ACC is a rare but aggressive endocrine disease with a poor prognosis, whose estimated five-year overall survival rate is < 50% [19]. The recurrence rate of ACC patients with radical resection is also up to 85%, which is usually with concurrent metastases [19]. Therefore, prognostic stratification is important for individualized therapies for ACC patients. At present, the tumor extension that is best reflected by the ENSAT stage, and the tumor proliferation which is estimated either by mitotic count or Ki67 proliferation index were the main prognostic factors used in clinical practice[20]. However, the prognosis still varies widely among tumors with the same tumor stage and proliferation index[21]. Currently, pan-genomic studies have shown that molecular classification is closely associated with prognosis in ACC, and targeted molecular markers have been proposed to complete the routine prognostic assessment[20, 22, 23]. Recently, m6A RNA methylation regulators play an important role in the prognostic assessment and add to a better understanding of the pathogenesis of cancer [24]. Therefore, it is necessary to explore the prognostic significance of m6A RNA methylation regulators in ACC.
In the present study, we grouped all patients with ACC into 2 distinctly different clusters with different clinical outcome based on the expression levels of the 13 m6A RNA methylation regulators. It implied a potential prognostic value of m6A RNA methylation regulators in ACC. Therefore, we finally selected four genes (RBM15, HNRNPC, WTAP, and METTL14) associated with prognosis by univariate cox regression and LASSO regression constructing a RS model for prognostic assessment of ACC. It revealed the evident difference in 5-year OS between the high and low risk groups. And the time-dependent AUC was 0.787. More importantly, it suggested that RS, stage status and T status were all associated with prognosis, but RS and T status were independent prognostic biomarkers. It also suggested RS was closely correlated to the stage status, T status, and N status. A previous study reported that patient age and initial stage at diagnosis were considered two of the most important predictors of survival, with a poorer prognosis in patients with advanced age or metastatic disease at the time of diagnosis [4]. Unfortunately, the age and M status were not included in our study because all these data were unknown. Accurate staging was also a crucial step in treatment planning and in determining the prognosis of ACC [21]. Survival dropped precipitously with advanced stages. The 1-year survival for Stage I was 90%, while it dropped to 29% for Stage IV disease[25]. Our study also demonstrated that stage status was associated with prognosis of ACC, although it seemed not an independent prognostic biomarker. More importantly, our study indicated T status was an independent prognostic biomarker and RS was closely associated with the stage status, T status, and N status in ACC. Thus, combination of RS, along with clinical features, might provide a better way for the prognostic assessment of ACC. It also offered a new understanding of the tumorigenesis and the correlation between the molecular, clinical, and pathological profile of ACC.
Emerging evidence has demonstrated that m6A RNA methylation regulators are involved in the occurrence and development of many cancers, such as hepatocellular carcinoma and colorectal cancer [26, 27]. However, the role of m6A RNA methylation regulators in ACC remains unknown. At the present study, we found the expression levels of RBM15, HNRNPC and WTAP were higher in the high risk group than the low risk group, while that of METTL14 was higher in the low risk group than the high risk group. It seemed that RBM15, HNRNPC and WTAP were poor prognostic factors, while METTL14 was a protective prognostic factor in ACC. Though there were few studies about these m6A methylation regulators in ACC, the role of them was investigated in other cancers. A recent study reported that HNRNPC was highly expressed in bladder cancer, and HNRNPC knockdown reduced the proliferation and tumor growth of breast cancer cells [6, 28]. HNRNPC was also proved to be an independent prognostic biomarker in oral squamous cell carcinoma, and HNRNPC overexpression promoted its carcinogenesis via epithelial- mesenchymal transition [29]. WTAP was evaluated in osteosarcoma with a poor prognosis, which served as an independent prognostic factor for osteosarcoma patients [30]. WTAP/HMBOX1 dramatically promoted osteosarcoma proliferation and metastasis in a PI3K/AKT-dependent pattern[30]. High expression of WTAP also resulted in a poor prognosis by influencing tumor-associated T lymphocyte infiltration in gastric cancer [31]. METTL14 was lowly expressed in bladder cancer and colorectal cancer, and METTL14 might inhibit the malignant process via regulating RNA stability of Notch1 mRNA in bladder cancer and SOX4-mediated EMT process and PI3K/Akt signals in colorectal cancer [32, 33]. knockdown of METTL14 drastically enhanced proliferative and invasive ability of colorectal cancer cells in vitro and promoted tumorigenicity and metastasis in vivo by substantially abolishing m6A level of lncRNA XIST and augmenting the expression of lncRNA XIST, which was a downstream target of METTL14 [34]. METTL14 was also an independent prognostic factor in colorectal cancer, and decreased METTL14 was associated with poor OS [32]. These findings not only presented a similar prognostic value of HNRNPC, WTAP and METTL14 in other cancers, but also implied their functions and mechanisms in ACC. However, more further investigations of m6A RNA methylation regulators were necessary in ACC.
Up to now, the role and specific mechanism of m6A RNA methylation regulators are not fully understood and lack specificity. Though we have demonstrated the prognostic values of m6A RNA methylation regulators in patients with ACC, the accurate mechanism of them in the tumorigenesis, progression and prognosis still needs further studies. Additionally, there were several limitations existing in our study. Firstly, this ACC cohort lacked the samples of normal adrenal tissues. As a result, the expression landscape of m6A RNA methylation regulators in ACC was unclear. Secondly, the expression levels and prognostic value of m6A RNA methylation regulators were not verified in our own ACC samples, which needed a future verification. Moreover, though we investigated the role of the m6A RNA methylation regulators in the prognosis of ACC, we did not further explore the accurate mechanism of them. Finally, some important clinical and pathological information in this ACC cohort was missing, mainly including age and M status. Therefore, m6A RNA methylation regulators play a crucial role in the prognostic assessment, and modulating the m6A RNA methylation modification might be a novel strategy to guide the therapy of patients with ACC, but more further investigations are still needed.
In summary, using RBM15, HNRNPC, WTAP, and METTL14, we constructed a four-m6A methylation regulator RS signature, which was calculated as follows: (-0.182757311315352 × METTL14) + (0.329207248192332 × RBM15) + (0.0218703531461402 × HNRNPC) + (0.00097106415792933 × WTAP). This RS signature could serve as an independent prognostic biomarker to predict the prognosis of patients with ACC.