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PubMed2001

A gain-of-function mutation in the sodium channel gene Scn2a results in seizures and behavioral abnormalities.

The GAL879-881QQQ mutation in the cytoplasmic S4-S5 linker of domain 2 of the rat brain IIA sodium channel (Na(v)1.2) results in slowed inactivation and increased persistent current when expressed in Xenopus oocytes. The neuron-specific enolase promoter was used to direct in vivo expression of the mutated channel in transgenic mice. Three transgenic lines exhibited seizures, and line Q54 was characterized in detail. The seizures in these mice began at two months of age and were accompanied by behavioral arrest and stereotyped repetitive behaviors. Continuous electroencephalogram monitoring detected focal seizure activity in the hippocampus, which in some instances generalized to involve the cortex. Hippocampal CA1 neurons isolated from presymptomatic Q54 mice exhibited increased persistent sodium current which may underlie hyperexcitability in the hippocampus. During the progression of the disorder there was extensive cell loss and gliosis within the hippocampus in areas CA1, CA2, CA3 and the hilus. The lifespan of Q54 mice was shortened and only 25% of the mice survived beyond six months of age. Four independent transgenic lines expressing the wild-type sodium channel were examined and did not exhibit any abnormalities. The transgenic Q54 mice provide a genetic model that will be useful for testing the effect of pharmacological intervention on progression of seizures caused by sodium channel dysfunction. The human ortholog, SCN2A, is a candidate gene for seizure disorders mapped to chromosome 2q22-24.

Animals↗
PubMed2002

Gain of function mutations of RTK conserved residues display differential effects on NTRK1 kinase activity.

Activation of tyrosine kinase receptors is associated with human tumors. Tumorigenic versions of several RTKs, such as Ret, Kit and Met carry activating mutations at highly conserved residues of the tyrosine kinase domain. We have investigated the effect of some of these mutations on the NTRK1/NGF receptor, for which no naturally occurring activating point mutations have been so far detected. We introduced the following mutations in NTRK1 tyrosine kinase domain: (i) D668N equivalent to Met D1246N associated to HPRC; (ii) D668V modelled on Kit D816V found in mastocytosis; (iii) M688T corresponding to Ret M918T associated to the cancer syndrome MEN2B. The Met-like mutation rendered the NTRK1 receptor more responsive to ligand, as observed for the corresponding mutation in Met. On the contrary the Kit-like D668V resulted as neutral mutation. Surprisingly, the MEN2B-like M688T completely abrogated NTRK1 receptor activity, resulting as a loss of function mutation. Our results show that the mutations tested, although involving conserved amino acids in highly homologous regions, exert distinct effects in different receptors, and suggest a very peculiar auto-inhibitory mechanism for NTRK1.

Amino Acid Sequence↗
PubMed1997

The association of atopy with a gain-of-function mutation in the alpha subunit of the interleukin-4 receptor.

BACKGROUND: Atopic diseases are very common, and atopy has a strong genetic predisposition. METHODS: Using single-strand conformation polymorphism analysis and DNA sequencing, we searched for mutations in the a subunit of the interleukin-4 receptor that would predispose persons to atopy. We examined the prevalence of the alleles among patients with allergic inflammatory disorders and among 50 prospectively recruited adults. Subjects with atopy were identified on the basis of an elevated serum IgE level (> or = 95 IU per milliliter) or a positive radioimmunosorbent test in response to standard inhalant allergens. The signaling function of mutant interleukin-4 receptor a was examined by flow cytometry, binding assays, and immunoblotting. RESULTS: A novel interleukin-4 receptor alpha allele was identified in which guanine was substituted for adenine at nucleotide 1902, causing a change from glutamine to arginine at position 576 (R576) in the cytoplasmic domain of the interleukin-4 receptor alpha protein. The R576 allele was common among patients with allergic inflammatory disorders (found in 3 of 3 patients with the hyper-IgE syndrome and 4 of 7 patients with severe atopic dermatitis) and among the 50 prospectively recruited adults (found in 13 of 20 subjects with atopy and 5 of 30 without atopy; P=0.001; relative risk of atopy among those with a mutant allele, 9.3). The R576 allele was associated with higher levels of expression of CD23 by interleukin-4 than the wild-type allele. This enhanced signaling was associated with a change in the binding specificity of the adjacent tyrosine residue at position 575 to signal-transducing molecules. CONCLUSIONS: The R576 allele of interleukin-4 receptor alpha is strongly associated with atopy. This mutation may predispose persons to allergic diseases by altering the signaling function of the receptor.

