Prevalence of Pseudotumor in Patients After Metal-On-Metal Hip Arthroplasty Evaluated with Metal Ion Analysis and MARS-MRI.

The purpose of this study is to quantify the prevalence of pseudotumors in patients with well-functioning and painful metal-on-metal total hip arthroplasty, to characterize the pseudotumor with the use of MARS-MRI, and to assess the relationship between pseudotumors and metal ions. We retrospectively reviewed 102 single surgeon patients. The results showed that 68.6% developed pseudotumor with 60.9% of the asymptomatic group developing pseudotumor. The symptomatic group had a higher proportion of patients with elevated serum cobalt levels (P=0.035). There was no difference found with elevated metal ions and prevalence of pseudotumor, but elevated cobalt levels were associated with larger pseudotumor size (P=0.001). The available evidence indicated that most patients that develop pseudotumors are asymptomatic, and that elevated serum cobalt levels may be associated with symptoms and pseudotumor size.

FACT facilitates chromatin transcription by RNA polymerases I and III.

Efficient transcription elongation from a chromatin template requires RNA polymerases (Pols) to negotiate nucleosomes. Our biochemical analyses demonstrate that RNA Pol I can transcribe through nucleosome templates and that this requires structural rearrangement of the nucleosomal core particle. The subunits of the histone chaperone FACT (facilitates chromatin transcription), SSRP1 and Spt16, co-purify and co-immunoprecipitate with mammalian Pol I complexes. In cells, SSRP1 is detectable at the rRNA gene repeats. Crucially, siRNA-mediated repression of FACT subunit expression in cells results in a significant reduction in 47S pre-rRNA levels, whereas synthesis of the first 40 nt of the rRNA is not affected, implying that FACT is important for Pol I transcription elongation through chromatin. FACT also associates with RNA Pol III complexes, is present at the chromatin of genes transcribed by Pol III and facilitates their transcription in cells. Our findings indicate that, beyond the established role in Pol II transcription, FACT has physiological functions in chromatin transcription by all three nuclear RNA Pols. Our data also imply that local chromatin dynamics influence transcription of the active rRNA genes by Pol I and of Pol III-transcribed genes.

RNA-polymerase-I-directed rDNA transcription, life and works.

In the extensive network of interdependent biochemical processes required for cell growth and division, there is mounting evidence that ribosomal DNA transcription by RNA polymerase I (pol I) not only drives cell growth via its direct role in production of the ribosomal RNA (rRNA) component of the protein-synthesis machinery, but that it is also crucial in determining the fate of the cell. Considerable progress has been made in recent years towards understanding both the function of components of the pol I transcription machinery and how cells accomplish the tight control of pol I transcription, balancing the supply of rRNA with demand under different growth conditions.

Casein kinase 2 associates with initiation-competent RNA polymerase I and has multiple roles in ribosomal DNA transcription.

Mammalian RNA polymerase I (Pol I) complexes contain a number of associated factors, some with undefined regulatory roles in transcription. We demonstrate that casein kinase 2 (CK2) in human cells is associated specifically only with the initiation-competent Pol Ibeta isoform and not with Pol Ialpha. Chromatin immunoprecipitation analysis places CK2 at the ribosomal DNA (rDNA) promoter in vivo. Pol Ibeta-associated CK2 can phosphorylate topoisomerase IIalpha in Pol Ibeta, activator upstream binding factor (UBF), and selectivity factor 1 (SL1) subunit TAFI110. A potent and selective CK2 inhibitor, 3,8-dibromo-7-hydroxy-4-methylchromen-2-one, limits in vitro transcription to a single round, suggesting a role for CK2 in reinitiation. Phosphorylation of UBF by CK2 increases SL1-dependent stabilization of UBF at the rDNA promoter, providing a molecular mechanism for the stimulatory effect of CK2 on UBF activation of transcription. These positive effects of CK2 in Pol I transcription contrast to that wrought by CK2 phosphorylation of TAFI110, which prevents SL1 binding to rDNA, thereby abrogating the ability of SL1 to nucleate preinitiation complex (PIC) formation. Thus, CK2 has the potential to regulate Pol I transcription at multiple levels, in PIC formation, activation, and reinitiation of transcription.

