Impaired autophagy: The collateral damage of lysosomal storage disorders

Lysosomal storage disorders (LSDs), which number over fifty, are monogenically inherited and caused by mutations in genes encoding proteins that are involved in lysosomal function. Lack of the functional protein results in storage of a distinctive material within the lysosomes, which for years was thought to determine the pathophysiology of the disorder. However, our current view posits that the primary storage material disrupts the normal role of the lysosome in the autophagic pathway resulting in the secondary storage of autophagic debris. It is this "collateral damage" which is common to the LSDs but nonetheless intricately nuanced in each. We have selected five LSDs resulting from defective proteins that govern widely different lysosomal functions including glycogen degradation (Pompe), lysosomal transport (Cystinosis), lysosomal trafficking (Danon), glycolipid degradation (Gaucher) and an unidentified function (Batten) and argue that despite the disparate functions, these proteins, when mutant, all impair the autophagic process uniquely.

Pompe disease: Shared and unshared features of lysosomal storage disorders

Pompe disease, an inherited deficiency of lysosomal acid α-glucosidase (GAA), is a severe metabolic myopathy with a wide range of clinical manifestations. It is the first recognized lysosomal storage disorder and the first neuromuscular disorder for which a therapy (enzyme replacement) has been approved. As GAA is the only enzyme that hydrolyses glycogen to glucose in the acidic environment of the lysosome, its deficiency leads to glycogen accumulation within and concomitant enlargement of this organelle. Since the introduction of the therapy, the overall understanding of the disease has progressed significantly, but the pathophysiology of muscle damage is still not fully understood. The emerging complex picture of the pathological cascade involves disturbance of calcium homeostasis, mitochondrial abnormalities, dysfunctional autophagy, accumulation of toxic undegradable materials, and accelerated production of lipofuscin deposits that are unrelated to aging. The relationship of Pompe disease to other lysosomal storage disorders and potential therapeutic interventions for Pompe disease are discussed.

Defects in calcium homeostasis and mitochondria can be reversed in Pompe disease

Mitochondria-induced oxidative stress and flawed autophagy are common features of neurodegenerative and lysosomal storage diseases (LSDs). Although defective autophagy is particularly prominent in Pompe disease, mitochondrial function has escaped examination in this typical LSD. We have found multiple mitochondrial defects in mouse and human models of Pompe disease, a life-threatening cardiac and skeletal muscle myopathy: a profound dysregulation of Ca(2+) homeostasis, mitochondrial Ca(2+) overload, an increase in reactive oxygen species, a decrease in mitochondrial membrane potential, an increase in caspase-independent apoptosis, as well as a decreased oxygen consumption and ATP production of mitochondria. In addition, gene expression studies revealed a striking upregulation of the β 1 subunit of L-type Ca(2+) channel in Pompe muscle cells. This study provides strong evidence that disturbance of Ca(2+) homeostasis and mitochondrial abnormalities in Pompe disease represent early changes in a complex pathogenetic cascade leading from a deficiency of a single lysosomal enzyme to severe and hard-to-treat autophagic myopathy. Remarkably, L-type Ca(2+)channel blockers, commonly used to treat other maladies, reversed these defects, indicating that a similar approach can be beneficial to the plethora of lysosomal and neurodegenerative disorders.

Autophagy and mitochondria in Pompe disease: nothing is so new as what has long been forgotten

Macroautophagy (often referred to as autophagy) is an evolutionarily conserved intracellular system by which macromolecules and organelles are delivered to lysosomes for degradation and recycling. Autophagy is robustly induced in response to starvation in order to generate nutrients and energy through the lysosomal degradation of cytoplasmic components. Constitutive, basal autophagy serves as a quality control mechanism for the elimination of aggregated proteins and worn-out or damaged organelles, such as mitochondria. Research during the last decade has made it clear that malfunctioning or failure of this system is associated with a wide range of human pathologies and age-related diseases. Our recent data provide strong evidence for the role of autophagy in the pathogenesis of Pompe disease, a lysosomal glycogen storage disease caused by deficiency of acid alpha-glucosidase (GAA). Large pools of autophagic debris in skeletal muscle cells can be seen in both our GAA knockout model and patients with Pompe disease. In this review, we will focus on these recent data, and comment on the not so recent observations pointing to the involvement of autophagy in skeletal muscle damage in Pompe disease.

