Substrate clearance assays for lysosomal enzymes
The present invention provides, among other things, improved substrate clearance assays for lysosomal enzyme that are particularly useful for measuring potency of lysosomal enzymes or other therapeutics for treatment of lysosomal storage diseases. In particular, the present invention combines a physiologically relevant substrate cell assay and an efficient scintillation based detection method.
1. A method of measuring enzymatic activity of a recombinant lysosomal enzyme, comprising steps of
obtaining a sample from a manufacturing process comprising a recombinant lysosomal enzyme of interest;
contacting the sample comprising the lysosomal enzyme of interest with cells containing a substrate of the lysosomal enzyme labeled with a radioactive isotope cleavable by the lysosomal enzyme, under conditions that allow the lysosomal enzyme to cleave the radioactive isotope from the substrate, wherein the cells are attached to a scintillant base such that scintillation events are indicative of radioactive emission level associated with the cells; measuring a change of radioactive emission level associated with the cells by detecting scintillation events as compared to baseline scintillation events before the contacting step, wherein a decrease in scintillation events after the contacting step relative to baseline scintillation is indicative of enzymatically active lysosomal enzyme; and
measuring the enzymatic activity of the lysosomal enzyme based on the change of radioactive emission level associated with the cells as compared to a control, wherein the control is a reference standard indicative of a predetermined enzymatic activity of the lysosomal enzyme of interest,
wherein the method does not involve lysis of the cells;
thereby measuring enzymatic activity of a recombinant lysosomal enzyme.
2. The method of claim 1 , wherein the cells lack endogenous lysosomal enzyme of interest.
3. The method of claim 1 , wherein the cells are fibroblasts derived from patients suffering from a lysosomal storage disease associated with a deficiency of the lysosomal enzyme of interest.
4. The method of claim 1 , wherein the substrate of the lysosomal enzyme labeled with the radioactive isotope is synthesized by the cells in the presence of the radioactive isotope.
5. The method of claim 1 , wherein the method further comprises steps of:
growing cells to a desired confluency;
treating the cells with the radioactive isotope such that the radioactive isotope is incorporated into newly synthesized substrate of the lysosomal enzyme; and
seeding the cells into a well with the scintillant base such that the cells attach to the scintillant base.
6. The method of claim 1 , wherein the substrate is glycosamine glycan (GAG).
7. The method of claim 6 , wherein the radioactive isotope is 35 S.
8. The method of claim 7 , wherein the substrate is labeled by treating the cells with 35 S-sodium sulfate.
9. The method of claim 1 , wherein the conditions that allow the lysosomal enzyme to cleave the radioactive isotope from the substrate comprises incubating the cells and the sample comprising the lysosomal enzyme at 37° C. with 5% CO 2 in atmosphere.
10. The method of claim 1 , wherein the method further comprises a step of washing the cells prior to the measuring step.
11. The method of claim 1 , wherein the contacting step is carried out in the presence of mannose-6-phosphate (M6P) and wherein the method further comprises a step of comparing the enzyme activity to a control measured without M6P to determine if cellular uptake is M6P dependent.