CDKN1B gene alterations and human cancers: mechanicistic investigations on G9R missense mutation

p27Kip1 (p27) was first discovered as a key regulator of cell proliferation, modulating Cyclin-dependent Kinases activity (1). Nearly two decades have elapsed since the discovery of p27 roles in the regulation of cytoskeletal dynamics and cellular plasticity, stem-cell proliferation and differentiation. This versatility has been attributed to its intrinsically unstructured nature that allows p27 to bind and modulate different proteins. Depending on the cell-specific context and its cellular compartimentalization, p27 acts both as a tumor-suppressor or tumor-promoter, and several post-synthetic modifications (mainly phosphorylations) control its commitment (1). Recently, mutations of CDKN1B (p27 encoding gene) have been found with a statistical significance in human cancers: along with frameshifts, some missense changes were associated to Multiple Endocrine Neoplasia, neuroendocrine tumours and other cancers (2). This study focuses on a germline mutation detected in sporadic parathyroid adenoma, Glycine9→Arginine (3). Glycine in position 9 is highly conserved in p27 across species and its substitution has been proposed to negatively affect the phosphorylation of adjacent Serine10 residue, the most abundant phosphorylation site of the protein. Therefore, we examined the phosphorylation pattern of this mutant protein by 2D-immunoblotting. Unexpectedly, Serine10 is still highly phosphorylated, at least as in the wild-type p27. Furthermore, a new phosphorylation site is probably generated by the missense mutation. Studies are in progress to identify the novel phosphorylation residue as well the protein kinase responsible for its modification. Functionally, despite its nuclear localization, G9Rp27 might contribute to enhance cell growth, motility and invasion; it also protects cells from apoptosis. Our studies indicate that G9Rp27 oncogenic activities depend on its phosphorylation pattern. In conclusion, this investigation confirms the importance of post-translational modifications in addressing the function of intrinsically unstructured proteins.