CDKN2A/p16 inactivation is related to pituitary adenoma type and size

Abstract

p16 (CDKN2A, MTS1, INK4A) status at genomic and protein levels was analysed and correlated with clinico-pathological features in 72 pituitary adenomas. Methylation of CpG islands of promoter/exon 1 sequences was found in most gonadotroph, lactotroph, plurihormonal, and null cell adenomas (36 of 44, 82%), but it was rare in somatotroph (1 of 13 cases, 8%) and corticotroph adenomas (1 of 15 cases, 7%). Homozygous CDKN2A deletion was restricted to rare somatotroph (15%) and corticotroph adenomas (13%). Immunohistochemical p16 protein expression was observed in the normal adenohypophysis, whereas it was absent in 60 of 72 (83%) tumours and reduced in another ten (14%) tumours. Staining for p16 was only seen in 5 of 15 (33%) corticotroph, 3 of 13 (23%) somatotroph, 3 of 5 (60%) plurihormonal, and 1 of 19 (5%) null cell adenomas. p16 immunonegativity without CDKN2A methylation or deletion occurred in 22 tumours, including most somatotroph and corticotroph adenomas (15 of 28, 54%). Both CDKN2A alterations and p16 negativity were related to larger tumour size. Patients with p16-negative tumours were older than patients with p16-positive tumours. These data suggest that p16 down-regulation is common in all adenoma types. The mechanisms of p16 down-regulation probably involve CDKN2A methylation in most types, but remain to be determined in somatotroph and corticotroph adenomas. These findings also suggest that p16 down-regulation is usually not an initial event, but is acquired during adenoma progression.