total of 91,995 somatic mutations (Supplementary Table 2) in all the cases, and the median number per case was 120.5. The median tumor mutation burden (TMB) was 2.0 megabase per case (/Mb) and the burden was comparable between pCCA (2.0/Mb) and iCCA (2.0/Mb) (Wilcoxon rank-sum test P = 0.48, Supplementary Fig. 1A). Among these mutations, we defined 28,254 nonsynonymous mutations, for which the median was 33 single nucleotide variants (SNVs) and 3 small insertion and deletions (INDELs) per patient. Interestingly, the nonsynonymous mutation burden (median 0.61/Mb) of iCCA was significantly higher than that (median 0.47/Mb) of pCCA (Wilcoxon rank-sum test P = 9.6 × 104, Fig. 1A). Similar to the non-synonymous mutation pattern, iCCA had a higher copy number alteration burden (CNAB) than pCCA (iCCA median = 25.0%, pCCA median = 8.4%, Wilcoxon rank-sum test P = 2.4 × 10−4), no matter the amplification or deletion (Fig. 1C). The CNAB based on purity-adjusted copy number in iCCA was also significantly higher than it in pCCA (All burden: Wilcoxon rank-sum test P = 8.0 × 104; Amplification burden: Wilcoxon rank-sum test P = 1.9 × 106; Deletion burden: Wilcoxon rank-sum test P = 4.3 × 102; Supplementary Table 3).

Fig. 1: Mutational spectrum of patients with CCA.
figure 1

A The tumor mutation burden (TMB) of nonsynonymous mutations (substitutions and indels) between pCCAs (presented in red) and iCCAs (presented in blue). P value was obtained from Wilcoxon rank-sum test. B Summary composition of COSMIC 30 mutation signatures in pCCAs (red) and iCCAs (blue). COSMIC Signature MMR included signatures 6, 15, and 26; Signature liver included signatures 12, 16, and 24; Signature APOBEC included signatures 2 and 13. C Difference of copy number alterations burden (CNAB) no matter amplification or deletion between pCCAs (red) and iCCAs (blue). Wilcoxon rank-sum test was performed to determine the P value. Box plots depicted the median, quartiles and range. The whiskers in box plots extended to the most extreme data point which is no more than 1.5 times IQR. Outliers were identified using upper/lower quartile ±1.5 times IQR. D Cox proportional hazard model adjusted for age, gender, and stage was conducted to obtain HR and P value (n = 348). E, F Prognostic effect of CNAB in all samples. E CNAB in 43 pCCAs. F CNAB in 126 iCCAs. *P < 0.05; **P < 0.01, ***P < 0.001; TMB Tumor mutation burden, CNV Copy number variation, Amp Amplification, Del Deletion.

Next, we investigated the genomic mutational signatures, which provide clues for the carcinogenesis of CCA. Firstly, we classified all the mutations into six classic subtypes. Consistent with previous studies17,18, C > T/G > A substitutions accounted for the predominant type of all SNVs in all of the patients (Supplementary Fig. 1B). The proportion of the mutation subtypes was similar between pCCA and iCCA (Wilcoxon rank-sum test P = 0.46, 0.09, 0.67, 0.62, 0.51, and 0.29 respectively for C > A, C > G, C > T, T > A, T > C, and T > G, Supplementary Fig. 1B). We further analyzed mutational signatures using R package SignatureEstimation, which accurately reconstructs the mutation profiles of all pCCA and iCCA patients based on a predefined mutational spectrum of 30 COSMIC signatures22. COSMIC signature 1 (Age, 44.71%), signatures associated with mismatch repair deficiency (MMR, Signatures 6, 15, and 26, 31.99%), signatures associated with activated APOBECs (Signatures 2 and 13, 6.48%), and signature 8 (6.41%) were dominant (proportion > 5%) in pCCA patients (Fig. 1B). For iCCA patients, COSMIC signature 1 (27.19%) and signature MMR (26.74%) were also identified as dominant, but the proportion was slightly lower than in pCCA patients. In addition, signature 4 related to smoking behavior (10.13%), signature 22 (9.96%) related to aristolochic acid, and signatures specific to liver cancer (Liver, Signatures 12, 16, and 24, 8.7%) were predominant in iCCA. Interestingly, signature Liver were more common in HBV-related iCCA (HBV: 26.0%, non-HBV: 3.8%, Supplementary Table 4). Aristolochic acid-related mutations were mainly found in iCCA patients from Zou et al.’s study, but this signature was also observed in pCCA patients (Supplementary Table 5). In addition, signature 3, related to homologous recombination deficiency (4.26%), and signature 10, related to altered POLE activity (2.44%) were only identified in iCCA (Fig. 1B), which suggested that multiple types of DNA damage occurred during the carcinogenesis progress of CCA.

