79577

CDC73

C1orf28|FIHP|HPTJT|HRPT1|HRPT2|HYX;cell division cycle 73;CDC73;cell division cycle 73

Paf1/RNA polymerase II complex component|cell division cycle 73, Paf1/RNA polymerase II complex component, homolog|cell division cycle protein 73 homolog|hyperparathyroidism 2 protein|parafibromin

1q25

protein-coding

Parafibromin is the 531-amino-acid protein product encoded by HRPT2, a putative tumor suppressor gene recently implicated in the autosomal dominant hyperparathyroidism-jaw tumor familial cancer syndrome, sporadic parathyroid cancer, and a minority of families with isolated hyperparathyroidism. Parafibromin contains no identified functional domains but bears sequence homology to Cdc73p, a budding yeast protein component of the RNA polymerase II-associated Paf1 complex. This study addressed the expression and functional properties of human parafibromin. A survey of human and mouse tissues analysed with polyclonal antibodies to parafibromin showed specific immunoreactivity in adrenal and parathyroid glands, kidney, heart, and skeletal muscle. Subcellular fractionation and laser confocal microscopy of normal human parathyroid gland demonstrated expression of parafibromin in both the cytoplasmic and nuclear compartments. Parafibromin was expressed in four parathyroid adenomas but was absent from two parathyroid carcinomas. Transient overexpression of wild-type parafibromin, but not its Leu64Pro missense mutant implicated in parathyroid cancer and familial isolated hyperparathyroidism, inhibited cell proliferation, and blocked expression of cyclin D1, a key cell cycle regulator previously implicated in parathyroid neoplasia. These results demonstrate that human parafibromin is a nucleocytoplasmic protein with functions consistent with its postulated role as a tumor suppressor protein.

Parafibromin, the product of the HRPT2 (hyperparathyroidism-jaw tumor syndrome 2) tumor suppressor gene, is the human homologue of yeast Cdc73, part of the yeast RNA polymerase II/Paf1 complex known to be important for histone modification and connections to posttranscriptional events. By purifying cellular parafibromin and characterizing its associated proteins, we have identified a human counterpart to the yeast Paf1 complex including homologs of Leo1, Paf1, and Ctr9. Like the yeast complex, the parafibromin complex associates with the nonphosphorylated and Ser2 and Ser5 phosphorylated forms of the RNA polymerase II large subunit. Immunofluorescence experiments show that parafibromin is a nuclear protein. In addition, cotransfection data suggest that parafibromin can interact with a histone methyltransferase complex that methylates histone H3 on lysine 4. Some mutant forms of parafibromin lack association with hPaf1 complex members and with the histone methyltransferase complex, suggesting that disruption of these complexes may correlate with the oncogenic process.

Inactivation of the HRPT2 tumor suppressor gene is associated with the pathogenesis of the hereditary hyperparathyroidism-jaw tumor syndrome and malignancy in sporadic parathyroid tumors. The cellular function of the HPRT2 gene product, parafibromin, has not been defined yet. Here we show that parafibromin physically interacts with human orthologs of yeast Paf1 complex components, including PAF1, LEO1, and CTR9, that are involved in transcription elongation and 3 end processing. It also associates with modified forms of the large subunit of RNA polymerase II, in particular those phosphorylated on serine 5 or 2 within the carboxy-terminal domain, that are important for the coordinate recruitment of transcription elongation and RNA processing machineries during the transcription cycle. These interactions depend on a C-terminal domain of parafibromin, which is deleted in ca. 80% of clinically relevant mutations. Finally, RNAi-induced downregulation of parafibromin promotes entry into S phase, implying a role for parafibromin as an inhibitor of cell cycle progression. Taken together, these findings link the tumor suppressor parafibromin to the transcription elongation and RNA processing pathway as a PAF1 complex- and RNA polymerase II-bound protein. Dysfunction of this pathway may be a general phenomenon in the majority of cases of hereditary parathyroid cancer.

Parafibromin is a putative tumor suppressor encoded by HRPT2, mutations in which have been implicated in the familial tumor syndrome hyperparathyroidism jaw tumor syndrome (HPT-JT), and sporadic parathyroid carcinoma. Recently, parafibromin has been shown to be an accessory factor for RNA polymerase II as part of the human Paf 1 complex, suggesting, as has been shown for its yeast homologue (Cdc 73), that it may have a role as an important regulator of transcription. Parafibromin has also been shown to interact with a histone methyltransferase complex that methylates histone H3 and to inhibit proliferation when overexpressed in mammalian cell lines. Despite these findings, the cellular localization of parafibromin has been controversial, with reports of both nuclear and nucleocytoplasmic localization. We have expressed wild-type and mutant parafibromin tagged with enhanced green fluorescent protein and have identified a functional bipartite nuclear localization signal (NLS) at residues 125-139 (nucleotides 373-417), KRAADEVLAEAKKPR, that is evolutionarily conserved and critical for the nuclear localization of parafibromin. We have also shown that the C-terminal arm of this bipartite NLS plays the primary role in nuclear localization. In support of these findings, specific HRPT2 mutations identified in HPT-JT or sporadic parathyroid carcinoma predicted to truncate parafibromin upstream of or within this NLS disrupt nuclear localization.

