Search

Mutation of C9orf72 is the most prevalent defect associated with amyotrophic lateral sclerosis and frontotemporal degeneration.sup.1. Together with hexanucleotide-repeat expansion.sup.2,3, haploinsufficiency of C9orf72 contributes to neuronal dysfunction.sup.4-6. Here we determine the structure of the C9orf72-SMCR8-WDR41 complex by cryo-electron microscopy. C9orf72 and SMCR8 both contain longin and DENN (differentially expressed in normal and neoplastic cells) domains.sup.7, and WDR41 is a [beta]-propeller protein that binds to SMCR8 such that the whole structure resembles an eye slip hook. Contacts between WDR41 and the DENN domain of SMCR8 drive the lysosomal localization of the complex in conditions of amino acid starvation. The structure suggested that C9orf72-SMCR8 is a GTPase-activating protein (GAP), and we found that C9orf72-SMCR8-WDR41 acts as a GAP for the ARF family of small GTPases. These data shed light on the function of C9orf72 in normal physiology, and in amyotrophic lateral sclerosis and frontotemporal degeneration. The cryo-electron microscopy structure of C9orf72-SMCR8-WDR41 suggests that this complex is a GTPase-activating protein for ARF-family small GTPases, which sheds light on the role of C9orf72 mutations in neuronal dysfunction., Author(s): Ming-Yuan Su [sup.1] [sup.2] , Simon A. Fromm [sup.1] [sup.2] , Roberto Zoncu [sup.1] [sup.2] , James H. Hurley [sup.1] [sup.2] [sup.3] Author Affiliations: (1) Department of Molecular and [...]

Author(s): Alexander D. Gitlin [sup.1] , Ziv Shulman [sup.1] , Michel C. Nussenzweig [sup.1] [sup.2] Author Affiliations: (1) Laboratory of Molecular Immunology, The Rockefeller University, New York, USA (2) Howard [...], Clonal expansion and hypermutation of B cells in the germinal centre are regulated by the amount of antigen that the B cells present to follicular helper T cells. Immune B-cell selection in the germinal centre Clonal expansion, in which B lymphocytes bearing antigen-specific immunoglobulins proliferate in specialized areas of lymphoid tissue known as germinal centres, is a vital part of the immune reaction. This study reveals how germinal-centre B lymphocytes with the highest affinity for antigen are selectively expanded and diversified. B-cell clonal expansion and hypermutation respond in proportion to the amount of antigen that the B cells present to follicular helper T cells. The selected cells both increase their rate of cell division and undergo increased somatic mutation, so the highest affinity cells are also the most diversified group. During immune responses, B lymphocytes clonally expand and undergo secondary diversification of their immunoglobulin genes in germinal centres (GCs).sup.1,2,3,4. High-affinity B cells are expanded through iterative interzonal cycles of division and hypermutation in the GC dark zone followed by migration to the GC light zone, where they are selected on the basis of affinity to return to the dark zone.sup.5,6,7,8,9,10. Here we combine a transgenic strategy to measure cell division and a photoactivatable fluorescent reporter to examine whether the extent of clonal expansion and hypermutation are regulated during interzonal GC cycles. We find that both cell division and hypermutation are directly proportional to the amount of antigen captured and presented by GC B cells to follicular helper T cells in the light zone. Our data explain how GC B cells with the highest affinity for antigen are selectively expanded and diversified.

The Mll gene regulates Hox gene expression, exhibits haplo-insufficiency and is necessary for segment identity in mammals. Researchers studied the involvement of Mll in pattern development in mutant mice. Mll -/- mice died as embryos. Mll +/- mice were deformed and growth retarded. They also had segment identity abnormalities aw well as caudal shifts. Mll-mutant mice have many features in common with trx mutants.

A study of mice containing mutations in T-cell factor-1 (Tcf-1), a thymocyte differentiation regulating gene, indicates the absence of transition of the thymocytes from the CD8+ stage to the CD4+/CD8+ stage, which is a crucial step in cell differentiation, in the mutant mice. This implies the importance of Tcf-1, which encodes HMG-box proteins, for cell differentiation. The mutants exhibit low levels of immunocompetent T cells in the peripheral lymphoid organs.

Deletions of the DAX-1 gene in patients and adrenal hypoplasia congenita (AHC) and analysis of point mutations in the coding region in DNA of the patients confirm that the DAX-1 gene causes X-linked AHC and the related hypogonadotropic hypogonadism (HHG). The DAX-1 gene has different point mutations and exhibits genetic heterogeneity or differential expression. This gene is characterized by a small size and a two exon structure.

A study of antigen-specific T cells that are recruited into splenic germinal centers shows that these T cells undergo mutations in the variable region of genes forming T-cell antigen receptor (TCR) alpha-chains but not TCR beta-chains. These hypermutations are locus-specific and their frequencies are similar to immunoglobulin VH which has undergone mutation. Identically mutated T cells are present in multiple germinal centers, suggesting the possibility of a reentry of TCRs into the peripheral lymphocyte pool.

Translocation chromosome breakpoint cloning in a sex-reversed patient suffering from campomelic dysplasia and analysis of mutations in SOX9, which is an SRY-related gene, shows that SOX9 takes part in both testis development and bone formation. The results support the view that campomelic dysplasia is an autosomal dominant disease. There might also be a close relationship between SOX9 and SRY gene because of their sequence similarities and the dosage sensitivity exhibited by SOX9 in sex determination.