Suppression of autophagy permits successful enzyme replacement therapy in a lysosomal storage disorder--murine Pompe disease

Autophagy, an intracellular system for delivering portions of cytoplasm and damaged organelles to lysosomes for degradation/recycling, plays a role in many physiological processes and is disturbed in many diseases. We recently provided evidence for the role of autophagy in Pompe disease, a lysosomal storage disorder in which acid alphaglucosidase, the enzyme involved in the breakdown of glycogen, is deficient or absent. Clinically the disease manifests as a cardiac and skeletal muscle myopathy. The current enzyme replacement therapy (ERT) clears lysosomal glycogen effectively from the heart but less so from skeletal muscle. In our Pompe model, the poor muscle response to therapy is associated with the presence of pools of autophagic debris. To clear the fibers of the autophagic debris, we have generated a Pompe model in which an autophagy gene, Atg7, is inactivated in muscle. Suppression of autophagy alone reduced the glycogen level by 50–60%. Following ERT, muscle glycogen was reduced to normal levels, an outcome not observed in Pompe mice with genetically intact autophagy. The suppression of autophagy, which has proven successful in the Pompe model, is a novel therapeutic approach that may be useful in other diseases with disturbed autophagy.

The values and limits of an in vitro model of Pompe disease: the best laid schemes o' mice an' men

In Pompe disease, a lysosomal glycogen storage disorder, cardiac and skeletal muscle abnormalities are responsible for premature death and severe weakness. Swollen glycogen-filled lysosomes, the expected pathology, are accompanied in skeletal muscle by a secondary pathology-massive accumulation of autophagic debris-that appears to contribute greatly to the weakness. We have tried to reproduce these defects in murine, Pompe myotubes derived from either primary myoblasts or myoblasts with extended proliferative capacity. The cells accumulated large lysosomes filled with glycogen, but, to our disappointment, did not have autophagic buildup even though basal autophagy was intact. When we suppressed autophagy by knocking down Atg7, we found that glycogen uptake by lysosomes was not affected, suggesting that macroautophagy is not the major pathway for glycogen delivery to lysosomes. But two apparently incidental observations-a peculiar distribution of both microinjected dextran and of small acidic structures adjacent to the interior membrane of large alkalinized glycogen containing lysosomes-raised the possibility that glycogen traffics to the lysosomes by microautophagy or/and by the engulfment of small lysosomes by large ones. The cultured myotubes, therefore, appear to be a useful model for studying the mechanisms involved in glycogen accumulation in Pompe disease and to test substrate deprivation approaches.

Murine muscle cell models for Pompe disease and their use in studying therapeutic approaches

Lysosomes filled with glycogen are a major pathologic feature of Pompe disease, a fatal myopathy and cardiomyopathy caused by a deficiency of the glycogen-degrading lysosomal enzyme, acid alpha-glucosidase (GAA). To facilitate studies germane to this genetic disorder, we developed two in vitro Pompe models: myotubes derived from cultured primary myoblasts isolated from Pompe (GAA KO) mice, and myotubes derived from primary myoblasts of the same genotype that had been transduced with cyclin-dependent kinase 4 (CDK4). This latter model is endowed with extended proliferative capacity. Both models showed extremely large alkalinized, glycogen-filled lysosomes as well as impaired trafficking to lysosomes. Although both Pompe tissue culture models were derived from fast muscles and were fast myosin positive, they strongly resemble slow fibers in terms of their pathologic phenotype and their response to therapy with recombinant human GAA (rhGAA). Autophagic buildup, a hallmark of Pompe disease in fast muscle fibers, was absent, but basal autophagy was functional. To evaluate substrate deprivation as a strategy to prevent the accumulation of lysosomal glycogen, we knocked down Atg7, a gene essential for autophagosome formation, via siRNA, but we observed no effect on the extent of glycogen accumulation, thus confirming our recent observation in autophagy-deficient Pompe mice [N. Raben, V. Hill, L. Shea, S. Takikita, R. Baum, N. Mizushima, E. Ralston, P. Plotz, Suppression of autophagy in skeletal muscle uncovers the accumulation of ubiquitinated proteins and their potential role in muscle damage in Pompe disease, Hum. Mol. Genet. 17 (2008) 3897-3908] that macroautophagy is not the major route of glycogen transport to lysosomes. The in vitro Pompe models should be useful in addressing fundamental questions regarding the pathway of glycogen to the lysosomes and testing panels of small molecules that could affect glycogen biosynthesis or speed delivery of the replacement enzyme to affected lysosomes.