RNA polymerase I-specific subunit CAST/hPAF49 has a role in the activation of transcription by upstream binding factor.

Eukaryotic RNA polymerases are large complexes, 12 subunits of which are structurally or functionally homologous across the three polymerase classes. Each class has a set of specific subunits, likely targets of their cognate transcription factors. We have identified and characterized a human RNA polymerase I (Pol I)-specific subunit, previously identified as ASE-1 (antisense of ERCC1) and as CD3epsilon-associated signal transducer (CAST), and here termed CAST or human Pol I-associated factor of 49 kDa (hPAF49), after mouse orthologue PAF49. We provide evidence for growth-regulated Tyr phosphorylation of CAST/hPAF49, specifically in initiation-competent Pol Ibeta complexes in HeLa cells, at a conserved residue also known to be important for signaling during T-cell activation. CAST/hPAF49 can interact with activator upstream binding factor (UBF) and, weakly, with selectivity factor 1 (SL1) at the rDNA (ribosomal DNA repeat sequence encoding the 18S, 5.8S, and 28S rRNA genes) promoter. CAST/hPAF49-specific antibodies and excess CAST/hPAF49 protein, which have no effect on basal Pol I transcription, inhibit UBF-activated transcription following functional SL1-Pol I-rDNA complex assembly and disrupt the interaction of UBF with CAST/hPAF49, suggesting that interaction of this Pol I-specific subunit with UBF is crucial for activation. Drawing on parallels between mammalian and Saccharomyces cerevisiae Pol I transcription machineries, we advance one model for CAST/hPAF49 function in which the network of interactions of Pol I-specific subunits with UBF facilitates conformational changes of the polymerase, leading to stabilization of the Pol I-template complex and, thereby, activation of transcription.

The RNA polymerase I transcription machinery.

The rRNAs constitute the catalytic and structural components of the ribosome, the protein synthesis machinery of cells. The level of rRNA synthesis, mediated by Pol I (RNA polymerase I), therefore has a major impact on the life and destiny of a cell. In order to elucidate how cells achieve the stringent control of Pol I transcription, matching the supply of rRNA to demand under different cellular growth conditions, it is essential to understand the components and mechanics of the Pol I transcription machinery. In this review, we discuss: (i) the molecular composition and functions of the Pol I enzyme complex and the two main Pol I transcription factors, SL1 (selectivity factor 1) and UBF (upstream binding factor); (ii) the interplay between these factors during pre-initiation complex formation at the rDNA promoter in mammalian cells; and (iii) the cellular control of the Pol I transcription machinery.

A simple and accurate method for determining leg length in primary total hip arthroplasty.

Reconstruction of appropriate leg length is an important part of soft-tissue balance in total hip arthroplasty (THA). Leg length discrepancy (LLD) is one of the more common reasons for litigation after otherwise successful THA. The purpose of the study reported here was to analyze the accuracy of using preoperative templating and intraoperative referencing of the well leg to determine postoperative leg length in unilateral primary THA. Seven-hundred primary THAs performed at an institution by 3 surgeons were randomly selected from a computerized database. Cases with significant bilateral disease, congenital dysplasia, acute fracture, or previous surgery or without complete preoperative and postoperative radiographs were excluded. Three reviewers used a standardized method to measure preoperative and postoperative LLD. Included in the review were 410 THAs. Mean postoperative LLD was 3.9 mm lengthening (SD, 7.5 mm). In 20 THAs (4.9%), lengthening was more than 15 mm. Lengthening was more than 20 mm (maximum, 22 mm) in 4 THAs (1%). Of the 20 THAs with LLD of more than 15 mm, 14 involved hips that were longer preoperatively. Thirteen of these hips were reconstructed to within 10 mm of preoperative LLD. Only 2 patients with radiographic LLD of more than 15 mm perceived LLD. There were no differences in gender, height, weight, or body mass index. This method of preoperative templating and referencing the well leg intraoperatively is an inexpensive, reliable, and accurate method for determining leg length in primary THA and has few significant radiographic or clinical outliers.

