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 (mucolipidosis 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 regenerated, and these enzymes are not targeted to lysosomes. We have developed a sensitive assay for the transferase that uses alpha-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 (less than 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 3392), the enzyme activity was normal when assayed by using alpha-methyl mannoside as acceptor whereas it was low when assayed with endogenous glycoprotein acceptors or with human placental beta-hexosaminidase A. The apparent Km values of the mutant enzyme toward alpha-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 lysosomal 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.