CASP9 Emerges as a Double-Edged Gene in Kidney Cancer, Study Finds

Kidney cancer researchers have long suspected that pyroptosis, the fiery form of programmed cell death that ruptures cells and floods tissues with inflammatory signals, plays a pivotal role in tumor biology. Now a new multi-omics study has zeroed in on one of its central players, the caspase-9 gene known as CASP9, and found that it behaves like a molecular double agent in clear cell renal cell carcinoma, the most common and aggressive form of kidney cancer. Depending on the stage of the disease, the same gene appears to protect patients or, paradoxically, to mark them for worse outcomes.

The study, published in Clinical Cancer Bulletin by Haojie Dai of Nanjing Medical University, took an unusually broad approach. Rather than starting with tumor tissue alone, the analysis began with genetics. Using Summary-data-based Mendelian Randomization, or SMR, the researcher linked genetic variants that regulate gene expression in kidney tissue to the risk of developing renal cell carcinoma. Among dozens of pyroptosis-related genes, CASP9 stood out with striking statistical significance. A follow-up two-sample Mendelian randomization analysis using independent data from the IEU GWAS and FinnGen databases confirmed the finding: higher genetically predicted expression of CASP9 was associated with a lower risk of renal cell carcinoma, with an odds ratio below one, marking the gene as a protective factor.

That genetic signal made biological sense. Caspase-9 is a key initiator of the intrinsic apoptotic pathway, the cellular machinery that drives damaged or stressed cells to self-destruct. Previous work has shown that activated caspase-9 can cleave Gasdermin E, triggering pyroptosis, and that caspase-9-dependent apoptosis can activate the NLRP3 inflammasome through the channel protein pannexin-1, further fueling inflammatory cell death. In principle, a tumor cell with abundant CASP9 should be more vulnerable to these destructive programs, which could explain why the gene appears protective at the population level.

But when the analysis moved from genetics to tumor tissue, the picture grew more complicated. Mining transcriptomic data from The Cancer Genome Atlas kidney clear cell carcinoma cohort, the study found that CASP9 expression was significantly downregulated in tumors compared with normal kidney tissue, a pattern confirmed in paired samples from the same patients and corroborated at the protein level by lighter immunohistochemical staining in tumor specimens from the Human Protein Atlas. Despite being expressed at lower levels in tumors, the gene showed real diagnostic power: a receiver operating characteristic analysis yielded an area under the curve of 0.784, suggesting CASP9 expression could help distinguish cancerous from healthy tissue.

The prognostic story was where the paradox deepened. Across the full TCGA cohort, patients with higher CASP9 expression fared worse on overall survival, disease-specific survival, and progression-free interval, with prediction accuracies for one-, three-, and five-year survival approaching 0.6. Yet when the researcher stratified patients by tumor stage, a clear pattern emerged. In early T1-stage tumors, high CASP9 expression was associated with better prognosis. In advanced T3 and T4 tumors, the relationship reversed, with high expression significantly predicting poorer survival. The authors propose that CASP9 acts as a protective factor in pre-tumor stages and early disease, but that as cancer progresses, the gene is effectively hijacked by tumor cells and becomes an adverse prognostic marker.

Two mechanisms may explain this stage-dependent flip. In early disease, high CASP9 may simply indicate that cancer cells remain sensitive to apoptotic signals, restraining tumor growth. Work in leukemia has also shown that caspase-9 can cleave vimentin, a structural protein, thereby inhibiting cancer cell migration and invasion, a mechanism that may apply to kidney cancer as well. In advanced disease, however, a study published in Nature Immunology offers a darker explanation: tumor cells can hijack caspase-9 signaling to suppress immune responses, sharply reducing type I interferon expression and weakening the ability of CD8 T cells to kill cancer cells. The new study’s immune analyses lend support to this idea in the kidney cancer context.

Single-cell transcriptomics added another layer of detail. Analyzing two public ccRCC single-cell datasets, the researcher found that CASP9 expression was spread broadly across cell types rather than confined to malignant cells, with M1 macrophages showing relatively high expression. Notably, CASP9 levels were higher in cells from early-stage T1a tumors than in cells from T3a tumors, particularly in B cells, M1 macrophages, monocytes, and natural killer cells, echoing the stage-dependent pattern seen in the bulk survival data.

