Data from Clinically Defined Mutations in <i>MEN1</i> Alter Its Tumor-suppressive Function Through Increased Menin Turnover

Abstract

<div><p>Loss of the tumor suppressor protein menin is a critical event underlying the formation of neuroendocrine tumors (NET) in hormone-expressing tissues including gastrinomas. While aberrant expression of menin impairs its tumor suppression, few studies explore the structure–function relationship of clinical multiple endocrine neoplasia, type 1 (<i>MEN1</i>) mutations in the absence of a complete LOH at both loci. Here, we determined whether clinical <i>MEN1</i> mutations render nuclear menin unstable and lead to its functional inactivation. We studied the structural and functional implications of two clinical <i>MEN1</i> mutations (R516fs, E235K) and a third variant (A541T) recently identified in 10 patients with gastroenteropancreatic (GEP)-NETs. We evaluated the subcellular localization and half-lives of the mutants and variant in <i>Men1</i>-null mouse embryo fibroblast cells and in hormone-expressing human gastric adenocarcinoma and NET cell lines. Loss of menin function was assessed by cell proliferation and gastrin gene expression assays. Finally, we evaluated the effect of the small-molecule compound MI-503 on stabilizing nuclear menin expression and function <i>in vitro</i> and in a previously reported mouse model of gastric NET development. Both the R516fs and E235K mutants exhibited severe defects in total and subcellular expression of menin, and this was consistent with reduced half-lives of these mutants. Mutated menin proteins exhibited loss of function in suppressing tumor cell proliferation and gastrin expression. Treatment with MI-503 rescued nuclear menin expression and attenuated hypergastrinemia and gastric hyperplasia in NET-bearing mice. Clinically defined <i>MEN1</i> mutations and a germline variant confer pathogenicity by destabilizing nuclear menin expression.</p>Significance:<p>We examined the function of somatic and germline mutations and a variant of <i>MEN1</i> sequenced from gastroenteropancreatic NETs. We report that these mutations and variant promote tumor cell growth and gastrin expression by rendering menin protein unstable and prone to increased degradation. We demonstrate that the menin-MLL (mixed lineage leukemia) inhibitor MI-503 restores menin protein expression and function <i>in vitro</i> and <i>in vivo</i>, suggesting a potential novel therapeutic approach to target MEN1 GEP-NETs.</p></div>