Disease relevance of GALC

• A large deletion together with a point mutation in the GALC gene is a common mutant allele in patients with infantile Krabbe disease [1].
• Krabbe disease (globoid cell leukodystrophy) is an autosomal recessive neurodegenerative disorder that affects both the central and peripheral nervous system due to an enzymatic defect of galactocerebrosidase (GALC) [2].
• The twitcher mouse is an animal model of galactosylceramidase deficiency, comparable to Krabbe's disease, a lysosomal storage disease in humans [3].
• BACKGROUND: Demyelination in globoid cell leukodystrophy (GLD) is due to a deficiency of galactocerebrosidase (GALC) activity [4].
• When TwS1 cells were infected with a retrovirus vector encoding GALC, GALC activity was markedly increased and psychosine levels were significantly decreased [5].

High impact information on GALC

• It is now reported that bone marrow transplantation results in increased galactosylceramidase levels in the central nervous system (CNS) [3].
• A 5-6-fold increase in galactosylceramidase activity was found in the peripheral nervous system [6].
• These observations could account for the formation of multinuclear globoid cells in the brains of patients with globoid cell leukodystrophy, a neurological disorder characterized by the accumulation of psychosine due to galactosylceramidase deficiency [7].
• The gene responsible for Krabbe disease, an autosomal recessive disorder caused by deficiency of galactocerebrosidase (GALC), was localized by multipoint linkage analysis on chromosome 14 [8].
• Localization of the Krabbe disease gene (GALC) on chromosome 14 by multipoint linkage analysis [8].

Chemical compound and disease context of GALC

• Globoid cell leukodystrophy (Krabbe disease) is an autosomal recessive disorder resulting from the deficiency of galactocerebrosidase (GALC) activity [9].
• Residual galactosylsphingosine (psychosine) beta-galactosidase activities and associated GALC mutations in late and very late onset Krabbe disease [10].
• In the galactosylceramide-loading tests using fibroblasts from patients with GLD and GM1 gangliosidosis, both cell lines hydrolyzed the incorporated galactosylceramide, with lower rates than control fibroblasts but higher than the fibroblasts from patients with I-cell disease, in which both galactosylceramidase I and II were deficient [11].
• Use of mixed dispersion of fluorescent galactosylceramide and sodium dodecylsulfate for assaying galactosylceramide-beta-galactosidase and diagnosing Krabbe disease [12].

Biological context of GALC

• The mild clinical phenotype was likely to be associated with the 809G>A, since residual GALC activity, about 17% of the control activity, was detected in the expression studies of this mutation [13].
• Since material suitable for mutation analysis was no longer available from the proband, her GALC genotype was reconstructed by analyzing this gene in her six obligate carrier offspring [13].
• Recently we cloned the full-length human GALC cDNA using amino acid sequence information obtained from GALC purified from human urine and brain [14].
• Eight mapped dinucleotide repeat polymorphisms were tested for linkage to GALC [8].
• Additional support for close linkage of GALC and D14S48 comes from the apparent linkage disequilibrium between these two loci in a consanguineous Druze community in Israel. These data localize GALC to 14q24.3-q32.1 [8].

Anatomical context of GALC

• Modification of the sequence surrounding the initiation codon to one more favorable for expression, resulted in a 6-fold increase in GALC activity in transfected COS-1 cells [9].
• GALC is responsible for the lysosomal catabolism of galactosylceramide, a major lipid in myelin, kidney and epithelial cells of small intestine and colon [9].
• Therefore continued myelination and/or remyelination in patients will require supplying GALC activity by transplanted cells or viral vectors to still functional endogenous oligodendrocytes or transplantation of normal oligodendrocytes or stem cells that can differentiate into oligodendrocytes [15].
• The pathological changes observed, including the presence of globoid cells and decreased myelin, appear to result from the toxic nature of psychosine and accumulation of galactosylceramide that cannot be degraded due to the GALC deficiency [15].
• METHODS: Activities of GALC towards galactosylceramide (GC) and galactosylsphingosine (psychosine; PS) were determined in white blood cells and cultured fibroblasts derived from GLD patients and controls using tritium-labelled natural substrates [10].

Associations of GALC with chemical compounds

• Galactocerebrosidase (GALC) activity is deficient in all patients with globoid cell leukodystrophy (GLD) [1].
• The Moslem Arabs were homozygous for two mutations in the GALC gene; a T-to-C transition at CDNA position 1637 (counting from the A of the initiation codon), which is considered a polymorphism and a G-to-A transition at position 1582, which changes the codon for aspartic acid to one for asparagine [16].
• Anisomycin, emetine and puromycin, inhibitors of NMD, effectively increased the level of GALC transcript in the TwS1 cells providing further support for nonsense-mediated mRNA decay being the mechanism by which no GALC protein is detected in these animals [17].
• Mannose-6-phosphate receptor-mediated uptake was only partially responsible for the efficient transfer of GALC to neighboring cells [18].
• After preliminary studies determined what effect each amino acid change had on mouse GALC activity in transient transfection experiments, mice containing a cysteine residue at codon 168 instead of histidine (H168C) were produced [19].

Co-localisations of GALC

• In fact immunocytochemical studies in transduced oligodendrocytes revealed that the GALC colocalizes in vesicles lysosomal-associated membrane protein-2 (LAMP2) (+) [20].

Other interactions of GALC

• Two-point linkage analysis demonstrated close linkage of GALC and D14S48, with Z = 13.69 at theta = 0 [8].
• CONCLUSIONS: Expression of this mutation in COS-1 cells using the pcDNA3 expression vector (Invitrogen, Carlsbad, Calif) resulted in low, although not null, GALC activity, which can explain the protracted clinical course in this patient [21].
• Quantification of cellular acid sphingomyelinase and galactocerebroside beta-galactosidase activities by electrospray ionization mass spectrometry [22].
• The activities of the galactolipid catabolic hydrolases, galactocerebroside-beta-galactosidase and arylsulfatase A, were reduced by 20% [23].

Analytical, diagnostic and therapeutic context of GALC

• The isolation of this clone will permit investigations into the causes for GALC deficiency in humans and available animal models, development of more accurate tests for patient and carrier identification, and evaluation of methods for effectively treating GALC deficiency, initially using the animal models [9].
• We describe the molecular cloning of human GALC cDNA and its expression in COS-1 cells [9].
• Regional mapping of the human galactocerebrosidase gene (GALC) to 14q31 by in situ hybridization [24].
• In addition, gel filtration of partially purified GALC after disassociation showed one peak of activity estimated to have a molecular mass near 50 kDa [25].
• Using either immunocytochemical approaches or Western blot methodology, we have been unable to detect the truncated form of GALC expected to be produced in these animals [17].

References