A single amino acid residue constitutes the third dimerization domain essential for the assembly and function of the tetrameric polycystin-2 (TRPP2) channel

Autosomal dominant polycystic kidney disease (ADPKD), the most common inherited cause of kidney failure, is caused by mutations in either PKD1 (85%) or PKD2 (15%). The PKD2 protein, polycystin-2 (PC2 or TRPP2), is a member of the transient receptor potential (TRP) superfamily and functions as a nonselective calcium channel. PC2 has been found to form oligomers in native tissues, suggesting that similar to other TRP channels, it may form functional homo- or heterotetramers with other TRP subunits. We have recently demonstrated that the homodimerization of PC2 is mediated by both N-terminal and C-terminal domains, and it is known that PC2 can heterodimerize with PC1, TRPC1, and TRPV4. In this paper, we report that a single cysteine residue, Cys(632), mutated in a known PKD2 pedigree, constitutes the third dimerization domain for PC2. PC2 truncation mutants lacking both N and C termini could still dimerize under nonreducing conditions. Mutation of Cys(632) alone abolished dimerization in these mutants, indicating that it was the critical residue mediating disulfide bond formation between PC2 monomers. Co-expression of C632A PC2 mutants with wild-type PC2 channels reduced ATP-sensitive endoplasmic reticulum Ca(2+) release in HEK293 cells. The combination of C632A and mutations disrupting the C-terminal coiled-coil domain (Val(846), Ile(853), Ile(860), Leu(867) or 4M) nearly abolished dimer formation and ATP-dependent Ca(2+) release. However, unlike the 4M PC2 mutant, a C632A mutant could still heterodimerize with polycystin-1 (PC1). Our results indicate that PC2 homodimerization is regulated by three distinct domains and that these events regulate formation of the tetrameric PC2 channel.     

Divergent function of polycystin 1 and polycystin 2 in cell size regulation

Autosomal dominant polycystic kidney disease (ADPKD) is caused by mutations in PKD1 or PKD2, the genes encoding polycystin 1 (PC1) and polycystin 2 (PC2), respectively. PC1 and PC2 localize to the primary cilium and form a protein complex, which is thought to regulate signaling events. PKD1 mutations are associated with a stronger phenotype than PKD2, suggesting the existence of PC1 specific functions in renal tubular cells. However, the evidence for diverging molecular functions is scant. The bending of cilia by fluid flow induces a reduction in cell size through a mechanism that involves the kinase LKB1 but not PC2. Here, using different in vitro approaches, we show that contrary to PC2, PC1 regulates cell size under flow and thus phenocopies the loss of cilia. PC1 is required to couple mechanical deflection of cilia to mTOR in tubular cells. This study pinpoints divergent functions of the polycystins in renal tubular cells that may be relevant to disease severity in ADPKD.     

Polycystic kidney disease: novel insights into polycystin function

Autosomal dominant polycystic kidney disease (ADPKD) is a life-threatening monogenic disease caused by mutations in PKD1 and PKD2 that encode polycystin 1 (PC1) and polycystin 2 (PC2). PC1/2 localize to cilia of renal epithelial cells, and their function is believed to embody an inhibitory activity that suppresses the cilia-dependent cyst activation (CDCA) signal. Consequently, PC deficiency results in activation of CDCA and stimulates cyst growth. Recently, re-expression of PCs in established cysts has been shown to reverse PKD. Thus, the mode of action of PCs resembles a ‘counterbalance in cruise control’ to maintain lumen diameter within a designated range. Herein we review recent studies that point to novel arenas for future PC research with therapeutic potential for ADPKD.     

Identification and functional characterization of an N-terminal oligomerization domain for polycystin-2

Autosomal dominant polycystic kidney disease (ADPKD), the most common inherited cause of kidney failure, is caused by mutations in either PKD1 (85%) or PKD2 (15%). The PKD2 protein, polycystin-2 (PC2 or TRPP2), is a member of the transient receptor potential (TRP) superfamily and functions as a non-selective calcium channel. PC2 has been found to form oligomers in native tissues suggesting that it may form functional homo- or heterotetramers with other subunits, similar to other TRP channels. Our experiments unexpectedly revealed that PC2 mutant proteins lacking the known C-terminal dimerization domain were still able to form oligomers and co-immunoprecipitate full-length PC2, implying the possible existence of a proximal dimerization domain. Using yeast two-hybrid and biochemical assays, we have mapped an alternative dimerization domain to the N terminus of PC2 (NT2-1-223, L224X). Functional characterization of this domain demonstrated that it was sufficient to induce cyst formation in zebrafish embryos and inhibit PC2 surface currents in mIMCD3 cells probably by a dominant-negative mechanism. In summary, we propose a model for PC2 assembly as a functional tetramer which depends on both C- and N-terminal dimerization domains. These results have significant implications for our understanding of PC2 function and disease pathogenesis in ADPKD and provide a new strategy for studying PC2 function.     

