Different mutations in the LMNA gene cause autosomal dominant and autosomal recessive Emery-Dreifuss muscular dystrophy

Emery-Dreifuss muscular dystrophy (EMD) is a condition characterized by the clinical triad of early-onset contractures, progressive weakness in humeroperoneal muscles, and cardiomyopathy with conduction block. The disease was described for the first time as an X-linked muscular dystrophy, but autosomal dominant and autosomal recessive forms were reported. The genes for X-linked EMD and autosomal dominant EMD (AD-EMD) were identified. We report here that heterozygote mutations in LMNA, the gene for AD-EMD, may cause diverse phenotypes ranging from typical EMD to no phenotypic effect. Our results show that LMNA mutations are also responsible for the recessive form of the disease. Our results give further support to the notion that different genetic forms of EMD have a common pathophysiological background. The distribution of the mutations in AD-EMD patients (in the tail and in the 2A rod domain) suggests that unique interactions between lamin A/C and other nuclear components exist that have an important role in cardiac and skeletal muscle function.     

Characterization of dominant and recessive assembly-defective mutations in mouse neurofilament NF-M

We have generated a set of amino- and carboxy-terminal deletions of the neurofilament NF-M gene and determined the molecular consequences of forced expression of these mutant constructs in mouse fibroblasts. To follow the expression of mutant NF-M subunits in transfected cells, a 12 amino acid epitope (from the human c-myc protein) was expressed at the carboxy terminus of each mutant. We show that NF-M molecules missing up to 90 or 70% of the nonhelical carboxy-terminal tail or amino-terminal head domains, respectively, incorporate readily into an intermediate filament network comprised either of vimentin or NF-L, whereas deletions into either the amino- or carboxy-terminal alpha- helical rod region generate assembly-incompetent polypeptides. Carboxy- terminal deletions into the rod domain invariably yield dominant mutants which rapidly disrupt the array of filaments comprised of NF-L or vimentin. Accumulation of these mutant NF-M subunits disrupts vimentin filament arrays even when present at approximately 1% the level of the wild-type subunits. In contrast, the amino-terminal deletions into the rod produce pseudo-recessive mutants that perturb the wild-type NF-L or vimentin arrays only modestly. The inability of such amino-terminal mutants to disrupt wild-type subunits defines a region near the amino-terminal alpha-helical rod domain (residues 75- 126) that is required for the earliest steps in filament assembly.     

Ectopic expression of a recessive resistance gene generates dominant potyvirus resistance in plants

Despite long-standing plant breeding investments and early successes in genetic engineering, plant viral pathogens still cause major losses in agriculture worldwide. Early transgenic approaches involved the expression of pathogen-derived sequences that provided limited protection against relatively narrow ranges of viral pathotypes. In contrast, this study demonstrates that the ectopic expression of pvr1 , a recessive gene from Capsicum chinense , results in dominant broad-spectrum potyvirus resistance in transgenic tomato plants ( Solanum lycopersicum ). The pvr1 locus in pepper encodes the eukaryotic translation initiation factor eIF4E. Naturally occurring point mutations at this locus result in monogenic recessive broad-spectrum potyvirus resistance that has been globally deployed via plant breeding programmes for more than 50 years. Transgenic tomato progenies that over-expressed the Capsicum pvr1 allele showed dominant resistance to several tobacco etch virus strains and other potyviruses, including pepper mottle virus, a range of protection similar to that observed in pepper homozygous for the pvr1 allele.     

Is ageing the result of dominant and co-dominant mutations?

A number of unexplained features of ageing can be accounted for if cellular ageing is due to dominant or co-dominant mutations. The experimental evidence both for and against this hypothesis is weak, but experiments involving direct testing are possible.     

Autosomal dominant polycystic kidney disease: Genetics, mutations and microRNAs

Autosomal dominant polycystic kidney disease (ADPKD) is a common, monogenic multi-systemic disorder characterized by the development of renal cysts and various extrarenal manifestations. Worldwide, it is a common cause of end-stage renal disease. ADPKD is caused by mutation in either one of two principal genes, PKD1 and PKD2, but has large phenotypic variability among affected individuals, attributable to PKD genic and allelic variability and, possibly, modifier gene effects. Recent studies have generated considerable information regarding the genetic basis and molecular diagnosis of this disease, its pathogenesis, and potential strategies for targeted treatment. The purpose of this article is to provide a comprehensive review of the genetics of ADPKD, including mechanisms responsible for disease development, the role of gene variations and mutations in disease presentation, and the putative role of microRNAs in ADPKD etiology. The emerging and important role of genetic testing and the advent of novel molecular diagnostic applications also are reviewed. This article is part of a Special Issue entitled: Polycystic Kidney Disease.     

