Abstract:
Fc fusion proteins of human growth hormone with good biological activities relative to rhGH on a molar basis are disclosed. The hGH-L-vFc fusion protein comprises hGH, a flexible peptide linker of about 20 or fewer amino acids, and a human IgG Fc variant. The Fc variant is of a non-lytic nature and shows minimal undesirable Fc-mediated side effects. A method is also disclosed to make or produce such fusion proteins at high expression levels. Such hGH-L-vFc fusion proteins exhibit extended or prolonged serum half-life and/or good biological activities relative to that of rhGH on a molar basis, leading to improved pharmacokinetics and pharmacodynamics, thus fewer injections will be needed within a period of time.

Description:
BACKGROUND 
       [0001]    Human growth hormone (“hGH”) is produced in and released from the pituitary gland. The 22-kDa peptide hormone participates in much of the regulation of normal human growth and development. This pituitary hormone exhibits a multitude of biological effects, including linear growth (somatogenesis), lactation, activation of macrophages, and insulin-like effects, among others. These biological effects are mediated by the interaction between hGH and specific hGH Receptors (hGHR) on the surface of target cells through a cascade of biochemical events (see, for example, Isaksson et al.,  Annu. Rev. Physiol.  47:483-499, 1985; Friedrichsen et al.,  Mol. Endocrino.  15:136-148, 2001; Herrington et al.,  Trends Endocrinol Metab.  12:252-257, 2001). 
         [0002]    Recombinant human Growth Hormone (rhGH) is used to treat growth failure due to deficiency of endogenous hGH secretion. Both children and adult patients with hGH deficiency have been treated successfully with rhGH. Other treatment applications for conditions that may involve short stature are in patients for Turner syndrome, chronic renal failure, Prader-Willi syndrome, and idiopathic short stature. 
         [0003]    Supplemental hGH is a desirable treatment in cases of growth hormone deficiency for both children and adults. The half-life for the serum clearance of intravenous (i.v.) rhGH in healthy adults is approximately 20 min. The peak serum concentration for subcutaneous (s.c.) rhGH occurs in several hours after injection with an elimination half-life of 3 to 8 h. Treatment with hGH requires s.c. injection three times a week, or once daily to maintain suitable serum levels of hGH. For patients chronically receiving hGH, the frequent injections often result in patient compliance problems. From 1999 to 2004, a sustained-release form of rhGH (Nutropin Depot by Genentech and Alkermes) was in the market, allowing for fewer injections every 2 or 4 weeks instead of daily. The product was withdrawn due to high manufacturing costs. Several versions of PEGylated hGH (see, for example, Cox et al.,  Endocrinol.  148:1590-1597, 2007; Webster et al.,  Xenobiotica.  38: 1340-1351, 2008) have been reported to show longer half-life than rhGH yet often at the expense of bioactivities as a result of the PEGylation of the protein. It is therefore desirable to have a long-acting hGH with high activity that can be produced at a reasonable cost. 
         [0004]    Immunoglobulins of IgG class are among the most abundant proteins in human blood. Their circulation half-lives can reach as long as 21 days. Fusion proteins have been reported to combine the Fc regions of IgG with the domains of another protein, such as various cytokines and soluble receptors (see, for example, Capon et al.,  Nature  337:525-531, 1989; Chamow et al.,  Trends Biotechnol.  14:52-60 (1996); U.S. Pat. Nos. 5,116,964 and 5,541,087). The prototype fusion protein is a homodimeric protein linked through cysteine residues in the hinge region of IgG Fc, resulting in a molecule similar to an IgG molecule without the CH1 domains and light chains. Due to the structural homology, Fc fusion proteins exhibit in vivo pharmacokinetic profile comparable to that of human IgG with a similar isotype. This approach has been applied to several therapeutically important cytokines, such as EPO, and G-CSF and soluble receptors, such as TNF-Rc and IL-5—Rc (see, for example, U.S. Pat. Nos. 5,349,053 and 6,224,867). To extend the circulating half-life of hGH and/or to increase its biological activity, it is desirable to make fusion proteins containing hGH linked to the Fc portion of the human IgG protein as disclosed or described in this invention. 
         [0005]    In most of the reported Fc fusion protein molecules, a hinge region serves as a spacer between the Fc region and the cytokine or soluble receptor at the amino-terminus, allowing these two parts of the molecule to function separately (see, for example, Ashkenazi et al.,  Current Opinion in Immunology,  9:195-200, 1997). It is desirable that a human GH (rHG) fusion protein with an appropriate peptide linker between the hGH and Fc moieties (hGH-L-Fc) may be as good as or more active than rhGH, with in vitro activity similar to, better than, or preferably at least 2-fold as that of rhGH on a molar basis. It is discovered according to this invention that an added peptide linker present between hGH and a human IgG Fc variant may enhance the in vitro biological activity of the hGH-L-Fc molecule in two ways: (1) keeping the Fc region away from the hGHR binding sites on hGH, and (2) keeping one hGH from the other hGH molecule, so both hGH molecules can interact with hGHR on cell surface independently. For the present invention, a flexible peptide linker of about 20 or fewer amino acids in length is preferred. It is also desirable and more preferable to use a peptide linker comprising of two or more of the following amino acids: glycine, serine, alanine, and threonine. 
