Introduction
Tarui’s disease is a genetic metabolic disorder that affects the utilization of carbohydrates for energy and the storage of excessive carbohydrates as glycogen. The disorder is a result of a deficiency of the enzyme phosphofructokinase (PFK), which is vital in the biochemical processes that lead to the conversion of stored glycogen into glucose. Therefore, insufficient quantities of this enzyme cause a buildup of glycogen, which comes along with fatal systemic consequences. The body of patients with this malady prefers the use of fats for energy instead of the conventional energy source of carbohydrates. Tarui’s disease was given the name after a Japanese scientist Seiichiro Tarui who first identified it in the year 1965 (Glycogen storage disease VII, 2014). The disease is also known as glycogen storage disorder type VIII (GSD VII).
GSD VIII is a rare disease with only about one hundred cases being documented so far in the entire world. The community largely affected by the ailment is the Ashkenazi Jews (Raben, & Sherman, 1995) though Japanese and Italians have also been affected.
Symptoms of Tarui’s Disease
The indicators of Tarui’s disease include intolerance to exercise, which often happens after taking a carbohydrate-rich meal. The presence of carbohydrates in the body reduces the quantities of fats moving in the blood. Myoglobin that colors muscles disintegrates at a very high rate and is later excreted in the urine leading to the production of urine with a brownish tinge (rust-like appearance). This occurrence is referred to as myoglobinuria. Muscular pain and cramps are also a common phenomenon in this condition (Finsterer & Stöllberger, 2008). General weakness of the body usually occurs during exercise and resolves itself during resting. In instances where the deficiency of phosphofructokinase spreads over to the erythrocytes, the red blood cells break down leading to elevated amounts of bilirubin in blood (Thomas & Frontera, 2007).
The clinical manifestations of Tarui’s disease lead to the categorization of the condition into four main groups namely infantile onset, late onset, hemolytic and classic (Toscano & Musumeci, 2007). Classic Tarui’s disease is typified by intolerance to working out, exhaustion, myoglobinuria, and nausea that may be accompanied by vomiting after an attempt to work out. There may also be incidences of jaundice as well as hyperuricemia. The infantile disease is exemplified by myopathy, psychomotor hindrance, cataracts, joint contractures, and infant mortality (Amit et al. 1992). In the late onset form of the disease, the symptoms reveal in later life and are similar to those of the classic form. The hemolytic form of the disease appears alongside hereditary nonspherocytic hemolytic anemia without muscle signs.
Causes of the Disease
Tarui’s disease is an outcome of a transformation in the “PFKM gene” responsible for the manufacture of phosphofructokinase (Raben, & Sherman, 1995). Phosphofructokinase is a multipart enzyme consisting of three different subunits specific to the liver (L), platelets (P) and muscle (M). The specific genes that code for each of these subunits are denoted PFK-M (muscle), PFK-L (liver) and PFK-P (platelets). These genes are located at different chromosomal positions. For example, the PFK-M gene is situated on chromosome 12 (Toscano & Musumeci, 2007). Red blood cells have the M and L variants of PFK and this is why a deficiency of PFK-M halves the activity of erythrocytes (Eber, 2003). Several studies show that the mutation happens on the fifth exon of the PFKM gene and that the mutation is homozygous. Tarui’s disease is passed on through the genes in an autosomal recessive fashion.
A mutated PFK-M gene implies that the enzyme PFK is not produced in the muscles, which have large energy requirements. Glycolysis is arrested and no energy is produced (Ozen, 2007). Therefore, a muscle-cell energy catastrophe occurs leading to the destruction and degeneration of muscle tissue, which manifests as Tarui’s disease.
The Biochemical Pathway Affected
Glycolysis is the initial stage in the production of energy from carbohydrate sources. A series of ten chemical reactions convert glucose into pyruvic acid that enters the tricarboxylic acid cycle. During glycolysis, glucose is phosphorylated by hexokinase to form glucose-6-phosphate, which is then converted to fructose-6-phosphate in a reaction catalyzed by the enzyme phosphoglucoisomerase. In the third step, fructose-6-phosphate is phosphorylated to yield fructose-1, 6-bisphosphate in a reaction mediated by the enzyme phosphofructokinase. This reaction is the rate-limiting step of glycolysis that is affected in Tarui’s disease due to the absence or insufficient amounts of phosphofructokinase. Consequently, glycolysis in the muscles becomes arrested at this point. Most of the fructose-6-phosphate produced enters the hexose monophosphate shunt leading to elevated amounts of 5-phosphoribosyl pyrophosphate (PRPP).