Global gene expression in a type 2 Gaucher disease brain

Gaucher disease is a member of a family of inherited disorders called sphingolipidoses that among others includes Tay-Sachs and Sandhoff diseases. It is caused by the accumulation of glucosylceramide (glucocerebroside) due to deficient activity of the enzyme glucosylceramide-beta-glucosidase (glucocerebrosidase). As with other glycosphingolipidoses, severe neurodegeneration is present in types 2 and 3 Gaucher disease. We have used Serial Analysis of Gene Expression (SAGE) to characterize the gene expression profiles in brain of patients with glycosphingolipid storage diseases to understand the molecular details of neurodegeneration. In the current study we have determined the gene expression profile from the brain of a patient with type 2 Gaucher disease, the acute neuronopathic form of the disorder. We found that the expression profile of the type 2 Gaucher brain is significantly altered relative to the normal control brain profile. There were also differences when compared with profiles from Tay-Sachs and Sandhoff patients, in particular in levels of genes related to macrophage activation. Intriguingly we found that gamma-synuclein, a family member of proteins involved the pathogenesis of other neurodegenerative disorders, was elevated in the one Gaucher type 2 patient brain we examined.

Molecular pathophysiology in Tay-Sachs and Sandhoff diseases as revealed by gene expression profiling

Tay-Sachs and Sandhoff diseases are lysosomal storage disorders characterized by the absence of beta-hexosaminidase activity and the accumulation of GM2 ganglioside in neurons. In each disorder, a virtually identical course of neurodegeneration begins in infancy and leads to demise generally by 4-6 years of age. Through serial analysis of gene expression (SAGE), we determined gene expression profiles in cerebral cortex from a Tay-Sachs patient, a Sandhoff disease patient and a pediatric control. Examination of genes that showed altered expression in both patients revealed molecular details of the pathophysiology of the disorders relating to neuronal dysfunction and loss. A large fraction of the elevated genes in the patients could be attributed to activated macrophages/microglia and astrocytes, and included class II histocompatability antigens, the pro-inflammatory cytokine osteopontin, complement components, proteinases and inhibitors, galectins, osteonectin/SPARC, and prostaglandin D2 synthase. The results are consistent with a model of neurodegeneration that includes inflammation as a factor leading to the precipitous loss of neurons in individuals with these disorders.

Tay-Sachs disease-causing mutations and neutral polymorphisms in the Hex A gene

Tay-Sachs disease is an autosomal recessive disorder affecting the central nervous system. The disorder results from mutations in the gene encoding the alpha-subunit of beta-hexosaminidase A, a lysosomal enzyme composed of alpha and beta polypeptides. Seventy-eight mutations in the Hex A gene have been described and include 65 single base substitutions, one large and 10 small deletions, and two small insertions. Because these mutations cripple the catalytic activity of beta-hexosaminidase to varying degrees, Tay-Sachs disease displays clinical heterogeneity. Forty-five of the single base substitutions cause missense mutations; 39 of these are disease causing, three are benign but cause a change in phenotype, and three are neutral polymorphisms. Six nonsense mutations and 14 splice site lesions result from single base substitutions, and all but one of the splice site lesions cause a severe form of Tay-Sachs disease. Eight frameshift mutations arise from six deletion- and two insertion-type lesions. One of these insertions, consisting of four bases within exon 11, is found in 80% of the carriers of Tay-Sachs disease from the Ashkenazi Jewish population, an ethnic group that has a 10-fold higher gene frequency for a severe form of the disorder than the general population. A very large deletion, 7.5 kilobases, including all of exon 1 and portions of DNA upstream and downstream from that exon, is the major mutation found in Tay-Sachs disease carriers from the French Canadian population, a geographic isolate displaying an elevated carrier frequency. Most of the other mutations are confined to single pedigrees. Identification of these mutations has permitted more accurate carrier information, prenatal diagnosis, and disease prognosis. In conjunction with a precise tertiary structure of the enzyme, these mutations could be used to gain insight into the structure-function relationships of the lysosomal enzyme.

