Resolution of sister telomere association is required for progression through mitosis

Cohesins keep sister chromatids associated from the time of their replication in S phase until the onset of anaphase. In vertebrate cells, two distinct pathways dissociate cohesins, one acts on chromosome arms and the other on centromeres. Here, we describe a third pathway that acts on telomeres. Knockdown of tankyrase 1, a telomeric poly(ADP-ribose) polymerase caused mitotic arrest. Chromosomes aligned normally on the metaphase plate but were unable to segregate. Sister chromatids separated at centromeres and arms but remained associated at telomeres, apparently through proteinaceous bridges. Thus, telomeres may require a unique tankyrase 1-dependent mechanism for sister chromatid resolution before anaphase.

Tankyrase 1 and tankyrase 2 are essential but redundant for mouse embryonic development

Tankyrases are proteins with poly(ADP-ribose) polymerase activity. Human tankyrases post-translationally modify multiple proteins involved in processes including maintenance of telomere length, sister telomere association, and trafficking of glut4-containing vesicles. To date, however, little is known about in vivo functions for tankyrases. We recently reported that body size was significantly reduced in mice deficient for tankyrase 2, but that these mice otherwise appeared developmentally normal. In the present study, we report generation of tankyrase 1-deficient and tankyrase 1 and 2 double-deficient mice, and use of these mutant strains to systematically assess candidate functions of tankyrase 1 and tankyrase 2 in vivo. No defects were observed in development, telomere length maintenance, or cell cycle regulation in tankyrase 1 or tankyrase 2 knockout mice. In contrast to viability and normal development of mice singly deficient in either tankyrase, deficiency in both tankyrase 1 and tankyrase 2 results in embryonic lethality by day 10, indicating that there is substantial redundancy between tankyrase 1 and tankyrase 2, but that tankyrase function is essential for embryonic development.

Tankyrase 1 as a target for telomere-directed molecular cancer therapeutics

Telomere elongation by telomerase is repressed in cis by the telomeric protein TRF1. Tankyrase 1 poly(ADP-ribosyl)ates TRF1 and releases it from telomeres, allowing access of telomerase to telomeres. Here we demonstrate that tankyrase 1 inhibition in human cancer cells enhances telomere shortening by a telomerase inhibitor and hastens cell death. Conversely, either tankyrase 1 upregulation or telomere shortening, each of which decreased TRF1 loading on a chromosome end, attenuated the impact of telomerase inhibition. These results are consistent with the idea that telomeres having fewer TRF1s increase the efficiency of their elongation by telomerase. This study implies that both enzyme activity and accessibility to telomeres can be targets for telomerase inhibition.

Insulin-stimulated exocytosis of GLUT4 is enhanced by IRAP and its partner tankyrase

The glucose transporter GLUT4 and the aminopeptidase IRAP (insulin-responsive aminopeptidase) are the major cargo proteins of GSVs (GLUT4 storage vesicles) in adipocytes and myocytes. In the basal state, most GSVs are sequestered in perinuclear and other cytosolic compartments. Following insulin stimulation, GSVs undergo exocytic translocation to insert GLUT4 and IRAP into the plasma membrane. The mechanisms regulating GSV trafficking are not fully defined. In the present study, using 3T3-L1 adipocytes transfected with siRNAs (small interfering RNAs), we show that insulin-stimulated IRAP translocation remained intact despite substantial GLUT4 knockdown. By contrast, insulin-stimulated GLUT4 translocation was impaired upon IRAP knockdown, indicating that IRAP plays a role in GSV trafficking. We also show that knockdown of tankyrase, a Golgi-associated IRAP-binding protein that co-localizes with perinuclear GSVs, attenuated insulin-stimulated GSV translocation and glucose uptake without disrupting insulin-induced phosphorylation cascades. Moreover, iodixanol density gradient analyses revealed that tankyrase knockdown altered the basal-state partitioning of GLUT4 and IRAP within endosomal compartments, apparently by shifting both proteins toward less buoyant compartments. Importantly, the afore-mentioned effects of tankyrase knockdown were reproduced by treating adipocytes with PJ34, a general PARP (poly-ADP-ribose polymerase) inhibitor that abrogated tankyrase-mediated protein modification known as poly-ADP-ribosylation. Collectively, these findings suggest that physiological GSV trafficking depends in part on the presence of IRAP in these vesicles, and that this process is regulated by tankyrase and probably its PARP activity.