ATM mutations and phenotypes in ataxia-telangiectasia families in the British Isles: expression of mutant ATM and the risk of leukemia, lymphoma, and breast cancer.

We report the spectrum of 59 ATM mutations observed in ataxia-telangiectasia (A-T) patients in the British Isles. Of 51 ATM mutations identified in families native to the British Isles, 11 were founder mutations, and 2 of these 11 conferred a milder clinical phenotype with respect to both cerebellar degeneration and cellular features. We report, in two A-T families, an ATM mutation (7271T-->G) that may be associated with an increased risk of breast cancer in both homozygotes and heterozygotes (relative risk 12.7; P=. 0025), although there is a less severe A-T phenotype in terms of the degree of cerebellar degeneration. This mutation (7271T-->G) also allows expression of full-length ATM protein at a level comparable with that in unaffected individuals. In addition, we have studied 18 A-T patients, in 15 families, who developed leukemia, lymphoma, preleukemic T-cell proliferation, or Hodgkin lymphoma, mostly in childhood. A wide variety of ATM mutation types, including missense mutations and in-frame deletions, were seen in these patients. We also show that 25% of all A-T patients carried in-frame deletions or missense mutations, many of which were also associated with expression of mutant ATM protein.     

Role of recessive and dominant mutations in adaptation the genus Rana to recent biosphere

The populations of three anuran amphibian species (Amphibia, Anura) of the genus Rana (R. ridibunda Pall., R. arvalis Nilss. and R. temporaria L.) inhabiting the Yekaterinburg urban agglomeration were examined. The frequencies of two traits, morph striata and iris depigmentation, were estimated in these populations. The former trait, so-called morph striata, is phenotypically expressed as a light dorsomedial stripe. It is controlled by a dominant allele of autosomal diallelic gene striata in some species of Rana genus, exhibiting complete dominance. Striata individuals have a number of physiological features that confer them an advantage under conditions of natural and artificial geochemical anomalies. The second trait, iris depigmentation, is the result of a recessive mutation. The individuals homozygous for this trait have low viability. Thus, the dominant mutations promote rapid adaptation of their carriers. Conversely, the recessive mutations may reduce viability of an individual.     

Role of recessive and dominant mutations in adaptation of the genus Rana to modern biosphere

The populations of three anuran amphibian species (Amphibia, Anura) of the genus Rana (R. ridibunda Pall., R. arvalis Nilss. and R. temporaria L.) inhabiting the Yekaterinburg urban agglomeration were examined. The frequencies of two traits, morph striata and iris depigmentation, were estimated in these populations. The former trait, so-called morph striata, is phenotypically expressed as a light dorsomedial stripe. It is controlled by a dominant allele of autosomal diallelic gene striata in some species of Rana genus, exhibiting complete dominance. Striata individuals have a number of physiological features that confer them an advantage under conditions of natural and artificial geochemical anomalies. The second trait, iris depigmentation, is the result of a recessive mutation. The individuals homozygous for this trait have low viability. Thus, the dominant mutations promote rapid adaptation of their carriers. Conversely, the recessive mutations may reduce viability of an individual.     

Simultaneous expression of both MAT loci in haploid cells suppresses mutations in yeast microtubule motor genes

The kinesin-related Cin8p and cytoplasmic dynein are microtubule-associated motor proteins required for anaphase spindle elongation in the yeast Saccharomyces cerevisiae. Cells deleted for DYN1 (the gene encoding the dynein heavy chain) and carrying the temperature-sensitive allele cin8-3 cannot grow at temperatures above 35 degrees C. Here, we report that the temperature sensitivity of haploid cin8-3 dyn1delta cells is suppressed by the simultaneous presence of the loci MATa and MATalpha, which contain the regulatory genes that determine mating-type and ploidy-dependent phenotypes. The presence of the two MAT loci also rendered haploid cells more resistant to the antimicrotubule drug benomyl. Our results suggest that, in preparation for handling double the amount of DNA in mitosis, properties of microtubules in diploid cells are modified in a pathway controlled by the mating-type regulatory genes.     

