Biomedical subjects

Ruth Arkell

Publications and source records attributed to Ruth Arkell.

8 recordsLinked to original sources

PubMed2006

Loss of Atrx affects trophoblast development and the pattern of X-inactivation in extraembryonic tissues.

ATRX is an X-encoded member of the SNF2 family of ATPase/helicase proteins thought to regulate gene expression by modifying chromatin at target loci. Mutations in ATRX provided the first example of a human genetic disease associated with defects in such proteins. To better understand the role of ATRX in development and the associated abnormalities in the ATR-X (alpha thalassemia mental retardation, X-linked) syndrome, we conditionally inactivated the homolog in mice, Atrx, at the 8- to 16-cell stage of development. The protein, Atrx, was ubiquitously expressed, and male embryos null for Atrx implanted and gastrulated normally but did not survive beyond 9.5 days postcoitus due to a defect in formation of the extraembryonic trophoblast, one of the first terminally differentiated lineages in the developing embryo. Carrier female mice that inherit a maternal null allele should be affected, since the paternal X chromosome is normally inactivated in extraembryonic tissues. Surprisingly, however, some carrier females established a normal placenta and appeared to escape the usual pattern of imprinted X-inactivation in these tissues. Together these findings demonstrate an unexpected, specific, and essential role for Atrx in the development of the murine trophoblast and present an example of escape from imprinted X chromosome inactivation.

Alleles↗

PubMed2005

An N-ethyl-N-nitrosourea screen for genes involved in variegation in the mouse.

We have developed a sensitized screen to identify genes involved in gene silencing, using random N-ethyl-N-nitrosourea mutagenesis on mice carrying a variegating GFP transgene. The dominant screen has produced six mutant lines, including both suppressors and enhancers of variegation. All are semidominant and five of the six are homozygous embryonic lethal. In one case, the homozygous lethality depends on sex: homozygous females die at midgestation and display abnormal DNA methylation of the X chromosome, whereas homozygous males are viable. Linkage analysis reveals that the mutations map to unique chromosomal locations. We have studied the effect of five of the mutations on expression of an endogenous allele known to be sensitive to epigenetic state, agouti viable yellow. In all cases, there is an effect on penetrance, and in most cases, parent of origin and sex-specific effects are detected. This screen has identified genes that are involved in epigenetic reprogramming of the genome, and the behavior of the mutant lines suggests a common mechanism between X inactivation and transgene and retrotransposon silencing. Our findings raise the possibility that the presence or absence of the X chromosome in mammals affects the establishment of the epigenetic state at autosomal loci by acting as a sink for proteins involved in gene silencing. The study demonstrates the power of sensitized screens in the mouse not only for the discovery of novel genes involved in a particular process but also for the elucidation of the biology of that process.

Alleles↗

PubMed2004

In vitro analysis of partial loss-of-function ZIC2 mutations in holoprosencephaly: alanine tract expansion modulates DNA binding and transactivation.

Heterozygous loss-of-function mutations in ZIC2 result in the severe brain malformation known as holoprosencephaly (HPE), indicating that forebrain development is exquisitely sensitive to the activity of this poorly understood transcription factor. To identify the regions of ZIC2 that are essential for activity, we have assessed the ability of a variety of ZIC2 mutant proteins to function in in vitro assays. Two sources of information were used to design relevant mutations. First, phenotype producing mutations in human and in mouse ZIC2 were mimicked and secondly, a comparative sequence analysis of the C-terminal was carried out. Analysis of these mutations suggests that either a decrease or an increase in ZIC2 mediated transcriptional activity can produce a forebrain phenotype. In addition, the analysis reveals that the C-terminal of ZIC2 contains both activation and repression domains. This region of ZIC2 contains an alanine-tract, and expansion of this domain is associated with HPE. In vitro analysis of proteins with alterations in alanine-tract length illustrates that the C-terminal alanine-tract of ZIC2 influences the strength of DNA binding and alters transcriptional activity in a promoter-specific manner. This finding provides a possible mechanism by which alanine-tract expansion mutations could alter the function of other transcription factors.

Alanine↗

PubMed2004

Organization and evolution of a gene-rich region of the mouse genome: a 12.7-Mb region deleted in the Del(13)Svea36H mouse.

Del(13)Svea36H (Del36H) is a deletion of approximately 20% of mouse chromosome 13 showing conserved synteny with human chromosome 6p22.1-6p22.3/6p25. The human region is lost in some deletion syndromes and is the site of several disease loci. Heterozygous Del36H mice show numerous phenotypes and may model aspects of human genetic disease. We describe 12.7 Mb of finished, annotated sequence from Del36H. Del36H has a higher gene density than the draft mouse genome, reflecting high local densities of three gene families (vomeronasal receptors, serpins, and prolactins) which are greatly expanded relative to human. Transposable elements are concentrated near these gene families. We therefore suggest that their neighborhoods are gene factories, regions of frequent recombination in which gene duplication is more frequent. The gene families show different proportions of pseudogenes, likely reflecting different strengths of purifying selection and/or gene conversion. They are also associated with relatively low simple sequence concentrations, which vary across the region with a periodicity of approximately 5 Mb. Del36H contains numerous evolutionarily conserved regions (ECRs). Many lie in noncoding regions, are detectable in species as distant as Ciona intestinalis, and therefore are candidate regulatory sequences. This analysis will facilitate functional genomic analysis of Del36H and provides insights into mouse genome evolution.

