A mutation in the 5' untranslated region of the human alpha-galactosidase A gene in high-activity variants inhibits specific protein binding

Misfolding of Lysosomal α-Galactosidase a in a Fly Model and Its Alleviation by the Pharmacological Chaperone Migalastat

Fabry disease, an X-linked recessive lysosomal disease, results from mutations in the GLA gene encoding lysosomal α-galactosidase A (α-Gal A). Due to these mutations, there is accumulation of globotriaosylceramide (GL-3) in plasma and in a wide range of cells throughout the body. Like other lysosomal enzymes, α-Gal A is synthesized on endoplasmic reticulum (ER) bound polyribosomes, and upon entry into the ER it undergoes glycosylation and folding. It was previously suggested that α-Gal A variants are recognized as misfolded in the ER and undergo ER-associated degradation (ERAD). In the present study, we used Drosophila melanogaster to model misfolding of α-Gal A mutants. We did so by creating transgenic flies expressing mutant α-Gal A variants and assessing development of ER stress, activation of the ER stress response and their relief with a known α-Gal A chaperone, migalastat. Our results showed that the A156V and the A285D α-Gal A mutants underwent ER retention, which led to activation of unfolded protein response (UPR) and ERAD. UPR could be alleviated by migalastat. When expressed in the fly’s dopaminergic cells, misfolding of α-Gal A and UPR activation led to death of these cells and to a shorter life span, which could be improved, in a mutation-dependent manner, by migalastat.

Multiple phenotypic domains of Fabry disease and their relevance for establishing genotype- phenotype correlations

Fabry disease (FD) is a rare X-linked glycosphingolipidosis resulting from deficient α-galactosidase A (AGAL) activity, caused by pathogenic mutations in the GLA gene. In males, the multisystemic involvement and the severity of tissue injury are critically dependent on the level of AGAL residual enzyme activity (REA) and on the metabolic load of the disease, but organ susceptibility to damage varies widely, with heart appearing as the most vulnerable to storage pathology, even with relatively high REA. The expression of FD can be conceived as a multidomain phenotype, where each of the component domains is the laboratory or clinical expression of the causative GLA mutation along a complex pathophysiologic cascade pathway. The AGAL enzyme activity is the most clinically useful marker of the protein phenotype. The metabolic phenotype and the pathologic phenotype are diverse expressions of the storage pathology, respectively, assessed by biochemical and histological/ultrastructural methods. The storage phenotypes are the direct consequences of enzyme deficiency and hence, together with the enzymatic phenotype, constitute the more specific diagnostic markers of FD. In the pathophysiology cascade, the clinical phenotypes are most distantly linked to the underlying genetic causation, being critically influenced by the patients’ gender and age, and modulated by the effects of variation in other genetic loci, of polygenic inheritance and of environmental risk factors. A major challenge in the clinical phenotyping of patients with FD is the differential diagnosis between its nonspecific, later-onset complications, particularly the cerebrovascular, cardiac and renal, and similar chronic illnesses that are common in the general population. Comprehensive phenotyping, whenever possible performed in hemizygous males, is therefore crucial for grading the severity of pathogenic GLA variants, to clarify the phenotypic correlations of hypomorphic alleles, to define benign polymorphisms, as well as to establish the pathogenicity of variants of uncertain significance.

The Modulatory Effects of the Polymorphisms in GLA 5'-Untranslated Region Upon Gene Expression Are Cell-Type Specific

