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.     

Assignment of the gene for methylthioadenosine phosphorylase to human chromosome 9 by mouse-human somatic cell hybridization.

The purine and polyamine metabolic enzyme methylthioadenosine (MeSAdo) phosphorylase is abundant in normal cells and tissues but is lacking from many human and murine malignant cell lines and from cells of some human leukemias in vivo. To explore the genetic control of MeSAdo phosphorylase expression, we measured levels of the enzyme in somatic cell hybrids prepared by fusing MeSAdo phosphorylase-deficient mouse L cell lines with human fibroblasts. In the hybrid clones, MeSAdo phosphorylase activity segregated concordantly with adenylate kinase 1, a marker for human chromosome 9, but not with enzyme markers for any other human chromosome. In hybrid clones derived from human fibroblasts with a reciprocal translocation between chromosomes 9 and 17, MeSAdo phosphorylase activity was confined to cells containing the 9pter----9q12 region. In every case, the enzyme-positive hybrid clones displayed bands of MeSAdo phosphorylase activity with isoelectric points characteristic of both the human and murine enzymes. These results indicate that the structural gene for human MeSAdo phosphorylase, designated MTAP, can be assigned to the 9pter----9q12 region of human chromosome 9. Furthermore, these studies with interspecies somatic cell hybrids show that the MeSAdo phosphorylase-deficient state is recessive in mouse L cell lines.     

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.     

Regional gene assignment of human porphobilinogen deaminase and esterase A4 to chromosome 11q23 leads to 11qter.

The regional gene assignments for human porphobilinogen deaminase (PBGD; EC 4.3.1.8) and esterase A4 (ESA4; EC3.1.1.1) chromosome 11 have been determined with somatic cell hybridization and immunologic, electrophoretic, and cytogenetic techniques. Dimethyl sulfoxide-induced erythroid differentiation of hybrid clones derived from the fusion of tetraploid Friend murine erythroleukemia (2S MEL) cells deficient in thymidine kinase and human Lesch--Nyhan fibroblasts (HLN) deficient in hypoxanthine phosphoribosyltransferase (HPRT-; EC 2.4.2.8) were examined for expression of human PBGD, ESA4, and lactate dehydrogenase A (LDHA; EC 1.1.1.27). Human PBGD was detected by rocket immunoelectrophoresis with rabbit anti-human PBGD IgG and by isoelectric focusing. The human chromosome complement of each clone was determined by cytogenetic and enzyme marker analyses. Of the five primary 2S MEL--HLN clones examined, three were positive for human PBGD. These were subcloned to yield a total of 10 secondary, tertiary, or quaternary clones. Analyses of these subclones permitted the regional assignment of human PBGD and ESA4 to the long arm of chromosome 11. Finer regional assignment of the loci for human PBGD and ESA4 was facilitated when two 2S MEL (HPRT-)--human fibroblast (HX/11) hybrids, each containing the X chromosome--autosome translocation (der11), t(X;11)(q25-26;q23) as the only human chromosome, were examined for expression of human PBGD, ESA4, and LDHA. One clone, HX/11-2, contained the intact X/11 translocated chromosome; in the other, HX/11-3, 11p was deleted, and the human X/11 derivative was translocated onto a mouse chromosome. HX/11-2 expressed human LDHA, but HX/11-3 did not, verifying that the latter human 11/X derivative did not include 11pter leads to 11p12; PBGD and ESA4 were not detected in either hybrid. These results confirm the location of the gene for human PBGD on chromosome 11 and establish the assignment of the loci for PBGD and ESA4 in the region 11q23 leads to 11qter.     

Assignment of the gene for the beta subunit of thyroid-stimulating hormone to the short arm of human chromosome 1.