         [0006]    The Fc region of human immunoglobulins plays a significant role in immune defense for the elimination of pathogens. Effector functions of IgG are mediated by the Fc region through two major mechanisms: (1) binding to the cell surface Fc receptors (Fc γ Rs) can lead to ingestion of pathogens by phagocytosis or lysis by killer cells via the antibody-dependent cellular cytotoxicity (ADCC) pathway, or (2) binding to the C1q part of the first complement component C1 initiates the complement-dependent cytotoxicity (CDC) pathway, resulting in the lysis of pathogens. Among the four human IgG isotypes, IgG1 and IgG3 are effective in binding to Fc γ R. The binding affinity of IgG4 to Fc γ R is an order of magnitude lower than that of IgG1 or IgG3, while binding of IgG2 to Fc γ R is below detection. Human IgG1 and IgG3 are also effective in binding to C1q and activating the complement cascade. Human IgG2 fixes complement poorly, and IgG4 appears quite deficient in the ability to activate the complement cascade (see, for example, Jefferis et al.,  Immunol. Rev.  163:59-76, 1998). For therapeutic use in humans, it is essential that when hGH-L-Fc binds to receptors on the cell surface of target cells, the Fc region of the fusion protein will not mediate undesirable effector functions, leading to the lysis or removal of these cells. Accordingly, the Fc region of hGH-L-Fc must be of a non-lytic nature, i.e. the Fc region must be inert in terms of binding to Fc γ Rs and C1q for the triggering of effector functions. It is clear that none of the naturally occurring IgG isotypes is suitable for use to produce the hGH-L-Fc fusion protein. To obtain a non-lytic Fc, certain amino acids of the natural Fc region have to be mutated for the attenuation of the effector functions. 
         [0007]    By comparing amino acid sequences of human and murine IgG isotypes, a portion of Fc near the N-terminal end of the CH2 domain is implicated to play a role in the binding of IgG Fc to Fc γ Rs. The importance of a motif at positions 234 to 237 has been demonstrated using genetically engineered antibodies (see, for example, Duncan et al.,  Nature  332:563-564, 1988). The numbering of the amino acid residues is according to the EU index as described in Kabat et al. (in  Sequences of Proteins of Immunological Interest,  5 th  Edition, United States Department of Health and Human Services, 1991). Among the four human IgG isotypes, IgG1 and IgG3 bind Fc γ Rs the best and share the sequence Leu234-Leu-Gly-Gly237 (only IgG1 is shown in  FIG. 1 ). In IgG4, which binds Fc γ Rs with a lower affinity, this sequence contains a single amino acid substitution, Phe for Leu at position 234. In IgG2, which does not bind Fc γ Rs, there are two substitutions and a deletion leading to Val234-Ala-Gly237 ( FIG. 1 ). To minimize the binding of Fc to Fc γ R and hence the ADCC activity, Leu235 in IgG4 has been replaced by Ala (see, for example, Hutchins et al.,  Proc. Natl. Acad. Sci. USA  92:11980-11984, 1995). IgG1 has been altered in this motif by replacing Glu233-Leu-Leu235 with Pro233-Val-Ala235, which is the sequence from IgG2. This substitution resulted in an IgG1 variant devoid of Fc γ R-mediated ability to deplete target cells in mice (see, for example, Isaacs et al.,  J. Immunol.  161: 3862-3869, 1998). 
         [0008]    A second portion that appears to be important for both Fc γ R and C1q binding is located near the carboxyl-terminal end of CH2 domain of human IgG (see, for example, Duncan et al.,  Nature  332:738-740, 1988). Among the four human IgG isotypes, there is only one site within this portion that shows substitutions: Ser330 and Ser331 in IgG4 replacing Ala330 and Pro331 present in IgG1, IgG2, and IgG3 ( FIG. 1 ). The presence of Ser330 does not affect the binding to Fc γ R or C1q. The replacement of Pro331 in IgG1 by Ser virtually abolished IgG1 ability to C1q binding, while the replacement of Ser331 by Pro partially restored the complement fixation activity of IgG4 (see, for example, Tao et al.,  J. Exp. Med.  178:661-667, 1993; Xu et al.,  J. Biol. Chem.  269:3469-3474, 1994). 
         [0009]    We discover that at least three Fc variants (vFc) can be designed for the production of hGH-L-vFc fusion proteins ( FIG. 1 ). Human IgG2 Fc does not bind Fc γ R but showed weak complement activity. An Fc γ2  variant with Pro331Ser mutation should have less complement activity than natural Fc γ2  while remain nonbonding to Fc γ R. IgG4 Fc is deficient in activating the complement cascade, and its binding affinity to Fc γ R is about an order of magnitude lower than that of the most active isotype, IgG1. An Fc γ4  variant with Leu235Ala mutation should exhibit minimal effector functions as compared to the natural Fc γ4 . The Fc γ1  variant with Leu234Val, Leu235Ala and Pro331Ser mutations also will exhibit much less effector functions than the natural Fc γ1 . These Fc variants are more suitable for the preparation of the hGH fusion proteins than naturally occurring human IgG Fc. It is possible that other replacements can be introduced for the preparation of a non-lytic Fc without compromising the circulating half-life or causing any undesirable conformational changes. 
