Human beta-D-N-acetylhexosaminidases A and B: expression and linkage relationships in somatic cell hybrids

Imaging manifestations of the leukodystrophies, inherited disorders of white matter

The leukodystrophies are a diverse set of inherited white matter disorders and are uncommonly encountered by radiologists in everyday practice. As a result, it is challenging to recognize these disorders and to provide a useful differential for the referring physician. In this article, leukodystrophies are reviewed from the perspective of 4 imaging patterns: global myelination delay, periventricular/deep white matter predominant, subcortical white matter predominant, and mixed white/gray matter involvement patterns. Special emphasis is placed on pattern recognition and unusual combinations of findings that may suggest a specific diagnosis.

Chronic GM2 gangliosidosis type Sandhoff associated with a novel missense HEXB gene mutation causing a double pathogenic effect

We identified a novel c.1556A>G transition in exon 12 of the HEXB gene associated with chronic Sandhoff's disease, changing a conserved aspartic acid to glycine at position 494 of the Hex beta-subunit; moreover, RT-PCR showed aberrant exon 12 skipping, causing a frame-shift and premature stop codon, consequent to the disruption of an exonic splicing enhancer motif by the mutation. These data suggest that the c.1556 A>G transition would affect both HEXB mRNA processing and biochemical properties of the beta-subunit.

Construction and screening of a cosmid library generated from a somatic cell hybrid bearing human chromosome 15

A cosmid library has been constructed with DNA isolated from a mouse/human hybrid cell line designated A15, which was previously characterized and shown to retain chromosome 15 as the only human material. The library was generated and stored as 34 independent pools of primary colonies at 8-10,000 colonies per pool. Screening colonies representing pools of this library by hybridization with a human-specific repetitive probe has facilitated the identification of random clones bearing human inserts. To data, 43 unique clones have been isolated and the inserts mapped by fluorescence in situ hybridization (FISH) to chromosome 15. An STS was generated for each of these clones by end sequencing of the inserts. Two of these clones, c36 and MR23, were mapped by FISH to 15q26.1, and end-sequence data revealed homology to different regions of the FES protooncogene. Sequence generated from the other end of the c36 insert was found to match the previously identified and unmapped coding sequence EST00075. In addition to the identification of such random clones, establishment of the library as pools was expected to facilitate the PCR-based identification of unique clones representing specific regions of chromosome 15. Screening of the library pools with primers specific to the FES region led to the recovery of five independent clones facilitating the development of a cosmid contig encompassing the FES gene. One of the cosmids isolated by PCR-based analysis was derived from the same pool as M23 and was subsequently shown to be identical to M23. The data offer the first report of a chromosome 15 cosmid library and demonstrate the value of utilizing pools to evaluate libraries generated from complex sources like somatic cell hybrids.

A common beta hexosaminidase gene mutation in adult Sandhoff disease patients

beta-Hexosaminidase gene mutations were analyzed in two adult-onset Sandhoff disease Italian patients by PCR analysis of a common known mutation (delta 5') and by heteroduplex analysis of genomic and RT-PCR DNA fragments, covering the whole gene. The patients' genotypes were delta 5'/C1214%, and G890A/C1214T, respectively. As mutation C1214T (Pro405Leu) is also present in the other two late-onset cases so far described, we suggest that C1214T is a common mutation in this type of Sandhoff disease. Mutation G890A (Cys297Tyr) is a novel mutation which presumably causes altered processing of the pro beta chain.

Tay-Sachs disease screening and diagnosis: evolving technologies

Tay-Sachs disease (TSD) is an autosomal recessive, progressive, and fatal neurodegenerative disorder. Within the last 25 years, the discovery of the enzymatic basis of the disease, the deficiency of the enzyme hexosaminidase A, has made possible both enzymatic diagnosis of TSD and heterozygote identification. TSD is the first genetic condition for which a community-based heterozygote screening program was attempted with the intention of reducing the incidence of a genetic disease. In this article we review the clinical, biochemical, and molecular features of TSD as well as the development of laboratory technology that has been deployed in community genetic screening programs. We describe the assay procedures used and some of the limitations in their accuracy. We consider the impact of DNA-based technology on the process of identification of individuals carrying mutant genes associated with TSD and we discuss the social context within which genetic screening occurs.

Chromosomal localization of the gene for a human cytosolic thyroid hormone binding protein homologous to the subunit of pyruvate kinase, subtype M2

A cDNA for the gene that encodes a human cytosolic thyroid hormone binding protein (p58) recently has been isolated and sequenced. Analysis of the p58 sequence indicates that it is identical to the subunit of pyruvate kinase, subtype M2. By in situ hybridization, the gene for p58 was mapped to 15q24-25. This localization shows that the p58 gene is not linked to the L-type of pyruvate kinase, which is located on chromosome 1. The p58 gene was found to be activated in several forms of cancer. Current localization will permit us to assess the effect of alterations involving chromosome 15 on the structure and activity of the p58 gene in neoplasms or chromosome syndromes.