The chromosomal locations of the genes for the beta subunit of human thyroid-stimulating hormone (TSH) and the glycoprotein hormone alpha subunit have been determined by restriction enzyme analysis of DNA extracted from rodent-human somatic cell hybrids. Human chorionic gonadotropin (CG) alpha-subunit cDNA and a cloned 0.9-kilobase (kb) fragment of the human TSH beta-subunit gene were used as hybridization probes in the analysis of Southern blots of DNA extracted from rodent-human hybrid clones. Analysis of the segregation of 5- and 10-kb EcoRI fragments hybridizing to CG alpha-subunit cDNA confirmed the previous assignment of this gene to chromosome 6. Analysis of the patterns of segregation of a 2.3-kb EcoRI fragment containing human TSH beta-subunit sequences permitted the assignment of the TSH beta-subunit gene to human chromosome 1. The subregional assignment of TSH beta subunit to chromosome 1p22 was made possible by the additional analysis of a set of hybrids containing partially overlapping segments of this chromosome. Human TSH beta subunit is not syntenic with genes encoding the beta subunits of CG, luteinizing hormone, or follicle-stimulating hormone and is assigned to a conserved linkage group that also contains the structural genes for the beta subunit of nerve growth factor (NGFB) and the proto-oncogene N-ras (NRAS).     

The alpha-spectrin gene is on chromosome 1 in mouse and man.

By using alpha-spectrin cDNA clones of murine and human origin and somatic cell hybrids segregating either mouse or human chromosomes, the gene for alpha-spectrin has been mapped to chromosome 1 in both species. This assignment of the mouse alpha-spectrin gene to mouse chromosome 1 by DNA hybridization strengthens the previous identification of the alpha-spectrin locus in mouse with the sph locus, which previously was mapped by linkage analysis to mouse chromosome 1, distal to the Pep-3 locus. By in situ hybridization to human metaphase chromosomes, the human alpha-spectrin gene has been localized to 1q22-1q25; interestingly, the locus for a non-Rh-linked form of elliptocytosis has been provisionally mapped to band 1q2 by family linkage studies.     

A gene on human chromosome 6 functions in assembly of tissue-specific adenosine deaminase isozymes

In human tissues, adenosine deaminase (ADA) (adenosine aminohydrolase; EC 3.5.4.4) activity can be separated by gel electrophoresis into several isozymes. A structural gene (ADA) on chromosome 20 codes for the "erythrocyte" isozyme, ADA-1, which is also expressed in some nonerythroid tissues. Nonerythroid cells also differentially express five ADA "tissue isozymes" of a greater molecular weight than ADA-1. Each ADA tissue isozyme has a characteristic electrophoretic mobility and tissue distribution. It has been suggested that these ADA tissue isozymes are composed of ADA-1 and other components. We report that the expression of one of these tissue isozymes, ADA-d, is dependent upon ADA on chromosome 20 and another gene on chromosome 6 which functions in the assembly of the ADA tissue isozymes. In human-mouse hybrids segregating human chromosomes, chromosome 6(+),20(+) hybrids express both ADA-1 and ADA-d; chromosome 6(-),20(+) hybrids express only ADA-1; while 6(+),20(-) hybrids have no human ADA activity. ADA-d formation also occurs in vitro by self-assembly when an extract of human erythrocytes or chromosome 6(-),20(+) hybrids is mixed with a homogenate of chromosome 6(+),20(-) hybrids. The gene on chromosome 6, designated ADCP, codes for an adenosine deaminase complexing protein. The product of ADCP presumably combines with ADA-1 to form the ADA tissue isozymes. The data are consistent with the hypothesis that the distribution of enzymatic activity between ADA-1 and the tissue isozymes depends on the expression of the gene for ADA complexing protein, while the differences in the electrophoretic mobilities of the ADA isozymes, except ADA-1, are generated, as suggested by others, by the degree of glycosylation of the complexing protein.     

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.     

Genetics of the connective tissue proteins: Assignment of the gene for human type I procollagen to chromosome 17 by analysis of cell hybrids and microcell hybrids

Somatic cell hybrids between mouse and human cell lines have been used to identify the specific chromosome that governs the synthesis of type I procollagen. Fourteen hybrid clones and subclones were derived independently from crosses between mouse parents [LM (thymidine kinase-negative) or A9 (hypoxanthine phosphoribosyltransferase-negative)] and human cells (human diploid lung fibroblasts WI-38 or diploid skin fibroblasts GM5, GM17, and GM9). The cultures were labeled with [(3)H]proline in modified Eagle's medium without serum. Radioactive procollagens were purified from the medium by the method of Church et al. [(1974) J. Mol. Biol. 86, 785-799]. DEAE-cellulose chromatography was used to separate collagen and type I and type III procollagen. Human type I procollagen was assayed by double immunodiffusion analysis with type I procollagen antibodies prepared by immunizing rabbits with purified human type I procollagen. These analyses combined with karyology and isozyme analyses of each hybrid line have produced evidence for the assignment of the gene for human type I procollagen to chromosome 17. A human microcell-mouse hybrid cell line containing only human chromosome 17 was positive for human type I procollagen, lending further support to the assignment of the human type I procollagen gene to chromosome 17. Finally, by using a hybrid line containing only the long arm of human chromosome 17 translocated onto a mouse chromosome, the type I procollagen gene can be assigned more specifically to the long arm of chromosome 17.     