The immune infiltration findings were among the most striking in the study. Using CIBERSORT and ssGSEA algorithms to deconvolve bulk tumor expression data, the analysis revealed that nearly all immune functions were downregulated in the CASP9 high-expression group. The negative correlation between CASP9 and natural killer cell activity was especially pronounced: high-expression tumors had more resting NK cells and fewer activated ones, suggesting a dampening of NK cell activation. Because reactive oxygen species help NK cells recognize and destroy tumor targets, and because the gene set enrichment analysis showed more vigorous ROS-related signaling in the low-CASP9 group, the authors argue that the suppressed NK cell activity in high-expression tumors is mechanistically traceable. The enrichment analysis also showed that epithelial-mesenchymal transition, Hedgehog signaling, and TNF-alpha signaling through NF-kappa-B were highly activated in the high-expression group, while oxidative phosphorylation, complement signaling, and ROS signaling dominated the low-expression group. Given that EMT is a well-established driver of invasion and metastasis in ccRCC, the authors hypothesize that CASP9 may promote malignant progression in advanced tumors partly through this pathway.

The study also connected CASP9 to treatment choices. Using the OncoPredict platform to estimate drug sensitivity from the GDSC2 pharmacogenomic dataset, the analysis found that patients with low CASP9 expression were more sensitive to the MEK inhibitor trametinib, while those with high expression showed lower predicted IC50 values for afatinib, zoledronate, ulixertinib, and cyclophosphamide. To identify compounds that might directly target CASP9, the researcher screened the DSigDB database and performed molecular docking simulations with AutoDock. All five top candidate compounds showed favorable binding energies with the CASP9 protein, with beta-solamarine achieving the strongest score at minus 9.5 kilocalories per mole, followed by alantolactone at minus 8.1. Both compounds have documented anticancer activity in other tumor types, inhibiting Akt and MAPK signaling in liver and gastric cancers and disrupting Wnt signaling in melanoma, which the author argues makes them promising starting points for kidney cancer drug development.

The author is candid about the study’s limitations. The genetic and transcriptomic cohorts analyzed were drawn primarily from European and American populations, raising the possibility of selection bias, and all findings remain at the bioinformatics level without experimental validation. The analysis also relied on GWAS data for renal cell carcinoma as a whole rather than ccRCC specifically, since no sufficiently large subtype-specific dataset exists, though ccRCC accounts for roughly 70 to 80 percent of cases. To address these gaps, the author outlines an ambitious validation roadmap: cross-ethnic cohorts spanning Asia, Africa, and Latin America, CRISPR and siRNA experiments to test CASP9’s effects on proliferation, apoptosis, and invasion in kidney cancer cell lines, chemical proteomics and biophysical binding assays to confirm drug-target interactions, and exploration of liquid biopsy markers such as circulating CASP9-related signals. For now, the study establishes CASP9 as a biomarker of unusual complexity, one that could guide diagnosis, refine prognosis by tumor stage, and help select chemotherapy regimens, while embodying the central lesson of modern cancer biology: that the same molecule can be friend and foe depending on the context in which it acts.

Subject of Research: The dual role of the pyroptosis gene CASP9 in clear cell renal cell carcinoma

Article Title: Friend or foe? Multi-omics analysis unravels the complex identity of key pyroptosis factor CASP9 in clear cell renal cell carcinoma

Article References: Dai, H. (2025). Friend or foe? Multi-omics analysis unravels the complex identity of key pyroptosis factor CASP9 in clear cell renal cell carcinoma. Clinical Cancer Bulletin, 4(1), Article 18. https://doi.org/10.1007/s44272-025-00047-x

Image Credits: AI Generated

DOI: 10.1007/s44272-025-00047-x

Keywords: CASP9, pyroptosis, clear cell renal cell carcinoma, Mendelian randomization, TCGA, biomarker, natural killer cells, epithelial-mesenchymal transition, molecular docking, drug sensitivity, tumor microenvironment, precision oncology