Localization of the gene encoding 3-hydroxy-3-methylglutaryl-coenzyme A synthase to human chromosome 5

A series of hybrids between primary human cells and a Chinese hamster somatic cell mutant (Mev-1), defective in expression of the enzyme 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) synthase [(S)-3-hydroxy-3-methylglutaryl-CoA acetoacetyl-CoA-lyase (CoA-acetylating, EC 4.1.3.5], has been prepared that complements the mutant defect. A technique based on differential sensitivity of this enzyme activity to inhibition by magnesium ion is described that allows the discrimination of expression of human and hamster HMG-CoA synthase in these hybrids. The results indicate a structural gene defect in expression of HMG-CoA synthase activity in Mev-1 cells. Segregation of human chromosomes that do not possess the complementing marker have allowed the assignment of human HMG-CoA synthase activity to chromosome 5. This is the second demonstrably transcriptionally regulated enzyme of cholesterologenesis to be assigned to chromosome 5, the other being HMG-CoA reductase.

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.

Diagnosis and carrier detection of Tay-Sachs disease: direct determination of hexosaminidase A using 4-methylumbelliferyl derivatives of beta-N-acetylglucosamine-6-sulfate and beta-N-acetylgalactosamine-6-sulfate

4-Methylumbelliferyl-6-sulfo-2-acetamido-2-deoxy derivatives of beta-D glucopyranoside and beta-D-galactopyranoside were prepared by direct sulfation of the commonly used unsulfated derivatives. Both sulfated substrates were highly specific for hexosaminidase A, and in fractionated serum, cells, and tissue preparations, less than 2.5% of these activities were associated with hexosaminidase B and the intermediate isozyme fractions. Serum and leukocytes from patients with infantile Tay-Sachs disease, including a patient with thermolabile hexosaminidase B, had less than 2% of noncarrier activities. Carrier values were clearly separated from those of noncarriers, and no problems were encountered in utilizing sera from pregnant women. The % hexosaminidase A values as derived from the ratio between the activities toward the sulfated and unsulfated substrates in the same specimen were comparable to those obtained by the heat-inactivation method (except for subjects with thermolabile hexosaminidase B) and may be helpful in genotype determination in borderline cases.

Human salivary proline-rich protein genes on chromosome 12

A DNA probe (PRP1) for the proline-rich protein (PRP) genes was used to analyze the segregation of human PRP genes in human X mouse somatic cell hybrids. Endonuclease restriction analysis of 22 independent hybrid clones segregating human chromosomes demonstrated that PRP genes segregate with human chromosome 12 only and were therefore assigned to that chromosome. The PRP1 probe should prove useful for further mapping studies of human chromosome 12.

Isolation of cDNA clones coding for the beta subunit of human beta-hexosaminidase

The major forms of beta-hexosaminidase (2-acetamido-2-deoxy-beta-D-glucoside acetamidodeoxyglucohydrolase, EC 3.2.1.30) occur as multimers of alpha and beta chains--hexosaminidase A (alpha beta a beta b) and hexosaminidase B 2(beta a beta b). To facilitate the investigation of beta-chain biosynthesis and the nature of mutation in Sandhoff disease, a human hexosaminidase beta-chain cDNA clone was isolated. Hexosaminidase B (10 mg) was treated with CNBr, five peptide fragments were isolated by reverse-phase HPLC, and their amino acid sequences were determined. One of these contained a string of six amino acids from which an oligonucleotide probe was defined. The simian virus 40-transformed human fibroblast cDNA library of Okayama and Berg was screened by colony hybridization with the radiolabeled probe. Thirteen probe-binding clones were selected out of 50,000 clones screened. Four of these designated pHex were shown to be identical at their 3' ends by restriction enzyme mapping, differing only in their 5' extensions (1.4-1.7 kilobases). The nucleotide sequence of a 174-base-pair segment contained the deduced amino acid sequence of two of the five CNBr peptides, indicating that the pHex clones encode the beta subunit of hexosaminidase. In addition, pHex cDNA was found homologous to multiple bands in digests of genomic human DNA totaling 43 kilobases (kb), all of which were mapped to chromosome 5 in somatic cell hybrids, as expected of the HEXB gene. The pHex cDNA also hybridized to a 2.2-kilobase RNA that apparently codes for the pre-beta-polypeptide of hexosaminidase. This RNA species was absent in the fibroblasts of one of three patients with Sandhoff disease examined. We anticipate that these clones will be of value to diagnosis and carrier detection of Sandhoff disease in affected families.

Mapping of the gene coding for the human GM2 activator protein to chromosome 5

The gene coding for the GM2 activator protein has been mapped to human chromosome 5, using an enzyme-linked immunoadsorbent assay (ELISA) to identify the human protein in human-mouse somatic cell hybrids.

