Patent Publication Number: US-8530624-B2

Title: Osteoprotegerin variant proteins

Description:
This application is a continuation of U.S. Ser. No. 11/303,169, filed Dec. 13, 2005, now U.S. Pat. No. 7,612,169, entitled “OSTEOPROTEGERIN VARIANT PROTEINS” by Anthony Roberts, et al., which claims benefit under 35 U.S.C. §119(e) of U.S. Provisional Patent Application Ser. No. 60/635,722, filed Dec. 13, 2004, which applications are incorporated herein by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to evolved proteins that are useful in the regulation of biological effects mediated through the interaction of RANK and RANKL. More particularly, the invention relates to novel osteoprotegerin variant proteins (OVPs) that have altered binding specificities when compared to wild-type osteoprotegerin. Nucleic acids which encode these OVPs are also provided. Recombinant vectors and host cells expressing these OVPs are also encompassed as are methods of producing recombinant OVPs. The present invention also relates to compositions comprising these OVPs, and to methods of treating diseases associated with the a biological effect mediated through the interaction of RANK and RANKL. 
     BACKGROUND OF THE INVENTION 
     The Rank ligand (RANKL)/OPG/RANK biochemical axis has been successfully targeted to treat osteoporosis, rheumatoid arthritis, cancer-induced bone destruction, metastasis, hypercalcemia, and pain (Hofbauer et al., Cancer 92(3): p. 460-470 (2001). Therapies utilizing OPG (Honore et al., Nature Medicine 6(5):521-528 (2000)), an antibody directed to RANKL, or the soluble RANK-Fc protein (Oyajobi et al., Cancer Res 61(6): p. 2572-8 (2001)) are also in development. OPG and soluble RANK-Fc protein constructs bind to RANKL, thereby decreasing amount of RANKL that is available for RANK receptor activation. 
     In addition to being important in bone biology, RANKL plays a role in the immune system by regulating antigen-specific T cell responses (Anderson et al., Nature 390(6656):175-9 (1997)). RANKL is highly expressed on activated T cells while the RANK receptor is expressed at high levels on mature dendritic cells (DC). The interaction between RANKL and RANK acts as a costimulatory signal, which enhances DC survival and T cell proliferation by inducing DC differentiation, cytokine production and reduced apoptosis in both cell types. Immunotherapy to produce tolerance to transplanted tissues and/or organs can be achieved by blocking the costimulatory signal using RANK antagonists. Blocking costimulation prevents T cell activation by DCs, and causes alloreactive T cells to become anergic and/or undergo apoptosis (Adler et al., Current Opinion in Immunology 14:660-665 (2002)). By a similar mechanism of action, antagonizing RANK signaling could be a treatment for autoimmune and immune-mediated inflammatory disorders such as systemic lupus erythematosus, inflammatory bowel disease, diabetes, multiple sclerosis, rheumatoid arthritis, and ankylosing spondylitis. 
     Osteoprotegerin (OPG) is a protein of the Tumour Necrosis Factor (TNF) receptor family, and was first described by Simonet et al. (Cell, 89, 309-319 (1997)). 
     OPG appears to be a crucial element in regulating the natural processes of bone production and turnover. Changes in the balance between OPG and its target receptor RANKL have been noted in a number of conditions associated with abnormal bone metabolism. 
     OPG has undergone preclinical and clinical testing with potential for application in various conditions associated with increased bone turnover and bone loss, including osteoporosis, rheumatoid arthritis, Paget&#39;s disease, periodontal disease, vascular disease and cancers that are located in or have metastasised to bone (For review, see Lorenz et al., J. Amer Med Assoc 292: 490-5 (2004) and references therein). Extensive animal testing of OPG is reported in the literature, indicating equal or superior performance compared with other therapies currently used for reducing bone loss (Simonet et al., Cell, 89, 309-319 (1997); Bolon et al., Cell Mol Life Sci 59, 1569-76 (2002)). OPG has also been shown to reduce pain associated with bone cancers (Luger et al., Cancer Research 61, 4038-4047 (2001)). 
     OPG is described in detail in U.S. Pat. No. 6,015,938, U.S. Pat. No. 6,284,740, U.S. Pat. No. 6,284,728, U.S. Pat. No. 6,613,544, U.S. Pat. No. 6,316,408, U.S. Pat. No. 6,288,032 and U.S. Pat. No. 6,369,027. Transgenic mice lacking expression of OPG are described in U.S. Pat. No. 6,087,555. 
     OPG has undergone Phase I trials in both multiple myeloma patients and in osteoporosis patients for reduction of bone turnover. In preliminary Phase I/II studies in post-menopausal women suffering from osteoporosis, a single injection of OPG was found to suppress markers of bone turnover by 30-80% for several days (Bekker et al., J. Bone Miner Res 16, 348-360 (2001)). Multiple myeloma patients were treated with a range of doses of OPG, and showed up to 50% decrease in bone resorption over 30 or more days (Body et al., Cancer 97, 887-892 (2003)). Thus there is very strong evidence, both from preclinical studies and clinical studies, that OPG is likely to be a highly effective treatment in a variety of conditions characterised by abnormal bone metabolism. 
     The tumour necrosis factor (TNF) family of cytokines and their corresponding receptors contains a considerable number of members and there is substantial cross reactivity among members in ligand-receptor interactions (for review, see Igney, F. H. and Krammer, P. H Nature Reviews Cancer 2, 277-288 (2002)). TNF receptors generally trigger one of two kinds of response, either an apoptotic (programmed cell death) response or an effect on cell metabolic pathways via activation of the transcription factor NFkB. The receptor family also contains members which act as decoys, to reduce the effectiveness of interaction between other family ligand-receptor pairs. OPG appears to fall into this receptor class. 
     Compared with many TNF receptor family members, OPG is relatively restricted in its target specificity. According to current knowledge, OPG binds to only two TNF-like ligands:
         1. RANKL (Receptor activator of NFkB ligand), a TNF-like cell surface molecule normally interacts with a TNF receptor family member, RANK, which is expressed on osteoclast precursors, dendritic cells T-cells and haematopoietic precursors (Kong et al., Nature 397:315-323 (1999). RANKL interacts with RANK on cell surfaces to stimulate the production and activity of osteoclasts, the principal cells involved in bone turnover (Hsu, H et al., Proc Natl Acad Sci USA 96, 3540-3545 (1999)). RANKL is expressed on stromal cells/osteoblasts (Kong et al., Nature 397:315-323 (1999)). The interaction of OPG with RANKL inhibits RANKL&#39;s ability to attach to RANK and stimulate osteoclasts and it is this activity of OPG that confers its ability to reduce bone loss (Lacey et al., Amer J Pathol 157, 435-448 (2000); Simonet et al., Cell, 89, 309-319 (1997)). The binding constant of dimeric OPG binding to RANKL has been reported as 6.7 nM (Willard et al., Protein Expr. Purif 20, 48-57 (2000)) with other reports placing the binding constant between 2 and 10 nM.   2. TRAIL (TNF-related apoptosis inducing ligand), a TNF-like cell surface molecule involved in the induction of apoptosis in cancer cells. OPG is seen as one of a small number of decoy receptors for TRAIL, acting to modulate its ability to target cancer cells (Reviewed in Igney and Krammer (2002) as referenced above). OPG would thus be expected to enhance cancer cell survival if present at a relevant site in sufficient quantities, and its ability to increase the survival of tumour cells has been documented (for example, Neville-Webbe et al., Breast Cancer Res Treat. 86, 271-82 (2004); Holen I et al., Cancer Res 62, 1619-1623 (2002)). The affinity of OPG for TRAIL has been reported as 3 nM (Emery et al., J. Biol. Chem. 273, 14363-7 (1998)) although this may vary with temperature (Truneh et al., J. Biol. Chem. 275, 23319-25 (2000)).       

