Identification of a variant of mucolipidosis III (pseudo Hurler polydystrophy): A catalytically active N-acetylglucosaminylphosphotransferase that fails to phosphorylate lysosomal enzymes

Fibroblasts from patients with I-cell disease (mucolipodosis II) or with pseudo Hurler polydystrophy (mucolipidosis III) are markedly deficient in UDP-N-acetylglucosamine:lysosomal enzyme N-acetylglucosamine-1-phosphotransferase. As a consequence, the common phosphomannosyl recognition marker of acid hydrolases is not generated, and these enzymes are not targeted to lysosomes. We have developed a sensitive assay for the transferase that uses α-methyl mannoside as an acceptor, and this has allowed us to distinguish between fibroblasts from these two types of patients. The enzyme activity is less in the former than in the latter (< 0.4 - 2.0 pmol/mg per hr vs 2.9 - 39.4). This may provide an explanation for the difference in clinical severity between the two syndromes. However, in two siblings with pseudo Hurler polydystrophy (GM 3391 and GM 3392), the enzyme activity was normal when assayed by using α-methyl mannoside as acceptor whereas it was low when assayed with endogenous glycoprotein acceptors or with human placental β-hexosaminidase A. The apparent K[m] values of the mutant enzyme toward α-methyl mannoside, high-mannose oligosaccharides, and UDP-GlcNAc were not different from those of the normal enzyme. Mixing experiments demonstrated that the mutant fibroblasts contained endogenous acceptors and were free of inhibitors. We conclude that the N-acetylglucosaminylphosphotransferase in the mutant fibroblasts has normal catalytic activity but is defective in the ability to recognize lyosomal enzymes as specific substrates for phosphorylation. This variant form of pseudo Hurler polydystrophy demonstrates the biological importance of this recognition mechanism in the generation of the phosphomannosyl marker.