Mutations in the alpha 1 subunit of the inhibitory glycine receptor cause the dominant neurologic disorder, hyperekplexia.

Hereditary hyperekplexia, or familial startle disease (STHE), is an autosomal dominant neurologic disorder characterized by marked muscle rigidity of central nervous system origin and an exaggerated startle response to unexpected acoustic or tactile stimuli. Linkage analyses in several large families provided evidence for locus homogeneity and showed the disease gene was linked to DNA markers on the long arm of chromosome 5. Here we describe the identification of point mutations in the gene encoding the alpha 1 subunit of the glycine receptor (GLRA1) in STHE patients from four different families. All mutations occur in the same base pair of exon 6 and result in the substitution of an uncharged amino acid (leucine or glutamine) for Arg271 in the mature protein.

A novel cDNA detects homozygous microdeletions in greater than 50% of type I spinal muscular atrophy patients.

Spinal muscular atrophy (SMA) is the second most common lethal, autosomal recessive disease in Caucasians (after cystic fibrosis). Childhood SMAs are divided into three groups (type I, II and III), which are allelic variants of the same locus in a region of approximately 850 kb in chromosome 5q12-q13, containing multiple copies of a novel, chromosome 5-specific repeat as well as many atypical pseudogenes. This has hampered the identification of candidate genes. We have identified several coding sequences unique to the SMA region. A genomic fragment detected by one cDNA is homozygously deleted in 17/29 (58%) of type I SMA patients. Of 235 unaffected individuals examined, only two showed the deletion and both are carriers of SMA. Our results suggest that deletion of at least part of this novel gene is directly related to the phenotype of SMA.

Thanatophoric dysplasia (types I and II) caused by distinct mutations in fibroblast growth factor receptor 3.

Thanatophoric dysplasia (TD), the most common neonatal lethal skeletal dysplasia, affects one out of 20,000 live births. Affected individuals display features similar to those seen in homozygous achondroplasia. Mutations causing achondroplasia are in FGFR3, suggesting that mutations in this gene may cause TD. A sporadic mutation causing a Lys650Glu change in the tyrosine kinase domain of FGFR3 was found in 16 of 16 individuals with one type of TD. Of 39 individuals with a second type of TD, 22 had a mutation causing an Arg248Cys change and one had a Ser371Cys substitution, both in the extracellular region of the protein. None of these mutations were found in 50 controls showing that mutations affecting different functional domains of FGFR3 cause different forms of this lethal disorder.

A missense mutation in the gene encoding the alpha 1 subunit of the inhibitory glycine receptor in the spasmodic mouse.

Hereditary hyperekplexia, an autosomal dominant neurologic disorder characterized by an exaggerated startle reflex and neonatal hypertonia, can be caused by mutations in the gene encoding the alpha 1 subunit of the inhibitory glycine receptor (GLRA1). Spasmodic (spd), a recessive neurologic mouse mutant, resembles hyperekplexia phenotypically, and the two disease loci map to homologous chromosomal regions. Here we describe a Glra1 missense mutation in spd that results in reduced agonist sensitivity in glycine receptors expressed in vitro. We conclude that spd is a murine homologue of hyperekplexia and that mutations in GLRA1/Glra1 can produce syndromes with different inheritance patterns.

Linkage in cultured Chinese hamster cells of two genes, emtB and leuS, involved in protein synthesis and isolation of cell lines with mutations in three linked genes.

We have determined via segregation analyses from appropriate hybrids that two genes involved in protein synthesis, one encoding for a ribosomal protein (emtB) and one encoding for leucyl-tRNA synthetase (leuS), cosegregate at a very high frequency and are linked in both Chinese hamster ovary and lung cells. In contrast, the emtA locus, defined by a second complementation group of emetine-resistant mutants which also have alterations affecting protein synthesis and probably the ribosome, is not linked to leuS. In addition, we have determined that a third gene, one that can be altered to give rise to chromate resistance, is syntenic with emtB and leuS. We have selected cell lines with mutations in each of these three linked genes and have shown that the three loci cosegregate at a high frequency. Because the mutations in these three linked genes provide easily distinguishable phenotypes, these cell lines should provide a powerful tool for examining several important questions concerning mitotic recombination in somatic cells.

Human chromosome 12 is required for elevated HIV-1 expression in human-hamster hybrid cells.

