[Hypoxanthine guanine phosphoribosyl transferase].

The purine salvage enzyme hypoxanthine guanine phosphoribosyl transferase (HPRT) catalyzes the transfer of phosphoribose from PRPP to the 9-position of hypoxanthine or guanine, yielding IMP or GMP, respectively. HPRT is a 217-amino acid protein and is expressed in all tissues at low levels. High levels are present in brain, specially in the basal ganglia. HPRT is encoded by a single gene on the X chromosome (Xq26), which contains nine exons, dispersed over 40 kb of genomic DNA. mRNA is about 1.6 kb in length and contains an open reading frame of 654 nucleotides. Four autosomal pseudogenes are present within the human genome. The promoter sequences lack CAAT and TATAA elements and have an extremely CpG rich promoter. Hypomethylation of CpG clusters within the first intron and hypermethylation of sites within the 3' portion of the gene correlate with gene activity. Two DNA polymorphysms are located within the HPRT gene. One is a three-allele Bam HI restriction fragment-length polymorphysm and the other is a highly informative tetranucleotide(AGAT) tandem repeat located with intron 3.