A mutation in the gene of a glycolipid-binding protein (GM2 activator) that causes GM2-gangliosidosis variant AB

GM2-gangliosidoses are neurological disorders caused by a genetic deficiency of either the β-hexosaminidase A or the GM2 activator, a glycolipid binding protein. In a patient with an immunologically proven GM2 activator protein deficiency, A T⁴¹² → C transition (counted from A of the initiation codon) was found in the coding sequence, which results in the substitution of Arg for the normal Cys¹⁰⁷ in the mature GM2 activator protein. The remainder of the coding sequence remained entirely normal.     

GM2-gangliosidosis B1 variant: analysis of beta-hexosaminidase alpha gene abnormalities in seven patients

A single nucleotide transition within exon 5 of the beta-hexosaminidase alpha chain gene was identified in a Puerto Rican patient with GM2-gangliosidosis B1 variant as the mutation responsible for the unusual enzymological characteristics of this variant (G533----A; Arg178----His) (the DN-allele). A total of seven patients with enzymological characteristics of B1 variant have since been studied. They were Puerto Rican (DN), Italian, French, Spanish, two patients of mixed ethnic origin (English/Italian/Hungarian and English/French/Azores), and a Czechoslovakian. In confirmation of our earlier finding based on screening with allele-specific probes, all patients except the one from Czechoslovakia carried the same DN-allele. A new point mutation found in this patient changed the same codon affected in the DN-allele (C532----T; Arg178----Cys). An asymptomatic Japanese individual included as a control also carried one allele with the DN-mutation. Site-directed mutagenesis and expression studies in COS I cells demonstrated that either of the two point mutations abolishes the catalytic activity of the alpha subunit. The Spanish patient was homozygous for the DN-allele, but others were all compound heterozygotes. The Puerto Rican patient was a compound heterozygote with the DN-mutation in one allele and with the four-base insertion in exon 11, one of the two mutations found in the classical Ashkenazi Jewish Tay-Sachs disease, in the other allele. Abnormalities of the other allele were not identified in all other compound heterozygous patients. In these patients, the level of mRNA derived from the other allele was variable, ranging from being undetectable to being much lower than normal. This series of studies uncovered a new B1 variant mutation, confirmed our preliminary finding that the DN-allele has a surprisingly wide geographic and ethnic distribution, and pointed out the highly complex nature of the molecular genetics of this rare disorder. They also support our working hypothesis that mutations responsible for the unique enzymological characteristics of the B1 variant should be located in or near exon 5 of the gene and that this region of the enzyme protein is critical for its catalytic function.     

Molecular genetics of GM2-gangliosidosis AB variant: a novel mutation and expression in BHK cells

The GM2 activator is a hexosaminidase A-specific glycolipid-binding protein required for the lysosomal degradation of ganglioside GM2. Genetic deficiency of GM2 activator leads to a neurological disorder, an atypical form of Tay-Sachs disease (GM2 gangliosidosis variant AB). Here, we describe a G⁵⁰⁶ to C transversion (Arg¹⁶⁹ to Pro) in the mRNA of an infantile patient suffering from GM2-gangliosidosis variant AB. Using the polymerase chain reaction amplification and direct-sequencing technique, we found the patient to be homozygous for the mutation, whereas the parents were, as expected, heterozygous. BHK cells transfected with a construct of mutant cDNA gave no GM2 activator protein detectable by the Western blotting technique, whereas those transfected by a wild-type cDNA construct showed a significant level of human GM2 activator protein. The substitution of proline for the normal Arg¹⁶⁹ therefore appears to result in premature degradation of the mutant GM2 activator, either during the post-translational processing steps or after reaching the lysosome. The basis for the phenotype of GM2 gangliosidosis variant AB may therefore be either inactivation of the physiological activator function by the point mutation or instability of the mutant protein.     

Detection of homozygotes and heterozygote carriers of GM2-gangliosidosis

11 patients with Tay-Sachs disease (TSD) and 4 patients with Sandhoff disease were identified using the methods of heat inactivation of hexosaminidase at 50 degrees C (3 and 4 hours) and electrofocusing on PAG-plates in the pH range 3.5-9.5. Ion exchange chromatography on DEAE cellulose DE-52 proved to be reliable for identification of heterozygotes in cases when the proband was not available. The incidence of TSD gene was estimated in 2 population samples--from the cities of Gomel and Kostroma. It was about 0.004 in the Gomel sample. No heterozygotes were detected in Kostroma.     

Differentiation of two variants of type-AB GM2-gangliosidosis using chromogenic substrates.