Autosomal dominant polycystic kidney disease: Disrupted pathways and potential therapeutic interventions

Autosomal dominant polycystic kidney disease (ADPKD) is a monogenic inherited renal cystic disease that occurs in different races worldwide. It is characterized by the development of a multitude of renal cysts, which leads to massive enlargement of the kidney and often to renal failure in adulthood. ADPKD is caused by a mutation in PKD1 or PKD2 genes encoding the proteins polycystin-1 and polycystin-2, respectively. Recent studies showed that cyst formation and growth result from deregulation of multiple cellular pathways like proliferation, apoptosis, metabolic processes, cell polarity, and immune defense. In ADPKD, intracellular cyclic adenosine monophosphate (cAMP) promotes cyst enlargement by stimulating cell proliferation and transepithelial fluid secretion. Several interventions affecting many of these defective signaling pathways have been effective in animal models and some are currently being tested in clinical trials. Moreover, the stem cell therapy can improve nephropathies and according to studies were done in this field, can be considered as a hopeful therapeutic approach in future for PKD. This study provides an in-depth review of the relevant molecular pathways associated with the pathogenesis of ADPKD and their implications in development of potential therapeutic strategies.     

Regulation of polycystin expression,maturation and trafficking

The major autosomal dominant polycystic kidney disease (ADPKD) genes, PKD1 and PKD2, are wildly expressed at the organ and tissue level. PKD1 encodes polycystin 1 (PC1), a large membrane associated receptor-like protein that can complex with the PKD2 product, PC2. Various cellular locations have been described for both PC1, including the plasma membrane and extracellular vesicles, and PC2, especially the endoplasmic reticulum (ER), but compelling evidence indicates that the primary cilium, a sensory organelle, is the key site for the polycystin complex to prevent PKD. As with other membrane proteins, the ER biogenesis pathway is key to appropriately folding, performing quality control, and exporting fully folded PC1 to the Golgi apparatus. There is a requirement for binding with PC2 and cleavage of PC1 at the GPS for this folding and export to occur. Six different monogenic defects in this pathway lead to cystic disease development, with PC1 apparently particularly sensitive to defects in this general protein processing pathway. Trafficking of membrane proteins, and the polycystins in particular, through the Golgi to the primary cilium have been analyzed in detail, but at this time, there is no clear consensus on a ciliary targeting sequence required to export proteins to the cilium. After transitioning though the trans-Golgi network, polycystin-bearing vesicles are likely sorted to early or recycling endosomes and then transported to the ciliary base, possibly via docking to transition fibers (TF). The membrane-bound polycystin complex then undergoes facilitated trafficking through the transition zone, the diffusion barrier at the base of the cilium, before entering the cilium. Intraflagellar transport (IFT) may be involved in moving the polycystins along the cilia, but data also indicates other mechanisms. The ciliary polycystin complex can be ubiquitinated and removed from cilia by internalization at the ciliary base and may be sent back to the plasma membrane for recycling or to lysosomes for degradation. Monogenic defects in processes regulating the protein composition of cilia are associated with syndromic disorders involving many organ systems, reflecting the pleotropic role of cilia during development and for tissue maintenance. Many of these ciliopathies have renal involvement, likely because of faulty polycystin signaling from cilia. Understanding the expression, maturation and trafficking of the polycystins helps understand PKD pathogenesis and suggests opportunities for therapeutic intervention.     