Tay-Sachs disease-causing mutations and neutral polymorphisms in the Hex A gene

Tay-Sachs disease is an autosomal recessive disorder affecting the central nervous system. The disorder results from mutations in the gene encoding the alpha-subunit of beta-hexosaminidase A, a lysosomal enzyme composed of alpha and beta polypeptides. Seventy-eight mutations in the Hex A gene have been described and include 65 single base substitutions, one large and 10 small deletions, and two small insertions. Because these mutations cripple the catalytic activity of beta-hexosaminidase to varying degrees, Tay-Sachs disease displays clinical heterogeneity. Forty-five of the single base substitutions cause missense mutations; 39 of these are disease causing, three are benign but cause a change in phenotype, and three are neutral polymorphisms. Six nonsense mutations and 14 splice site lesions result from single base substitutions, and all but one of the splice site lesions cause a severe form of Tay-Sachs disease. Eight frameshift mutations arise from six deletion- and two insertion-type lesions. One of these insertions, consisting of four bases within exon 11, is found in 80% of the carriers of Tay-Sachs disease from the Ashkenazi Jewish population, an ethnic group that has a 10-fold higher gene frequency for a severe form of the disorder than the general population. A very large deletion, 7.5 kilobases, including all of exon 1 and portions of DNA upstream and downstream from that exon, is the major mutation found in Tay-Sachs disease carriers from the French Canadian population, a geographic isolate displaying an elevated carrier frequency. Most of the other mutations are confined to single pedigrees. Identification of these mutations has permitted more accurate carrier information, prenatal diagnosis, and disease prognosis. In conjunction with a precise tertiary structure of the enzyme, these mutations could be used to gain insight into the structure-function relationships of the lysosomal enzyme.

Evidence for the involvement of Glu-355 in the catalytic action of human beta-hexosaminidase B

In a previous study the photoactivable affinity probe, 3-azi-1-[([6-3H]2-acetamido-2-deoxy-1-beta-D-galactopyranosyl)thio ]-b utane, was used to identify the active site of beta-hexosaminidase B, a beta-subunit dimer (Liessem, B., Glombitza, G. J., Knoll, F., Lehmann, J., Kellermann, J., Lottspeich, F., and Sandhoff, K. (1995) J. Biol. Chem. 270, 23693-23699). The probe predominately labeled Glu-355, a highly conserved residue among hexosaminidases. To determine if Glu-355 has a role in catalysis, beta-subunit mutants were prepared with the Glu-355 codon altered to either Ala, Gln, Asp, or Trp. After expression of mutant proteins using recombinant baculovirus, the enzyme activity associated with the beta-subunits was found to be reduced to background levels. Although catalytic activity was lost, the mutations did not otherwise affect the folding or assembly of the subunits. The mutant beta-subunits could be isolated using substrate affinity chromatography, indicating they contained intact substrate binding sites. As shown by cross-linking with disuccinimidyl suberate, the mutant beta-subunits were properly assembled. They could also participate in the formation of functional beta-hexosaminidase A activity as indicated by activator-dependent GM2 ganglioside degradation activity produced by co-expression of the mutant beta-subunits with the alpha-subunit. Finally, the mutant subunits showed normal lysosomal processing in COS-1 cells, demonstrating that a transport-competent protein conformation had been attained. Collectively the results provide strong support for the intimate involvement of Glu-355 in beta-hexosaminidase B-mediated catalysis.

An unusual genotype in an Ashkenazi Jewish patient with Tay-Sachs disease

The Ashkenazi Jewish population is enriched for carriers of a fatal form of Tay-Sachs disease, a recessive inherited disorder caused by mutations in the alpha-chain of the lysosomal enzyme beta-hexosaminidase A. Approximately 20% of the Ashkenazi carriers harbor a splice junction defect while about 78% bear a 4 base pair (bp) insertion. However, the Ashkenazi Jewish patient used in the original description of the 4 bp insertion carried this lesion in only 1 allele and was negative for the splice junction mutation. We cloned the insertion negative allele and by sequence analysis of the exons found a point mutation in exon 11 that results in substitution of Trp392 with a premature termination codon. Nine Ashkenazi Jewish carriers that tested negative for the major and minor mutations as well as for a lesion causing an adult form of Tay-Sachs disease did not carry the base change defect, suggesting that it may be a recent and/or rare mutation. This finding also indicates that screening the Ashkenazi population solely by recombinant DNA methods for the splice junction, 4 bp insertion, and adult mutations may result in occasional false negatives.