         [0010]    There are many advantages with the present invention. The good activity and/or preferably prolonged presence of the hGH-L-vFc fusion protein in the serum can lead to lower dosages as well as less frequent injections. Less fluctuations of the drug in serum concentrations also means improved safety and tolerability. Less frequent injections may result in better patient compliance and quality of life. The hGH-L-vFc fusion protein containing a non-lytic Fc variant will therefore contribute significantly to the long term management of hGH deficiency in patients that may involve short stature and other conditions including Turner syndrome, chronic renal failure, Prader-Willi syndrome, and idiopathic short stature. 
       SUMMARY OF THE INVENTION 
       [0011]    One aspect of the present invention relates to a hGH-L-vFc fusion protein. The hGH-L-vFc fusion protein comprises hGH, a peptide linker, and a human IgG Fc variant. It is preferable that the human IgG Fc variant comprises a hinge, a CH2 domain, and a CH3 domain selected from the group consisting of (A) human IgG1 with Leu234Val, Leu235Ala, and Pro331Ser mutations, (B) human IgG2 with Pro331Ser mutation, and (C) human IgG4 with Ser228Pro and Leu235Ala mutations. It is preferably to use a flexible peptide linker of 20 or fewer amino acids in length which comprises of two or more of the following amino acids: glycine, serine, alanine, and threonine. The IgG Fc variants are preferably of non-lytic nature and contain amino acid mutations as compared to naturally occurring IgG Fc. It is disclosed herein as an aspect of the present invention that the hGH-L-vFc fusion protein exhibits good in vitro biological activity relative to that of rhGH on a molar basis. Further, it is also an embodiment of the invention that the hGH-L-vFc fusion protein exhibits a plasma half-life that is comparable to or equal to, preferably longer than [i.e. prolonged], and more preferably at least 2 times as long as that of rhGH when administered in vivo. 
         [0012]    It is another embodiment of the present invention that the human Ig Fc comprises a hinge, CH2 domain and CH3 domain of human IgG, such as human IgG1, IgG2, and IgG4. The CH2 domain contains amino acid mutations at positions 228, 234, 235, and 331 (defined by the EU numbering system) to attenuate the effector functions of Fc. 
         [0013]    In yet another embodiment of the present invention, a method is disclosed to generate, make or produce such fusion proteins from a mammalian cell line such as a CHO-derived cell line. Growing transfected cell lines under conditions such that the recombinant fusion protein is expressed in its growth medium in excess of 10 μg, preferably 30 μg, per million cells in a 24 hour period. These hGH-L-vFc fusion proteins exhibit good biological activity and extended or prolonged serum half-life without undesirable side effects, leading to improved pharmacokinetics and pharmacodynamics, thus lower dosages and fewer injections would be needed to achieve similar efficacies. Another aspect of the invention involves a recombinant hGH-L-vFc fusion protein which exhibits a plasma half-life that is comparable to or equal to, preferably longer than, and more preferably at least 2 times as long as that of rhGH when administered in vivo. Further, the present invention discloses peptide linkers which contain about 20 or fewer amino acids are present between hGH and the human IgG Fc variant; and the peptide linker comprises two or more amino acids selected from the group consisting of glycine, serine, alanine, and threonine. 
         [0014]    A further aspect of the present invention also involves a method for making a recombinant fusion protein comprising hGH, a flexible peptide linker, and a human IgG Fc variant, which method comprises: (a) generating a CHO-derived cell line; (b) growing the cell line under conditions the recombinant fusion protein is expressed in its growth medium in excess of 10 μg, more preferably 30 μg, per million cells in a 24 hour period; and (c) purifying the expressed protein from step (b), wherein the recombinant fusion protein exhibits a good in vitro biological activity on a molar basis relative to that of rhGH, and/or a preferably prolonged plasma half-life that is comparable to, or equal to, preferably longer than, and more preferably at least 2 times as long as that of rhGH when administered in vivo, or both, wherein the flexible peptide linker containing about 20 or fewer amino acids is present between hGH and the human IgG Fc variant; and the peptide linker comprises two or more amino acids selected from the group consisting of glycine, serine, alanine, and threonine; wherein the human IgG Fc variant comprises a hinge, CH2, and CH3 domains selected from the group consisting of human IgG2 with Pro331Ser mutation, human IgG4 with Ser228Pro and Leu235Ala mutations, and human IgG1 with Leu234Val, Leu235Ala, and Pro331Ser mutations. 
         [0015]    A further aspect of the present invention relates to a CHO-derived cell line producing the hGH-L-vFc fusion protein as disclosed herein. The human IgG-Fc variant comprises a hinge, CH2, and CH3 domains of human IgG selected from the group consisting of IgG1, IgG2, and IgG4. The IgG Fc contains amino acid mutations to attenuate effector functions, and a flexible peptide linker containing about 20 or fewer amino acids is present between hGH and human IgG Fc variant. And the hGH-L-vFc fusion protein exhibits a good in vitro biological activity relative to that of rhGH on a molar basis. Another aspect also involves such a CHO-derived cell line producing the hGH-L-vFc fusion protein wherein the protein exhibits a plasma half-life that is comparable to or equal to, preferably longer than [i.e. prolonged], and more preferably at least 2 times as long as that of rhGH when administered in vivo. 
     
