In this study, to better assess the survival of HCC patients, we established a
prognostic model based on panel of 5 ferroptosis-related genes and clinical characteristic using machine learning methods to analyze the data of a large number of HCC patients. The model was established and validated through multiple databases, showing good discrimination and calibration in predicting survival. Finally, the connection between the model and immunity has been partially confirmed.
As a major leading cause of cancer-related mortality worldwide, HCC causes a major health burden for society18. Considering the increasing number of HCC patients, it is very important to predict the survival of patients. The ferroptosis proposed by Dixon et al.4, as a new type of programmed cell death, has been considered to be closely related to HCC19, 20. Although some studies have shown that several genes might regulate drug-induced ferroptosis in HCC, but their relationship with the prognosis of HCC patients is still unknown. Therefore, it has great potential to construct a models based on ferroptosis-related genes to predict the overall survival of HCC.
Through the differential gene analysis of HCC patients, we identified 70 ferroptosis-related DEGs from 5266 DEGs between normal and tumor tissues. Then, 5 genes were obtained, after univariate and LASSO Cox regression analysis. These genes have been confirmed to be closely related to cancer. G6PD, which is involved in the pentose phosphate pathway, has been reported to involved in erastin-induced ferroptosis in non-small cell lung cancer cells4. Similarly, inhibition of SLC7A11, a subunit of system Xc to import cystine in the cell, sensitized fibrosarcoma cells to erastin-induced death4. Furthermore, we constructed these 5 genes into a signature to better predict the overall survival, and the superiority of the model was confirmed by the HCC patients in TCGA and ICGC.
The clinical characteristics of HCC patients are closely related to the prognosis. In order to better optimize the model and strengthen the survival prediction of patients, TNM stage and cirrhosis were selected, through univariate and multivariate Cox analysis. TNM stage is an important criterion for the current staging of cancer patients and is widely considered to be the standard approach for predicting prognosis in most solid tumour systems21, 22. In addition, cirrhosis, as a major risk factor for HCC, has been confirmed to be closely related to the occurrence and development of HCC23. Every year, a large number of cirrhosis patients infected with viral hepatitis progress to HCC, which greatly shortens the life span of patients24.
Based on these meaningful variables closely related to the prognosis of HCC, we have incorporated clinical information combined with gene signature to construct a nomogram. In order to understand the performance of the model, we evaluated its discrimination and calibration ability. C-index and ROC curve are used to evaluate the discrimination ability of nomogram. In predicting 1-year, 3-year, and 5-year survival of HCC patients in TCGA, the C index is 0.732, 0.705, and 0.774 respectively. At the same time, AUC is 0.745, 0.740, and 0.756. Similarly, in the HCC patients in the GEO database, these indexes also perform relatively well, indicating that the model has a good discrimination ability for predicting survival. The calibration curve for evaluating calibration also shows that the nomogram has good calibration, whether in TCGA or GEO database.
Although the mechanism of ferroptosis in HCC has been extensively studied, the relationship between ferroptosis and tumor immunity is not yet clear. We performed functional enrichment analysis on the DEGs between the high-risk group and the low-risk group. Interestingly, most of the enriched items are immune-related pathways. This may be closely related to the difference in survival rates between subgroups. With this purpose, we compared the composition of different types of immune cells between the groups. The results show that tumor-associated macrophages and Treg cells, clued to poor prognosis in HCC patients due to their role in immune invasion25–27, are higher in the high-risk group. This may be that the ferroptosis-related genes affect tumor cells by affecting immune cells. Moreover, the expression of immune checkpoint, including CTLA4, LAG3, TIGIT, and IDO1, of the HCC patients in high-risk group was significantly higher than that in the low-risk group, indicating that lower overall survival of patients may be due to immunosuppressive microenvironment.
The important finding in this paper is that the nomogram constructed based on the risk score related to ferroptosis genes can well predict the prognosis of HCC patients. Although some prognostic models have been used to predict the overall survival of HCC patients28–30, there are few nomograms based on ferroptosis-related genes. This model incorporates a large number of HCC patients to construct, and has been validated by multiple databases, showing a good ability to predict survival. At the same time, the 5 ferroptosis-related genes identified can also be used as biomarkers for predicting the survival of HCC patients.
However, there are still some limitations to our study. Due to the lack of information in the database, we did not include the cause of HCC. The number of HCC patients caused by non-alcoholic fatty liver disease and alcoholic liver is gradually increasing, though the number of HCC patients induced by viral hepatitis is gradually decreasing. For HCC caused by different causes, the prognosis and treatment of patients are different31–34. The lack of relevant information led us to not include HCC-related imaging data and serum indicators into the nomogram. The inclusion of more indicators helps increase the specificity and sensitivity of the prognostic model. In addition, the RNA-seq data included in this model are all derived from liver specimens, not blood and other easier-to-obtain specimens, which increases the difficulty of sampling.
In conclusion, our study defined a novel prognostic model of 5 ferroptosis-related genes. This model proved to have good discrimination and calibration ability, by analyzing a large number of HCC patients from multiple databases, providing a new insight into the evaluation the prognosis of HCC patients.