Two variants of type-ABGM2-gangliosidosis can be distinguished by using p-nitrophenyl-6-sulfo-2-acetamido-2-deoxy-beta-D-glucopyranoside (PNP-GlcNAc-6-SO4) as substrate. One of the variants is caused by a deficiency of the activator for the hydrolysis of GM2-ganglioside. The beta-hexosaminidase A from this variant has a normal activity toward both PNP-GlcNAc and PNP-GlcNAc-6-SO4. A second variant caused by a defect in the enzyme, beta-hexosaminidase A, exhibits severely attenuated activity toward PNP-GlcNAc-6-SO4 but normal activity toward PNP-GlcNAc.     

GM2-gangliosidosis B1 variant: analysis of beta-hexosaminidase alpha gene mutations in 11 patients from a defined region in Portugal.

The GM2-gangliosidosis B1 variant occurs at an exceptionally high frequency in the northern part of Portugal. In most patients, the disease manifests itself as a juvenile form, as opposed to the late-infantile form described for many patients from other parts of the world. We have analyzed the beta-hexosaminidase alpha gene in 11 patients, as well as in some relatives, in order to characterize the underlying abnormalities. They were screened for the two previously identified mutations responsible for the B1 variant phenotype (G533----A, also designated as the "DN allele," and C532---T) by PCR amplification of an 800-bp DNA fragment and subsequent dot-blot hybridization with allele-specific oligonucleotides. The fragment amplified from one patient was also subcloned and sequenced. Ten patients, constituting a clinically and biochemically homogeneous group, were found to be homozygous for the DN allele. The other, whose clinical profile more resembled the late-infantile phenotype often described in the literature, was a compound heterozygote carrying the DN allele and another, as yet unidentified, abnormal allele. Our results, corroborated by previously published data, suggest that homozygotes and compound heterozygotes for the DN allele may be distinguishable at the phenotypic level, depending on the nature of the abnormality in the other allele. A common ancestral origin for the DN allele can also be postulated.     

Hepatic glycosphingolipid abnormalities in a patient with GM1-gangliosidosis

Glycosphingolipids from the liver, kidney, and spleen of a patient with type 1 II3-N-acetylneuraminosylgangliotetraosylceramide (GM1)-gangliosidosis were quantitatively analyzed. It was noted that large amounts of unusual glycosphingolipids other than GM1 ganglioside or gangliotetrasylceramide accumulated in the liver of the patient. Particularly, the prominent accumulation of III3-alpha-fucosylneolactotetraosylceramide, galactosylceramide I3-sulfate and cholesterol sulfate was observed in addition to a small but significant increase of galabiosylceramide and neolacto-or lactotetraosylceramide. None of these lipids except cholesterol sulfate can be detected in normal liver. None of the lipids accumulated in the liver can be the direct substrates for acid beta-galactosidase which is deficient in the patient. Thus, it was suggested that secondary effects due to the defect in acid beta-galactosidase might cause the abnormal accumulation of various lipids in the liver.     

GM1-gangliosidosis. Defective recognition site on beta-galactosidase precursor

Cultured fibroblasts from different variants of GM1-gangliosidosis synthesize normal amounts of 88-kDa beta-galactosidase precursor. Yet the amount of the mature 64-kDa form is reduced to 5-15% of normal values. In this communication it is shown that the mutation in the infantile and adult form of GM1-gangliosidosis interferes with the phosphorylation of precursor beta-galactosidase. As a result the precursor is secreted instead of being compartmentalized into the lysosomes and further processed. The impaired phosphorylation might be due to conformational changes of the precursor molecule.     

GM2-gangliosidosis B1 variant: a wide geographic and ethnic distribution of the specific beta-hexosaminidase alpha chain mutation originally identified in a Puerto Rican patient

A point mutation within exon 5 of beta-hexosaminidase alpha chain gene was identified earlier in a Puerto Rican patient with GM2-gangliosidosis B1 variant (the DN-allele) [K. Ohno and K. Suzuki: J. Neurochem. 50:316-318, 1988]. Oligonucleotide probes designed to detect either the normal or the DN-allele showed that four additional patients carried the same mutation. These patients were of Italian, French, Spanish, and English/Italian/Hungarian origin. Three of them, as well as our original patient, were compound heterozygotes with positive signals for both the mutant and normal probes, while the Spanish patient was positive only for the DN-allele. A patient from Czechoslovakia was negative for the DN-allele. Thus, the specific mutation originally found in the Puerto Rican patient has a surprisingly wide geographic and ethnic distribution. This mutation can account for the B1 variant phenotype in five of the six B1 variant patients so far examined.     