Protein requirements for sister telomere association in human cells

Previous studies in human cells indicate that sister telomeres have distinct requirements for their separation at mitosis. In cells depleted for tankyrase 1, a telomeric poly(ADP-ribose) polymerase, sister chromatid arms and centromeres separate normally, but telomeres remain associated and cells arrest in mitosis. Here, we use biochemical and genetic approaches to identify proteins that might mediate the persistent association at sister telomeres. We use immunoprecipitation analysis to show that the telomeric proteins, TRF1 (an acceptor of PARsylation by tankyrase 1) and TIN2 (a TRF1 binding partner) each bind to the SA1 ortholog of the cohesin Scc3 subunit. Sucrose gradient sedimentation shows that TRF1 cosediments with the SA1-cohesin complex. Depletion of the SA1 cohesin subunit or the telomeric proteins (TRF1 and TIN2) restores the normal resolution of sister telomeres in mitosis in tankyrase 1-depleted cells. Moreover, depletion of TRF1 and TIN2 or SA1 abrogates the requirement for tankyrase 1 in mitotic progression. Our studies indicate that sister telomere association in human cells is mediated by a novel association between a cohesin subunit and components of telomeric chromatin.

Tankyrase 1 inhibitior XAV939 increases chemosensitivity in colon cancer cell lines via inhibition of the Wnt signaling pathway

Aberrant Wnt signaling pathway is associated with a wide array of tumor types and plays an important role in the drug resistance of cancer stem cells (CSCs). To explore the effects and mechanism of WNT signaling pathway inhibitor XAV939 on drug resistance in colon cancer cells, the colon cancer cells SW480 and SW620 were treated with 5-fluorouracil (5-FU)/cisplatin (DDP) alone or combined with XAV939. Cell cycle distribution, apoptosis level and the percentage of CD133+ cells were detected by flow cytometry. The protein expression of Axin, β-catenin, EpCAM, TERT and DCAMKL-1 was detected by western blotting. XAV939 upregulated Axin , decreased the total and nuclei of β-catenin in SW480 and SW620 cells. Furthermore, XAV939 significantly downregulated the CSC markers EpCAM, TERT and DCAMKL-1 in SW480 cells, as well as EpCAM in SW620 cells. No significant difference was found in the apoptosis of SW480 and SW620 cells with XAV939 treatment, but XAV939 significantly increased apoptosis induced by 5-FU/DDP in SW480 cells, whereas, the effects were slight in SW620 cells. Collectively, we show for the first time that the WNT signaling pathway inhibitor XAV939 was able to significantly increase the apoptosis induced by 5-FU/DDP, accompanied by the protein expression level alternation of β-catenin, Axin and CSC markers in colon cancer cells. Axin, an important component of Wnt/β-catenin signaling pathway could be a potential molecular target for reversing multidrug resistance in colon cancer.

Tankyrase inhibitors attenuate WNT/β-catenin signaling and inhibit growth of hepatocellular carcinoma cells

Deregulated WNT/β-catenin signaling contributes to the development of a subgroup of hepatocellular carcinoma (HCC), the second leading cause of cancer deaths worldwide. Within this pathway, the tankyrase enzymes (TNKS1 and TNKS2) degrade AXIN and thereby enhance β-catenin activity. We evaluate TNKS enzymes as potential therapeutic targets in HCC, and the anti-tumor efficacy of tankyrase inhibitors (XAV939, and its novel nitro-substituted derivative WXL-8) in HCC cells. Using semi-quantitative RT-PCR, we found significantly elevated levels of TNKS1/2 mRNA in tumor liver tissues compared to adjacent non-tumor livers, at protein levels only TNKS1 is increased. In HepG2, Huh7cells, siRNA-mediated knockdown suppression of endogenous TNKS1 and TNKS2 reduced cell proliferation, together with decreased nuclear β-catenin levels. XAV939 and WXL-8 inhibited cell proliferation and colony formation in HepG2, Huh7, and Hep40 cells (p < 0.05), with stabilization of AXIN1 and AXIN2, and decreased β-catenin protein levels. XAV939 and WXL-8 also attenuated rhWNT3A-induced TOPflash luciferase reporter activity in HCC cells, indicating reduced β-catenin transcriptional activity, consistent with decreased nuclear β-catenin levels. In vivo, intra-tumor injections of XAV939 or WXL-8 significantly inhibited the growth of subcutaneous HepG2 xenografts (P < 0.05). We suggest that tankyrase inhibition is a potential therapeutic approach for treating a subgroup HCC with aberrant WNT/β-catenin signaling pathway.

