Frequent Loss of Heterozygosity at the Retinoblastoma Susceptibility Gene (RB) Locus in Aggressive Pituitary Tumors: Evidence for a Chromosome 13 Tumor Suppressor Gene Other Than RB1

Mice bearing retinoblastoma susceptibility gene (RB) germ-line muta tions almost invariably develop pituitary neoplasms. We therefore tested 17 patients with pituitary tumors for loss of heterozygosity (LOH) using an RB sequence polymorphism and 5 polymorphic microsatellite markers surrounding the RB gene on the long arm of chromosome 13. In all of the 13 malignant or highly invasive pituitary tumor cases, and in 4 of their respective métastases,a RB alÃ-elewas lost. In contrast, no LOH at the Rii locus was detected in 4 benign pituitary adenoma cases. Three invasive tumors also lost a portion of 13q, which included DI3sl37, D13sI33, and D13sll8 telomeric and centromeric to RB, respectively. Immunohistochemical analysis, however, revealed the presence of RB protein in tumors with LOH and the RB locus. Therefore, although inactivation of RB may play a role in the development of invasive pituitary adenomas and carci nomas in mice, another tumor suppressor gene on 13q is likely involved in human pituitary tumor progression. LOH of 13q markers may also be of predictive value in determining the biological behavior of pituitary macroadenomas and their progression to invasiveness and frank malignancy.