A cancer‐associated CDKN1B mutation induces p27 phosphorylation on a novel residue: a new mechanism for tumor suppressor loss‐of‐function

CDKN1B haploinsufficiency promotes the development of several human cancers. The gene encodes p27(Kip1), a protein playing pivotal roles in the control of growth, differentiation, cytoskeleton dynamics, and cytokinesis. CDKN1B haploinsufficiency has been associated with chromosomal or gene aberrations. However, very few data exist on the mechanisms by which CDKN1B missense mutations facilitate carcinogenesis. Here, we report a functional study on a cancer‐associated germinal p27(Kip1) variant, namely glycine9‐>arginine‐p27(Kip1) (G9R‐p27(Kip1)) identified in a parathyroid adenoma. We unexpectedly found that G9R‐p27(Kip1) lacks the major tumor suppressor activities of p27(Kip1) including its antiproliferative and pro‐apoptotic functions. In addition, G9R‐p27(Kip1) transfection in cell lines induces the formation of more numerous and larger spheres when compared to wild‐type p27(Kip1)‐transfected cells. We demonstrated that the mutation creates a consensus sequence for basophilic kinases causing a massive phosphorylation of G9R‐p27(Kip1) on S12, a residue normally never found modified in p27(Kip1). The novel S12 phosphorylation appears responsible for the loss of function of G9R‐p27(Kip1) since S12AG9R‐p27(Kip1) recovers most of the p27(Kip1) tumor suppressor activities. In addition, the expression of the phosphomimetic S12D‐p27(Kip1) recapitulates G9R‐p27(Kip1) properties. Mechanistically, S12 phosphorylation enhances the nuclear localization of the mutant protein and also reduces its cyclin‐dependent kinase (CDK)2/CDK1 inhibition activity. To our knowledge, this is the first reported case of quantitative phosphorylation of a p27(Kip1) variant on a physiologically unmodified residue associated with the loss of several tumor suppressor activities. In addition, our findings demonstrate that haploinsufficiency might be due to unpredictable post‐translational modifications due to generation of novel consensus sequences by cancer‐associated missense mutations.