Functional subdomain in the ankyrin domain of tankyrase 1 required for poly(ADP-ribosyl)ation of TRF1 and telomere elongation

In human cells, telomere elongation by telomerase is repressed in cis by the telomeric protein TRF1. Tankyrase 1 binds TRF1 via its ankyrin domain and poly(ADP-ribosyl)ates it. Overexpression of tankyrase 1 in telomerase-positive cells releases TRF1 from telomeres, resulting in telomere elongation. The tankyrase 1 ankyrin domain is classified into five conserved subdomains, ARCs (ankyrin repeat clusters) I to V. Here, we investigated the biological significance of the ARCs. First, each ARC worked as an independent binding site for TRF1. Second, ARCs II to V recognized the N-terminal acidic domain of TRF1 whereas ARC I bound a discrete site between the homodimerization and the Myb-like domains of TRF1. Inactivation of TRF1 binding in the C-terminal ARC, ARC V, either by deletion or point mutation, significantly reduced the ability of tankyrase 1 to poly(ADP-ribosyl)ate TRF1, release TRF1 from telomeres, and elongate telomeres. In contrast, other ARCs, ARC II and/or IV, inactivated by point mutations still retained the biological function of tankyrase 1. On the other hand, ARC V per se was not sufficient for telomere elongation, suggesting a structural role for multiple ARCs. This work provides evidence that specific ARC-TRF1 interactions play roles in the essential catalytic function of tankyrase 1.

Tankyrase polymerization is controlled by its sterile alpha motif and poly(ADP-ribose) polymerase domains

Tankyrases are novel poly(ADP-ribose) polymerases that have SAM and ankyrin protein-interaction domains. They are found at telomeres, centrosomes, nuclear pores, and Golgi vesicles and have been shown to participate in telomere length regulation. Their other function(s) are unknown, and it has been difficult to envision a common role at such diverse cellular locations. We have shown that tankyrase 1 polymerizes through its sterile alpha motif (SAM) domain to assemble large protein complexes. In vitro polymerization is reversible and still allows interaction with ankyrin-domain binding proteins. Polymerization can also occur in vivo, with SAM-dependent association of overexpressed tankyrase leading to formation of large tankyrase-containing vesicles, disruption of Golgi structure, and inhibition of apical secretion. Finally, tankyrase polymers are dissociated efficiently by poly(ADP-ribosy)lation. This disassembly is prevented by mutation of the PARP domain. Our findings indicate that tankyrase 1 has the unique capacity to promote both assembly and disassembly of large protein complexes. Thus, tankyrases appear to be master scaffolding proteins that regulate the formation of dynamic protein networks at different cellular locations. This implies a common scaffolding function for tankyrases at each location, with specific tankyrase interaction partners conferring location-specific roles to each network, e.g., telomere compaction or regulation of vesicle trafficking.

Functional characterization of the poly(ADP-ribose) polymerase activity of tankyrase 1, a potential regulator of telomere length

Poly(ADP-ribose) polymerases (PARPs) comprise a growing family of enzymes known to be involved in genotoxic signaling and metabolic regulation. One of the latest family members, tankyrase 1, was shown to be involved in maintenance of telomere integrity. Here we expressed full-length tankyrase 1 and a fragment, termed T-PARP, spanning the poly(ADP-ribose) polymerase domain and characterized the enzymatic properties of the two proteins. Both, tankyrase 1 and T-PARP catalyze an auto poly(ADP-ribosyl)ation reaction with comparable catalytic activity. In contrast, (ADP-ribosyl)ation of TRF1, a previously described substrate, is strongly performed only by the full-length enzyme but not by T-PARP. Characterization of the poly(ADP-ribose) products reveals that tankyrase 1 synthesizes polymers with an average chain length of 20 units and no detectable branching of the polymers. Finally, we show that the catalytic efficiency of tankyrase 1, as expressed by the k(cat)/K(m) value, is approximately 150-fold lower compared to the basal activity of the poly(ADP-ribose) polymerase, PARP 1.

