Gaucher’s Disease is an inborn error of metabolism passed from generation to generation through an autosomal recessive mode of inheritance. It is caused by a mutation in the GBA gene that results in a defective GlcCerase, resulting in a buildup of the GlcCer substrate (Brady et al. 1965). GlcCerase is a lysosomal enzyme, explaining the cellular localization of GlcCer in patients with Gaucher’s Disease. The most prevalent GBA mutation among type I Gaucher patients is a missense mutation that results in an N370S substitution (Dvir et al. 2003).
Dvir et al. crystallized GlcCerase in 2003, which paved the way for understanding the molecular mechanism of Gaucher’s Disease. X-ray crystallography data showed that GlcCerase contained three domains. Notably, the catalytic domain III of GlcCerase resembles a TIM barrel structure and glycosylation is present in domain I (Dvir et al. 2003). Dvir et al. also employed site-directed mutagenesis modeling and suggested that E235 serves as the acid-base catalyst while E340 serves as the nucleophile for GlcCerase’s hydrolysis reaction (Dvir et al. 2003). The authors placed the three-dimensional structure of GlcCerase in conversation with the most common mutation in type I Gaucher patients: an N370S substitution. They proposed that position 370 is too far away from domain III to be involved in direct catalysis, so this substitution may have an effect on the association of GlcCerase to other biomolecules that enhance its catalytic activity (Dvir et al. 2003).
Effects of Mutation
Salvioli et al. investigated the effects of defective GlcCerase with an N370S substitution and proposed a molecular mechanism for reduced GlcCerase activity. This team of scientists previously demonstrated that saposin C is an activator of GlcCerase and enhances the ability of normal GlcCerase to associate with the plasma membrane (Salvioli et al. 2005). However, under conditions of reduced lipid content similar to the lysosome, N370S GlcCerase associates with the plasma membrane to a lesser extent than normal GlcCerase even after interaction with a saposin C activator (Salvioli et al. 2005). This phenomenon was observed through co-localization experiments involving murine antibodies. The nature of the amino acid substitution likely explains this phenomenon – a positively charged Asn residue at position 370 allows GlcCerase to more readily associate with the negatively charged plasma membrane than a polar Ser residue. A relative activity of 10% was observed for N370S GlcCerase compared to normal GlcCerase (Salvioli et al. 2005). The authors were also careful to point out that the levels of GlcCerase, saposin C and anionic phospholipid content were similar in wild type cells and Gaucher cells (ie. cells with a high concentration of GlcCer), thus attributing the differential association between GlcCerase and its activators to the substitution.
Cellular Trafficking Problems
GlcCerase is synthesized on an endoplasmic reticulum-bound polyribosome and is subsequently glycosylated for trafficking to the lysosome. Ron and Horowitz attribute the severity of Gaucher’s Disease to the ability for the endoplasmic reticulum to retain GlcCerase, as opposed to being degraded by the proteasome (Ron and Horowitz 2005). This is most likely due to the improper folding intrinsic to defective GlcCerase, and this improper folding is detected by ER quality control measures, including the ubiquitin-proteasome system (Ron and Horowitz 2005). Differential levels of ER retention were confirmed by lower levels of GlcCerase in Gaucher cells than normal cells. This phenomenon was also supported by the association of mutant GlcCerase with calnexin, a protein localized in the endoplasmic reticulum (Ron and Horowitz 2005).
The mechanism through which the symptoms of Gaucher’s Disease present in a patient is a difficult concept to investigate. The tell-tale clinical presentation of Gaucher cells (ie. macrophages with large concentrations of GlcCer) was well documented, but the pathophysiology still remained unclear. Gery et al. studied how an accumulation of GlcCer in macrophages leads to physiological effects at the cellular level (Gery et al. 1981). Using a mouse model, these scientists observed a high concentration of lymphocyte activating factor released from macrophages. When GlcCer was added to epithelial cells, however, no cytotoxic effects were detected (Gery et al. 1981). This was the first study that evaluated the pathophysiology of Gaucher’s Disease.
The pathophysiology of Gaucher’s Disease was also addressed in a recent study by Pavlova et al. As part of normal intracellular turnover, macrophages recycle leukocytes of the immune system and engulf large quantities of GlcCer present on the plasma membrane (Pavlova et al. 2014). When GlcCerase of macrophages is defective, GlcCer accumulates in these cells and presents with the clinical manifestation of Gaucher cells. The result of GlcCer accumulation is a release of cytokines and chemokines, as well as macrophage invasion of tissues of other organ systems (Pavlova et al. 2014). A consequence of high levels of cytokine release is an activation of the inflammatory response in different tissues, explaining the clinical presentation of hepatosplenomegaly. Another consequence is a higher likelihood of developing lymphoma. Pavlova et al. reported a decreased rate of lymphoma and B cell proliferation among mice treated with GENZ 112638, an inhibitor of GlcCer synthesis (Pavlova et al. 2014). Their results also demonstrated a role for sphingolipids in the propensity of B cell proliferation and other Gaucher-associated symptoms (Pavlova et al. 2014).
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