Host cell factors act together with regulatory genes of the human immunodeficiency virus (HIV) to control virus production. Human-Chinese hamster ovary hybrid cell clones were used to probe for human chromosomes involved in regulating HIV gene expression. DNA transfection experiments showed that 4 of 18 clones had high levels of HIV gene expression measured by both extracellular virus production and transactivation of the HIV long terminal repeat in the presence of the trans-activator (tat) gene. Karyotype analyses revealed a 94% concordance (17/18) between human chromosome 12 and HIV gene expression. Other chromosomes had an 11 to 72% concordance with virus production.

Isolation of Chinese hamster ovary cells that overproduce asparaginyl-tRNA synthetase.

Temperature-resistant revertants, derived from the temperature-sensitive CHO asparaginyl-tRNA synthetase mutant, Asn-5, were isolated and characterized. Several lines of evidence indicate that the temperature-resistant phenotype of the revertants is due to their overproducing the same altered enzyme present in the Asn-5 parent.

Selective linkage disruption in human-Chinese hamster cell hybrids: deletion mapping of the leuS, hexB, emtB, and chr genes on human chromosome 5.

Chinese hamster-human interspecific hybrid cells, which contain human chromosome 5 and express four genes linked on that chromosome, were subjected to selective conditions requiring them to retain one of the four linked genes, leuS (encoding leucyl-tRNA synthetase), but lose another, either emtB (encoding ribosomal protein S14) or chr. Cytogenetic and biochemical analyses of spontaneous segregants isolated by using these unique selective pressures have enabled us to determine the order and regional location of the leuS, hexB, emtB, and chr genes on human chromosome 5. These segregants arise primarily by terminal deletions of various portions of the long arm of chromosome 5. Our results indicate that the order of at least three of these genes is the same on human chromosome 5 and Chinese hamster chromosome 2. Thus, there appears to be extensive homology between Chinese hamster chromosome 2 and human chromosome 5, which represents an extreme example of the conservation of gene organization between very divergent mammalian species. In addition, these hybrids and selective conditions provide a very simple and quantitative means to assess the potency of various agents suspected of inducing gross chromosomal damage.

Genetic and biochemical distinction among Chinese hamster cell emtA, emtB, and emtC mutants.

Genetic and biochemical experiments have enabled us to more clearly distinguish three genetic loci, emtA, emtB, and emtC, all of which can be altered to give rise to resistance to the protein synthesis inhibitor, emetine, in cultured Chinese hamster cells. Genetic experiments have demonstrated that, unlike the emtB locus, neither the emtA locus nor the emtC locus is linked to chromosome 2 in Chinese hamster cells, clearly distinguishing the latter two genes from emtB. emtA mutants can also be distinguished, biochemically, from emtB and emtC mutants based upon different degrees of cross-resistance to another inhibitor of protein synthesis, cryptopleurine. Two-dimensional gel electrophoretic analysis of ribosomal proteins failed to detect any electrophoretic alterations in ribosomal proteins from emtA or emtC mutants that could be correlated with emetine resistance. However, a distinct electrophoretic alteration in ribosomal protein S14 was observed in an emtB mutant. In addition, the parental Chinese hamster peritoneal cell line of an emtC mutant, and the emtC mutant itself, are apparently heterozygous for an electrophoretic alteration in ribosomal protein L9.

Construction and characterization of Chinese hamster cell EmtA EmtB double mutants.

Starting with hybrid cell lines between a Chinese hamster cell EmtA mutant and a Chinese hamster cell EmtB mutant, we have constructed cell lines that are homozygous for mutant alleles at both the emtA locus and the emtB locus, by using a two-step segregation protocol. The EmtA EmtB double mutants are approximately 10-fold more resistant to emetine inhibition than either of the parental mutants. Having both the EmtA mutation and the EmtB mutation expressed in the same cell also results in a level of resistance to cryptopleurine that is significantly higher than a simple additive effect of the two mutations alone. Analysis of ribosomal proteins by two-dimensional polyacrylamide gel electrophoresis demonstrated that a parental hybrid and a first-step segregant, which has lost the wild-type emtA allele, synthesize both a normal and an altered form of ribosomal protein S14, whereas an EmtA EmtB double mutant synthesizes only the altered form of this ribosomal protein. This result confirms that the emtB locus is the structural gene for ribosomal protein S14. Our results also suggest that the products of the emtA and emtB loci interact directly, indicating that the emtA locus, like the emtB locus, encodes a component of the ribosome.

Identification and characterization of a third complementation group of emetine-resistant Chinese hamster cell mutants.

