Human hypoxanthine-guanine phosphoribosyltransferase: a single nucleotide substitution in cDNA clones isolated from a patient with Lesch-Nyhan syndrome (HPRT Midland)

Abstract

We have determined the molecular basis for hypoxanthine-guanine phosphoribosyltransferase (HPRT) deficiency in a patient J.H., with Lesch-Nyhan syndrome. Radioimmunoassay of lysates of erythrocytes or cultured B-lymphoblasts showed that this patient had no detectable HPRT enzyme activity or HPRT protein. HPRT-specific mRNA levels were normal by Northern analysis. We created a cDNA library from mRNA isolated from cultured lymphoblasts derived from this patient. Nucleotide sequencing of full-length HPRT cDNA clones revealed a single nucleotide (nt) substitution: a T-to-A transversion at nt 389. We have designated this variant HPRT Midland. The predicted amino acid (aa) substitution in HPRT Midland is a valine to aspartic acid at aa 130. This substitution is within 2 aa of the amino acid substitution in a previously defined HPRT variant, HPRT Ann Arbor. Both mutations are within a highly conserved sequence in the putative 5-phosphoribosyl-l-pyrophosphate-binding domain. The amino acid substitution in HPRT Midland causes a significant perturbation in the predicted secondary structure of this region. The HPRT Midland mutation affects a different domain of HPRT than the HPRT Flint mutation located at 167 nt away.