Diagnosis and Management of Pituitary Tumors

74 SHIMON AND MELMEDnal allelic mutation, and leads to inactivation of the suppressorgene and to unrestrained tumor growth.MEN-1 Gene Multiple endocrine neoplasia type 1 (MEN-1)is an autosomal dominant disorder characterized by the combinedoccurrence of parathyroid, pancreatic islet, and anterior pituitarytumors. The MEN-1 gene was mapped to chromosome 11q13(47,48). LOH of the MEN-1 locus on the pericentromeric regionof the long arm of chromosome 11 has been demonstrated in themajority of parathyroid tumors removed from MEN-1 patients(49–51), in 25% of sporadic parathyroid adenomas (51), and inpancreatic islet tumors (47,52). These findings indicate that inactivationof a tumor suppressor gene on chromosome 11 ispathogenetically related to these MEN-1 associated endocrinetumors. Allelic losses for chromosome 11q markers were determinedin both MEN-1 associated and sporadic pituitary adenomas(52). However, most sporadic pituitary tumors studied (40,51) didnot demonstrate LOH of 11q13 loci.Recently, the gene for MEN-1 has been cloned and characterized(53,54). The MEN-1 gene contains 10 exons and encodes apredicted 610-amino acid protein product, termed menin. Mutationalanalysis of the gene-coding sequence in unrelated MEN-1families revealed different heterozygous mutations, includingmissense, nonsense, insertional and deletional frameshifts alterationsin almost all cases (53–55), most of which predict a prematuretruncation of the menin protein. Thus, this confirms thatMEN-1 is suppressor gene according to Knudson’s two-hit theory,where the first hit are the germ-line MEN-1 mutations, and thesecond hit is a somatic deletion of the second MEN-1 allele onchromosome 11q13, resulting in LOH. MEN-1 gene mutationswere also identified in most cases of sporadic MEN-1 syndrome(55,56), but not in cases of familial pituitary adenomas (56). Incontrast to MEN-1 syndrome, patients with sporadic pituitaryadenomas, both secreting and nonfunctional, usually do not demonstratepathogenic changes, neither germ-line nor somatic, in thecoding sequence of MEN-1 gene, even in tumors with LOH of11q13 (57,58), and only two cases of sporadic pituitary adenomas(of 94 tumors studied) had specific MEN-1 mutations (57,58).Thus, MEN-1 gene mutations do not play a role in pituitary tumorigenesisin most sporadic adenomas.RETINOBLASTOMA GENE The retinoblastoma (Rb) geneis a tumor suppressor gene located at chromosome 13q14 that wasfirst characterized because of its association with human retinoblastomas.Inactivation of both Rb alleles by somatic mutationsleads to sporadic retinoblastomas (59), and LOH on chromosome13q is associated with other types of tumors, including breastcancer, bladder, esophageal, renal cell, and prostate carcinoma.Subjects with germline mutations in one Rb allele develop thehereditary form of retinoblastoma at an early age. The Rb proteinregulates the cell cycle, and is important for cell survival and differentiation.This protein is missing from retinoblastoma cells,and reintroducing a nonmutant Rb gene into these cells suppressestumor formation.Studies using transgenic disruptions of the Rb gene in micerevealed that embryos homozygous for the disrupted Rb gene diewithin days 14–15 of gestation (60). Mice heterozygous for the Rbmutation do not develop retinoblastomas, but have a high frequency(approx 90%) of pituitary tumors associated with LOH atthe Rb locus (60). These tumors originate from the intermediatelobe of the pituitary, and are classified histologically as adenocarcinomas.However, no Rb allelic deletions were detected in benignpituitary adenomas studied by four different groups (61–64). Peiet al. (64) showed LOH in proximity to the Rb locus in 13 malignantor highly invasive pituitary tumors, and in their respectivemetastases. Immunohistochemical studies, however, revealed thepresence of Rb protein in tumors with chromosome 13 allelic loss(64). In another study of 24 benign pituitary tumors, no Rb mutationsin exons 20–24 of the Rb gene were found, and the expressionof Rb protein was confirmed (65). Thus, inactivation of Rb suppressorgene does not appear to play a role in pathogenesis ofbenign human pituitary adenomas. Another tumor suppressor genelocated on chromosome 13 adjacent to the Rb locus may beinvolved in the development of invasive pituitary adenomas andcarcinomas.p53 GENE The p53 suppressor gene encodes a sequencespecificDNA binding protein that stimulates expression of downstreamgenes, which negatively control cell proliferation. p53 islocated on chromosome 17p13.1, and is the most common genemutated and/or deleted in human neoplasia, associated with 50%of cancer types. Characteristically, one allele of the p53 is lost, andthe remaining allele is mutated and results in a mutant p53 protein.This altered protein fails to suppress transformation and contributesto the development of malignancy. The wild-type p53 proteincannot be detected by immunohistochemistry owing to its shorthalf-life, but the mutated protein is more stable and can be detectedby specific antibodies.p53 mutations were not detected using cycle sequencing of exons5–8 (where the p53 mutations usually occur) in 22 nonsecretingand 22 GH-secreting pituitary adenomas (40), or in 29 adenomas(including all major types) (66) or in 4 pituitary carcinomas andtheir respective metastases (42). In addition, immunohistochemicalanalysis of p53 protein in 40 pituitary adenomas also detectedno alteration of this protein (67). However, Buckley et al. (68)detected p53 protein accumulation by immunohistochemistry ofsporadic pituitary adenomas in 16% of invasive tumors, but inonly ACTH-producing and nonfunctional tumors, and in 50% ofnoninvasive ACTH-secreting tumors.Interestingly, transgenic mice homozygous or heterozygousfor p53 mutations develop lymphomas and sarcomas, but notpituitary tumors (69). However, mice heterozygous for both p53and Rb mutations develop pituitary tumors, as well as lymphomasand sarcomas (69). These observations suggest that p53 probablyhas no role in pituitary adenoma formation. Its suggested involvementin Cushing’s adenoma pathogenesis, and the progression ofsome nonfunctional tumors toward invasiveness still requireselucidation.nm23 The purine binding factor (nm23) gene was identifiedas a tumor suppressor gene on the basis of reduced expression inhighly metastatic cancer, including breast, hepatic, and colorectalcarcinomas (70), compared with its abundant expression in lowmetastatic potential tumors. Recently, nm23 RNA expression wasstudied in 22 pituitary tumors, and the H2 isoform expression wassignificantly reduced in invasive adenomas, and correlatedinversely with cavernous sinus invasion (71). nm23 H2 proteinimmunoreactivity was also reduced in invasive tumors (71). However,nm23 gene mutations were not detected in those aggressivetumors.CYCLIN-DEPENDENT KINASE (CDK) INHIBITORSThe cyclin-dependent kinase (CDK) complexes play a centralrole in the regulation of cell-cycle progression. The Rb protein is