Lesch-Nyhan Disease and Its Variants: Phenotypic and Mutation Spectrum of Hypoxanthine-Guanine Phosphoribosyltransferase Deficiency in Argentine Patients

Abstract Hypoxanthine-guanine phosphoribosyltransferase (HPRT) deficiency is a disorder of purine metabolism responsible for Lesch-Nyhan Disease (LND) and its variants, HPRT-related hyperuricemia with neurologic dysfunction (HND) and HPRT-related hyperuricemia (HRH). The objective of this study was to characterize a cohort of Argentine patients with HPRT deficiency diagnosed in a single center. Results: Twenty nine patients were studied, including 12 LND, 15 HND and 2 HRH. The average onset age was 0.64 years for LND with motor delay as the main manifestation, 8.84 years for HND and 2.5 years for HRH; nephrological manifestations predominated as presenting features in these variants. The average diagnosis age was 3.58 years for LND, 17.21 years for HND and 2.5 years for HRH. Clinical heterogeneity was more evident in HND, even in members of the same family. All patients presented hyperuricemia and no detectable HPRT activity in erythrocyte lysate. The molecular study allowed to identify 9 different mutations in HPRT1 gene from 24 patients (11 independent pedigrees) and to establish genotype-phenotype correlation. In conclusion, this study describes the genotypic/phenotypic spectrum of HPRT deficiency in Argentine patients and highlights the need to increase awareness about the suspicion of these diseases, especially the LND variants with high clinical heterogeneity.

HPRT1 activity loss is associated with resistance to thiopurine in ALL.

Acute lymphoblastic leukemia (ALL) is an aggressive hematological tumor resulting from the malignant transformation of lymphoid progenitors. Thiopurine is a widely used drug in the maintaining treatment of ALL. After a period of chemotherapy, 20% of pediatric patients and over 50% of adult patients will relapse. To investigate the mechanisms of drug resistance in vitro, we established the thiopurine resistant cell lines Reh-6MPR (6-MP Resistant cell) and Reh-6TGR (6-TG Resistant cell) by stepwise selection of the ALL cell line Reh. Cell viability assay revealed that 6MPR and 6TGR cells were almost 1000-fold more resistant to thiopurine comparing with the control Reh cells, and thiopurine conversion was significantly impaired in the resistant cells. Mechanistically, a same novel hypoxanthine phosphoribosyl transferase 1 (HPRT1) mutation c.495_496insA (p.V165fs) was found by whole exome sequencing in both resistant cells. The HPRT1 mutation dramaticly decreased the production of [13C5,15N4]-IMP from [13C5,15N4]-hypoxanthine (HX), showed a loss-of-funciton mechanism. Notably, re-expression the wildtype HPRT1 in Reh-6MPR cell can reverse the drug resistance and thiopurine conversion in Reh-6MPR cells. These results highlight the importance of HPRT1's activity in thiopurine resistance.