The high mortality associated with CCA is the result of the late presentation common in the disease, and resistance to current treatment strategies [1,44]. With the development of sequencing, data regarding tumour development is accumulating and demonstrates the clear heterogeneity of CCA. How to use this information to benefit patients is of highest practical significance. In this study, the most frequently altered genes in CCA were chosen to produce a predictive model for overall survival. Results demonstrate that genetic alteration of APC, PTPRT, SRC, TP53, BRAF, CSMD1, FBXW7, MACROD2, RBFOX1, SYNE1, TTN, and WWOX are associated with altered patient overall survival in CCA. These results are clinically meaningful as they can be used to predict patient outcome and inform treatment strategies.
Patient characteristics and outcome
As cBioPortal data is a collective of multiple studies, several clinical details such as stage at presentation or treatment regimens were not available for assessment for all patients. The stage at diagnosis was available for 818 patients, while the BMI was available for 229 patients. Whilst the data is clearly not complete, sufficient data was available for assessment of these characteristics. Large impacts on overall survival would have been observed based on the data presented here. Non-significance observed is the result of genuine features of CCA or potentially the result of characteristics unique to the database itself.
The most significant patient characteristics at presentation observed in this cohort was age and the location of the cancer. Patients over 60 years of age were associated with a worse overall survival, and this is in line with the current understanding in CCA [45,46]. For location, right sided CCA was associated with a worse outcome as has been described elsewhere [38].
Whilst TMB showed no observed effect in this study, this may be the result of how this was calculated here. The appropriate definition of TMB is the total number of mutations per coding area of a tumour genome [47]. It is widely hypothesised that higher levels of mutations in tumour cells produce more neoantigens, which will be recognised by T-cells and destroyed [48]. This has been demonstrated to lead to a higher sensitivity to treatment with immune checkpoint inhibitors [49,50]. It is interesting to note that alterations in TTN were associated with worse overall survival in this study (Table 2) and has been closely associated with TMB in other studies [51]. Su et al. (2021) demonstrated that a TTN mutation was associated with a greater response to immune checkpoint inhibitors, supporting previous comments [52]. Furthermore, in CRC patients with low TMB, therapy based on irinotecan was associated with improved progression free survival compared to oxaliplatin-based chemotherapy (11.9 vs. 6.5 months) [53].
Tumour suppressor genes and patient outcome
Alterations in tumour suppressor genes, such as APC, TP53, and PTPRT, resulted in an improved 5-year survival. APC encodes for a large multidomain protein that plays a role in intercellular adhesion and microtubule function [54,55]. The primary function of APC is to negatively regulate the wingless (Wnt)-pathway by forming a complex with β-catenin [55,56,32]. This complex will phosphorylate β-catenin for proteasomal degradation, reducing transcription activation and controlling cell growth (Figure 13) [32,54].
This study observed APC alteration in 57% of CCA patients, in line with the literature (45 to 80%), APC is the most common mutated gene in CCA [56,55,32,57]. This study and the wider literature observed truncation mutations as the dominant alteration [32,54]. Truncation leads to activation of the Wnt-pathway and pooling of β-catenin, leading to activation of T-cell factor and lymphoid enhancer factor families, resulting in activation of cellular MYC Proto-Oncogene [55,32,58]. The improved survival associated with APC alteration is supported by the literature, with wild type APC more commonly associated with right sided CCA and a poorer outcome [59,22,60].
TP53 encodes for P53 protein, which halts cell division in the G1 phase. This results from activating p21, which inhibits cyclin-dependent kinases [33,61,62]. TP53 suppresses tumour formation by inducing cell apoptosis by activating Bax protein and death receptors, such as death receptor 5 (Figure 14) [62]. Missense mutations in the central DNA binding domain are the most common alterations in TP53 [62,33]. Losing TP53 function leads to the transition of benign adenoma to invasive carcinoma [63]. The expression of ATP-binding cassette subfamily B member 1 increased by altered TP53, leads to enhanced drug efflux and chemoresistance [64]. In this study, TP53 alteration significantly improved overall survival which is in line with previous studies [65,66].
PTPRT inhibits signal transducer and activator of transcription 3 (STAT3) and paxillin function (Figure 15) [67-69]. STAT3 is an oncogene that promotes cell proliferation and metastasis [67,68]. STAT3 function by promoting Bcl-XL and Cyclin D1 [67,68]. Paxillin is a signal transduction adaptor protein that promotes tumour growth, cell migration, and metastasis [67,69]. 27% of CRC have PTPRT alterations, with missense mutations as the most common alteration, leading to loss of its function and decrease the overall survival by 45% [67,70,68,34,69]. In contrast, PTPRT amplification will potentiate its tumour suppressive role and inhibit tumour growth [34]. Overall, this study observed a survival advantage when PRPRT alterations were investigated as a collective group, however it is important to note the specific difference that is attributed to types of mutation.