Inactivation of tankyrases reduces experimental fibrosis by inhibiting canonical Wnt signalling

OBJECTIVES

Canonical Wnt signalling has recently emerged as a key mediator of fibroblast activation and tissue fibrosis in systemic sclerosis. Here, we investigated tankyrases as novel molecular targets for inhibition of canonical Wnt signalling in fibrotic diseases.

METHODS

The antifibrotic effects of the tankyrase inhibitor XAV-939 or of siRNA-mediated knockdown of tankyrases were evaluated in the mouse models of bleomycin-induced dermal fibrosis and in experimental fibrosis induced by adenoviral overexpression of a constitutively active TGF-β receptor I (Ad-TBRI).

RESULTS

Inactivation of tankyrases prevented the activation of canonical Wnt signalling in experimental fibrosis and reduced the nuclear accumulation of β-catenin and the mRNA levels of the target gene c-myc. Treatment with XAV-939 or siRNA-mediated knockdown of tankyrases in the skin effectively reduced bleomycin-induced dermal thickening, differentiation of resting fibroblasts into myofibroblasts and accumulation of collagen. Potent antifibrotic effects were also observed in Ad-TBRI driven skin fibrosis. Inhibition of tankyrases was not limited by local or systemic toxicity.

CONCLUSIONS

Inactivation of tankyrases effectively abrogated the activation of canonical Wnt signalling and demonstrated potent antifibrotic effects in well-tolerated doses. Thus, tankyrases might be candidates for targeted therapies in fibrotic diseases.

Factor-inhibiting hypoxia-inducible factor (FIH) catalyses the post-translational hydroxylation of histidinyl residues within ankyrin repeat domains

Factor-inhibiting hypoxia-inducible factor (FIH) is an Fe(II)/2-oxoglutarate-dependent dioxygenase that acts as a negative regulator of the hypoxia-inducible factor (HIF) by catalysing β-hydroxylation of an asparaginyl residue in its C-terminal transcriptional activation domain (CAD). In addition to the hypoxia-inducible factor C-terminal transcriptional activation domain (HIF-CAD), FIH also catalyses asparaginyl hydroxylation of many ankyrin repeat domain-containing proteins, revealing a broad sequence selectivity. However, there are few reports on the selectivity of FIH for the hydroxylation of specific residues. Here, we report that histidinyl residues within the ankyrin repeat domain of tankyrase-2 can be hydroxylated by FIH. NMR and crystallographic analyses show that the histidinyl hydroxylation occurs at the β-position. The results further expand the scope of FIH-catalysed hydroxylations.

Hypermetabolism, hyperphagia, and reduced adiposity in tankyrase-deficient mice

OBJECTIVE

Tankyrase (TNKS) is a Golgi-associated poly-ADP-ribose polymerase that is implicated in the regulation of GLUT4 trafficking in 3T3-L1 adipocytes. Its chromosomal locus 8p23.1 is linked to monogenic forms of diabetes in certain kindred. We hypothesize that TNKS is involved in energy homeostasis in mammals.

RESEARCH DESIGN AND METHODS

Gene-trap techniques were used to ablate TNKS expression in mice. Homozygous and wild-type littermates maintained on standard chow were compared.

RESULTS

Wild-type mice express the TNKS protein abundantly in adipose tissue, the brain, and the endocrine pancreas but scarcely in the exocrine pancreas and skeletal muscle. TNKS-deficient mice consume increased amounts of food (by 34%) but have decreased plasma leptin levels and a >50% reduction in epididymal and perirenal fat pad size. Their energy expenditure is increased as assessed by metabolic cage studies and core body temperatures. These changes are not attributable to an increase in physical activity or uncoupled respiration (based on oxygraph analyses of mitochondria isolated from brown fat and skeletal muscle). The heightened thermogenesis of TNKS-deficient mice is apparently fueled by increases in both fatty acid oxidation (based on muscle and liver gene expression analyses and plasma ketone levels) and insulin-stimulated glucose utilization (determined by hyperinsulinemic-euglycemic clamps). Although TNKS deficiency does not compromise insulin-stimulated GLUT4 translocation in primary adipocytes, it leads to the post-transcriptional upregulation of GLUT4 and adiponectin in adipocytes and increases plasma adiponectin levels.

