Mice with Adenine Phosphoribosyltransferase Deficiency Develop Fatal 2,8-Dihydroxyadenine Lithiasis

Abstract

Deficiencies in different steps of purine metabolism give rise to a number of human inherited disorders. Lesch-Nyhan syndrome is a severe neurological disorder, caused by a deficiency in the purine salvage enzyme hypoxanthine phosphoribosyltransferase (HPRT). HPRT-deficient mice have been generated, but have proved to be an unsuccessful model of the human disease. We have suggested that this may be due to a greater dependency in rodents on the other purine salvage enzyme, adenine phosphoribosyltransferase (APRT). We have generated an APRT-deficient mouse line by gene targeting, with a phenotype that closely resembled the symptoms of APRT deficiency in man. APRT null mice were viable, but 90% died prematurely before 6 months of age, displaying highly abnormal kidney morphology, with pathology characteristic of tubule obstruction. These mice have elevated urinary levels of adenine and 2,8-dihydroxyadenine, a highly insoluble adenine derivative, plus birefringent crystalline deposits and calculi within tubules throughout the kidney. A standard therapy for APRT-deficient human patients is the administration of the xanthine oxidase inhibitor, allopurinol. This has proved an effective therapy for APRT null mice, preventing accumulation of 2,8-dihydroxyadenine and much of the resultant renal obstruction, allowing us to establish a breeding line. We believe that these mice should provide a useful model for further study of APRT deficiency in humans. Furthermore, by generating APRT and HPRT double mutants, we will be able to test our hypothesis that both genes must be inactivated in mice before a model for Lesch-Nyhan syndrome can be obtained.

Overview summary

Mice with engineered genetic alterations are used as experimental models of human inherited disorders. There are metabolic differences between humans and rodents, and some models do not show the expected phenotype, for example, the hypoxanthine phosphoribosyltransferase (HPRT)-deficient mouse model of Lesch-Nyhan syndrome. However, we have generated an adenine phosphoribosyltransferase (APRT-deficient mouse model which closely resembled the human disease, 2,8-dihydroxyadenine (2,8-DHA) lithiasis. The null mice died prematurely, presumably due to renal failure. The kidneys of null mice were abnormal with pathology characteristic of the accumulation of crystals and calculi of the insoluble adenine derivative, 2,8-DHA. Administration of allopurinol, as used in therapy of the human disease, has proved effective in the treatment of these mice. Life was prolonged and the accumulation of crystals of 2,8-DHA in the kidneys was largely prevented. Continued study of mice with APRT and HPRT deficiency may produce a model for Lesch-Nyhan syndrome that could be used to investigate gene therapy for this untreatable disorder.