The results mentioned above revealed the nature of genomic instability in CCA patients. We further investigated the association between TMB/CNAB and the overall survival of patients. We found that the patients carrying higher loads of mutations showed worse outcomes after adjusting for age, sex, and stage at diagnosis (Hazard ratio (HR) = 1.71, P = 3.9 × 104, Fig. 1D). The association was consistent in pCCA and iCCA patients (Supplementary Fig. 1C). When examining CNAB, we noticed that the survival of pCCA patients with higher CNAB was significantly worse than for patients with lower CNAB (HR = 2.70, P = 3.6 × 102, Fig. 1E); However, there was no significant difference between iCCA patients with distinct CNAB (HR = 0.91, P = 6.7 × 101, Fig. 1F). Consistent findings were obtained from the prognostic effect of CNAB based on purity-adjusted CNAB (pCCA: HR = 2.89, P = 3.6 × 102; iCCA: HR = 0.84, P = 4.5× 101; Supplementary Table 6).

Crucial potential driver genes in CCA

We identified 36 significantly mutated genes (SMGs, Fig. 2), also described as mut-drivers, in all patients by MutSig2CV and IntOGen, including 24 reported SMGs and 12 SMGs not highlighted in previous CCA publications (Supplementary Table 7). Among these SMGs, MACF1 (5.17%, 18/348) and AXIN1 (0.86%, 3/348, all of the three patients were with iCCA), which is commonly considered as one of the potential drivers in hepatocellular carcinoma (HCC), are two interactive components of the β-catenin/Wnt signaling pathway (Fig. 2). Although the mutation rate of AXIN1 was low, we noticed that there were multiple non-synonymous mutations in two iCCA patients (Fig. 2). The mutations of PIK3R1, a component of the PI3K pathway, was displayed in 2.59% (9/348) of patients. Post-transcriptional modification genes RBM10 (3.16%, 11/348) and METTL14 (0.86%, 3/348) act as well-known alternative splicing regulators and m6A writers. Importantly, we identified recurrent mutations in METTL14, and all mutations in this gene affected the same amino acid (Supplementary Fig. 2B), which were also predicted as potentially deleterious variants by several bioinformatic pathogenicity prediction tools (Supplementary Table 7). We observed the same mutation rate (1.44%, 5/348) of chromatin modifiers SMARCA4 and WHSC1. Interestingly, mutations in these genes were prone to co-occur in the same patient (Fisher’s exact test OR = 660, P = 2.29 × 106, Fig. 2). In addition, classic cancer driver genes identified in other cancers were also identified as SMGs in this study, including ATM (3.45%, 12/348), BRCA2 (2.01%, 7/348), and MLLT4 (2.01%, 7/348). Although BRCA2, EPHA2, and ATM were mentioned in previously published work due to their same position recurrent inactivating mutations, our integrated analysis with more sample size identified them as SMGs.

Fig. 2: Mutation landscapes of curated significantly mutated genes (SMGs) and recurrent mutation genes in CCA.
figure 2

Each column represents one CCA patient and each row represents a gene. The gene symbols in red are our newly identified CCA-related genes, and genes in black have been reported in CCA. The bar chart at the left panel indicates the overall mutational rate of curated genes in all CCAs. The right panel is the mutation spectrum and mutation rate for pCCAs and iCCAs, respectively. The asterisks following gene symbol indicate there are recurrent mutations in the relevant genes.

