xNgn2 induces expression of predominantly sensory neuron markers in Xenopus whole embryo ectoderm but induces mixed subtype expression in isolated ectoderm explants

Proneural basic-helix-loop-helix (bHLH) proteins, such as Neurogenin2 (Ngn2) and Ascl1, are critical regulators at the onset of neuronal differentiation. Endogenously they have largely complementary expression patterns, and have conserved roles in the specification of distinct neuronal subtypes. InXenopusembryos, xNgn2 is the master regulator of primary neurogenesis forming sensory, inter- and motor neurons within the neural plate, while xAscl1 is the master regulator of autonomic neurogenesis, forming noradrenergic neurons in the antero-ventral region of the embryo. Here we characterise neuronal subtype identity of neurons induced by xNgn2 in the ectoderm of wholeXenopusembryos in comparison with xAscl1, and in ectodermal “animal cap” explants. We find that the transcriptional cascades mediating primary and autonomic neuron formation are distinct, and while xNgn2 and xAscl1 can upregulate genes associated with a non-endogenous cascade, this expression is spatially restricted within the embryo. xNgn2 is more potent than xAscl1 at inducing primary neurogenesis as assayed by neural-β-tubulin. In ectoderm of the intact embryo, these induced primary neurons have sensory characteristics with no upregulation of motor neuron markers. In contrast, xNgn2 is able to up-regulate both sensory and motor neuron markers in naïve ectoderm of animal cap explants, suggesting a non-permissive environment for motor identity in the patterned ectoderm of the whole embryo.

Interaction between opposing modes of phospho-regulation of the proneural proteins Ascl1 and Ngn2

From the relatively simple nervous system of Drosophila to the elaborate mammalian cortex, neurogenesis requires exceptional spatial and temporal precision to co-ordinate progenitor cell proliferation and subsequent differentiation to a diverse range of neurons and glia. A limited number of transiently expressed proneural basic-helix-loop-helix (bHLH) transcription factors, for example achaete-scute-complex (as-c) and atonal (ato) in Drosophila and the vertebrate homologues Ascl1 and Neurogenin2 (Ngn2), are able to orchestrate the onset of neuronal determination, context-dependent subtype selection and even influence later aspects of neuronal migration and maturation. Within the last decade, two models have emerged to explain how the temporal activity of proneural determination factors is regulated by phosphorylation at distinct sites. One model describes how cell-cycle associated phosphorylation on multiple sites in the N and C termini of vertebrate proneural proteins limits neuronal differentiation in cycling progenitor cells. A second model describes phosphorylation on a single site in the bHLH domain of Drosophila atonal that acts as a binary switch, where phosphorylation terminates proneural activity. Here we combine activating mutations of phosphorylation sites in the N- and C- termini with an inhibitory phospho-mimetic mutation in the bHLH domain of Ascl1 and Ngn2 proteins, and test their functions in vivo using Xenopus embryos to determine which mode of phospho-regulation dominates. Enhancing activity by preventing N- and C terminal phosphorylation cannot overcome the inhibitory effect of mimicking phosphorylation of the bHLH domain. Thus we have established a hierarchy between these two modes of proneural protein control and suggest a model of temporal regulation for proneural protein activity.

The N terminus of Ascl1 underlies differing proneural activity of mouse and Xenopus Ascl1 proteins

The proneural basic-helix-loop-helix (bHLH) transcription factor Ascl1 is a master regulator of neurogenesis in both central and peripheral nervous systems in vivo, and is a central driver of neuronal reprogramming in vitro. Over the last three decades, assaying primary neuron formation in Xenopus embryos in response to transcription factor overexpression has contributed to our understanding of the roles and regulation of proneural proteins like Ascl1, with homologues from different species usually exhibiting similar functional effects. Here we demonstrate that the mouse Ascl1 protein is twice as active as the Xenopus protein in inducing neural-β-tubulin expression in Xenopus embryos, despite there being little difference in protein accumulation or ability to undergo phosphorylation, two properties known to influence Ascl1 function. This superior activity of the mouse compared to the Xenopus protein is dependent on the presence of the non-conserved N terminal region of the protein, and indicates species-specific regulation that may necessitate care when interpreting results in cross-species experiments.

Genetic mapping of susceptibility loci in the genes involved in rheumatoid arthritis

Eighty-nine multicase rheumatoid arthritis families, each containing at least one affected sib pair, have been studied for evidence of genetic linkage to a panel of 315 microsatellite DNA markers. The families were located through the UK national data repository of the Arthritis and Rheumatism Council. Microsatellites were genotyped by semiautomated technology using Applied Biosystems sequencers (ABI 373). Using the SIBPAIR statistical package, linkage to HLA was confirmed (p < 0.0003). Several possible linkages outside HLA were noted, including at least one (p < 0.004) that merits further investigation.