Polycystin 2: A calcium channel,channel partner,and regulator of calcium homeostasis in ADPKD

Polycystin 2 (PC2) is one of two main protein types responsible for the underlying etiology of autosomal dominant polycystic kidney disease (ADPKD), the most prevalent monogenic renal disease in the world. This debilitating and currently incurable condition is caused by loss-of-function mutations in PKD2 and PKD1, the genes encoding for PC2 and Polycystin 1 (PC1), respectively. Two-hit mutation events in these genes lead to renal cyst formation and eventual kidney failure, the main hallmarks of ADPKD. Though much is known concerning the physiological consequences and dysfunctional signaling mechanisms resulting from ADPKD development, to best understand the requirement of PC2 in maintaining organ homeostasis, it is important to recognize how PC2 acts under normal conditions. As such, an array of work has been performed characterizing the endogenous function of PC2, revealing it to be a member of the transient receptor potential (TRP) channel family of proteins. As a TRP protein, PC2 is a nonselective, cation-permeant, calcium-sensitive channel expressed in all tissue types, where it localizes primarily on the endoplasmic reticulum (ER), primary cilia, and plasma membrane. In addition to its channel function, PC2 interacts with and acts as a regulator of a number of other channels, ultimately further affecting intracellular signaling and leading to dysfunction in its absence. In this review, we describe the biophysical and physiological properties of PC2 as a cation channel and modulator of intracellular calcium channels, along with how these properties are altered in ADPKD.     

Post-translational modifications of the polycystin proteins

Autosomal dominant polycystic kidney disease (ADPKD) is the most common inherited cause of kidney failure and affects up to 12 million people worldwide. Germline mutations in two genes, PKD1 or PKD2, account for almost all patients with ADPKD. The ADPKD proteins, polycystin-1 (PC1) and polycystin-2 (PC2), are regulated by post-translational modifications (PTM), with phosphorylation, glycosylation and proteolytic cleavage being the best described changes. A few PTMs have been shown to regulate polycystin trafficking, signalling, localisation or stability and thus their physiological function. A key challenge for the future will be to elucidate the functional significance of all the individual PTMs reported to date. Finally, it is possible that site-specific mutations that disrupt PTM could contribute to cystogenesis although in the majority of cases, confirmatory evidence is awaited.     

Characterization of the murine polycystic kidney disease (Pkd2) gene

Autosomal dominant polycystic kidney disease (ADPKD) is one of the most frequent genetically transmitted disorders among Europeans with an attributed frequency of 0.1%. The two most common genetic determinants for ADPKD are the PKD1 and PKD2 genes. In this study we report the genomic structure and pattern of expression of the Pkd2 gene, the murine homolog of the human PKD2 gene. Pkd2 is localized on mouse Chromosome (Chr) 5 proximal to anchor marker D5Mit175, spans at least 35 kb of the mouse genome, and consists of 15 exons. Its translation product consists of 966 amino acids, and the peptide shows a 95% homology to human polycystin2. Functional domains are particularly well conserved in the mouse homolog. The expression of mouse polycystin2 in the developing embryo at day 12.5 post conception is localized in mesenchymally derived structures. In the adult mouse, the protein is mostly expressed in kidney, which suggests its functional relevance for this organ. Received: 13 March 1998 / Accepted: 11 May 1998     

ATP-2 interacts with the PLAT domain of LOV-1 and is involved in Caenorhabditis elegans polycystin signaling

Caenorhabditis elegans is a powerful model to study the molecular basis of autosomal dominant polycystic kidney disease (ADPKD). ADPKD is caused by mutations in the polycystic kidney disease (PKD)1 or PKD2 gene, encoding polycystin (PC)-1 or PC-2, respectively. The C. elegans polycystins LOV-1 and PKD-2 are required for male mating behaviors and are localized to sensory cilia. The function of the evolutionarily conserved polycystin/lipoxygenase/alpha-toxin (PLAT) domain found in all PC-1 family members remains an enigma. Here, we report that ATP-2, the beta subunit of the ATP synthase, physically associates with the LOV-1 PLAT domain and that this interaction is evolutionarily conserved. In addition to the expected mitochondria localization, ATP-2 and other ATP synthase components colocalize with LOV-1 and PKD-2 in cilia. Disrupting the function of the ATP synthase or overexpression of atp-2 results in a male mating behavior defect. We further show that atp-2, lov-1, and pkd-2 act in the same molecular pathway. We propose that the ciliary localized ATP synthase may play a previously unsuspected role in polycystin signaling.     