Synthesis of a human lysosomal enzyme, beta-hexosaminidase B, using the baculovirus expression system

Human lysosomal beta-hexosaminidase exists in two major forms: the A isoform is composed of both alpha and beta chains, while the B form is a homopolymer of beta chains. Deficiency of beta-hexosaminidase underlies the GM2 gangliosidoses. We have produced active beta-hexosaminidase B in cultured insect (Sf9) cells by isolation of a recombinant insect virus (baculovirus) containing the cDNA for the beta chain within the viral polyhedron gene and infection of Sf9 cells with this construct. That portion of the enzyme secreted into the medium, 50%, was purified with concanavalin A Sepharose and subsequent affinity chromatography to yield beta-hexosaminidase B that is 75% pure. The product has an N-terminal amino acid sequence, specific activity, and size (M(r) 62,000) similar to that of the enzyme present in cultured human fibroblasts. However, endo H sensitivity studies revealed that the oligosaccharide structures present on recombinant beta-hexosaminidase B differ from those found on the enzyme synthesized in the human system. In addition, these structures lack the mannose 6-phosphate recognition marker that targets degradative hydrolases to lysosomes. Despite these differences, recombinant beta-hexosaminidase B does serve as a specific substrate for the mannose phosphorylating enzyme, N-acetylglucosaminyl phosphotransferase. Furthermore, the oligosaccharide moieties phosphorylated in vitro match those phosphorylated in vivo, pointing to the conformational integrity of the recombinant enzyme. Generous amounts of easily obtained, easily purified, and properly folded beta-hexosaminidase B will facilitate physical structural analysis of the enzyme.

Polymerase chain reaction-generated heteroduplexes from Ashkenazi Tay-Sachs carriers with an insertion mutation can be detected on agarose gels

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A gel electrophoretic assay for detecting the insertion defect in Ashkenazi Jewish carriers of Tay-Sachs disease

A simple, rapid, nonradioactive assay for detecting the 4-bp insertion defect found in the beta-hexosaminidase alpha-chain gene of 70% of the Ashkenazi Jewish carriers of Tay-Sachs disease is described. In this assay, DNA derived from such carriers serves as a template for the polymerase chain reaction. Following amplification of a 159-bp fragment of exon 11 inclusive of the insertion, a portion of the product is subjected to electrophoresis in a 4% NuSieve agarose minigel. Visualization of the DNA with ethidium bromide demonstrates that heterozygote carriers for the defect display two distinct bands. In contrast, DNA from carriers of the splice junction defect, a mutation found in 30% of the Ashkenazi Jewish carriers of Tay-Sachs disease, displays only one band.

Fine assignment of beta-hexosaminidase A alpha-subunit on 15q23-q24 by high resolution in situ hybridization

Tay-Sachs disease results from mutation in the gene encoding beta-hexosaminidase A alpha-subunit. Although some reports have suggested the locus on 15q, we tried to determine the finer gene locus using high resolution in situ hybridization. cDNA probe, p beta H alpha-5, containing the full-length sequence for the enzyme subunit, was 3H-labeled within 1-4 x 10(7) cpm/micrograms of cDNA by nick-translation. After molecular hybridization and autoradiography, prometaphases were G-banded by Hoechst 33258, UV-exposure and Giemsa. A total of 227 silver grains on chromosomes within 115 prometaphase spreads were analyzed. The region 15q23-q24 had 27 grains, corresponding to 11.9% of the total grains and to 77.1% of the grains on chromosome 15. 20.9% of prometaphases were observed with a grain at 15q23-q24. According to several previous reports, the shortest region of overlap (SRO) of the locus has been 15q22-q25.1. Here we have assigned the gene locus to the narrower region 15q23-q24 by high-resolution in situ hybridization, which is one of the most powerful strategy for the completion of human gene map.

