applicants have discovered that the prenatal delivery of a chimeric protein containing the igg fc - domain ( genbank accession no . x68090 ), which after infusion into the maternal circulation , mediates delivery of the protein across the placenta into the circulation of the fetus . specifically , applicants have invented a b - glucuronidase / fc chimeric protein and used it for prenatal enzyme replacement therapy in the mps vii mouse . the chimeric enzyme , after intravenous infusion into the pregnant female on embryonic days 17 and 18 , was delivered across the placenta utilizing the interaction between the fc tag and the neonatal fc receptor ( fcrn ). this enzyme was transported into the circulation of the fetuses , and due to the presence of the m6p recognition marker on the enzyme , was taken up by the m6p - receptor and delivered to the lysosomes of numerous tissues . here it was able to clear lysosomal storage material characteristic for this disease . applicants have shown that the invention can be applied to the prenatal treatment of mps vii and certainly could be expanded to prenatally treat any of the numerous lysosomal storage diseases . however , it is envisioned that the invention can be used to deliver any protein , peptide , drug or therapeutic across the placenta to the fetus . thus , in one embodiment , the invention is directed to a chimeric protein in which human gus contains a carboxy terminal tag consisting of the ch2 - ch3 domain of human igg . this chimera is an active form of b - glucuronidase containing the m6p lysosomal targeting signal . when this chimera was intravenously infused into a pregnant mouse , the fc domain mediates transport across the placenta into the circulation of the fetus . from there , the enzyme was taken up via the m6p receptor present in the tissues of the fetus and delivered to the lysosomes of those tissues . enzyme delivered in this manner was able to clear accumulated glycosaminoglycans from the lysosomes of the mps vii knock - out mouse . applicants have used the mps vii mouse model to provide proof - of - concept for this transplacental delivery system . one skilled in the art would reasonably expect that this method can be used in any and all lysosomal storage diseases that are amenable to enzyme replacement therapy . applicants further envision that the instant fc delivery system can be used to deliver many other therapeutic proteins , peptides and biologics across the placenta . in another embodiment , the invention is directed to systems and methods for delivery therapeutic polypeptides across the placenta and into the fetus . preferably ( but not limited to ) an object of this invention is to deliver corrective enzyme to a mps vii fetus by infusing a chimeric enzyme containing the fc domain from human igg fused to the c - terminus of recombinant human gus into the pregnant mother . to test this hypothesis , the c - terminal fusion protein gus - fc was compared to native phosphorylated recombinant gus ( pgus ) for clearance from the murine maternal circulation , delivery to the fetus and reduction of lysosomal storage . ( the murine system is used experimentally to model the human , hence the invention is broadly applicable to any and all mammals , most preferably humans .) after administering the gus - fc enzyme ( 4 - 6 mg / kg ) to the pregnant mother on gestational days 17 and 18 , applicants found clear evidence that the gus - fc was transported across the placenta . enzyme activity was present in newborn mps vii ( mps −/−) mice plasma at 2000 - 5000 u / ml , 1000 times more than background levels ( 2 - 5 u / ml ) seen following administration of non - fc tagged enzyme , which was not detectably transported . these elevated levels in treated ( mps −/−) newborn mice were roughly 100 times those seen in untreated wild type newborns . reduction of lysosomal storage in storage in heart valves , liver and spleen provided evidence that the administered gus - fc was corrective in the mps vii fetus . using dna cloning techniques applicants added the cdna sequence encoding the ch2 - ch3 domains of human immunoglobulin g ( genbank accession #) to the carboxy terminus of the fall length cdna for human b - glucuronidase ( genbank accession # nm — 000181 )( gus - fc , fig1 ). the polynucleotide sequence is set forth in seq id no : 1 and its corresponding polypeptide sequence is set forth in seq id