Analysis of three deletion breakpoints in Xp21.1 and the further localization of RP3

The gene responsible for X-linked retinitis pigmentosa (xlRP) in Xp21.1 (RP3) was initially localized by deletion analysis to within a 150- to 170-kb region between the CYBB locus and the proximal deletion junction (BBJPROX) from a patient, BB, who suffered from Duchenne muscular dystrophy (DMD), McLeod syndrome, chronic granulomatous disease (CGD), and xlRP. This gene has recently been isolated and was found to be located outside and 400 kb proximal to the BB deletion. Further analysis of BBJPROX has identified the breakpoint junction sequence, showing that it occurs within an Alu repetitive element on the proximal side but with no significant homology to the distal sequence in dystrophin intron 30. Analysis of an overlapping deletion in patient NF, who suffered from DMD, CGD, and McLeod syndrome, shows that this deletion is within 4 kb but extends centromeric to BBJPROX, consistent with the location of RP3 outside the BB deletion region. A sequence with strong homology to a THE-1 transposon-like element was identified 7-13 kb from the proximal BB and NF breakpoints. These elements have been implicated in several highly unstable genomic regions. A third overlapping deletion, in a patient, SB, who suffered from CGD, McLeod syndrome, and xlRP, has here been shown to extend 380 kb proximal to the NF breakpoint, consistent with the finding that RP3 lies outside the BB deletion. This deletion has now been shown to disrupt the RP3 (RPGR) gene. The reason for the retinitis pigmentosa phenotype in patient BB remains unclear, but the most likely explanations include a long-range chromosomal position effect, a small secondary rearrangement, and the presence of a coincident autosomal form of retinitis pigmentosa.

Identification of a novel gene, ETX1 from Xp21.1, a candidate gene for X-linked retintis pigmentosa (RP3)

A novel gene encoding a 2.2 kilobase transcript has been isolated from the Xp21.1 region of the human X chromosome by exon amplification. The gene, called EXT1, spans 80 kilobases and contains 12 exons, at least two of which are alternatively spliced and have predicted products of 464 and 471 amino acids respectively. Conceptual translation of the open reading frames shows one product with a 30 amino acid signal peptide, which is absent from the alternative transcript, followed by three complement control protein domains, a hydrophobic region with a possible role in membrane anchorage and short 17 amino acid putative cytoplasmic carboxyl terminus. An alternative first exon contains a 39 amino acid open reading frame which is rich in serine and threonine residues and contains a potential chondroitin/dermatan sulphate attachment site. Northern analysis showed ETX1 expression within the retina and heart with lower levels in several other tissues. Since ETX1 lies within the region thought to contain the x-linked retinitis pigmentosa (xIRP) gene, RP3, it was screened for mutation within a set of 45 xIRP patients using single strand conformation analysis and/or chemical cleavage of mismatch using reverse transcription/polymerase chain reaction amplification of polyA+RNA from blood cells. Three low frequently variants (17-23Ldel, P225S, S413F) were found in both patients and controls; one of which (P225S) was found in four of 45 unrelated patient chromosomes and one of 178 control chromosomes (p <0.001). The allelic association between P225S and xIRP alleles suggests a common ancestral chromosome bearing the P225S variant and an RP3 mutation at a neighbouring locus.

Linkage of the gene that encodes the alpha 1 chain of type V collagen (COL5A1) to type II Ehlers-Danlos syndrome (EDS II)

Ehlers-Danlos syndrome (EDS) is a group of heritable disorders of connective tissue with skin, ligaments and blood vessels being the main sites affected. The commonest variant (EDS II) exhibits an autosomal dominant mode of inheritance and is characterized by joint hypermobility, cigarette paper scars, lax skin and excessive bruising. As yet no gene has been linked to EDS II, nor has linkage been established to a specific region of the genome. However, several candidate genes encoding proteins of the extracellular matrix have been excluded. Using an intragenic simple sequence repeat polymorphism, we report linkage of the COL5A1 gene, which encodes the alpha 1(V) chain of type V collagen, to EDS II. A maximum LOD score (Zmax) for linkage of 8.3 at theta = 0.00 was generated for a single large pedigree.

