A GERMLINE MUTATION ABOLISHING THE ORIGINAL STOP CODON OF THE HUMAN ADENINE PHOSPHORIBOSYLTRANSFERASE (APRT) GENE LEADS TO COMPLETE LOSS OFTHE ENZYME PROTEIN

Adenine phosphoribosyltransferase (APRT) is a purine metabolic enzyme and a homozygous deficiency in this enzyme causes 2,8-dihydroxyadenine urolithiasis. Various germline abnormalities have been described, but we report here a unique type of germline mutation in a homozygous ind ividual (SY) who had excreted 2,8-dihydroxyadenine crystals. In SY, TC A was substituted for the physiological stop codon TGA. This base subs titution generates a new HinfI restriction site, and, using the polyme rase chain reaction and subsequent digestion by this enzyme, it was co nfirmed that SY is homozygous for the base substitution. This base cha nge is unique in that it generates an open reading frame that extends to the poly(A) addition site. The amount of mRNA in transformed B cell s from SY was approximately a quarter of that in control subjects and no APRT proteins were detected. In eukaryotes, unlike in prokaryotes, no rescue systems for defective polypeptide termination caused by a mi ssing stop codon have been found, Therefore, the outcome of the defect of SY is unclear from present knowledge about termination of polypept ide synthesis, Investigations into the mechanisms of the absence of pr otein in the cells of SY may lead to a better understanding of the phy siological and nonphysiological termination of polypeptide synthesis i n eukaryotic cells.