     The ability of OPG to inhibit TRAIL activity, resulting in a possible increase in risk of cancer development, has been noted in the medical literature as a likely deterrent to the use of this agent as a therapeutic. Among the application areas for agents inhibiting bone loss are conditions such as osteoporosis and Paget&#39;s disease, which are not in themselves life threatening, but require long term therapy. Furthermore, one of the most important potential uses of OPG is in adjunct treatment of cancer patients with bone metastases. In all of these potential application areas, any indication of increased growth and survival of cancer cells associated with treatment would be a deterrent to use. Hence there is a need to generate a novel variant of OPG that substantially lacks TRAIL-binding capabilities. Such a variant would lack ability to significantly interfere with natural anti-cancer mechanisms while retaining its ability to interfere with the RANK/RANKL interaction that stimulates osteoclast formation and activity, and consequent bone loss. 
     SUMMARY OF THE INVENTION 
     The present inventors have now generated novel OPG Variant Proteins (OVPs) which demonstrate reduced binding affinity for TRAIL and reduced ability to inhibit the normal biological effects resulting from the interaction of TRAIL with its receptor. 
     Analysis of the OVPs of the present invention has revealed a focussing of point mutations in specific regions of the wild-type OPG protein sequence. In particular, the mutations are clustered in the region encompassed by OPG residues 102-130. Modelling studies conducted on the OVPs of the present invention support the idea that this region of OPG/OVP forms an interface with both TRAIL and RANKL proteins during binding. The identification of mutations within this region which can substantially alter the binding affinity for TRAIL without affecting attachment to RANKL is therefore surprising. 
     Importantly, the OVPs of the present invention retain the ability to bind to RANKL, and to reduce or inhibit the RANK-RANKL interaction and the consequent biological effects of this interaction. It is envisaged that preferred OVPs of the present invention will retain the ability of wild-type OPG to modulate bone metabolism, without having the undesired effect of enhancing survival of cancer cells through inhibition of the action of TRAIL. 
     Accordingly, the present invention provides an OPG variant protein comprising at least one modification when compared to wild-type OPG, wherein the OPG variant protein retains binding affinity for RANKL but exhibits reduced binding affinity for TRAIL when compared to wild-type OPG. 
     An OPG variant of the present invention includes any variant protein of OPG, whether truncated or full-length, in monomer or dimer form, which has significantly reduced attachment to TRAIL as a result of a modification to the protein, while substantially retaining its attachment to RANKL. Where comparisons are made between the activity of an OVP and that of wild type OPG, it will be appreciated that the comparison is made on the basis of proteins in a similar form with respect to size and dimerisation state. 
     In one preferred embodiment the OPG variant protein comprises at least one modification in the region encompassed by amino acid residues 102-130. For example, the OPG variant protein may comprise at least one amino acid substitution at position 102, 111, 115, 122, 128 or 130. 
     The present invention also provides an OVP protein which has been modified to induce dimerisation, or to maintain or improve its stability, expression or biological half life. Such modifications may include addition of an albumin moiety, an Fc region or substitution with polyethylene glycol, or addition of extensions which serve to maintain the OVP in a dimeric form. 
     The present invention also provides an isolated polynucleotide encoding an OPG variant protein of the present invention. Preferably, the polynucleotide is a variant of SEQ ID NO: 1, wherein the variant comprises nucleic acid changes such that it encodes the desired OPG variant protein of the invention. Methods of producing such nucleic acid changes are known in the art and are also described herein. 
     Variations on this sequence where the encoded protein maintains the same spectrum of biological activities are also provided. The nucleotide sequence may be altered to improve expression of the encoded protein in one or more expression systems, to increase RNA stability, or serendipitously, containing changes which do not diminish biological function of the nucleic acid or its encoded protein. 
     The present invention also provides a vector which comprises a polynucleotide according to the invention. Preferably, the vector is suitable for the replication and/or expression of a polynucleotide. The vectors may be, for example, a plasmid, virus or phage vector provided with an origin of replication, and preferably a promoter for the expression of the polynucleotide and optionally a regulator of the promoter. The vector may contain one or more selectable markers, for example an ampicillin resistance gene in the case of a bacterial plasmid or a neomycin resistance gene for a mammalian expression vector. The vector may be used in vitro, for example for the production of RNA or used to transfect or transform a host cell. 
     The present invention also provides a host cell which comprises a vector according to the invention, or contains the gene encoding one of the proteins of the invention under conditions that allow expression and production of the subject protein. 
     The present invention also provides a process for preparing an OPG variant protein of the invention, the process comprising cultivating a host cell of the invention under conditions which allow production of the OPG variant protein, and recovering the OPG variant protein. Such cells can be used for the production of commercially useful quantities of the encoded OPG variant protein. 
     The present invention also provides a method for reducing the binding affinity of OPG for TRAIL, the method comprising modifying at least one amino acid within an OPG protein or fragment thereof to produce an OPG variant protein, and testing the OPG variant protein for binding affinity for TRAIL. 
     In one preferred embodiment the method comprises modifying at least one amino acid within the region encompassed by residues 102-130 of wild type OPG to produce an OPG variant protein, and testing the OPG variant protein for binding affinity for TRAIL. 
     It will be appreciated that an OVP of the present invention can be used to reduce or inhibit the binding or RANK to RANKL in vivo, and reduce or inhibit the interaction between RANK and RANKL and the biological effects mediated through this interaction. 
     Accordingly, the present invention also provides a composition for inhibiting or reducing binding of RANK to RANKL, the composition comprising an OPG variant protein according to the invention, and one or more acceptable carriers. 
     The present invention also provides a pharmaceutical composition comprising a therapeutically effective amount of an OPG variant protein according to the invention and one or more of a pharmaceutically acceptable carrier, solubilizer, stabilizer and anti-oxidant. 
     The present invention also provides a method for inhibiting or reducing a biological effect mediated through the interaction of RANK and RANKL, the method comprising exposing a RANKL molecule to an OPG variant protein according to the present invention. 
     The present invention also provides a method for inhibiting or reducing a biological effect mediated through the interaction of RANK and RANKL in a subject, the method comprising administering to a subject an OPG variant protein according to the invention. 
     By “subject” herein is meant both humans and other animals, particularly mammals as outlined herein, with humans being preferred. 
     The present invention also provides a method for inhibiting or reducing differentiation, activation and/or survival of osteoclasts by RANKL, the method comprising exposing tissue containing osteoclasts or osteoclast precursor cells to an OPG variant protein according to the invention. 
     The present invention also provides a method for inhibiting or reducing differentiation, activation and/or survival of osteoclasts by RANKL in a subject, the method comprising administering to a subject an OPG variant protein according to the invention. 
     The present invention also provides method for preventing or treating a bone disorder comprising administering to a patient the pharmaceutical composition of the present invention. 
     Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps. 
     The various features and embodiments of the present invention, referred to in individual sections herein, also apply as appropriate, to other sections. Consequently features specified in relation to one embodiment of the invention may be combined with features specified in relation to other embodiments, as appropriate. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1 : A. OPG gene structure. CRD-cysteine-rich domain; DD-death domain; HBD: heparin binding domain. B. pEGX254 mutagenesis and display cassette. 
         FIG. 2 : ELISA comparing binding of purified OVPs and OPG to RANKL and TRAIL. Binding curves are labelled according to the variant number being tested. WT refers to unmutated OPG. Curve labels ending in R refer to RANKL binding; curve labels ending in T refer to TRAIL binding. These studies were carried out using monomeric OPG 22-194 expressed in  E. coli.    
         FIG. 3 : ELISA comparing binding of purified OVPs and OPG to RANKL and TRAIL. Curve labelling and conditions as for  FIG. 3 . 
         FIG. 4 : ELISA of purified mammalian-expressed OVPs binding to RANKL (A) and TRAIL (B). These studies were carried out using dimeric Fc-linked OPG or OVP 22-194. 
         FIG. 5 : (A) Inhibition of TRAIL-induced apoptosis in HCT116 cells by OPG wild type proteins. (B) Inhibition of TRAIL-induced apoptosis in HCT116 cells by OVPs. 
         FIG. 6 : (A) OPG wild type inhibition of RANKL-mediated Osteoclastic Activation of RAW264.7 cells. (B) OVP inhibition of RANKL-mediated Osteoclastic Activation of RAW264.7 cells. 
         FIG. 7 : OVP inhibition of RANKL-induced osteoclast mediated resorption. All OVPs used at 1 nM. UT—untreated with OVP 
         FIG. 8 : Alignments of human OPG wild-type and preferred OVP amino acid sequences. 
         FIG. 9 : Structural model of OPG bound to mouse RANKL (pdb file 1JTZ). The RANKL trimer is visualized as the upper light gray ribbon diagram, OPG as dark gray ribbon diagram. OPG residues with at least one atom within 5 Angstrom of RANKL are highlighted as spacefilling spheres. Light spheres represent potential contact residues of OPG to RANKL in the N-terminal receptor domain (residues 42 to 91 of OPG, with specific contact residues scored as 42, 43, 49, 50, 52, 53, 69, 71-82, 85-91), dark spheres show the C-terminal receptor domain (OPG residues 105 to 130 with specific contact residues scored as 105, 106, 107, 108, 110-130). 
         FIG. 10 : Partial view of modelled OPG in contact with TRAIL focusing on the second TNF receptor domain. In the upper half TRAIL is shown as ribbon diagram. OPG is depicted as alpha carbon trace in light grey. In black, an OPG variant is aligned with OPG, featuring mutation of Ile 115 to Glu. Side chains for residues 115 and 122 are shown as stick figures. The modelling of the OVP suggests that mutation of residue 115 results in an altered conformation of the 107-118 loop, with realigned contacts in the interface. The conformational change may alter additional contacts, as highlighted by the shift of the side chain of residue 122, potentially abolishing contacts in the region of residues 120-130. These contacts appear more crucial for TRAIL than for RANKL binding. This would be consistent with the nearly complete abolishment of TRAIL binding seen for this mutant in our assays. 
         FIG. 11 : Model predicting the association between OPG (bottom) and TRAIL (top as ribbon diagram). Residues 102-130 are shown in space-filling form. According to this model, the loop containing residue 115 of OPG (long arrow), and residues C-terminal to this (to the right of the 115 loop) make multiple contacts with TRAIL (short arrow). The protruding loop 195-202 of TRAIL (short arrow) appears to allow additional or more extensive contacts with OPG than are possible for RANKL in particular with OPG residues 120-130 (compare to  FIG. 12 ). This confirms the region as an area where mutations are likely to alter TRAIL binding with the potential for unaltered RANKL binding. 
         FIG. 12 : Model predicting the association between OPG (bottom) and RANKL (top as ribbon diagram), using the mouse RANKL structure, aligned to modelled OPG-TRAIL complex. Residues 102-130 of OPG are shown in space-filling form. The loop containing residue 115 long arrow) points towards and contacts RANKL, but residues C-terminal to this area (to the right of this loop) appear to have few contacts with RANKL (short arrow-RANKL residues 225-233, which corresponds to the protruding loop 195-202 in TRAIL of  FIG. 11 ). This distinguishes the contact of OPG with RANKL from that of TRAIL shown in the previous diagram. 
     
    
    
