The
tumor suppressor HRPT2/CDC73 is mutated in constitutive
DNA from patients with the familial disorder
hyperparathyroidism-jaw tumor syndrome and in approximately 70% of all
parathyroid carcinomas. In a number of HRPT2 mutant
tumors however, expression of the encoded
protein parafibromin is lost in the absence of a clear second event such as HRPT2 allelic loss or the presence of a second mutation in this tumor suppressor gene. We sought to determine whether hypermethylation of a 713 bp CpG island extending 648
nucleotides upstream of the HRPT2 translational start site and 65
nucleotides into exon 1 might be a mechanism contributing to the loss of expression of parafibromin in parathyroid
tumors. Furthermore, we asked whether mutations might be present in the 5'-untranslated region (5'-UTR) of HRPT2. We investigated a pool of tissue from 3 normal parathyroid glands, as well as 15 individual parathyroid
tumor samples including 6
tumors with known HRPT2 mutations, for hypermethylation of the HRPT2 CpG island. Methylation was not identified in any specimens despite complete loss of parafibromin expression in two
parathyroid carcinomas with a single detectable HRPT2 mutation and retention of the wild-type HRPT2 allele. Furthermore, no mutations of a likely pathogenic nature were identified in the 5'-UTR of HRPT2. These data strongly suggest that alternative mechanisms such as mutation in HRPT2 intronic regions, additional epigenetic regulation such as histone modifications, or other regulatory inactivation mechanisms such as targeting by
microRNAs may play a role in the loss of parafibromin expression.