Expression of p16^INK4A gene in human pituitary tumours

Pituitary adenomas comprise 10–15% of primary intracranial tumours but the mechanisms leading to tumour development are yet to be clearly established. The retinoblastoma pathway, which regulates the progression through the cell cycle, is often deregulated in different types of tumours. We studied the cyclin-dependent kinase inhibitor p16^INK4A gene expression at mRNA level in human pituitary adenomas. Forty-six tumour specimens of different subtypes, 21 clinically non-functioning, 12 growth hormone-secreting, 6 prolactin-secreting, 6 adrenocorticotropin-secreting, and 1 thyrotropin-secreting tumours were studied. All clinically non-functioning and most of the hormone-secreting tumours were macroadenomas (38/46). The RT–PCR assay and electrophoresis of the PCR-products showed that p16^INK4A mRNA was undetectable in: 62% of non-functioning, 8% of growth hormone-secreting, 17% of prolactin-secreting and 17% of adrenocorticotropin-secreting adenomas. Forty percent of all macroadenomas and 25% of microadenomas had negative p16^INK4A mRNA, the latter results suggest that the absence of p16^INK4A product might be an early event in tumours with no expression of this suppressor gene. Within the non-functioning adenomas 63% were “null cell” and 37% were positive for some hormone, both subgroups showed similar percentage of cases with absence of p16^INK4A mRNA. Our results show that clinically non-functioning macroadenomas have impaired p16^INK4A expression in a clearly higher proportion than any other pituitary tumour subtype investigated. Other regulatory pathways may be implicated in the development of tumours with positive p16^INK4A expression.