    
     
       BRIEF DESCRIPTIONS OF THE DRAWINGS 
         [0016]      FIG. 1  shows the amino acid sequence alignment from the hinge and CH2 regions of human IgG1, IgG2, IgG4 and their variants. Three portions are compared: amino acid position 228, 234-237, and 330-331. Amino acid mutations of the variants are indicated in bold italics. The EU numbering system is used for the amino acid residues. 
           [0017]      FIG. 2  shows the nucleotide sequence and deduced amino acid sequence of (A) hGH-L-vFc γ2 , (B) hGH-L-vFc γ4 , and (C) hGH-L-vFc γ 1 as the HindIII-EcoRI fragment in the respective pGFP expression vector. The peptide from amino acid residues −26 to −1 is the leader peptide of hGH. The mature protein contains hGH (amino acid residues 1 to 191), a peptide linker (amino acid residues 192 to 207), and a Fc variant (amino acid residues 208 to 430 of vFc 12 , 208 to 436 of vFc γ4 , and 208 to 434 of vFc γ1 ). In the Fc regions, nucleotide and corresponding amino acid mutations in bold are also underlined. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    1. Construction of the Gene Encoding the hGH-L-vFc γ2 /Fusion Protein 
         [0019]    A fusion protein is assembled from several DNA segments. To obtain the gene encoding the leader peptide and mature protein of human GH, a synthetic gene is prepared to have the same sequence as human growth hormone 1 of NCBI Reference Sequence: NM — 000515.3. Using the DNA sequence from 5′- to 3′-terminus of the hGH gene, four oligonucleotides, each of about 180 nucleotides in length containing overlapping sequences, are synthesized. Using these four synthetic oligonucleotides, DNA fragments of approximately 650-bp in length are assembled by PCR. For the convenience of cloning, SEQ ID NO:1 (Table 1), which incorporates a restriction enzyme cleavage site (HindIII) is used as the 5′ oligonucleotide primer. Table 1 shows the sequences of oligonucleotides used for the cloning of the fusion proteins. The 3′ primer (SEQ ID NO:2) incorporates another restriction enzyme cleavage site (BamHI site). The resulting DNA fragments of approximately 650 bp in length are inserted into a holding vector such as pUC19 at the HindIII and BamHI sites to give the phGH plasmid. The sequence of the hGH gene is confirmed by DNA sequencing. 
         [0020]    The hinge domain of human IgG2 heavy chain contains 12 amino acid residues (GluArgLysCysCysValGluCysProProCysPro) including 4 cysteine residues. Out of these 4 cysteine residues, the 3 rd  and 4 th  are involved in the formation of disulfide bonding between two heavy chains. The 1 st  and 2 nd  cysteine residues are deleted to avoid nonspecific disulfide bonding. The hinge domain of Fc γ2  can be truncated to contain 7 amino acids (ValGluCysProProCysPro). The gene encoding the Fc region of human IgG2 (Fc γ2 ) is obtained by reverse transcription and PCR using RNA prepared from human leukocytes and appropriate 5′ (SEQ ID NO:3) and 3′ (SEQ ID NO:4) primers. Resulting DNA fragments of Fc γ2  containing complete sequences of the truncated hinge, CH2 and CH3 domains of IgG2 will be used as the template to generate the Fc γ2  Pro331Ser variant (vFc 72 ) in which Pro at position 331 of Fc γ2  is replaced with Ser. To incorporate this mutation, two segments are produced and then assembled by using the natural Fc γ2  as the template in overlapping PCR. The 5′ segment is generated by using SEQ ID NO:3 as the 5′ primer and SEQ ID NO:5 as the 3′ primer. The 3′ segment is generated by using SEQ ID NO:6 as the 5′ primer and SEQ ID NO:4 as the 3′ primer. These two segments are then joined at the region covering the Pro331Ser mutation by using SEQ ID NO:7 as the 5′ primer and SEQ ID NO:4 as the 3′ primer. The SEQ ID NO:7 primer contains sequences encoding a 16-amino acid Gly-Ser peptide linker including a BamHI restriction enzyme site. The resulting DNA fragments of approximately 700 bp in length are inserted into a holding vector such as pUC19 at the BamHI and EcoRI sites to give the pL-vFcγ2 plasmid. The sequence of the gene is confirmed by DNA sequencing. 
         [0021]    To prepare the hGH-L-vFc γ2  fusion gene, the hGH fragment is excised from the phGH plasmid with HindIII and BamHI and is purified by agarose gel electrophoresis. The purified fragment is then inserted to the 5′-end of the peptide linker in the pL-vFcγ2 plasmid to give the phGH-L-vFcγ2 plasmid. The fusion gene comprises hGH, a Gly-Ser peptide linker and the Fc γ2  variant gene. 
         [0022]    The presence of a peptide linker between the hGH and Fc moieties may increase the flexibility of hGH and enhance its biological activity (see, for example, U.S. Pat. Nos. 6,797,493 and 6,900,292). For the present invention, a peptide linker of about 20 or fewer amino acids in length is preferred. Peptide linker comprising two or more of the following amino acids: glycine, serine, alanine, and threonine can be used. An example of the peptide linker contains Gly-Ser peptide building blocks, such as GlyGlyGlyGlySer.  FIG. 2A  shows a fusion gene containing sequences encoding hGH, a 16-amino acid peptide linker (GlySerGlyGlyGlySerGlyGlyGlyGlySerGlyGlyGlyGlySer), and the Fc γ2  Pro331 Ser variant. 