Lysosomal α-galactosidase A (αGal) is the enzyme deficient in Fabry disease (FD). The 5'-untranslated region (5'UTR) of the αGal gene (GLA) shows a remarkable degree of variation with three common single nucleotide polymorphisms at nucleotide positions c.-30G>A, c.-12G>A and c.-10C>T. We have recently identified in young Portuguese stroke patients a fourth polymorphism, at c.-44C>T, co-segregating in cis with the c.-12A allele. In vivo, the c.-30A allele is associated with higher enzyme activity in plasma, whereas c.-10T is associated with moderately decreased enzyme activity in leucocytes. Limited data suggest that c.-44T might be associated with increased plasma αGal activity. We have used a luciferase reporter system to experimentally assess the relative modulatory effects on gene expression of the different GLA 5'UTR polymorphisms, as compared to the wild-type sequence, in four different human cell lines. Group-wise, the relative luciferase expression patterns of the various GLA variant isoforms differed significantly in all four cell lines, as evaluated by non-parametric statistics, and were cell-type specific. Some of the post hoc pairwise statistical comparisons were also significant, but the observed effects of the GLA 5'UTR polymorphisms upon the luciferase transcriptional activity in vitro did not consistently replicate the in vivo observations.These data suggest that the GLA 5'UTR polymorphisms are possible modulators of the αGal expression. Further studies are needed to elucidate the biological and clinical implications of these observations, particularly to clarify the effect of these polymorphisms in individuals carrying GLA variants associated with high residual enzyme activity, with no or mild FD clinical phenotypes.

The g.1170C>T polymorphism of the 5' untranslated region of the human alpha-galactosidase gene is associated with decreased enzyme expression--evidence from a family study

Subnormal leukocyte α-galactosidase (α-Gal) activity was found during evaluation of an adolescent male with cryptogenic cerebrovascular small-vessel disease. The only molecular abnormality found was the g.1170C>T single-nucleotide polymorphism (SNP) in the 5' untranslated region of exon 1 in the α-Gal gene (GLA). Historically, this polymorphism has been considered to be biologically neutral. To test the hypothesis that the g.1170T allele might be associated with lower α-Gal expression, we genotyped GLA exon 1 and measured leukocyte and plasma α-Gal in the parents, brother and sister of the index case. The g.1170T allele co-segregated with a subnormal leukocyte α-Gal activity in the three siblings. Although plasma enzyme activities were within the normal range in all five relatives, the ranking of their values suggested a dosage effect of the g.1170T allele. Western blotting assays of leukocyte protein extracts showed that the relative expression of α-Gal in both the patient and his sister was significantly lower than in sex-matched hemizygous or homozygous controls for the g.1170C allele, either normalized to the β-actin immunoblot expression or standardized to a known amount of recombinant human α-Gal. These family data, in combination with results from a recent GLA SNP screening study among healthy Portuguese individuals, suggest that the g.1170C>T SNP may be co-dominantly associated with a relatively decreased GLA expression at the transcription and/or translation level. Larger population studies are needed to confirm these findings and to test the hypothesis that the GLA g.1170C>T may contribute to the multifactorial risk of ischaemic small-vessel cerebrovascular disease.

Effect of single-nucleotide polymorphisms of the 5' untranslated region of the human α-galactosidase gene on enzyme activity, and their frequencies in Portuguese caucasians

BACKGROUND

The α-galactosidase gene (GLA) has three single-nucleotide polymorphisms in the 5' untranslated region of exon 1, respectively g.1150G>A, g.1168G>A, g.1170C>T. The g.1150A allele is associated with increased plasma α-galactosidase (α-Gal) activity in hemizygotes, while the others are regarded as biologically neutral. The primary goal of this investigation was to test the hypothesis, raised by a clinical observation and results of a family study, that the g.1170T allele polymorphism is associated with lower α-Gal expression.

SUBJECTS AND METHODS

Plasma and leukocyte α-Gal activities were assayed in unrelated healthy young adults of both sexes, who had been genotyped for GLA exon 1, and enzyme activity values in carriers of any of the polymorphisms were compared to those of individuals with the standard genotype; GLA exon 1 was genotyped in males who had α-Gal activity in dried blood spots lower than 2 SD below the cohort average.

RESULTS AND CONCLUSIONS

Mean α-Gal leukocyte activity was ∼ 25% higher in subjects with the g.1170C or CC genotype than in those with the alternative genotypes (p < 0.05). The frequency of the g.1170T allele in subjects with low α-Gal activity in dried blood spots was 4-fold higher (p < 0.05) than in the general population. As in hemizygotes, the g.1150A heterozygote identified in this study had plasma α-Gal activity more than 2-fold above the normal mean. The g.1168A allele did not affect enzyme activity. Surprisingly, females with the standard GLA exon 1 genotype had significantly higher plasma α-Gal activity than genetically comparable males.