We further defined the genes with a distinct mutation rate between pCCA and iCCA. TP53, ARID1A, PBRM1, MACF1, EPHA2, ARID2, IDH1, PTEN, RB1, BRAF, NRAS, SLC8A1, AXIN, and MLLT4 were iCCA-enriched genes, while RBM10, TGFBR2, PIK3R1, ELF3, NACC1, and METTL14, were pCCA-enriched genes. The mutation rates of TP53 and IDH1 were significantly higher in iCCA (Fisher’s exact test ORTP53= 1.3, PTP53 = 8.8 × 103; ORIDH1 = + ∞ , PIDH1 = 4.4 × 102, Fig. 2). Importantly, IDH1 only mutated in iCCA, but not in pCCA. RBM10, however, carried significantly more mutations in pCCA than in iCCA (Fisher’s exact test ORRBM10 = 3.8, PRBM10 = 3.2 × 102).

We further identified 22 regions by focal copy number alteration (CNAs) in CCAs (Fig. 3A, Supplementary Table 8), including 16 regions overlapping with previously reported peaks, and six significantly altered regions not reported before (amplification at 7q31.2, 22q11.21, and deletion at 1p36.13, 2p24.1, 7q35, and 12q24.33, Fig. 3A). The cancer-related genes located at these peaks were considered to be CNA-driver genes. The gain of a 0.8-Mb region at 7q31.2 occurred in 45 samples (26.6%), which contains the classic MET oncogene in the RTK/RAS pathway. However, the amplification of 7q31.2 was found rare previously in Caucasian population. The loss at 1p36.13 involved tumor suppressor gene SHDB and affected 76 samples (45.0%). DNA Polymerase Epsilon (POLE) at 12q24.33, where was also considered lost in HCC, lost one copy in 40 patients (23.7%). In addition, important DNA methylation factor DNMT3A (2p24.1) and chromatin methylation factor EZH2 (7q35) also show loss in 20 (11.8%) and 19 (11.24%) patients (including iCCA and pCCA), respectively. Whereas these two regions were also only considered deleted in iCCA based on a previous single nucleotide polymorphism array analysis. We only observed heterogeneous loss in CCA patients (Fig. 3A). Among all of the regions, the frequencies of amplification/gain at 8q24.21 (MYC), 1q21.3 (S100A7), 5p15.33 (SDHHA), and 7q31.2 (MET) were significantly higher in iCCA (Supplementary Table 8). The frequency of high-level amplification was significantly higher in pCCA than in iCCA, although the frequency of amplification/gain was comparable across the subtypes (Fig. 3A). In addition, we found that copy number loss and nonsynonymous mutations of classic tumor suppressors (TP53, CDKN2A, SMAD4, PTEN, and ATM) were prone to co-occur in the same patients (Fisher’s exact test OR = 8.4, P = 8.2 × 106, Fig. 3B).

Fig. 3: Frequently altered focal CNA regions and characteristics of mutation-driver and CNA-driver genes.
figure 3

A Shows 22 curated frequently-altered focal CNA regions involving cancer-related regions. Each column represents one of the CCA patients and each row represents a focal region. The bar chart at the left panel indicates the overall CNA rate in all CCAs. The right panel is the CNA spectrum and rate for pCCAs and iCCAs, respectively. The top panel represents amplification and the bottom represents deletion regions. Region names in red and blue indicate the novel alterations the plus sign following the gene symbol indicates a significant alteration rate difference between pCCAs and iCCAs. Asterisk means the genes in focal region were also identified in SMGs. B The co-occurrence of copy number losses and mutations of relevant tumor suppressor genes. CPathway enrichment results of mutation-driver and CNA- driver genes in TCGA pancancer ten pathways, P value of hypergeometric test. *P < 0.05; **P < 0.01, ***P < 0.001.