The major defect in Ashkenazi Jews with Tay-Sachs disease is an insertion in the gene for the alpha-chain of beta-hexosaminidase

The Ashkenazi Jewish population is enriched for carriers of a fatal form of Tay-Sachs disease, an inherited disorder caused by mutations in the alpha-chain of the lysosomal enzyme, beta-hexosaminidase A. Until recently it was presumed that Tay-Sachs patients from this ethnic isolate harbored the same alpha-chain mutation. This was disproved by identification of a splice junction defect in the alpha-chain of an Ashkenazi patient which could be found in only 20-30% of the Ashkenazi carriers tested. In this study we have isolated the alpha-chain gene from an Ashkenazi Jewish patient, GM515, with classic Tay-Sachs disease who was negative for the splice junction defect. Sequence analysis of the promoter region, exon and splice junctions regions, and polyadenylation signal area revealed a 4-base pair insertion in exon 11. This mutation introduces a premature termination signal in exon 11 which results in a deficiency of mRNA in Ashkenazi patients. A dot blot assay was developed to screen patients and heterozygote carriers for the insertion mutation. The lesion was found in approximately 70% of the carriers tested, thereby distinguishing it as the major defect underlying Tay-Sachs disease in the Ashkenazi Jewish population.

Splice junction mutation in some Ashkenazi Jews with Tay-Sachs disease: evidence against a single defect within this ethnic group

Tay-Sachs disease is an inherited disorder in which the alpha chain of the lysosomal enzyme beta-N-acetylhexosaminidase A bears the mutation. Ashkenazi Jews are found to be carriers for a severe type of Tay-Sachs disease, the classic form, 10 times more frequently than the general population. Ashkenazi Jewish patients with classic Tay-Sachs disease have appeared to be clinically and biochemically identical, and the usual assumption has been that they harbor the same alpha-chain mutation. In this study I have isolated the alpha-chain gene from an Ashkenazi Jewish patient, GM2968, with classic Tay-Sachs disease and compared its nucleotide sequences with that of the normal alpha-chain gene in the promoter region, exon and splice junction regions, and polyadenylylation signal area. Only one difference was observed between these sequences: at the 5' boundary of intron 12, a guanosine in the conserved splice junction dinucleotide sequence G-T had been altered to a cytidine. The alteration is presumed to be functionally significant and to result in aberrant mRNA splicing. Utilizing the polymerase chain reaction to amplify the region encompassing the mutation, I developed an assay to screen patients and heterozygote carriers for this mutation. Surprisingly, in each of two Ashkenazi patients, only one alpha-chain allele harbored the splice junction mutation. Only one parent of each of these patients was positive for the defect. Another Ashkenazi patient did not bear this mutation at all nor did either of the subject's parents. In addition, 30% of obligate heterozygotes tested carried the splice junction mutation, whereas 20 Ashkenazi Jews designated noncarriers by enzymatic assay were negative for this alteration. The data are consistent with the presence of more than one mutation underlying the classic form of Tay-Sachs disease in the Ashkenazi Jewish population.

A deletion involving Alu sequences in the beta-hexosaminidase alpha-chain gene of French Canadians with Tay-Sachs disease

French Canadians living in eastern Quebec are carriers of a severe type of Tay-Sachs disease, known as the classic form, 10 times more often than the general population. The alpha-chain of beta-hexosaminidase A, a lysosomal enzyme composed of two chains (alpha, beta), bears the mutation in this inherited disorder. We previously reported that the 5' end of the alpha-chain gene was deleted in two such patients (Myerowitz, R., and Hogikyan, N.D. (1986) Science, 232, 1646-1648). The present study reports the size, precise location, and environment of the deletion. A clone encompassing the deletion was isolated from a genomic library constructed in lambda EMBL3 with DNA from a patient's fibroblasts. Comparison of the restriction maps of the clone with that of the normal gene (Proia, R.L., and Soravia, E. (1987) J. Biol. Chem. 262, 5677-5681) showed that the deletion was 7.6 kilobases long and included part of intron 1, all of exon 1 and extended 2000 base pairs upstream past the putative promotor region of the alpha-chain gene. These data are consistent with the inability to detect mRNA and immunoprecipitable alpha-chain protein in this mutant. Sequence analysis of the deletion junction in the mutant and corresponding regions of the normal gene demonstrated the presence of similarly oriented Alu sequences at the 5' and 3' deletion boundaries. The data are in accord with the possibility that the deletion may have arisen during homologous recombination from unequal crossing over between Alu sequences.