We have isolated emetine-resistant cell lines from Chinese hamster peritoneal fibroblasts and have shown that they represent a third distinct class or complementation group of emetine-resistant mutants, as determined by three different criteria. These mutants, like those belonging to the two other complementation groups we have previously defined, which were isolated from Chinese hamster lung and Chinese hamster ovary cells, have alterations that directly affect the protein biosynthetic machinery. So far, there is absolute cell line specificity with respect to the three complementation groups, in that all the emetine-resistant mutants we have isolated from Chinese hamster lung cells belong to one complementation group, all those we have isolated from Chinese hamster ovary cells belong to a second complementation group, and all those isolated from Chinese hamster peritoneal cells belong to a third complementation group. Thus, in cultured Chinese hamster cells, mutations in at least three different loci, designated emtA, emtB, and emtC, encoding for different components of the protein biosynthetic machinery, can give rise to the emetine-resistant phenotype.

Efficient procedure for transferring specific human genes into Chinese hamster cell mutants: interspecific transfer of the human genes encoding leucyl- and asparaginyl-tRNA synthetases.

We have developed a simple and efficient procedure for transferring specific human genes into mutant Chinese hamster ovary cell recipients that does not rely on using calcium phosphate-precipitated high-molecular-weight DNA. Interspecific cell hybrids between human leukocytes and temperature-sensitive Chinese hamster cell mutants with either a thermolabile leucyl-tRNA synthetase or a thermolabile asparaginyl-tRNA synthetase were used as the starting material in these experiments. These hybrids contain only one or a few human chromosomes and require expression of the appropriate human aminoacyl-tRNA synthetase gene to grow at 39 degrees C. Hybrids were exposed to very high doses of gamma-irradiation to extensively fragment the chromosomes and re-fused immediately to the original temperature-sensitive Chinese hamster mutant, and secondary hybrids were isolated at 39 degrees C. Secondary hybrids, which had retained small fragments of the human genome containing the selected gene, were subjected to another round of irradiation, refusion, and selection at 39 degrees C to reduce the amount of human DNA even further. Using this procedure, we have constructed Chinese hamster cell lines that express the human genes encoding either asparaginyl- or leucyl-tRNA synthetase, yet less than 0.1% of their DNA is derived from the human genome, as quantitated by a sensitive dot-blot nucleic acid hybridization procedure. Analysis of these cell lines with Southern blots confirmed the presence of a small number of restriction endonuclease fragments containing human DNA specifically. These cell lines represent a convenient and simple means to clone the human genomic sequences of interest.

Chimeras of the native form or achondroplasia mutant (G375C) of human fibroblast growth factor receptor 3 induce ligand-dependent differentiation of PC12 cells.

Mutations in the gene for human fibroblast growth factor receptor 3 (hFGFR3) cause a variety of skeletal dysplasias, including the most common genetic form of dwarfism, achondroplasia (ACH). Evidence indicates that these phenotypes are not due to simple haploinsufficiency of FGFR3 but are more likely related to a role in negatively regulating skeletal growth. The effects of one of these mutations on FGFR3 signaling were examined by constructing chimeric receptors composed of the extracellular domain of human platelet-derived growth factor receptor beta (hPDGFR beta) and the transmembrane and intracellular domains of hFGFR3 or of an ACH (G375C) mutant. Following stable transfection in PC12 cells, which lack platelet-derived growth factor (PDGF) receptors, all clonal cell lines, with either type of chimera, showed strong neurite outgrowth in the presence of PDGF but not in its absence. Antiphosphotyrosine immunoblots showed ligand-dependent autophosphorylation, and both receptor types stimulated strong phosphorylation of mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase, an event associated with the differentiative response of these cells. In addition, ligand-dependent phosphorylation of phospholipase Cgamma and Shc was also observed. All of these responses were comparable to those observed from ligand activation, such as by nerve growth factor, of the native PC12 cells used to prepare the stable transfectants. The cells with the chimera bearing the ACH mutation were more rapidly responsive to ligand with less sustained MAPK activation, indicative of a preactivated or primed condition and consistent with the view that these mutations weaken ligand control of FGFR3 function. However, the full effect of the mutation likely depends in part on structural features of the extracellular domain. Although FGFR3 has been suggested to act as a negative regulator of long-bone growth in chrondrocytes, it produces differentiative signals similar to those of FGFR1, to which only positive effects have been ascribed, in PC12 cells. Therefore, its regulatory effects on bone growth likely result from cellular contexts and not the induction of a unique FGFR3 signaling pathway.