A simple method for rapid isolation of microsatellites from yeast artificial chromosomes

Microsatellites are widely recognised as providing a rich source of polymorphic markers for genetic mapping. Consequently, highly polymorphic CA repeats tightly linked to a disease locus are invaluable tools in linkage studies. We have developed an efficient technique for cloning microsatellite repeats from a region of interest contained within a yeast artificial chromosome (YAC). The YAC material is digested with a frequent cutting restriction endonuclease and ligated to polymerase chain reaction (PCR) amplifiable catch-linkers. A 5' biotinylated (CA)11 oligonucleotide is then used to select fragments containing a complementary repeat by binding to streptavidin-coated magnetic beads. The catch-linkers enable these fragments to be PCR amplified, cloned and sequenced. Primers are then designed to amplify the repeat locus and to confirm its genomic localization and heterozygosity. We have successfully used this technique to clone a new (CA)18 microsatellite from a 360-kb YAC. The YAC contains the CYBB locus in Xp21.1 and is thought to contain at least part of the RP3 gene responsible for X-linked retinitis pigmentosa. This new CA repeat is highly polymorphic with nine alleles identified so far and a heterozygosity of 0.75.

Genetic analysis of a kindred with X-linked mental handicap and retinitis pigmentosa

A kindred is described in which X-linked nonspecific mental handicap segregates together with retinitis pigmentosa. Carrier females are mentally normal but may show signs of the X-linked retinitis pigmentosa carrier state and become symptomatic in their later years. Analysis of polymorphic DNA markers at nine loci on the short arm of the X chromosome shows that no crossing-over occurs between the disease and Xp11 markers DXS255, TIMP, DXS426, MAOA, and DXS228. The 90% confidence limits show that the locus is in the Xp21-q21 region. Haplotype analysis is consistent with the causal gene being located proximal to the Xp21 loci DXS538 and 5'-dystrophin on the short arm of the X chromosome. The posterior probability of linkage to the RP2 region of the X chromosome short arm (Xp11.4-p11.23) is .727, suggesting the possibility of a contiguous-gene-deletion syndrome. No cytogenetic abnormality has been identified.

Heterogeneity analysis in 40 X-linked retinitis pigmentosa families

Analysis of genetic heterogeneity in 40 kindreds with X-linked retinitis pigmentosa (XLRP), with 20 polymorphic markers, showed that significant heterogeneity is present (P = .001) and that 56% of kindreds are of RP3 type and that 26% are of RP2 type. The location of the RP3 locus was found to be 0.4 cM distal to OTC in the Xp21.1 region, and that of the RP2 locus was 6.5 cM proximal to DXS7 in Xp11.2-p11.3. Bayesian probabilities of linkage to RP2, RP3, or to neither locus were calculated. This showed that 20 of 40 kindreds could be assigned to one or the other locus, with a probability > .70 (14 kindreds with RP3 and 6 kindreds with RP2 disease). A further three kindreds were found to be unlinked to either locus, with a probability > .8. The remaining 17 kindreds could not be classified unambiguously. This highlights the difficulty of classifying families in the presence of genetic heterogeneity, where the two loci are separated by an estimated 16 cM.

Linkage analysis in a family with complete type congenital stationary night blindness with and without myopia

A family is described with X-linked congenital stationary night blindness of the complete type (CSNB1) in which clinical variation between affected males resulted in diagnostic difficulties. In two affected male cousins, one had congenital nystagmus and myopia, while the other was initially thought to have retinitis pigmentosa with optic atrophy and was hyperopic. The diagnosis of X-linked congenital stationary night blindness was established by clinical, psychophysical and electrophysiological criteria, and DNA markers flanking the CSNB1 locus were analysed in the family. The results show that both affected males have inherited the same haplotype from their carrier mothers, excluding the possibility that a myopia gene in linkage disequilibrium with CSNB1 has recombined with this locus.

Linkage analysis in X-linked congenital stationary night blindness

X-linked congenital stationary night blindness (XL-CSNB) is a nonprogressive disorder of the retina, characterized by night blindness, reduced visual acuity, and myopia. Previous studies have localized the CSNB1 locus to the region between OTC and TIMP on the short arm of the X chromosome. We have carried out linkage studies in three XL-CSNB families that could not be classified as either complete or incomplete CSNB on the criteria suggested by Miyake et al. (1986. Arch. Ophthalmol. 104: 1013-1020). We used markers for the DXS538, DMD, OTC, MAOA, DXS426, and TIMP loci. Two-point analyses show that there is close linkage between CSNB and MAOA (theta max = 0.05, Zmax = 3.39), DXS426 (theta max = 0.06, Zmax = 2.42), and TIMP (theta max = 0.07, Zmax = 2.04). Two multiply informative crossovers are consistent with CSNB lying proximal to MAOA and distal to DXS426, respectively. Multipoint analysis supports this localization, giving the most likely order as DMD-17 cM-MAOA-7.5 cM-CSNB-7.5 cM-DXS426/TIMP-cen, and thus refines the localization of CSNB.