Abstract

RSK4 belongs to the p90 Ribosomal S6 Kinase family which lies downstream of the MAPK pathway. While we previously revealed this kinase to be a therapeutic target in lung and bladder cancers, there is conflicting evidence for its wider role in other cancer types. Indeed, RSK4 was instead suggested to be a tumour suppressor in colorectal and gastric cancers, and reports of its role in breast malignancies are contradictory. One possible explanation for these discrepancies may be the expression of different RSK4 isoforms across cancers. Four RNAs are produced from the RSK4 gene with two being protein-coding. Here, we analysed the expression of the two RSK4 protein-coding mRNAs across 30 normal and 33 cancer tissue types from the combined GTEx/TCGA dataset and correlated it with associated clinical features. This analysis revealed the mRNA expression of RSK4 isoform 1 and 2 to be independent prognostic factors for patient survival, pathological stage, cancer metastasis, recurrence, and immune infiltration in brain, stomach, cervical, and kidney cancers. However, we found that the upregulation of either RSK4 isoform can equally be associated with good or bad prognosis depending on the cancer type considered, and changes in the expression ratio of isoforms fail to predict clinical outcome. Taken together, we show that differential isoform expression alone cannot explain the contradictory roles of RSK4 in cancers and that further research is needed to highlight the underlying mechanisms for the context-dependent function of this kinase.