Synteny on mouse chromosome 5 of homologs for human DNA loci linked to the Huntington disease gene

Comparative mapping in man and mouse has revealed frequent conservation of chromosomal segments, offering a potential approach to human disease genes via their murine homologs. Using DNA markers near the Huntington disease gene on the short arm of chromosome 4, we defined a conserved linkage group on mouse chromosome 5. Linkage analyses using recombinant inbred strains, a standard outcross, and an interspecific backcross were used to assign homologs for five human loci, D4S43, D4S62, QDPR, D4S76, and D4S80, to chromosome 5 and to determine their relationships with previously mapped markers for this autosome. The relative order of the conserved loci was preserved in a linkage group that spanned 13% recombination in the interspecific backcross analysis. The most proximal of the conserved markers on the mouse map, D4S43h, showed no recombination with Emv-1, an endogenous ecotropic virus, in 84 outcross progeny and 19 recombinant inbred strains. Hx, a dominant mutation that causes deformities in limb development, maps approximately 2 cM proximal to Emv-1. Since the human D4S43 locus is less than 1 cM proximal to HD near the telomere of chromosome 4, the murine counterpart of the HD gene might lie between Hx and Emv-1 or D4S43h. Cloning of the region between these markers could generate new probes for conserved human sequences in the vicinity of the HD gene or possibly candidates for the murine counterpart of this human disease locus.     

Mapping of body weight loci on mouse Chromosome X

Inheritance of overweight in humans appears to be under polygenic control. Study on the mouse model may help to determine candidate regions in human genome for the search of overweight genes. Inbred mouse strains showed wide variation in body weight and can provide an experimental model for the study of inheritance of overweight. By genetic linkage analysis, we report the mapping of two loci, named Bw1 and Bw2 (body weight 1 and 2), on Chromosome (Chr) X that strongly affect adult body weight in two interspecific testcross male populations (HSB and ASB) of mice. In addition, another locus, named Bw3, is also mapped on Chr X in ASB populations. These loci account for up to 24% of the phenotypic variation in both populations. Considering the conserved synteny between mouse and human Chr X, these results provide candidate regions on Chr X that can be tested for linkage with overweight in humans.     

A genetic linkage map of rat Chromosome 5 reveals extensive linkage conservation with mouse Chromosome 4

Linkages among three biochemical loci (Acol, Ahd2, and Mup1) and four microsatellite loci (A8, Glut1, Jun, and Pnd) were determined to construct a linkage map of rat Chromosome (Chr) 5. Consequently, an extensive linkage map on rat Chr 5 was constructed with the following gene order: A8-Aco1-Mup1-Jun-Glut1-Ahd2-Pnd. In this linkage map, the Jun and A8 loci are newly placed, and two previously reported linkage groups on rat Chr 5 are connected by the Jun locus. The linkage map indicates an extensive linkage conservation between the loci on rat Chr 5 and those on mouse Chr 4.     

Localization of thereeler gene relative to flanking loci on mouse chromosome 5

The location of thereeler (rl) locus in mice in the paracentromeric part of chromosome (Chr) 5, proximal to theT(5;12)31H translocation breakpoint, has been confirmed. Analysis of DNA from animals with different doses of the proximal part of Chr 5 and from congenic mice showed that thePgy-1 locus is the closest marker torl, whereasEn-2 is located farther, distal to theT31H breakpoint. Together with recently published evidence (Martin et al. 1989), our data suggest the following order:Cen-rl/Pgy-1-T31H-En-2.     

Chromosome assignment of the owl monkey MOS and MYC gene loci

Southern blot hybridizations of rodent x owl monkey hybrid DNAs with human cDNA probes allowed the mapping of the MOS and MYC gene loci to owl monkey chromosome 16 of karyotype VI (2n = 49 male/50 female) and to the homologous chromosome 15 of karyotype V (2n = 46). Synteny of MOS and MYC gene loci in both man and owl monkey suggests this chromosome segment's conservation in primates, contrasting with its disruption in the mouse.     

Localization of the bronx waltzer (bv) deafness gene to mouse Chromosome 5

The bronx waltzer (bv) mutation is an autosomal recessive mutation that is manifested as head tossing and circling in the mouse. The mutation affects the inner hair cells (IHCs) and pillar cells in the organ of Corti of the cochlea and the maculae and cristae of the vestibular part of the inner ear. IHCs begin to degenerate by a controlled mechanism of cell death as early as gestational day 17 (G17) in the basal coil of the cochlea, and few surviving IHCs are seen in the adult. As a first step towards the identification of bv, we analyzed a total of 20 loci in 118 mice from an intraspecific backcross giving the gene order: centromere–D5Mit1–D5Mit73–D5Mit55–[D5Mit12, Nds4 (Afp)]–D5Mit87–[D5Mit205, 20, 88, 208, 93–D5Mit338]–D5Mit25–D5Mit209–bv–D5Mit188–D5Mit367–D5Mit95–D5Mit43–D5Mit102. A total of 701 mice were then analyzed for the markers D5Mit93 and D5Mit95, defining a region of 12.08 cM flanking bv. Mice that were recombinant between D5Mit93 and D5Mit95 were analyzed for D5Mit338, D5Mit25, D5Mit209, bv, D5Mit188, and D5Mit367. bv maps 0.14 cM distal of the marker D5Mit209 and 1.14 cM proximal of the marker D5Mit188 in 701 backcross progeny. Received: 3 March 1997 / Accepted: 30 May 1997     

Early alterations in gene expression and cell morphology in a mouse model of Huntington's disease

