Cloning of human DAB1 and mapping to chromosome 1p31-p32

Disabled-1 is a large common fragile site gene, inactivated in multiple cancers

Common fragile sites (CFS) are large, genomically unstable regions, which are hot-spots for deletions and other alterations, especially in cancer cells. Several have been shown to contain genes that span large genomic regions, such as FHIT (1.5 Mb), WWOX (1.0 Mb), GRID2 (1.36 Mb), PARK2 (1.3 Mb), and RORA (730 kb). These genes are frequently inactivated in multiple different cancers, and FHIT and WWOX are shown to function as tumor suppressors. The disabled-1 gene (DAB1) is one of the human homologs of the Drosophila disabled locus, which in mammals is involved in neuronal migration and lamination in the developing cerebral cortex. Mice DAB1 inactivation results in the neurological mutant Scrambler, having similarities to mice with the inactivation of PARK2 (Quaker), GRID2 (Lurcher), and RORA (Staggerer). We were interested in whether DAB1 was another large CFS gene that could have cancer development importance. We demonstrated here that the human DAB1 gene (spanning 1.25 Mb) mapped within FRA1B CFS region on chromosomal band 1p32.2. Real-time RT-PCR analysis revealed that the expression level of DAB1 was decreased in many human cancer samples, including primary tumor tissues and cancer-derived cell lines, from several different cancers, especially in brain and endometrial cancer. Additionally, the introduction of an over-expression DAB1 plasmid into two different cell lines, having insignificant endogenous DAB1 expression, resulted in decreased cell growth. In summary, DAB1 is another gene that resides within an unstable CFS region and might play a role in human tumorigenesis. These data may provide further linkage between neurological development and cancer.

The gene encoding disabled-1 (DAB1), the intracellular adaptor of the Reelin pathway, reveals unusual complexity in human and mouse

The Disabled-1 (Dab1) gene encodes a key regulator of Reelin signaling. Reelin is a large glycoprotein secreted by neurons of the developing brain, particularly Cajal-Retzius cells. The DAB1 protein docks to the intracellular part of the Reelin very low density lipoprotein receptor and apoE receptor type 2 and becomes tyrosine-phosphorylated following binding of Reelin to cortical neurons. In mice, mutations of Dab1 and Reelin generate identical phenotypes. In humans, Reelin mutations are associated with brain malformations and mental retardation; mutations in DAB1 have not been identified. Here, we define the organization of Dab1, which is similar in human and mouse. The Dab1 gene spreads over 1100 kb of genomic DNA and is composed of 14 exons encoding the major protein form, some alternative internal exons, and multiple 5'-exons. Alternative polyadenylation and splicing events generate DAB1 isoforms. Several 5'-untranslated regions (UTRs) correspond to different promoters. Two 5'-UTRs (1A and 1B) are predominantly used in the developing brain. 5'-UTR 1B is composed of 10 small exons spread over 800 kb. With a genomic length of 1.1 Mbp for a coding region of 5.5 kb, Dab1 provides a rare example of genomic complexity, which will impede the identification of human mutations.

Disabled-1 mRNA and protein expression in developing human cortex

Disabled-1 (Dab1) forms part of the Reelin-Dab1 signalling pathway that controls neuronal positioning during brain development; Dab1 deficiency gives rise to a reeler-like inversion of cortical layers. To establish a timetable of Dab1 expression in developing human brain, Dab1 mRNA and protein expression were studied in prenatal human cortex. The earliest Dab1 signal was detected at 7 gestational weeks (GW), the stage of transition from preplate to cortical plate, suggesting a role of the Reelin-Dab1 signalling pathway in preplate partition. From 12 to 20 GW, the period of maximum cortical migration, Dab1 expression was prominent in the upper tiers of the cortical plate, to decline after midgestation. Radially orientated apical dendrites of Dab1-expressing neurons indicated a predominant pyramidal phenotype. Pyramidal cells in hippocampus and entorhinal cortex displayed a more protracted time of Dab1 expression compared to neocortex. In addition, at later stages (18-25 GW), Dab1 was also expressed in large neurons scattered throughout intermediate zone and subplate. From 14 to 22 GW, particularly high levels of Dab1 mRNA and protein were observed in cells of the ventricular/subventricular zone displaying the morphology of radial glia. The partial colocalization of vimentin and Dab1 in cells of the ventricular zone supported a radial glia phenotype. The concentration of Dab1 protein in ventricular endfeet and initial portions of radial processes of ventricular-zone cells points to a possible involvement of Dab1 in neurogenesis. Furthermore, a subset of Cajal-Retzius cells in the marginal zone colocalized Dab1 and Reelin, and may thus represent a novel target of the Reelin-Dab1 signalling pathway.