Diagnosis and Treatment of Tarui’s Disease
Laboratory tests are employed in the detection of Tarui’s disease. Such tests include muscle and red blood cell enzyme assay for phosphofructokinase. The muscle tissue is usually obtained from the outer thigh. It is realized that the muscles show no activity of the enzyme at all, whereas the red blood cells display only half of the normal PFK activity (Lerardi-Curto, 2013). The quantities of glycogen, as well as hexose monophosphates in the muscles, also become elevated. It is also noted that the amount of 2,3-diphosphoglycerates is reduced. The levels of serum creatine kinase also surpass the normal values. An examination of PAS-stained muscle tissue of affected individuals under the electron microscope reveals deposits of glycogen in “the sub-sarcolemmal and inter-myofibrillar areas” (Toscano & Musumeci, 2007, p. 107). The precise transformation can be obtained by investigating the gene sequence. During the diagnosis, it is imperative for patients to be offered genetic counseling. In families with known cases of mutation, carrier, prenatal and pre-implantation genetic diagnostics are often performed prior to carrying out in vitro fertilization (Ronquist, 2002). Such tests help minimize the chances of having offspring with the infantile form of GSD VII that is fatal.
There is no lasting therapy for this genetic disease. However, taking an appropriate diet goes a long way in minimizing the manifestation of the disease symptoms. For example, it is advisable to consume foods rich in fats so that muscles can break down glycerol instead of glucose for energy. The destruction of muscle cells causes the leakage of myoglobin into the blood hence the production of stained urine. This happening has serious implications on the kidney and requires instant medical attention. Dialysis is then performed to rid the blood plasma of noxious substances. Medications that bring down the quantities of blood uric acid are usually administered to prevent the development of gout. GSD VII patients ought to avoid strenuous activities to avoid the detrimental effects of exercise intolerance.
Approaches under Investigation
The lack of a permanent cure for Tarui’s disease has led to extensive research on the condition. The research aims at finding a cure as well as improving the quality of life for the affected individuals. Some of the issues being investigated include improved methods of diagnosis to identify the condition before it progresses to advanced stages. It is imperative that the mechanisms of the disease are well understood to determine a disease cure. Therefore, there is ongoing research to establish the precise association between exercise and eating habits in metabolic ailments. Animal models are being used to attain this objective as well as to test the efficacy of treatment regimens. It is also hypothesized that providing the missing enzyme can improve the outcomes of Tarui’s disease. Consequently, enzyme replacement therapies are being studied for their effectiveness. Interchanging the defective gene with a normal one in a procedure known as gene therapy is also being studied.
Conclusion
Tarui’s disease is a manageable condition apart from the infantile form that is fatal. Minimizing the intake of carbohydrates and increasing the amount of fats in the diet of affected individuals can help reduce the symptoms of the disease. It is also important for GSD VII patients to avoid strenuous activities.
References
Amit, R., Bashan, N., Abarbanel, J. M., Shapira, Y., Sofer, S., Moses, S. (1992). Fatal familial infantile glycogen storage disease: Multisystem phosphofructokinase deficiency. Muscle Nerve, 15(4), 455-458.
Eber, S. W. (2003). Disorders of erythrocyte glycolysis and nucleotide metabolism. In Handin, I. R, Lux, S. E., & Stossel, T. P. (Eds.), Blood: Principles and practice of hematology, volume 1 (pp. 1887-1920). Philadelphia, PA: Lippincott William & Wilkins.
Finsterer J., &Stöllberger C. (2008). Progressive mitral valve thickening and progressive muscle cramps as manifestations of glycogenosis VII (Tarui’s disease). Cardiology, 110(4), 238-240.
Glycogen storage disease VII. (2014). Web.
Lerardi-Curto, L. (2013). Genetics of glycogen-storage disease type VII. Web.
Ozen H. (2007). Glycogen storage diseases: New perspectives. World Journal of Gastroenterology, 13(18), 2541-2553.
Raben, N., & Sherman, J. B. (1995). Mutations in muscle phosphofructokinase gene. Human Mutations, 6(1), 1-6.
Ronquist, G. (2002). Glycogenosis type VII (Tarui’s disease); diagnostic considerations and late sequelae. Southern Medical Journal, 95(12), 1361-1362.
Thomas, A-M., & Frontera, W. R. (2007). Muscle disease and dysfunction. In Magee, D. J., Zachazewski, J. E. & Quillen, W. S. (Eds.), Pathology and intervention in musculoskeletal rehabilitation (pp. 912-936).USA: Elsevier Saunders.
Toscano, A. & Musumeci, O. (2007). Tarui disease and distal glycogenoses: Clinical and genetic update. Acta Myologica, 26(2), 105-107.