Assignment of the human gene for liver-type 6-phosphofructokinase isozyme (PFKL) to chromosome 21 by using somatic cell hybrids and monoclonal anti-L antibody.

Human 6-phosphofructokinase (PFK; ATP:D-fructose-6-phosphate 1-phosphotransferase, EC 2.7.1.11) is under the control of structural loci that code for muscle (M), liver (L), and platelet (P) subunits, which are variably expressed in different tissues; human diploid fibroblasts and leukocytes express all three genes. Random tetramerization of these subunits produces various isozymes, which can be distinguished from one another by ion exchange chromatography or by subunit-specific monoclonal antibodies. We have examined 17 somatic cell hybrids established between Chinese hamster cells and human diploid fibroblasts or leukocytes for the expression of L-type subunits of human PFK. As electrophoresis does not distinguish between Chinese hamster PFKs and human PFKs, we used an anti-human L-subunit-specific monoclonal antibody, which does not react with chinese hamster PFKs. The expression of human L subunits in the hybrids was detected by the enzyme-immunoprecipitation technique using staphylococci bearing protein A as an immunoadsorbent. Twelve out of 17 hybrids expressed human L subunits and retained chromosome 21, as determined by chromosome and isozyme marker analysis, whereas 5 did not express human PFKL and lacked chromosome 21. The mean erythrocyte PFK of seven individuals with trisomy 21 was found to be elevated (147% of normal). A specific increase in L subunits in trisomic erythrocytes was evident chromatographically by a striking increase in L4 species (50%; normal 10%) and immunologically by decreased precipitation with anti-M monoclonal antibody (50%; normal 80%). We conclude from these data that PFKL is located on chromosome 21 and that the previously noted elevation of erythrocyte PFK activity in individuals with trisomy 21 is due to a gene-dosage effect.     

Assignment of the human gene for galactose-1-phosphate uridyltransferase to chromosome 9: studies with Chinese hamster-human somatic cell hybrids.

Chinese hamster-human somatic cell hybrids were analyzed for the expression of human galactose-1-phosphate uridyltransferase (GALT; UDPglucose:alpha-D-galactose-1-phosphate uridyltransferase, EC 2.7.7.12) by electrophoresis and for the presence of human chromosomes cytogenetically with the aid of Q-banding. Three of the 10 randomly chosen independently derived primary hybrid lines showed the presence of human GALT. Human chromosome 9 was consistently present in the hybrid lines expressing human GALT and consistently absent in the lines not expressing it. Biochemical analysis alone of 11 independently derived hybrid lines showed human GALT to be syntenic with known chromosome 9 markers (soluble aconitase, adenylate kinase 1, and adenylate kinase 3). Previous studies on chromosome assignment of this locus, utilizing somatic cell hybrids, have yielded inconsistent results; one group assigned GALT to chromosome 2, and another assigned it to chromosome 3. However, we believe that, based on our results and other published evidence, the correct assignment of the human GALT locus is to chromosome 9.     

Human chromosome 7 carries the beta 2 interferon gene.

A cDNA clone (pAE20-4) corresponding to the 1.3-kilobase human beta 2 interferon mRNA was used as a probe in blot-hybridization experiments of DNA from a panel of human-rodent somatic cell hybrids containing overlapping subsets of human chromosomes. The DNA hybridization experiments showed that the human beta 2 interferon gene is located on human chromosome 7. This assignment is consistent with previous experimental data in which the expression of the translationally active 1.3-kilobase beta 2 interferon mRNA was assayed in various somatic cell hybrids. Blot-hybridization experiments using DNA from different human cell strains and cell lines reveal distinct EcoRI restriction fragment length polymorphisms of the human beta 2 interferon gene.     