TRPP2 channels regulate apoptosis through the Ca2+ concentration in the endoplasmic reticulum

Ca²⁺ is an important signalling molecule that regulates multiple cellular processes, including apoptosis. Although Ca²⁺ influx through transient receptor potential (TRP) channels in the plasma membrane is known to trigger cell death, the function of intracellular TRP proteins in the regulation of Ca²⁺‐dependent signalling pathways and apoptosis has remained elusive. Here, we show that TRPP2, the ion channel mutated in autosomal dominant polycystic kidney disease (ADPKD), protects cells from apoptosis by lowering the Ca²⁺ concentration in the endoplasmic reticulum (ER). ER‐resident TRPP2 counteracts the activity of the sarcoendoplasmic Ca²⁺ ATPase by increasing the ER Ca²⁺ permeability. This results in diminished cytosolic and mitochondrial Ca²⁺ signals upon stimulation of inositol 1,4,5‐trisphosphate receptors and reduces Ca²⁺ release from the ER in response to apoptotic stimuli. Conversely, knockdown of TRPP2 in renal epithelial cells increases ER Ca²⁺ release and augments sensitivity to apoptosis. Our findings indicate an important function of ER‐resident TRPP2 in the modulation of intracellular Ca²⁺ signalling, and provide a molecular mechanism for the increased apoptosis rates in ADPKD upon loss of TRPP2 channel function.     

A Pathogenic C Terminus-truncated Polycystin-2 Mutant Enhances Receptor-activated Ca2+ Entry via Association with TRPC3 and TRPC7

Mutations in PKD2 gene result in autosomal dominant polycystic kidney disease (ADPKD). PKD2 encodes polycystin-2 (TRPP2), which is a homologue of transient receptor potential (TRP) cation channel proteins. Here we identify a novel PKD2 mutation that generates a C-terminal tail-truncated TRPP2 mutant 697fsX with a frameshift resulting in an aberrant 17-amino acid addition after glutamic acid residue 697 from a family showing mild ADPKD symptoms. When recombinantly expressed in HEK293 cells, wild-type (WT) TRPP2 localized at the endoplasmic reticulum (ER) membrane significantly enhanced Ca²⁺ release from the ER upon muscarinic acetylcholine receptor (mAChR) stimulation. In contrast, 697fsX, which showed a predominant plasma membrane localization characteristic of TRPP2 mutants with C terminus deletion, prominently increased mAChR-activated Ca²⁺ influx in cells expressing TRPC3 or TRPC7. Coimmunoprecipitation, pulldown assay, and cross-linking experiments revealed a physical association between 697fsX and TRPC3 or TRPC7. 697fsX but not WT TRPP2 elicited a depolarizing shift of reversal potentials and an enhancement of single-channel conductance indicative of altered ion-permeating pore properties of mAChR-activated currents. Importantly, in kidney epithelial LLC-PK1 cells the recombinant 679fsX construct was codistributed with native TRPC3 proteins at the apical membrane area, but the WT construct was distributed in the basolateral membrane and adjacent intracellular areas. Our results suggest that heteromeric cation channels comprised of the TRPP2 mutant and the TRPC3 or TRPC7 protein induce enhanced receptor-activated Ca²⁺ influx that may lead to dysregulated cell growth in ADPKD.     

Polycystin-2 is a novel cation channel implicated in defective intracellular Ca(2+) homeostasis in polycystic kidney disease

Mutations in polycystins-1 and -2 (PC1 and PC2) cause autosomal dominant polycystic kidney disease (ADPKD), which is characterized by progressive development of epithelial renal cysts, ultimately leading to renal failure. The functions of these polycystins remain elusive. Here we show that PC2 is a Ca(2+)-permeable cation channel with properties distinct from any known intracellular channels. Its kinetic behavior is characterized by frequent transitions between closed and open states over a wide voltage range. The activity of the PC2 channel is transiently increased by elevating cytosolic Ca(2+). Given the predominant endoplasmic reticulum (ER) location of PC2 and its unresponsiveness to the known modulators of mediating Ca(2+) release from the ER, inositol-trisphosphate (IP(3)) and ryanodine, these results suggest that PC2 represents a novel type of channel with properties distinct from those of the other Ca(2+)-release channels. Our data also show that the PC2 channel can be translocated to the plasma membranes by defined chemical chaperones and proteasome modulators, suggesting that in vivo, it may also function in the plasma membrane under specific conditions. The sensitivity of the PC2 channel to changes of intracellular Ca(2+) concentration is deficient in a mutant found in ADPKD patients. The dysfunction of such mutants may result in defective coupling of PC2 to intracellular Ca(2+) homeostasis associated with the pathogenesis of ADPKD.     