     KEY TO SEQUENCE LISTING 
     SEQ ID NO: 1—Human wild-type osteoprotegerin gene; 
     SEQ ID NO: 2—Human wild-type osteoprotegerin; 
     SEQ ID NO: 3—Rat wild-type osteoprotegerin; 
     SEQ ID NO: 4—Mouse wild-type osteoprotegerin; and 
     SEQ ID NOs: 5 to 47—Human osteoprotegerin variant proteins (OVPs); and 
     SEQ ID NOs: 48 and 49—Primers used to isolate the OPG gene. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     General Techniques 
     Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art (e.g., in cell culture, molecular genetics, nucleic acid chemistry, hybridization techniques, chemistry and biochemistry). 
     Unless otherwise indicated, the recombinant DNA techniques utilized in the present invention are standard procedures, well known to those skilled in the art. Such techniques are described and explained throughout the literature in sources such as, J. Perbal, A Practical Guide to Molecular Cloning, John Wiley and Sons (1984), J. Sambrook et al., Molecular Cloning: A Laboratory Manual, Cold Spring Harbour Laboratory Press (1989), T. A. Brown (editor), Essential Molecular Biology: A Practical Approach, Volumes 1 and 2, IRL Press (1991), D. M. Glover and B. D. flames (editors), DNA Cloning: A Practical Approach, Volumes 1-4, IRL Press (1995 and 1996), and F. M. Ausubel et al. (Editors), Current Protocols in Molecular Biology, Greene Pub. Associates and Wiley-Interscience (1988, including all updates until present). 
     Osteoprotegerin (OPG) 
     OPG is a member of the TNF receptor superfamily, having an activity associated with bone metabolism and in particular having the activity of inhibiting bone resorption thereby increasing bone density. In particular, the interaction of OPG with RANKL inhibits RANKL&#39;s ability to stimulate the formation and activity of osteoclasts via its interaction with RANK, and it is this activity of OPG that confers its ability to reduce bone loss. 
     The biological activities of OPG also include activity associated with binding to TRAIL. TRAIL (TNF-related apoptosis inducing ligand) is a TNF-like cell surface molecule involved in the induction of apoptosis in cancer cells. OPG is seen as one of a small number of decoy receptors for TRAIL, acting to modulate its ability to target cancer cells. OPG is thus expected to have the undesirable effect of enhancing survival of cancer cells if present at a relevant site in sufficient quantities. 
     Various active fragments of OPG have been described. Yamaguchi et al. (J. Biol. Chem. 273, 5117-5123 (1998)) demonstrated that truncated and deleted variants of OPG retained OPG-like activity, while OPG in both monomer or dimer form retained biological activity (Tomoyasu, A et al., Biochem Biophys Res Commun 245, 382-387 (1998)). Schneeweis et al., (J. Biol. Chem. Oct. 7 2005 (epub)) examined the assembly, state and affinity of OPG for RANKL. They reported that dimerisation of OPG either in full length form where the dimerisation is mediated by non-covalent interactions within the death domain regions, or in truncated form where dimerisation may be mediated by Fc attachment, is required for high affinity attachment of OPG to RANKL. 
     The amino acid sequences of wild-type human, rat and mouse osteoprotegerin are shown in SEQ ID NOs 2 to 4 respectively. 
     Wild-type OPG sequences also encompass those that have the amino terminal leader sequence of 21 amino acids removed and/or where amino acids are removed from the C-terminus up to and including amino acid 185. 
     OPG Variant Proteins 
     By “protein” herein is meant at least two covalently attached amino acids, which includes proteins, polypeptides, oligopeptides and peptides. The protein may be made up of naturally occurring amino acids and peptide bonds, or synthetic peptidomimetic structures, i.e., “analogs” such as peptoids (see Simon et al., Proc. Natl. Acd. Sci. U.S.A. 89(20:9367-71 (1992)), generally depending on the method of synthesis. For example, homo-phenylalanine, citrulline, and noreleucine are considered amino acids for the purposes of the invention. “Amino acid” also includes imino acid residues such as proline and hydroxyproline. Both D- and L-amino acids may be utilized. 
     The present inventors have now generated novel OPG Variant Proteins (OVPs) which demonstrate reduced binding affinity for TRAIL. Importantly, the OVPs of the present invention retain the ability to bind to RANKL. Accordingly, it is envisaged that the OVPs of the present invention will retain the ability of wild-type OPG to modulate bone metabolism, without having the undesired TRAIL-associated characteristic of enhancing survival of cancer cells. 
     Accordingly, the present invention provides an OPG variant protein comprising at least one modification when compared to wild-type OPG, wherein the OPG variant protein retains binding affinity for RANKL but exhibits reduced binding affinity for TRAIL when compared to wild-type OPG. 
     By “retains binding affinity for RANKL” we mean that the OPG variant protein exhibits at least 20% binding affinity for RANKL when compared to wild-type OPG. 
     The wild type OPG may be derived from any species, preferably a mammalian species. In a preferred embodiment, the wild-type OPG is a prevalent human OPG protein. More preferably, the wild type OPG is a protein having an amino acid sequence as is shown in SEQ ID NO:2. 
     The OPG variant proteins of the present invention preferably exhibit at least one biological activity of wild-type OPG. 
     In one example, the biological activity of the wild-type OPG relates to bone metabolism, and in particular the ability to inhibit or reduce stimulation and/or activity of osteoclasts by RANKL. Preferably, OPG variant proteins of the present invention retain the ability to prevent disease related decreases in bone density or abnormal bone turnover. 
     In another example, the biological activity of the wild-type OPG relates to T cell activity, and in particular the ability to reduce or inhibit T cell activation. Preferably, OPG variant proteins of the present invention are useful treatments for autoimmune disorders such as systemic lupus erythematosus, inflammatory bowel disease, diabetes, multiple sclerosis, rheumatoid arthritis, and ankylosing spondylitis. 
     In further examples, OPG variant proteins of the present invention are useful for treating cardiovascular disease, preventing or reducing calcification of blood vessels and valves, or enhancing blood vessel growth. 
     In one embodiment the OPG variant protein is a modified version of a full length wild type OPG sequence. In another embodiment, the OPG variant protein contains one or more additional insertions or deletions at the N-terminus, C-terminus, or internally. For example, the OPG variant protein may consist of any modified active fragment of OPG. The active fragment may consist of, for example, amino acids 22-294, amino acids 22-201, 22-194 or amino acids 1-197 
     In a preferred embodiment, OPG variant proteins have at least 1 amino acid residue that differs from a wild-type OPG sequence, with at least 2, 3, 4, or 5 different residues being more preferred. The modifications are preferably amino acid substitutions and may include those to surface or exposed areas of OPG. OPG variant proteins may comprise one domain or multiple domains connected by linker sequences. OPG variant proteins may contain further modifications, for instance modifications that alter stability or immunogenicity or which enable posttranslational modifications such as PEGylation or glycosylation. 
     In one preferred embodiment the OPG variant protein comprises at least one modification in the region encompassed by amino acid residues 102-130. For example, the OPG variant protein may comprise at least one amino acid substitution at position 102, 111, 115, 122, 128 or 130. 
     In one preferred embodiment, the OPG variant protein comprises at least one modification within the loop structure comprising residues 107-118. In a particularly preferred embodiment, the modification in this region involves substitution of Ile at position 115. Ile at position 115 may be substituted by amino acids that are more polar or have shorter side chains. Preferably, Ile at position 115 is substituted with Thr, Met, Val, Asp, Gly. Ser or Arg. Thus, preferred modifications include I115T, I115M, I115V, I115D, I115G, I115S and I115R. 
     In one preferred embodiment the OPG variant protein comprises at least one modification in the region encompassed by amino acid residues 120-130. In another preferred embodiment, the modification involves substitution of Arg at position 122. Preferably, Arg at position 122 is substituted with Gly, Gln, Ser, Asn or Glu. Thus, preferred modifications include R122G, R122Q, R122S, R122N and R122E. 
     In yet a further particularly preferred embodiment, the modification involves substitution of Phe at position 128. Preferably, Phe at position 128 is substituted with Val, Ala, Leu, Ile or Ser. Thus, preferred modifications include F128V, F128A, F128L, F128 I and F128S. 
     In still a further preferred embodiment, the modification involves substitution of Val at position 130. Preferably, Val at position 130 is substituted with Glu or Ala. 
     In a further preferred embodiment, the OPG variant protein comprises at least two modifications. 
     The at least two modifications may both occur in the region encompassed by residues 102-130. For example, the at least two modifications may involve substitutions at positions 115 and 122. Examples of suitable double modifications within this region are R122N and I115M; R122N and I115M; F128S and I115M; F128I and I115M; and F128L and I115M. 
     Alternatively, the at least two modifications may involve one or more modifications within the region encompassed by amino acids 102-130 and one or more modifications outside of this region. 
     The modification outside of this region may occur at, for example, any one or more of residues 31, 40, 51, 100, 155, 167 or 168. In one preferred embodiment modification occurs at residue 40. Preferably, the modification involves substitution of Leu at position 40 with Ser. 
     In another embodiment, the OPG variant protein of the present invention comprises one or more modifications within the region encompassed by amino acids 102-130 and a modification to any one or more of the following amino acid residues: Gln21, Glu22, Thr23, Phe24, Pro25, Pro26, Lys27, Tyr28, Leu29, His30, Tyr31, Asp32, Glu33, Glu34, Thr35, Ser36, His37, Gln38, Asp42, Lys43, Pro45, Pro46, Thr48, Lys51, Gln52, His53, Cys54, Thr55, Ala56, Lys57, Trp58, Lys59, Thr60, Val61, Ala63, Pro64, Pro66, Asp67, His68, Tyr69, Asp72, Ser73, Trp74, Thr76, Ser77, Asp78, Glu79, Leu81, Tyr82, Ser84, Pro85, Val86, Lys88, Glu89, Leu90, Tyr92, Val93, Lys94, Gln95, Glu96, Asn98, Arg99, Thr100, His101, Val131, Gln132, Ala133, Gly134, Thr135, Pro136, Glu137, Arg138, Val141, Lys143, Arg144, Cys145, Pro146, Asp147, Gly148, Phe149, Phe150, Ser151, Asn152, Glu153, Thr154, Ser155, Ser156, Lys157, Ala158, Pro159, Cys160, Arg161, Lys162, His163, Thr164, Asn165, Cys166, Ser167, Val168, Phe169, Gly170, Leu171, Leu172, Leu173, Thr174, Gln175, Lys176, Gly177, Asn178, Ala179, Thr180, His181, Asp182, Asn183, Ile184, Cys185, Ser186, Gly187, Asn188, Ser189, Glu190, Ser191, Thr192, Gln193, Lys194, Cys195, Gly196, Ile197, Asp198, Val199, Thr200 and Leu201. 
     In another embodiment, the OPG variant protein of the present invention comprises one or more modifications within the region encompassed by amino acids 102-130 and a modification to any one or more of the following amino acid residues: Tyr28, His30, Tyr31, Glu34, Thr35, Ser36, His 37, Lys43, Tyr49, Gln52, His53, Pro66, Asp67, His68, Tyr69, Tyr70, Thr71, Asp72, Ser73, Trp74, His75, Thr76, Ser77, Asp78, Glu79, Cys80, Leu81, Tyr82, Cys83, Ser84, Pro85, Val86, Cys87, Lys88, Glu89, Leu90, Gln91, Asn139 and Glu153. 
     The present invention also provides an OPG variant protein comprising a modification at residue 40, wherein the OPG variant protein retains binding affinity for RANKL but exhibits reduced binding affinity for TRAIL when compared to wild-type OPG. Preferably, the modification involves substitution of Leu at position 40 with Ser. 
     An OPG variant protein of the present invention may involve any one or more of the modifications shown in Table 1. 
     In a further preferred embodiment the OPG variant protein comprises a sequence as shown in any one of SEQ ID Nos: 5 to 47. 
     In a further preferred embodiment of the present invention, the OVP exhibits a binding affinity for RANKL that is at least 50%, more preferably at least 60%, more preferably at least 70%, more preferably at least 80%, more preferably at least 90% and more preferably at least 95% that of wild type OPG. 
     In one embodiment, the OVP exhibits increased binding affinity for RANKL when compared to wild-type OPG. 
     In a further preferred embodiment, the OVP has an EC50 (nM) for attachment to RANKL of less than 5 nM, more preferably less than 1 nM under assay conditions as described in Example 8. 
     In a further preferred embodiment of the present invention, the OVP exhibits a binding affinity for TRAIL that is less than 50%, more preferably less than 40%, more preferably less than 30%, more preferably less than 20%, more preferably less than 10% and more preferably less than 5% that of wild type OPG. 
     In a further preferred embodiment, the OVP has an EC50 (nM) for attachment to TRAIL of greater than 5 nM, more preferably greater than 10 nM, more preferably greater than 25 nM, more preferably greater than 50 nM under assay conditions as described in Example 8. 
     Also encompassed are OPG variant proteins having deletions or carboxy-terminal truncations of part or all of amino acids residues 195-401 or 198-401 of OPG; one or more amino acid changes in residues 195-401 or 198-401; deletion of part or all of a cysteine-rich domain of OPG, in particular deletion of the distal (carboxy-terminal) cysteine-rich domain; and one or more amino acid changes in a cysteine-rich domain, in particular in the distal (carboxy-terminal) cysteine-rich domain. 
     Preferred amino acid sequence deletions generally range from about 1 to 30 residues, more preferably about 1 to 10 residues and typically about 1 to 5 contiguous residues. 
     It will be understood that the amino acid numbering used herein to refer to sites for mutagenesis relates to numbering of the human OPG sequence shown in SEQ ID NOs: 1 and 2. Corresponding sites in homologues, derivatives and allelic variants are also encompassed by the present invention. 
     The term “corresponding to” is used in the context of the present invention to refer to amino acid residues of OPG polypeptides related to SEQ ID NOs 1 and 2 (for example greater than 70% identical, greater than 80% identical, greater than 90% identical or greater than 95% identical to SEQ ID NOs 1 and 2), however, the relative residue numbering of the related polypeptide may be different to that of SEQ ID NOs 1 and 2. 
     Also encompassed are OPG variant proteins described herein comprising additional conservative substitutions throughout the sequence. Such conservative substitutions are as follows: 
     
       
         
           
               
               
               
             
               
                   
                   
               
               
                   
                 Original 
                 Exemplary 
               
               
                   
                 Residue 
                 Substitutions 
               
               
                   
                   
               
             
            
               
                   
                 Ala (A) 
                 val; leu; ile; gly 
               
               
                   
                 Arg (R) 
                 lys 
               
               
                   
                 Asn (N) 
                 gln; his; 
               
               
                   
                 Asp (D) 
                 glu 
               
               
                   
                 Cys (C) 
                 ser 
               
               
                   
                 Gln (Q) 
                 asn; his 
               
               
                   
                 Glu (E) 
                 asp 
               
               
                   
                 Gly (G) 
                 pro, ala 
               
               
                   
                 His (H) 
                 asn; gln 
               
               
                   
                 Ile (I) 
                 leu; val; ala 
               
               
                   
                 Leu (L) 
                 ile; val; met; ala; phe 
               
               
                   
                 Lys (K) 
                 arg 
               
               
                   
                 Met (M) 
                 leu; phe; 
               
               
                   
                 Phe (F) 
                 leu; val; ala 
               
               
                   
