Homologous modeling of the lysosomal protective protein/carboxypeptidase L: structural and functional implications of mutations identified in galactosialidosis patients

Galactosialidosis: historic aspects and overview of investigated and emerging treatment options

INTRODUCTION

Galactosialidosis is a glycoprotein storage disease caused by mutations in the CTSA gene, encoding lysosomal protective protein/cathepsin A (PPCA). The enzyme's catalytic activity is distinct from its protective function towards β-galactosidase (β-GAL) and neuraminidase 1 (NEU1), with which PPCA forms a complex. In this configuration the two glycosidases acquire their full activity and stability in lysosomes. Deficiency of PPCA results in combined NEU1/β-GAL deficiency. Because of its low incidence, galactosialidosis is considered an orphan disorder with no therapy yet available.

AREAS COVERED

This review gives a historic overview on the discovery of PPCA, which defined galactosialidosis as a new clinical entity; the evidence for the existence of the PPCA/NEU1/β-GAL complex; the clinical forms of galactosialidosis and disease-causing CTSA mutations. Ppca^-/- mice have proven to be a suitable model to test different therapeutic approaches, paving the way for the development of clinical trials for patients with galactosialidosis.

EXPERT OPINION

Improved understanding of the molecular bases of disease has sparked renewed incentive from clinicians and scientists alike to develop therapies for rare conditions, like GS, and has increased the willingness of biotech companies to invest in the manufacturing of new therapeutics. Both ERT and gene therapy may become available to patients in the near future.

Intrauterine onset of acute neuropathic type 2 Gaucher disease: identification of a novel insertion sequence

A subset of patients with type 2 Gaucher disease is characterized by intrauterine onset of rapidly progressive neuropathic disease, arthrogryposis, hydrops fetalis and in some cases restrictive dermopathy. beta-Glucocerebrosidase (beta-glucosidase) activity is usually low or undetectable. In most cases death ensues either in-utero or within hours or days after birth. We report on an infant born to non-consanguineous parents of Caucasian origin presenting at birth with hydrops, arthrogryposis, severe respiratory distress, hepatosplenomegaly, and liver failure. Death occurred within several hours after delivery and autopsy revealed typical Gaucher cells in multiple organs in combination with severe apoptotic neurodegeneration throughout the brain. beta-Glucocerebrosidase activity was 1% of the norm in fibroblasts and a novel heterozygous insertion c.1515_1516insAGTGAGGGCAAT was identified by genomic sequencing and an insertion-specific seminested PCR. In addition, molecular studies revealed a previously described in type 1 Gaucher disease missense mutation c.476G --> A which results in a heterozygous substitution of R120Q. Our observations confirm considerable genotypic heterogeneity in patients with type 2 Gaucher disease. The transheterozygous combination of a mutation, previously described in type 1 Gaucher disease, together with a newly identified insertion may result in this severe phenotype.

Lysosomal multienzyme complex: biochemistry, genetics, and molecular pathophysiology

Lysosomal enzymes sialidase (alpha-neuraminidase), beta-galactosidase, and N-acetylaminogalacto-6-sulfate sulfatase are involved in the catabolism of glycolipids, glycoproteins, and oligosaccharides. Their functional activity in the cell depends on their association in a multienzyme complex with lysosomal carboxypeptidase, cathepsin A. We review the data suggesting that the integrity of the complex plays a crucial role at different stages of biogenesis of lysosomal enzymes, including intracellular sorting and proteolytic processing of their precursors. The complex plays a protective role for all components, extending their half-life in the lysosome from several hours to several days; and for sialidase, the association with cathepsin A is also necessary for the expression of enzymatic activity. The disintegration of the complex due to genetic mutations in its components results in their functional deficiency and causes severe metabolic disorders: sialidosis (mutations in sialidase), GM1-gangliosidosis and Morquio disease type B (mutations in beta-galactosidase), galactosialidosis (mutations in cathepsin A), and Morquio disease type A (mutations in N-acetylaminogalacto-6-sulfate sulfatase). The genetic, biochemical, and direct structural studies described here clarify the molecular pathogenic mechanisms of these disorders and suggest new diagnostic tools.