CONCLUSIONS

TNKS-deficient mice exhibit increases in energy expenditure, fatty acid oxidation, and insulin-stimulated glucose utilization. Despite excessive food intake, their adiposity is substantially decreased.

Vertebrate tankyrase domain structure and sterile alpha motif (SAM)-mediated multimerization

Tankyrases 1 and 2 are two highly related poly(ADP-ribose) polymerases that interact with a variety of cytoplasmic and nuclear proteins. Both proteins have been implicated in telomere length regulation, insulin signalling and centrosome function. To learn more about their mode of action, we have isolated the chicken tankyrase homologues and examined their interaction partners and subcellular location. Cross-species sequence comparison indicated that tankyrase domain structure is highly conserved and supports division of the ankyrin domain into five subdomains, which are each separated by a highly conserved LLEAAR/K motif. Glutathione S-transferase pull-down experiments demonstrated that the ankyrin domains of both proteins interact with chicken telomere repeat factor 1 (TRF1). Analysis of total cellular and nuclear proteins revealed that cells contain approximately twice as much tankyrase 1 as tankyrase 2. Although > or = 90% of each protein is present in the cytoplasm, both tankyrase 1 and 2 were detected in the nucleus. The nuclear location together with its ability to interact with TRF1, point to tankyrase 2 having a telomeric function. Yeast two-hybrid and cross-linking experiments show that both tankyrases can multimerize through their sterile-alpha motif domains. These results indicate that tankyrases may be master scaffolding proteins, capable of regulating assembly of large protein complexes.

Tankyrase Requires SAM Domain-Dependent Polymerization to Support Wnt-β-Catenin Signaling

The poly(ADP-ribose) polymerase (PARP) Tankyrase (TNKS and TNKS2) is paramount to Wnt-β-catenin signaling and a promising therapeutic target in Wnt-dependent cancers. The pool of active β-catenin is normally limited by destruction complexes, whose assembly depends on the polymeric master scaffolding protein AXIN. Tankyrase, which poly(ADP-ribosyl)ates and thereby destabilizes AXIN, also can polymerize, but the relevance of these polymers has remained unclear. We report crystal structures of the polymerizing TNKS and TNKS2 sterile alpha motif (SAM) domains, revealing versatile head-to-tail interactions. Biochemical studies informed by these structures demonstrate that polymerization is required for Tankyrase to drive β-catenin-dependent transcription. We show that the polymeric state supports PARP activity and allows Tankyrase to effectively access destruction complexes through enabling avidity-dependent AXIN binding. This study provides an example for regulated signal transduction in non-membrane-enclosed compartments (signalosomes), and it points to novel potential strategies to inhibit Tankyrase function in oncogenic Wnt signaling.

Tankyrase 2 poly(ADP-ribose) polymerase domain-deleted mice exhibit growth defects but have normal telomere length and capping

Regulation of telomere length maintenance and capping are a critical cell functions in both normal and tumor cells. Tankyrase 2 (Tnks2) is a poly(ADP-ribose) polymerase (PARP) that has been shown to modify itself and TRF1, a telomere-binding protein. We show here by overexpression studies that tankyrase 2, like its closely related homolog tankyrase 1, can function as a positive regulator of telomere length in human cells, dependent on its catalytic PARP activity. To study the role of Tnks2 in vivo, we generated mice with the Tnks2 PARP domain deleted. These mice are viable and fertile but display a growth retardation phenotype. Telomere analysis by quantitative fluorescence in situ hybridization (FISH), flow-FISH, and restriction fragment analysis showed no change in telomere length or telomere capping in these mice. To determine the requirement for Tnks2 in long-term maintenance of telomeres, we generated embryonic stem cells with the Tnks2 PARP domain deleted and observed no change, even upon prolonged growth, in telomere length or telomere capping. Together, these results suggest that Tnks2 has a role in normal growth and development but is not essential for telomere length maintenance or telomere capping in mice.

