Failure of Mutant Menin Protein to Block Hedgehog Signaling Triggers a Type of Endocrine Cancer

Mutations in the gene that encodes the protein menin cause a hereditary cancer syndrome called MEN1 (multiple endocrine neoplasia type 1) due to the inability of the mutant menin protein to block the oncogenic effects of enhanced Hedgehog signaling.

The Hedgehog signaling pathway transmits information to embryonic cells required for proper development. Different parts of the embryo have different concentrations of Hedgehog signaling proteins. The pathway takes its name from its polypeptide ligand, an intercellular signaling molecule called Hedgehog (Hh) found in fruit flies of the genus Drosophila. Hh is one of Drosophila's segment polarity-gene products, involved in establishing the basis of the fly body plan. The molecule remains important during later stages of embryogenesis and metamorphosis, and diseases associated with the malfunction of this pathway include basal cell carcinoma.

Individuals with MEN1 are at a substantially increased risk of developing neuroendocrine tumors, including cancer of the pancreatic islet cells that secrete insulin.

Investigators at the University of Pennsylvania (Philadelphia, USA) performed DNA microarray analysis studies on murine pancreatic islets with mutated menin genes. They reported in the April 15, 2013, online edition of the journal Cancer Research that elimination of menin enhanced pro-proliferative and oncogenic Hedgehog signaling activity.

The investigators showed that menin functioned by physically interacting with a second protein, PRMT5 (protein arginine methyltransferase 5). The menin-PRMT5 complex bound to the promoter of the Gas1 (growth arrest-specific protein 1) gene, where PRMT5 functioned as an epigenetic inhibitor by methylating histone proteins, which inhibited gene transcription. The Gas1 protein normally promoted Hedgehog signaling, and thus by inhibiting Gas1 expression, menin and PRMT5 effectively blocked the pathway's ability to induce cell proliferation. In MEN1 cells mutant menin failed to bind to PRMT5, and Gas1 continued to enhance Hedgehog signaling.

In another set of experiments the investigators treated a mouse model of human MEN1 syndrome with the Hedgehog pathway inhibitor Erivedge, a drug approved by the [US] Food and Drugs Administration for the treatment of metastatic or locally advanced basal cell carcinoma. They reported that the drug reduced proliferation of tumor cells and lowered blood insulin levels.

"Because we show in this mouse tumor model that we can significantly suppress proliferation of tumor cells in pancreatic islets and that we can reduce the higher insulin levels with a drug, which was just clinically approved to be safe, that naturally raises the question of whether, in patients who have mutation in this gene or enhanced Hedgehog signaling, this drug can improve patient symptoms to reduce tumor progression or insulinemia," said senior author Dr Xianxin Hua, associate professor of cancer biology at the University of Pennsylvania.

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