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Purines are key components of cellular energy systems (eg, ATP, NAD), signaling (eg, GTP, cAMP, cGMP), and, along with pyrimidines, RNA and DNA production. Purines and pyrimidines may be synthesized de novo or recycled by a salvage pathway from normal catabolism. The end product of complete catabolism of purines is uric acid; catabolism of pyrimidines produces citric acid cycle intermediates. For a more complete listing of disorders of purine and pyrimidine metabolism, see Table Disorders of Purine and Pyrimidine Metabolism.
Disorders
of Purine Salvage
Lesch-Nyhan
syndrome:
This is a rare, X-linked, recessive disorder caused by deficiency of hypoxanthine-guanine phosphoribosyl transferase (HPRT); degree of deficiency (and hence manifestations) vary with the specific mutation. HPRT deficiency results in failure of the salvage pathway for hypoxanthine and guanine. These purines are instead degraded to uric acid. Additionally, a decrease in inositol monophosphate and guanosyl monophosphate leads to an increase in conversion of 5-phosphoribosyl-1-pyrophosphate (PRPP) to 5-phosphoribosylamine, which further exacerbates uric acid overproduction. Hyperuricemia predisposes to gout and its complications. Patients also have a number of cognitive and behavioral dysfunctions, etiology of which is unclear; they do not appear related to uric acid.
The disease usually manifests between 3 mo and 12 mo of age with the appearance of orange sandy precipitate (xanthine) in the urine; it progresses to CNS involvement with mental retardation, spastic cerebral palsy, involuntary movements, and self-mutilating behavior (particularly biting). Later, chronic hyperuricemia causes symptoms of gout (eg, urolithiasis, nephropathy, gouty arthritis, tophi).
Diagnosis is suggested by the combination of dystonia, retardation, and self-mutilation. Serum uric acid levels are usually elevated, but confirmation by HPRT enzyme assay is usually done.
CNS dysfunction has no known treatment; management is supportive. Self-mutilation may require physical restraint, dental extraction, and sometimes drug therapy; a variety of agents has been used. Hyperuricemia is treated with a low-purine diet (eg, avoiding organ meats, beans, sardines) and allopurinol , a xanthine oxidase inhibitor (the last enzyme in the purine catabolic pathway). Allopurinol prevents conversion of accumulated hypoxanthine to uric acid; because hypoxanthine is highly soluble, it is excreted.
Adenine
phosphoribosyltransferase deficiency:
This is a rare autosomal recessive disorder that results in the inability to salvage adenine for purine synthesis. Accumulated adenine is oxidized to 2,8-dihyroxyadenine, which precipitates in the urinary tract, causing problems similar to those of uric acid nephropathy (eg, renal colic, frequent infections, and, if diagnosed late, renal failure). Onset can occur at any age.
Diagnosis is by demonstrating elevated levels of 2,8-dihyroxyadenine, 8-hyroxyadenine, and adenine in urine and confirmed by enzyme assay; serum uric acid is normal.
Treatment is with dietary purine restriction, high fluid intake, and avoidance of urine alkalinization. Allopurinol can prevent oxidation of adenine; renal transplantation may be needed for end-stage renal disease.
Disorders of
Purine Nucleotide Synthesis
Phosphoribosylpyrophosphate
synthetase superactivity:
This is an X-linked, recessive disorder that causes purine overproduction. Excess purine is degraded, resulting in hyperuricemia and gout, and neurologic and developmental abnormalities. Diagnosis is by enzyme studies on RBCs and cultured skin fibroblasts. Treatment is with allopurinol and a low-purine diet.
Adenylosuccinase
deficiency:
This is an autosomal recessive disorder causing profound mental retardation, autistic behavior, and seizures. Diagnosis is by identifying elevated levels of succinylaminoimidazole carboxamide riboside and succinyladenosine in CSF and urine. There is no effective treatment.
Disorders of
Purine Catabolism
Myoadenylate
deaminase deficiency (or muscle adenosine monophosphate deaminase
deficiency):
This enzyme converts AMP to inosine and ammonia. Deficiency may be asymptomatic or it may cause exercise-induced myalgias or cramping; expression seems to be variable because, despite the high frequency of the mutant allele (10 to 14%), the frequency of the muscle phenotype is quite low in patients homozygous for the mutant allele. When symptomatic patients exercise, they do not accumulate ammonia or inosine monophosphate as do normal people; this is how the disorder is diagnosed. Treatment is exercise modulation as appropriate.
Adenosine
deaminase deficiency:
Adenosine deaminase converts adenosine and deoxyadenosine to inosine and deoxyinosine, which are further broken down and excreted. Enzyme deficiency (from 1 of > 60 known mutations) results in accumulation of adenosine , which is converted to its ribonucleotide and deoxyribonucleotide (dATP) forms by cellular kinases. The dATP increase results in inhibition of ribonucleotide reductase and underproduction of other deoxyribonucleotides. DNA replication is compromised as a result. Immune cells are especially sensitive to this defect; adenosine deaminase deficiency causes one form of severe combined immunodeficiency (see Immunodeficiency Disorders: Severe Combined Immunodeficiency (SCID)). Diagnosis is by low RBC and WBC enzyme activity. Treatment is by bone marrow or stem cell transplantation and enzyme replacement therapy. Somatic cell gene therapy is being evaluated as well.
Purine
nucleoside phosphorylase deficiency:
This rare, autosomal recessive deficiency is characterized by immunodeficiency with severe T-cell dysfunction and often neurologic symptoms. Manifestations are lymphopenia, thymic deficiency, recurrent infections, and hypouricemia. Many have developmental delay, ataxia, or spasticity. Diagnosis is by low enzyme activity in RBCs. Treatment is with bone marrow or stem cell transplantation.
Xanthine
oxidase deficiency:
This enzyme catalyzes uric acid production from xanthine and hypoxanthine. Deficiency causes buildup of xanthine, which may precipitate in the urine, causing symptomatic stones with hematuria, urinary colic, and UTIs. Diagnosis is by low plasma uric acid and high urine and plasma hypoxanthine and xanthine. Enzyme determination requires liver or intestinal mucosal biopsy and is rarely indicated. Treatment is high fluid intake to minimize likelihood of stone formation and allopurinol in some patients.
Disorders
of Pyrimidine Metabolism
Uridine monophosphate
synthase deficiency (hereditary orotic aciduria):
This enzyme catalyzes orotate phosphoribosyltransferase and orotidine-5′-monophosphate decarboxylase reactions. With deficiency, orotic acid accumulates, causing clinical manifestations of megaloblastic anemia, orotic crystalluria and nephropathy, cardiac malformations, strabismus, and recurrent infections.
Diagnosis is by enzyme assay in a variety of tissues. Treatment is with oral uridine supplementation.
Last full review/revision November 2005
Content last modified November 2005
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