Transport function of the naturally occurring pathogenic polycystin-2 mutant, R742X

Most patients with autosomal dominant polycystic kidney disease (ADPKD) harbor mutations truncating polycystin-1 (PC1) or polycystin-2 (PC2), products of the PKD1 and PKD2 genes, respectively. A third member of the polycystin family, polycystin-L (PCL), was recently shown to function as a Ca(2+)-modulated nonselective cation channel. More recently, PC2 was also shown to be a nonselective cation channel with comparable properties to PCL, though the membrane targeting of PC2 likely varies with cell types. Here we show that PC2 expressed heterologously in Xenopus oocytes is targeted to intracellular compartments. By contrast, a truncated form of mouse PC2 corresponding to a naturally occurring human mutation R742X is targeted predominantly to the plasma membrane where it mediates K(+), Na(+), and Ca(2+) currents. Unlike PCL, the truncated form does not display Ca(2+)-activated transport activities, possibly due to loss of an EF-hand at the C-terminus. We propose that PC2 forms ion channels utilizing structural components which are preserved in the R742X form of the protein. Implications for epithelial cell signaling are discussed.     

Identification of a Polycystin-1 Cleavage Product,P100, That Regulates Store Operated Ca2+ Entry through Interactions with STIM1

Autosomal Dominant Polycystic Kidney Disease (ADPKD) is a genetic disorder resulting in large kidney cysts and eventual kidney failure. Mutations in either the PKD1 or PKD2/TRPP2 genes and their respective protein products, polycystin-1 (PC1) and polycystin-2 (PC2) result in ADPKD. PC2 is known to function as a non-selective cation channel, but PC1''s function and the function of PC1 cleavage products are not well understood. Here we identify an endogenous PC1 cleavage product, P100, a 100 kDa fragment found in both wild type and epitope tagged PKD1 knock-in mice. Expression of full length human PC1 (FL PC1) and the resulting P100 and C-Terminal Fragment (CTF) cleavage products in both MDCK and CHO cells significantly reduces the store operated Ca²⁺ entry (SOCE) resulting from thapsigargin induced store depletion. Exploration into the roles of P100 and CTF in SOCE inhibition reveal that P100, when expressed in Xenopus laevis oocytes, directly inhibits the SOCE currents but CTF does not, nor does P100 when containing the disease causing R4227X mutation. Interestingly, we also found that in PC1 expressing MDCK cells, translocation of the ER Ca²⁺ sensor protein STIM1 to the cell periphery was significantly altered. In addition, P100 Co-immunoprecipitates with STIM1 but CTF does not. The expression of P100 in CHO cells recapitulates the STIM1 translocation inhibition seen with FL PC1. These data describe a novel polycystin-1 cleavage product, P100, which functions to reduce SOCE via direct inhibition of STIM1 translocation; a function with consequences for ADPKD.     

The gating of polycystin signaling complex

Mutations in either polycystin-2 (PC2) or polycystin-1 (PC1) proteins cause severe, potentially lethal, kidney disorders (autosomal dominant polycystic kidney disease, ADPKD) and multiple extrarenal disease phenotypes. PC2, a member of the transient receptor potential channel superfamily and PC1, an orphan membrane receptor of largely unknown function, are thought to be part of a common signalling pathway. Here, I show that co-assembly of full-length PC with PC2 forms an ion channel signalling complex in which PCI regulates PC2 channel gating through a structural rearrangement of the polycystin complex (Delmas et al., 2004a). These polycystin complexes function either as a receptor-cation channel or as a G-protein-coupled receptor. Thus, PC acts as a prototypical membrane receptor that regulates G-proteins and plasmalemmal PC2, a bimodal mechanism that may account for the multifunctional roles of polycystin proteins in various cell types. Genetic alteration of polycystin proteins such as those occurring in kidney diseases may impede polycystin signalling, thereby providing a likely mechanistic explanation to the pathogenesis of ADPKD. Our proposed mechanism may also be paradigmatic for the function of polycystin orthologues and other polycystin-related proteins in a variety of nonrenal cell types, including sperm, muscle cells and sensory neurons.     