Next, we performed pathway enrichment analysis on all potential driver genes identified in this study, which included SMGs (mut-drivers) and cancer genes in the frequently altered focal CNA regions (CNA-drivers) described above, and found that the potential driver genes of CCA were significantly enriched in the RTK-RAS, Wnt, PI3K, Cell Cycle, TP53, TGF-beta, and HIPPO pathways (Fig. 3C). Including the identified MET amplification, 34.3% of CCA patients harbored mutations and CNAs in oncogenes from the RTK-RAS pathway, and these alterations occurred in a mutually exclusive manner, as reported in other cancers (Supplementary Fig. 2A).

Functional recurrent mutations in METTL14

Among the potential driver genes mentioned above, we identified a potential driver gene, METTL14, the main factor involved in aberrant m6A modification of various cancers, with recurrent and deleterious mutations23,24 (Supplementary Fig. 2B, Supplementary Table 9). All three mutations affected the same 298 amino acid residue (METTL14 p.R298H and p.R298C) and two of them were in the same genomic position (two patients from NMU and ICGC respectively) (Supplementary Fig. 2B). We further conducted sanger sequencing in an independent pCCA cohort with extra 40 subjects from NMU cohort and identified an additional p.R298H carrier (Supplementary Fig. 2C, D). In the COSMIC database, we also found that the same p.R298H also occurred in three pancreatic ductal carcinomas (Supplementary Fig. 2B) and all the three patients were East Asian.

Crystal structures of the METTL3-METTL14 complex have revealed that p.R298 lies close to the putative RNA-binding groove of the complex which may have a complex role to affect methylation activity25. The recurrent and deletions mutant p.R298H suggested us its possibly positive selection and the need of METTL14’s normal action in antitumor progress. However, the relevance of this hotspot mutation and m6A mRNA methylation to the CCA has not yet been established. Dysfunction of METTL14, the key catalytic protein forming the core m6A methyltransferase complex, has shown fundamental biological effects in cancer initiation and progression24,26. Thus, we hypothesized that CCA could be associated with METTL14 that regulated m6A mRNA methylation. Hence, we first examined METTL14 expression in 69 CCA pairs and matched adjacent normal tissues. The results of qRT-PCR and western blot revealed that the expression of METTL14 was significantly downregulated in tumors (P < 0.01; Fig. 4A, B and Supplementary Fig. 7A). The immunohistochemistry (IHC) staining on tissue microarray also confirmed that METTL14 staining was decreased in CCA at the protein level (Supplementary Fig. 3A) and its downregulation displayed a significant association with poor cancer-specific survival in CCA (Log-Rank P = 0.045; Supplementary Fig. 3B). Consistently, we found that the m6A level of total RNA was significantly decreased in CCA tissues (Fig. 4C). Interestingly, we also observed the m6A modification level was significantly decreased in METTL14 low expression CCA group compared with METTL14 high expression CCA group (Supplementary Fig. 3C). These results suggested that METTL14 and it mediated m6A modification were frequently downregulated or disturbed in CCA.

Fig. 4: METTL14R298H mutation reduced the METTL14wt tumor-suppressive effect in CCA.
figure 4