Chromosomal assignments of human genes for serine proteases trypsin, chymotrypsin B, and elastase

The genes for the serine proteases trypsin, chymotrypsin B, and elastase were chromosomally assigned in man using cDNA probes that have been isolated from a rat pancreatic cDNA library. DNA from human X rodent somatic cell hybrids was cleaved with BamHI or EcoRI and analyzed by Southern filter hybridization methods for the segregation of the genes for trypsin-1 (TRY1), chymotrypsin B (CTRB), and elastase-1 (ELA1). TRY1 was assigned to human chromosome 7q22----qter, CTRB to chromosome 16, and ELA1 to chromosome 12. Although the three genes are members of the same gene family, they are dispersed over different chromosomes.

Thermal activation of hexosaminidase A in a genetic compound with Tay-Sachs disease

Increase in total hexosaminidase activity has been observed during heat treatment of serum and leukocyte specimens from a 1-year-old boy with cherry-red spot and severe and progressive mental and motor deterioration. The activity increased 40% in the first 40-70 min of incubation at 50 degrees C and pH 4.3, but declined thereafter and was only slightly above the initial activity in the final 2-3 h of incubation. Heat treatment of specimens from family members revealed very reduced rates of inactivation of hexosaminidase in the proband's father and some paternal relatives, whereas those of the mother and some maternal relatives were indistinguishable from those of Tay-Sachs carriers. Mixing experiments with enzyme preparations from the proband, normal controls and patients with Tay-Sachs disease resulted in additive values and did not support the possibility of inhibitor- or activator-related defect. Fractionation of heat-treated samples by ion exchange chromatography and electrophoresis, as well as examination of the separated fractions for their thermostability, have shown that hexosaminidase A is the activated component and hexosaminidases B, I1 and I2 are not affected. These findings suggest that the patient is a genetic compound and the apparent thermal activation is probably due to formation of hexosaminidase A from altered alpha-subunits produced by the paternal mutant alpha-allele and beta-subunits produced by the normal beta-alleles.

Evidence for two dissimilar polypeptide chains in the beta 2 subunit of hexosaminidase

The major isoenzymes of human hexosaminidase have the structures alpha beta 2 (hex A) and 2 beta 2 (hex B). In this study, we present evidence that the beta 2 subunit of hex B and hex BA (the form of hex B derived from hex A) is composed of two nonidentical polypeptide chains. We have called these chains beta a and beta b. They have similar molecular weights (25,000) but have pI values that differ by 1 unit. We have used a two-dimensional analytical gel electrophoresis method in combination with peptide mapping to compare the primary sequence structure of the two beta chains. In this method, the polypeptide chains of hex B or hex BA were first separated by isoelectric focusing in 8.5 M urea. The separated chains were subjected to partial proleolytic digestion in the stacking gel of a second NaDodSO4/polyacrylamide gel with subsequent separation of peptides by electrophoresis into the second gel. Partial digestion by protease V8 or papain showed that the beta a and beta b species have distinct primary structures, neither of which was similar to that of the alpha chain. On the basis of these results, we suggest that the beta 2 subunit of hexosaminidase has the structure of beta a beta b. The possibility that the distinct beta chains are encoded by a single gene is discussed in the light of genetic and other data.

The expression of hex A and hex B isozymes of hexosaminidase in parental and experimental human fibroblast cells and their components

The expression of the two major isozyme forms of hexosaminidase (EC 3.2.1.30), hexosaminidase A and hexosaminidase B, has been examined. The parental cells and/or cellular components of parental cells are individually fused using inactivated Sendai virus with the aid of a micromanipulator. The progeny cells produced from such hybrids are subjected to a microenzymatic assay which allows measurements at the single cell level. The lysosomal-deficient cells used in this study are Tay-Sachs and Sandhoff fibroblasts, and the normal cells used are WI-38 (fetal lung fibroblasts), amniotic fluid cells (GM 473), and JASD3 (normal human foreskin). The results show that the ratio of cell components which are fused to form the experimental cell affects the percentage of hexosaminidase A expressed in the progeny cells. Furthermore, our results imply the presence of a "factor" in the Sandhoff cell's cytoplasm which, together with the Tay-Sachs nucleus, is necessary for hexosaminidase A expression in the experimental cell's progeny.

Lipochromosome mediated gene transfer: identification and probable specificity of localization of human chromosomal material and stability of the transferents

Using lipochromosomes (phospholipid-entrapped chromosomes) were have transferred the human HGPRT gene into HGPRT deficient mouse cells (A9) with a frequency of approximately 1 x 19(-5) (Mukherjee et al. Proc. Natl. Acad. Sci. USA 75: 1361-1365; 1978). Two other genes located on the long arm of the human X-chromosome were also expressed two independently derived populations of transferents (A9/GT3 and A9/GT4). We report here the chromosomal and enzymatic composition of human HGPRT-positive clones from each subpopulation analyzed in detail with alkaline Giemsa-11 staining. All the clones expressed human PGK and HGPRT, but one (A9/GT4C6) lacked human G6PD. In each of four clone examined microscopically, a small piece of presumptive human chromatin was visible in the karyotypes of most cells. The chromatin fragment was free or attached in each cell of an individual clone. When integrated, the human chromosomal fragment in each clone appeared associated with the centromere of the same telocentric A9 chromosome (No. 6 Q-banding). These data suggest that: (a)substantial human chromosomal fragments can be transferred into recipient cell using the lipochromosome technique; (b) clones from human HGPRT positive A9 transferent subpopulations may or may not possess other human X-linked markers: (c) the stability of lipochromosomally transferred genes varied from clone to clone and stability is generally poor in the absence of continuous selection pressure (e.g., HAT); (d) when multiple X-linked human genes were transferred to mouse cells a cytologically detectable human chromosomal fragment was identified free or attached to a host chromosome; and (e) integration of transferred human chromosomal material into mouse chromosomes may occur at preferential site(s) in the recipient genome.