Adult↗
PubMed2003

Estrogen excess associated with novel gain-of-function mutations affecting the aromatase gene.

BACKGROUND: Gynecomastia of prepubertal onset may result from increased estrogen owing to excessive aromatase activity in extraglandular tissues. A gene in chromosome 15q21.2 encodes aromatase, the key enzyme for estrogen biosynthesis. Several physiologic tissue-specific promoters regulate the expression of aromatase, giving rise to messenger RNA (mRNA) species with an identical coding region but tissue-specific 5'-untranslated regions in placenta, gonads, brain, fat, and skin. METHODS: We studied skin, fat, and blood samples from a 36-year-old man, his 7-year-old son, and an unrelated 17-year-old boy with severe gynecomastia of prepubertal onset and hypogonadotropic hypogonadism caused by elevated estrogen levels. RESULTS: Aromatase activity and mRNA levels in fat and skin and whole-body aromatization of androstenedione were severely elevated. Treatment with an aromatase inhibitor decreased serum estrogen levels and normalized gonadotropin and testosterone levels. The 5'-untranslated regions of aromatase mRNA contained the same sequence (FLJ) in the father and son and another sequence (TMOD3) in the unrelated boy; neither sequence was found in control subjects. These 5'-untranslated regions normally make up the first exons of two ubiquitously expressed genes clustered in chromosome 15q21.2-3 in the following order (from telomere to centromere): FLJ, TMOD3, and aromatase. The aromatase gene is normally transcribed in the direction opposite to that of TMOD3 and FLJ. Two distinct heterozygous inversions reversed the direction of the TMOD3 or FLJ promoter in the patients. CONCLUSIONS: Heterozygous inversions in chromosome 15q21.2-3, which caused the coding region of the aromatase gene to lie adjacent to constitutively active cryptic promoters that normally transcribe other genes, resulted in severe estrogen excess owing to the overexpression of aromatase in many tissues.

Adipose Tissue↗
PubMed2003

A gain-of-function mutation in ftsA bypasses the requirement for the essential cell division gene zipA in Escherichia coli.

ZipA and FtsA are recruited independently to the FtsZ cytokinetic ring (Z ring) and are essential for cell division of Escherichia coli. The molecular role of FtsA in cell division is unknown; however, ZipA is thought to stabilize the Z ring, anchor it to the membrane, and recruit downstream cell division proteins. Here we demonstrate that the requirement for ZipA can be bypassed completely by a single alteration in a conserved residue of FtsA (FtsA*). Cells with ftsA* in single copy in place of WT ftsA or with ftsA* alone on a multicopy plasmid divide mostly normally, whether they are zipA+ or zipA-. Experiments with ftsQAZ and ftsQA*Z on multicopy plasmids indicate that ftsQAZzipA+ and ftsQA*ZzipA- cells divide fairly normally, whereas ftsQAZzipA- cells divide poorly and ftsQA*ZzipA+ cells display a phenotype that suggests their septa are unusually stable. In support of the idea that ftsA* stabilizes Z rings, single-copy ftsA* confers resistance to excess MinC, which destabilizes Z rings. The inhibitory effect of excess ZipA on division is also suppressed by ftsA*. These results suggest that the molecular mechanism of the FtsA* bypass is to stabilize FtsZ assembly via a parallel pathway and that FtsA* can replace the multiple functions of ZipA. This is an example of a complete functional replacement of an essential prokaryotic cell division protein by another and may explain why most bacteria can divide without an obvious ZipA homolog.

