We describe a family of seven boys affected by Lesch-Nyhan disease with various 
phenotypes. Further investigations revealed a mutation c.203T>C in the gene 
encoding HGprt of all members, with substitution of leucine to proline at 
residue 68 (p.Leu68Pro). Thus patients from this family display a wide variety 
of symptoms although sharing the same mutation. Mutant HGprt enzyme was prepared 
by site-directed mutagenesis and the kinetics of the enzyme revealed that the 
catalytic activity of the mutant was reduced, in association with marked 
reductions in the affinity towards phosphoribosylpyrophosphate (PRPP). Its Km 
for PRPP was increased 215-fold with hypoxanthine as substrate and 40-fold with 
guanine as substrate with associated reduced catalytic potential. Molecular 
modeling confirmed that the most prominent defect was the dramatically reduced 
affinity towards PRPP. Our studies suggest that the p.Leu68Pro mutation has a 
strong impact on PRPP binding and on stability of the active conformation. This 
suggests that factors other than HGprt activity per se may influence the 
phenotype of Lesch-Nyhan patients.