Cell cycle regulation by MEN syndrome genes

Karla R Smith, Purdue University

Abstract

Multiple Endocrine Neoplasia (MEN) is an inherited human cancer syndrome characterized by the formation of tumors in several endocrine organs within the same patient or between several family members. There are several different kinds of MEN syndromes, including, MEN1, MEN2, VHL, NF-1, Cowden syndrome and Carney complex. MEN1 and MEN2 are the most prevalent of all MEN syndromes. MEN1 syndrome is linked with inactivating mutations of the MEN1 tumor suppressor gene. MEN2 syndromes arise from activating mutations of the RET proto-oncogene (eg. RET C634R, or RET2A). Cross-phosphorylation of the RET receptor at Y905 triggers autophosphorylation of tyrosine residues within the RET kinase domain to activate signaling of several well-characterized signal transduction pathways. Mice deficient for both p18 and p27 develop multiple endocrine tumors that mimic the tumor spectrum of MEN1 and MEN2 syndromes combined. We hypothesize that MEN1 and RET normally function to regulate growth by mediating proper expression of p18 and p27. To address these possibilities, we assessed the cell cycle effects of expression of MENIN, the protein product of the MEN1 gene, and MENINA242V, a common MEN1 syndrome mutation. Transient expression of MENIN correlates with cell cycle arrest and an increase in p18 and p27. Conversely, expression of MENINA242V does not correlate with cell cycle arrest and is unable to upregulate p27. This suggests that MENIN confers cell cycle arrest by upregulating p18 and p27. MENIN A242V may promote tumorigenesis because it is unable to upregulate p27. To examine the role of phosphorylation-induced signaling by RET2A, we analyzed the cell cycle profiles of RET2A and several RET2A tyrosine mutants that do not become phosphorylated. We have identified a RET2A tyrosine phosphorylation mutant, Y1062F, that prevents downregulation of p18 due to loss of transcriptional regulation, and correlates with a decreased proliferative index compared to RET2A. Our results demonstrate that phosphorylation of tyrosine 1062 is required for several growth properties of RET2A and the downstream signaling pathways triggered by phosphorylation of this tyrosine are necessary for downregulation of p18. These data provide the early framework for mechanisms of tumorigenesis by MEN genes involving the loss of important G1 CDKIs, p18INK4c and p27Kip1.

Degree

Ph.D.

Advisors

Franklin, Purdue University.

Subject Area

Molecular biology|Cellular biology|Oncology

COinS