Parafibromin is a tumor suppressor protein encoded by HRPT2, a gene recently implicated in the hereditary hyperparathyroidism-jaw tumor syndrome, parathyroid cancer, and a subset of kindreds with familial isolated hyperparathyroidism. Human parafibromin binds to RNA polymerase II as part of a PAF1 transcriptional regulatory complex. The mechanism by which loss of parafibromin function can lead to neoplastic transformation is poorly understood. Because the subcellular localization of parafibromin is likely to be critical for its function with the nuclear PAF1 complex, we sought to experimentally define the nuclear localization signal (NLS) of parafibromin and examine its potential role in parafibromin function. Using site-directed mutagenesis, we define a dominant bipartite NLS and a secondary NLS, both in the NH(2)-terminal region of parafibromin whose combined mutation nearly abolishes nuclear targeting. The NLS-mutant parafibromin is significantly impaired in its association with endogenous Paf1 and Leo1. We further report that overexpression of wild-type but not NLS-mutant parafibromin induces apoptosis in transfected cells. Inhibition of endogenous parafibromin expression by RNA interference inhibits the basal rate of apoptosis and apoptosis resulting from DNA damage induced by camptothecin, a topoisomerase I inhibitor. These experiments identify for the first time a proapoptotic activity of endogenous parafibromin likely to be important in its role as a tumor suppressor and show a functional role for the NLS of parafibromin in this activity.

Parafibromin (PF) is a 531-amino acid protein encoded by HRPT2, a putative tumor suppressor gene recently implicated in the autosomal-dominant hyperparathyroidism-jaw tumor familial cancer syndrome and sporadic parathyroid carcinoma. To investigate effects of PFs overexpression on cell proliferation, we performed assays in four different cell lines. The transient overexpression of PF inhibited cell growth in HEK293 and NIH3T3 cells, but enhanced cell growth in the SV40 large T antigen-expressing cell lines such as 293FT and COS7 cells. In 293FT cells, PF was found to interact with SV40 large T antigen and its overexpression promoted entry into the S phase, implying that the interaction enhanced progression through the cell cycle. The tumor suppressor protein PF acts as a positive regulator of cell growth similar to an oncoprotein in the presence of SV40 large T antigen.

BACKGROUND: Germline and somatic inactivating mutations in the HRPT2 gene occur in the inherited hyperparathyroidism-jaw tumor syndrome, in some cases of parathyroid cancer and in some cases of familial hyperparathyroidism. HRPT2 encodes parafibromin. To identify parafibromin interacting proteins we used the yeast two-hybrid system for screening a heart cDNA library with parafibromin as the bait. RESULTS: Fourteen parafibromin interaction positive preys representing 10 independent clones encoding actinin-2 were isolated. Parafibromin interacted with muscle alpha-actinins (actinin-2 and actinin-3), but not with non-muscle alpha-actinins (actinin-1 and actinin-4). The parafibromin-actinin interaction was verified by yeast two-hybrid, GST pull-down, and co-immunoprecipitation. Yeast two-hybrid analysis revealed that the N-terminal region of parafibromin interacted with actinins. In actin sedimentation assays parafibromin did not dissociate skeletal muscle actinins from actin filaments, but interestingly, parafibromin could also bundle/cross-link actin filaments. Parafibromin was predominantly nuclear in undifferentiated proliferating myoblasts (C2C12 cells), but in differentiated C2C12 myotubes parafibromin co-localized with actinins in the cytoplasmic compartment. CONCLUSION: These data support a possible contribution of parafibromin outside the nucleus through its interaction with actinins and actin bundling/cross-linking. These data also suggest that actinins (and actin) participate in sequestering parafibromin in the cytoplasmic compartment.

Parafibromin is a tumor suppressor protein encoded by HRPT2, a gene recently implicated in the hereditary hyperparathyroidism-jaw tumor syndrome, parathyroid cancer, and a subset of kindreds with familial isolated hyperparathyroidism. Human parafibromin binds to RNA polymerase II as part of a PAF1 transcriptional regulatory complex. The physiologic targets of parafibromin and the mechanism by which its loss of function can lead to neoplastic transformation are poorly understood. We show here that RNA interference with the expression of parafibromin or Paf1 stimulates cell proliferation and increases levels of the c-myc proto-oncogene product, a DNA-binding protein and established regulator of cell growth. This effect results from both c-myc protein stabilization and activation of the c-myc promoter, without alleviation of the c-myc transcriptional pause. Chromatin immunoprecipitation demonstrates the occupancy of the c-myc promoter by parafibromin and other PAF1 complex subunits in native cells. Knockdown of c-myc blocks the proliferative effect of RNA interference with parafibromin or Paf1 expression. These experiments provide a previously uncharacterized mechanism for the anti-proliferative action of the parafibromin tumor suppressor protein resulting from PAF1 complex-mediated inhibition of the c-myc proto-oncogene.

