2,8-dihydroxyadenine urolithiasis in a patient with considerable residual adenine phosphoribosyltransferase activity in cell extracts but with mutations in both copies of APRT

We have examined the mutational basis of adenine phosphoribosyltransferase (APRT, EC 2.4.2.7) deficiency (MIM 102600) in a patient of Polish origin wh o has been passing 2,8-dihydroxyadenine (DKA) stones since birth, but has c onsiderable residual enzyme activity in lymphocyte extracts. The five exons and flanking regions of APRT were amplified by PCR and then sequenced. A s ingle T insertion was identified at the intron 4 splice donor site (TG-gtaa to TGgttaa:IVS4+2insT) in one allele from the proband, his mother, and bro ther. A G-to-T transversion in exon 5 (GTC-to-TTC:c.448G > T, V150F) was id entified in the other allele, and this mutation was also present in one all ele from the father and the paternal grandmother. Tru91 and AvaII digestion s of PCR products spanning exons 4 and 5, respectively, confirmed the mutat ions. The mother was heterozygous for an intragenic TaqI site, but all othe r family members were homozygous for the presence of this site. IVS4+2insT, located on the allele containing the TaqI site, has been identified previo usly in several families from Europe, suggesting a founder effect, but the substitution in exon 5 is a novel mutation. IVS4+ainsT is known to result i n complete loss of enzyme activity, and our results suggest that V150F prod uces an enzyme that is nonfunctional in vivo but has considerable residual activity in vitro. (C) 2001 Academic Press.