Orotic Aciduria

At a glance

Very rare congenital disorder characterized by an inborn error of pyrimidine metabolism resulting in hematologic (megaloblastic anemia unresponsive to vitamin C, vitamin B₁₂, or folic acid) and neurologic (growth and mental retardation) manifestations. Onset in childhood.

Synonyms

Hereditary Orotic Aciduria; Uridine Monophosphate Synthase Deficiency.

Incidence

Very rare, fewer than 30 patients are reported. Only one patient with Type 2 has been described.

Genetic inheritance

Autosomal recessive.

Pathophysiology

Compound heterozygous mutation of the UMPS (Uridine Monophosphate Synthase) gene on chromosome 3q21.2. Uridine monophosphate synthase is a bifunctional sequential enzyme involved in the de novo synthesis of pyrimidines. These functions are also distinguished clinically. Type I is a deficiency of the OMP-PP portion of uridine monophosphate synthetase. This reaction catalyzed by orotate phosphoribosyltransferase converts orotic acid into orotidine monophosphate. Type II is a deficiency of the OMP-DC portion of uridine monophosphate synthetase. This reaction, catalyzed by orotidine decarboxylase, transforms orotidine monophosphate into uridine monophosphate. The classic Type I disease is caused by diminished activity of both enzymes; Type II disease could be caused by diminished activity of orotidine decarboxylase only.

Diagnosis

Urinalysis shows massive oversecretion of orotic acid (up to 1.5 g/day) and crystalluria. However, orotic aciduria can also result from some urea cycle defects, parenteral nutrition, or essential amino acid deficiency. The enzymatic diagnosis can be performed on red blood cells. A deficiency of orotidylic acid pyrophosphorylase and/or decarboxylase activities in leukocytes, erythrocytes, hepatic cells, and cultured skin fibroblasts is characteristic. The presence in the serum of megaloblastic anemia is typical.

Clinical aspects

Megaloblastic anemia with anisocytosis and hypochromia appearing a few weeks or months after birth is the first manifestation. It does not respond to iron, folic acid, or vitamin B₁₂. If the disorder is unrecognized, failure to thrive with growth and developmental retardation ensues. Urinary obstruction (renal, ureteral, or urethral) from orotic acid crystals has been reported. For treatment, the enzymatic defect can be bypassed by the administration of uridine, which is converted into uridine monophosphate by uridine kinase. The hematologic response is quick. The dosage is adapted in order to achieve the lowest possible urinary output of orotic acid.

Precautions before anesthesia

Check hemoglobin level.

Anesthetic considerations

Perioperative hydration should be sufficient to maintain a high urine output to prevent increasing the urinary concentration of orotic acid and the ensuing risk of urinary stones.

Pharmacological implications

No specific implications.

References