1/5. A glycine250--> aspartate substitution in the alpha-subunit of hexosaminidase a causes juvenile-onset tay-sachs disease in a Lebanese-Canadian family.The mutation causing juvenile tay-sachs disease (TSD) in two sibs of Lebanese-Maronite origin is described. An mRNA-containing extract of cultured fibroblasts obtained from one of the probands was used as a template to amplify the coding sequence of the hexosaminidase a (Hex A) alpha-subunit. Sequencing of amplified cDNA fragments revealed a single alteration, guanine to adenine at nt 749 creating a G250D mutation. The mutation introduces a new recognition site for the restriction enzyme Eco RV, permitting identification of heterozygotes for this allele following PCR amplification and Eco RV digestion of exon 7 sequences from genomic dna templates. In order to test the effect of this substitution, an in vitro mutagenized cDNA construct was introduced into a mammalian expression vector and transfected into monkey Cos-1 cells separately or along with a beta-cDNA expression vector. When the mutant alpha-cDNA was the only gene introduced into cos cells no enzymatic activity above endogenous COS cell activity was detected. Cotransfection of normal alpha-cDNA and beta-cDNA followed by immunoprecipitation of human Hex A resulted in 20-fold increase in the ratio between positive and negative (mock transfection) control values. This allowed the detection of some residual activity (12% of the positive control) when the mutant alpha-cDNA replaced its wild-type counterpart. The predicted protein environment in which the mutation occurs is compared to that of the adult-onset tay-sachs disease mutation caused by a Gly269-->Ser substitution in exon 7.(ABSTRACT TRUNCATED AT 250 WORDS)- - - - - - - - - - ranking = 1keywords = juvenile-onset (Clic here for more details about this article) |
2/5. The mutation mechanism causing juvenile-onset tay-sachs disease among Lebanese.Expression of the hexosaminidase isozymes was evaluated in fibroblast cell lines obtained from two sibs of Lebanese-Christian origin who presented with juvenile-onset tay-sachs disease. In the normal control fibroblasts the alpha subunit of hexosaminidase a (hex A) is synthesized as a 67 KD precursor which is cleaved in lysosomes to a mature 54 KD peptide. The patients' fibroblasts were capable of synthesizing the 67 KD precursor but failed to convert it to the mature subunit. The alpha subunit precursor synthesized by patients' cells could not be phosphorylated, nor was the patients' alpha subunit precursor secreted into the medium in response to NH4Cl, which caused accumulation of both alpha and beta subunit precursor in the medium of the normal control fibroblasts. The measurement of residual enzyme activity in the fibroblasts of patients which best correlated with the onset of the illness was the ion exchange chromatographic separation of Hex A-associated hydrolysis of the synthetic substrate 4-methylumbelliferyl N-acetyl-beta-D-glucosamine-6-sulfate (4MUGS). The patients had 0.32% and 0.36% of Hex A-associated 4MUGS cleaving activity compared to normal control fibroblasts as compared to less than 0.016% for infantile tay-sachs disease fibroblasts. The residual Hex A activity in patients' cells had a pH optimum identical with normal enzyme (pH 3.9-4.0), a reduced specific activity for 4MUGS (relative to hydrolysis of unsulfated synthetic substrate), and a greatly enhanced thermal stability. The occurrence of this form of tay-sachs disease in lebanon, the fact that the condition has been described in three unrelated Lebanese immigrant families in canada, together with the fact that the grandparents of the unrelated probands come from villages in both the northern and southern regions of lebanon, leads us to speculate that a gene causing juvenile-onset tay-sachs disease may not be infrequent in lebanon.- - - - - - - - - - ranking = 1.5keywords = juvenile-onset (Clic here for more details about this article) |