no : 2 , which are incorporated herein . both the wild type and fc - tagged cdnas were subcloned into the mammalian expression vector pcxn . ( 4 ) this expression vector contains an expression cassette consisting of the chicken beta - actin promoter coupled to the cmv intermediate - early ( cmv - ie ) enhancer . pcxn also contains a selectable marker for g418 allowing selection of stably expressing mammalian cells . these plasmids were introduced into the chinese hamster ovary cell line , cho - k1 ( 5 ) by electroporation ( 6 ). after selection in growth medium consisting of minimal essential medium + 35 μg / ml proline + 15 % fetal bovine serum ( fbs ) + 400 μg g418 , colonies were picked and grown to confluency in 48 - well plates . high level expressing clones were identified by measuring gus activity secreted into the conditioned medium from these clones ( ref ). the highest - producing clone was scaled up and secreted enzyme was collected in low serum collection medium consisting of waymouth mb 752 / 1 + 2 % fbs . conditioned medium collected in this way was pooled , centrifuged at 5000 × g for 20 minutes and the supernatant was collected and frozen at − 20 ° f . until sufficient quantities were accumulated for purification . gus activity was measured using the 10 mm 4 - methyl - umbelliferyl β - d - glucuronide as substrate in 0 . 1m sodium acetate buffer ph 4 . 8 , 1 mg / ml crystalline bsa as previously described ( 7 ). affinity chromatography procedure was essentially as described ( 8 ). conditioned medium from cho cells overexpressing the gus or gus - fc fusion protein was filtered through a 0 . 22μ filter . sodium chloride ( crystalline ) was added to a final concentration of 0 . 5m , and sodium azide was added to a final concentration of 0 . 025 % by adding 1 / 400 volume of a 10 % stock solution . the medium was applied to a 5 ml column of anti - human β - glucuronidase - affigel 10 ( pre - equilibrated with antibody sepharose wash buffer : 10 mm tris ph 7 . 5 , 10 mm potassium phosphate , 0 . 5 m nacl , 0 . 025 % sodium azide ) at a rate of 25 ml / hour at 4 ° c . the column was washed at 36 ml / hour with 10 - 20 column volumes of antibody sepharose wash buffer . the column was eluted at 36 ml / hour with 50 ml of 10 mm sodium phosphate ph 5 . 0 + 3 . 5 m mgcl 2 . 4 ml fractions were collected and assayed for gus activity . fractions containing the fusion protein were pooled , diluted with an equal volume of p6 buffer ( 25 mm tris ph 7 . 5 , 1 mm β - glycerol phosphate , 0 . 15 mm nacl , 0 . 025 % sodium azide ) and desalted over a biogel p - 6 column ( pre - equilibrated with p6 buffer ) to remove the mgcl 2 and to change the buffer to p6 buffer for storage . the fusion protein was eluted with p6 buffer , fractions containing gus activity were pooled and assayed for gus activity and protein . both purified gus and gus - fc were stored frozen at − 80 ° c . in p6 buffer for long - term stability . for mouse infusions , the enzymes were highly concentrated in centricon ym - 30 concentrators and the buffer was changed to p6 buffer without azide . these concentrates were frozen in small aliquots at − 80 ° c . until use . gus is a 300 kda protein that exists as a homotetramer consisting of four identical monomers of apparent molecular weight of 75 kda . in fig2 , two different amounts of purified gus were analyzed by sds - page under reducing conditions ( 9 ). the apparent molecular weight is 75 kda as expected . when gus - fc was analyzed by sds - page the apparent molecular weight as seen below is approximately 104 kda indicating an increase of ˜ 29 kda . the addition of the fc domain would be expected to add approximately 29 kda to the gus monomer . there is a small amount of 75 kda protein present also which might indicate a small percentage of protein from which the fc domain has been removed by proteolysis . the addition of the fc domain to gus would not only confer the ability to bind to the fcrn , but should also allow the binding to the bacterial protein g . protein g immobilized on various resins have long been used to precipitate igg / antigen complexes during immunoprecipitation reactions . this technique can be used to analyze the proportion of gus - fc that contains a functional fc domain quite easily . only protein that contains functional fc will be precipitated by immobilized protein g . in this case , the gus activity serves as the reporter in the assay . briefly , 500 units of gus or gus - fc were combined with protein g - sepharose in phosphate buffered saline ( pbs ) ph 7 . 