Human metabolic pathways are comprised of many complex and specific reactions. To carry out these specific reactions, the body is highly dependent on enzymes, the workers of the cell. Malfunction in an enzyme can cause mild to severe disruption of the metabolic pathway, where the severity of the damage is dependent on the stage of the metabolic pathway where the malfunction occurs. For example, a malfunctioning enzyme may trigger a cascade effect that can cause the cell significant damage. Certain malfunctions can be caused by exposure to mutagens, such as radiation, ultraviolet light, and carcinogens. Other malfunctions have their root in inherited genetic abnormalities. Fabry disease is one such genetic disorder. A rare X-linked lysosomal storage disorder, Fabry disease is caused by a deficiency in α-galactosidase A (GLA). [1] α-galactosidase A is member of a family of enzymes that catalyzes glycosides, molecules to which sugars are attached. A malfunction in the protein structure of α-galactosidase A causes a buildup of glycolipids that can cause kidney failure, cardiovascular dysfunction, neuropathy, stroke and dermatological manifestations. [2] However, there is hope for individuals who are diagnosed with this disorder. Molecular chaperone therapy, substrate reduction therapy, gene therapy, and other medical procedures and treatments are available for enhancing the quality of life for affected individuals.
Lysosomes
The complications of Fabry disease begin in the lysosome. Lysosomes function as the digestive system of the cell, serving both to degrade extracellular material and to digest obsolete components of the cell itself. [3] The lysosome functions by storing multiple enzymes in a stable state until they are activated to breakdown targeted molecules. Thus, a malfunctioning targeting system can be disastrous to the cell, resulting in two possible scenarios: either the lysosome spills its contents into the cytoplasm, or proteins build up within the lysosome. The former poses less of a threat because the physiology of the enzymes renders them inactive in intracellular pH solutions(the pH of the cytoplasm); only in an acidic environment are lysosomal enzymes active. The latter poses a greater threat, as a buildup of substrate can be toxic to the lysosome and the entire cell.
Enzymatics
Most success in understanding the chemical behavior of the enzyme responsible for Fabry disease came from studying the enzyme’s interaction with a synthetic substrate. Investigation of α-galactosidase activity in normal individuals and residual activity in patients with Fabry disease revealed that there were two α-galactosidase isoenzymes, A and B, with activity towards synthetic substrates (i.e. p-nitrophenyl–α-d-galactoside). [8] The former enzyme, α-galactosidase A, accounts for most of the activity in normal cells and is mostly inactive in patients diagnosed with Fabry disease. The later enzyme, α-galactosidase B, is active only in patients diagnosed with Fabry disease.
α-galactosidase A is a relatively large protein, and, like most eukaryotic proteins, undergoes post-translational modifications. The following is the structure of recombinant monomer α-galactosidase A [8]:
There are two distinct domains present in the monomer of the enzyme. Domain 1 is responsible for all the nucleophilic and acid/base reactions. More specifically, two aspartic acid residues are involved in the reaction center. The pKa of aspartic acid is 4.0, making it a very useful amino acid being used in the acidic environment of the lysosome. The following is a homodimer molecule of α-galactosidase A, which is how it is found naturally in the active form [8]:
There are two major reasons that may cause the malfunction of α-galactosidase A: either a mis-sence mutation, or, a lack of Saposin B molecule. Saposin B is a coenzyme for α-galactosidase A and is needed in the lysosome to help α-galactosidase A bind to the substrate. A lack of Saposin B molecule has been associated with a buildup of globotriaosylceramide (Gb3) in patients diagnosed with Fabry disease. The Saposin B molecule is very simple. Each monomer is composed of three alpha helixes. The following is the dimer molecule of Saposin B[9]:
There are several types of mutations that can affect genes: substitution, insertion, deletion, and frameshift. Each can potentially be very harmful to a cell. However, only the mutations that have a phenotypic effect are important for analysis. The mutation of the α-galactosidase A must occur on the X chromosome, since it is an X-linked disorder. Based on the number of different changes at any nucleotide position (polymorphism), there is no obvious mutational 'hot spot', when considering all 288 point mutations (mis-sense and nonsense) scattered over the 1290 bp coding region (q22.1). [10] Single or multiple mutations can lead to a misfolded and malfunctioning enzyme. Different combinations of mutations observed in patients diagnosed with Fabry disease have made this disorder more difficult to fight. Since a single mutation is not responsible for the disorder, multiple approaches are undertaken to combat the inhibition of α-galactosidase A.
Diagnosis
Diagnosing Fabry disease is difficult due to the vague symptoms the disease produces. The symptoms for Fabry disease include acroparaesthesia (constant feeling of numbness and/or tingling) and pain, which can be triggered by heat and fever. Unfortunately, these symptoms are often misinterpreted and only occasionally lead to the correct diagnosis. [11]
The following is a graph illustrating the frequency of misdiagnosis for Fabry disease compared to other disorders [11]:
Cardiovascular and Renal Effects
As with many diseases, an early diagnosis of Fabry disease gives patients more time to prepare for the effects of the disease. If proper precautions are not taken, the disease can be debilitating and even fatal. Cardiac involvement is one of the three major causes of morbidity and mortality in Fabry disease, together with end-stage renal failure and cerebrovascular events. [12] The build up of Gb3 in cardiac muscle can cause severe heart problems, such as cardiac hypertrophy, ischemia, and other coronary diseases.
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The darkening of the walls around the left ventricle is a good indicator of cardiac hypertrophy. |
Treatment
Since Fabry disease is a genetic disorder, enzyme replacement therapy (ERT) is a good candidate for treatment if diagnosed at an early age. Data from FOS – the Fabry Outcome Survey –and from clinical trials on the effects of enzyme replacement therapy in children with Fabry disease have demonstrated that ERT with agalsidase alfa is well tolerated and has beneficial clinical effects on pain and gastrointestinal symptoms. [16] Although a cure for Fabry disease is still in the future, medical procedures and medications are currently available to prevent organ failure. Replagal (Agalsidase alfa), a synthetic α-galactosidase A enzyme, treats some of the renal effects of Fabry disease.[17] Enzyme replacement therapy can also help bring neurological relief to the patients, such as decreasing neuropathic pain and increasing peripheral nerve sensory function.
Molecular Chaperone Therapy (MCT) is also a valuable tool for the treatment of Fabry disease. Molecular chaperones and foldases are a diverse group of proteins that bind to misfolded or unfolded proteins (non-native or unstable proteins). [19] In Fabry disease, MCT therapy introduces a chaperone that is able to bind to improperly folded α-galactosidase A or β-galactosidase A enzymes and fold it into the proper confirmation. In addition, the chaperone can be used to help Saposin B, the coenzyme for α-galactosidase A, bind more effectively to the enzyme. This chaperone can also be used to prevent inhibitors of α-galactosidase A from binding to the enzyme.
Conclusion
Fabry disease is a genetic disorder that expresses a malfunctioning protein in numerous individuals. It is significantly important for people to understand the disorder, so an early diagnosis can prevent organ failures in adulthood. The biochemical understanding allows different treatment methods for this disorder, however the treatment methods must be more efficient and more specific to fully cure the disease. With combination of biotechnology and information gather via biochemistry, the end of Fabry disease must be near.
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