Different mutations in Ashkenazi Jewish and non-Jewish French Canadians with Tay-Sachs disease

Tay-Sachs disease patients of Ashkenazi Jewish and non-Jewish French Canadian origin are affected with a clinically identical form of this inherited disease. Both have a similar gene frequency for the disorder, which is tenfold higher than that found in the general population. Unlike other patients with the disease, who often display variation at the clinical or biochemical level, the absence of such differences between these two groups has prompted the idea that they may harbor the same mutation. In this report, a complementary DNA clone coding for the alpha chain of human beta-hexosaminidase has been used to analyze the genetic lesions in the alpha-chain locus of two patients with Tay-Sachs disease from each of these groups. On the basis of DNA hybridization analyses, the alpha-chain gene of the Ashkenazi patients appears intact while the alpha-chain gene of French Canadian patients has a 5' deletion of approximately 5 to 8 kilobases.

Human beta-hexosaminidase alpha chain: coding sequence and homology with the beta chain

We have isolated a cDNA clone, p beta H alpha-5, from an adult human liver library that contains the entire coding sequence of the alpha chain of beta-hexosaminidase. The cDNA insert of p beta H alpha-5 is 1944 base pairs long and contains a 168-base-pair 5' untranslated region, a 186-base-pair 3' untranslated region, and an open reading frame of 1587 base pairs corresponding to 529 amino acids (Mr, 60,697). The first 17-22 amino acids satisfy the requirements of a signal sequence. A striking sequence homology with a published partial amino acid sequence for the beta chain [O'Dowd, B. F., Quan, F., Willard, H. F., Lamhonwah, A. M., Korneluk, R. G., Lowden, J. A., Gravel, R. A. & Mahuran, D. J. (1985) Proc. Natl. Acad. Sci. USA 82, 1184-1188] suggests that both chains may have evolved from a common ancestor. A shorter alpha-chain cDNA was found to hybridize to the long arm of chromosome 15, the known location for the alpha-chain gene. In addition, we isolated another alpha-chain cDNA clone, p beta H alpha-4, from a simian virus 40-transformed human fibroblast library that contained an extra 453-base-pair piece at its 3' end. A probe consisting of this additional sequence hybridized exclusively to a single mRNA species (2.6 kilobases) in mRNA preparations from cultured human fibroblasts. In contrast, p beta H alpha-5 hybridized to both a 2.1-kilobase major and a 2.6-kilobase minor mRNA species in these same mRNA preparations, indicating the presence of two distinct alpha-chain mRNA species differing at the 3' end. Fibroblasts from an Ashkenazi Jewish patient with classic Tay-Sachs disease were deficient in both species of mRNA, confirming their genetic relationship.

cDNA clone for the alpha-chain of human beta-hexosaminidase: deficiency of alpha-chain mRNA in Ashkenazi Tay-Sachs fibroblasts

We have isolated a cDNA clone containing sequences complementary to mRNA encoding the alpha-chain of the lysosomal enzyme beta-hexosaminidase. RNA from a human lung fibroblast strain, IMR90, was enriched for beta-hexosaminidase messenger by polysome immunoselection with antiserum against beta-hexosaminidase A. This preparation was used to construct cDNA recombinant plasmids by the Okayama-Berg vector primer procedure. After transformation of Escherichia coli, 385 ampicillin-resistant colonies were obtained, 44 of which contained inserts in the plasmid DNA. Differential hybridization, with cDNA probes prepared from polysomal RNA enriched or depleted for beta-hexosaminidase messenger, was used to screen the recombinant plasmids for sequences encoding beta-hexosaminidase. One clone, p beta H alpha-1, containing a cDNA insert of approximately equal to 240 base pairs, was identified in this manner. The plasmid hybrid-selected a messenger from placental RNA that programed a translation system to synthesize the alpha-chain of beta-hexosaminidase. p beta H alpha-1 hybridized to an mRNA of approximately equal to 1.9 kilobases in preparations enriched separately in messenger for the alpha-chain or for both alpha- and beta-chains (by polysome immunoselection with antiserum against isolated alpha-chain or against beta-hexosaminidase A, respectively). It did not hybridize to an RNA preparation enriched for messenger of beta-chain by immunoselection with antiserum against beta-hexosaminidase B. The 1.9-kilobase mRNA was observed in poly(A)+ RNA preparations from control fibroblasts and from fibroblasts of a Tay-Sachs patient that synthesize an altered alpha-chain; however, it was not seen in similar preparations from fibroblasts of four Ashkenazi Tay-Sachs patients.

