Pompe disease, also known as acid maltase deficiency or glycogenosis type II, is an inherited neuro-muscular disease. This disease disrupts the breakdown of glycogen in certain cells due to a mutated gene coding the enzyme acid alpha-glucosidase (also known as acid maltase).
This is the first type of glycogen storage disease described in medical literature, first described in 1932.
Symptoms
Symptoms of GSD2 are like those of other glycogen storage diseases. Therefore, careful differential diagnosis is highly important for providing the most suitable treatment.
This disease has two main forms, differentiated by the severity of heart impairment and age of onset:
Infantile-onset (Early-onset)
This disease affects infants within the first months of life up to one year of age. It is expressed by very low enzyme activity (up to one percent of normal activity in healthy people) and causes severe myopathy (muscle weakness). This weakness creates difficulties in the lifting of the head, feeding difficulties, and delayed motor development.
In addition, babies with infantile onset Pompe disease suffer from chronic respiratory tract infections, an enlarged heart, and an enlarged liver. In untreated cases, the median age of death is around eight months. Prevalence of this disease is 1 in 14,000 people.
Late-onset
Late-onset Pompe develops in late childhood or adulthood. Enzyme activity in these patients is more than sixty percent lower than in healthy people. These cases often do not involve heart defects.
However, late-onset Pompe patients experience muscle weakness in the shoulders, waist, and limbs. Patients also suffer from respiratory tract infections. Respiratory failure is often the cause of death. Prevalence of this form is 1:60,000.
Causes
The disease is caused by a mutation in the GAA gene that provides instructions for producing acid maltase and is located on chromosome 17. As of today, 289 different gene mutations were identified, 67 of them have no impact on patient’s lives.
Acid maltase is a lysosomal enzyme that hydrolyzes glycogen after it penetrates the cell. Maltase deficiency causes parts of the glycogen to remain whole and accumulate in the cytoplasm and lysosome, impairing cell activity.
Diagnosis
People with a family history of GSD2 (who have at least one family member with this disorder) can undergo genetic screening tests to detect the relevant gene mutation. Prenatal genetic counselling is also recommended.
Genetic testing includes taking a biopsy and testing the level of the enzyme acid alpha-glucosidase in muscle or skin samples.
Treatment
In the past, Pompe disease had no cure and only supportive treatment was available. However, in 2006, the Food and Drug Administration (FDA) approved a new drug for treating this disease.
The drug, marketed as Myozyme, replaces the missing enzyme with biological recombinant enzymes. It is given as an infusion (drip) once every two weeks. Recent studies showed that with early treatment, results are encouraging.
Older patients showed a significant improvement in walking abilities and lung function, and younger patients showed a decreased heart size and improvement in respiratory function following treatment. In addition, the drug improves patients’ life expectancy.