Human lysosomal genes: arylsulfatase A and beta-galactosidase

The segregation of human lysosomal arylsulfatase A (ARS-A) has been evaluated in 50 primary hybrid clones derived from four separate fusions involving WBCs from two unrelated individuals and three hamster cell lines. ARS-A was expressed in the hybrids as a dimeric molecule of very similar or identical subunits. The expression of this enzyme was concordant with that of mitochondrial aconitase (ACON-M), an isozyme assigned to chromosome 22, in all 50 clones and with chromosome 22 segregation in all but one of the 29 karyotyped hybrids. No other human chromosome cosegregated with 22 in these clones, suggesting that this enzyme is specified in hybrid cells by a locus (or loci) on a single chromosome. beta-Galactosidase (B-GAL) expression was analyzed with two different electrophoresis systems and with a number of cell extract preparation methods in 39 of the primary hybrid clones. The B-GAL isozyme expressed in these hybrid cells was concordant with the expression of glutathione peroxidase-1 (GPX-1), an isozyme assigned to chromosome 3, in all 39 clones and with the segregation of this chromosome in 97% of the 29 karyotyped hybrids. These observations substantiate the prior tentative assignments of an ARS-A locus to chromosome 22 and a B-GAL locus to chromosome 3 (Bruns et al., 1978a, b). The implications of the chromosome assignments of loci for 12 human lysosomal enzymes for the cellular assembly of these organelles are discussed.

Demonstration of cross-reacting material in Tay-Sachs disease

Antibodies against placental hexosaminidase A and kidney alpha-subunits were raised in rabbits after cross-linking the antigens with glutaraldehyde. Anti-(alpha(n)-subunit) antiserum (anti-alpha(n)) precipitated hexosaminidase A but not hexosaminidase B, whereas anti-(hexosaminidase A) antiserum precipitated both hexosaminidases A and B. Specific anti-(hexosaminidase A) antiserum was prepared by absorbing antiserum with hexosaminidase B. Both anti-alpha(n) and anti-(hexosaminidase A) antisera precipitated the CR (cross-reacting) material from eight unrelated patients with Tay-Sachs disease. Immunotitration, immunoelectrophoresis, double-immunodiffusion and radial-immunodiffusion techniques were used to demonstrate the presence of CR material. The CR-material-antibody complex was enzymically inactive. Antiserum raised against kidney or placental hexosaminidase A, without cross-linking with glutaraldehyde, failed to precipitate the CR material, implying that treatment of the protein with glutaraldehyde exposes antigenic determinants that are hidden in the native protein. Since anti-(hexosaminidase B) antiserum did not precipitate the CR material during the immunoelectrophoresis of Tay-Sachs liver extracts, it is suggested that altered alpha-subunits do not combine with beta-subunits. By using immunotitration we have demonstrated the competition between the hexosaminidase B-free Tay-Sachs liver extract and hexosaminidase A for the common binding sites on monospecific anti-(cross-linked hexosaminidase A) antiserum. The amount of CR material in the liver samples from seven cases of Tay-Sachs desease was found to be in the same range as theoretically calculated alpha-subunits in normal liver samples. Similar results were obtained by the radial-immunodiffusion studies. The present studies therefore suggest that Tay-Sachs disease is caused by a structural-gene mutation.

Lysosomal arylsulfatase deficiencies in humans: chromosome assignments for arylsulfatase A and B