         [0023]    The complete gene encoding the hGH-L-vFc γ2  fusion protein is then inserted at the HindIII and EcoRI sites of a mammalian expression vector, such as pcDNA3 (Invitrogen). The final expression vector plasmid, named pGFP2, contains the cytomegalovirus early gene promoter-enhancer which is required for high level expression in mammalian cells. The plasmid also contains selectable markers to confer ampicillin resistance in bacteria, and G418 resistance in mammalian cells. In addition, the pGFP2 expression vector contains the dihydrofolate reductase (DHFR) gene to enable the co-amplification of the hGH-L-vFc γ2  fusion gene and the DHFR gene in the presence of methotrexate (MTX) when the host cells are deficient in the DHFR gene expression (see, for example, U.S. Pat. No. 4,399,216). 
         [0000]    2. Construction of the Gene Encoding the hGH-L-vFc γ4  Fusion Protein 
         [0024]    Human IgG4 is observed partly as half antibody molecules due to the dissociation of the inter-heavy chain disulfide bonds in the hinge domain. This is not seen in the other three human IgG isotypes. A single amino acid substitution replacing Ser228 with Pro, which is the residue found at this position in IgG2 and IgG4, leads to the formation of IgG4 complete antibody molecules (see, for example, Angal et al.,  Molec. Immunol.  30:105-108, 1993; Owens et al.,  Immunotechnology  3:107-116, 1997; U.S. Pat. No. 6,204,007). The Fc 74  variant containing Leu235Ala mutation for the attenuation of FcR binding will also give rise to a homogeneous fusion protein preparation with this additional Ser228Pro mutation. 
         [0025]    The gene encoding the Fc region of human IgG4 (Fc 74 ) is obtained by reverse transcription and PCR using RNA prepared from human leukocytes and appropriate 5′ primer (SEQ ID NO:8) and 3′ primer (SEQ ID NO:9). Resulting DNA fragments of Fc 74  containing complete sequences of the hinge, CH2 and CH3 domains of IgG4 is used as the template to generate the Fc 74  variant with Ser228Pro and Leu235Ala mutations (vFc γ4 ) in which Ser228 and Leu235 are replaced with Pro and Ala, respectively. The CH2 and CH3 regions is amplified using the 3′ primer (SEQ ID NO:9) and a 5′ primer containing the Leu235Ala mutation (SEQ ID NO:10). This amplified fragment, together with a synthetic oligonucleotide of 60 bases in length (SED ID NO:10) containing both Ser228Pro and Leu235Ala mutations, are joined in PCR by using SEQ ID NO:12 as the 5′ primer and SEQ ID NO:9 as the 3′ primer. The SEQ ID NO:12 primer contains sequences encoding a 16-amino acid Gly-Ser peptide linker including the BamHI site. The resulting DNA fragments of approximately 700 bp in length are inserted into a holding vector such as pUC19 at the BamHI and EcoRI sites to give the pL-vFcγ4 plasmid. The sequence of the gene is confirmed by DNA sequencing. 
         [0026]    To prepare the hGH-L-vFc 74  fusion gene, the hGH fragment is excised from the phGH plasmid with HindIII and BamHI and then inserted to the 5′-end of the peptide linker in the pL-vFcγ4 plasmid to give the phGH-L-vFcγ4 plasmid. This fusion gene comprising hGH, a 16-amino acid Gly-Ser peptide linker and the Fc γ4  variant gene is then inserted at the HindIII and EcoRI sites of a mammalian expression vector, such as pcDNA3, as described for the hGH-L-vFc γ2  fusion protein. The final expression vector plasmid is designated as pGFP4.  FIG. 2B  shows a fusion gene containing sequences encoding hGH, a 16-amino acid peptide linker (GlySerGlyGlyGlySerGlyGlyGlyGlySerGlyGlyGlyGly Ser), and the Fc γ4  variant with Ser228Pro and Leu235Ala mutations. 
       3. Construction of the Gene Encoding the HuGH-L-vFc γ1  Fusion Protein 
       [0027]    The hinge domain of human IgG1 heavy chain contains 15 amino acid residues (GluProLysSerCysAspLysThrHisThrCysProProCysPro) including 3 cysteine residues. Out of these 3 cysteine residues, the 2 nd  and 3 rd  are involved in the formation of disulfide bonding between two heavy chains. The 1 st  cysteine residue may pair with other cysteine residues, leading to nonspecific disulfide bonding. The hinge domain of Fc γ1  can be truncated to eliminate the 1 st  cysteine residue (AspLysThrHisThrCysProProCysPro). The gene encoding the Fc γ1  region is obtained by reverse transcription and PCR using RNA prepared from human leukocytes and appropriate 5′ primer (SEQ ID NO:13) and 3′ primer (SEQ ID NO:4). Resulting DNA fragments containing the truncated hinge and complete sequences of CH2 and CH3 domains of Fc γ1  is used as the template to generate the Fc γ1  variant with Leu234Val, Leu235Ala, and Pro331Ser mutations (vFc γ1 ). 
         [0028]    One way to incorporate these mutations is as follows: two segments are produced and then assembled by using the natural Fc γ1  as the template in overlapping PCR. The 5′ segment is generated by using SEQ ID NO:14 as the 5′ primer and SEQ ID NO:5 as the 3′ primer. This 5′ primer contains the Leu234Val, Leu235Ala mutations and the 3′ primer contains the Pro331Ser mutation. The 3′ segment is generated by using SEQ ID NO:6 as the 5′ primer and SEQ ID NO:4 as the 3′ primer. These 5′ and 3′ segments are then joined at the region covering the Pro331Ser mutation by using SEQ ID NO:14 as the 5′ primer and SEQ ID NO:4 as the 3′ primer. This amplified fragment of approximately 650 bp in length, together with a synthetic oligonucleotide of 55 bases (SED ID NO:15) containing Leu234Val and Leu235Ala, are joined in PCR by using SEQ ID NO:16 as the 5′ primer and SEQ ID NO:4 as the 3′ primer. The SEQ ID NO:16 primer contains sequences encoding a 16-amino acid Gly-Ser peptide linker including the BamHI site. The resulting DNA fragments of approximately 700 bp in length are inserted into a holding vector such as pUC19 at the BamHI and EcoRI sites to give the pL-vFcγ1 plasmid. The sequence of the gene is confirmed by DNA sequencing. 