Animals↗

PubMed2004

New semidominant mutations that affect mouse development.

Dominantly acting mutations that produce visible phenotypes are frequently recovered, either during routine maintenance of colonies or from mutagenesis experiments. We have studied 12 dominant mouse mutations that cause a tail dysmorphology, a coat spotting phenotype, or a combination of these. The majority of these mutations act in a semidominant manner with the homozygous state associated with embryonic lethality and a visible phenotype at or before midgestation. The homozygous phenotypes include axis truncation and neural crest cell defects, as may be expected from the heterozygous phenotypes. The majority of mutations, however, also produced other phenotypes that include neural tube closure defects and aberrant heart looping. In one coat spotting mutant the homozygous condition is lethal before neural crest cell production commences. The mutated genes often function in processes additional to those alluded to by the heterozygous phenotype.

Alkylating Agents↗

PubMed2004

Overlapping and distinct expression domains of Zic2 and Zic3 during mouse gastrulation.

The Zic genes are the vertebrate homologues of the Drosophila Odd-paired gene. Mutations in two of these genes are associated with human congenital genetic disorders. Mutation of human and mouse Zic2 is associated with holoprosencephaly which is caused by a defect of ventral forebrain development and mutation of human and mouse Zic3 is associated with a X-linked heterotaxy syndrome that results from a failure of left-right axis formation. The embryological role of the Zic genes in these disorders is not well understood. Here we show that both of these genes are expressed prior to and throughout gastrulation. The genes show some broad similarities in their expression domains. Both genes however are also uniquely expressed in some tissues and these unique domains correlate with regions that potentially play a role in the aetiology of the respective genetic disorders. During primitive streak stages Zic2 is expressed transiently and uniquely in the node and the head process mesendoderm. The head process is known to be required for the establishment or maintenance of the ventral forebrain, which is the region disrupted in holoprosencephaly. Zic3 is not expressed in the node during primitive streak stages but is expressed in and around the node beginning from the head fold stages of development. This expression of Zic3 correlates well with the first steps in the establishment of the left-right axis. We also examined the expression of the closely related gene, Zic1, and did not detect any transcripts in gastrulation stage embryos.

Animals↗

PubMed2003

Zic2 is required for neural crest formation and hindbrain patterning during mouse development.

The Zic genes are the vertebrate homologues of the Drosophila pair rule gene odd-paired. It has been proposed that Zic genes play several roles during neural development including mediolateral segmentation of the neural plate, neural crest induction, and inhibition of neurogenesis. Initially during mouse neural development Zic2 is expressed throughout the neural plate while later on expression in the neurectoderm becomes restricted to the lateral region of the neural plate. A hypomorphic allele of Zic2 has demonstrated that in the mouse Zic2 is required for the timing of neurulation. We have isolated a new allele of Zic2 that behaves as a loss of function allele. Analysis of this mutant reveals two further functions for Zic2 during early neural development. Mutation of Zic2 results in a delay of neural crest production and a decrease in the number of neural crest cells that are produced. These defects are independent of mediolateral segmentation of the neurectoderm and of dorsal neurectoderm proliferation, both of which occur normally in the mutant embryos. Additionally Zic2 is required during hindbrain patterning for the normal development of rhombomeres 3 and 5. This work provides the first genetic evidence that the Zic genes are involved in neural crest production and the first demonstration that Zic2 functions during hindbrain patterning.

Animals↗

PubMed2003

Disruption of scribble (Scrb1) causes severe neural tube defects in the circletail mouse.

Circletail is one of only two mouse mutants that exhibit the most severe form of neural tube defect (NTD), termed craniorachischisis. In this disorder, almost the entire brain and spinal cord is affected, owing to a failure to initiate neural tube closure. Craniorachischisis is a significant cause of lethality in humans, yet the molecular mechanisms involved remain poorly understood. Here, we report the identification of the gene mutated in circletail (Crc), using a positional cloning approach. This gene, Scrb1, encodes a member of the LAP protein family related to Drosophila scribble, with 16 leucine rich repeats and four PDZ domains. The Crc mutant contains a single base insertion that creates a frame shift and leads to premature termination of the Scrb1 protein. We report the expression pattern of Scrb1 during embryonic and fetal development, and show that Scrb1 expression closely mirrors the phenotypic defects observed in Crc/Crc mutants. In addition, circletail genetically interacts with the loop-tail mutant, and we reveal overlapping expression of Scrb1 with Vangl2, the gene mutated in loop-tail. The identification of the Crc gene further defines the nature of the genetic pathway required for the initiation of neural tube closure and provides an important new candidate that may be implicated in the aetiology of human NTDs.

Amino Acid Sequence↗