Gary Lee

Pharmacological Chaperones as Therapeutics for Lysosomal Storage Diseases

Lysosomal enzymes are responsible for the degradation of a wide variety of glycolipids, oligosaccharides, proteins, and glycoproteins. Inherited mutations in the genes that encode these proteins can lead to reduced stability of newly synthesized lysosomal enzymes. While often catalytically competent, the mutated enzymes are unable to efficiently pass the quality control mechanisms of the endoplasmic reticulum, resulting in reduced lysosomal trafficking, substrate accumulation, and cellular dysfunction. Pharmacological chaperones (PCs) are small molecules that bind and stabilize mutant lysosomal enzymes, thereby allowing proper cellular translocation. Such compounds have been shown to increase enzyme activity and reduce substrate burden in a number of preclinical models and clinical studies. In this Perspective, we review several of the lysosomal diseases for which PCs have been studied and the SAR of the various classes of molecules.

Identification and Characterization of Pharmacological Chaperones to Correct Enzyme Deficiencies in Lysosomal Storage Disorders

Many human diseases result from mutations in specific genes. Once translated, the resulting aberrant proteins may be functionally competent and produced at near-normal levels. However, because of the mutations, the proteins are recognized by the quality control system of the endoplasmic reticulum and are not processed or trafficked correctly, ultimately leading to cellular dysfunction and disease. Pharmacological chaperones (PCs) are small molecules designed to mitigate this problem by selectively binding and stabilizing their target protein, thus reducing premature degradation, facilitating intracellular trafficking, and increasing cellular activity. Partial or complete restoration of normal function by PCs has been shown for numerous types of mutant proteins, including secreted proteins, transcription factors, ion channels, G protein-coupled receptors, and, importantly, lysosomal enzymes. Collectively, lysosomal storage disorders (LSDs) result from genetic mutations in the genes that encode specific lysosomal enzymes, leading to a deficiency in essential enzymatic activity and cellular accumulation of the respective substrate. To date, over 50 different LSDs have been identified, several of which are treated clinically with enzyme replacement therapy or substrate reduction therapy, although insufficiently in some cases. Importantly, a wide range of in vitro assays are now available to measure mutant lysosomal enzyme interaction with and stabilization by PCs, as well as subsequent increases in cellular enzyme levels and function. The application of these assays to the identification and characterization of candidate PCs for mutant lysosomal enzymes will be discussed in this review. In addition, considerations for the successful in vivo use and development of PCs to treat LSDs will be discussed.

Pharmacological Chaperones Restore Function to MC4R Mutants Responsible for Severe Early-Onset Obesity

Heterozygous null mutations in the melanocortin-4 receptor (MC4R) cause early-onset obesity in humans, indicating that metabolic homeostasis is sensitive to quantitative variation in MC4R function. Most of the obesity-causing MC4R mutations functionally characterized so far lead to intracellular retention of receptors by the cells quality control system. Thus, recovering cell surface expression of mutant MC4Rs could have a beneficial therapeutic value. We tested a pharmacological chaperone approach to restore cell surface expression and function of 10 different mutant forms of human melanocortin-4 receptor found in obese patients. Five cell-permeant MC4R-selective ligands were tested and displayed pharmacological chaperone activities, restoring cell surface targeting and function of the receptors with distinct efficacy profiles for the different mutations. Such mutation-specific efficacies suggested a structure-activity relationship between compounds and mutant receptor conformations that may open a path toward personalized therapy. In addition, one of the five pharmacological chaperones restored function to most of the mutant receptors tested. Combined with its ability to reach the central nervous system and its selectivity for the MC4R, this pharmacological chaperone may represent a candidate for the development of a targeted therapy suitable for a large subset of patients with MC4R-deficient obesity.