Tankyrase 1 interacts with Mcl-1 proteins and inhibits their regulation of apoptosis

Mcl-1L (myeloid cell leukemia-1 long) is an antiapoptotic Bcl-2 family protein discovered as an early induction gene during leukemia cell differentiation. Previously, we identified Mcl-1S (short) as a short splicing variant of the Mcl-1 gene with proapoptotic activity. To identify Mcl-1-interacting proteins, we performed yeast two-hybrid screening and found cDNAs encoding tankyrase 1. This protein possesses poly(ADP-ribose) polymerase activity and presumably facilitates the turnover of substrates following ADP-ribosylation. In yeast and mammalian cells, tankyrase 1 interacts with both Mcl-1L and Mcl-1S, but does not bind to other Bcl-2 family proteins tested. Analysis of truncated tankyrase 1 mutants indicated that the first 10 ankyrin repeats are involved in interaction with Mcl-1. In the N terminus of Mcl-1, a stretch of 25 amino acids is sufficient for binding to tankyrase 1. Overexpression of tankyrase 1 antagonizes both Mcl-1L-mediated cell survival and Mcl-1S-induced cell death. Furthermore, coexpression of tankyrase 1 with Mcl-1L or Mcl-1S decreased the levels of Mcl-1 proteins. Although tankyrase 1 down-regulates Mcl-1 protein expression, no ADP-ribosylation of Mcl-1 was detected. In contrast, overexpression of Mcl-1 proteins suppressed the ADP-ribosylation of the telomeric repeat binding factor 1, another tankyrase 1-interacting protein. Thus, interaction of Mcl-1L and Mcl-1S with tankyrase 1 could serve as a unique mechanism to decrease the expression of these Bcl-2 family proteins, thereby leading to the modulation of the apoptosis pathway.

Novel binding mode of a potent and selective tankyrase inhibitor

Tankyrases (TNKS1 and TNKS2) are key regulators of cellular processes such as telomere pathway and Wnt signaling. IWRs (inhibitors of Wnt response) have recently been identified as potent and selective inhibitors of tankyrases. However, it is not clear how these IWRs interact with tankyrases. Here we report the crystal structure of the catalytic domain of human TNKS1 in complex with IWR2, which reveals a novel binding site for tankyrase inhibitors. The TNKS1/IWR2 complex provides a molecular basis for their strong and specific interactions and suggests clues for further development of tankyrase inhibitors.

circ5615 functions as a ceRNA to promote colorectal cancer progression by upregulating TNKS

Circular RNAs (circRNAs), non-coding RNAs generated by precursor mRNA back-splicing of exons, have been reported to fulfill multiple roles in cancer. However, the role of quite a lot circRNAs in colorectal cancer (CRC) remains mostly unknown. Herein, we explored the expression profiles of circRNAs in 5 paired samples of CRC patients by microarray and noted a circRNA, hsa_circ_0005615 (circ5615), was significantly upregulated in CRC tissues. Circ5615 was derived from exon 2 of NFATC3 and its upregulation was tightly correlated with higher T stage and poor prognosis in CRC patients. Studies in vitro and in vivo demonstrated that knockdown of circ5615 in cancer cells inhibited proliferation and cell cycle acceleration, while overexpression promoted malignant phenotypes. Mechanistically, RNA immunoprecipitation, biotin-coupled probe pull-down and luciferase reporter assays revealed circ5615 effectively bound to miR-149-5p and might play a role like miR-149-5p sponge. Additionally, tankyrase (TNKS), regulator of β-catenin stabilization, was identified as circ5615 downstream and the potential miR-149-5p targets by RNA-seq and bioinformatics analysis. We further verified the upregulation of β-catenin and cyclin D1 induced by circ5615. Our results indicated that circ5615 exerted oncogenic function as competing endogenous RNA (ceRNA) of miR-149-5p to release TNKS and activated Wnt/β-catenin pathway.

