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

Abstract

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.

Keywords: CTSA; PPCA; galactosialidosis; lysosomal storage disease; therapy.

Figure 1

Limited proteolysis with trypsin of 54-kDa precursor and 34- and 20-kDa reconstituted two-chain protein. Aliquots of medium concentrates containing the 54-kDa precursor and 34- and 20-kDa associated protein were incubated at 37 °C with 1 mg of trypsin in the presence of bovine serum albumin (1 mg/ml) for the indicated periods of time. Reactions were stopped with 3 mg of trypsin inhibitor. Samples in lanes 1 and 9 were untreated. At time 0 (lanes 2 and 10), the samples were treated with trypsin inhibitor prior to the addition of trypsin. A portion of each sample was separated by SDS-polyacrylamide gel electrophoresis, followed by electroblotting and immunostaining with anti-54 and anti-pep antibodies. Cathepsin A activity toward the acylated dipeptide benzyloxycarbonyl-phenylalanyl-alanine was measured in each aliquot. One milliunit (mU) of activity is defined as the enzyme activity that releases one nanomole of alanine/minute. Adapted from Bonten et al JBC 1995, with permission of JBC.

Figure 2

Gross phenotypic appearance of a PPCA −/− mouse at 7 months of age, compared to a wild-type littermate. The affected mouse has a broad face, disheveled coat, and swollen limbs and eyelids. Mouse model generated in Zhou et al 1995 .

Figure 3

Low magnification images of a cerebellar lobe show progressive loss of Purkinje cells in the PPCA^−/− mouse. Sections were immunostained with an antibody against the PEP 19 marker. Size bar 100 μm. Adapted from de Geest et al. Hum Mol Genet 2010, with permission of Oxford Journal.

Figure 4. Histology of systemic organs and cerebellum from BM-transplanted GS mice

Top panels: Organs from PPCA ^−/− mice transplanted with total ^−/− BM transduced with the MSCV-PPCA (BMT-PPCA) retrovirus were isolated at different time points after treatment. Hematoxylin and eosin-stained tissue sections of the liver (LIV), kidney (KID), and spleen (SP) from a BMT-PPCA–treated PPCA ^−/− mouse sacrificed 9 months after treatment, and from age-matched wild-type and PPCA ^−/− mice revealed the complete restoration of normal tissue morphology with BM expressing PPCA, compared to the extensive vacuolation present in the PPCA ^−/− control mouse. Size bar corresponds to 30 μm. Lower panels: Serial sections of the cerebellum from a 9-month-old GS mouse transplanted with MSCV-PPCA–marked BM cells were stained with anti-PEP19 antibody. Note the dramatic loss of Purkinje cells in an age-matched GS mouse and the significant number of these cells that are retained in the treated animal. Size bars correspond to 60 μm and 30 μm Adapted from Leimig et al 2002 Blood, with permission of the American Society of Hematology.

Figure 5. Immunostaining of tissue sections from mice after dual-ERT

A) Numerous cells expressing both PPCA and Neu1 were detected with anti-PPCA and anti-Neu1 antibodies in liver (Li), spleen (Sp), and kidney (Ki). B) Cells expressing only PPCA were detected in adrenal gland (AG), heart (He), intestine (In), testis (Te), and choroid plexus (CP). Punctuated staining demonstrates internalization of corrective enzyme mostly by Kuppfer cells in the liver and resident macrophages in other tissues. Magnification = x400 Adapted from Bonten et al. Faseb Journal 2004, with permission of FASEB.