A The protein level of METTL14 was validated in CCA tissues. B Downregulated METTL14 mRNA expression was detected in 69 pairs of iCCA and pCCA tumor tissues as well as the adjacent normal tissues by qRT-PCR (p = 0.0064). C The level of m6A modification in tumor and adjacent normal tissues (n = 8). D The transfection efficiency of lentiviral constructs expressing METTL14wtand METTL14R298H in RBE cell line (n = 3). E METTL14R298H reduced METTL14wt-mediated m6A modification detected by m6A colorimetric quantification in RBE cell line (n = 3). F Proliferation curve of RBE cells with METTL14R298H, METTL14wt or negative control (n = 3). G Apoptotic assay of RBE cells with METTL14R298H, METTL14wt, or negative control were determined by a PI and annexin V double-staining assay and analysis by flow cytometry (n = 3); p(NC vs WT) < 0.001, p(WT vs R298H) < 0.001). H Colony formation assay of RBE cells with METTL14R298H, METTL14wt, or negative control. The numbers of colonies were counted and presented in the histogram (n = 3). I Representative images (left) and quantification (right) of transwell migration (p(NC vs WT) = 0.0024; p(WT vs R298H) = 0.0194) and invasion (p(NC vs WT) < 0.001, p(WT vs R298H) < 0.001) assays in RBE cells with METTL14wt, METTL14R298H or negative control (n = 3). J Top panel is a photograph of excised tumor tissues from mice. Bottom panel is the average weight (p(NC vs WT) = 0.0011; p(WT vs R298H) = 0.0066)and volume (p(NC vs WT) < 0.001; p(WT vs R298H) = 0.0005) of harvested tumors in the NC, METTL14wt and METTL14R298H groups (n = 5). The data are presented as the mean ± standard deviation. K METTL14R298H reversed the tumor-suppressive effect of METTL14wt in the lung metastasis experiment in vivo. Left panel is representative images taken at six weeks in NC, METTL14wt and METTL14R298H groups (n = 3). Data are shown as mean ± SEM. The P values were calculated using unpaired two-sided Student’s t test with no correction for multiple comparison. *P < 0.05, **P < 0.01, ***P < 0.001; T, Tumor tissue, N, normal tissue, R298H, METTL14R298H; WT, METTL14wt; NC, negative control.

To determine the “driver” role of METTL14 and R298H mutations during CCA development, we first verified that there were no R298H mutations of METTL14 in CCA cell lines (RBE and HCCC9810) using sanger sequencing (Supplementary Fig. 3G), and then confirmed the transfection efficiency of lentiviral constructs expressing METTL14wt and METTL14R298H in RBE and HCCC9810 cell lines (Fig. 4D, Supplementary Figs. 3D, E, and 7C). In contrast to the cells stably overexpressing METTL14wt, cells overexpressing METTL14R298H showed significantly decreased overall m6A modification ability (Fig. 4Eand Supplementary Fig. 3F). Subsequently, while overexpression of wild-type METTL14 affected cell proliferation and apoptosis, overexpression of the mutation had no noticeable effect on proliferation and apoptosis (Fig. 4F–H, Supplementary Figs. 3H, I, L, and 4A). In addition, overexpression of METTL14wt resulted in a decrease in cell migration and invasion, and METTL14R298H remarkably reversed the gene’s ability to inhibit migration and invasion in CCA cells (Fig. 4I and Supplementary Fig. 3J, K). Taken together, these results provided us evidence for loss of function caused by METTL14R298Hmutation, suggesting that METTL14R298H mutation could reduce the tumor-suppressive effect of METTL14wt in CCA.

To further verify the functional roles of METTL14wt and METTL14R298H in vivo, we observed that METTL14wt effectively suppressed tumor growth as reflected by the significant reduction of tumor size and weight in nude mice, and in a lung metastasis model, METTL14wt could also restrain pulmonary metastasis significantly. Consistent with results from in vitro experiments, the inhibition of METTL14R298H on tumor proliferation and lung metastasis was apparently attenuated in vivo when compared to the METTL14wt group (Fig. 4J, K). Therefore, these data implied that loss of function caused by METTL14R298Hmutation was sufficient to repress the tumor suppressor function of METTL14wt in CCA.

