Adenosine Monophosphate Deaminase Deficiency
A rare metabolic disorder for which two forms have been described. Lack of activity of the erythrocyte isoform of adenosine monophosphate (AMP) deaminase has been described in subjects with low plasma uric acid levels without obvious clinical relevance and will not be described further. Myoadenylate deaminase deficiency is an inherited disorder of muscular energy metabolism with a lack of AMP deaminase activity in skeletal muscle. It is characterised by exercise-induced muscle pain, cramps and/or early fatigue.
About 1-2% of the Caucasian population carries the genetic defect causing myoadenylate deaminase deficiency, but only a minority of carriers develop symptoms. The prevalence is unknown but several hundred patients with the disorder have been reported in case reports and patient series. Men and women are equally affected.
The vast majority of patients suffer from post-exercise symptoms: rapid fatigue, cramps or myalgias. Approximately equal proportions of the patients first develop symptoms during childhood, adolescence, or as young or older adults. After progression of the symptoms over the first few years, the clinical course usually stabilises. There is no evidence of muscular dystrophy or muscular wasting. The disorder exclusively affects skeletal muscle. Smooth muscle or other organs are not affected as the disorder is associated with a specific lack of skeletal muscle adenylate deaminase activity.
The vast majority of patients with this disease are homozygous for the nonsense C34-T mutation in the AMPD1 (adenosine monophosphate deaminase 1) gene. This mutation creates an early stop codon thus preventing the synthesis of an enzymatically active protein. The deficiency disrupts the purine nucleotide cycle, and thus muscle energy production. Surprisingly, however, asymptomatic AMP deaminase-deficient subjects have been reported, indicating that additional factors are likely to be involved in the development of myopathic symptoms.
The diagnosis is based on histochemical staining or biochemical analysis of a muscle biopsy showing a lack of muscle adenylate deaminase activity, or on molecular identification of the disease-causing mutation.
Transmission is autosomal recessive.
Management and treatment
Unfortunately, there is no medical cure for this disorder. Symptoms improve with administration of D-ribose. However, the effects of this sugar are only short-tem and it has no beneficial effect during subsequent days.