Amino Acid Sequence↗
PubMed1995

Gain of function mutations for paralogous Hox genes: implications for the evolution of Hox gene function.

To investigate the functions of paralogous Hox genes, we compared the phenotypic consequences of altering the embryonic patterns of expression of Hoxb-8 and Hoxc-8 in transgenic mice. A comparison of the phenotypic consequences of altered expression of the two paralogs in the axial skeletons of newborns revealed an array of common transformations as well as morphological changes unique to each gene. Divergence of function of the two paralogs was clearly evident in costal derivatives, where increased expression of the two genes affected opposite ends of the ribs. Many of the morphological consequences of expanding the mesodermal domain and magnitude of expression of either gene were atavistic, inducing the transformation of axial skeletal structures from a modern to an earlier evolutionary form. We propose that regional specialization of the vertebral column has been driven by regionalization of Hox gene function and that a major aspect of this evolutionary progression may have been restriction of Hox gene expression.

Animals↗
PubMed2000

A gain-of-function mutation in STAT6.

Interleukin-4 (IL-4) is a cytokine that plays a crucial role in the pathophysiology of asthma and allergic diseases. IL-4-induced gene expression is largely mediated through the activation of the latent transcription factor STAT6. We identified a STAT6 mutant (STAT6VT)) that is activated independently of IL-4 stimulation. STAT6VT carries two amino acid changes in the SH2 domain that affect the overall structure and stability of the monomeric and dimeric protein. When overexpressed in mammalian cells, STAT6VT undergoes tyrosine phosphorylation, binds DNA, and activates transcription in the absence of IL-4 stimulation. Using the Jak1- and Jak3-deficient fibroblast line U4A, we demonstrate that phosphorylation is mediated by an IL-4-independent tyrosine kinase that is not able to activate the wild-type STAT6 protein. These results suggest that small changes in STAT6 could result in hyperactivation of the protein and constitutive expression of STAT6-dependent genes. Such a mutation, if found in vivo, could cause genetic predisposition for atopic diseases.

Cell Line↗
PubMed2004

Gain of function mutation in the mineralocorticoid receptor of the Brown Norway rat.

The aim of this research was to identify the molecular bases of differences in sensitivity to corticosteroid hormones between Brown Norway and Fischer 344 rats. We previously showed an apparent insensitivity to adrenalectomy in Brown Norway rats. Based on our first hypothesis of a different activity/reactivity of the mineralocorticoid signaling pathway between the two rat strains, we sequenced Brown Norway and Fischer 344 mineralocorticoid receptor cDNA and identified a tyrosine to cysteine substitution (Y73C) in the N-terminal part of the Brown Norway mineralocorticoid receptor. As a first step, this substitution gave us a means to distinguish the Brown Norway allele from the Fischer 344 at the mineralocorticoid receptor locus in an F2 population. We showed a strong genetic linkage between the mineralocorticoid receptor genotype and sensitivity to adrenalectomy. A subsequent genome-wide linkage analysis confirmed the involvement of the mineralocorticoid receptor locus and implicated other loci, including one on chromosome 4, which collectively explain a large part of the strain differences in corticosteroid receptor responses. In vitro studies further revealed that the Y73C substitution induces greater transactivation of the mineralocorticoid receptor by aldosterone, and surprisingly by progesterone as well, which could substitute for aldosterone after adrenalectomy in Brown Norway rats. We challenged this hypothesis in vivo and showed that plasma progesterone is higher in Brown Norway male rats and partially compensates for aldosterone after adrenalectomy. This work illustrates the interest of a pluristrategic approach to explore the mineralocorticoid receptor signaling pathway and its implication in the regulation of hydroelectrolytic homeostasis and blood pressure.