                 Pro (P) 
                 gly 
               
               
                   
                 Ser (S) 
                 thr 
               
               
                   
                 Thr (T) 
                 ser 
               
               
                   
                 Trp (W) 
                 tyr 
               
               
                   
                 Tyr (Y) 
                 trp; phe 
               
               
                   
                 Val (V) 
                 ile; leu; met; phe, ala 
               
               
                   
                   
               
            
           
         
       
     
     Furthermore, if desired, non-naturally occurring amino acids or chemical amino acid analogues can be introduced as a substitution or addition into the polypeptide of the present invention. Such amino acids include, but are not limited to, the D-isomers of the common amino acids, 2,4-diaminobutyric acid, α-amino isobutyric acid, 4-aminobutyric acid, 2-aminobutyric acid, 6-amino hexanoic acid, 2-amino isobutyric acid, 3-amino propionic acid, ornithine, norleucine, norvaline, hydroxyproline, sarcosine, citrulline, homocitrulline, cysteic acid, t-butylglycine, t-butylalanine, phenylglycine, cyclohexylalanine, β-alanine, fluoro-amino acids, designer amino acids such as β-methyl amino acids, Cα-methyl amino acids, Nα-methyl amino acids, and amino acid analogues in general. 
     Also provided by the invention are chemically modified derivatives of OPG variant proteins which may provide additional advantages such as increasing stability and circulating time of the polypeptide, or decreasing immunogenicity (see U.S. Pat. No. 4,179,337). The chemical moieties for derivitization may be selected from water soluble polymers such as polyethylene glycol, ethylene glycol/propylene glycol copolymers, carboxymethylcellulose, dextran, polyvinyl alcohol and the like. Also included are OVPs of the present invention which are differentially modified during or after synthesis, e.g., by biotinylation, benzylation, glycosylation, acetylation, phosphorylation, amidation, derivatization by known protecting/blocking groups, proteolytic cleavage, etc. The polypeptides may be modified at random positions within the molecule, or at predetermined positions within the molecule and may include one, two, three or more attached chemical moieties. These modifications may serve to increase the stability and/or bioactivity of the polypeptide of the invention. 
     Additional modifications of OPG variant proteins encompassed by the invention include post-translational modifications (e.g., N-linked or O-linked carbohydrate chains, processing of N-terminal or C-terminal ends), chemical modifications of N-linked or O-linked carbohydrate chains, and addition of an N-terminal methionine residue as a result of procaryotic host cell expression. The OVPs may also be modified with a detectable label, such as an enzymatic, fluorescent, isotopic or affinity label to allow for detection and isolation of the protein. 
     Further modifications of OPG variant proteins include chimeric proteins wherein the OPG variant protein is fused to a heterologous amino acid sequence. The heterologous sequence may be any sequence which allows the resulting fusion protein to retain at least the activity of the OPG variant protein. The heterologous sequences include for example, immunoglobulin fusions, such as Fc fusions, or fusions to other cellular ligands which may increase stability or aid in purification of the protein. 
     In one embodiment, OPG variant proteins of the invention may be produced as dimeric or even as oligomeric single-chain molecules, with two, three or possibly more monomers. 
     The monomers may be joined by a peptide bond or a peptide linker, or for example, by means of a PEG molecule. 
     Dimerisation can also be achieved by producing the compound as a fusion protein with the Fc-portion of Ig gamma 1 (GenPept accession No. M87789.1). The molecules can be expressed as fusion proteins with a C-terminal Fc-part or with a N-terminal Fcpart. 
     Dimerisation can also be achieved by fusing the product candidate to a GCN4 leucine zipper, which has been reported to induce dimerisation of fusion proteins (Donate, et al., Biochemistry, 39 11467-76 (2000)). 
     Alternatively, dimeric molecules may be produced by mutagenizing one of the last five, or alternatively one of the first five amino acid residues to a cysteine residue. An unpaired cysteine residue of the purified compound can then be attached to a “di-active” PEG group by using existing thiol reactive attachment groups. Alternatively, dimeric molecules can be produced by inserting two candidate molecules (identical or even different) in-frame with a suitable flexible polypeptide linker in an appropriate expression vector. 
     Still further modifications of OPG variant proteins include dimeric or multimeric forms where the monomeric units of the OPG variant protein may be linked together chemically or genetically by appropriate linker molecules which may include peptide or chemical linkers, or disulphide bridges between cysteine residues. Additionally, dimeric or multimeric forms of the subject proteins may be achieved by linkages between proteins to which the OPG protein is fused, such as Fc regions. 
     The OPG variant proteins of the invention are preferably isolated and purified from other polypeptides present in tissues, cell lines and transformed host cells expressing the OVP, or purified from components in cell cultures containing the secreted protein. In one embodiment, the OVP is free from association with other proteins, such as the expression product of a bacterial host cell. 
     OPG variant proteins of the present invention can be prepared using any technique known in the art. For example, the polynucleotide provided as SEQ ID NO: 1 can be subjected to in vitro mutagenesis. Such in vitro mutagenesis techniques include sub-cloning the polynucleotide into a suitable vector, transforming the vector into a “mutator” strain such as the  E. coli  XL-1 red (Stratagene) and propagating the transformed bacteria for a suitable number of generations. In another example, the polynucleotides of the invention are subjected to DNA shuffling techniques as broadly described by Harayama (1998). Protein products derived from mutated/altered DNA can readily be screened using techniques described herein to determine if they have enhanced and/or altered substrate specificity. 
     Amino acid sequence mutants of the polypeptides of the present invention can also be prepared by introducing appropriate nucleotide changes into a nucleic acid sequence, or by in vitro synthesis of the desired polypeptide. Such mutants include, for example, deletions, insertions or substitutions of residues within the amino acid sequence. A combination of deletion, insertion and substitution can be made to arrive at the final construct, provided that the final protein product possesses the desired characteristics. 
     Substitution mutants have at least one amino acid residue in the wild-type OPG sequence removed and a different residue inserted in its place. The sites of greatest interest for substitutional mutagenesis include those sites exemplified herein. The sites for mutation can be modified individually or in series, e.g., by (1) substituting first with conservative amino acid choices and then with more radical selections depending upon the results achieved, (2) deleting the target residue, or (3) inserting other residues adjacent to the located site. 
     Alternatively, OPG variant proteins can be generated using a cell-free in vitro evolution mutagenesis system. In one example of cell-free in vitro evolution, a system similar to that described in WO 99/58661 or WO 2004/039995 is utilized. 
     A cell-free in vitro evolution method comprises exposing mutant OPG RNA molecules, produced directly or indirectly by the action of a polymerase in the presence of a mutagen (such QB replicase or ribavirin, or a derivative/analogue thereof) to a translation system under conditions which result in the production of a population of mutant proteins. These mutant OPG proteins are linked to the RNA from which they were translated forming a population of mutant protein/RNA complexes. This population of mutant OPG protein/RNA complexes is screened for a desired biological activity such as binding to RANKL without binding to TRAIL. A mutant protein/RNA complex with the desired activity can be isolated and the sequence of the protein encoded by the RNA characterized by standard techniques. 
     The present invention also provides a process for preparing an OPG variant protein of the invention, the process comprising cultivating a host cell of the invention under conditions which allow production of the OPG variant protein, and recovering the OPG variant protein. Such cells can be used for the production of commercially useful quantities of the encoded OPG variant protein. 
     OPG variant proteins of the present invention can be produced in a variety of ways, including production and recovery of natural proteins, production and recovery of recombinant proteins, and chemical synthesis of the proteins. In one embodiment, an OPG variant protein of the present invention is produced by culturing a cell capable of expressing the OPG variant protein under conditions effective to produce the OPG variant protein, and recovering the OPG variant protein. Effective culture conditions include, but are not limited to, effective media, bioreactor, temperature, pH and oxygen conditions that permit protein production. An effective medium refers to any medium in which a cell is cultured to produce a polypeptide of the present invention. Such medium typically comprises an aqueous medium having assimilable carbon, nitrogen and phosphate sources, and appropriate salts, minerals, metals and other nutrients, such as vitamins. Cells of the present invention can be cultured in conventional fermentation bioreactors, shake flasks, test tubes, microtiter dishes, and petri plates. Culturing can be carried out at a temperature, pH and oxygen content appropriate for a recombinant cell. Such culturing conditions are within the expertise of one of ordinary skill in the art. 
     A method for the purification of OPG variant proteins from transfected host cells is also included. The purification process may employ one or more standard protein purification steps, such as chromatography, in an appropriate order to obtain purified protein. The chromatography steps can include ion exchange, gel filtration, hydrophobic interaction, reverse phase, chromatofocusing, affinity chromatography employing an anti-OPG antibody or biotin-streptavidin affinity complex and the like. 
     The present invention also provides a method for reducing the binding affinity of OPG for TRAIL, the method comprising modifying at least one amino acid within an OPG protein or fragment thereof to produce an OPG variant protein, and testing the OPG variant protein for binding affinity for TRAIL. 
     In one preferred embodiment the method comprises modifying at least one amino acid within the region encompassed by residues 102-130 of wild type OPG to produce an OPG variant protein, and testing the OPG variant protein for binding affinity for TRAIL. 
     Methods for testing the OPG variant protein for binding affinity for TRAIL will be known to those skilled in the art. Suitable assays include, but are not limited to, quantitative comparisons comparing kinetic and equilibrium binding constants. The kinetic association rate (Kon) and dissociation rate (Koff), and the equilibrium binding constants (Kd) may be determined using surface plasmon resonance on a BIAcore instrument following the standard procedure in the literature (Pearce et al., Biochemistry 38:81-89 (1999)). Several alternative methods can also be used to determine binding affinity and kinetics, examples of which are described in the Example section herein. 
     Nucleic Acids, Vectors and Host Cells 
     The present invention also provides an isolated polynucleotide encoding an OPG variant protein of the present invention. Examples of polynucleotides of the invention include cDNA, genomic DNA, synthetic DNA and RNA. 
     Preferably, the polynucleotide is a variant of SEQ ID NO: 1, wherein the variant comprises nucleic acid changes such that it encodes the desired OPG variant protein of the invention. Methods of producing such nucleic acid changes are known in the art and are also described herein. 
     The present invention also provides a vector which comprises a polynucleotide according to the invention. Preferably, the vector is suitable for the replication and/or expression of a polynucleotide. The vectors may be, for example, a plasmid, virus or phage vector provided with an origin of replication, and preferably a promoter for the expression of the polynucleotide and optionally a regulator of the promoter. The vector may contain one or more selectable markers, for example an ampicillin resistance gene in the case of a bacterial plasmid or a neomycin resistance gene for a mammalian expression vector. The vector may be used in vitro, for example for the production of RNA or used to transfect or transform a host cell. 
     The present invention also provides a host cell which comprises a vector according to the invention, or contains the gene encoding one of the proteins of the invention under conditions that allow expression and production of the subject protein. 
     The nucleic acids of the invention may be constructed using evolutionary or site-directed mutagenesis techniques available to the skilled worker. The starting material may be, for example, cDNA or genomic DNA encoding wild-type OPG. cDNA is obtained from libraries prepared from mRNA isolated from various tissues expressing Osteoprotegerin. In humans, tissue sources for Osteoprotegerin include kidney, liver, placenta and heart. Genomic DNA encoding Osteoprotegerin is obtained from genomic libraries which are commercially available from a variety of species. 
     Alternatively, the nucleic acids of the invention may be synthetic nucleic acids. Synthetic DNA, for example, is obtained by chemical synthesis of overlapping oligonucleotide fragments followed by assembly of the fragments to reconstitute part or all of the coding region and flanking sequences (see U.S. Pat. No. 4,695,623 describing the chemical synthesis of interferon genes). RNA is obtained most easily by procaryotic expression vectors or by in vitro constructs which direct high-level synthesis of mRNA, such as vectors using T7 promoters and RNA polymerase. 
     Expression vectors containing nucleic acid sequences encoding OVPs, host cells transformed with said vectors and methods for the production of OVPs are also provided by the invention. An overview of expression of recombinant proteins is found in Methods of Enzymology Vol. 185 (Goeddel, D. V. ed.) Academic Press (1990). 
     Host cells for the production of OVPs include procaryotic host cells, such as  E. coli , yeast, plant, fungal, insect and mammalian host cells.  E. coli  strains such as HB101 or JM101 are suitable for expression. Preferred mammalian host cells include COS, CHOd-, 293, CV-1, 3T3, baby hamster kidney (BHK) cells and others. Mammalian host cells are preferred when post-translational modifications, such as glycosylation and polypeptide processing, are important for OVP activity. Mammalian expression allows for the production of secreted polypeptides which may be recovered from the growth medium. 
     Vectors for the expression of OVPs preferably contain at a minimum sequences required for vector propagation and for expression of the cloned insert. These sequences include a replication origin, selection marker, promoter, ribosome binding site, enhancer sequences, RNA splice sites and transcription termination site. Vectors suitable for expression in the aforementioned host cells are readily available and the nucleic acids of the invention are inserted into the vectors using standard recombinant DNA techniques. Vectors for tissue-specific expression of Osteoprotegerin are also included. Such vectors include promoters which function specifically in liver, kidney or other organs for production in mice, and viral vectors for the expression of Osteoprotegerin in targeted human cells. 
     Using an appropriate host-vector system, an OVP may be produced recombinantly by culturing a host cell transformed with an expression vector containing nucleic acid sequences encoding the OVP under conditions such that the OVP is produced, and isolating the product of expression. The OVP may be produced in the supernatant of transfected mammalian cells or in inclusion bodies of transformed bacterial host cells. OVP so produced may be purified by procedures known to one skilled in the art as described below. The expression of OVP in  E. coli  is described in Example 2. The specific plasmids and host cells described in this Example are for illustrative purpose only, however, and it will be appreciated that other available plasmids and host cells could also be used to express OVPs of the invention. 
     Pharmaceutical Compositions 
     The present invention provides a composition for inhibiting or reducing binding of RANK to RANKL, the composition comprising an OPG variant protein according to the invention, and one or more acceptable carriers. 
     The present invention also provides a composition for inhibiting or reducing a biological effect mediated through the interaction of RANK and RANKL, the composition comprising an OPG variant protein according to the invention, and one or more acceptable carriers. 
     The present invention also provides a composition for inhibiting or reducing stimulation of osteoclasts by RANKL, the composition comprising an OPG variant protein according to the invention, and one or more acceptable carriers. 
     The present invention also provides a pharmaceutical composition comprising a therapeutically effective amount of an OPG variant protein according to the invention and one or more of a pharmaceutically acceptable carrier, solubilizer, stabilizer and anti-oxidant. 
     The term “therapeutically effective amount” means an amount which provides a therapeutic effect for a specified condition and route of administration. The composition may be in a liquid or lyophilized form and comprises a diluent (Tris, acetate or phosphate buffers) having various pH values and ionic strengths, solubilizer such as Tween or Polysorbate, carriers such as human serum albumin or gelatin, preservatives such as thimerosal or benzyl alcohol, and antioxidants such as ascorbic acid or sodium metabisulfite. Also encompassed is a composition comprising an OVP modified with water soluble polymers to increase solubility or stability. Compositions may also comprise incorporation of OPVs into liposomes, microemulsions, micelles or vesicles for controlled delivery over an extended period of time. Selection of a particular composition will depend upon a number of factors, including the condition being treated, the route of administration and the pharmacokinetic parameters desired. A more extensive survey of component suitable for pharmaceutical compositions is found in Remington&#39;s Pharmaceutical Sciences, 18th ed. A. R. Gennaro, ed. Mack, Easton, Pa. (1980). 
     Compositions of the invention may be administered by injection, either subcutaneous, intravenous or intramuscular, or by oral, nasal, pulmonary or rectal administration. The route of administration eventually chosen will depend upon a number of factors and may be ascertained by one skilled in the art. 
     The invention also provides for pharmaceutical compositions comprising a therapeutically effective amount of the nucleic acids of the invention together with a pharmaceutically acceptable adjuvant. Nucleic acid compositions will be suitable for the delivery of part or all of the OVP to cells and tissues as part of a gene therapy regimen. 
     By “gene therapy” herein is meant the one time or repeated administration of a therapeutically effective DNA, mRNA, or other nucleic acid. In one embodiment, genes are introduced into cells in order to achieve in vivo synthesis of a therapeutically effective genetic product. 
     Methods of Treatment 
     OPG has undergone preclinical and clinical testing with potential for application in various conditions associated with increased bone turnover and bone loss, including osteoporosis, rheumatoid arthritis, Paget&#39;s disease, periodontal disease, vascular disease and cancers that are located in or have metastasised to bone (For review, see Lorenz et al., J. Amer Med Assoc 292: 490-5 (2004) and references therein). 
     It is also known that antagonizing RANK signaling could be a treatment for autoimmune disorders such as systemic lupus erythematosus, inflammatory bowel disease, diabetes, multiple sclerosis, rheumatoid arthritis, and ankylosing spondylitis. 
     Recent reports also indicate that OPG may also be used in the treatment or prevention of cardiovascular disease, to prevent or reduce calcification of blood vessels and valves (Kaden et al., J. Mol. Cardiol. 36: 17-19 (2004) and for encouraging blood vessel growth (Cross et al; Int J Cancer Nov. 14, 2005 (epub)). Polymorphisms in the OPG gene have been associated with the occurrence of Juvenile Pagets disease (Daroszewska et al., J. Bone Miner. Res 19: 1506-11 (2004)) and with Caucasian men at high risk of developing coronary artery disease (Soufi et al., J. Clin. Endocrinol. Metab. 89: 3764-8 (2004)). 
     It is clear, therefore, that the OPG variant proteins of the present invention will have a range of therapeutic uses. 
     Accordingly, the present invention provides a method for inhibiting or reducing binding of RANK to RANKL, the method comprising exposing a RANKL molecule to an OPG variant protein according to the present invention. 
     The present invention also provides a method for inhibiting or reducing a biological effect mediated through the interaction of RANK and RANKL in a cell, the method comprising exposing the cell to an OPG variant protein according to the present invention. 
     The present invention also provides a method for inhibiting or reducing a biological effect mediated through the interaction of RANK and RANKL in a subject, the method comprising administering to a subject an OPG variant protein according to the invention. 
     The present invention also provides a method for treatment or prevention of an autoimmune or immune-mediated inflammatory disease in a subject, the method comprising administering to a subject an OPG variant protein according to the invention. 
     The present invention also provides a method for treatment or prevention of cardiovascular disease in a subject, the method comprising administering to a subject an OPG variant protein according to the invention. 
     The present invention also provides a method for inhibiting or reducing differentiation, activation and/or survival of osteoclasts by RANKL, the method comprising exposing tissue containing osteoclasts or osteoclast precursors to an OPG variant protein according to the invention. 
     The present invention also provides a method for inhibiting or reducing differentiation, activation and/or survival of osteoclasts by RANKL in a subject, the method comprising administering to a subject an OPG variant protein according to the invention. 
     By “subject” herein is meant both humans and other animals, particularly mammals as outlined herein, with humans being preferred. 
     The present invention also provides method for preventing or treating a bone disorder comprising administering to a patient the pharmaceutical composition of the present invention. 
     The terms “treating” and “treatment” herein are meant to include therapeutic treatment, as well as prophylactic, or suppressive measures for the disease or disorder. Thus, for example, successful administration of an OPG variant protein prior to onset of the disease may result in treatment of the disease. As another example, successful administration of an OPG variant protein after clinical manifestation of the disease to combat the symptoms of the disease comprises “treatment” of the disease. “Treatment” also encompasses administration of an OPG variant protein after the appearance of the disease in order to eradicate the disease. Successful administration of an agent after onset and after clinical symptoms have developed, with possible abatement of clinical symptoms and perhaps amelioration of the disease, further comprises “treatment” of the disease. 
     Conditions which are treatable with OVPs of the present invention include the following: 
     Osteoporosis, such as primary osteoporosis, endocrine osteoporosis (hyperthyroidism, hyperparathyroidism, Cushing&#39;s syndrome, and acromegaly), hereditary and congenital forms of osteoporosis (osteogenesis imperfecta, homocystinuria, Menkes&#39; syndrome, and Riley-Day syndrome) and osteoporosis due to immobilization of extremities. 
     Paget&#39;s disease of bone (osteitis deformans) in adults and juveniles. 
     Osteomyelitis, or an infectious lesion in bone, leading to bone loss. 
     Hypercalcemia resulting from solid tumors (breast, lung and kidney) and hematologic malignancies (multiple myeloma, lymphoma and leukemia); idiopathic hypercalcemia, and hypercalcemia associated with hyperthyroidism and renal function disorders. 
     Osteopenia following surgery, induced by steroid administration, and associated with disorders of the small and large intestine and with chronic hepatic and renal diseases. 
     Osteonecrosis, or bone cell death, associated with traumatic injury or non-traumatic necrosis associated with Gaucher&#39;s disease, sickle cell anaemia, systemic lupus erythematosus and other conditions. 
     Bone loss due to rheumatoid arthritis, ankylosing spondylitis and other immune-mediated inflammatory disorders. 
     Calcification of the vascular system. 
     Periodontal bone loss. 
     Cancers that affect bone including multiple myeloma, primary cancer of the bone and osteolytic metastasis such as from metastasised breast and prostate cancer. 
     Cardiovascular disease. 
     Coronary artery disease. 
     Autoimmune disorders such as systemic lupus erythematosus, inflammatory bowel disease, diabetes, multiple sclerosis, rheumatoid arthritis, and ankylosing spondylitis. 
     It will be appreciated that an OVP of the invention may be used alone or in conjunction with other therapeutic agents. For example, an OVP of the present invention may be used in conjunction with a therapeutically effective amount of a TRAIL activator antibody or an anti-TNF antibody. 
     In one preferred embodiment the OPG variant protein of the present invention is used to treat a bone disorder that is associated with excessive bone turnover or loss. The bone disorder may include, but is not limited to, osteoporosis, Paget&#39;s disease of the bone, hypercalcemia, hyperpararthyroidism, steroid-induced osteopenia, rheumatoid arthritis, osteomyelitis, multiple myeloma, bone cancer, vascular calcification, osteolytic metastasis, and periodontal bone loss. 
     It is understood that the OVP of the invention may be used alone or in conjunction with other factors for the treatment of bone disorders. 
     In one embodiment, the OVP is used in conjunction with a therapeutically effective amount of a factor which stimulates bone formation. Such factors include but are not limited to the bone morphogenic factors designated BMP-1 through BMP-12, transforming growth factor-β. (TGF-β) and TGF-β family members, interleukin-1 inhibitors, TNFα inhibitors, parathyroid hormone and analogs thereof, parathyroid related protein and analogs thereof, Vitamin E series prostaglandins, bisphosphonates (such as alendronate and others), and bone-enhancing minerals such as fluoride and calcium. 
     The invention will now be described in more detail by reference to the following non-limiting Examples. 
     EXAMPLE 1 
     Cloning the Human OPG Gene 
     The gene for OPG was isolated from a human fetal kidney cDNA library (Spring Bioscience, Fremont, Calif.). Primers to the 5′ and 3′ ends of the OPG sequence were designed and standard PCR used to amplify the gene. The primers used to isolate the OPG gene were: 
     
       
         
           
               
            
               
                 Forward: 
               
            
           
           
               
            
               
                 (SEQ ID NO: 48) 
               
            
           
           
               
            
               
                 5′TATTACTCGCGGCCCAGCCGGCCATGAACAAGTTGCTGTGCTGCGCGC 
               
               
                   
               
               
                 TCG 3′ 
               
               
                   
               
               
                 Reverse: 
               
            
           
           
               
            
               
                 (SEQ ID NO: 49) 
               
            
           
           
               
            
               
                 5′CATCTTTATAATCTGCGGCCGCTAAGCAGCTTATTTTTACTGATTGGA 
               
               
                   
               
               
                 CC 3′. 
               
