CDKN3 in cancer: context-dependent cell-cycle control and implications for genome stability and therapy

Cyclin-dependent kinase inhibitor 3 (CDKN3), also known as kinase-associated phosphatase (KAP), dephosphorylates activating cyclin-dependent kinase 2 (CDK2) Thr160, yet high expression is associated with aggressive features in some cancers. This review distinguishes messenger RNA (mRNA) expression, protein abundance, phosphatase activity, functional dependency, prognostic association, and treatment-response prediction. Renal studies include selected perturbation evidence, whereas lung studies cited here primarily report prognostic or computational associations. Hepatocellular carcinoma (HCC) findings remain directionally discordant across analytes, etiologies, models, and endpoints. Recent triple-negative breast cancer (TNBC) studies include transcriptomic and cell-perturbation evidence, as well as a separate cell and murine mammary fat-pad graft study, while an ultraviolet (UV)-induced skin-cancer study identifies a sex- and context-specific protective axis. These results do not support a universal CDKN3 role. A CDKN3-specific connection to chromosomal instability (CIN) and cyclic GMP–AMP synthase (cGAS)–stimulator of interferon genes (STING) signaling remains unvalidated. Micronuclei and CIN do not invariably activate cGAS–STING. Bulk immune signatures and survival associations alone cannot establish direct immune regulation, functional dependency, or treatment sensitivity. This focused narrative review compares molecular measurements, genetic perturbations, clinical associations, and selected computational studies. We outline matched assays, genetic rescue, longitudinal CIN measurements, and pathway-specific tests needed to determine whether CDKN3 is a context-specific dependency, a biomarker, or a therapeutic target.