Sunqu Zhang

Impaired Elastic-Fiber Assembly by Fibroblasts from Patients with Either Morquio B Disease or Infantile GM1-Gangliosidosis Is Linked to Deficiency in the 67-kD Spliced Variant of β-Galactosidase

We have previously shown that intracellular trafficking and extracellular assembly of tropoelastin into elastic fibers is facilitated by the 67-kD elastin-binding protein identical to an enzymatically inactive, alternatively spliced variant of beta-galactosidase (S-Gal). In the present study, we investigated elastic-fiber assembly in cultures of dermal fibroblasts from patients with either Morquio B disease or GM1-gangliosidosis who bore different mutations of the beta-galactosidase gene. We found that fibroblasts taken from patients with an adult form of GM1-gangliosidosis and from patients with an infantile form, carrying a missense mutations in the beta-galactosidase gene-mutations that caused deficiency in lysosomal beta-galactosidase but not in S-Gal-assembled normal elastic fibers. In contrast, fibroblasts from two cases of infantile GM1-gangliosidosis that bear nonsense mutations of the beta-galactosidase gene, as well as fibroblasts from four patients with Morquio B who had mutations causing deficiency in both forms of beta-galactosidase, did not assemble elastic fibers. We also demonstrated that S-Gal-deficient fibroblasts from patients with either GM1-gangliosidosis or Morquio B can acquire the S-Gal protein, produced by coculturing of Chinese hamster ovary cells permanently transected with S-Gal cDNA, resulting in improved deposition of elastic fibers. The present study provides a novel and natural model validating functional roles of S-Gal in elastogenesis and elucidates an association between impaired elastogenesis and the development of connective-tissue disorders in patients with Morquio B disease and in patients with an infantile form of GM1-gangliosidosis.

Identification of the Gene Encoding the Enzyme Deficient in Mucopolysaccharidosis IIIC (Sanfilippo Disease Type C)

Mucopolysaccharidosis IIIC (MPS IIIC), or Sanfilippo C, represents the only MPS disorder in which the responsible gene has not been identified; however, the gene has been localized to the pericentromeric region of chromosome 8. In an ongoing proteomics study of mouse lysosomal membrane proteins, we identified an unknown protein whose human homolog, TMEM76, was encoded by a gene that maps to 8p11.1. A full-length mouse expressed sequence tag was expressed in human MPS IIIC fibroblasts, and its protein product localized to the lysosome and corrected the enzymatic defect. The mouse sequence was used to identify the full-length human homolog (HGSNAT), which encodes a protein with no homology to other proteins of known function but is highly conserved among plants and bacteria. Mutational analyses of two MPS IIIC cell lines identified a splice-junction mutation that accounted for three mutant alleles, and a single base-pair insertion accounted for the fourth.

The biochemistry and clinical features of galactosialidosis

Galactosialidosis is a heterogeneous disorder that is manifested in infantile, late infantile, juvenile/adult, and atypical forms. In every instance the primary defect is in the ability of protective protein to associate with beta-galactosidase and neuraminidase to protect them from intralysosomal proteolysis. The protective protein is in reality a serine protease that displays both cathepsin A and C-terminal deamidase activity. We summarize the major clinical features of each form, and the range of storage products accumulated. The concept of an intralysosomal complex containing beta-galactosidase and neuraminidase in addition to protective protein seems irrefutable but major gaps exist in our understanding of how the complex is formed and in what subcellular organelles, how it is sustained, and the protein domains contributed by the constituent enzymes that play a pivotal role in this process.

Novel mutations (Asn 484 Lys, Thr 500 Ala, Gly 438 Glu) in Morquio B disease.

Primary deficiency of beta-galactosidase results in GM1 gangliosidosis and Morquio B disease. Of the more than 40 disease-causing mutations described in the Gal gene to date, about 75% are of the missense type and are scattered along the length of the gene. No single, major common mutation has been associated with GM1 gangliosidosis. However, a Trp 273 Leu mutation has been commonly found in the majority of patients with Morquio B disease defined genotypically to date. We now report three new mutations in three Morquio B patients where the Trp 273 Leu mutation is absent. Two of the mutations, C1502G (Asn 484 Lys) and A1548G (Thr 500 Ala), were found in twins (one male, one female) who display a mild form of Morquio B disease and keratan sulfate in the urine. In their fibroblasts, residual activity was 1.9% and 2.1% of controls. On Western blots, the 84-kDa precursor and the 64-kDa mature protein were barely detectable. The occurrence of a 45-kDa degradation product indicates that the mutated protein reached the lysosome but was abnormally processed. In the third case, we identified only a G1363A (Gly 438 Glu) mutation (a major deletion on the second allele has not been ruled out). This female patient too displays a very mild form of the disease with a residual activity of 5.7% of control values. In fibroblasts from this case, the 84-kDa precursor and the 45-kDa degradation product were present, while the mature 64-kDa form was barely detectable. The occurrence of these three mutations in the same area of the protein may define a domain involved in keratan sulfate degradation.

Maturation and degradation of β-galactosidase in the post-Golgi compartment are regulated by cathepsin B and a non-cysteine protease

Lysosomal β‐galactosidase precursor is processed to a mature form and associated with protective protein in lysosomes. In this study we used two cysteine protease proinhibitors, E64‐d for cathepsins B, S, H, and L, and CA074Me for cathepsin B. They are converted intracellularly to active forms, E‐64c and CA074, respectively. Both active compounds inhibited maturation of the exogenous β‐galactosidase precursor, but E‐64c did not inhibit further degradation to an inactive 50‐kDa product. We concluded that cathepsin B participated exclusively in maturation of β‐galactosidase, and a non‐cysteine protease was involved in further degradation and inactivation of the enzyme molecule.