Allopurinol-induced xanthine lithiasis in a patient with partial deficiency of hypoxanthine-guanine phosphoribosyltransferase

The deficiency of hypoxanthine-guanine phosphoribosyltransferase (HGPRT) is a very rare hereditary disorder causing excessive purine production (1). When the enzyme deficiency is virtually complete it causes the pediatric neurological LeschNyhan syndrome (LNS); when partial, it causes severe gout and uric acid lithiasis. In most purine overproducers, including the HGPRT deficient patients, the pathology caused by the excessive amounts of uric acid is avoided successfully by the treatment with the xanthine oxidase inhibitor allopurinol, which effectively reduces uric acid levels. In overproducers with normal HGPRT activity, allopurinol administration is associated also with a significant deceleration of purine synthesis, manifest in a marked reduction in the amount of total purines excreted. In HGPRT deficiency this effect does not occur and the amount of the excreted oxypurines, replacing the uric acid, is markedly increased, rendering such patients to be at risk for xanthine stone formation. Xanthine lithiasis was indeed documented in allopurinol-treated children with the complete enzyme deficiency (2-4). The following is the first case of xanthine stones formation in an allopurinol-treated gouty patient with partial HGPRT deficiency (5).