The hyperparathyroidism-jaw tumor syndrome (HPT-JT) is a familial cancer syndrome that can result from germline inactivation of HRPT2/CDC73, a putative tumor suppressor gene that encodes parafibromin, a component of the transcriptional regulatory PAF1 complex with homology to the yeast protein Cdc73p. The vast majority of HRPT2/CDC73 germline mutations identified have been truncation or frameshift mutations, and loss of function due to missense mutation is rare. We report here a kindred with HPT-JT due to a germline L95P missense mutation in parafibromin. The mutant parafibromin was studied in vitro to understand the basis of its presumed loss-of-function. When transfected in cultured cells, the L95P mutant was expressed to a lower level than wild-type (wt) parafibromin, a difference that was not overcome by inhibition of the proteasomal degradation pathway. The L95P mutant parafibromin retained the ability to assemble with endogenous PAF1 complex components as evidenced by co-immunoprecipitation. Analysis of subcellular localization showed that the L95P mutant was markedly deficient in nucleolar localization compared to the wt, an impairment likely resulting from disruption of a putative nucleolar localization signal immediately upstream of the L95P mutation. Transfection of the L95P parafibromin mutant, but not the wt, enhanced cell cycle progression and increased cell survival in NIH-3T3 and HEK 293 cells, resulting apparently from dominant interference with endogenous parafibromin action. The simultaneous loss of nucleolar localization and acquisition of a growth stimulatory phenotype with the L95P mutation raise the possibility that parafibromin must interact with targets in the nucleolus to fully execute its tumor suppressor functions.

Monoubiquitination of histone H2B is a dynamic post-translational histone modification associated with transcriptional elongation and the DNA damage response. To date, dysregulation of histone monoubiquitination has not been linked to pathogenic mutations in genes encoding proteins, or co-factors, catalyzing this modification. The tumor suppressor cell division cycle 73 (CDC73) is mutated and/or down-regulated in parathyroid carcinoma, renal, breast, gastric and colorectal tumors, as well as in the germline of patients with the familial disorder-hyperparathyroidism jaw tumor syndrome. Using CDC73 as bait in a yeast two-hybrid assay, we identified the ring finger proteins RNF20 and RNF40 as binding partners of this tumor suppressor. These polypeptides constitute a heterodimeric complex that functions as the E3 ubiquitin ligase for monoubiquitination of histone H2B at lysine 120 (H2B-K120). We show that RNF20 and RNF40 bind to discrete, but closely located, residues on CDC73. Monoubiquitinated H2B-K120 was significantly reduced after loss of nuclear CDC73, both in vitro upon down-regulation of CDC73, and in CDC73 mutant parathyroid tumors. A second histone modification, trimethylation of histone 3 at lysine 4 (H3-K4me3), remained unchanged in the presence of mutant or down-regulated CDC73, suggesting that H3-K4me3 is not always tightly linked to H2B-K120 monoubiquitination for transcription as previously described. This is the first report of pathogenic mutations affecting histone monoubiquitination. We conclude that CDC73 is required for the maintenance of H2B-K120 monoubiquitination and propose that reduction in levels of monoubiquitinated H2B-K120 is a major mechanism whereby mutations in CDC73 exert their tumorigenic effect.

The CDC73 gene is mutationally inactivated in hereditary and sporadic parathyroid tumors. It negatively regulates beta-catenin, cyclin D1, and c-MYC. Down-regulation of CDC73 has been reported in breast, renal, and gastric carcinomas. However, the reports regarding the role of CDC73 in oral squamous cell carcinoma (OSCC) are lacking. In this study we show that CDC73 is down-regulated in a majority of OSCC samples. We further show that oncogenic microRNA-155 (miR-155) negatively regulates CDC73 expression. Our experiments show that the dramatic up-regulation of miR-155 is an exclusive mechanism for down-regulation of CDC73 in a panel of human cell lines and a subset of OSCC patient samples in the absence of loss of heterozygosity, mutations, and promoter methylation. Ectopic expression of miR-155 in HEK293 cells dramatically reduced CDC73 levels, enhanced cell viability, and decreased apoptosis. Conversely, the delivery of a miR-155 antagonist (antagomir-155) to KB cells overexpressing miR-155 resulted in increased CDC73 levels, decreased cell viability, increased apoptosis, and marked regression of xenografts in nude mice. Cotransfection of miR-155 with CDC73 in HEK293 cells abrogated its pro-oncogenic effect. Reduced cell proliferation and increased apoptosis of KB cells were dependent on the presence or absence of the 3-UTR in CDC73. In summary, knockdown of CDC73 expression due to overexpression of miR-155 not only adds a novelty to the list of mechanisms responsible for its down-regulation in different tumors, but the restoration of CDC73 levels by the use of antagomir-155 may also have an important role in therapeutic intervention of cancers, including OSCC.