High overexpression of the human alpha-galactosidase A gene driven by its promoter in transgenic mice: implications for the treatment of Fabry disease

BACKGROUND

Human alpha-galactosidase A (alpha-Gal A) is the lysosomal enzyme that cleaves alpha-galactosyl residues from glycoconjugates and is the deficient enzyme in Fabry disease. To date, there have been no studies on the regulation of this "housekeeping" gene.

METHODS

Transgenic mice were established with either 1) a 13.3-kilobase (kb) human genomic fragment that contained 246 bp of 5'- and approximately 2.8 kb of 3'- untranslated sequences, or 2) an "intronless" construct derived from the genomic sequence with the 5' and 3' flanking regions intact. Tissues that expressed high levels of alpha-Gal A activity were examined by light and electron microscopy.

RESULTS

Transgenic mice were generated with 2 and 12 copies of the genomic sequence (Lines 1 and 2) or about 60 copies of the intronless construct (Lines 3 and 4). In mice hemizygous for the genomic transgene (Lines 1 and 2), tissue alpha-Gal A activities were 12 to 155 times higher than those in the respective wild-type tissue, depending on tissue and transgene copy number. Of note, the high overexpression did not alter the cellular or subcellular cytoarchitecture. In contrast, alpha-Gal A activities expressed by mice that carried the intronless construct were only two- to sixfold more than in wild-type tissues in which the genomic transgene was highly expressed.

CONCLUSIONS

The remarkably high levels of alpha-Gal A expression in transgenic mice carrying the intact genomic sequence versus the intronless construct suggested that the genomic sequence contained a strong intronic enhancer element. Identification of this regulatory element or elements may be useful in efforts to overexpress human alpha-Gal A for gene therapy endeavors. In addition, overexpression of human alpha-Gal A did not affect cellular morphology, which indicates that its overexpression in gene therapy endeavors should be safe.

Genomic structure of the human DNA methyltransferase gene

We determined the genomic structure of the gene encoding human DNA methyltransferase (DNA MTase). Six overlapping human genomic DNA clones which include all of the known cDNA sequence were isolated. Analysis of these clones demonstrates that the human DNA MTase gene consists of at least 40 exons and 39 introns spanning a distance of 60 kilobases. Elucidation of the chromosomal organization of the human DNA MTase gene provides the template for future structure-function analysis of the properties of mammalian DNA MTase.

Human alpha-galactosidase A: high plasma activity expressed by the -30G-->A allele

Human alpha-galactosidase A (EC 3.2.1.22; alpha-Gal A) is the lysosomal exoglycosidase responsible for the hydrolysis of terminal alpha-galactosyl residues from glycoconjugates and is the defective enzyme causing Fabry disease (McKusick 301500). An unusally elevated level of plasma alpha-Gal A activity (> 2.5 times the normal mean) was detected in two unrelated normal males and the elevated activities were inherited as X-linked traits in their families. Sequencing of the alpha-Gal A coding region, intron/exon boundaries and 5'-flanking region from the proband identified a single mutation, a G-->A transition 30 nt upstream from the initiation of translation codon in exon 1. The -30G-->A mutation occurred in a putative NF kappa B/Ets consensus binding site that was recently shown to inhibit protein binding to the 5'-untranslated region of the gene, providing a possible explanation for its high activity. To further characterize the mutation, the mRNA and protein expressed by this variant allele were studied. Purified plasma and lymphoblast alpha-Gal A activity from individuals with the -30G-->A mutation had normal physical and kinetic properties. In vitro translation of mRNAs from the cloned normal and high plasma activity alleles resulted in similar levels of alpha-Gal A protein, indicating that this mutation did not enhance translation. These findings suggest that the -30G-->A mutation in the 5'-untranslated region of the alpha-Gal A gene enhances transcription, presumably by interfering with the binding of negatively-acting transcription factors which normally decrease alpha-Gal A expression in various cells. Preliminary studies of the frequency of the -30G-->A mutation in 395 unrelated normal males of mixed ancestry revealed two additional unrelated individuals who had high plasma enzymatic activity and the mutation, confirming the effect of this mutation on enzyme expression and suggesting that about 0.5% of normal individuals have high plasma alpha-Gal A activity due to this variant allele.