A novel germline CDKN1B mutation causing multiple endocrine tumors: clinical, genetic and functional characterization

Multiple endocrine neoplasia (MEN) syndromes are characterized by tumors involving two or more endocrine glands. Two MEN syndromes have long been known: MEN1 and MEN2,caused by germline mutations in MEN1 or RET, respectively. Recently, mutations in CDKN1B,encoding the cyclin-dependent kinase (Cdk) inhibitor p27, were identified in patients having a MEN1-like phenotype but no MEN1 gene mutations. Currently, the molecular mechanisms mediating the role of p27 in tumor predisposition are ill defined. We here report a novel germline missense variant in CDKN1B (c.678C>T, p.P69L) found in a patient with multiple endocrine tumors. We previously reported a nonsense p27 mutation (c.692G>A, p.W76X) in two patients with MEN1-like phenotype. Functional assays were used to characterize p27P69L and p27W76X in vitro. We show that p27P69L is expressed at reduced level and is impaired in both binding toCdk2 and inhibiting cell growth. p27W76X, which is mislocalized to the cytoplasm, can no longer efficiently bind Cyclins-Cdks, nor inhibit cell growth or induce apoptosis. In the patient’s tumor tissues, p27P69L associates with reduced/absent p27 expression and in one tumor with loss-of heterozygosity.Our results extend previous findings of CDKN1B mutations in patients with MEN1-related states and support the hypothesis of a tumor suppressor role for p27 in neuroendocrine cells.