The gene for multiple endocrine neoplasia type 1 (MEN-1) has recently been cloned and encodes a putative tumor suppressor protein named menin. We have previously reported inactivating MEN-1 gene mutations associated with loss of heterozygosity (LOH) of the normal allele in tumors of patients with MEN-1 and in some sporadic pituitary tumors. These genetic alterations, however, are noted in no more than 10% of sporadic adenomas. To investigate whether other mechanisms may result in down-regulation of menin gene expression in pituitary adenomas, we examined menin gene expression by semiquantitative RT-PCR in 60 sporadic pituitary adenomas. Ribonucleic acid (RNA) was extracted from surgically resected, morphologically characterized tumors. Primers were designed to amplify a 257-bp fragment spanning exons 4–6 of the MEN-1 gene. A product of the predicted size was amplified from normal pituitary samples as well as from adenomas. Competitive PCR was performed with the housekeeping gene PGK-1 to quantitate menin gene expression. A comparable ratio of menin/PGK-1 messenger RNA was identified in all but three samples; in two tumors with LOH, menin expression was weak, and in one tumor, menin messenger RNA was undetectable, associated with LOH and mutation of the other allele.

Reduced expression of menin in some sporadic adenomas is consistent with a putative tumor suppressor role for this gene product. However, lack of menin down-regulation in the majority of these tumors, which exhibit LOH at 11q13 in up to 20% of cases, provides compelling evidence for an additional tumor suppressor gene at this locus, which is more commonly involved in the pathogenesis of pituitary neoplasms.