4 , 1 mg / ml crystalline bovine serum albumin ( c - bsa ). this reaction mixture was mixed by rotation for 4 hours at 4 ° c . after centrifugation at 10 , 000 × g for 2 minutes , the supernatant was removed and saved for assay . the pellets were washed 2 times with 1 ml each of pbs then resuspended in 1 ml of pbs + c - bsa . as shown in table i , only 5 % of wild type gus was precipitated by protein g - sepharose when analyzed in this manner . this is considered to be in the range of non - specific binding to the resin . in contrast , both unpurified gus - fc from the secretion medium and purified gus - fc were precipitated 85 % and 74 %, respectively by protein g - sepharose . these results indicate that the fc domain on the fusion protein does confer the ability to function similar to the fc domain on igg . they also indicate the majority of the purified gus - fc seems to contain a functional fc domain . eventual delivery of any lysosomal enzyme to the lysosome requires the presence of the mannose 6 - phosphate ( m6p ) recognition signal on the enzyme . this signal is recognized by two separate mannose 6 - phosphate receptors that can mediate delivery of the lysosomal enzyme to the lysosome ( 10 ). postranslational modification of lysosomal enzymes is required to add m6p to their n - linked oligosaccharides . this process is accomplished by a two - step procedure utilizizing the udp - glcnac lysosomal phosphotransferase which transfers glcnac — po 4 to terminal mannose residues on the oligosaccharides . the second step utilizes the phosphodiesterase ( uce ) that removes the glcnac cap exposes the m6p . with previous gus fusion proteins applicants have made , applicants have seen various reductions in the m6p content . applicants attribute this to some steric or conformational change induced by the addition of the fusion tag that affects the efficiency of either the phosphotransferase or uce to produce a functional m6p recognition signal . in order to assess the amount of m6p recognition signal that gus fc contains relative to gus , applicants measured the rate at which the enzyme is taken up by human fibroblasts ( 11 ). human fibroblasts contain mannose 6 - phosphate receptors on their cell surfaces which mediate the endocytosis of m6p - containing ligands . table 1 shows the results of an uptake experiment in which 8000 units of gus or gus fc were added to the medium of human fibroblasts in the presence and absence of 2 mm m6p . results are expressed as m6p specific enzyme uptake per mg of cell protein per hour of uptake at 37 ° c . as can be seen , gus is taken up quite well in a m6p - specific manner . however , the uptake of gus - fc is reduced to about 14 % of that seen for gus . even though this reduction is significant , the enzyme still has the ability to be delivered to lysosomes by the m6p receptor . in addition to mediating transplacental transfer , it has been well documented that the fc domain is responsible for maintaining high levels of igg in the circulation . the fcrn , which is also expressed in endothelial cells in adults recaptures igg which has been endocytosed or pinocytosed out of the circulation . the fcrn which resides in the endosomes binds the fc moiety at igg at an acid ph in the endosome and transports it back out into the circulation . this has the net effect of returning igg that would normally be delivered to lysosomes and degraded , back into the circulation . because of this , one skilled in the art would predict that gus - fc , which would normally be rapidly cleared from the circulation by mannose and m6p - receptors , would be recaptured and returned to the circulation . this would have the net effect of prolonging the clearance times and maintaining higher levels of gus - fc in the circulation for a longer time . this should increase the opportunity for gus - fc to be recognized by fcrn receptors in the placenta and thus increase the amount of enzyme transported across to the fetus . in order to test these hypotheses , fig3 shows the clearance from the circulation in either mannose receptor positive mps vii mice ( mr +/+) or mannose receptor negative mps vii mice ( mr −/−) ( 12 ). untagged gus is cleared fairly rapidly from mr +/+ mice as one would expect . the same enzyme , when infused into the mr −/− mouse is cleared significantly more slowly as it is being cleared only by the m6p receptor . in contrast , gus - fc is cleared substantially less slowly in both the mr +/+ and mr −/− mice . applicants also tested gus that had been treated with periodate and borohydride ( pb - gus ) which destroys all carbohydrate on the enzyme ( 13 ). since pb - gus is no longer a ligand for either the mannose or m6p receptors its clearance time should be extremely prolonged . as seen in fig3 , the clearance of pb - gus is extremely low during the time frame of these experiments . these results would indicate that the fc domain is functional and is able mediate recapture of gus - fc by fcrn . this also makes it highly likely , that the fc domain on gus - fc would mediate binding to the fcrn in the placenta and therefore transcytosis of the tagged enzyme across the placenta into the fetal circulation . in order to test for transplacental transport of gus - fc applicants did the experiments summarized in table 2 . timed pregnancies were set up using either mr +/+ or mr −/− females . on calculated embryonic days 17 and 18 , these pregnant mice were infused with 380 , 000 units of gus , gus - fc , pb - gus or pbs by bolus infusion into the tail vein . pups that were delivered , that were later analyzed by pcr to confirm their homozygous status ) were collected on newborn day 1 , sacrificed and blood collected . after centrifugation , plasma was collected and assayed for b - glucuronidase as described above . both mr +/+ and mr −/− mice infused with pbs had very low levels consistant with being b - glucuronidase deficient . similarly , both types of mice have low levels of enzyme when the mothers had bee infused with untagged gus . in contrast , pups from both the mr +/+ and mr −/− pregnant females infused with gus - fc contained highly elevated levels of enzyme in their plasma . this could only be attributed to transplacental transport of the fc - tagged enzyme . the levels in the pups from the mr −/− moms are about 2 - fold higher than from the mr +/+ moms . this would seem to correlate with differences in clearance rates from the circulation in the pregnant females . the next question applicants asked was if the gus - fc could be delivered to tissues of the fetus once it had been delivered into the fetal circulation . two lines of evidence support this . the first is summarized in table 3 below . tissues were collected from the pups after perfusion to remove any enzyme still contained in the blood . the tissues were homogenized and assayed for b - glucuronidase and protein ( 14 ). enzyme levels are expressed as units / mg protein . enzyme levels were compared to wild type b6 pups that were harvested in the same way . as can be seen , pups from pregnant mice that had been infused with untagged gus were very low , essentially the same as for pups from non - infused pregnant mice . however , pups from pregnant mice infused with gus - fc contained significant amounts of activity in many tissues . while these levels are not nearly as high as in wild type pups , they are close to the 5 - 10 % of normal levels which have been documented to be clinically effective in treating this disease . the second line of evidence that enzyme has been delivered to the fetal tissues is presented in fig4 . lysosomes in untreated mps vii mice accumulate undegraded glycosaminylglycans and become distended in appearance . these distended lysosomes are evident in spleen , liver and heart from pups from untreated mps vii mice shown in the first column of fig4 . in contrast , the same tissues from pups from both mr −/− and mr +/+ females treated with gus - fc , showed substantial clearance in the former and partial clearance in the latter . this is considered definitive proof that gus - fc infused into the pregnant female was able to cross the placenta and to ultimately be delivered to the lysosomes in tissues of the fetuses .
the invention is directed to chimeral fusion proteins having an igg1 antibody fc portion and a lysosomal storage enzyme , particularly a fc - gus fusion protein useful in treating sly &# 39 ; s disease in an embryo or fetus . the invention is also directed to methods of treating in born errors of metabolism , particularly sly &# 39 ; s disease , in a fetus by delivering to a pregnant mother a fc - mps emzyme fusion protein .