            
           
         
       
     
     Overhangs in the primers incorporated restriction sites allowing cloning into suitable vectors for propagation in  E. coli  such as pGC (Coia et al., (1996),  J. Immunol. Meth.  192:13-23) with successful cloning confirmed by DNA sequencing. In addition to the 1203 nucleotides encoding the full-length 401 as OPG protein, the OPG sequence excluding its signal peptide (aa 1-21) was amplified (OPG 22-401). The N-terminal half of the protein to aa 194, comprising the 4 Cys-rich domains, with and without its signal peptide, was also amplified. A schematic representation of the OPG gene structure is shown in  FIG. 1A . 
     The fragment OPG 22-194, which encompasses the part of the protein responsible for binding to RANKL and TRAIL (Simonet et al., (1997), Yamaguchi et al., (1998) as referenced above) was expressed in 2 L of 2×YT/100 μg/mL ampicillin with IPTG induction out of the pGC vector. This produces the protein with a C-terminal double FLAG peptide tag fusion for detection and affinity purification from  E. coli  periplasmic extracts. Induction was carried out for 3-4 h at 20-22° C. Affinity purified material was further peak purified by gel filtration chromatography on a Superdex 200 column. Peaks corresponding in size to an OPG monomer were collected. A smaller peak, corresponding in size to an OPG dimer was also purified. The purified peaks could be detected by Western blot using an anti-FLAG antibody migrating at approximately the expected size. The purified monomeric protein was detected by specific antibodies supplied with an OPG detection kit (R&amp;D systems, Minneapolis, Minn.). The identity of this protein was confirmed by N-terminal amino acid sequencing of its first 19 residues that correlate with the published sequence. 
     EXAMPLE 2 
     Generation of OPG Variant Proteins (OVPs) 
     To create a library of OPG variants, the OPG(22-194) DNA sequence was cloned into the pEGX mutagenesis and display vector ( FIG. 1B ) with confirmation by DNA sequencing. The OPG-containing plasmid (˜0.5-1.0 μg) was linearised by SmaI digestion and 100-200 ng of this was used directly as template for T7 RNA polymerase transcription at 37° C. for 16-24 h. The success of the transcription was verified by agarose gel electrophoresis loading 1 μL, of the 20 μL reaction. Where possible, all manipulations involving RNA, including ribosome display, use DEPC-treated buffers, reagents, and tubes to minimise RNA degradation. 
     The RNA transcript is used directly as template for a replication reaction by Qβreplicase at 37° C. for 16-24 h: 
     
       
         
           
               
               
             
               
                   
                   
               
               
                   
                 volume (μL) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 T7 RNA transcript 
                   0.5 
               
               
                   
                 25 mM rNTPs 
                 1 
               
               
                   
                 5 X T7 polymerase buffer 
                 4 
               
               
                   
                 100 mM DTT 
                 2 
               
               
                   
                 100 mM MgSO 4   
                   0-1.6 
               
               
                   
                 Qβreplicase 
                 1 
               
               
                   
                 H 2 0 
                 10.5-8.9 
               
               
                   
                   
               
            
           
         
       
     
     The success of the replication was assessed by agarose gel electrophoresis loading 1 μL of the reaction. This process generates a double stranded RNA library encoding variants of OPG. Mutation conditions were adjusted to obtain 1-3 random mutations per gene copy. At this stage 1 μg of the OPG RNA library being prepared for display is equivalent to approximately 7×10 11  molecules. The concentration of the sample is then estimated spectrophotometrically. 
     The OPG RNA library was then used for ribosome display. The library was first translated for 30 min at 30° C. using a rabbit reticulocyte lysate translation system (Promega): 
     
       
         
           
               
               
             
               
                   
                   
               
               
                   
                 volume (μL) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 Reticulocyte lysate 
                 33 
               
               
                   
                 Amino acid mix minus Leu 
                 0.5 
               
               
                   
                 Amino acid mix minus Met 
                 0.5 
               
               
                   
                 25 mM MgCl 2   
                 0.5 
               
               
                   
                 RNasin 
                 0.5 
               
               
                   
                 KCl 
                 1.5 
               
            
           
           
               
               
               
               
            
               
                   
                 OPG RNA 
                 1-2 
                 μg 
               
               
                   
                 H 2 O 
                 to 50 
                 μL 
               
               
                   
                   
               
            
           
         
       