Adrenal Cortex Hormones↗
PubMed2004

Dominant gain-of-function mutations in Hsp104p reveal crucial roles for the middle region.

Heat-shock protein 104 (Hsp104p) is a protein-remodeling factor that promotes survival after extreme stress by disassembling aggregated proteins and can either promote or prevent the propagation of prions (protein-based genetic elements). Hsp104p can be greatly overexpressed without slowing growth, suggesting tight control of its powerful protein-remodeling activities. We isolated point mutations in Hsp104p that interfere with this control and block cell growth. Each mutant contained alterations in the middle region (MR). Each of the three MR point mutations analyzed in detail had distinct phenotypes. In combination with nucleotide binding site mutations, Hsp104p(T499I) altered bud morphology and caused septin mislocalization, colocalizing with the misplaced septins. Point mutations in the septin Cdc12p suppressed this phenotype, suggesting that it is due to direct Hsp104p-septin interactions. Hsp104p(A503V) did not perturb morphology but stopped cell growth. Remarkably, when expressed transiently, the mutant protein promoted survival after extreme stress as effectively as did wild-type Hsp104p. Hsp104p(A509D) had no deleterious effects on growth or morphology but had a greatly reduced ability to promote thermotolerance. That mutations in an 11-amino acid stretch of the MR have such profound and diverse effects suggests the MR plays a central role in regulating Hsp104p function.

Adenosine Triphosphatases↗
PubMed2005

Gain-of-function mutation in Nav1.7 in familial erythromelalgia induces bursting of sensory neurons.

Erythromelalgia is an autosomal dominant disorder characterized by burning pain in response to warm stimuli or moderate exercise. We describe a novel mutation in a family with erythromelalgia in SCN9A, the gene that encodes the Na(v)1.7 sodium channel. Na(v)1.7 produces threshold currents and is selectively expressed within sensory neurons including nociceptors. We demonstrate that this mutation, which produces a hyperpolarizing shift in activation and a depolarizing shift in steady-state inactivation, lowers thresholds for single action potentials and high frequency firing in dorsal root ganglion neurons. Erythromelalgia is the first inherited pain disorder in which it is possible to link a mutation with an abnormality in ion channel function and with altered firing of pain signalling neurons.

Action Potentials↗
PubMed1996

Both loss-of-function and gain-of-function mutations in snf define a role for snRNP proteins in regulating Sex-lethal pre-mRNA splicing in Drosophila development.

The Drosophila snf gene encodes a protein with functional homology to the mammalian U1A and U2B" snRNP proteins. Studies, based on the analysis of three viable alleles, have suggested a role for snf in establishing the female-specific splicing pattern of the sex determination switch gene, sex-lethal. Here, we show that the non-sex-specific lethal null allele is required for female sex determination, arguing against the formal possibility that the viable alleles disrupt a function unrelated to snf's wild-type function. Moreover, we find snf is required for normal cell growth and/or survival, as expected for a protein involved in a cell-vital process such as RNA splicing. We also show that of the three viable alleles only one, snfJA2, is a partial loss-of-function mutation. The other two viable alleles, snf1621 and snfe8H, encode antimorphic proteins. We find the antimorphic proteins are mislocalized and correlate their mislocalization with their molecular lesions and mutant phenotypes. Finally, we provide genetic evidence that the antimorphic alleles interfere with the autoregulatory splicing function of the Sex-lethal protein. Based on these studies we suggest a model in which the snRNP protein, Snf, functions with Sex-lethal to block recognition of the regulated male-specific exon.

Alleles↗
PubMed2002

Gain-of-function mutation in ADULT syndrome reveals the presence of a second transactivation domain in p63.