Topoisomerase IIα promotes activation of RNA polymerase I transcription by facilitating pre-initiation complex formation.

Type II DNA topoisomerases catalyse DNA double-strand cleavage, passage and re-ligation to effect topological changes. There is considerable interest in elucidating topoisomerase II roles, particularly as these proteins are targets for anti-cancer drugs. Here we uncover a role for topoisomerase IIα in RNA polymerase I-directed ribosomal RNA gene transcription, which drives cell growth and proliferation and is upregulated in cancer cells. Our data suggest that topoisomerase IIα is a component of the initiation-competent RNA polymerase Iß complex and interacts directly with RNA polymerase I-associated transcription factor RRN3, which targets the polymerase to promoter-bound SL1 in pre-initiation complex formation. In cells, activation of rDNA transcription is reduced by inhibition or depletion of topoisomerase II, and this is accompanied by reduced transient double-strand DNA cleavage in the rDNA-promoter region and reduced pre-initiation complex formation. We propose that topoisomerase IIα functions in RNA polymerase I transcription to produce topological changes at the rDNA promoter that facilitate efficient de novo pre-initiation complex formation.

TAF1B is a TFIIB-like component of the basal transcription machinery for RNA polymerase I.

Transcription by eukaryotic RNA polymerases (Pols) II and III and archaeal Pol requires structurally related general transcription factors TFIIB, Brf1, and TFB, respectively, which are essential for polymerase recruitment and initiation events. A TFIIB-like protein was not evident in the Pol I basal transcription machinery. We report that TAF1B, a subunit of human Pol I basal transcription factor SL1, is structurally related to TFIIB/TFIIB-like proteins, through predicted amino-terminal zinc ribbon and cyclin-like fold domains. SL1, essential for Pol I recruitment to the ribosomal RNA gene promoter, also has an essential postpolymerase recruitment role, operating through TAF1B. Therefore, a TFIIB-related protein is implicated in preinitiation complex assembly and postpolymerase recruitment events in Pol I transcription, underscoring the parallels between eukaryotic Pol I, II, and III and archaeal transcription machineries.

A novel TBP-associated factor of SL1 functions in RNA polymerase I transcription.

In mammalian RNA polymerase I transcription, SL1, an assembly of TBP and associated factors (TAFs), is essential for preinitiation complex formation at ribosomal RNA gene promoters in vitro. We provide evidence for a novel component of SL1, TAF(I)41 (MGC5306), which functions in Pol I transcription. TAF(I)41 resides at the rDNA promoter in the nucleolus and co-purifies and co-immunoprecipitates with SL1. TAF(I)41 immunodepletion from nuclear extracts dramatically reduces Pol I transcription; addition of SL1 restores the ability of these extracts to support Pol I transcription. In cells, siRNA-mediated decreased expression of TAF(I)41 leads to loss of SL1 from the rDNA promoter in vivo, with concomitant loss of Pol I from the rDNA and reduced synthesis of the pre-rRNA. Extracts from these cells support reduced levels of Pol I transcription; addition of SL1 to the extracts raises the level of Pol I transcription. These data suggest that TAF(I)41 is integral to transcriptionally active SL1 and imply a role for SL1, including the TAF(I)41 subunit, in Pol I recruitment and, therefore, preinitiation complex formation in vivo.

UBF activates RNA polymerase I transcription by stimulating promoter escape.

Ribosomal RNA gene transcription by RNA polymerase I (Pol I) is the driving force behind ribosome biogenesis, vital to cell growth and proliferation. The key activator of Pol I transcription, UBF, has been proposed to act by facilitating recruitment of Pol I and essential basal factor SL1 to rDNA promoters. However, we found no evidence that UBF could stimulate recruitment or stabilization of the pre-initiation complex (PIC) in reconstituted transcription assays. In this, UBF is fundamentally different from archetypal activators of transcription. Our data imply that UBF exerts its stimulatory effect on RNA synthesis, after PIC formation, promoter opening and first phosphodiester bond formation and before elongation. We provide evidence to suggest that UBF activates transcription in the transition between initiation and elongation, at promoter escape by Pol I. This novel role for UBF in promoter escape would allow control of rRNA synthesis at active rDNA repeats, independent of and complementary to the promoter-specific targeting of SL1 and Pol I during PIC assembly. We posit that stimulation of promoter escape could be a general mechanism of activator function.