     
     The absence of a stop codon at the end of the C L  spacer domain stalls the ribosome at the end of translation leading to an RNA/ribosome/protein complex. On completion of translation these complexes were stabilised by keeping cold (on ice) and with the addition of magnesium acetate (MgOAc). The translation was diluted to 300 μL with addition of 60 μL ice-cold PBS/50 mM MgOAc/10% skim milk (biotin-free) and 190 μL PBS/50 mM MgOAc/0.05% Tween/2.5 mg/mL heparin. This mix was then split into 3×100 μL aliquots. 
     EXAMPLE 3 
     Panning OVPs 
     The complexes generated in Example 2 were panned against RANKL (Roche) and TRAIL (Peprotech) in solution. Biotinylated versions of RANKL and TRAIL were prepared using Sulfo-NHS-LC-biotin (Pierce) to enable capture of selected complexes in solution using streptavidin coated magnetic beads (Dynal). 
     A panning reaction was carried out in each of the three translation mix aliquots. Two different strategies were employed to create a selection pressure favouring the isolation of OVP sequences that have amino acid changes in residues that reduce binding affinity to TRAIL, whilst retaining RANKL-binding affinity. One strategy used was to first pan against soluble RANKL, and then TRAIL binders were competed off with an excess of free TRAIL protein. In one translation aliquot, biotinyl-RANKL was added at a concentration that is approximately equimolar to the amount of input RNA. This mix was then rotated for 40-60 min at 4° C. Streptavidin coated magnetic beads (Dynal) were washed and resuspended in PBS/50 mM MgOAc/0.05% Tween. After the RANKL pan, 10 μL of these beads were added to the mix for 10 min at 4° C. The beads that captured biotinyl-RANKL in complex with ribosome-displayed OPG were then washed using a magnet, twice with PBS/50 mM MgOAc/0.05% Tween, twice with PBS/50 mM MgOAc, then resuspended in 100 μL of this buffer. An excess of soluble TRAIL was added to this mix at 2 or more times the molar concentration of the added RANKL to compete off TRAIL binders and this was rotated at 4° C. for 30 min. Any complexes eluted from the beads during this procedure were then washed away as above and the beads with bound RANKL-binding complexes resuspended in 20 μL of H 2 O. This panning strategy was denoted “RANKL-1 st ”. 
     The other strategy used was to first pan against soluble TRAIL to remove the stronger TRAIL binders, and then pan the mix against RANKL to isolate the RANKL binders remaining. To a second translation aliquot, biotinyl-TRAIL was added at approximately ½ to ¼ of the molar amount of input RNA. This mix was rotated for 40-60 min at 4° C., then 10 μL of streptavidin beads added for 10 min. The beads that captured biotinyl-TRAIL in complex with ribosome-displayed OVPs were removed using a magnet, thereby ridding the mix of the better TRAIL binders. Biotinyl-RANKL was then added to the mix at approximately ½ the molar amount of input RNA and this was rotated at 4° C. for 30-40 min for selection of OPG variants that retain RANKL-binding affinity. The beads were then washed as above and resuspended in H 2 O. This panning strategy was denoted “TRAIL-1 st ”. A control pan was carried out in a third translation aliquot as performed for the RANKL-1 st  pan but biotinyl-IGF-I (Gropep) was used in place of biotinyl-RANKL. A variation of the RANKL-1 st  and TRAIL-1 st  selection strategies described above was also performed where the antigens used were coupled to epoxy magnetic beads (Dynal) instead of using the antigens in soluble biotinylated form. 
     EXAMPLE 4 
     Cloning of selected OVPs 
     RNA selected with the complexes was converted to cDNA using RT-PCR by standard methods using 5 μL of the bead suspension as template. The RT-PCR products were isolated by agarose electrophoresis. As well as bands for the TRAIL-1 st  and RANKL-1 st  pans, a weak band can be seen generated from the IGF-I pan, suggesting that there is some non-specific selection inherent within this selection procedure. The pool of selected DNAs were then purified from the gel and restriction digested and ligated into the expression vector pGC so that screening for the desired binding properties could proceed. The ligations were electroporated into an appropriate  E. coli  strain to produce transformant colonies. 
     EXAMPLE 5 
     Screening OVP Clones 
     About 100 clones from each display and selection experiment were screened. Clones carrying the genes for OVPs are grown as starter cultures in 200 μL of 2×YT supplemented with ampicillin and glucose in wells of microtitre plates and incubated overnight at 37° C. These cultures are used as inoculum for expression cultures in 200 μL of 2×YT supplemented with ampicillin. When the optical densities at 600 nm of the cultures reach approximately 1.0, the expression of variants out of pGC is induced with addition of IPTG to 0.5-1 mM and incubation for 16 h at 20-22° C. The culture supernatants are then harvested for ELISA binding to RANKL and TRAIL coated overnight on well surfaces of microtitre plates. The surfaces are blocked with 0.5% casein for 2 h. Supernatants are filtered using microtitre plates fitted with 100 kD molecular weight cut-off (MWCO) membranes to remove higher molecular weight species and aggregates which helps reduce background signal in the ELISA. These filtered supernatants are applied to the antigen-coated and blocked wells. Detection of bound OVPs is achieved using mouse anti-FLAG antibody, then goat anti-mouse-peroxidase conjugate (BioRad). TMB (KPL) is used as peroxidase substrate with the binding signal measured by reading absorbance at 450 nm. Washes in between steps are done 3 times with PBS/0.05% Tween, then 3 times with PBS and dilutions are made in 0.25% casein. This first crude screen was assessed, with the most promising clones (based on RANKL-binding signal being retained at or close to the level of wild type OPG and TRAIL-binding signal being substantially reduced) selected for a secondary screen. 
     The secondary screen involves expressing selected clones in 10 ml cultures with an induction period of 3 h at 20-22° C. The periplasmic extracts from these cells are harvested, filtered as above, and then tested at a range of dilutions (generally from 1:2 to 1:32) by ELISA as above. The best of these proteins were chosen for large-scale expression and purification (0.5-2 L) as described above for OPG wild type. The purified proteins are quantitated by determination of extinction coefficients due to Trp and Tyr residues at A 280nm . The OVPs were then tested at equivalent concentrations in comparison to OPG wild type protein by ELISA as described above. The proteins were tested for binding in a doubling dilution series generally from a concentration of 0.5 μM to 0.031 μM. Most of the proteins which are in monomer format show retention of RANKL-binding at or close to that of OPG wild type, and substantial reduction in TRAIL-binding signal. An example of results from such an ELISA is shown in  FIG. 2 . Variants with the required profile of binding properties were produced from all selection strategies. 
     EXAMPLE 6 
     Repeat Ribosome Display and Selection Processes 
     Subsequent rounds of ribosome display and selection were undertaken to further improve some of the best candidate OVPs. To achieve this, genes for these OVPs were cloned into the pEGX vector and mutagenesis and display carried out as described above with the molar amounts of the soluble antigens used in selection altered to force selection pressure towards further reduction of TRAIL binding and maintenance of RANKL binding. Screening for the best variants was undertaken as described above with further mutants isolated with apparent improvements in the desired properties. The sequences of the best ribosome display-selected OVPs from all rounds of selection are shown in  FIG. 3 . Mutations at aa 115 within the Cys107-Cys118 loop were observed in many of the selected variants, as were several mutations in amino acids nearby in the area encompassing aas 102-130. This suggests that this region, and aa 115 in particular, is important for TRAIL binding and that some changes can be tolerated here without loss of RANKL binding. 
     EXAMPLE 7 
     Site Directed Mutagenesis 
     The locations of mutations pinpointed by ribosome display selection shown to be beneficial for altering the specificity of OPG were investigated further by employing site-directed mutagenesis at these positions. Additional amino acid changes were sampled at residues 115, 122, and 128 and the L40S mutation was combined with some of the mutations at these positions. Forward and reverse primers were designed that encoded the particular mutant residue, allowing PCR amplification of front and back fragments of the mutant OVP gene with complementary overhangs regions. PCR was used to join the two DNA segments together by priming off each other for an initial 10 cycles without primers, and then 30 cycles with flanking 5′ and 3′ primers for amplification of the full mutant gene with restriction sites for cloning. 
     A mammalian expression system was employed to allow for production of higher levels of the more biologically relevant stable dimeric form of the proteins for further ELISA and cell-based assays. The site-directed mutants and the best ribosome display-selected candidate OVP(1-194) genes (residues 1-21 constitute the signal peptide) were cloned into the mammalian expression vector pAPEX-3.Fc. This allows expression of the OVPs as fusions with a human Fc domain to form stable dimers. A C-terminal FLAG tag is also added. HEK 293 EBNA cells are used for expression. These adherent cells are passaged in DMEM media supplemented with 2 mM Glutamax (Invitrogen), 10% bovine calf serum, and 200 μg/mL geneticin and incubated at 37° C. in 5% CO 2 . For transient transfection using lipofectamine (Invitrogen), near-confluent cells are harvested and 5×10 6  cells are used to seed a 10 cm tissue culture dish to a volume of 10 mL. The cells are allowed to adhere and grow for 24 h. 24 mg of pAPEX-3.Fc plasmid DNA harbouring the OVP of interest is mixed with 1.5 mL of DMEM. 60 ul lipofectamine is diluted in 1.5 mL DMEM and incubated for 5 mins at room temperature. The diluted DNA and lipofectamine are mixed, incubated for 20 min at room temperature, and then added dropwise to the culture dish. After overnight growth total cells from one dish are harvested and used to seed a T175 dish in 25-30 mL. Once cells have reached near-confluence (1-2 days) the serum-containing DMEM is replaced with 25-30 mL of Ex-cell 293T serum-free media (JRH Biosciences) supplemented with 4 mM Glutamax. After 3-4 days the media is collected, centrifuged, and filtered through a 0.22 μm syringe filter. 
     For purification of proteins secreted into the media the supernatants are applied to 1 mL of protein A (Sigma) resin equilibrated with PBS. Proteins bound to the protein A via their Fc portion are eluted with 0.1 M Gly, pH 3 and this is monitored at A 280nm . The eluate is neutralized by immediate dialysis against PBS. Dialysed protein is concentrated to 0.5 mL and subjected to gel filtration chromatography using a Superdex 200 column. The dominant peak corresponding to the OVP-Fc dimer is collected and identity confirmed by Western blot using anti-FLAG antibody and N-terminal sequencing which reveals OPG with signal peptide having been processed. Ion-exchange chromatography of gel filtration-purified OVP elutes one protein peak, suggesting homogeneity. 
     EXAMPLE 8 
     Screening Site-Directed OVP Mutants 
     Purified mammalian-expressed OVP proteins are tested for binding to RANKL and TRAIL by ELISA. Wells are coated with RANKL at 5 μg/mL or TRAIL at 10 μg/mL in PBS O/N at 4° C., rinsed with PBS, blocked with 0.5% casein in PBS at room temp for 2 h, then rinsed again with PBS. Concentrations of OVPs to be tested are normalised to around 10 nM and then 50 μL applied to the antigen-coated wells in serial doubling dilutions over 6-7 wells for 1 h. All dilutions are made in 0.25% casein. All washes in between binding incubations are 3 times in PBS/0.05% Tween, then 3 times with PBS. Detection of bound OVPs is achieved with incubation of 50 μL anti-FLAG at 1.2 mg/mL for 20 min then 50 μl goat anti-mouse-peroxidase conjugate (BioRad) at 1:1500 for 20 min. TMB is used as enzyme substrate and then binding signal is measured at A 450nm . All OVPs or OPG wt are tested in the C-terminal Fc-linked format. An example of results from such an ELISA comparing binding of several OVPs to the OPG(22-194) wild type is shown in  FIGS. 4A and 4B . The data shows that OPG(22-194) wt binds to both RANKL and TRAIL and that most OVPs maintain RANKL binding at or close to the level of the wild type protein. All the OVPs tested here show at least some reduction of binding to TRAIL compared to wt, with the ribosome display selected 1R17 (I115M), and site-directed mutants at position 115-115D, 115G, and 115R, showing very low levels of TRAIL binding. To enable approximate comparisons of relative binding of OVPs in ELISA, EC 50  values from the data are summarised in Table 1. EC 50  values were estimated by plotting binding responses against OVP dose on a logarithmic scale, with extrapolations made when the data does not reach the EC50 range. Some OVPs showed slight increase in binding to RANKL while in some RANKL binding was reduced by up to 3 fold. 
     EXAMPLE 9 