3/5. Late-onset hexosaminidase a and hexosaminidase a and B deficiency: family study and review.Five children from two non-consanguineous Asian families with juvenile-onset hexosaminidase deficiency are presented. Two have juvenile tay-sachs disease with hexosaminidase a deficiency and three have juvenile sandhoff disease with hexosaminidase a and B deficiency. The contributing factors in the spectrum of the hexosaminidase deficiency disease are outlined, and previously reported cases of late-onset Tay-Sachs and sandhoff disease are reviewed. The heterogeneity of the effects of hexosaminidase deficiency is discussed, with the recommendation that the diagnosis be considered, in its various forms, when there is no other obvious explanation.- - - - - - - - - - ranking = 0.25keywords = juvenile-onset (Clic here for more details about this article) |
4/5. Juvenile GM2 gangliosidosis (AMB variant): inability to activate hexosaminidase a by activator protein.Two sibling from a consanguineous Puerto Rican marriage were found to have a juvenile-onset type of lipidosis first noted at age 2 1/2 by expressing difficulties with motor function and developmental delay. They continued to deteriorate, showing muscle atrophy, spasticity, and loss of speech, and death occurred at ages 7 and 8. Examination of the brains from these patients revealed that the concentration of GM2 ganglioside was about 56% of the total gangliosides. Hexosaminidase and percent hexosaminidase a (HEX A) and other lysosomal enzymes were normal in cultured skin fibroblasts, liver, and brain. The concentration of the activator protein required for the enzymatic hydrolysis of GM2 ganglioside was in high normal levels in the brain of the patient available. However, the HEX A from the patient's brain and liver as well as from skin fibroblast lysates could not be activated to hydrolyze GM2 ganglioside by the activator protein from a control or himself. The HEX A from a control could be activated by the activator protein from controls or this patient. These patients appear to have a defect in HEX A, which does not affect it heat stability, electrophoretic migration, and activity toward fluorogenic substrates, but may affect the binding of the activator protein required for GM2 ganglioside hydrolysis. We propose to call these patients the AMB variant of GM2 gangliosidosis to denote the mutation in HEX A but with normal levels of HEX A and B with synthetic substrates. This is to distinguish these patients from those missing the activator protein and normal HEX A and B levels.- - - - - - - - - - ranking = 0.25keywords = juvenile-onset (Clic here for more details about this article) |
5/5. W474C amino acid substitution affects early processing of the alpha-subunit of beta-hexosaminidase a and is associated with subacute G(M2) gangliosidosis.Mutations in the HEXA gene, encoding the alpha-subunit of beta-hexosaminidase a (Hex A), that abolish Hex A enzyme activity cause tay-sachs disease (TSD), the fatal infantile form of G(M2) gangliosidosis, Type 1. Less severe, subacute (juvenile-onset) and chronic (adult-onset) variants are characterized by a broad spectrum of clinical manifestations and are associated with residual levels of Hex A enzyme activity. We identified a 1422 G-->C (amino acid W474C) substitution in the first position of exon 13 of HEXA of a non-Jewish proband who manifested a subacute variant of G(M2) gangliosidosis. On the second maternally inherited allele, we identified the common infantile disease-causing 4-bp insertion, TATC 1278, in exon 11. pulse-chase analysis using proband fibroblasts revealed that the W474C-containing alpha-subunit precursor was normally synthesized, but not phosphorylated or secreted, and the mature lysosomal alpha-subunit was not detected. When the W474C-containing alpha-subunit was transiently co-expressed with the beta-subunit to produce Hex A (alphabeta) in COS-7 cells, the mature alpha-subunit was present, but its level was much lower than that from normal alpha-subunit transfections, although higher than in those cells transfected with an alpha-subunit associated with infantile TSD. Furthermore, the precursor level of the W474C alpha-subunit was found to accumulate in comparison to the normal alpha-subunit precursor levels. We conclude that the 1422 G-->C mutation is the cause of Hex A enzyme deficiency in the proband. The resulting W474C substitution clearly interferes with alpha-subunit processing, but because the base substitution falls at the first position of exon 13, aberrant splicing may also contribute to Hex A deficiency in this proband.- - - - - - - - - - ranking = 0.25keywords = juvenile-onset (Clic here for more details about this article) |