Potential driver gene MACF1 served as the target of METTL14

We then conducted RNA-Seq and MeRIP-Seq assays in negative control, METTL14wt, and METTL14R298H cells. A total of 2,601 peaks involving classic transcripts of 1,586 genes were robustly identified by exomePeak2 in all cells. The most common m6A motif, GGAC, was significantly enriched in the m6A peaks identified (Supplementary Fig. 4B), and the m6A peaks were especially enriched in the vicinity of the stop codon. Next, we performed differential m6A-methylation analysis between control and METTL14wt cells, as well as between METTL14wt and METTL14R298H cells. Because of the writer role of METTL14 during the m6A methylation modification, we included only m6A peaks (712) with increased abundance in METTL14wt cells as compared to control cells, and peaks (990) with decreased abundance in METTL14R298H as compared to METTL14wt cells. A total of 237 peaks were shared by both analyses, including four peaks on the two potential driver genes mentioned above (MACF1, MET) (Fig. 5A, B, Supplementary Fig. 4C). MACF1 was also the potential driver gene identified in this study (Supplementary Table 7). We then verified whether m6A-modified MACF1 was susceptible to decay. The lifetime of MACF1 was prolonged in METTL14R298H cells and shortened in METTL14wt cells after actinomycin D treatment (Fig. 5C, and Supplementary Fig. 4D). We further immunoprecipitated m6A from the RNAs of METTL14wt and METTL14R298H cells and found that METTL14R298H significantly decreased the amount of MACF1 modified by m6A compared to METTL14wt (Fig. 5D, and Supplementary Fig. 4E). According to the previous study, p.R298 of METTL14 belongs to the groove between METTL3 and METTL14 which might be responsible for RNA binding, hence we supposed that METTL14R298H may regulate the enrichment of MACF1 mRNA25,27. And we found that METTL14-specific antibody obversely enriched MACF1 mRNA in METTL14wt compared to the METTL14R298H group using RNA immunoprecipitation (Fig. 5E, and Supplementary Fig. 4F). We then applied immunofluorescence assays and western blot to further confirm that METTL14wt mediated MACF1 degradation, and METTL14R298H showed increase expression of MACF1 compared to METTL14wt (Fig. 5F, G, Supplementary Figs. 4H, I, and 7B). We also found that METTL14wt cells decreased the expression of MACF1, and no noticeable effect on MACF1 expression was observed in METTL14R298H cells using qRT-PCR (Supplementary Fig. 4J).

Fig. 5: M6A modification mediated expression of MACF1 in β-catenin nucleocytoplasmic transport.
figure 5

A Overlap of significantly different m6A modification peaks identified from METTL14R298H compared with METTL14wt, METTL14wt compared with NC and mutation-driver genes. B The log2 scaled fold change of different m6A modification peaks in WT versus NC and R298H versus WT. C RNA lifetime for MACF1 in RBE cells transfected with METTL14R298H, METTL14wt, or NC. D The m6A modification level of MACF1 in RBE cells was validated in MeRIP (n = 5). E Immunoprecipitation of METTL14-related RNA in NC, METTL14wt and METTL14R298H was conducted in RBE cells followed by RT-qPCR to detect the amount of MACF1 mRNA binding to METTL14 (n = 3). F Representative images of MACF1 immunofluorescence in METTL14wt and METTL14R298H (n = 3). GExpression of MACF1 following NC, METTL14wt and METTL14R298H was evaluated by western blotting. All data are representative of at least two independent experiments with similar results. HColony formation assay of RBE cells with NC and si-MACF1 (n = 3). I Apoptotic assay of RBE cells with NC and si-MACF1 were determined (n = 3; p(NC vs si#1) = 0.0004; p(NC vs si#2) < 0.0001). J Proliferation curve of RBE cells with MACF1 and si-MACF1 (n = 3; p(NC vs si#1) < 0.0001; p(NC vs si#2) = 0.0001). K Transwell migration and invasion assays in RBE cells with MACF1 and si-MACAF1 (n = 3; Migration:p(NC vs si#1) = 0.0076; p(NC vs si#2) = 0.003; Invasion: p(NC vs si#1) = 0.0417; p(NC vs si#2) = 0.0430)). L QRT-PCR results revealed that expression of METTL14 and MACF1 was negatively interrelated in CCA tissues (n = 82). M Representative images of β-catenin immunofluorescence showed nuclear β-catenin tended to be increased by expression of METTL14R298H compared to METTL14wt in RBE cells. All data are representative of at least two independent experiments with similar results. N Western blot analysis was performed to analyze β-catenin in the cytoplasmic, membrane and nuclear extracts, and expression of E-cadherin, N-cadherin, PCNA, and CyclinD1 in total RBE cells extracts. All data are representative of at least two independent experiments with similar results. The P values were calculated using unpaired two-sided Student’s t test with no correction for multiple comparison. Data are shown as mean ± SEM. *P < 0.05, **P < 0.01, ***P < 0.001.