The Ras GTPase-activating-protein-related human protein IQGAP2 harbors a potential actin binding domain and interacts with calmodulin and Rho family GTPases.

We previously described IQGAP1 as a human protein related to a putative Ras GTPase-activating protein (RasGAP) from the fission yeast Schizosaccharomyces pombe. Here we report the identification of a liver-specific human protein that is 62% identical to IQGAP1. Like IQGAP1, the novel IQGAP2 protein harbors an N-terminal calponin homology motif which functions as an F-actin binding domain in members of the spectrin, filamin, and fimbrin families. Both IQGAPs also harbor several copies of a novel 50- to 55-amino-acid repeat, a single WW domain, and four IQ motifs and have 25% sequence identity with almost the entire S. pombe sar1 RasGAP homolog. As predicted by the presence of IQ motifs, IQGAP2 binds calmodulin. However, neither full-length nor truncated IQGAP2 stimulated the GTPase activity of Ras or its close relatives. Instead, IQGAP2 binds Cdc42 and Racl but not RhoA. This interaction involves the C-terminal half of IQGAP2 and appears to be independent of the nucleotide binding status of the GTPases. Although IQGAP2 shows no GAP activity towards Cdc42 and Rac1, the protein did inhibit both the intrinsic and RhoGAP-stimulated GTP hydrolysis rates of Cdc42 and Rac1, suggesting an alternative mechanism via which IQGAPs might modulate signaling by these GTPases. Since IQGAPs harbor a potential actin binding domain, they could play roles in the Cdc42 and Rac1 controlled generation of specific actin structures.

Association of a mature B cell leukemia with a 4p+ chromosomal abnormality: derivation and characterization of a cell line.

We have found a single 4p+ chromosomal abnormality, 46,XX, -4, +der(4)t(3;4)(q13.3;p16), in a patient with an unusual B cell leukemia of mature phenotype characterized by a high white cell count, tartrate-resistant acid phosphatase-positive malignant cells, splenic white pulp proliferation, and a serum IgM monoclonal gammopathy. The malignant cells were characterized by surface expression of CD19 (B4), CD20 (B1), IgM, IgD, kappa, and HLA-DR. They were weakly positive for CD21 (B2) and negative for CD25 (interleukin-2 receptor). The malignant cells also showed clonal rearrangement of the immunoglobulin heavy chain and kappa light chain genes. A cell line, designated HCLW-3B, was derived from unstimulated peripheral blood obtained during the leukemic phase and was found to contain the same 4p+ chromosomal abnormality as well as genomic sequences of the Epstein-Barr virus nuclear antigen. A somatic cell hybrid constructed from HCLW-3B containing the derivative chromosome 4 was used to confirm that chromosome 3q was the source of the translocated material. The availability of a cell line which is clonally derived from the patient's circulating leukemia cells should permit further characterization of this translocation at the molecular level.

Molecular genetic reevaluation of the Dutch hyperekplexia family.

OBJECTIVES: To confirm linkage of the locus of the major form of hyperekplexia to markers on chromosome 5q, to screen for a point mutation in the gene encoding the alpha 1 subunit of the glycine receptor, and to investigate whether the putative "minor" form of hyperkeplexia consisting of an excessive startle response without stiffness, is based on the same genetic defect as the major form. DESIGN: A survey of various symptoms of hyperekplexia was performed in the Dutch pedigree. Linkage studies were performed for these symptoms. SETTING: Subjects were visited at home, and the genetic study was performed at University Hospital Leiden, (the Netherlands). PATIENTS: A history was taken from 76 subjects in the pedigree, and neurologic examinations were performed on 61 subjects from four generations of the pedigree. MAIN OUTCOME MEASURES: The main outcome measures were lod scores for markers on chromosome 5q for the major and minor forms of hyperekplexia and periodic leg movements during sleep. Mutations in the alpha 1 subunit of the glycine receptor were detected by screening the exons with denaturing gradient gel electrophoresis. RESULTS: Exaggerated startle responses were reported in 44 patients. The major form consisted of stiffness in addition to the excessive startle reaction and occurred in 28 subjects. Sixteen of 44 subjects had startle responses without stiffness, indicating the minor form. Linkage was found between markers CSF1-R, D5S209, and D5S119 and the disease locus for the major form, but not for the minor form. The alpha 1 subunit of the glycine receptor showed a G to A transition mutation in codon 271 for the major form, but not for the minor form. CONCLUSIONS: Linkage and an abnormal glycine receptor were found only in the major form of hyperekplexia. Recognition of a major form is based on additional stiffness. This is therefore the most important diagnostic symptom. The minor form is not a different expression of the same genetic defect and may represent a normal but pronounced startle response.

