KDM4C inhibition blocks tumor growth in basal breast cancer by promoting cathepsin L-mediated histone H3 cleavage

Basal breast cancer is a subtype with a poor prognosis in need of more effective therapeutic approaches. Here we describe a unique role for the KDM4C histone lysine demethylase in KDM4C-amplified basal breast cancers, where KDM4C inhibition reshapes chromatin and transcriptomic landscapes without substantial alterations of its canonical substrates, trimethylated histone H3 lysine 9 (H3K9me3) and lysine 36 (H3K36me3). Rather, KDM4C loss causes proteolytic cleavage of histone H3 mediated by cathepsin L (CTSL), resulting in decreased glutamate–cysteine ligase expression and increased reactive oxygen species. CTSL is recruited to the chromatin by the grainyhead-like 2 (GRHL2) transcription factor that is methylated at lysine 453 following KDM4C inhibition, triggering CTSL histone clipping activity. Deletion of CTSL rescued KDM4-loss-mediated tumor suppression. Our study reveals a function for KDM4C that connects cellular redox regulation and chromatin remodeling. KDM4C regulates cathepsin L-mediated histone H3 N-terminal tail clipping through grainyhead-like 2 (GRHL2) methylation in breast cancer. This link between the cellular redox state and chromatin remodeling might represent a therapeutic target in KDM4C-amplified tumors.