Expression and functional analysis of menin in a multiple endocrine neoplasia type 1 (MEN1) patient with somatic loss of heterozygosity in chromosome 11q13 and unidentified germline mutation of the MEN1 gene

Menin and bone metabolism

Menin, a product of the MEN1 gene, is related to the ontogeny of several cancers such as MEN1 and sporadic endocrine tumors, although it is considered to be a tumor suppressor. Many proteins interact with menin, and it is involved in various biological functions in several tissues. Menin plays some physiological and pathological roles related to transforming growth factor-beta (TGF-β) signaling pathway in the parathyroid, and it is implicated in the tumorigenesis of parathyroid tumors. In bone, the bone phenotype was observed in some menin-deleted mice. Menin is considered to support BMP-2- and Runx2-induced differentiation of mesenchymal cells into osteoblasts by interacting with Smad1/5, Runx2, β-catenin and LEF-1, although it has different effects on osteoblasts at later differentiation stages through TGF-β-Smad3 and AP-1 pathways. Further research is expected to shed more light on the role of menin in bone.

Menin missense mutants encoded by the MEN1 gene that are targeted to the proteasome: restoration of expression and activity by CHIP siRNA

CONTEXT

In multiple endocrine neoplasia type 1 (MEN1) characterized by tumors of parathyroid, enteropancreas, and anterior pituitary, missense mutations in the MEN1 gene product, menin, occur in a subset of cases. The mutant proteins are degraded by the proteasome. However, whether their expression and activity can be restored is not known.

OBJECTIVE

Our objective was to functionally characterize a panel of 16 menin missense mutants, including W423R and S443Y identified in new MEN1 families, with respect to protein stability, targeting to the proteasome and restoration of expression by proteasome inhibitors and expression and function by small interfering RNA technology.

METHODS

Flag-tagged wild-type (WT) and missense menin mutant expression vectors were transiently transfected in human embryonic kidney (HEK293) and/or rat insulinoma (Rin-5F) cells.

RESULTS

The majority of mutants were short-lived, whereas WT menin was stable. Proteasome inhibitors MG132 and PS-341 and inhibition of the chaperone, heat-shock protein 70 (Hsp70), or the ubiquitin ligase, COOH terminus of Hsp70-interacting protein (CHIP), by specific small interfering RNA, restored the levels of the mutants, whereas that of WT menin was largely unaffected. Inhibition of CHIP restored the ability of mutants to mediate normal functions of menin: TGF-β up-regulation of the promoters of its target genes, the cyclin-dependent kinase inhibitors p15 and p21 as well as TGF-β inhibition of cell numbers.

CONCLUSION

When the levels of missense menin mutants that are targeted to the proteasome are normalized they may function similarly to WT menin. Potentially, targeting specific components of the proteasome chaperone pathway could be beneficial in treating a subset of MEN1 cases.

Impaired transforming growth factor-β (TGF-β) transcriptional activity and cell proliferation control of a menin in-frame deletion mutant associated with multiple endocrine neoplasia type 1 (MEN1)

Multiple endocrine neoplasia type 1 (MEN1) is characterized by tumors of the parathyroid, enteropancreas, and anterior pituitary. The MEN1 gene encodes the tumor suppressor menin of 610 amino acids that has multiple protein partners and activities. The particular pathways that, when lost, lead to tumorigenesis are not known. We demonstrated that members of a three-generation MEN1 kindred are heterozygous for a donor splice site mutation at the beginning of intron 3 (IVS3 + 1G→A). Lymphoblastoid cells of a mutant gene carrier had, in addition to the wild-type menin transcript, an aberrant transcript resulting from use of a cryptic splice site within exon III that splices to the start of exon IV. The predicted menin Δ(184-218) mutant has an in-frame deletion of 35 amino acids but is otherwise of wild-type sequence. The transfected menin Δ(184-218) mutant was well expressed and fully able to mediate the normal inhibition of the activity of the transcriptional regulators JunD and NF-κB. However, it was defective in mediating TGF-β-stimulated Smad3 action in promoter-reporter assays in insulinoma cells. Importantly, lymphoblastoid cells from an individual heterozygous for the mutation had reduced TGF-β-induced (Smad3) transcriptional activity but normal JunD and NF-κB function. In addition, the mutant gene carrier lymphoblastoid cells proliferated faster and were less responsive to the cytostatic effects of TGF-β than cells from an unaffected family member. In conclusion, the menin mutant exhibits selective loss of the TGF-β signaling pathway and loss of cell proliferation control contributing to the development of MEN1.

Caspase 8 and menin expressions are not correlated in human parathyroid tumors

Menin is lost by the sequential inactivation of both MEN1 alleles in subsets of non-hereditary endocrine tumors as well as those associated with multiple endocrine neoplasia type 1 (MEN1), an autosomal dominant hereditary cancer syndrome characterized by multiple tumors including parathyroid, pituitary and enteropancreatic endocrine tumors. Loss of menin has been reported to be associated with lowered caspase 8 expression and resistance to apoptosis in murine fibroblasts and in pancreatic islet tumors arising in heterozygous MEN1 gene knockout mice, the animal model of the human MEN1 syndrome. We confirmed by menin-knockdown experiments with specific siRNA that menin is crucial for caspase 8 expression in human culture cells while overexpression of menin did not increase caspase 8 protein over basal levels. We then examined expression of menin, caspase 8 and cyclin-dependent kinase inhibitors p27(Kip1) and p15(Ink4b) by Western blotting in human parathyroid tumors surgically resected from patients with MEN1 and those with non-hereditary primary hyperparathyroidism. The menin and p27(Kip1) expression levels were correlated with MEN1 mutation status that was confirmed by DNA analysis. The caspase 8 and p15(Ink4b) protein levels were variable among tumors, and were not correlated with menin protein levels. These findings suggest that human endocrine tumors lacking menin may not always exhibit lowered caspase 8 expression and hence may not be resistant to apoptosis-inducing therapy.

The role of menin in parathyroid tumorigenesis

Primary hyperparathyroidism is a common disorder that involves the pathological enlargement of one or more parathyroid glands resulting in excessive production of parathyroid hormone (PTH). The exact pathogenesis of this disease remains to be fully understood. In recent years interest has focussed on the interaction between menin protein and the transforming growth factor (TGF)-beta/Smad signalling pathway. In vitro experimentation has demonstrated that the presence of menin is required for TGF-beta to effectively inhibit parathyroid cell proliferation and PTH production. This observation correlates with the almost universal occurrence of parathyroid tumors accompanying the inactivation of menin in multiple endocrine neoplasia Type 1 (MEN1) syndrome and the high rate of somatic menin gene mutations seen in sporadic parathyroid adenomas. This chapter aims to review the role of menin in primary hyperparathyroidism and parathyroid hormone-regulation, including the influences of MEN1 gene mutations on parathyroid cell proliferation, differentiation and tumorigenesis.