Tarui disease, more commonly known as glycogen storage disease type VII, or GSD7, is an inherited disease caused by a deficiency in an enzyme responsible for breaking down glycogen in our body.
Glycogen storage diseases
Glycogen is a form of energy storage in the body. The main energy supply to the cells is glucose – a monosaccharide molecule. When glucose accumulates in the body, certain cells can convert it into a long chain of glycogen which is used to store excess glucose and release it when needed.
Glycogen storage diseases are a group of diseases that disrupt the glycogen breakdown process and impair the body’s ability to utilize energy. Since glycogen serves as a form of energy storage in the liver, muscles, kidneys, and other organs, these diseases cause impairments to organs such as the kidneys, heart, and muscles.
In Tarui’s disease, the deficient enzyme is called phosphofructokinase. This enzyme participates in glycolysis (glycogen breakdown) and is involved in changes undergone by glucose that are necessary for its utilization by the cells.
When this enzyme is defected, the glycolysis process is impaired, and glucose is left unused in the cells. The presence of glucose encourages creating more glycogen to store it for future use. Since phosphofructokinase is deficient, glucose utilization in the cells becomes abnormal, glycogen accumulates in the cells and cannot be utilized effectively.
Several types of the enzyme phosphofructokinase exist in our body. One type of this enzyme is found in muscle cells, another type exists in liver cells, and both types are found in the red blood cells.
In most cases of Tarui’s disease, the deficient enzyme is muscle phosphofructokinase, while the liver enzyme functions normally. As a result, this disease’s main effect is on muscle function and not on liver function. Since our liver is responsible for the general metabolism of nutrients throughout our body, the disease mainly affects the muscles.
GSD type VII has an autosomal recessive inheritance pattern. Therefore, two copies of the mutated gene must be inherited for the disease to develop. The mutated gene is coded to the enzyme muscle phosphofructokinase. This gene is located on chromosome 12 (12q13).
Several gene mutations were found to cause this disease. The disease is more frequent in people of Ashkenazi Jewish (eastern European) ancestry, and in people of Japanese ancestry.
Signs and symptoms
Symptoms first appear during childhood and include muscle pains and cramps (caused by energy deficiency in the muscles), exercise intolerance, and fatigue. Foods rich in sugar may worsen the symptoms due to the muscle’s inability to break down glucose into accessible energy.
Some patients suffer from damage to their red blood cells, with signs of hemolysis (destruction of red blood cells). A milder form of the disease usually develops only in later stages of life, during adulthood.
Patient’s blood tests show elevated levels of the enzyme muscle creatine kinase. Lab test results indicate signs of hemolysis. In addition, high levels of uric acid (hyperuricemia) can be found in the blood. Genetic testing can also be used to confirm mutations in the relevant gene.
There is currently no cure or available treatment for Tarui’s disease. Patients are advised to avoid heavy exercise due to muscle impairment.