A 360-kb interchromosomal duplication of the human HYDIN locus.

The HYDIN gene located in human chromosome band 16q22.2 is a large gene encompassing 423 kb of genomic DNA that has been suggested as a candidate for an autosomal recessive form of congenital hydrocephalus. We have found that the human HYDIN locus has been very recently duplicated, with a nearly identical 360-kb paralogous segment inserted on chromosome 1q21.1. The duplication, among the largest interchromosomal segmental duplications described in humans, is not accounted for in the current human genome assembly and appears to be part of a greater than 550-kb contig that must lie within 1 of the 11 sequence gaps currently remaining in 1q21.1. Both copies of the HYDIN gene are expressed in alternatively spliced transcripts. Elucidation of the role of HYDIN in human disease susceptibility will require careful discrimination among the paralogous copies.

Recombineered Xenopus tropicalis BAC expresses a GFP reporter under the control of Arx transcriptional regulatory elements in transgenic Xenopus laevis embryos.

The aristaless-related homeobox (Arx) gene is expressed in a dynamic pattern in the developing vertebrate forebrain. We identified a bacterial artificial chromosome (BAC) containing the Xenopus tropicalis Arx gene and replaced a portion of the first coding exon with a green fluorescent protein (GFP) expression cassette by homologous recombination in bacteria (recombineering). Transgenic X. laevis embryos obtained by microinjecting the modified BAC expressed GFP in the developing forebrain in a pattern identical to that of the endogenous Arx gene. Thus, this BAC contains transcriptional regulatory elements necessary for regulating proper expression in transgenic frogs. This work demonstrates that transgenesis using recombineered BACs is a viable technique in Xenopus.

Neural precursors express multiple chondroitin sulfate proteoglycans, including the lectican family.

Chondroitin sulfate proteoglycans (CSPGs) abnormally accumulate in cerebrospinal fluid (CSF) of both human neonates with preterm hydrocephalus, and P8 hydrocephalic mice. We hypothesized CSF CSPGs are synthesized by neural precursors, separated from ventricular CSF by ependyma, which is often disrupted in hydrocephalus. Western blotting demonstrates that neural precursors cultured as neurospheres secrete CSPGs (> 30 microg/ml) into their media which appear to be very similar to these CSF CSPGs. Some CSPGs bear the stage-specific embryonic antigen-1 (ssea-1), associated with embryonic/neural stem cells. Neurospheres transcribe many CSPG genes, including the entire aggrecan/lectican family, phosphacan, and tenascin. Phosphacan can be detected in media by Western blotting. Aggrecan can be detected in media after purification using hyaluronic acid affinity chromatography. During differentiation, neurospheres downregulate CSPGs. This is the first report to show that proliferating neural precursors synthesize lecticans, including aggrecan, which are downregulated with differentiation. These observations suggest novel links between CSPGs and CNS precursor biology.

Congenital hydrocephalus in hy3 mice is caused by a frameshift mutation in Hydin, a large novel gene.

The autosomal-recessive mutation hydrocephalus3 (hy3) results in lethal communicating hydrocephalus with perinatal onset. We recently described a hydrocephalus-inducing transgenic insertional mutation, OVE459, which represents a new allele of hy3. Direct cDNA selection performed on a wild-type mouse BAC clone spanning the OVE459 insertion locus on chromosome 8 led to the identification of two novel candidate genes, Hydin and Vac14. The transgene insertion resulted in a rearrangement of Hydin exons in OVE459 mice. Hydin consists of at least 86 exons spanning over 340 kb of genomic DNA. The full-length Hydin transcript is nearly 16 kb, encoding a putative 5099 amino acid protein. Northern analysis revealed a marked reduction of Hydin mRNA in both OVE459 and hy3 homozygotes relative to wild-type littermates. A single CG base-pair deletion in exon 15 of Hydin was discovered specifically in mice carrying the spontaneous hy3 mutant allele. This deletion creates a premature termination signal two codons downstream of the mutation, likely resulting in the loss of 89% of the full-length gene product. Within the neonatal brain, Hydin expression is confined to the ciliated ependymal cell layer lining the lateral, third and fourth ventricles. Other sites of Hydin expression include the ciliated epithelial cells lining the bronchi and oviduct, as well as in the developing spermatocytes in the testis. The Hydin gene product is not closely related to any previously identified protein, with the exception of a 314 amino acid domain with homology to caldesmon, an actin-binding protein, suggesting an interaction with the cytoskeleton.

Genetic mapping of an insertional hydrocephalus-inducing mutation allelic to hy3.

The transgenic mouse line OVE459 carries a transgene-induced insertional mutation resulting in autosomal recessive congenital hydrocephalus. Homozygous transgenic animals experience ventricular dilation with perinatal onset and are noticeably smaller than hemizygous or non-transgenic littermates within a few days after birth. Fluorescence in situ hybridization (FISH) revealed that the transgene inserted in a single locus on mouse Chromosome (chr) 8, region D2-E1. Genetic crosses between hemizygous OVE459 mice and mice heterozygous for the spontaneous mutation hydrocephalus-3 (hy3) produced hydrocephalic offspring with a frequency of 22%, demonstrating that these two mutations are allelic. A genomic library was made by using DNA from homozygous OVE459 mice, and genomic DNA flanking the transgene insertion site was isolated and sequenced. A PCR polymorphism between C57BL/6 DNA and Mus spretus was used to map the location of the transgene insert to 1.06 cM +/- 0.75 proximal to D8Mit152 by using the Jackson Laboratory Backcross DNA Panel Mapping Resource. Furthermore, sequence analysis from a mouse bacterial artificial chromosome (BAC) clone, positive for unique markers on both sides of the transgene insertion site, demonstrated that the genomic DNAs flanking each side of the transgene insertion are physically separated by approximately 51 kb on the wild-type mouse chromosome.