Compartmentalized zinc deficiency and toxicities caused by ZnT and Zip gene over expression result in specific phenotypes in Drosophila

Movement of zinc ions across cellular membranes is achieved mainly by two families of zinc transport genes encoding multi-transmembrane domain proteins. Members of the Zip family generally transport zinc into the cytosol, either from outside the cell or from the lumen of subcellular organelles such as the endoplasmic reticulum, Golgi, endosomes or storage vacuoles. ZnT proteins move zinc in the opposite direction, resulting in efflux from the cell or uptake into organelles. Zinc homeostasis at both the cellular and systemic level is achieved by the coordinated action of numerous Zip and ZnT proteins, twenty-four in mammals and seventeen in the vinegar fly Drosophila melanogaster. Previously, we have identified a zinc toxicity phenotype in the Drosophila eye, caused by targeted over expression of dZip42C.1 (dZip1) combined with knockdown of dZnT63C (dZnT1). In general, this phenotype was rescued by increased zinc efflux or decreased uptake and was exacerbated by decreased efflux or increased uptake. Now we have identified three additional zinc dyshomeostasis phenotypes caused by over expression of dZnT86D, dZnT86D^eGFP and dZip71B^FLAG. Genetic and dietary manipulation experiments showed that these three phenotypes all differ both from each other and from our original zinc toxicity phenotype. Based on these data and the approximate subcellular localization of each zinc transport protein, we propose that each phenotype represents a different redistribution of zinc within these cells, resulting in a Golgi zinc toxicity, a Golgi zinc deficiency and a combined Golgi/other organelle zinc toxicity respectively. We are able to group the remaining Drosophila Zip and ZnT genes into several functional categories based on their interaction with the three novel zinc dyshomeostasis phenotypes, allowing the role of each zinc transport protein to be defined in greater detail. This research highlights the differential effects that redistribution of zinc can have within a particular tissue and identifies the Golgi as being particularly sensitive to both excess and insufficient zinc.     

Assembly properties of dominant and recessive mutations in the small mouse neurofilament (NF-L) subunit

We have generated a set of amino- and carboxy-terminal deletions of the NF-L neurofilament gene and determined the assembly properties of the encoded subunits after coexpression with vimentin or wild-type NF-L. NF- L molecules missing greater than 30% (31 amino acids of the head) or 90% (128 amino acids of the tail) failed to incorporate into intermediate filament networks. Carboxy-terminal deletions into the rod domain yield dominant mutants that disrupt arrays assembled from wild- type subunits, even when present at levels of approximately 2% of the wild-type subunits. Even mutants retaining 55% of the tail (61 amino acids) disrupt normal arrays when accumulated above approximately 10% of wild-type subunits. Since deletion of greater than 90% of the head domain produces "recessive" assembly incompetent subunits that do not affect wild-type filament arrays, whereas smaller deletions yield efficient network disruption, we conclude that some sequence(s) in the head domain (within residues 31-87) are required for the earliest steps in filament assembly. Insertional mutagenesis in the nonhelical spacer region within the rod domain reveals that as many as eight additional amino acids can be tolerated without disrupting assembly competence.     

A high frequency of distinct ATM gene mutations in ataxia-telangiectasia.

The clinical features of the autosomal recessive disorder ataxia-telangiectasia (AT) include a progressive cerebellar ataxia, hypersensitivity to ionizing radiation, and an increased susceptibility to malignancies. Epidemiological studies have suggested that AT heterozygotes may also be at increased risk for malignancy, possibly as a consequence of radiation exposure. A gene mutated in AT patients (ATM) has recently been isolated, making mutation screening in both patients and the general population possible. Because of the relatively large size of the ATM gene, the design of screening programs will depend on the types and distribution of mutations in the general population. In this report, we describe 30 mutations identified in a panel of unrelated AT patients and controls. Twenty-five of the 30 were distinct, and most patients were compound heterozygotes. The most frequently detected mutation was found in three different families and had previously been reported in five others. This corresponds to a frequency of 8% of all reported ATM mutations. Twenty-two of the alterations observed would be predicted to lead to protein truncation at sites scattered throughout the molecule. Two fibroblast cell lines, which displayed normal responses to ionizing radiation, also proved to be heterozygous for truncation mutations of ATM. These observations suggest that the carrier frequency of ATM mutations may be sufficiently high to make population screening practical. However, such screening may need to be done prospectively, that is, by searching for new mutations rather than by screening for just those already identified in AT families.     

Gene inactivation as a mechanism for the expression of recessive phenotypes.