Isolated liner exchange using the anterolateral approach is associated with a low risk of dislocation.

UNLABELLED: Authors of reports on the outcome of isolated liner exchange for osteolysis and wear have reported high dislocation rates. Twenty-six patients (27 hips) with a minimum of 2 years of followup had isolated liner exchange for wear and osteolysis done using the abductor splitting anterolateral approach. The mean followup was 41 months. The average age at time of surgery was 51 years. Preoperative Harris hip scores averaged 70, and increased to 82 at the most recent followup. We observed improvements in pain and functional scores. The average operating time was 82 minutes, and the average blood loss was 255 mL. Only three (12%) patients required transfusion. No components were rerevised for aseptic loosening, and one patient (one hip) had a dislocation (3.7%). Isolated liner exchange for osteolysis and wear done using the anterolateral approach has a lower risk of dislocation than previously reported and provides substantial improvements in pain, function, and Harris hip score. LEVEL OF EVIDENCE: Therapeutic study, Level IV (case series). See the Guidelines for Authors for a complete description of levels of evidence.

TBP-TAF complex SL1 directs RNA polymerase I pre-initiation complex formation and stabilizes upstream binding factor at the rDNA promoter.

Knowledge of the role of components of the RNA polymerase I transcription machinery is paramount to understanding regulation of rDNA expression. We describe key findings for the roles of essential transcription factor SL1 and activator upstream binding factor (UBF). We demonstrate that human SL1 can direct accurate Pol I transcription in the absence of UBF and can interact with the rDNA promoter independently and stably, consistent with studies of rodent SL1 but contrary to previous reports of human SL1. UBF itself does not bind stably to rDNA but rapidly associates and dissociates. We show that SL1 significantly reduces the rate of dissociation of UBF from the rDNA promoter. Our findings challenge the idea that UBF activates transcription through recruitment of SL1 at the rDNA promoter and suggest that the rate of pre-initiation complex (PIC) formation is primarily determined by the rate of association of SL1, rather than UBF, with the promoter. Therefore, we propose that SL1 directs PIC formation, functioning in core promoter binding, RNA polymerase I recruitment, and UBF stabilization and that SL1-promoter complex formation is a necessary prerequisite to the assembly of functional and stable PICs that include the UBF activator in mammalian cells.

The long-term outcome of 755 consecutive constrained acetabular components in total hip arthroplasty examining the successes and failures.

Constrained acetabular components can treat or prevent instability after total hip arthroplasty (THA). We examine long-term results of 755 consecutive constrained THA in 720 patients (1986-1993; 62 primary, 59 conversion, 565 revision, 60 reimplantation, and 9 total femur). Eighty-three patients (88 THAs) were lost before 10-year follow-up, leaving 639 patients (667 THAs) available for study. Dislocation occurred in 117 hips (17.5%), in 37 (28.9%) of 128 constrained for recurrent dislocation, and 46 (28.2%) of 163 with dislocation history. Other reoperations were for aseptic loosening (51, 7.6% acetabular; 28, 4.2% stem; 16, 2.4% combined), infection (40, 6.0%), periprosthetic fracture (19, 2.8%), stem breakage (2, 0.3%), cup malposition (1, 0.1%), dissociated insert (1, 0.1%), dissociated femoral head (1, 0.1%), and impingement of 1 broken (0.1%) and 4 (0.6%) dissociated constraining rings. Although constrained acetabular components prevented recurrent dislocation in 71.1%, they should be used cautiously, with a 42.1% long-term failure rate observed in this series. Dislocation was common despite constraint with previous history as a significant risk.