22q11.2 deletion syndrome
Jun. 01, 2020
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Panel A (left) shows the processing of normal alpha-galactosidase A. Newly synthesized alpha-galactosidase A is translocated into the endoplasmic reticulum, where molecular chaperones facilitate its proper folding and dimerization by specialized processing enzymes. The molecular chaperones then dissociate from the folded, dimerized enzyme, which moves to the Golgi apparatus and then to lysosomes, where the enzyme is stable and active in the acidic environment of these organelles. Panel B (right) shows the processing of mutant alpha-galactosidase A. Most mutations in the alpha-galactosidase A gene encode alpha-galactosidase A molecules that are misfolded, misassembled, or aggregated in the endoplasmic reticulum, where they are degraded, presumably by the ubiquitin-proteasome pathway. However, certain missense mutations decrease the stability of the enzyme, but the conformation of the active site is retained. Most of this type of mutant enzyme is degraded in the endoplasmic reticulum. However, these mutant forms of alpha-galactosidase A may be stabilized by chemical chaperones, such as galactose, that bind to the active site of the enzyme, promote folding, and stabilize the mutant enzyme. Some of the enzyme then reaches the lysosomes, where it retains low levels of activity. In the lysosomes, the accumulated glycosphingolipid substrates displace the chemical chaperones and are hydrolyzed by the enzyme. (Modified from Kuznetsov and Nigam. Source: Frustaci A, Chimenti C, Ricci R, et al. Improvement in cardiac function in the cardiac variant of Fabry's disease with galactose-infusion therapy. N Engl J Med 2001;345:25-32. Wikimedia Commons. Creative Commons Attribution-Share Alike 4.0 International License, https://creativecommons.org/licenses/by-sa/4.0/deed.en.)