Processing of human beta-galactosidase in GM1-gangliosidosis and Morquio B syndrome

The nature of the molecular defect resulting in the beta-galactosidase deficiency in different forms of GM1-gangliosidosis and mucopolysaccharidosis IV B (Morquio B syndrome) was investigated. Normal and mutant cultured skin fibroblasts were labeled in vivo with [3H]leucine and immunoprecipitation studies with human anti-beta-galactosidase antiserum were performed, followed by polyacrylamide gel electrophoresis and fluorography. In Morquio B syndrome, the mutation does not interfere with the normal processing and intralysosomal aggregation of beta-galactosidase. In cells from infantile and adult GM1-gangliosidosis, 85-kDa precursor beta-galactosidase was found to be synthesized normally but more than 90% of the enzyme was subsequently degraded at one of the early steps in posttranslational processing. The residual 5-10% beta-galactosidase activity in adult GM1-gangliosidosis is 64-kDa mature lysosomal enzyme with normal catalytic properties but with a reduced ability of the monomeric form to aggregate into high molecular weight multimers. Knowledge of the exact nature of the molecular defect underlying beta-galactosidase deficiency in man may lead to a better understanding of the clinical and pathological heterogeneity among patients with different types of GM1-gangliosidosis and Morquio B syndrome.     

Prenatal diagnosis of GM1-gangliosidosis: Biochemical manifestations in fetal tissues

Summary A prenatal diagnosis of G_M1-gangliosidosis was made in a pregnancy at risk, on the basis of a deficiency of -galactosidase activity demonstrated in cultured aminiotic fluid cells. Biochemical analyses were performed in the aborted fetus. G_M1-ganglioside -galactosidase activity was reduced to 1% of the control value in both the brain and liver of the affected fetus. Lamellar bodies suggestive of membranous cytoplasmic bodies were found in cells of basal ganglions, while the accumulation of G_M1-ganglioside in the brain was not remarkable.     

GM1-gangliosidosis: Accumulation of ganglioside GM1 in cultured skin fibroblasts and correlation with clinical types

Summary Uptake of radioactivity from ¹⁴C-galactose into gangliosides by cultured skin fibroblasts was studied. G_M3 was the major ganglioside in control human fibroblasts. An increase of G_M1 was demonstrated in G_M1-gangliosidosis fibroblasts. The degree of G_M1 accumulation was correlated with the clinical types of this disease. The fibroblasts from an infantile-type patient showed a marked increase of G_M1. In late-onset types the amount of total gangliosides was only slightly increased, but the distribution of individual gangliosides was definitely abnormal; a relative increase of G_M1 was demonstrated in these cases. G_M1 -galactosidase activities were not detectable in either infantile or late-onset cases.     

A Cys138-to-Arg substitution in the GM2 activator protein is associated with the AB variant form of GM2 gangliosidosis.

The AB-variant form of GM2 gangliosidosis is an inherited lysosomal storage disease. Biochemical data have linked its cause to the lack of a functional GM2 activator protein (activator). In the present study we identify a mutation in the gene encoding the activator protein of an AB-variant patient. These data represent direct evidence that the disease in the patient described here is a result of mutations at the Activator gene locus. A T412----C transition was found in the homozygous form in cDNA and genomic DNA from the patient. This nucleotide change would result in the substitution of Cys138 by an Arg residue in the activator protein. Whereas the patient's fibroblasts produce apparently normal levels of activator mRNA, they lack a functional activator protein. Transfection of either a construct containing the normal activator cDNA, pAct1, or a cDNA construct containing the T----C transition caused COS-1 cells to transcribe high levels of activator mRNA. Lysates from cells transfected with pAct1 produced an elevated level of both pro- and mature forms of the activator protein, with an accompanying 11-fold enhancement in the ability of purified hexosaminidase A to hydrolyze GM2 ganglioside. However, lysates from cells transfected with the mutant cDNA construct contained only low levels of the pro-activator protein, which failed to enhance hexosaminidase A activity significantly above the endogenous level of mock transfected COS cells. We conclude that the T412----C transition in the GM2 Activator gene of the patient is responsible for the disease phenotype.     

Mutations in acid beta-galactosidase cause GM1-gangliosidosis in American patients.

We describe four new mutations in the beta-galactosidase gene. These are the first mutations causing infantile and juvenile GM1-gangliosidosis to be described in American patients. Cell lines from two patients with juvenile and from six patients with infantile GM1-gangliosidosis were analyzed. Northern blot analysis showed the acid beta-galactosidase message to be of normal size and quantity in two juvenile and four infantile cases and of normal size but reduced quantity in two infantile cases. The mutations are distinct from the Japanese mutations. All are point mutations leading to amino acid substitutions: Lys577-->Arg, Arg590-->His, and Glu632-->Gly. The fourth mutation, Arg208-->Cys, accounts for 10 of 16 possible alleles. Two infantile cases from Puerto Rico of Spanish ancestry are homozygous for this mutation, suggesting that this allele may have come to South America and North America via Puerto Rico. That these mutations cause clinical disease was confirmed by marked reduction in catalytic activity of the mutant proteins in the Cos-1 cell expression system.