Alternatively Spliced Citrate Synthase Supports Colorectal Cancer

Metabolic changes are a major hallmark of cancer with the mitochondrial tricarboxylic acid (TCA) cycle playing a central role in this process. Remodeling of the TCA cycle occurs in cancer cells to sustain the increased anabolic and energetic demands required to grow, proliferate, and metastasize. Alternative splicing (AS) is increasingly recognized as a key regulator of cancer metabolism, yet its specific impact on TCA cycle enzymes remains unclear. In this issue of Cancer Research, Cheung and colleagues describe a novel splicing isoform of citrate synthase (CS), termed CS-ΔEx4, which is highly expressed in colorectal cancer. This CS-ΔEx4 isoform forms heterocomplexes with full-length CS, enhancing CS activity and promoting the metabolic reprogramming characteristic of malignancy. Overexpression of CS-ΔEx4 increases mitochondrial respiration and drives glycolytic carbon flux toward TCA intermediates, resulting in elevated levels of the metabolite 2-hydroxyglutarate. Mechanistically, this increase in 2-hydroxyglutarate, facilitated by increased activity of phosphoglycerate dehydrogenase, leads to epigenetic alterations that support oncogenic gene expression and tumor progression. Suppression of CS-ΔEx4 or pharmacologic inhibition of its activity reverts these metabolic and epigenetic changes, reducing cancer cell survival and metastatic potential. These findings establish a direct link between AS of a core metabolic enzyme and the emergence of cancer hallmarks, suggesting that targeting AS-derived variants like CS-ΔEx4 may represent a promising therapeutic strategy for colorectal cancer and potentially other malignancies in which such isoforms are expressed. See related article by Cheung et al., p. 4450.