     Biological Testing of OVPs in an Assay of TRAIL-Induced Apoptosis of HCT116 Cells 
     The reduction of TRAIL-binding activity of OVPs observed in ELISA is further tested in an assay of TRAIL-induced apoptosis of HCT116 cells in the presence of these OVPs. These cells are cultured in DMEM containing 10% foetal calf serum (FCS) and 1% penicillin-streptomycin at 37° C. in 5% CO 2 . HCT116 cells are plated in 12 well plates at 7.5×10 4  cells/well in 1 ml DMEM+FCS and allowed to attach and grow for 24 hours. Growth medium is then removed and replaced with 500 μL fresh medium for 1 hour at 37° C. OVPs or recombinant human OPG.Fc (R&amp;D Systems) are added at 0.56, 1.69, 5.6 and 11.2 nM and the cells incubated a further 15 minutes at 37° C. before the addition of recombinant human TRAIL (R&amp;D Systems) to 10 ng/ml (0.53 nM). The cells are then incubated for 16 hours at 37° C. before being harvested to score the percentage of apoptotic cells in each well. The maximum dose of OVP was tested in the absence of TRAIL for toxicity to cells. All cells (attached and floating) are harvested and resuspended in 20 μL of 1 mg/ml propidium iodide (PI), 0.1% Triton X-100, 0.1% sodium acetate in PBS. Apoptosis is scored by assessing nuclear morphology by fluorescence microscopy following PI staining with a cell scored as apoptotic if it displays one or more of the following: nuclear margination, chromatin condensation, or formation of apoptotic bodies. To avoid bias, samples are scored without knowing the treatment given to the sample. Examples of such an assay are shown in  FIGS. 5A and 5B . The effect of wild type OPG proteins is shown in  FIG. 5A  where the proportion of cells exhibiting TRAIL-induced apoptosis is reduced from greater about 70% to under 10% at an OPG dose of 5.6 nM. The OPG(22-194).Fc appears to be similarly effective to the full-length OPG.Fc (R&amp;D systems) molecule while an N-terminally linked-Fc version of the wild type protein is a little less effective. All the OVPs shown in  FIG. 5B  have little or no ability to inhibit TRAIL-induced apoptosis. Even at a dose four-fold higher than shown here there appears to be negligible TRAIL blocking activity with these selected OVPs. 
     EXAMPLE 10 
     Biological Testing of OVPs in an Assay of RANKL-Mediated Differentiation of RAW264.7 Cells 
     The murine leukaemia virus-induced tumour monocyte cell line RAW264.7 differentiates into osteoclasts under RANKL stimulation. To further test the ability of the OVPs to bind RANKL and block its biological effects mediated through binding to RANK, the RANKL-mediated differentiation of RAW264.7 cells in the presence of OVPs was assayed. RAW264.7 cells are cultured in DMEM containing 5% FCS and 1% penicillin-streptomycin at 37° C. in 5% CO2. RAW264.7 cells (3×10 3  cells/well) are seeded into 96-well plates, in 100 μL DMEM and 5% FCS, and allowed to grow for 24 hours. An additional 100 μL of medium containing 15 ng/mL (0.75 nM) human RANKL (Roche) and OVPs from 0 to 0.5 nM is added to each well and the cells incubated a further 3 days. Osteoclast formation is evaluated by measuring tartrate-resistant acid phosphatase (TRAP) activity. The cells are fixed, dried, and 100 μL 50 mM citrate buffer pH 4.6, 10 mM tartrate, 1 mg/mL p-nitrophenylphosphate is added to measure TRAP activity. After incubation for 30 min, the reactions are stopped and absorbance measured at 405 nm. Such an assay is shown in  FIG. 6A . At a dose of 0.4 nM, all of the OVPs shown are able to totally block the osteoclastic differentiation of RAW264.7 cells induced by 0.75 nM RANKL.  FIG. 6B  also shows such an assay involving OVPs generated by site-directed mutagenesis at aa115. Over this narrow range of low doses we can see a discrimination of RANKL-binding activities despite these proteins all showing substantial loss of the ability to inhibit TRAIL-induced apoptosis ( FIG. 5B ). It should be noted that some variants showed activity in this assay equivalent to or better than that of OPG. On the other hand, some variants that showed a relatively modest reduction in binding to RANKL (Table 1) showed significantly decreased ability to block RANKL mediated osteoclast activation. 
     EXAMPLE 11 
     Biological Testing of OVPs in a Giant Cell Tumour Osteoclast Resorption Assay 
     A giant cell tumour osteoclast resorption assay was also used as a means of measuring the biological effect of OVPs binding to RANKL. This is an assay (Atkins et al., Bone 28:370-37 (2001)) for RANKL-mediated osteoclastogenesis by measuring the level of resorption pit formation in dentine slices. The assay can be used to show the OPG-mediated inhibition of this effect. Both the number of resorption pits and area of resorption can be quantitated. An example of this assay testing several OVPs is shown in  FIG. 7 . All constructs showed inhibition, with the OPG wt at about 55% inhibition and the best OVP, R23 (I115M, L40S), showing 75% inhibition. 
     EXAMPLE 12 
     Conclusions from Biological Testing of OVPs 
     The sequences of selected OVPs isolated by ribosome and display selection are shown in  FIG. 8 . Results of binding studies conducted on these OVPs and additional OVPs generated by site directed mutagenesis are shown in Table 1. 
     It is clear from the data shown in Table 1 that substitutions at residue position 115 have a major impact on the ability of the OVP to bind to TRAIL. In particular, substitution of residues such as Met, Gly, Asn, Asp and Glu at this position lead to a substantial reduction in binding to TRAIL without significantly affecting binding to RANKL. Further, none of the OVPs generated with these substitutions showed any activity in an assay of TRAIL-induced apoptosis of tumour cells. 
     Mutations at residue 122 also have an impact on the ability of OVPs to bind to TRAIL. For example, double mutants 1I115M, R122N and I115M, R122E showed substantially reduced binding affinity for TRAIL. 
     Mutations at residue 128 also have an impact on the ability of OVPs to bind to TRAIL. For example, double mutants 1I115M, F128L and I115M, F1281 showed substantially reduced binding affinity for TRAIL. 
     The double mutant I115M, L40S resulted in a 75% inhibition of RANKL-mediated osteoclasogenesis which highlights the importance of mutations at residue 40. 
     A summary of biological activity of selected OVPs in cell-based assays is provided in Table 2. 
     EXAMPLE 13 
     OPG/OPV Three Dimensional Structure Modelling 
     OPG is classified as a member of the TNF-receptor superfamily of proteins. Members most notably feature multiple repeats of a approximately 40 amino acids, cysteine-rich domains, that are involved in ligand binding. Known ligands include the structurally homologous TRAIL and RANKL proteins. 
     No three dimensional structure has been published for OPG alone or in combination with either of its ligands RANKL or TRAIL. A structural model was therefore established of the two cysteine-rich domains in the amino terminal fragment of OPG, in an attempt to examine the likely effect of mutations contained in preferred OVPs. 
     From a BLAST search of the Brookhaven protein structure database (PDB), three close homologues of TNF-receptor related proteins were selected for which three-dimensional structures had been determined, namely polypeptides in pdb files 1 SG1 (Receptor-Ligand Complex between Nerve Growth Factor and the Common Neurotrophin Receptor p75), and Death Receptor 5 structures from the files 1D4V and 1DU3. In addition, a structure from file 1D0G was added. The TNF-receptor superfamily member DR5 (a TRAIL receptor protein) is represented in this structure complexed to a trimer of TRAIL that was exploited for the design of models for the OPG ligand complex. The modelling was performed using the module Modeller within the InsightII software package (Molecular Simulations Inc.). The structure of 1SG1 X chain was furthermore divided into an N-terminal and a C-terminal domain and the two domains were separately aligned with the remaining templates, as both domains had shown homology to OPG. Compared to the other three templates there appeared to be a hinge region in the protein allowing the C-terminal domain to be displaced. Fifty models were generated for the wild type OPG fragment used as the starting point for selection experiments as well as for a number of the selected OVP sequences. The models were ranked according to scores from Modeller, and the top 10 ranked models were aligned to DR5 in the 1D0G structure. The receptor contact regions of the TRAIL component of this structure, was in turn used to model a possible contact structure of receptors on RANKL, using the determined structure of mouse RANKL (pdb file 1JTZ). This process allowed a comparison of modelled OPG when binding TRAIL and RANKL structures. The resulting models of the complex of OPG, in its modelled structure, with TRAIL or RANKL respectively, were examined for steric clashes and potential binding interactions. 
     Our models of the OPG protein suggested that binding of both RANKL and TRAIL could occur with the extensive interface found in similar TNF-receptor-Ligand interactions (Hymowitz et al., Mol. Cell 4:563-571 (1999); He et al., Science 304: 870-875 (2004). As predicted from our modelling, two TNF-receptor like domains of OPG appear to be involved in binding both RANKL and TRAIL (see  FIGS. 9 to 12 ), the N-terminal domain having contacts within the region of residues 42 to 92 and the C-terminal domain within the region of residues 107 to 154. Two subregions were detected which appeared particularly significant—107-118 and 120-142. The N-terminal domain contacts with OPG appear similar between RANKL and TRAIL, therefore mutations in this region would be more likely result in effects on both binding interactions. This is consistent with the results of the selection experiments reported herein, as this region was not one where frequent mutation was found in OVPs selected for reduced binding to TRAIL but not to RANKL. 
     There appears to be a hinge around the disulfide bond between residues 124 to 142, allowing the C-terminal domain to either bend away from the ligand, thereby limiting interactions or bend towards the ligand to form more interactions. There are furthermore differences in conformation between the two ligands examined, TRAIL and RANKL. Residues Glu195 to Asn202 in TRAIL, in particular Asn199, Thr200 and Lys201 protrude as a loop region towards the OPG binding site, potentially contacting OPG within the region of residues Leu119 to Arg143 in OPG, with the most likely contacted residues Lys120, His121, Arg122, Cys124, Pro125, Pro126, Gly127, Phe128, Cys141, Lys142. 
     However, the precise positioning of this region appears to be additionally guided by contacts within the protruding loop N-terminal to the hinge region, which is enclosed between Cys107 and Cys118. Within this loop, Glu116 and Phe117 appear capable of forming sets of favourable interactions in at least two orientations. Mutations in the neighbouring residue Ile115, for example I115G or I115E, appear to bias for one orientation, which could translate to a more open position and thereby weaken the interaction of C-terminal contacts to the 195-202 loop of TRAIL while maintaining the RANKL interaction. Some changes at residue 115, however, were found in these studies to disrupt binding to RANKL as well as to TRAIL. These changes were ones where the substituted amino acid had a larger side chain in combination with a positive or negative charge. This suggests that residue 115 is also involved in contacting RANKL or that unfavourable changes to this residue disrupt important contacts made by neighbouring residues such as 116. 
     Modelling of changes within the region 120-142 suggest that alterations in any one of a number of residues in this region might influence binding to TRAIL specifically, and a number of such changes have been detected amongst the OVPs examined. 
     Within the N-terminal binding region, residues 42-92 of OPG form a broad contact region with both RANKL and TRAIL. Both ligand structures appear similar in this interface. Since this was not a region where mutations leading to loss of TRAIL binding were observed in the work described herein, we conclude that residues in this region may in reality contribute more weakly to TRAIL binding, or that mutations in this region affect binding to TRAIL and RANKL in parallel, so that changes in this area were not selected during our screening strategy. Nonetheless, subtle conformational changes due to mutations in this or a nearby area may possibly alter the properties of the molecule favourably for its RANKL-mediated effects, as it has been suggested above for a conformational shift due to mutation of residue 115. Of particular interest is a change at residue 40 identified among OVPs examined. Since this residue lies adjacent to this contact region, it is likely that changes in nearby residues may subtly influence the shape of this loop, thereby preferentially biasing binding towards RANKL at the expense of TRAIL attachment. It is likely that such subtle shifts in shape may be achieved by mutation in other OPG residues that can affect folding of this domain. 
     All references cited above are incorporated herein in their entirety by reference. 
     Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application. 
     It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 OVP binding studies 
               
