334 ADENOVIRAL VECTOR-MEDIATED EXPRESSION OF RECOMBINANT HUMAN GLUCOCEREBROSIDASE IN THE MAMMARY GLAND OF RATS

Abstract

Glucocerebrosidase is a lysosomal enzyme that plays a key role in sphingolipid cleavage, an intermediate in glycolipid metabolism. A recessive mutation in the glucocerebrosidase gene leads to the accumulation of glucosylceramide in macrophages (sphingolipidosis), a lysosomal storage disease known in humans as the Gaucher disease. The enzyme replacement treatment with recombinant human glucocerebrosidase (hGCase) dramatically reduces and reverses symptoms, with the need of lifelong treatment for patients to attain a normal life. Currently, hGCase is very costly, being produced through in vitro expression in Chinese hamster ovary cells or in vivo, in plants. The aim of this study was to develop a model for the production of hGCase in the mammary gland of rats transiently transduced with recombinant adenovirus. A replication-defective adenovirus carrying hGCase was generated using the AdEasy™ adenoviral vector system (Stratagene, La Jolla, CA, USA). The hGCase cDNA (NM_001005741) was in vitro-synthesized and ligated in the XhoI site of the pAdTrack-CMV vector (pAdT-hGCase). The resulting plasmid was recombined with the pAdEasy™ vector in BJ5183 electro-competent cells. The purified pAdE-pAdT-hGCase vector was linearized and transfected into HEK-293 cells for the production of a primary viral stock. Further amplifications and the titration assay were done in HEK-293 cells, monitoring the transduction by the qualitative evaluation of green fluorescent protein (GFP) expression. Following transfection, the HEK-293 cells increasingly expressed the GFP reporter, regulated by a CMV promoter, in tandem with the hGCase cDNA, under another CMV promoter. On Day 18 of gestation, a female rat (Rattus norvegicus) was anesthetized and the 2 left caudal mammary glands were infused with 10⁹ GTU mL^–1 of the pAdE-pAdT-hGCase in PBS solution supplemented with 36 mM EGTA. The 2 right caudal mammary glands were infused only with PBS-EGTA (control milk). Milk samples collected from Days 2 through 9 post-partum were mixed with separation buffer (10 mM Tris-HCl, pH 8.0; 10 mM CaCl₂) and centrifuged, with the supernatant assayed for hGCase by Western blot using a monoclonal anti-human glucocerebrosidase antibody (sc-166407, Santa Cruz Biotechnology, Santa Cruz, CA, USA). Relative quantification of the hGCase expression was done using the FluorChem FC2 system (Alpha Innotech, San Leandro, CA, USA), with hGCase band intensity being normalized against GAPDH expression. The in vivo expression assay confirmed the production of hGCase in the secreted portion of the rat milk, with a specific band between 50 to 60 kDa observed on the Western blot, and no detection of the protein in the control milk. The hGCase peak production occurred in Days 5 and 6 of lactation, with levels being 35 times greater than on Day 9. An ELISA quantification assay and an enzymatic activity assay for the recombinant hGCase are currently in development. In conclusion, the use of the rat for hGCase transient expression in the milk was proven a valid model for testing the potential use of a mammary gland expression system for the production of a functional human glucocerebrosidase protein.