Chromosomal location of murine disabled-2 gene and structural comparison with its human ortholog

Disabled-2 (Dab2) is one of the two mammalian orthologs of the Drosophila Disabled. The three spliced forms, p96, p93, and p67 of murine Dab2 cDNAs were first isolated as phosphoproteins functioning in the macrophage CSF-1 signal transduction pathway. Subsequently, the involvement of Dab2 in ovarian cancer development has been investigated: Dab2 expression is lost or greatly diminished in breast and ovarian cancers, and gene deletions have been found. Regulation of Disabled-2 expression is also found to be important in development and physiological functions. Structural information of the murine Dab2 gene is essential for studies of transcription regulation and gene function in mouse models. In this study, the mouse Dab2 gene coding sequence was identified and sequenced from three lambda phage clones containing the gene. Two BAC clones of mouse genomic DNA were also used to identify the sequences of the non-coding first exon and promoter. The first exon is separated from the second exon by a large (15 kb) intron. The mouse gene is about 40 kb in size and consists of 15 exons, producing a 3.6 kb message. The translation initiation site resides in the middle of the second exon. The mouse Dab2 gene structure is very similar to that of its human ortholog in exon/intron sizes and promoter sequences. The chromosomal localization of mouse Dab2 was mapped by FISH to chromosome 15A2, a site of syntax with the human 5p12 where human Dab2 gene resides. The information on the mouse Dab2 gene structure and promoter will be invaluable in studies of the involvement of Dab2 gene in cancer, expression, physiological function, and development in mouse models.

Neuronal migration

Like other motile cells, neurons migrate in three schematic steps, namely leading edge extension, nuclear translocation or nucleokinesis, and retraction of the trailing process. In addition, neurons are ordered into architectonic patterns at the end of migration. Leading edge extension can proceed at the extremity of the axon, by growth cone formation, or from the dendrites, by formation of dendritic tips. Among both categories of leading edges, variation seems to be related to the rate of extension of the leading process. Leading edge extension is directed by microfilament polymerization following integration of extracellular cues and is regulated by Rho-type small GTPases. In humans, mutations of filamin, an actin-associated protein, result in heterotopic neurons, probably due to defective leading edge extension. The second event in neuron migration is nucleokinesis, a process which is critically dependent on the microtubule network, as shown in many cell types, from slime molds to vertebrates. In humans, mutations in the PAFAH1B1 gene (more commonly called LIS1) or in the doublecortin (DCX) gene result in type 1 lissencephalies that are most probably due to defective nucleokinesis. Both the Lis1 and doublecortin proteins interact with microtubules, and two Lis1-interacting proteins, Nudel and mammalian NudE, are components of the dynein motor complex and of microtubule organizing centers. In mice, mutations of Cdk5 or of its activators p35 and p39 result in a migration phenotype compatible with defective nucleokinesis, although an effect on leading edge formation is also likely. The formation of architectonic patterns at the end of migration requires the integrity of the Reelin signalling pathway. Other known components of the pathway include members of the lipoprotein receptor family, the intracellular adaptor Dab1, and possibly integrin alpha 3 beta 1. Defective Reelin leads to poor lamination and, in humans, to a lissencephaly phenotype different from type 1 lissencephaly. Although the action of Reelin is unknown, it may trigger some recognition-adhesion among target neurons. Finally, pattern formation requires the integrity of the external limiting membrane, defects of which lead to overmigration of neurons in meninges and to human type 2 lissencephaly.

Autosomal recessive lissencephaly with cerebellar hypoplasia is associated with human RELN mutations

Normal development of the cerebral cortex requires long-range migration of cortical neurons from proliferative regions deep in the brain. Lissencephaly ("smooth brain," from "lissos," meaning smooth, and "encephalos," meaning brain) is a severe developmental disorder in which neuronal migration is impaired, leading to a thickened cerebral cortex whose normally folded contour is simplified and smooth. Two identified lissencephaly genes do not account for all known cases, and additional lissencephaly syndromes have been described. An autosomal recessive form of lissencephaly (LCH) associated with severe abnormalities of the cerebellum, hippocampus and brainstem maps to chromosome 7q22, and is associated with two independent mutations in the human gene encoding reelin (RELN). The mutations disrupt splicing of RELN cDNA, resulting in low or undetectable amounts of reelin protein. LCH parallels the reeler mouse mutant (Reln(rl)), in which Reln mutations cause cerebellar hypoplasia, abnormal cerebral cortical neuronal migration and abnormal axonal connectivity. RELN encodes a large (388 kD) secreted protein that acts on migrating cortical neurons by binding to the very low density lipoprotein receptor (VLDLR), the apolipoprotein E receptor 2 (ApoER2; refs 9-11 ), alpha3beta1 integrin and protocadherins. Although reelin was previously thought to function exclusively in brain, some humans with RELN mutations show abnormal neuromuscular connectivity and congenital lymphoedema, suggesting previously unsuspected functions for reelin in and outside of the brain.