Genetic heterogeneity of I‐cell disease is demonstrated by complementation of lysosomal enzyme processing mutants

Abstract

I-cell disease (mucolipidosis II) is a fatal childhood disorder affecting the expression of multiple lysosomal acid hydrolases. The disorder is characterized by clinical and biochemical heterogeneity which may reflect different mutants with a similar phenotype. Genetic complementation studies demonstrating genetic heterogeneity within this disorder are described utilizing cultured fibroblasts from 11 different patients. Fibroblasts from I-cell disease (ICD) and from five different lysosomal storage diseases with single structural gene enzyme deficiencies were fused in different combinations, and fractions enriched for multinucleated heterokaryons were isolated and tested for acid hydrolase activity and electrophoretic mobility. In fusions of ICD fibroblasts and various single lysosomal enzyme-deficient fibroblasts, the activity of the deficient enzyme and of the other ICD hydrolases were restored, demonstrating that ICD is not a lysosomal enzyme structural gene defect and that the ICD defect, and not just the single enzyme deficiency, is corrected. In fusions involving only I-cell fibroblasts, at least two complementation groups were identified by the recovery of activities of all lysosomal enzymes tested in heterokaryons. These results demonstrate the existence of genetic heterogeneity within the disorder and suggest that different mutations can result in the I-cell clinical and biochemical phenotype. The data support an altered post-translational processing of lysosomal enzymes as the cause of ICD and suggest that at least two genes participate in this pathway.

Molecular basis of variant pseudo-Hurler polydystrophy (mucolipidosis IIIC)

“…The genetic relationships between the multiple variants of MLII and MLIII are complex and have been difficult to understand in the absence of information about the gene(s) involved (14)(15)(16)35). This study demonstrates the MLIII(C) phenotype can result from a mutation in the GlcNAc-phosphotransferase γ subunit.…”

Mucolipidosis II and III. The genetic relationships between two disorders of lysosomal enzyme biosynthesis.

“…The results of complementation analysis within ML 11 (24), between ML II and ML III, and within ML III (25) are summarized in Fig. 3.…”

“…Human skin fibroblasts were derived from normal or clinically diagnosed ML II (24) and ML III (25) (26). The fused culture was harvested after an additional 1-3 d culture, and the multinucleated cells were enriched by sedimentation velocity as previously described (27).…”

“…The intracellular activities of lysosomal f,-hexosaminidase (#HEX), ,B-galactosidase (,3GAL), a-fucosidase (aFUC), agalactosidase (aGAL), and a-mannosidase (aMAN) were measured on aqueous fibroblast homogenates with appropriate fluorigenic 4-methylumbelliferyl derivatives as described (24) and are expressed in nanomoles of substrate enzymatically hydrolyzed per hour per milligram of homogenate protein. Protein concentration was determined using a method modified from Lowry et al (29).…”

“…Previously, we and others have used genetic complementation analysis to demonstrate that both ML II and ML III are genetically heterogeneous (23)(24)(25). We now present complementation studies that explore the genetic relationship between these two disorders.…”