Parathyroid

We report here the identification of a gene associated with the hyperparathyroidism-jaw tumor (HPT-JT) syndrome. A single locus associated with HPT-JT (HRPT2) was previously mapped to chromosomal region 1q25-q32. We refined this region to a critical interval of 12 cM by genotyping in 26 affected kindreds. Using a positional candidate approach, we identified thirteen different heterozygous, germline, inactivating mutations in a single gene in fourteen families with HPT-JT. The proposed role of HRPT2 as a tumor suppressor was supported by mutation screening in 48 parathyroid adenomas with cystic features, which identified three somatic inactivating mutations, all located in exon 1. None of these mutations were detected in normal controls, and all were predicted to cause deficient or impaired protein function. HRPT2 is a ubiquitously expressed, evolutionarily conserved gene encoding a predicted protein of 531 amino acids, for which we propose the name parafibromin. Our findings suggest that HRPT2 is a tumor-suppressor gene, the inactivation of which is directly involved in predisposition to HPT-JT and in development of some sporadic parathyroid tumors.

We looked for mutations of the HRPT2 gene, which encodes the parafibromin protein, in sporadic parathyroid carcinoma because germ-line inactivating HRPT2 mutations have been found in a type of familial hyperparathyroidism--hyperparathyroidism-jaw tumor (HPT-JT) syndrome--that carries an increased risk of parathyroid cancer. We directly sequenced the full coding and flanking splice-junctional regions of the HRPT2 gene in 21 parathyroid carcinomas from 15 patients who had no known family history of primary hyperparathyroidism or the HPT-JT syndrome at presentation. We also sought to confirm the somatic nature of the identified mutations and tested the carcinomas for tumor-specific loss of heterozygosity at HRPT2. Parathyroid carcinomas from 10 of the 15 patients had HRPT2 mutations, all of which were predicted to inactivate the encoded parafibromin protein. Two distinct HRPT2 mutations were found in tumors from five patients, and biallelic inactivation as a result of a mutation and loss of heterozygosity was found in one tumor. At least one HRPT2 mutation was demonstrably somatic in carcinomas from six patients. Unexpectedly, HRPT2 mutations in the parathyroid carcinomas of three patients were identified as germ-line mutations. Sporadic parathyroid carcinomas frequently have HRPT2 mutations that are likely to be of pathogenetic importance. Certain patients with apparently sporadic parathyroid carcinoma carry germ-line mutations in HRPT2 and may have the HPT-JT syndrome or a phenotypic variant. Copyright 2003 Massachusetts Medical Society

Parathyroid carcinoma is one of the rarest known malignancies that may occur sporadically or as a part of a genetic syndrome. It accounts for approximately 1% of patients with primary hyperparathyroidism. The majority (90%) of parathyroid cancer tumors are hormonally functional and hypersecrete parathyroid hormone (PTH). Thus, most patients exhibit strong symptomatology of hypercalcemia at presentation. Sometimes, it can be difficult to diagnose parathyroid cancer preoperatively due to clinical features shared with benign causes of hyperparathyroidism. Imaging techniques such as neck ultrasound and 99mTc sestamibi scan can help localize disease, but they are not useful in the assessment of malignancy potential. Fine needle aspiration (FNA) prior to initial operation is not recommended due to technical difficulty in differentiating benign and malignant disease on cytology specimens and the possible associated risk of tumor seeding from the needle track. Complete surgical resection with microscopically negative margins is the recommended treatment and offers the best chance of cure. Persistent or recurrent disease occurs in more than 50% of patients with parathyroid carcinoma. Surgical resection is also the primary mode of therapy for recurrence since it can offer significant palliation for the metabolic derangement caused by hyperparathyroidism and allows hypercalcemia to become more medically manageable. However, reoperation is rarely curative and eventual relapse is likely. Chemotherapy and external beam radiation treatments have been generally ineffective in the treatment of parathyroid carcinoma. Typically, these patients require repeated operations that predispose them to accumulated surgical risks with each intervention. In inoperable cases, few palliative treatment options exist, although treatment with calcimimetics can effectively control hypercalcemia in some patients. Most patients ultimately succumb to complications of hypercalcemia rather than from tumor burden or infiltration.