Genetics of human lysosomal arylsulfatases A and B (aryl-sulfate sulfohydrolase, EC 3.1.6.1), associated with childhood disease, has been studied with human-rodent somatic cell hybrids. Deficiency of arylsulfatase A (ARS(A)) in humans results in a progressive neurodegenerative disease, metachromatic leukodystrophy. Deficiency of arylsulfatase B (ARS(B)) is associated with skeletal and growth malformations, termed the Maroteaux-Lamy syndrome. Simultaneous deficiency of both enzymes is associated with the multiple sulfatase deficiency disease, suggesting a common relationship for ARS(A) and ARS(B). The genetic and structural relationships of human ARS(A) and ARS(B) have been determined by the use of human-Chinese hamster somatic cell hybrids. Independent enzyme segregation in cell hybrids demonstrated different chromosome assignments for the structural genes, ARS(A) and ARS(B), coding for the two lysosomal enzymes. ARS(A) activity showed concordant segregation with mitochondrial aconitase encoded by a gene assigned to chromosome 22. ARS(B) segregated with beta-hexosaminidase B encoded by a gene assigned to chromosome 5. These assignments were confirmed by chromosome analyses. The subunit structures of ARS(A) and ARS(B) were determined by their electrophoretic patterns in cell hybrids; a dimeric structure was demonstrated for ARS(A) and a monomeric structure for ARS(B). Although the multiple sulfatase deficiency disorder suggests a shared relationship between ARS(A) and ARS(B), independent segregation of these enzymes in cell hybrids did not support a common polypeptide subunit or structural gene assignment. The evidence demonstrates the assignment of ARS(A) to chromosome 22 and ARS(B) to chromosome 5. A third gene that affects ARS(A) and ARS(B) activity is suggested by the multiple sulfatase deficiency disorder.

The biochemical genetics of the hexosaminidase system in man

Tay-Sachs disease and related GM2 ganglioside storage disorders result from the absence of one form of hexosaminidase, HEX A. The persistence of a second major hexosaminidase isozyme, HEX B, does not protect against the lethal accumulation of GM2 ganglioside in the central nervous system. Using immunologic and biochemical techniques, it has been demonstrated that the two major isozymes of hexosaminidase, HEX A and HEX B, share a common subunit, the structure of HEX A being designated (alpha beta)n and the structure of HEX B being designated as (beta2)n. The minor isozyme, HEX S, is an alpha chain homopolymer designated (alpha2)n, and HEX C seems unrelated to the HEX A, B, S system. The structures of other minor isozymes have not been totally resolved, but HEX I1, I2, and P (which may be identical to I2) appear to represent forms of HEX B.

Carbohydrate composition of human placental N-acetylhexosaminidase A and B

The carbohydrate composition of N-acetyl-beta-D-hexosaminidases (EC 3.2.1.52) A, B and heat-converted B was determined by g.l.c. Similar quantities of mannose, N-acetyl-glucosamine and galactose are present in the A and B isoenzymes, whereas N-acetyl-neuraminic acid is found in significant amount in only the A isoenzyme. The heat-converted hexosaminidase B also contains only trace amounts of N-acetylneuraminic acid, but is about 1.5-fold richer in mannose and N-acetylglucosamine and nearly 2-fold richer in galactose than native hexosaminidase B. Since native and converted hexosaminidase B are thought to be composed of four identical protein chains, our results suggest that there may be variable glycosylation of these chains.

Biochemistry and genetics of gangliosidoses

The gangliosidoses comprise an-ever increasing number of biochemically and phenotypically variant diseases. In most of them an autosomal recessive inherited deficiency of a lysosomal hydrolase results in the fatal accumulation of glucolipids (predominantly in the nervous tissue) and of oligosaccharides. The structure, substrate specificity, immunological properties of and genetic studies on the relevant glycosidases, ganglioside GM1 beta-galactosidase and beta-hexosaminidase isoenzymes, are reviewed in this paper. Contrary to general expectation, only a poor correlation is observed between the severity of the disease and residual activity of the defective enzyme when measured with synthetic or natural substrates in the presence of detergents. For the understanding of variant diseases and for their pre- and postnatal diagnosis, the necessity of studying the substrate specificity of normal and mutated enzymes under conditions similar to the in vivo situation, e.g., with natural substrates in the presence of appropriate activator proteins, is stressed. The possibility that detergents may have adverse affects on the substrate specificity of the enzymes is discussed for the beta-hexosaminidases. The significance of activator proteins for the proper interaction of lipid substrates and water-soluble hydrolases is illustrated by the fatal glycolipid storage resulting from an activator protein deficiency in the AB variant of GM2-gangliosidosis. Recent somatic complementation studies have revealed the existence of a presumably post-translational modification factor necessary for the expression of ganglioside GM1 beta-galactosidase activity. This factor is deficient in a group of variants of GM1-glangliosidosis. Among the possible reasons for the variability of enzyme activity levels in heterozygotes and patients, allelic mutations, formation of hybrid enzymes, and the existence of patients as compound heterozygotes are discussed. All these may result in the production of mutant enzymes with an altered specificity for a variety of natural substrates.