         [0029]    To prepare the hGH-L-vFc γ1  fusion gene, the hGH fragment is excised from the phGH plasmid with HindIII and BamHI and inserted to the 5′-end of the peptide linker in the pL-vFcγ1 plasmid to give the phGH-L-vFcγ1 plasmid. The fusion gene comprising hGH, a 16-amino acid Gly-Ser peptide linker, and the Fc γ1  variant gene is then inserted at the HindIII and EcoRI sites of a mammalian expression vector, such as pcDNA3, as described for the hGH-L-vFc γ2  fusion protein. The final expression vector plasmid is designated as pGFP1.  FIG. 2C  shows a fusion gene containing sequences encoding hGH, a 16-amino acid peptide linker (GlySerGlyGlyGlySerGlyGlyGlyGlySerGlyGlyGlyGlySer), and the Fc γ1  variant with Leu234Val, Leu235Ala and Pro331Ser mutations. 
       4. Expression of the Fusion Protein in Transfected Cell Lines 
       [0030]    The recombinant pGFP1, pGFP2 or pGFP4 expression vector plasmid is transfected into a mammalian host cell line to achieve the expression of the hGH-L-vFc fusion protein. For stable high levels of expression, a preferred host cell line is Chinese Hamster Ovary (CHO) cells deficient in the DHFR enzyme (see, for example, U.S. Pat. No. 4,818,679). A preferred method of transfection is electroporation. Other methods, including calcium phosphate co-precipitation, lipofectin, and protoplast fusion, can also be used. For electroporation, 10 μg of plasmid DNA linearized with BspCI is added to 2 to 5×10 7  cells in a cuvette using Gene Pulser Electroporator (Bio-Rad Laboratories, Hercules, Calif.) set at an electric field of 250 V and a capacitance of 960 μFd. Two days following the transfection, the media are replaced with growth media containing 0.8 mg/ml of G418. Transfectants resistant to the selection drug are tested for the secretion of the fusion protein by anti-human IgG Fc ELISA. Quantitation of the expressed fusion protein can also be carried out by ELISA using anti-hGH assays. The wells producing high levels of the Fc fusion protein are subcloned by limiting dilutions on 96-well tissue culture plates. 
         [0031]    To achieve higher levels of the fusion protein expression, co-amplification is preferred by utilizing the gene of DHFR which can be inhibited by the MTX drug. In growth media containing increasing concentrations of MTX, the transfected fusion protein gene is co-amplified with the DHFR gene. Transfectants capable of growing in media with up to 1 μg/ml of MTX are again subcloned by limiting dilutions. The subcloned cell lines are further analyzed by measuring the secretion rates. Several cell lines yielding secretion rate levels over about 10 μg, preferably about 30 μg/10 6  [i.e. million] cells/24 h, are adapted to suspension culture using serum-free growth media. The conditioned media are then used for the purification of the fusion protein. 
       5. Purification and Characterization of the Fusion Protein 
       [0032]    Conditioned media containing the fusion protein are titrated with 1 N NaOH to a pH of 7 to 8 and filtered through a 0.45 micron cellulose nitrate filter. The filtrate is loaded onto a Prosep A column equilibrated in phospate-buffered saline (PBS). After binding of the fusion protein to Prosep A, the flow-through fractions are discarded. The column is washed with PBS until OD at 280 nm is below 0.01. The bound fusion protein is then eluted with 0.1 M citrate buffer at pH 3.75. After neutralizing with 0.4 volume of 1 M K 2 HPO 4 , fractions containing purified protein are pooled and dialyzed against PBS. The solution is then filtered through a 0.22 micron cellulose nitrate filter and stored at 4° C. The molecular weight of purified hGH-L-vFc protein is in the range of 90 to 100 kDa by SDS-PAGE under non-reducing conditions. Under reducing conditions, the purified protein migrates around approximately 50 kDa. The fusion protein is quantitated by BCA protein assay using BSA as the standard. 
       6. In Vitro Biological Potency Assays 
       [0033]    Supernatants of transfectants or purified proteins can be tested for their ability to stimulate the proliferation of Nb2 rat lymphoma cells. Although Nb2 cells proliferate in response to hGH via the lactogenic receptors on the cells, the Nb2 cell bioassay can be a useful tool for evaluating the biological activity of hGH (for example, Uchida et al.,  J. Mol. Endocrinol.  