Molecular pathology of galactosialidosis in a patient affected with two new frameshift mutations in the cathepsin A/protective protein gene

Galactosialidosis is a recessively inherited lysosomal storage disease characterized by the combined deficiency of neuraminidase and beta-galactosidase secondary to the genetic deficiency of cathepsin A/protective protein. In lysosomes, cathepsin A forms a high-molecular-weight complex with beta-galactosidase and neuraminidase that protects these enzymes against intralysosomal proteolysis. In a patient affected with late infantile form of galactosialidosis, we found two new cathepsin A mutations, a two-nucleotide deletion, c517delTT and an intronic mutation, IVS8+9C-->G resulting in abnormal splicing and a five-nucleotide insertion in the cathepsin A cDNA. Both mutations cause frameshifts and result in the synthesis of truncated cathepsin A proteins, which, as suggested by structural modeling, are incapable of dimerization, complex formation, and catalysis. However, enzymatic assays, gel-filtration, and Western blot analysis of the patient's cultured skin fibroblast extracts showed the presence of a small amount of normal-size, catalytically active cathepsin A and cathepsin A-beta-galactosidase 680 kDa complex, suggesting that a low amount of cathepsin A mRNA is spliced normally and produces the wild-type protein. This may contribute to the relatively mild phenotype of the patient and illustrates the importance of critically comparing molecular results with clinical and biochemical phenotypes.

Molecular basis of GM1 gangliosidosis and Morquio disease, type B. Structure-function studies of lysosomal beta-galactosidase and the non-lysosomal beta-galactosidase-like protein

GM1 gangliosidosis and Morquio B disease are distinct disorders both clinically and biochemically yet they arise from the same beta-galactosidase enzyme deficiency. On the other hand, galactosialidosis and sialidosis share common clinical and biochemical features, yet they arise from two separate enzyme deficiencies, namely, protective protein/cathepsin A and neuraminidase, respectively. However distinct, in practice these disorders overlap both clinically and biochemically so that easy discrimination between them is sometimes difficult. The principle reason for this may be found in the fact that these three enzymes form a unique complex in lysosomes that is required for their stability and posttranslational processing. In this review, I focus mainly on the primary and secondary beta-galactosidase deficiency states and offer some hypotheses to account for differences between GM1 gangliosidosis and Morquio B disease.

The atomic model of the human protective protein/cathepsin A suggests a structural basis for galactosialidosis

Human protective protein/cathepsin A (PPCA), a serine carboxypeptidase, forms a multienzyme complex with beta-galactosidase and neuraminidase and is required for the intralysosomal activity and stability of these two glycosidases. Genetic lesions in PPCA lead to a deficiency of beta-galactosidase and neuraminidase that is manifest as the autosomal recessive lysosomal storage disorder galactosialidosis. Eleven amino acid substitutions identified in mutant PPCAs from clinically different galactosialidosis patients have now been modeled in the three-dimensional structure of the wild-type enzyme. Of these substitutions, 9 are located in positions likely to alter drastically the folding and stability of the variant protein. In contrast, the other 2 mutations that are associated with a more moderate clinical outcome and are characterized by residual mature protein appeared to have a milder effect on protein structure. Remarkably, none of the mutations occurred in the active site or at the protein surface, which would have disrupted the catalytic activity or protective function. Instead, analysis of the 11 mutations revealed a substantive correlation between the effect of the amino acid substitution on the integrity of protein structure and the general severity of the clinical phenotype. The high incidence of PPCA folding mutants in galactosialidosis reflects the fact that a single point mutation is unlikely to affect both the beta-galactosidase and the neuraminidase binding sites of PPCA at the same time to produce the double glycosidase deficiency. Mutations in PPCA that result in defective folding, however, disrupt every function of PPCA simultaneously.

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

Lysosomal beta-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 beta-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 beta-galactosidase, and a non-cysteine protease was involved in further degradation and inactivation of the enzyme molecule.