When we successfully transfected CCA cells with siRNA pools targeting MACF1 (Supplementary Figs. 4K, L, and 7D), we found that knockdown of MACF1 significantly increased cell apoptosis and reduced CCA cell proliferation and metastasis in functional assays (Fig. 5H–K, and Supplementary Fig. 5A–F). Recent studies reported that MACF1 was involved in tumor metastasis and cytoskeleton, and that it played crucial roles in the nucleus translocation of β-catenin28,29. In our findings, we first validated MACF1 upregulation in CCA using qRT-PCR(Supplementary Fig. 4G), and noticed that lower METTL14 mRNA level exhibited stronger MACF1 expression (Fig. 5L). Given the essential role of MACF1 in regulating the nucleus translocation of β-catenin, immunofluorescence assays and western blotting analysis showed that the increase of nuclear β-catenin was correlated with the expression of METTL14R298H rather than METTL14wt (Fig. 5M, N, Supplementary Figs. 5G, H, and 7E). To ascertain the role of MACF1 in METTL14-mediated nucleus translocation of β-catenin, we transfected MACF1 siRNA in METTL14R298H-overexpressing cells, and observed that the nucleus translocation of β-catenin was decreased compared to only METTL14R298H-overexpressing cells (Supplementary Fig. 5I).

Additionally, METTL14R298H resulted in reversing the expression level of E-cadherin, N-cadherin, PCNA, and CyclinD1, which were the downstream targets of β-catenin, relevant to METTL14wt (Fig. 5N, Supplementary Figs. 5G, and 7B, E). These results implied that METTL14-mediated m6A modification repressed the MACF1/β-catenin pathway in CCA, while METTL14R298H mutation disrupted this mechanism.

Discussion

In this study, we included genomic data from 348 CCA samples (including 87 from pCCA and 261 from iCCA) to investigate the genomic landscapes of both pCCA and iCCA. We found the shared and distinct features between these two anatomical subtypes. Currently, there is no effective biomarker that can accurately predict the prognosis of CCA patients30,31. Our study found that TMB has a good predictive effect on the prognosis of East-Asian CCA patients. It is worth noting that pCCA patients with higher CNAB showed worse survival outcomes compared to those with lower CNAB. These findings suggest that we can explore prognostic markers of patients with CCA from a more macroscopic genetic perspective.

In addition to several significant driver genes in classic pathways, we identified driver genes participating in post-transcriptional modification (i.e., METTL14 and RBM10), which carried more mutations in pCCA patients. With in vitro experiments, we confirmed the role of METTL14, as well as the role of this mutation in CCA development. We further determined that m6A modification level and expression of potential driver gene MACF1 could be regulated by METTL14, which can influence the proliferation and metastasis ability of CCA cells. Notably, although we provide evidence to demonstrated that METTL14-mediated m6A modification repressed the MACF1/β-catenin pathway in CCA, while METTL14R298H mutation disrupted this mechanism, m6A-related key catalytic protein would be more complicated on the WNT pathway and the underlying mechanism is not fully characterized. Previously, ALKBH5 reducing m6A levels of WIF-1 and hindering activation of Wnt signaling32. YTHDF bound METTL3-mediated m6A of APC mRNA and upregulated Wnt/β-catenin pathway33. Given the importance of Wnt signaling in regulating cancer tumorigenesis, METTL14-MACF1-WNT would be more complicated, which deserves further investigation.

