Structural and functional analysis of mutations at the human hypoxanthine phosphoribosyl transferase (HPRT1) locus

Hypoxanthine phosphoribosyl transferase (HPRT, also known as HGPRT) is an often-used genetic marker in eukaryotic cells. The gene is conserved from bacteria to human, with retained catalytic activity, although substrate specificity may have changed, and the enzyme is essential in malaria-causing protozoans. Inherited mutations in the human HPRT1 gene result in three different phenotypes: Lesch-Nyhan syndrome (LNS or LND), LND variants, and HPRT-related hyperuricemia (HRH). In cultured cells, loss of HPRT activity gives rise to 6-thioguanine (6-TG) resistance. In general, cells from LND patients are also 6-TG resistant, whereas cells from HRH patients are not, with some interesting exceptions. Using modeling methods, we have studied the correlation between the mutable and nonmutated amino acid residues on one hand, and sequence conservation and predicted phenotypic effects on the other hand. Our results demonstrate that most of the mutations are explainable by the predicted effect on protein structure and function. They are also consistent with sequence conservation. Moreover, the mutational profiles of TG-resistant cells and LND overlap to a great extent, while most of the mutations in HRH are unique to that condition. We have also noticed a strong correlation between mutations in the tetramer interfaces and observed phenotypes, suggesting a functional role for a tetramer transition during catalysis.