Inhibition of cardiac delayed rectifier K+ current by overexpression of the long-QT syndrome HERG G628S mutation in transgenic mice.

Mutations in the HERG gene are linked to the LQT2 form of the inherited long-QT syndrome. Transgenic mice were generated expressing high myocardial levels of a particularly severe form of LQT2-associated HERG mutation (G628S). Hearts from G628S mice appeared normal except for a modest enlargement seen only in females. Ventricular myocytes isolated from adult wild-type hearts consistently exhibited an inwardly rectifying E-4031-sensitive K+ current resembling the rapidly activating cardiac delayed rectifier K+ current (Ikr) in its time and voltage dependence; this current was not found in cells isolated from G628S mice. Action potential duration was significantly prolonged in single myocytes from G628S ventricle (cycle length=1 second, 26 degrees C) but not in recordings from intact ventricular strips studied at more physiological rates and temperature (200 to 400 bpm, 37 degrees C). ECG intervals, including QT duration, were unchanged, although minor aberrancies were noted in 20% (16/80) of the G628S mice studied, primarily involving the QRS complex and, more rarely, T-wave morphology. The aberrations were more commonly observed in females than males but could not be correlated with sex-based differences in action potential duration. These results establish the presence of IKr in the adult mouse ventricle and demonstrate the ability of the G628S mutation to exert a dominant negative effect on endogenous IKr in vivo, leading to the expected LQT2 phenotype of prolonged repolarization at the single cell level but not QT prolongation in the intact animal. The model may be useful in dissecting repolarization currents in the mouse heart and as a means of examining the mechanism(s) by which the G628S mutation exerts its dominant negative effect on native cardiac cells in vivo.

Integration of physical, breakpoint and genetic maps of chromosome 22. Localization of 587 yeast artificial chromosomes with 238 mapped markers.

Detailed physical maps of the human genome are important resources for the identification and isolation of disease genes and for studying the structure and function of the genome. We used data from STS content mapping of YACs and natural and induced chromosomal breakpoints to anchor contigs of overlapping yeast artificial chromosome (YAC) clones spanning extensive regions of human chromosome 22. The STSs were assigned to specific regions (bins) on the chromosome using cell lines from a somatic hybrid mapping panel defining a maximum of 25 intervals. YAC libraries were screened by PCR amplification of hierarchical pools of yeast DNA with 238 markers, and a total of 587 YAC clones were identified. These YACs were assembled into contigs based upon their shared STS content using a simulated annealing algorithm. Fifteen contigs, containing between 2 and 74 STSs were assembled, and ordered along the chromosome based upon the cytogenetic breakpoint, meiotic and PFG maps. Additional singleton YACs were assigned to unique chromosomal bins. These ordered YAC contigs will be useful for identifying disease genes and chromosomal breakpoints by positional cloning and will provide the foundation for higher resolution physical maps for large scale sequencing of the chromosome.

A YAC contig of the region containing the spinal muscular atrophy gene (SMA): identification of an unstable region.

We report a 3.0-Mb YAC contig of the region 5q11.2-q13.3, which is where the spinal muscular atrophy gene has been localized. Three total genomic YAC libraries were screened by the polymerase chain reaction (PCR), and 45 YACs were recovered. These YACs were characterized for sequence tag site (STS) content, and overlaps were confirmed by vectorette PCR. Of the 45 YACs, 20 were isolated with the polymorphic marker CATT-1, which demonstrates significant allelic association with the SMA gene and maps within the 850-kb interval defined by the markers D5S557 and D5S823. Haplotyping of these YACs and their mother cell line indicates that the majority of YACs from this region contain deletions. Furthermore, a 1.9-Mb CATT-1 YAC that was negative for MAP1B and D5S435 and nonchimeric by FISH analysis provides a minimum distance between MAP1B and D5S435.

Universal signals control slime mold stalk formation.

The primitive slime mold Dictyostelium minutum does not display oscillations during aggregation, cannot form migrating slugs, and does not form a prestalk/prespore pattern, all of which are characteristic for development of its advanced relative Dictyostelium discoideum. We used D. minutum to investigate whether slime molds share common mechanisms controlling development. In D. discoideum, the morphogen differentiation inducing factor (DIF) can induce stalk-cell differentiation in vitro. However, stalk formation in vivo is supposedly triggered by local depletion of DIF antagonists such as ammonia or cAMP. A homologue of the D. discoideum stalk gene ecmB was cloned in D. minutum that encodes a 3.4-kb mRNA, and its deduced amino acid sequence shows repeats of 24 amino acids that are characteristic for the D. discoideum ecmB gene. Remarkably, DIF effectively induces expression of the D. minutum ecmB gene and ammonia inhibits its expression. D. discoideum cells were transformed with a construct of the D. minutum ecmB promoter fused to the lacZ reporter gene and showed expression in the stalk, but not in the upper and lower cup of the fruiting body, which also express the D. discoideum ecmB gene. In D. discoideum, the D. minutum ecmB and the ecmB promoter are similarly activated by DIF and repressed by both cAMP and ammonia, suggesting that additional signaling is required for ecmB expression in upper and lower cup cells. Our data indicate that the extracellular signals controlling stalk formation and their intracellular signaling cascades including gene regulatory proteins remained highly conserved during slime mold evolution.

A yeast artificial chromosome contig spanning the Charcot-Marie-Tooth disease type 1A duplication region.

A contiguous set of 43 overlapping yeast artificial chromosome (YAC) clones has been developed for the Charcot-Marie-Tooth disease type 1A (CMT1A) duplication region of chromosome 17p11.2. The contig spans approximately 2.0 Mb and can be represented in a minimum of five overlapping YACs. The YAC clones were isolated from two total human genomic YAC libraries and from YAC libraries made from rodent-human hybrid cell lines. YAC clones were isolated from the libraries by polymerase chain reaction (PCR) technique. Localization to chromosome 17p11.2 was confirmed by fluorescence in situ hybridization. Overlap between the YAC clones was detected by inter-Alu PCR amplification of the YACs and by cross hybridization of the YACs with YAC insert ends obtained by Vectorette PCR. This YAC contig is a useful resource for analyzing and mapping all the genes contained within the CMT1A duplication.

The peripheral myelin protein gene PMP-22 is contained within the Charcot-Marie-Tooth disease type 1A duplication.

Charcot-Marie-Tooth disease (CMT1) is the most common form of inherited peripheral neuropathy. Although the disease is genetically heterogeneous, it has been demonstrated that the gene defect is the most frequent type (CMT1A) is the result of a partial duplication of band 17p11.2. Recent studies suggested that the peripheral hypomyelination syndrome in the trembler (Tr) mouse, a possible animal model for CMT1 disease, is associated with a point mutation in the peripheral myelin protein-22 gene (pmp-22). Expression of pmp-22 is particularly high in Schwann cells, and the protein is found in peripheral myelin. We now report that the human PMP-22 gene is contained within the CMT1A duplication. We therefore, suggest that increased dosage of the PMP-22 gene may be the cause of CMT1A neuropathy.