Purine nucleoside phosphorylase deficiency

Overview
Purine nucleoside phosphorylase deficiency, often called PNP-deficiency, is a rare congenital immunodeficiency of purine nucleoside phosphorylase. This enzyme is important in the purine degradation pathway. A deficiency of it causes T-cell immunodeficiency. It is also often associated with neurological disorders such as mental retardation. This autosomal recessive metabolic disorder results in severe combined immunodeficiency.

Pathophysiology
The disorder is caused by a disruption of the purine nucleoside phosphorylase, a key enzyme in the purine salvage pathway.

This enzyme is required for purine degradation. Specifically, it catalyzes the conversion of inosine and guanosine to hypoxanthine. A deficiency of it leads to build up of elevated deoxy-GTP (dGTP) levels resulting in T-cell toxicity and deficiency.

In contrast to adenosine deaminase deficiency (another deficiency of purine metabolism), there is minimal disruption to B cells.

Symptoms
In addition to the symptoms associated with immunodeficiency, such as depletion of T-cells, decline of lymphocyte activity, and an abrupt proliferation of both benign and opportunistic infections, PNP-deficiency is often characterized by the development of autoimmune disorders. Lupus-erythematosis, autoimmune hemolytic anemia, and idiopathic thrombocytopenic purpura have been reported with PNP-deficiency.

Neurological symptoms, such as developmental decline, hypotonia, and mental retardation have also been reported.

Genetic prevalence


PNP-deficiency is extremely rare. Only 33 patients with the disorder in the United States have been documented. In the United Kingdom only one child has been diagnosed with this disorder.