Cell biology of polycystin-2

Naturally occurring mutations in two separate, but interacting loci, pkd1 and pkd2 are responsible for almost all cases of autosomal dominant polycystic kidney disease (ADPKD). ADPKD is one of the most common genetic diseases resulting primarily in the formation of large kidney, liver, and pancreatic cysts. Homozygous deletion of either pkd1 or pkd2 results in embryonic lethality in mice due to kidney and heart defects illustrating their indispensable roles in mammalian development. However, the mechanism by which mutations in these genes cause ADPKD and other developmental defects are unknown. Research in the past several years has revealed that PKD2 has multiple functions depending on its subcellular localization. It forms a receptor-operated, non-selective cation channel in the plasma membrane, a novel intracellular Ca2+ release channel in the endoplasmic reticulum (ER), and a mechanosensitive channel in the primary cilium. This review focuses on the functional compartmentalization of PKD2, its modes of activation, and PKD2-mediated signal transduction.     

Polycystin-2 regulates proliferation and branching morphogenesis in kidney epithelial cells

Autosomal dominant polycystic kidney disease (ADPKD) is characterized by the formation of multiple fluid-filled cysts that expand over time and destroy the renal architecture. Loss or mutation of polycystin-1 or polycystin-2, the respective proteins encoded by the ADPKD genes PKD1 and PKD2, is associated with most cases of ADPKD. Thus, the polycystin proteins likely play a role in cell proliferation and morphogenesis. Recent studies indicate that polycystin-1 is involved in these processes, but little is known about the role played by polycystin-2. To address this question, we created a number of related cell lines variable in their expression of polycystin-2. We show that the basal and epidermal growth factor-stimulated rate of cell proliferation is higher in cells that do not express polycystin-2 versus those that do, indicating that polycystin-2 acts as a negative regulator of cell growth. In addition, cells not expressing polycystin-2 exhibit significantly more branching morphogenesis and multicellular tubule formation under basal and hepatocyte growth factor-stimulated conditions than their polycystin-2-expressing counterparts, suggesting that polycystin-2 may also play an important role in the regulation of tubulogenesis. Cells expressing a channel mutant of polycystin-2 proliferated faster than those expressing the wild-type protein, but exhibited blunted tubule formation. Thus, the channel activity of polycystin-2 may be an important component of its regulatory machinery. Finally, we show that polycystin-2 regulation of cell proliferation appears to be dependent on its ability to prevent phosphorylated extracellular-related kinase from entering the nucleus. Our results indicate that polycystin-2 is necessary for the proper growth and differentiation of kidney epithelial cells and suggest a possible mechanism for the cyst formation seen in ADPKD2.     

Polycystin‐1 affects cancer cell behaviour and interacts with mTOR and Jak signalling pathways in cancer cell lines

Polycystic Kidney Disease (PKD), which is attributable to mutations in the PKD1 and PKD2 genes encoding polycystin‐1 (PC1) and polycystin‐2 (PC2) respectively, shares common cellular defects with cancer, such as uncontrolled cell proliferation, abnormal differentiation and increased apoptosis. Interestingly, PC1 regulates many signalling pathways including Jak/STAT, mTOR, Wnt, AP‐1 and calcineurin‐NFAT which are also used by cancer cells for sending signals that will allow them to acquire and maintain malignant phenotypes. Nevertheless, the molecular relationship between polycystins and cancer is unknown. In this study, we investigated the role of PC1 in cancer biology using glioblastoma (GOS3), prostate (PC3), breast (MCF7), lung (A549) and colorectal (HT29) cancer cell lines. Our in vitro results propose that PC1 promotes cell migration in GOS3 cells and suppresses cell migration in A549 cells. In addition, PC1 enhances cell proliferation in GOS3 cells but inhibits it in MCF7, A549 and HT29 cells. We also found that PC1 up‐regulates mTOR signalling and down‐regulates Jak signalling in GOS3 cells, while it up‐regulates mTOR signalling in PC3 and HT29 cells. Together, our study suggests that PC1 modulates cell proliferation and migration and interacts with mTOR and Jak signalling pathways in different cancer cell lines. Understanding the molecular details of how polycystins are associated with cancer may lead to the identification of new players in this devastating disease.