In contrast to the previous genes, alteration in tumour suppressor genes, such as FBXW7 resulted in a worse overall survival. FBXW7 encode for the recognition components in a complex that ubiquitinates defective proteins, especially proto-oncogenes, leading to its degradation (Figure 16) [23,71]. In the current study, 10.3% of FBXW7 were altered, aligning with the reported range of 6-10% in CRC cases [41,72,73]. Missense mutations accounted for 70% of FBXW7 alterations, with R465 being the most common site [74,23,75]. Loss of FBXW7 function due to missense mutations will accumulate its targeted proto-oncogenes, resulting in cancer development [41-43].
Oncogenes and patient outcome
The major oncogene investigated in this study was BRAF, encoding a serine/threonine-protein kinase [35,76]. It is part of the mitogen-activated protein kinase (MAPK) pathway, promoting cellular proliferation and survival and preventing apoptosis (Figure 17) [36]. BRAF is activated by the kinase activity of KRAS Proto-Oncogene in response to extracellular stimuli such as EGFR [35]. Activated BRAF will phosphorylate and activate MAPK 1 and 2, activating extracellular signal-regulated kinases (ERK). ERK will regulate several cellular activities and activate multiple transcription factors [37]. Aligning with the current study, BRAF is altered in around 10% of CRCs, with missense mutations as the most common alteration (~90%). The substitution of valine to glutamic acid at codon 600 (V600E) is the most predominant substitution [35,76,36]. Missense mutations will lead to constitutively active BRAF, activating downstream pathways, promoting cellular proliferation, survival, and resistance to chemotherapy [35-37].
Other gene alterations and patient outcome
RBFOX1, an RNA splicing promoter, encodes the splicing factor ataxin-2 binding protein 1 (A2BP1) [77-79]. A2BP1 binds to ataxin 2, which regulates microRNA processing by increasing mRNA stability and promoting translation [79,80]. This study observed deletions in 90.9% of the RBFOX1 alterations, which align with previous studies [77,39]. Deletions in RBFOX1 will lead to loss of gene function and abnormal post-transcriptional processing of many tumour suppressors and oncoproteins [39,40,78,81].
From these results it is important to not only consider genetic alteration, but also the specific alteration present. For example, PTPRT amplification was associated with improved overall survival and not missense mutation. Other studies have observed this for many of the genes described here, such as TP53 in non-small cell lung cancer [82].
Predictive model of overall survival and targeted therapies
Biological processes are deregulated in cancer due to accumulated genetic alterations. The mechanisms that cause cancer are numerous, but as shown here often converge on tumour suppressor gene inactivation or oncogenic activation. Genetic alterations can co-occur, working synergistically to drive tumour development and this work demonstrates that this can be used to predict overall survival [25]. The predictive model for CCA described here combines the effect of 12 genetic alterations, patient age at presentation, and location of tumour. Individually these factors have a small impact on overall survival, however in combination a more complete prediction of survival appears. Many cancers are associated with the use of predictive models as a clinical tool. For example, Non-Hodgkin's Lymphoma use the International Prognostic Index in order to stratify patients in clinical trials [83,84]. This work supports the use of a similar stratification occurring in trials surrounding CCA in order to more completely assess treatment success.
Chemotherapy represents the predominant treatment option for colorectal cancer (CRC), but only half of the patients benefit from these regimens [44]. With the development of targeted therapies, the treatment of cancer has entered the era of precision medicine. For instance, APC alterations can be targeted through ICG-001, a β-catenin inhibitor, halting the Wnt pathway activated in these cancers [85]. Similarly, TP53 alterations can be targeted by a bispecific antibody, which targets the mutant p53 peptide–HLA complex [86,87]. It is hoped that a more complete understanding of the relationship between genetic alteration and survival will lead to the identification of novel therapies that specifically target these changes in CCA. However, a potential limitation is that these identified targets may not translate to successful therapies. This work provides strong preclinical evidence for targeting these pathways in CCA. Therapies that account for cancer genetics are already commonly in use, and this is likely to be a substantial area of development over the coming years. The key evidence for genetic targeted therapies is summarised in Table 9 and 10 and represent a practical application of this work.