Specific chemical modification of the readily nitrated tyrosine of the RTEM beta-lactamase and of bacillus cereus beta-lactamase I. The role of the tyrosine in beta-lactamase catalysis

The function of the hydroxyl group of the tyrosine residue readily nitrated by tetranitromethane (tyrosine-105) in the RTEM plasmid-derived beta-lactamase (penicillinase; penicillin amido beta-lactam-hydrolase, EC 3.5.1.6) from E. coli and in Bacillus cereus beta-lactamase I has been investigated by chemical modification methods. In the case of B. cereus beta-lactamase I the nitrated tyrosine can be acetylated by acetic anhydride without effect on beta-lactamase activity The nitrated tyrosine of the E. coli enzyme can also be acetylated but in this case beta-lactamase activity is lost in a manner which directly correlates with extent of acetylation. However, deacetylation of the nitrotyrosine does not restore activity. The dilemma created by the latter result has been resolved by development of a new method of tyrosine hydroxyl modification at low pH. The nitrated enzyme is reduced by dithionite and then treated with either carbonyldiimidazole or N-(2.2.2-trifluoroethoxycarbonyl)imidazole, both of which convert 3-aminotyrosine into benzoxazolinonylalanine. That the final modification has been achieved is demonstrated both by classical chemical methods and by employment of Fourier transform infrared spectroscopy to detect the characteristic benzoxazolinone carbonyl absorption. Further, it is shown that no significant loss of beta-lactamase activity is associated with this modification. Hence in neither the B. cereus or the E. coli enzyme does the readily nitrated tyrosine residue have a direct chemical function at the beta-lactamase active site.

The mannose 6-phosphate receptor of Chinese hamster ovary cells. Compartmentalization of acid hydrolases in mutants with altered receptors

The localization of acid hydrolases was examined in Chinese hamster ovary cells with defective mannose 6-phosphate receptors; these mutants had been shown to exhibit reduced uptake and altered binding of exogenously added acid hydrolase (Robbins, A. R., Myerowitz, R., Youle, R. J., Murray, G. J., and Neville, D. M., Jr. (1981) J. Biol. Chem. 256, 10618-10622). Cells were grown in the presence of [3H]mannose, alpha-L-iduronidase and beta-hexosaminidase were immunoprecipitated sequentially, electrophoresed on polyacrylamide gels containing sodium dodecyl sulfate, and detected by fluorography. About 55% of the alpha-L-iduronidase and beta-hexosaminidase synthesized by the mutants in 12 h was found in the growth medium; parental cells secreted only approximately 15%. The mutants also secreted 2 to 6 times more alpha-mannosidase, beta-glucuronidase, and alpha-L-fucosidase than the parent as determined by measurements of enzyme activity. Intracellular levels of these enzymes were reduced in the mutants. The mutants secreted acid hydrolases in the precursor forms, within the cells these enzymes resided in lysosomes and were processed normally; thus, the mutants appeared aberrant only with respect to distribution of hydrolases between intracellular and extracellular compartments. [35S]methionine-labeled beta-hexosaminidase and alpha-L-iduronidase secreted by the mutants were taken up normally by both human fibroblasts and wild type CHO cells, and this uptake was inhibited by mannose 6-phosphate. Thus, the elevated secretion of acid hydrolases was not due to alteration of the mannose 6-phosphate recognition marker on the enzymes, but appears to result from alterations in the mannose 6-phosphate receptor.

Selective noncompetitive assimilation of bovine testicular beta-galactosidase and bovine liver beta-glucuronidase by generalized gangliosidosis fibroblasts

Bovine liver beta-glucuronidase and testicular beta-galactosidase were assimilated by generalized gangliosidosis fibroblasts at respectively rates of 90 and 464 times the rate of assimilation of horseradish peroxidase. Assimilation of either of the two enzymes by the fibroblasts was saturable, suggesting the participation of receptor-mediated adsorptive endocytosis for internalization. The rate of assimilation of either enzyme was not affected by high levels of the other enzyme, suggesting that distinct receptors for each enzyme occur on the fibroblasts' cell surface. Furthermore, although assimilation of beta-galactosidase was inhibited by mannose, methyl mannosides, mannosyl alpha 1 leads to 2 mannose, and mannose-6-phosphate, these compounds did not detectably inhibit the assimilation of beta-glucuronidase. These results suggest that testicular beta-galactosidase was assimilated by the well-established phosphomannosyl recognition system. However, liver beta-glucuronidase was assimilated by a distinct, noncompeting, and as yet undefined, recognition system.