Menin Directly Represses Gli1 Expression Independent of Canonical Hedgehog Signaling

Multiple Endocrine Neoplasia Type I (MEN1), a familial tumor syndrome results from mutations in the MEN1 gene, which encodes a tumor suppressor, menin. It has been previously shown that menin plays an important role in both repressing and activating gene expression. However, it is not well understood how menin represses expression of multiple genes. Here we show that upon Men1 excision, Gli1 and its target genes including PTCH1 and C-MYC are elevated in the absence of an apparent Hedgehog (Hh) pathway-activating ligand or when Smoothened (SMO), a key component of the pathway, is inhibited. Menin binds to the Gli1 promoter and recruits PRMT5, a histone arginine methyltransferase associated with transcriptional repression. Both PRMT5 binding and histone H4 arginine 3 methylation (H4R3m2s) are decreased at the Gli1 promoter in Men1-excised cells. Moreover, Men1 ablation results in increased binding of transcriptionally active GLI1 at the Gli1 promoter, in a manner not influenced by the canonical Hedgehog signaling pathway. Inhibition of GLI1 by a small molecule inhibitor, GANT 61, leads to decrease in expression of Gli1, and its target genes in Men1-excised cells. Furthermore, GANT-61 more potently suppresses proliferation of Men1-excised cells than control wild type cells. These findings uncover a novel link whereby menin directly represses Gli1 expression independent of the canonical Hedgehog (Hh) signaling pathway epigenetically via PRMT5 and its repressive H4R3m2s mark. These results suggest that inhibition of Gli1 can lead to suppression of neuroendocrine tumors harboring mutations in the MEN1 gene.