A reappraisal of the Rb1 gene abnormalities in the diagnosis of parathyroid cancer

OBJECTIVES Some histological features may suggest the malignant nature of a parathyroid tumour. However, the diagnosis of parathyroid cancer can only be definitively established in the presence of local invasion or metastases. DESIGN We further investigated the role of the retinoblastoma gene (Rb1) and the breast cancer susceptibility gene (BRCA2) in the differential diagnosis between benign and malignant parathyroid tumours by evaluating loss of heterozygosity (LOH) at these loci and Rb protein (pRb) immunohistochemistry. PATIENTS AND MEASUREMENTS Fifty-three parathyroid adenomas from patients with sporadic primary hyperparathyroidism (PHPT) and 10 parathyroid cancer specimens were studied. Microsatellite polymorphisms at the Rb1 and BRCA2 loci were polymerase chain reaction (PCR) amplified from each patient's paired tumour and leucocyte DNA samples, using oligonucleotide primers flanking the repeat sequence. Immunohistochemical staining of pRb was carried out using a monoclonal antibody. RESULTS All but one of the 53 tumour-leucocyte pairs was informative for at least one of the three polymorphic markers of the Rb1 gene. Fifteen adenomas (28.8%) showed LOH. Regarding the BRCA2 gene, 46 tumour-leucocyte pairs were informative and LOH was present in eight (17.4%). All six carcinomas had LOH for at least one marker at the Rb1 locus. LOH for the BRCA2 microsatellite was found in three of the five informative primary tumour samples. Immunohistochemical analysis revealed that all adenomas were positive and the number of pRb-positive cells varied significantly among different samples. The mean percentage of stained cells was 15.7%. Eleven of the 30 (36.7%) adenomas showed sparse positive staining, 13 (43.3%) intermediate staining and six (20%) extensive staining. All parathyroid cancers were entirely negative for pRb immunostaining. CONCLUSIONS Inactivation of the Rb1 gene is a common event in parathyroid tumorigenesis. Retention of heterozygosity seems to exclude parathyroid malignancy, which is suggested by the combined finding of LOH and lack of protein expression.