Argininosuccinic aciduria: assignment of the argininosuccinate lyase gene to the pter to q22 region of human chromosome 7 by bioautography

Argininosuccinic aciduria, an autosomal recessive disorder of the urea cycle in humans, is associated with a deficiency of argininosuccinate lyase (ASL; L-argininosuccinate arginine-lyase, EC 4.3.2.1). ASL activity was visualized on gels after electrophoresis by a new method, termed bioautography. Bioautography involves the use of mutant bacteria to visualize the location of mammalian enzymes after zone electrophoresis. By this technique, human ASL migrated to a position different from mouse ASL, while a survey of mouse strains, tissues, and tissue culture cell extracts demonstrated the same electrophoretic form and no genetic variants of mouse ASL. Identifying human ASL, by bioautography in human-mouse somatic cell hybrids has made it possible to regionally locate the ASL gene on human chromosome 7. The human ASL phenotype segregated concordantly with the human enzyme beta-glucoronidase (GUS; beta-D-glucoronide glucuronosohydrolase, EC 3.2.1.31) in cell hybrids, but showed discordant segregation with 32 other enzyme markers representing 23 linkage groups. The gene for GUS has been assigned to chromosome 7 in humans, and cosegregation (synteny) of ASL and GUS demonstrates the assignment of ASL to chromosome 7. Regional location of ASL and GUS to the pter to q22 region of chromosome 7 was achieved in hybrids segregating a 7/9 translocation.

A new immunochemical method for the quantitative measurement of specific gene products in man-rodent somatic cell hybrids

An immunochemical method has been developed for the quantitative determination of species-specific gene products, for instance alpha-galactosidase and N-acetyl-alpha-galactosaminidase, in man-rodent hybrid cells and in the parental cell lines. Antisera raised against the purified enzymes are covalently coupled to Sepharose 4B. The gene products are specifically removed from a cell lysate by incubating with the appropriate Sepharose-coupled antiserum. After centrifugation followed by washing of the precipitated Sepharose, the enzymic activity can be quantitatively measured on the Sepharose beads. With this technique it has been demonstrated that the ability of human N-acetyl-alpha-galactosaminidase (also known as alpha-galactosidase B) to hydrolyze alpha-galactosidic linkages is lost when the enzyme is expressed in man-Chinese hamster hybrid cells.

Genetics of the large, external, transformation-sensitive (LETS) protein: assignment of a gene coding for expression of LETS to human chromosome 8

Techniques have been developed to analyze the genetics of the large, external, transformation-sensitive (LETS) protein (fibronectin). External membrane proteins of human-mouse somatic cell hybrids with reduced numbers of human but not mouse chromosomes were labeled by lactoperoxidase-catalyzed iodination. Cell surface proteins were identified after sodium dodecyl sulfate/polyacrylamide gel electrophoresis by autoradiography of the dried gel. The LETS protein was identified in parental human cells, and LETS segregated in human-mouse cell hybrids formed from human WI-38 fibroblasts and a mouse L-cell line not expressing LETS. The LETS protein segregated concordantly with the chromosome 8 enzyme marker glutathione reductase (EC 1.6.4.2) and human chromosome 8. These findings demonstrate that a gene, LETS, encoded on chromosome 8, is responsible for the LETS protein expression in humans. Because LETS has been implicated in tumorigenicity and cellular transformation, it is of interest that rearrangement or modifications in the number of chromosome 8 have been associated with certain forms of cancer.

Homologous genes for enolase, phosphogluconate dehydrogenase, phosphoglucomutase, and adenylate kinase are syntenic on mouse chromosome 4 and human chromosome 1p

It is possible to generate interspecific somatic cell hybrids that preferentially segregate mouse chromosomes, thus making possible mapping of mouse genes. Therefore, comparison of the linkage relationships of homologous genes in man and mouse is now possible. Chinese hamster x mouse somatic cell hybrids segregating mouse chromosomes were tested for the expression of mouse enolase (ENO-1; EC 4.2.1.11, McKusick no. 17245), 6-phosphogluconate dehydrogenase [PGD; EC 1.1.1.44, McKusick no. 17220], phosphoglucomutase-2 (PGM-2; EC 2.7.5.1, McKusick no. 17190), and adenylate kinase-2 (AK-2; EC 2.7.4.3, McKusick no. 10302). In man, genes coding for the homologous forms of these enzymes have been assigned to the short arm of human chromosome 1. Analysis of 41 primary, independent, hybrid clones indicated that, in the mouse, ENO-1 and AK-2 are syntenic with PGD and PGM-2 and therefore can be assigned to mouse chromosome 4. In contrast, they were asyntenic with 21 other enzymes including mouse dipeptidase-1 (DIP-1, human PEP-C; EC 3.4.11.(*), McKusick no. 17000) assigned to human chromosome arm 1q and mouse chromosome 1. Karyologic analysis confirmed this assignment. These data demonstrate that a large autosomal region (21 map units in the mouse and 51 map units in the human male) has been conserved in the evolution of mouse chromosome 4 and the short arm of human chromosome 1. Identification of such conserved regions will contribute to our understanding of the evolution of the mammalian genome and could suggest gene location by homology mapping.