Human chromosome-dependent and -independent pathways for HIV-2 trans-activation.

Human immunodeficiency virus (HIV types 1 and 2) replication is controlled by the interaction of viral-encoded regulatory proteins and host cellular proteins with the viral long terminal repeat (LTR). The presence of HIV-1 and HIV-2 trans-activator proteins, tat1 and tat2, respectively, greatly increases viral gene expression from their homologous LTRs. It is unclear if the cellular factors that support tat1-directed trans-activation of the HIV-1 LTR are the same for tat2 trans-activation of the HIV-2 LTR. Human-Chinese hamster ovary hybrid cell clones were used to probe for human chromosomes involved in regulating HIV-1 and HIV-2 tat-directed transactivation. DNA transfection experiments showed that the presence of human chromosome 12 in human-hamster hybrid clones was necessary for high-level tat-directed trans-activation of the HIV-1 and -2 LTR. Cross-trans-activation of the HIV-2 LTR by tat1 was found to be chromosome 12 independent. In addition, chromosome 12 did not support trans-activation of another human retrovirus (human T-cell leukemia virus type I). Our results suggest that HIV-1 and -2 have evolved to employ a cellular pathway(s) encoded on human chromosome 12 for supporting homologous tat-directed trans-activation. Trans-activation of the HIV-2 LTR by tat1 in chromosome 12-minus cells suggests that multiple cellular pathways can be recruited to trans-activate the HIV-2 LTR and that these pathways may have been important in an HIV-like progenitor virus.

Identification of a cryptic t(5;7) reciprocal translocation by fluorescent in situ hybridization.

Fluorescent in situ hybridization (FISH) identified a cryptic balanced reciprocal translocation in the mother of an infant with the cri-duchat syndrome. A biotinylated probe from a flow-sorted chromosome 5 cosmid library was used to show the distal deletion of 5p15.2 in the propositus and a translocation of this segment to the distal end of 7 at 7p21 in his mother. In a subsequent pregnancy, the fetus was shown to have normal chromosomes using the same 5 cosmid library probe and a locus-specific probe derived from the 5p15.3 region. The importance of incorporating FISH into the routine diagnostic laboratory is discussed.

Molecular detection of a 4p deletion using PCR-based polymorphisms: a technique for the rapid detection of the Wolf-Hirschhorn syndrome.

Wolf-Hirschhorn syndrome (WHS) results from a deletion of part of chromosome 4p. The region of 4p consistently deleted in WHS is near the tip of 4p. Two loci in this region D4S95 and D4S125 are associated with highly informative VNTR polymorphisms and were recently converted to allow PCR-based screening. PCR analysis was used successfully to identify a small de novo deletion of 4p in a patient suspected of having WHS. This procedure allows a rapid and accurate confirmation of 4p deletions in cases where cytogenetics alone cannot provide a clear answer.

Molecular, radiologic, and histopathologic correlations in thanatophoric dysplasia.

Various mutations in the fibroblast growth factor receptor 3 (FGFR3) gene have recently been reported in thanatophoric dysplasia (TD). We examined the clinical, radiographic, and histologic findings in 91 cases from the International Skeletal Dysplasia Registry and correlated them with the specific FGFR3 mutation. Every case of TD examined had an identifiable FGFR3 mutation. Radiographically, all of the cases with the Lys650Glu substitution demonstrated straight femora with craniosynostosis, and frequently a cloverleaf skull (CS) was demonstrated. In all other cases, the femora were curved, and CS was infrequently present but was occasionally as severe as TD with the Lys650Glu substitution. Histopathologically, all of the cases shared similar abnormalities, but cases with the Lys650Glu substitution had better preservation of the growth plate. Cases with the Tyr373Cys substitution tended to have more severe radiographic manifestations than the Arg248Cys cases, but there was overlap in the phenotypic spectrum between them. One common classification of TD distinguishes affected infants based on the presence or absence of CS. In contrast, and as originally proposed by Langer et al. [1987: Am J Med Genet 3: 167-179], our data suggest that TD can be divided into at least two groups (TD1 and TD2) based on the presence of straight or curved femora. The variable presence of CS and severity of the radiologic and histologic findings in the other substitutions may be due to other genetic, environmental, or stochastic factors.