23:347-353, 1999). Approximately 48 hours before the start of the assay, the cells are transferred to the pre-assay medium (Fisher&#39;s Medium supplemented with 1% horse serum, and 50 μM 2-ME) to slow down the rate of cell replication. After incubation, cells are collected and resuspended in assay medium (Fisher&#39;s Medium supplemented with 10% horse serum, and 50 μM 2-ME) at a concentration of 1×10 5  cells/ml. Two-hundred μl aliquots are distributed in each well of a 96-well microtiter plate. The plate is incubated in a CO 2  incubator (5% CO 2 +95% air) for 48 hours at 37° C. To each well is added 50 μl culture supernatant containing various concentrations of the hGH-L-vFc fusion protein or rhGH control from 0.01 to 100 nM each. The plate is incubated in a CO 2  incubator for 48 h at 37° C. before 10 μl of MIT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl-tetrazolium bromide) (2.5 mg/ml in PBS) is added to each well. After another 4 h in a CO 2  incubator at 37° C., the cells and formazan are solubilized by adding 100 μl per well of 10% SDS in 0.01 N HCl. The plate is then read at 550 nm with the reference beam set at 690 nm. The OD reading is plotted against the concentration of rhGH or the fusion protein. The biological activity of hGH-L-vFc relative to that of rhGH can therefore be compared quantitatively. The biological activity is good. A good activity for the present invention means an activity that is significant when comparing with, preferably the same as, and more preferably better than that of rhGH when measured on a molar basis. 
         [0034]    Although the Nb2 cell bioassay can be a useful tool for evaluating the biological activity of hGH, cell proliferation depends on the cross reactivity of hGH with the lactogenic receptors on Nb2 cells. The Ba/F3-hGHR cell line, derived from mouse pro-B cell lymphoma cell line, is stably transfected with the hGHR gene and has been shown to express hGHR on cell surface. Cell proliferation assay using Ba/F3-hGHR cell line is mediated via the binding of hGH to hGHR on cell surface (For example, Ishikawa et al.,  J. Clin. Endocrinol . &amp;  Metabol.  85: 4274-4279, 2000). Approximately 4-6 hours before the start of the assays, the Ba/F3-hGHR cells are washed twice with assay medium (RPMI 1680, supplemented with 5% FCS, and 50 μM 2-ME) and are incubated in the assay medium for 4-6 h to slow down the rate of cell replication. After incubation, the cells are collected and resuspended at a concentration of 1×10 5  cells/ml. Two-hundred μl aliquots are distributed in each well of 96-well microtiter plate. To each well is added 50 μl culture supernatant containing various concentrations of the hGH-L-vFc fusion protein or recombinant hGH control from 0.01 to 100 nM each. The plate is incubated in a CO 2  incubator for 48 h at 37° C. At the end of the incubation, the colorimetric end point is determined as described above in the bioassay using the Nb2 cells. The biological activity is good. A good activity for the present invention means an activity that is significant when comparing with, preferably the same as, and more preferably better than that of rhGH when measured on a molar basis. 
       7. Pharmacokinetic Studies in Rats 
       [0035]    Rats are divided into two groups to receive rhGH or purified hGH-L-vFc protein for a comparison of their plasma profiles. Female Sprague-Dawley rats (approximately 300 g) are administered a single s.c. injection of rhGH (100 μg/kg) or hGH-L-vFc (1 mg/kg) in PBS. In addition to pre-treatment sample, blood samples of 0.25 ml are collected in EDTA-coated micro tubes. Time points are collected at 1, 4, 8, 12, 24, 48, and 96 h after injection. Samples are stored on wet ice for up to 1 h prior to centrifugation and plasma harvest. Plasma samples are stored at −20° C. prior to analysis. 
         [0036]    Concentration of hGH in plasma samples can be determined by using a commercial sandwich ELISA kit for detection of hGH (Invitrogen or Abnova). This kit detects hGH as well as hGH-L-vFc by means of an antibody sandwich ELISA format. Another ELISA format can also be used for the detection of hGH-L-vFc in rat plasma samples. In addition to using the anti-hGH antibody on the solid phase as included in the kit, anti-human IgG-heavy chain antibody is used in the antibody-enzyme (horseradish peroxidase) conjugate. By using the combination of anti-hGH and anti-human IgG-heavy chain detector antibodies, the ELISA measures specifically the concentration of hGH-L-vFc in plasma samples from the rats. The concentration of hGH is derived from a standard curve using materials supplied in the kit. The concentration of hGH-L-vFc is derived from a standard curve using hGH-L-vFc solutions ranging from 1 to 100 ng/ml (or approximately 0.01 to 1 nM). 
         [0037]    The hGH-L-vFc or rhGH concentrations of serum samples are plotted against time points for the calculation of the circulation time. From these curves, standard pharmacokinetic parameters, including clearance (CL), half-life (t½), area under the plasma concentration versus time curve (AUC), and maximal observed plasma concentration (C max ) can be obtained. The concentration of hGH-L-vFc decreases much slower than that of rhGH, indicating longer half-life of the fusion protein in rats. It is within the scope of the present invention that a plasma half-life that is comparable to or equal to, preferably longer than [i.e. prolonged], and more preferably at least 2 times as long as that of rhGH when administered in vivo. 
         [0038]    The examples described above are for illustration purposes only. They are not intended and should not be interpreted to limit either the scope or the spirit of this invention. It can be appreciated by those skilled in the art that many other variations or substitutes can be used as equivalents for the purposes of this invention, which is defined solely by the written description and the following claims. 
         [0000]    
       