The transcriptional co-activator p63 is of crucial importance for correct development of the limbs, ectodermal appendages (skin, nails, teeth, hair, glands), lip and palate. Mutations in the p63 gene are found in a number of human syndromes, including ectrodactyly-ectodermal dysplasia-cleft lip/palate (EEC) syndrome, limb-mammary syndrome (LMS), Hay-Wells syndrome and in non-syndromic split-hand/split-foot malformation (SHFM). Each syndrome has a specific pattern of mutations with different functional effects in in vitro functional assays. We report a mutation R298Q in acro-dermato-ungual-lacrimal-tooth (ADULT) syndrome, another EEC-like condition. The mutation is located in the DNA binding domain of p63, which harbors almost all EEC associated mutations. However, unlike mutations in EEC syndrome, the R298Q ADULT syndrome mutation does not impair DNA binding. Rather, the mutation confers novel transcription activation capacity on the DeltaN-p63gamma isoform, which normally does not possess such activity. These results confirm that ADULT syndrome is a clinically as well as molecularly distinct member of the expanding p63 mutation family of human malformation syndromes. Our results further show that p63 contains a second transactivation domain which is normally repressed and can become activated by mutations in the DNA binding domain of p63.

Abnormalities, Multiple↗
PubMed2003

A gain-of-function mutation in a plant disease resistance gene leads to constitutive activation of downstream signal transduction pathways in suppressor of npr1-1, constitutive 1.

Plants have evolved sophisticated defense mechanisms against pathogen infections, during which resistance (R) genes play central roles in recognizing pathogens and initiating defense cascades. Most of the cloned R genes share two common domains: the central domain, which encodes a nucleotide binding adaptor shared by APAF-1, certain R proteins, and CED-4 (NB-ARC), plus a C-terminal region that encodes Leu-rich repeats (LRR). In Arabidopsis, a dominant mutant, suppressor of npr1-1, constitutive 1 (snc1), was identified previously that constitutively expresses pathogenesis-related (PR) genes and resistance against both Pseudomonas syringae pv maculicola ES4326 and Peronospora parasitica Noco2. The snc1 mutation was mapped to the RPP4 cluster. In snc1, one of the TIR-NB-LRR-type R genes contains a point mutation that results in a single amino acid change from Glu to Lys in the region between NB-ARC and LRR. Deletions of this R gene in snc1 reverted the plants to wild-type morphology and completely abolished constitutive PR gene expression and disease resistance. The constitutive activation of the defense responses was not the result of the overexpression of the R gene, because its expression level was not altered in snc1. Our data suggest that the point mutation in snc1 renders the R gene constitutively active without interaction with pathogens. To analyze signal transduction pathways downstream of snc1, epistasis analyses between snc1 and pad4-1 or eds5-3 were performed. Although the resistance signaling in snc1 was fully dependent on PAD4, it was only partially affected by blocking salicylic acid (SA) synthesis, suggesting that snc1 activates both SA-dependent and SA-independent resistance pathways.

Amino Acid Sequence↗
PubMed2006

A gain-of-function mutation in a cytokinin receptor triggers spontaneous root nodule organogenesis.

Legume root nodules originate from differentiated cortical cells that reenter the cell cycle and form organ primordia. We show that perception of the phytohormone cytokinin is a key element in this switch. Mutation of a Lotus japonicus cytokinin receptor gene leads to spontaneous development of root nodules in the absence of rhizobia or rhizobial signal molecules. The mutant histidine kinase receptor has cytokinin-independent activity and activates an Escherichia coli two-component phosphorelay system in vivo. Mutant analysis shows that cytokinin signaling is required for cell divisions that initiate nodule development and defines an autoregulated process where cytokinin induction of nodule stem cells is controlled by shoots.

Alleles↗
PubMed1998

Gain-of-function mutations of c-kit in human gastrointestinal stromal tumors.

Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal tumors in the human digestive tract, but their molecular etiology and cellular origin are unknown. Sequencing of c-kit complementary DNA, which encodes a proto-oncogenic receptor tyrosine kinase (KIT), from five GISTs revealed mutations in the region between the transmembrane and tyrosine kinase domains. All of the corresponding mutant KIT proteins were constitutively activated without the KIT ligand, stem cell factor (SCF). Stable transfection of the mutant c-kit complementary DNAs induced malignant transformation of Ba/F3 murine lymphoid cells, suggesting that the mutations contribute to tumor development. GISTs may originate from the interstitial cells of Cajal (ICCs) because the development of ICCs is dependent on the SCF-KIT interaction and because, like GISTs, these cells express both KIT and CD34.

Amino Acid Sequence↗
PubMed2003

Formation of Hirano bodies induced by expression of an actin cross-linking protein with a gain-of-function mutation.

Hirano bodies are paracrystalline actin filament-containing structures reported to be associated with a variety of neurodegenerative diseases. However, the biological function of Hirano bodies remains poorly understood, since nearly all prior studies of these structures were done with postmortem samples of tissue. In the present study, we generated a full-length form of a Dictyostelium 34-kDa actin cross-linking protein with point mutations in the first putative EF hand, termed 34-kDa DeltaEF1. The 34-kDa DeltaEF1 protein binds calcium normally but has activated actin binding that is unregulated by calcium. The expression of the 34-kDa DeltaEF1 protein in Dictyostelium induces the formation of Hirano bodies, as assessed by both fluorescence microscopy and transmission electron microscopy. Dictyostelium cells bearing Hirano bodies grow normally, indicating that Hirano bodies are not associated with cell death and are not deleterious to cell growth. Moreover, the expression of the 34-kDa DeltaEF1 protein rescues the phenotypes of cells lacking the 34-kDa protein and cells lacking both the 34-kDa protein and alpha-actinin. Finally, the expression of the 34-kDa DeltaEF1 protein also initiates the formation of Hirano bodies in cultured mouse fibroblasts. These results show that the failure to regulate the activity and/or affinity of an actin cross-linking protein can provide a signal for the formation of Hirano bodies. More generally, the formation of Hirano bodies is a cellular response to or a consequence of aberrant function of the actin cytoskeleton.

Actin Cytoskeleton↗
PubMed2006

Loss- and gain-of-function mutations in the F1-HAMP region of the Escherichia coli aerotaxis transducer Aer.

The Escherichia coli Aer protein contains an N-terminal PAS domain that binds flavin adenine dinucleotide (FAD), senses aerotactic stimuli, and communicates with the output signaling domain. To explore the roles of the intervening F1 and HAMP segments in Aer signaling, we isolated plasmid-borne aerotaxis-defective mutations in a host strain lacking all chemoreceptors of the methyl-accepting chemotaxis protein (MCP) family. Under these conditions, Aer alone established the cell's run/tumble swimming pattern and modulated that behavior in response to oxygen gradients. We found two classes of Aer mutants: null and clockwise (CW) biased. Most mutant proteins exhibited the null phenotype: failure to elicit CW flagellar rotation, no aerosensing behavior in MCP-containing hosts, and no apparent FAD-binding ability. However, null mutants had low Aer expression levels caused by rapid degradation of apparently nonnative subunits. Their functional defects probably reflect the absence of a protein product. In contrast, CW-biased mutant proteins exhibited normal expression levels, wild-type FAD binding, and robust aerosensing behavior in MCP-containing hosts. The CW lesions evidently shift unstimulated Aer output to the CW signaling state but do not block the Aer input-output pathway. The distribution and properties of null and CW-biased mutations suggest that the Aer PAS domain may engage in two different interactions with HAMP and the HAMP-proximal signaling domain: one needed for Aer maturation and another for promoting CW output from the Aer signaling domain. Most aerotaxis-defective null mutations in these regions seemed to affect maturation only, indicating that these two interactions involve structurally distinct determinants.

Amino Acid Substitution↗