Genotype-phenotype correlations in Lesch-Nyhan disease: moving beyond the gene

Lesch-Nyhan disease and its attenuated variants are caused by mutations in the HPRT1 gene, which encodes the purine recycling enzyme hypoxanthine-guanine phosphoribosyltransferase. The mutations are heterogeneous, with more than 400 different mutations already documented. Prior efforts to correlate variations in the clinical phenotype with different mutations have suggested that milder phenotypes typically are associated with mutants that permit some residual enzyme function, whereas the most severe phenotype is associated with null mutants. However, multiple exceptions to this concept have been reported. In the current studies 44 HPRT1 mutations associated with a wide spectrum of clinical phenotypes were reconstructed by site-directed mutagenesis, the mutant enzymes were expressed in vitro and purified, and their kinetic properties were examined toward their substrates hypoxanthine, guanine, and phosphoribosylpyrophosphate. The results provide strong evidence for a correlation between disease severity and residual catalytic activity of the enzyme (k(cat)) toward each of its substrates as well as several mechanisms that result in exceptions to this correlation. There was no correlation between disease severity and the affinity of the enzyme for its substrates (K(m)). These studies provide a valuable model for understanding general principles of genotype-phenotype correlations in human disease, as the mechanisms involved are applicable to many other disorders.