Lysosomal Storage Disorders: Trafficking Assays
Lysomal Storage Disorders are a large class of monogenic disorders involving disruption of enzymes normally targeted to the lysosome (a highly acidic compartment in cells where specialized reactions take place). In the absence of the normal enzyme, the enzyme’s substrate builds up in the lysosome, often with toxic effects. The more prevalent LSDs include Gaucher’s (a defect in Glucosylceramide β-glucosidase), MPSI Hurler-Shae’s (a defect in α-Iduronase), Tay Sach’s (a defect in β-hexosaminidase A) and Fabry’s disease (a defect in trihexosylceramide α-galactosidase).
Enzyme-replacement therapy (ERT) has led to life-saving treatments for many of these conditions, but ERT has some well-known limitations. Infused enzyme does not cross the blood-brain barrier, and so the CNS manifestations of the diseases are not ameliorated by ERT. ERT is also typically quite expensive, and monthly intravenous infusions are inconvenient for patients.
Abberant trafficking of Glucosylceramide β-glucosidase (above) is the defect caused by many Gaucher’s mutations. Such detailed structural knowledge makes this an attractive drug discovery target.
In general, a lysosomal protein needs to cross several hrudles in order reach the lysosome and perform its normal function. First, the gene’s RNA must be synthesized in the nuclues. Several LSD mutants prevent this process by introducing premature stop codons into the gene, resulting in truncated RNA which are generally not expressed (the Truncation Mutants). If a full-length protein is made, it must traverse the Golgi/ER and get trafficked into the lysosome. The majority of LSD mutations disrupt that process. Finally, if a full length protein is made, it must function proprely, and some LSD mutations make the protein less active once it gets to its destination in the lysosome.
Many of the mutations that lead to these LSDs introduce trafficking defects into the protein, rather than disrupting the enzyme active site. This has led to several important studies showing that (counter-intuatively) lysosomal activity of these defective enzymes can be restored through the addition of enzyme inhibitors. These molecules act as chemical chaperones, allowing more protein to be trafficked to the lysosome, and, when the inhibitor concentration is just right, the enzyme activity is partially restored, even in the presence of inhibitor. Several companies are developing compounds based on this observation.
Through our specialization in protein trafficking, Sharp Edge Labs has developed a direct assay for lysosomal trafficking of these LSD-related enzymes that enables the screening and discovery of compounds that aid trafficking without effecting the enzyme’s activity. Our technology has allowed the discovery of new classes of chemical correctors, compounds that are specific for the defective protein, but don’t require exquisite dose-control to avoid inhibition, and that can also cross the blood-brain barrier to address the CNS manifestations of LSDs.