Gene assignment of alpha-fucosidase and glucose dehydrogenase to the p21 leads to pter region of chromosome 1 in man

Assignment of human genes coding for alpha-fucosidase (alphaFUC) and glucose dehydrogenase (GDH) to chromosome 1 has been confirmed and a location in the p21 leads to pter region demonstrated using man-mouse somatic cell hybrids. The regional location of alphaFUC and GDH was established in cell hybrids using human cells possessing 1/2 translocation chromosomes [46,XX,t(1;2)(p21;q37)]. Hybrids which retained the 2q+ chromosome carrying the 1p21 leads to 1pter region concordantly expressed alphaFUC, GDH, and the short-arm markers ENO1, AK2, and PGM1. Hybrids which retained the 1p21 leads to 1qter region only expressed human PEPC and FH. Data obtained from hybrids in which spontaneous breaks in chromosome 1 had occurred indicate that the gene order in 1p21 leads to 1pter is (ENO1,GDH)-alphaFUC-AK2-PGM1.

Entrapment of metaphase chromosomes into phospholipid vesicles (lipochromosomes): carrier potential in gene transfer

Transfer of genes from one type of cultured mammalian cell to another by using isolated metaphase chromosomes has been reported with a frequency of one per 10(6)-10(8) cells. Very recently a rate of 16/10(6) has been reported with Chinese hamster ovary cells [Spandidos, D. A. & Siminovitch, L. (1977) Proc. Natl. Acad. Sci. USA 74, 3480-3484]. To increase the frequency of gene transfer, we isolated metaphase chromosomes from hypoxanthine guanine phosphoribosyltransferase (HGPRT) positive cells, entrapped them in liposomes, and fused the lipochromosomes with HGPRT-negative cells. Lipochromosomes were prepared with cholesterol and egg lecithin, using isolated metaphase chromosomes from a mouse-human somatic hybrid cell line (A9/HRBC2); the entire X chromosome, including the HGPRT, glucose-6-phosphate dehydrogenase, and phosphoglycerate kinase genes, is the only recognizable human genetic material retained by the hybrids. Enclosure of the chromosomes in the lipid envelope was confirmed by electron and fluorescence microscopy and differential centrifugation. These lipochromosomes were fused with HGPRT(-) mouse cells (A9) in the presence or absence of polyethylene glycol and transferents were selected in hypoxanthine/aminopterin/thymidine (HAT) medium. The frequency of transfer was at least once per 10(5) cells, a minimum 10-fold improvement over previous methods. The selected cells contained HGPRT activity similar to the amount found in the A9/HRBC2 cells. Starch gel electrophoresis verified that the observed HGPRT activity in the transferents is due to the human enzyme. Human glucose-6-phosphate dehydrogenase and phosphoglycerate kinase were also identified electrophoretically in the transferents. Karyotyping with C and Q banding did not reveal the presence of the whole human X chromosome or a visible extra fragment of a human chromosome associated with the mouse genome. The biochemical data strongly suggest, however, that transfer of a portion of the human X chromosome has occurred in these transferents. Thus, at least three X-linked genes have been transferred from one cell to another with high frequency, using metaphase chromosomes.

Characterization of residual hexosaminidase activity in Sandhoff's disease using man-Chinese hamster cell hybrids

To obtain information about the nature of the residual hexosaminidase activity in Sandhoff's disease, hybrid cell lines between fibroblasts from a patient with Sandhoff's disease and Chinese hamster cells were isolated. In these hybrid cell lines, a heteropolymeric isoenzyme was detected that is composed of human alpha- and Chinese hamster hexosaminidase subunits. Due to the electrophoretic and immunological behavior of the heteropolymeric molecules in interspecies hybrids with normal fibroblasts and with cells from a patient with Sandhoff's disease, we conclude that Sandhoff cells contain an alpha-subunit of hexosaminidase with normal characteristics.

Genetic linkage studies of the human glycosphingolipid beta-galactosidases

The genetic linkage relationships of the human glycosphingolipid beta-galactosidases were determined using human--mouse somatic cell hybrids. A new method was devised for the estimation of human galactosylceramide, lactosylceramide, and GMI-ganglioside beta-galactosidase activities in the presence of their mouse counterparts, which takes advantage of the reproducible specific activity of lysosomal hydrolases under a given set of culture conditions and is based on differences in both pH optima and sensitivity to chloride ion. Human and mouse chromosomes were identified by their characteristic banding patterns obtained after quinacrine staining, and the optimum glycolipid beta-galactosidase activity was determined for three different substrates. A ratio was defined for each activity which was the specific activity at the human pH optimum divided by the specific activity at the mouse pH optimum. Linear regression analysis was used to test for concordant segregation between pH ratios for each enzyme and the frequency of occurrence of different human chromosomes in the man--mouse somatic hybrid clones. The results obtained from two independent series of hybrid clones indicated that human beta-galactosidase activities consistently segregated with human chromosome 12 in these somatic cell hybrids.

