Aniridia in man and Small eye in mice are semidominant developmental disorders caused by mutations within the paired box gene PAX6. Whereas heterozygotes suffer from iris hypoplasia, homozygous mice lack eyes and nasal cavities and exhibit brain abnormalities. To investigate the role of gene dosage in more detail, we have generated yeast artificial chromosome transgenic mice carrying the human PAX6 locus. When crossed onto the Small eye background, the transgene rescues the mutant phenotype. Strikingly, mice carrying multiple copies on a wild-type background show specific developmental abnormalities of the eye, but not of other tissues expressing the gene. Thus, at least five different eye phenotypes are associated with changes in PAX6 expression. We provide evidence that not only reduced, but also increased levels of transcriptional regulators can cause developmental defects.

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The pseudoautosomal region (PAR) is a segment of shared homology between the sex chromosomes. Here we report additional probes for this region of the mouse genome. Genetic and fluorescence in situ hybridization analyses indicate that one probe, PAR-4, hybridizes to the pseudoautosomal telomere and a minor locus at the telomere of chromosome 9 and that a PCR assay based on the PAR-4 sequence amplifies only the pseudoautosomal locus (DXYHgul). The region detected by PAR-4 is structurally unstable; it shows polymorphism both between mouse strains and between animals of the same inbred strain, which implies an unusually high mutation rate. Variation occurs in the region adjacent to a (TTAGGG). array. Two pseudoautosomal probes can also hybridize to the distal telomeres of chromosomes 9 and 13, and all three telomeres contain DXYMov1S. The similarity between these telomeres may reflect ancestral telomere-telomere exchange.Pairing and exchange of DNA between the human X and Y chromosomes, which are largely different in sequence, is restricted to two small telomeric regions of homology termed pseudoautosomal regions (PARs) (1). Genetic analyses indicate that an obligatory crossover occurs in male meiosis within the main PAR (2). Failure to form the X-Y chiasma can disrupt progress of the entire chromosome set through the subsequent stages of segregation; mice with unpaired sex chromosomes show developmental arrest at metaphase I and subsequent spermatocyte degeneration (2, 3). The main human PAR extends 2.6 Mb from the short-arm telomere (1). The obligatory crossover within this PAR causes genetic markers located at the Xp/Yp telomere to show 50% recombination with sex in male meiosis (1). Occasional exchange occurs at a second human PAR found at the Xq/Yq telomere (4, 5).The mouse PAR, located at the distal telomere, is poorly understood at the molecular level. Although its cytological behavior and role in hybrid sterility have been extensively analyzed (2), only two DNA probes for this region have been reported. The first is the (TTAGGG), telomere repeat, which detects restriction fragment length polymorphisms that map to the PAR (6, 7). The second probe was isolated from the Movl5 transgenic mouse, which has a single Moloney murine leukemia virus integrated in the PAR (8). Proviral DNA is transferred from the X to Y chromosomes in 10-20% of male meioses, suggesting an integration site close to the PAR boundary. A sequence flanking the virus integration site, DXYMovl5, has been cloned (9) and is a tandem repeat array of 31-bp monomer units. It is detected by the probe pMovl5/1, which hybridizes to the distal telomeres of chromosomes X, Y, 4, 9, and 13 (9-12). The sex-reversed (Sxr) mutation (13), which involves a translocation of the male-determining region

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The low-affinity leukaemia inhibitory factor receptor (LIF-R) is a component of cell-surface receptor complexes for the multifunctional cytokines leukaemia inhibitory factor, ciliary neurotrophic factor, oncostatin M and cardiotrophin-1. Both soluble and transmembrane forms of the protein have been described and several LIF-R mRNAs have been reported previously. In order to determine the coding potential of LIF-R mRNAs we have isolated and characterized the mouse LIF-R gene. mRNA encoding soluble LIF-R (sLIF-R) is formed by inclusion of an exon in which polyadenylation signals are provided by a B2 repeat. This exon is located centrally within the LIF-R gene but is excluded from the transmembrane LIF-R mRNA by alternative splicing. The transmembrane receptor is encoded by 19 exons distributed over 38 kb. Two distinct 5' non-coding exons have been identified, indicating the existence of alternative promoters. One of these is G/C rich and possesses a consensus initiator sequence as well as potential Sp1 binding sites. Expression of exon 1 from this promoter occurs in a wide variety of tissues, whereas expression of the alternative 5' untranslated region (exon 1a) is normally restricted to liver, the principal source of sLIF-R. During pregnancy expression of exon 1a becomes detectable also in the uterus. Expression of exon 1a increases dramatically during gestation and is accompanied by a similar quantitative rise in expression of sLIF-R mRNA. These findings establish that expression of LIF-R is under complex transcriptional control and indicate that regulated expression of the soluble cytokine receptor isoform may be due principally to an increase in the activity of a dedicated promoter.

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We describe the characterisation of three gene trap integrations in embryonic stem cells in which the lacZ reporter gene is repressed by retinoic acid (RA) in vitro and is expressed in the developing heart in vivo. In one of these, the gene trap vector has integrated into a gene that is located on chromosome 17 and is homologous to the human transcription factor gene, TFEB. Embryonic and adult cardiac expression of both the fusion transcript and the endogenous gene was confirmed. However, we show that the integration has not resulted in a null allele, because wild type transcripts, possibly resulting from splicing around the vector, are observed in homozygous tissue. The other two cardiac‐expressing gene trap integrations have occurred into exons on chromosomes 1 and 5 and have used cryptic donor sites within the vector to generate functional fusion transcripts. One of these exon integrations results in a lethal neonatal phenotype. Dev. Dyn. 1998;212:267–276. © 1998 Wiley‐Liss, Inc.