A study of the 77K subunit of the human cleavage stimulation factor (CstF) revealed that it is homologous to the Drosophila melanogaster suppressor for forked (su(f)) gene. The 77K subunit predicts the nature of the human CstF while it also forms the link between 64K and 50K units. The similar homology in su(f) protein and Saccharomyces cerevisiae RNA14 also confirms the evolutionary conservation of polyadenylation mechanism from yeast to man.

A study of insulin-receptor substrate-1 (IRS-1)-deficient mice made by targeted gene mutation shows the existence of IRS-1-dependent and IRS-1-independent channels of insulin/IGF-1 signalling and confirms the presence of an alternative substrate of these receptor kinases. The alternative substrate IRS-2 takes over the function of IRS-1, binding to PI(3)K, but does not react with anti-IRS-1 antibodies. The IRS-1 deficient mice have a 50% reduction in intrauterine growth, impaired glucose tolerance and a decrease of insulin/IGF-1-stimulated glucose intake.

TBP mutations disrupt TFIIB recruitment to early preinitiation complexes and affect interactions of GAL4-VP16 with VP16, TFIIB or an unidentified component. Mutations in yeast TBP selectively block activator GAL4-VP16-dependent are identified. GAL4-VP16 mechanism involves both direct interactions with TBP and a corresponding induction of an activation-specific TBP-TFIIB-promoter complex.

Self-replication of palindromic duplex DNA such as 24 monomers long oligonucleotide can be extended to include non-symmetrical sequences, selection and mutation. A sequence-selective chemical process involving the production of an intermediate triplex structure follows replication. The solutions suggest that DNA-like double-helical molecules can replicate without protein-assistance.

The HLA-DMA and -DMB genes structure the peptide sequence of the major histocompatibility complex (MHC) molecules. The HLA-DMA and -DMB genes consist of heterodimer subunits, necessary for the antigen presentation pathway. The new 2.2.93 mutant contains the HLA-DMA and -DMB mutations and a c2p-1 mutant, and is isolated from the somatic cell mutants of antigen pressing cells.

The tyrosine kinase receptor Ret is a vital component of the pathway involved in renal organogenesis and enteric neurogenesis in mice. Mice, homozygous for a targeted C-ret mutation, die soon after birth due to renal agenesis, severe dysgenesis and absence of enteric neurons in the digestive tract. Mutations of RET genes cause inheritable defects in excretory and nervous systems.

A new kind of ion channel which integrates the characteristics of voltage- and ligand-gated channels is expressed by Drosophila ether-a-go-go (eag) RNA in Xenopus oocytes. Voltage and cyclic AMP determines the permeability of these eag channels of potassium and calcium. The modulation of synaptic efficiency in central and peripheral nervous systems is probably influenced by channel characteristics that result from the capacity to mediate potassium-outward and calcium-inward currents.

A fresh approach using a targeted heat-inducible ribozyme to generate conditional knock-out mutations to evaluate their biological functions, is suggested for the Drosophila fushi tarazu (ftz) gene. Transgenic flies bearing a ribozyme are generated against the ftz gene. The timed induction of the ribozyme can be used to differentiate the two developmental stages of ftz function.

Experimental studies of the beta-2-subunit of neuronal nicotinic acetylcholine receptors (nAChR) suggest that mutations in this subunit cause absence of nicotine binding sites, leading to unusual nicotine-independent avoidance. The mutant mice exhibit better passive avoidance than normal mice. Nicotine administration does not elicit any response from thalamic neurons, as revealed by electrophysiological studies.

The activity of segment polarity genes regulates cell patterning in the body segments of Drosophila embryo. The segment polarity genes are molecularly heterogeneous groups and are described by phenotypes of generic mutants. The wingless and hedgehog (hh) genes encode the proteins which are found in the pathway of secretion. This encoding determines the fate of the nearby cells. The Hsp70 promoter induces the strange expression of the hh gene.

Genetic studies on the Usher syndrome type I (USH-1), which causes the dual sensory defect deaf-blindness in humans, through an analysis of mutations containing the shaker-1 mutation, indicate myosin VIIA encoded gene to be the cause of USH1B. A primary cytoskeletal protein deficiency due to the mutations in the protein's motor domain N-terminal causes USH1B, which induces cytoskeletal defects in the affected.

The effectiveness of the agonist, gamma-aminobutyric acid (GABA), is decreased by mutations in the aminobutyric terminal region of GABA receptor subunit. The mutated amino acids constitute the agonist binding site which reduces the affinity. The discovery of this binding arises from pharmacological experiments rather than experiments on binding. The binding of an agonist to the receptor also enables the agonist to open its channels after it is bound.

Understanding the solution structure of a DNA and DNA-binding domain of SRY complex is essential for the molecular interpretation of mutations which are responsible for the clinical effects. SRY is a DNA-binding protein determining the male gender in mammals and affiliated with abnormal sexual development. A mutation that incorporates an isoleucine residue lowers the specificity of the DNA target and causes DNA deformation.

Defects in three fibroblast growth factor receptor (FGFR) genes lead to hereditary bone disorders. A missense mutation in the region encoding the transmembrane domain causes achondroplasia, while Crouzon syndrome results from mutations in the alternatively spliced exon of FGFR2 on chromosome 10. In Pfeiffer syndrome mutation occurs in the extracellular position of FGFR1. Mutations of a few other genes such as CAG and SCA5 are responsible for many congenital bone defects.