Regulation of chemosensory and GABAergic motor neuron development by the C. elegans Aristaless/Arx homolog alr-1

Mutations in the highly conserved Aristaless-related homeodomain protein ARX have been shown to underlie multiple forms of X-linked mental retardation. Arx knockout mice exhibit thinner cerebral cortices because of decreased neural precursor proliferation, and also exhibit defects in the differentiation and migration of GABAergic interneurons. However, the role of ARX in the observed behavioral and developmental abnormalities is unclear. The regulatory functions of individual homeodomain proteins and the networks in which they act are frequently highly conserved across species, although these networks may be deployed in different developmental contexts. In Drosophila, aristaless mutants exhibit defects in the development of terminal appendages, and Aristaless has been shown to function with the LIM-homeodomain protein LIM1 to regulate leg development. Here, we describe the role of the Aristaless/Arx homolog alr-1 in C. elegans. We show that alr-1 acts in a pathway with the LIM1 ortholog lin-11 to regulate the development of a subset of chemosensory neurons. Moreover, we demonstrate that the differentiation of a GABAergic motoneuron subtype is affected in alr-1 mutants, suggesting parallels with ARX functions in vertebrates. Investigating ALR-1 functions in C. elegans may yield insights into the role of this important protein in neuronal development and the etiology of mental retardation.     

The pattern of gene expression in mouse Gr-1(+) myeloid progenitor cells

To understand the pattern of gene expression in mouse myeloid progenitor cells, we carried out a genome-wide analysis of gene expression in mouse bone marrow Gr-1(+) cells using SAGE and GLGI techniques. We identified 22,033 unique SAGE tags with quantitative information from 73,869 collected SAGE tags. Among these unique tags, 64% match known sequences, including many genes important for myeloid differentiation, and 36% have no matches to known sequences and are likely to represent novel genes. We compared the expression of mouse Gr-1(+) and human CD15(+) myeloid progenitor cells and showed that the pattern of gene expression of these two cell populations had some similarities. We also compared the expression of mouse Gr-1(+) myeloid progenitor cells with that of mouse brain tissue and found a highly tissue-specific manner of gene expression in these two samples. Our data provide a basis for studying altered gene expression in myeloid disorders using mouse models.     

The rhombotin gene family encode related LIM-domain proteins whose differing expression suggests multiple roles in mouse development.

The rhombotin (RBTN1 or Ttg-1) gene was first identified at a chromosome translocation in a T-cell acute leukaemia and later used to isolate two related genes (RBTN2 or Ttg-2 and RBTN3). Complete characterization of these genes in man and mouse shows that all three encode cysteine-rich proteins with typical LIM domains. RBTN1 and RBTN3-derived proteins have 98% identity in the LIM domains but are located on separate chromosomes in man and in mouse while RBTN1 and RBTN2, both located on human chromosome 11p but are on separate chromosomes in mouse, are only 48% identical in this part of the protein. The exon organization of RBTN1 and RBTN3 genes are similar, both having an intron, absent from the RBTN2 gene, in the LIM2-encoding region. The remarkable similarity between rbtn-1 and rbtn-3 proteins is parallelled in their expression patterns in mouse development, since both genes show high expression in restricted areas of the brain, but little lymphoid expression. rbtn-2 expression, however, is more ubiquitous. This gene shows a low level of thymus expression but high expression in fetal liver, adult spleen and B-cell lines, consistent with a role in B-cell development. These results suggest multiple cellular targets for the action of these proteins during development.     

Mutations of CDKL5 cause a severe neurodevelopmental disorder with infantile spasms and mental retardation

Rett syndrome (RTT) is a severe neurodevelopmental disorder caused, in most classic cases, by mutations in the X-linked methyl-CpG-binding protein 2 gene (MECP2). A large degree of phenotypic variation has been observed in patients with RTT, both those with and without MECP2 mutations. We describe a family consisting of a proband with a phenotype that showed considerable overlap with that of RTT, her identical twin sister with autistic disorder and mild-to-moderate intellectual disability, and a brother with profound intellectual disability and seizures. No pathogenic MECP2 mutations were found in this family, and the Xq28 region that contains the MECP2 gene was not shared by the affected siblings. Three other candidate regions were identified by microsatellite mapping, including 10.3 Mb at Xp22.31-pter between Xpter and DXS1135, 19.7 Mb at Xp22.12-p22.11 between DXS1135 and DXS1214, and 16.4 Mb at Xq21.33 between DXS1196 and DXS1191. The ARX and CDKL5 genes, both of which are located within the Xp22 region, were sequenced in the affected family members, and a deletion of nucleotide 183 of the coding sequence (c.183delT) was identified in CDKL5 in the affected family members. In a screen of 44 RTT cases, a single splice-site mutation, IVS13-1G-->A, was identified in a girl with a severe phenotype overlapping RTT. In the mouse brain, Cdkl5 expression overlaps--but is not identical to--that of Mecp2, and its expression is unaffected by the loss of Mecp2. These findings confirm CDKL5 as another locus associated with epilepsy and X-linked mental retardation. These results also suggest that mutations in CDKL5 can lead to a clinical phenotype that overlaps RTT. However, it remains to be determined whether CDKL5 mutations are more prevalent in specific clinical subgroups of RTT or in other clinical presentations.     

BAC contig from a 3-cM region of mouse chromosome 11 surrounding Brca1

Even with the completion of a draft version of the human genome sequence only a fraction of the genes identified from this sequence have known functions. Chromosomal engineering in mouse cells, in concert with gene replacement assays to prove the functional significance of a given genomic region or gene, represents a rapid and productive means for understanding the role of a given set of genes. Both techniques rely heavily on detailed maps of chromosomal regions, initially to understand the scope of the regions being modified and finally to provide the cloned resources necessary to allow both finished sequencing and large insert complementation. This report describes the creation of a BAC clone contig on mouse chromosome 11 in a region showing conservation of synteny with sequences on human chromosome 17. We have created a detailed map of an approximately 3-cM region containing at least 33 genes through the use of multiple BAC mapping strategies, including chromosome walking and multiplex oligonucleotide hybridization and gap filling. The region described is one of the targets of a large effort to create a series of mice with regional deletions on mouse chromosome 11 (33-80 cM) that can subsequently be subjected to further mutagenesis.     

Arx acts as a regional key selector gene in the ventral telencephalon mainly through its transcriptional repression activity

The homeobox-containing gene Arx is expressed during ventral telencephalon development and required for correct GABAergic interneuron tangential migration from the ganglionic eminences to the olfactory bulbs, cerebral cortex and striatum. Its human ortholog is associated with a variety of neurological clinical manifestations whose symptoms are compatible with the loss of cortical interneurons and altered basal ganglia-related activities. Herein, we report the identification of a number of genes whose expression is consistently altered in Arx mutant ganglionic eminences. Our analyses revealed a striking ectopic expression in the ganglionic eminences of several of these genes normally at most marginally expressed in the ventral telencephalon. Among them, Ebf3 was functionally analyzed. Thus, its ectopic expression in ventral telencephalon was found to prevent neuronal tangential migration. Further, we showed that Arx is sufficient to repress Ebf3 endogenous expression and that its silencing in Arx mutant tissues partially rescues tangential cell movement. Together, these data provide new insights into the molecular pathways regulated by Arx during telencephalon development.