Dissecting the molecular determinants of clinical PARP1 inhibitor selectivity for tankyrase1

Poly-ADP-ribosyltransferases play a critical role in DNA repair and cell death, and poly(ADP-ribosyl) polymerase 1 (PARP1) is a particularly important therapeutic target for the treatment of breast cancer because of its synthetic lethal relationship with breast cancer susceptibility proteins 1 and 2. Numerous PARP1 inhibitors have been developed, and their efficacy in cancer treatment is attributed to both the inhibition of enzymatic activity and their ability to trap PARP1 on to the damaged DNA, which is cytotoxic. Of the clinical PARP inhibitors, talazoparib is the most effective at trapping PARP1 on damaged DNA. Biochemically, talazoparib is also suspected to be a potent inhibitor of PARP5a/b (tankyrase1/2 [TNKS1/2]), which is an important regulator of Wnt/β-catenin pathway. Here we show using competition experiments in cell lysate that, at a clinically relevant concentration, talazoparib can potentially bind and engage TNKS1. Using surface plasmon resonance, we measured the dissociation constants of talazoparib, olaparib, niraparib, and veliparib for their interaction with PARP1 and TNKS1. The results show that talazoparib has strong affinity for PARP1 as well as uniquely strong affinity for TNKS1. Finally, we used crystallography and hydrogen deuterium exchange mass spectroscopy to dissect the molecular mechanism of differential selectivity of these PARP1 inhibitors. From these data, we conclude that subtle differences between the ligand-binding sites of PARP1 and TNKS1, differences in the electrostatic nature of the ligands, protein dynamics, and ligand conformational energetics contribute to the different pharmacology of these PARP1 inhibitors. These results will help in the design of drugs to treat Wnt/β-catenin pathway-related cancers, such as colorectal cancers.

Generation and characterization of telomere length maintenance in tankyrase 2-deficient mice

Telomere length and function are crucial factors that determine the capacity for cell proliferation and survival, mediate cellular senescence, and play a role in malignant transformation in eukaryotic systems. The telomere length of a specific mammalian species is maintained within a given range by the action of telomerase and telomere-associated proteins. TRF1 is a telomere-associated protein that inhibits telomere elongation by its binding to telomere repeats, preventing access to telomerase. Human TRF1 interacts with tankyrase 1 and tankyrase 2 proteins, two related members of the tankyrase family shown to have poly(ADP-ribose) polymerase activity. Human tankyrase 1 is reported to ADP-ribosylate TRF1 and to down-regulate the telomeric repeat binding activity of TRF1, resulting in telomerase-dependent telomere elongation. Human tankyrase 2 is proposed to have activity similar to that of tankyrase 1, although tankyrase 2 function has been less extensively characterized. In the present study, we have assessed the in vivo function of mouse tankyrase 2 by germ line gene inactivation and show that inactivation of tankyrase 2 does not result in detectable alteration in telomere length when monitored through multiple generations of breeding. This finding suggests that either mouse tankyrases 1 and 2 have redundant functions in telomere length maintenance or that mouse tankyrase 2 differs from human tankyrase 2 in its role in telomere length maintenance. Tankyrase 2 deficiency did result in a significant decrease in body weight sustained through at least the first year of life, most marked in male mice, suggesting that tankyrase 2 functions in potentially telomerase-independent pathways to affect overall development and/or metabolism.

Tankyrase-1 Ankyrin Repeats Form an Adaptable Binding Platform for Targets of ADP-Ribose Modification

The poly(ADP-ribose) polymerase enzyme Tankyrase-1 (TNKS) regulates multiple cellular processes and interacts with diverse proteins using five ankyrin repeat clusters (ARCs). There are limited structural insights into functional roles of the multiple ARCs of TNKS. Here we present the ARC1-3 crystal structure and employ small-angle X-ray scattering (SAXS) to investigate solution conformations of the complete ankyrin repeat domain. Mutagenesis and binding studies using the bivalent TNKS binding domain of Axin1 demonstrate that only certain ARC combinations function together. The physical basis for these restrictions is explained by both rigid and flexible ankyrin repeat elements determined in our structural analysis. SAXS analysis is consistent with a dynamic ensemble of TNKS ankyrin repeat conformations modulated by Axin1 interaction. TNKS ankyrin repeat domain is thus an adaptable binding platform with structural features that can explain selectivity toward diverse proteins, and has implications for TNKS positioning of bound targets for poly(ADP-ribose) modification.