The previous study showed that there was lack of evidence that anatomical site determines molecular subtypes according to integration analysis of transcriptomic and methylation data34. Our study identified four potential driver genes that were related to chromatin and methylation modification (i.e., mut-drivers: SMARCA4 and WHSC1; CNA-drivers: DNMT3A and EZH2), further supporting the finding that driver genes cause disruption at in the transcriptional level. Consistent with the previous study’s findings, we found comparable frequency between these alterations of potential driver genes. However, our analysis identified two genes as drivers (METTL14 and RBM10) and found that they are related to post-transcription modification. Mutations in these two genes were more common in pCCA patients. This evidence suggested that the investigation of the difference between pCCA and iCCA should be extended to the post-transcriptional area. METTL14 engages in m6A modification35,36, which modulates alternative splicing, export, stability, and translation of mRNA37. Interestingly, we found that two potential driver genes (MACF1 and MET) could be modulated by the m6A alteration caused by METTL14 mutations. It was worthy of note that all of these genes had a higher mutation rate or copy number altered rate in iCCA than in pCCA. Thus, they were crucial potential driver genes in both iCCA and pCCA. However, they could be activated through different mechanisms. Although MACF1 and MET could serve as shared therapeutic targets for pCCA and iCCA, we should not ignore the fact that METTL14 could modulate m6A modification of broader genes other than the two drivers, which may lead to unexpected drug resistance or side effects, and therefore, further investigation in pCCA patients is warranted. Another gene, RMB10, is an RNA binding protein and alternative splicing regulator frequently mutated in lung adenocarcinomas38. Similar to lung cancer, the majority of RBM10 mutations (63.6%) in CCA truncate the protein and may alter downstream splicing of specific genes38,39. Thus, the post-transcriptional events in CCA could be highly disturbed and further study may help determine new molecular subtypes and optimize therapeutic strategy in the clinical setting.

Our study also identified a number of crucial potential driver genes that were not reported in previous studies, involving multiple important pathways. MACF1 and AXIN1 were identified as SMGs of CCA in our study. They are involved in a complex that also contained CTNNB1, GSK3B, and APC, and can contribute to the activation of β-catenin/Wnt signaling pathways28,40. Consistently, the mutations in the two genes were mutually exclusive. CNA-driver MET belongs to the classic RTK-RAS signaling pathway, and its inhibitor has been widely used in other cancers41,42. Although the MET amplification has been reported in CCA, our study highlighted the presence of MET amplification is a genomic feature of pCCA, and reported with rare amplification frequency in Caucasian iCCA43,44, suggesting that it may occur more frequently in Chinese patients. According to OncoKB, nearly 106 (30.5%) of all CCA patients carried actionable alterations in the RTK-RAS signaling pathway. PIK3R1, phosphatidylinositol 3-kinase regulatory subunit alpha, is the predominant regulatory isoform of PI3K45 and is frequently mutated in multiple cancers46,47. ATM served as the activator of the TP53 tumor suppressor protein and somatic ATM mutations or deletions that are commonly found in lymphoid malignancies, pancreatic cancer, and lung adenocarcinoma48. Notably, our results suggested that ATM could be affected by both truncating mutations and deletion, resulting in the bi-allelic inactivation. Several known tumor suppressors, such as TP53, SMAD4, PTEN, and CDKN2A, showed a similar bi-allelic loss pattern. Germline mutations in BRCA2, which encodes a BRCA-associated protein, were reported in familial CCA cases49,50. Our study also suggested that BRCA2 could be affected by somatic mutations. MLLT4, also known as AF6, commonly fused with KMT2A in acute myeloid leukemia. It has also been identified as a SMGs in breast cancers51. Few genomic alterations has ever been reported in NACC1, but it is activated in ovarian serous carcinomas and influences cell apoptosis, senescence, and cytokinesis in cancer cells52,53. Integrated whole-exome analysis of Asian CCAs identified significant CCA driver genes and emphasized the similarities and distinctions genomic characteristics between pCCA and iCCA. Importantly, our study also highlighted the effect of underlying post-transcriptional modification in CCA occurrence and development. These results provide a better understanding of the CCA mutational landscape that may drive improvements in clinical practice.


Article classification: Biological abstract
Share to:
Tel:+86-027-87610298
Tel:+86-027-87610297
Add:Room A11-329, 1st Floor, No.1, SBI Venture Street, Optics Valley, East Lake
New Technology Development Zone, Wuhan, China.
Certificate NO.:U18Q28010569R0S