p53 gene mutations in pituitary adenomas: rare events

Occult pituitary adenomas are said to occur in up to 20% of random autopsy examinations, yet the only oncogene known to be associated with pituitary adenomas, gsp, is found in only 40% of somatotrophinomas, a subtype that accounts for a minority of pituitary tumours. Mutations of the p53 tumour suppressor gene are thought to be involved in the pathogenesis of as many as 50% of all human cancers, including tumours of the central nervous system. The objective of this study was to determine whether p53 gene mutations are associated with pituitary adenomas.

Fragments of pituitary adenoma tissue from 29 patients undergoing routine hypophysectomy for pituitary tumour were coated in cryostat embedding medium and frozen at -80 degrees C within 24 hours of resection. They consisted of 9 somatotroph, 4 corticotroph, 1 mammotroph and 15 endocrinologically inactive adenomas, all of the non-invasive clinical phenotype. Sequential frozen sections were subjected to in situ hybridization analysis for anterior pituitary hormone transcripts and examined histologically to ensure that the frozen sections used to generate DNA templates for polymerase chain reaction amplification were not contaminated with non-tumour tissue.

p53 exons 7 and 8, within which 98% of substitution mutations are thought to occur, and exons 4-6 in tumours immunopositive for p53, were amplified by polymerase chain reaction and ligated into the vector pCR2. DNA from small-scale plasmid preparations of pCR2 containing cloned p53 exons from human pituitary adenomas was sequenced using an automated fluorescence-based system (DuPont Genesis 2000) and compared with wild-type sequence. Apparent mutations were confirmed or refuted by sequencing a further 2-4 clones isolated from the same template.

Although immunocytochemical staining patterns for wild-type p53 varied markedly between different tumours, no mutations were identified in any of the exonic sequences examined.

p53 mutations, at least within the high mutation domains of p53, occur infrequently in human pituitary adenomas. Increased steady-state levels of p53 protein identified immunocytochemically may be a consequence of binding to other cellular proteins in these tumours.