Regional mapping of human genes for hexosaminidase B and diphtheria toxin sensitivity on chromosome 5 using mouse X human hybrid cells

Mouse 3T3 (TK-) cells were fused to human leukocytes containing a balanced translocation [ins(3;5) (q27;q13q15)] in which part of the long arm of a chromosome 5 has been inserted into the long arm of a chromosome 3. Two independent, primary hybrid clones (XVI-10C;XVI-18A) retained the deleted chromosome 5 [del(5) (q13q15)] translocation product and were informative for regional mapping on chromosome 5 of genes involved in expression of hexosaminidase B (HEXB) and diphtheria toxin sensitivity (DTS). Both XVI-10C and XVI-18A clones were sensitive to diphtheria toxin. Toxin-resistant derivatives of these clones (XVI-10C DTR; XVI-18A DTR) were analyzed for chromosome content and expression of Hex B activity, as were XVI-10C and XVI-18A cells which had not been exposed to diphtheria toxin. The results of this study provide evidence for localization of DTS to region 5q15 leads to 5 qter on the long arm of chromosome 5, and localization of HEXB to region 5pter leads to 5q13.

Assignment of the structural genes for the alpha subunit of hexosaminidase A, mannosephosphate isomerase, and pyruvate kinase to the region q22-qter of human chromosome 15

Concordant segregation of the expression of the alpha subunit of human hexosaminidase A, human mannosephosphate isomerase, and pyruvate kinase was observed in somatic cell hybrids between either thymidine kinase-deficient mouse cells or thymidine kinase-deficient Chinese hamster cells and human white blood cells carrying a translocation of the distal half (q 22-qter) of the long arm of chromosome 15 to chromosome 17. A positive correlation was established between the expression of these human phenotypes and the presence of the distal half of the long arm of human chromosome 15.

Glycosphingolipid hydrolases: properties and molecular genetics

This is a review of the properties and molecular genetics of six lysosomal hydrolases: beta-galactosidase, hexosaminidases A and B, alpha-galactosidase, beta-glucosidase and alpha-fucosidase. Each enzyme is discussed with regards to isoenzymes and substrate specificity, subunit structure, genetic relationship of isoenzymes and genetic variants. The molecular genetics of human diseases caused by deficiencies of each enzyme are discussed.

The deficiency of a lysosomal acid hydrolase in two clones derived from the human lymphoblastoid line F137 after mutagen treatment

Two clones (out of a total of 181 clones tested) derived from the human lymphoblastoid (lymphoid) line F137 after mutagen treatment were found to be deficient in a lysosomal acid hydrolase. The clone N32 derived from EMS-treated F137 is deficient in N-acetyl hexosaminidase A and B but contains normal levels of N-acetyl hexosaminidase C and low levels of an enzyme resembling N-acetyl hexosaminidase S. Thus the enzyme deficiency in this clone appears to resemble the so-called Sandhoff variant of Tay-Sachs disease, a disease inherited as an autosomal recessive condition. The clone G3 derived from MNNG treated F137 is deficient in alpha-galactosidase A. This clone resembles the situation in X-linked Fabry's disease. Karyotype analysis of the clones failed to reveal any chromosome rearrangement or losses of chromosomal material that might have accounted for the mutations and it is suggested that a single point mutation might in each case account for the loss of enzyme activity. No storage of the natural substrates of the two enzymes could be demonstrated in the clones.

Mannosidosis: assignment of the lysosomal alpha-mannosidase B gene to chromosome 19 in man

Human alpha-mannosidase activity (alpha-D-mannoside mannohydrolase, EC 3.2.1.24) from tissues and cultured skin fibroblasts was separated by gel electrophoresis into a neutral, cytoplasmic form (alpha-mannosidase A) and two closely related acidic, lysosomal components (alpha-mannosidase B). Human mannosidosis, an inherited glycoprotein storage disorder, has been associated with severe deficiency of both lysosomal alpha-mannosidase B molecular forms. Chromosome assignment of the gene coding for human alpha-mannosidase B (MANB) has been determined in human-mouse and human-Chinese hamster somatic cell hybrids. The human alpha-mannosidase B phenotype showed concordant segregation with the human enzyme glucosephosphate isomerase (GPI) (D-glucose-6-phosphate ketolisomerase, EC 5.3.1.9) but discordant segregation with 30 other enzyme markers representing 20 linkage groups. The glucose-phosphate isomerase gene has been assigned to chromosome 19 in man. This MANB-GPI linkage and confirming chromosome studies demonstrate assignment of the alpha-mannosidase B structural gene to chromosome 19 in man. Since mannosidosis is believed to result from a structural defect in alpha-mannosidase B, these findings suggest that the mannosidosis mutation is located on chromosome 19 in man.

Expression of alpha-D-mannosidase in man-hamster somatic cell hybrids

Two types of alpha-D-mannosidase isozymes are present in human white blood cells, human diploid fibroblasts, and HeLa cells. One of these (the S isozyme) constitutes the major alpha-D-mannosidase of the human cells, has a pH optimum of 4.4, and is associated with lysosomes. The other (the F isozyme) is most active at pH 6, is acid labile, and is located in the soluble portion of the cytoplasm. The expression of human lysosomal alpha-D-mannosidase was examined in man-hamster hybrid clones, and was found to be concordant with that of phosphohexose isomerase in 54 of 55 primary clones. A locus specifying human lysosomal alpha-D-mannosidase has therfore been assigned to chromosome 19.