Glycosaminoglycan storage begins in prenatal life in patients with
mucopolysaccharidosis (MPS). In fact, prenatal
hydrops is a common manifestation of MPS VII because of
beta-glucuronidase (GUS) deficiency. One way to address prenatal storage might be to deliver the missing
enzyme across the placenta into the fetal circulation. Maternal
IgG is transported across the placenta by the
neonatal Fc receptor (FcRn), which recognizes the Fc domain of
IgG and mediates transcytosis from maternal to fetal circulation. We hypothesized that we could exploit this process to deliver corrective
enzyme to the fetus. To test this hypothesis, the C-terminal fusion
protein, GUS-Fc, was compared with native, untagged, recombinant GUS for clearance from the maternal circulation, delivery to the fetus, and reduction of lysosomal storage in offspring of MPS VII mice. We observed that GUS-Fc, infused into pregnant mothers on embryonic days 17 and 18, was transported across the placenta. Similarly infused untagged GUS was not delivered to the fetus. GUS-Fc plasma
enzyme activity in newborn MPS VII mice was 1,000 times that seen after administration of untagged GUS and approximately 100 times that of untreated WT newborns. Reduced lysosomal storage in heart valves, liver, and spleen provided evidence that in utero
enzyme replacement therapy with GUS-Fc targeted sites of storage in the MPS VII fetus. We hypothesize that this noninvasive approach could deliver the missing lysosomal
enzyme to a fetus with any
lysosomal storage disease. It might also provide a method for inducing immune tolerance to the missing
enzyme or another foreign
protein.