Accelerated transcription of PRPS1 in X-linked overactivity of normal human phosphoribosylpyrophosphate synthetase

Phosphoribosylpyrophosphate (PRPP) synthetase (PRS) superactivity is an X-l inked disorder characterized by gout with overproduction of purine nucleoti des and uric acid. Study of the two X-linked PRS isoforms (PRS1 and PRS2) i n cells from certain affected individuals has shown selectively increased c oncentrations of structurally normal PRS1 transcript and isoform, suggestin g that this form of the disorder involves pretranslational dysregulation of PRPS1 expression and might be more appropriately termed overactivity of no rmal PRS. We applied Southern and Northern blot analyses and slot blotting of nuclear runoffs to delineate the process underlying aberrant PRPS1 expre ssion in fibroblasts and lymphoblasts from patients with overactivity of no rmal PRS, Neither PRPS1 amplification nor altered stability or processing o f PRS1 mRNA was identified, but PRPS1 transcription was increased relative to GAPDH (3- to 4-fold normal in fibroblasts; 1.9 to 2.4-fold in lymphoblas ts) and PRPS2, Nearly coordinate relative increases in each process mediati ng transfer of genetic information from PRPS1 transcription to maximal PRS1 isoform expression in patient fibroblasts further supported the idea that accelerated PRPS1 transcription is the major aberration leading to PRS1 ove rexpression. In addition, modulated relative increases in PRS activities at suboptimal Pi concentration and in rates of PRPP and purine nucleotide syn thesis in intact patient fibroblasts indicate that despite an intact allost eric mechanism of regulation of PRS activity, PRPS1 transcription is a majo r determinant of PRPP and purine synthesis. The genetic basis of disordered PRPS1 transcription remains unresolved; normal- and patient-derived PRPS1s share nucleotide sequence identity at least 850 base pairs 5' to the conse nsus transcription initiation site.