Isolation of human-mouse somatic cell hybrids producing human prolactin: dominant expression of hormone secretion and regulation

Human PRL (hPRL)-secreting adenoma cells obtained at hypophysectomy were fused with a mutant mouse fibroblast line (LMTK-) which is aminopterin sensitive due to a deficiency in the enzyme thymidine kinase. After fusion with polyethylene glycol, cells containing nuclear material from the two parental lines (heterokaryons) were selected in medium containing hypoxanthine, aminopterin, and thymidine, and resultant clones were screened for hPRL secretion. Functional human X mouse somatic cell hybrid clones secreting hPRL were isolated in order to study hPRL gene expression and regulation. Positive hybrid clones were subcultured and have sustained hPRL secretion. The hybrid nature of the cells was confirmed by fibroblastic morphology resembling the mouse parental cell, mixed karyotype of mouse and human chromosomes, and mixed isozyme banding pattern for human and mouse glucose-6-phosphate dehydrogenase and malic enzyme. Specific expression of the hPRL gene was demonstrated by the presence of electron microscopic secretory granules (650-800 nm), positive immunoperoxidase staining using anti-hPRL serum, and sustained secretion of immunoreactive hPRL, which comigrated with [125I] hPRL standard on Sephadex chromatography. Hormonal modulation of hPRL gene expression by TRH was dominantly expressed in the hybrid cell. Human chromosome 6 was identified in hybrid cells secreting hPRL, and the cells expressed human malic enzyme, a marker for this chromosome, thus confirming the chromosome assignment of the hPRL gene. The results show that functional replicating hybrids secreting hPRL can be isolated. The technique provides a useful in vitro model for the study of hPRL gene expression and modulation.     

Human chromosome 16 encodes a factor involved in induction of class II major histocompatibility antigens by interferon gamma

Interferon gamma (IFN-gamma) induces expression of class II major histocompatibility complex (MHC)-encoded antigens in immunocompetent cells. To gain further insight into the mechanism of this induction, we prepared somatic cell hybrids between different human cell lines and a murine cell line, RAG, that does not express murine class II MHC antigens before or after treatment with murine IFN-gamma. Some of the resulting cell hybrids express murine class II MHC antigens when treated with murine IFN-gamma. This inducible phenotype is correlated with the presence of human chromosome 16. It has been shown previously that the induction of class I MHC antigens by human IFN-gamma in human-rodent hybrids requires the presence of species-specific factors encoded by chromosome 6, which bears the gene for the human IFN-gamma receptor, and chromosome 21, whose product(s) is necessary for the transduction of human IFN-gamma signals. In this report, we show that the induction of murine class II MHC antigens by human IFN-gamma in the human-RAG cell hybrids requires, likewise, the presence of human chromosomes 6 and 21, in addition to chromosome 16. In some of these hybrids, when all three of these human chromosomes were present, induction of cell-surface HLA-DR antigens was also observed. Our results demonstrate that human chromosome 16 encodes a non-species-specific factor involved in the induction of class II MHC antigens by IFN-gamma.     

Variable numbers of pepsinogen genes are located in the centromeric region of human chromosome 11 and determine the high-frequency electrophoretic polymorphism.

A panel of 26 mouse-human somatic cell hybrids containing different human chromosome complements was analyzed with a cloned human pepsinogen cDNA probe to determine the chromosomal location and the number of genes encoding these proteins. A complex containing variable numbers of pepsinogen genes was localized to the centromeric region of human chromosome 11 (p11----q13). Examination of somatic cell hybrids containing single copies of chromosome 11 and the corresponding human parental cell lines revealed a restriction fragment length polymorphism determined by pepsinogen haplotypes that contained two or three genes, respectively. Concurrent studies of DNA from individuals exhibiting the most common pepsinogen electrophoretic phenotypes with exon-specific probes demonstrated that the absence of one gene among the different restriction fragment patterns correlated with the absence of one specific isozymogen (Pg 5). Thus, our studies demonstrate that this genetic polymorphism involving intensity variation of individual pepsinogen isozymogens results from chromosome haplotypes that contain different numbers of genes. The regional localization of this polymorphic gene complex will facilitate detailed linkage analysis of human chromosome 11.     

Somatic cell genetics of adenosine deaminase expression and severe combined immunodeficiency disease in humans.