         
               
             
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Sequences of Oligonucleotides. 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 SEQ ID NO: 1 
               
               
                 5′-cccaagcttggcgcggagatggctacaggctcccgga-3′ 
               
               
                   
               
               
                 SEQ ID NO: 2 
               
               
                 5′-cggatccgaagccacagctgccctcca-3′ 
               
               
                   
               
               
                 SEQ ID NO: 3 
               
               
                 5′-gtcgagtgcccaccgtgccca-3′ 
               
               
                   
               
               
                 SEQ ID NO: 4 
               
               
                 5′-ggaattctcatttacccggagacaggga-3′ 
               
               
                   
               
               
                 SEQ ID NO: 5 
               
               
                 5′-tggttttctcgatggaggctgggaggcct-3′ 
               
               
                   
               
               
                 SEQ ID NO: 6 
               
               
                 5′-aggcctcccagcctccatcgagaaaacca-3′ 
               
               
                   
               
               
                 SEQ ID NO: 7 
               
               
                 5′-cggatccggtggcggttccggtggaggcggaagcggcggtggagga 
               
               
                 tcagtcgagtgcccaccgtgccca-3′ 
               
               
                   
               
               
                 SEQ ID NO: 8 
               
               
                 5′-gagtccaaatatggtccccca-3′ 
               
               
                   
               
               
                 SEQ ID NO: 9 
               
               
                 5′-ggaattctcatttacccagagacaggga-3′ 
               
               
                   
               
               
                 SEQ ID NO: 10 
               
               
                 5′-cctgagttcgcggggggacca-3′ 
               
               
                   
               
               
                 SEQ ID NO: 11 
               
               
                 5′-gagtccaaatatggtcccccatgcccaccatgcccagcacctgagt 
               
               
                 tcgcggggggacca-3′ 
               
               
                   
               
               
                 SEQ ID NO: 12 
               
               
                 5′-cggatccggtggcggttccggtggaggcggaagcggcggtggagga 
               
               
                 tcagagtccaaatatggtccccca-3′ 
               
               
                   
               
               
                 SEQ ID NO: 13 
               
               
                 5′-gacaaaactcacacatgccca-3′ 
               
               
                   
               
               
                 SEQ ID NO: 14 
               
               
                 5′-acctgaagtcgcggggggaccgt-3′ 
               
               
                   
               
               
                 SEQ ID NO: 15 
               
               
                 5′-gacaaaactcacacatgcccaccgtgcccagcacctgaagtcgcgg 
               
               
                 ggggaccgt-3′ 
               
               
                   
               
               
                 SEQ ID NO: 16 
               
               
                 5′-cggatccggtggcggttccggtggaggcggaagcggcggtggagga 
               
               
                 tcagacaaaactcacacatgccca-3′