A series of Chinese hamster ovary cell hybrids were constructed which were heterozygous at the emtB and chr loci. These loci encode two recessive drug-resistance genes (emetine resistance and chromate resistance, respectively) located on a structurally hemizygous region on the long arm of chromosome 2. These heterozygous hybrids therefore exhibit wild-type sensitivity to both emetine and chromate. Drug-resistant variants were then selected in medium containing either emetine or chromate, and the mechanism of reexpression of the recessive drug-resistant allele was determined by karyotypic analysis of the resultant colonies. In previous studies at these loci we have determined that segregation of the recessive phenotype occurs primarily by (1) the loss of the chromosome 2 carrying the wild-type, drug-sensitive, allele, (2) deletion of the long arm of chromosome 2, or (3) loss of one chromosome 2 followed by duplication of the remaining homologue. However, a small proportion of segregants have also been detected which may have arisen by the mechanisms of de novo gene inactivation or mutation. In this report, hybrids are described which were constructed to allow selection for the retention of the chromosome carrying the wild-type allele and which therefore optimize isolation of these rare segregants. We demonstrate by karyotypic analysis, mutation frequency analysis, and microcell-mediated chromosome transfer that these rare segregants occur primarily by gene inactivation. We also demonstrate a dramatic increase in the proportion of segregants occurring by gene inactivation in two of these hybrids as compared with those previously reported, indicating that this mechanism may be an important mode of phenotype segregation in diploid cells and, therefore, in the development of cancers--such as the childhood tumors retinoblastoma and Wilms tumor--resulting from recessive alleles     

The evolving role of microRNAs in animal gene expression

MicroRNAs (miRNAs) constitute an abundant family of 22-nucleotide RNAs that base-pair to target mRNAs and typically inhibit their expression. To assess the global impact of animal miRNAs on gene regulation, the expression of predicted targets and their cognate miRNAs was extensively analyzed in mammals and Drosophila. In general, targets are co-expressed at relatively low or undetectable levels in the same tissues as the miRNAs predicted to regulate them. Additionally, genes that are highly co-expressed with miRNAs usually lack target sites. The authors conclude that many animal genes are under evolutionary pressure to maintain or avoid complementary sites to miRNAs. Thus, the miRNA pathway broadly contributes to the complex gene regulatory networks that shape animal tissue development and identity.     

Genetic suppression of phenotypes arising from mutations in dystrophin-related genes in Caenorhabditis elegans

BACKGROUND: Dystrophin is the product of the gene that is mutated in Duchenne muscular dystrophy (DMD), a progressive neuromuscular disease for which no treatment is available. Mice carrying a mutation in the gene for dystrophin (mdx mice) display only a mild phenotype, but it is aggravated when combined with a mutation in the MyoD gene. The nematode worm Caenorhabditis elegans has a dystrophin homologue (dys-1), but null mutations in dys-1 do not result in muscle degeneration.RESULTS: We generated worms carrying both the dys-1 null mutation cx18, and a weak mutation, cc561ts, of the C. elegans MyoD homologue hlh-1. The double mutants displayed a time-dependent impairment of locomotion and egg laying, a phenotype not seen in the single mutants, and extensive muscle degeneration. This result allowed us to look for genes that, when misexpressed, could suppress the dys-1; hlh-1 phenotype. When overexpressed, the dyc-1 gene - whose loss-of-function phenotype resembles that of dys-1 - partially suppressed the dys-1; hlh-1 phenotype. The dyc-1 gene encodes a novel protein sharing similarities with the mammalian neural nitric oxide synthase (nNOS)-binding protein CAPON, and is expressed in the muscles of the worm. CONCLUSIONS: As a C. elegans model for dystrophin-dependent myopathy, the dys-1; hlh-1 worms should permit the identification of genes, and ultimately drugs, that would reverse the muscle degeneration in this model.     

LIS1 missense mutations: variable phenotypes result from unpredictable alterations in biochemical and cellular properties

Mutations in one allele of the human LIS1 gene cause a severe brain malformation, lissencephaly. Although most LIS1 mutations involve deletions, several point mutations with a single amino acid alteration were described. Patients carrying these mutations reveal variable phenotypic manifestations. We have analyzed the functional importance of these point mutations by examining protein stability, folding, intracellular localization, and protein-protein interactions. Our data suggest that the mutated proteins were affected at different levels, and no single assay could be used to predict the lissencephaly phenotype. Most interesting are those mutant proteins that retain partial folding and interactions. In the case of LIS1 mutated in F31S, the cellular phenotype may be modified by overexpression of specific interacting proteins. Overexpression of the PAF-AH alpha1 subunit dissolved aggregates induced by this mutant protein and increased its half-life. Overexpression of NudE or NudEL localized this mutant protein to spindle poles and kinetochores but had no effect on protein stability. Our results implicate that there are probably different biochemical and cellular mechanisms obstructed in each patient yielding the varied lissencephaly phenotypes.