            
           
           
               
               
               
               
            
               
                   
                   
                 RANKL fold 
                 TRAIL fold 
               
               
                   
                 Variants 
                 change cf wt 
                 change cf wt 
               
               
                   
                   
               
               
                   
                 I115V 
                 1.5 
                 (4) 
               
               
                   
                 I115L 
                  1.75 
                   (4.3) 
               
               
                   
                 I115M 
                 1.2 
                 (25)  
               
               
                   
                 I115W 
                 (1.4) 
                 (5) 
               
               
                   
                 I115A 
                  2.25 
                   (2.5) 
               
               
                   
                 I115G 
                 2.1 
                 (&gt;125#)  
               
               
                   
                 I115N 
                 (1.1) 
                 (390#)  
               
               
                   
                 I115S 
                 (1.4) 
                   (3.1) 
               
               
                   
                 I115R 
                 (2.3) 
                 (50#) 
               
               
                   
                 I115K 
                 (60)   
                 No binding 
               
               
                   
                 I115Y 
                 1.1 
                 (3) 
               
               
                   
                 I115D 
                 1.5 
                 (&gt;125)   
               
               
                   
                 I115E 
                  (2.25) 
                 (&gt;250)   
               
               
                   
                 N102D 
                 1.1 
                 (1) 
               
               
                   
                 I115M, L40S 
                 1.1 
                 (&gt;250)   
               
               
                   
                 I115M, N139D 
                  (1.75) 
                 (60)  
               
               
                   
                 I115T, K51R 
                  (1.75) 
                   (0.8) 
               
               
                   
                 I115M plus: 
               
               
                   
                 R122G 
                 1   
                  (12.5) 
               
               
                   
                 R122N 
                 (1.1) 
                 (&gt;250)   
               
               
                   
                 R122Q 
                 2.5 
                 (3) 
               
               
                   
                 R122S 
                 1.5 
                   (3.5) 
               
               
                   
                 R122D 
                 (6)   
                 (23)  
               
               
                   
                 R122E 
                 (1.2) 
                 (&gt;250)   
               
               
                   
                 I115M plus: 
               
               
                   
                 F128L 
                 1.2 
                 (250#)  
               
               
                   
                 F128A 
                 1.2 
                 (2) 
               
               
                   
                 F128S 
                 1   
                 (50)  
               
               
                   
                 F128T 
                 (3.3) 
                 (7) 
               
               
                   
                 F128I 
                 (2.1) 
                 (250#)  
               
               
                   
                 I115T, K51R, R111H 
                 1.5 
                   (7.5) 
               
               
                   
                 I115T, K51R, S167G 
                 1.5 
                 (15)  
               
               
                   
                   
               
               
                   
                 # by extrapolation 
               
               
                   
                 increase in binding = lower EC50; 
               
               
                   
                 ( ) reduction in binding − higher EC50 
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Summary of biological activity in cell-based assays 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                 TRAIL- 
                   
                   
               
               
                   
                   
                 induced 
                 RAW 
                 GCT 
               
               
                   
                   
                 apoptosis 
                 cells 
                 resorption 
               
               
                   
                 OPG variant protein 
                 assay* 
                 assay 
                 assay 
               
               
                   
                   
               
               
                   
                 OPG wt 
                 +++++ 
                 +++++ 
                 +++ 
               
               
                   
                 OPG (R&amp;D systems) 
                 +++++ 
                 ++++ 
               
               
                   
                 I115T, K51R 
                 +++ 
                 +++++ 
               
               
                   
                 I115M 
                 ++ 
                 +++ 
                 ++ 
               
               
                   
                 I115M, N139D 
                 ++/+++ 
                 +++++ 
                 ++ 
               
               
                   
                 I115T, K51R, S167G 
                 +++ 
                 +++++ 
               
               
                   
                 I115T, F128V 
                 ++ 
                 +++++ 
               
               
                   
                 I115V 
                 ++++ 
                 +++++ 
                 +++ 
               
               
                   
                 I115M, L40S 
                 ++ 
                 +++++ 
                 ++++ 
               
               
                   
                 I115D 
                 + 
                 ++++ 
               
               
                   
                 I115G 
                 + 
                 +++++ 
               
               
                   
                 I115R 
                 − 
                 ++ 
               
               
                   
                 I115E 
                 − 
                 + 
               
               
                   
                   
               
               
                   
                 *− least ability to block TRAIL-induced apoptosis +++++ greatest ability to block TRAIL-induced apoptosis