The somatic cell hybrid method has been used to study the number and different types of human genes involved in the expression of adenosine deaminase (ADA; adenosine aminohydrolase, EC 3.5.4.4) in normal cells and cells from a patient with ADA-deficient severe combined immunodeficiency disease (SCID). Genetic and biochemical characterization of ADA in SCID and the ADA tissue-specific isozymes in normal human cells indicates that additional genes, besides the ADA structural gene on chromosome 20, are involved in ADA expression. Human chromosome 6 encodes a gene, ADCP-1, whose presence is necessary for the expression of an ADA-complexing protein in human-mouse somatic cell hybrids [Koch, G. & Shows, T. B. (1978) Proc. Natl. Acad. Sci. USA 75, 3876-3880]. We report the identification of a second gene, ADCP-2, on human chromosome 2, that is also involved in the expression of the ADA-complexing protein. The data indicate that these two ADCP genes must be present in the same cell for that cell to express the complexing protein. Human-mouse somatic cell hybrids, in which the human parental cells were fibroblastss from an individual with ADA-deficient SCID, also required human chromosomes 2 and 6 to express the ADA-complexing protein, indicating that neither ADCP-1 nor ADCP-2 is involved in the ADA deficiency in SCID. The SCID-mouse hybrid cells expressed no human ADA even when human chromosome 20 had been retained. The deficiency of human ADA in these hybrids maps to human chromosome 20, and therefore is not due to the repression or inhibiton of ADA or its product by unlinked genes or gene products. We propose that the expression of the polymeric ADA tissue isozymes in human cells requires at least three genes: ADA on chromosome 20, ADCP-1 on chromosome 6, and ADCP-2 on chromosome 2. A genetic scheme is presented and the different genes involved in ADA expression and their possible functions are discussed.     

Assignment of the gene for beta 2-microglobulin (B2m) to mouse chromosome 2.

We have assigned the gene (B2m) coding for murine beta 2-microglobulin (B2M) to mouse chromosome 2 by using a novel panel of Chinese hamster-mouse somatic cell hybrid clones. Because of 35 independent primary hybrids used in this study were derived from two types of feral mice, each with a different combination of Robertsonian translocation chromosomes, as well as from mice with a normal complement of acrocentric chromosomes, analysis of 16 selected mouse enzyme markers provided data on the segregation of all 20 mouse chromosomes in these hybrids. Mouse B2M was identified in cell hybrids by immunoprecipitation with a species-specific anti-mouse B2M antiserum followed by two-dimensional polyacrylamide gel electrophoresis of the immunoprecipitated polypeptides. Enzyme analysis of the segregant clones excluded all chromosomes for B2m assignment except mouse chromosome 2, and karyotype analysis of nine informative hybrid clones confirmed the assignment of B2m to this chromosome. These results demonstrate that, in the mouse, as in man, B2m is not linked to the major histocompatibility or immunoglobulin loci.     

Leukocyte and fibroblast interferon genes are located on human chromosome 9.

At least eight leukocyte interferon genes (IFL) and the single fibroblast interferon gene (IFF) have been located on chromosome 9 in humans. In somatic cell hybrids of human and mouse cells containing a normal complement of mouse parental cell chromosomes but reduced numbers of human chromosomes, the human leukocyte and fibroblast interferon DNA sequences were present only when human chromosome 9 was also present.     

Assignment of the human gene for the low density lipoprotein receptor to chromosome 19: synteny of a receptor, a ligand, and a genetic disease.

The availability of a species-specific monoclonal antibody that recognizes the low density lipoprotein (LDL) receptor of human but not hamster origin permitted assignment of the structural gene for the human receptor to chromosome 19. The antibody was used to detect the human LDL receptor in a series of hamster-human somatic cell hybrids by two assays: (i) a structural assay that measured cellular incorporation of [35S]methionine into immunoprecipitable receptor and (ii) a functional assay that measured the rate of receptor-dependent uptake and degradation of the 125I-labeled anti-receptor monoclonal antibody. Both assays showed that the human LDL receptor was expressed in 15 out of 20 hybrid cell lines. Expression of the human LDL receptor was 100% concordant with the presence of human chromosome 19; all other human chromosomes showed at least 25% discordance. As expected, the gene for the LDL receptor (LDLR) is located on the same chromosome as the gene for the disease familial hypercholesterolemia, which has been previously mapped to chromosome 19 by pedigree studies and is caused by allelic mutations at the LDL receptor locus. The gene for apolipoprotein E, a ligand for the LDL receptor, is also known to be located on chromosome 19, raising the possibility of an evolutionary link between a protein ligand and its receptor.