A Longitudinal Imaging Genetics Study of Neuroanatomical Asymmetry in Alzheimer's Disease

BACKGROUND

Contralateral brain structures represent a unique, within-patient reference element for disease, and asymmetries can provide a personalized measure of the accumulation of past disease processes. Neuroanatomical shape asymmetries have recently been associated with the progression of Alzheimer's disease (AD), but the biological basis of asymmetric brain changes in AD remains unknown.

METHODS

We investigated genetic influences on brain asymmetry by identifying associations between magnetic resonance imaging-derived measures of asymmetry and candidate single nucleotide polymorphisms (SNPs) that have previously been identified in genome-wide association studies for AD diagnosis and for brain subcortical volumes. For analyzing longitudinal neuroimaging data (1241 individuals, 6395 scans), we used a mixed effects model with interaction between genotype and diagnosis.

RESULTS

Significant associations between asymmetry of the amygdala, hippocampus, and putamen and SNPs in the genes BIN1, CD2AP, ZCWPW1, ABCA7, TNKS, and DLG2 were found.

CONCLUSIONS

The associations between SNPs in the genes TNKS and DLG2 and AD-related increases in shape asymmetry are of particular interest; these SNPs have previously been associated with subcortical volumes of amygdala and putamen but have not yet been associated with AD pathology. For AD candidate SNPs, we extend previous work to show that their effects on subcortical brain structures are asymmetric. This provides novel evidence about the biological underpinnings of brain asymmetry as a disease marker.

Genetic modifiers of the severity of sickle cell anemia identified through a genome-wide association study

We conducted a genome-wide association study (GWAS) to discover single nucleotide polymorphisms (SNPs) associated with the severity of sickle cell anemia in 1,265 patients with either "severe" or "mild" disease based on a network model of disease severity. We analyzed data using single SNP analysis and a novel SNP set enrichment analysis (SSEA) developed to discover clusters of associated SNPs. Single SNP analysis discovered 40 SNPs that were strongly associated with sickle cell severity (odds for association >1,000); of the 32 that we could analyze in an independent set of 163 patients, five replicated, eight showed consistent effects although failed to reach statistical significance, whereas 19 did not show any convincing association. Among the replicated associations are SNPs in KCNK6 a K(+) channel gene. SSEA identified 27 genes with a strong enrichment of significant SNPs (P < 10(-6)); 20 were replicated with varying degrees of confidence. Among the novel findings identified by SSEA is the telomere length regulator gene TNKS. These studies are the first to use GWAS to understand the genetic diversity that accounts the phenotypic heterogeneity sickle cell anemia as estimated by an integrated model of severity. Additional validation, resequencing, and functional studies to understand the biology and reveal mechanisms by which candidate genes might have their effects are the future goals of this work.

Telomerase activity in B-cell non-Hodgkin lymphomas is regulated by hTERT transcription and correlated with telomere-binding protein expression but uncoupled from proliferation

Telomere maintenance is a prerequisite for immortalisation, and in most malignant cells is carried out by telomerase, an enzyme that synthesis new telomeric repeats on the chromosome ends. In normal or reactive tissues with a high regenerative capacity, telomerase is regulated according to the telomere loss that occurs during proliferation. To evaluate the interaction of proliferation and telomerase activity in malignant lymphomas, we quantified telomerase expression in different non-Hodgkin lymphomas in comparison to normal or reactive lymph nodes. Surprisingly, the activity levels were the same in most of the lymphomas analysed as compared to reactive lymph nodes. Significantly higher activity was detected only in Burkitt's lymphoma. Telomerase activity correlated well with hTERT and c-myc expression, but was independent of proliferation. To evaluate interactions of telomere-binding protein expression on telomerase expression in non-Hodgkin lymphoma, the mRNA levels of TRF1, TRF2, tankyrase and hPif1 were assessed by real-time RT-PCR. We demonstrate here that the magnitude of telomerase upregulation does not necessarily reflect the requirement of telomere compensation caused by proliferation. Telomerase regulation in non-Hodgkin lymphomas is therefore uncoupled from proliferative stimuli found in reactive lymphoid tissue. We suggest that the upregulation of specific telomere-binding proteins like TRF2 may contribute to telomere maintenance in malignant lymphoma.