The p27/Kip1 locus shows no loss of heterozygosity in human pituitary adenomas

MEN-4 and other multiple endocrine neoplasias due to cyclin-dependent kinase inhibitors (p27(Kip1) and p18(INK4C)) mutations

Cyclin-dependent kinase inhibitors (CDKIs) are known targets to become deregulated in various tumour types, including endocrine tumours. Typically, these cell cycle regulators are somatically inactivated in sporadic endocrine tumours. Recently, it became known that certain CDKI genes cause inherited susceptibility to endocrine neoplasia. Multiple endocrine neoplasia type 4 (MEN4) emerged as a novel form of multiple endocrine neoplasia, caused by mutations in the CDKI gene CDKN1B/p27(Kip1). The MEN4 phenotype remains unclear, but all MEN4 patients identified thus far present with parathyroid involvement, and less typically with pituitary adenomas and other endocrine features. Moreover, the CDKI gene CDKN2C/p18(INK4C) has been also implicated in endocrine neoplasia susceptibility. This review presents the recent advances in these novel MEN-related states and summarises the current knowledge of how these CDKIs may be implicated in endocrine neoplasia. In addition, it briefly presents data from Cdkn1b/p27(Kip1) and Cdkn2c/p18(INK4C) murine models, which strongly support the protective role of these inhibitors against endocrine tumourigenesis.

Modeling sporadic loss of heterozygosity in mice by using mosaic analysis with double markers (MADM)

The initiation and progression of many human cancers involve either somatic activation of protooncogenes or inactivation of tumor-suppressor genes (TSGs) in sporadic cells. Although sporadic gain-of-function of protooncogenes has been successfully modeled in mice [e.g., Johnson L, Mercer K, Greenbaum D, Bronson RT, Crowley D, Tuveson DA, Jacks T (2001) Nature 410:1111-1116], generating a similar degree of sparseness of TSG loss-of-function remains a challenge. Here, we use mosaic analysis with double markers (MADM) to achieve TSG inactivation and concurrent labeling in sporadic somatic cells of mice, closely mimicking loss of heterozygosity as occurs in human cancers. As proof of principle, we studied the consequence of sporadic loss of p27kip1, a cyclin-dependent kinase inhibitor. MADM-mediated loss of p27kip1 results in mutant cell expansion markedly greater than that observed in conventional p27kip1 knockouts. Moreover, the direct comparison of WT and mutant cells at single-cell resolution afforded by MADM reveals that p27kip1 regulates organ size in vivo by cell-autonomous control of cell cycle exit timing. These studies establish MADM as a high-resolution method for modeling sporadic loss of heterozygosity in mice, providing insights into TSG function.

p27(Kip1): regulation and function of a haploinsufficient tumor suppressor and its misregulation in cancer

A major function of p27, also known as Kip1, is to bind and inhibit cyclin/cyclin-dependent kinase complexes, thereby blocking cell cycle progression. As p27 operates at the heart of the cell cycle, it is perhaps not surprising that it is emerging as a key player in multiple cell fate decisions including proliferation, differentiation, and cell death. The central role of p27 makes it important in a variety of disease processes that involve aberrations in cellular proliferation and other cell fates. Most notable among these processes is neoplasia. A large number of studies have reported that p27 expression is frequently downregulated in human tumors. In most tumor types, reduced p27 expression correlates with poor prognosis, making p27 a novel and powerful prognostic marker. In addition to these practical implications, murine and tissue culture models have shown that p27 is a potent tumor suppressor gene for multiple epithelially derived neoplasias. Loss of p27 cooperates with mutations in several oncogenes and tumor suppressor genes to facilitate tumor growth, indicating that p27 may be a "nodal point" for tumor suppression. In contrast to most tumor suppressor genes studied to date, which are recessive at the cellular level, p27 is haploinsufficient for tumor suppression. The fact that tumor suppression by p27 is critically dependent on the absolute level of p27 expression indicates that p27 acts as a rheostat rather than as an on/off switch to control growth and neoplasia.