Several mouse models for Huntington's disease (HD) have been produced to date. Based on differences in strain, promoter, construct, and number of glutamines, these models have provided a broad spectrum of neurological symptoms, ranging from simple increases in aggressiveness with no signs of neuropathology, to tremors and seizures in absence of degeneration, to neurological symptoms in the presence of gliosis and TUNEL (terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling) positivity, and finally to selective striatal damage associated with electrophysiological and behavioral abnormalities. We decided to analyze the morphology of striatum and hippocampus from a mouse transgenic line obtained by microinjection of exon 1 from the HD gene after introduction of a very high number of CAG repeat units. We found a massive darkening and compacting of striatal and hippocampal neurons in affected mice, associated with a lower degree of more classical apoptotic cell condensation. We then explored whether this morphology could be explained with alterations in gene expression by hybridizing normal and affected total brain RNA to a panel of 588 known mouse cDNAs. We show that some genes are significantly and consistently up-regulated and that others are down-regulated in the affected brains. Here we discuss the possible significance of these alterations in neuronal morphology and gene expression.     

Efficient repetitive alteration of the mouse Huntington's disease gene by management of background in the tag and exchange gene targeting strategy

The introduction of subtle mutations to predetermined locations in the mouse genome has aided in the assessment of gene function and the precise modeling of inherited disorders. Subtle mutations can be engineered into the mouse genome by the tag and exchange gene targeting strategy (Askew et al., 1993; Stacey et al., 1994; Wu et al., 1994). This two-step method involves both a positive and a negative selection. The negative selection step typically generates a large amount of undesired background that may prevent the practical recovery of gene targeted clones (Vazquez et al., 1998). In this work we describe a strategy to effectively manage this background by calculation of a tolerable level of background for a specific targeting event, pre-screening for clones with low background, subcloning and growth of cell lines under selection. This strategy was used to repeatedly and efficiently alter the mouse Huntington's disease homologue (Hdh) resulting in an average of 15 percent of the clones having the desired modification. Analysis of the remaining background clones showed they arose de novo by a mechanism that involved physical loss of the marker rather than mutation or inactivation. We calculated the rate of loss of this marker as 8.3×10^–6 events/cell/generation. We further show that the exchanged clones retained the capacity to contribute to the mouse germline demonstrating the utility of this strategy in the production of mouse lines with Hdh variants.     

Evolutionary conservation and selection of human disease gene orthologs in the rat and mouse genomes

Background

Model organisms have contributed substantially to our understanding of the etiology of human disease as well as having assisted with the development of new treatment modalities. The availability of the human, mouse and, most recently, the rat genome sequences now permit the comprehensive investigation of the rodent orthologs of genes associated with human disease. Here, we investigate whether human disease genes differ significantly from their rodent orthologs with respect to their overall levels of conservation and their rates of evolutionary change.

Results

Human disease genes are unevenly distributed among human chromosomes and are highly represented (99.5%) among human-rodent ortholog sets. Differences are revealed in evolutionary conservation and selection between different categories of human disease genes. Although selection appears not to have greatly discriminated between disease and non-disease genes, synonymous substitution rates are significantly higher for disease genes. In neurological and malformation syndrome disease systems, associated genes have evolved slowly whereas genes of the immune, hematological and pulmonary disease systems have changed more rapidly. Amino-acid substitutions associated with human inherited disease occur at sites that are more highly conserved than the average; nevertheless, 15 substituting amino acids associated with human disease were identified as wild-type amino acids in the rat. Rodent orthologs of human trinucleotide repeat-expansion disease genes were found to contain substantially fewer of such repeats. Six human genes that share the same characteristics as triplet repeat-expansion disease-associated genes were identified; although four of these genes are expressed in the brain, none is currently known to be associated with disease.

Conclusions

Most human disease genes have been retained in rodent genomes. Synonymous nucleotide substitutions occur at a higher rate in disease genes, a finding that may reflect increased mutation rates in the chromosomal regions in which disease genes are found. Rodent orthologs associated with neurological function exhibit the greatest evolutionary conservation; this suggests that rodent models of human neurological disease are likely to most faithfully represent human disease processes. However, with regard to neurological triplet repeat expansion-associated human disease genes, the contraction, relative to human, of rodent trinucleotide repeats suggests that rodent loci may not achieve a 'critical repeat threshold' necessary to undergo spontaneous pathological repeat expansions. The identification of six genes in this study that have multiple characteristics associated with repeat expansion-disease genes raises the possibility that not all human loci capable of facilitating neurological disease by repeat expansion have as yet been identified.     

Human and mouse amelogenin gene loci are on the sex chromosomes

Enamel is the outermost covering of teeth and is the hardest tissue in the vertebrate body. The enamel matrix is composed of enamelin and amelogenin classes of protein. We have determined the chromosomal locations for the human and mouse amelogenin (AMEL) loci using Southern blot analyses of DNA from human, mouse, or somatic cell hybrids by hybridization to a characterized mouse amelogenin cDNA. We have determined that human AMEL sequences are located on the distal short arm of the X chromosome in the p22.1----p22.3 region and near the centromere on the Y chromosome, possibly at the proximal long arm (Yq11) region. These chromosomal assignments are consistent with the hypothesis that perturbation of the amelogenin gene is involved in X-linked types of amelogenesis imperfecta, as well as with the Y-chromosomal locations for genes that participate in regulating tooth size and shape. Unlike the locus in humans, the mouse AMEL locus appears to be assigned solely to the X chromosome. Finally, together with the data on other X and Y chromosome sequences, these data for AMEL mapping support the notion of a pericentric inversion occurring in the human Y chromosome during primate evolution.