Molecular Genetics of Mammals

Recent advances in molecular genetics of mammals,including construction of genetic linkage maps, identification of mutations responsible for inheritable diseases, mapping of quantitative trait loci,and findings about sex determination are reviewed in the present paper. Information about genomes of humans,laboratory animals and domestic animals has been rapidly accumulated in the last decade. Establishment of polymorphic linkage markers and construction of chromosomal linkage maps of mammalian species enable us to identify the genes responsible for particular genetic traits. Various mutations have been found in genes responsible for human inheritable diseases. We have constructed genetic linkage maps for rats and identified a mutation responsible for mucopoly saccharidosis in rats. Mapping of loci affecting the quantitative traits of domestic animals, including growth rate, litter size, and milk production, as well as identification of the genes responsible for inheritable diseases of domestic animals, are also being carried out.Recently, the testis-determining gene on the Y chromosome, which determine the sex of mammals by defferentiating the testis in the embryos, have been identified in humans and mice. Functions of the sex-determining gene in sexual differentiation of the mammalian embryos have been intensively analyzed. We have found a sexual difference in the growth rate of mouse preimplantation embryos. An application of the recent findings in the molecular genetics of mammals will contribute to animal science and agricultural science.近年の分子生物学の発展は、ヒトを含む哺乳類の遺伝学にも大きな変化をもたらしている。すなわち、表現型を主な対象とした従来の遺伝学から、DNAをはじめとする分子を対象とした遺伝学へとその中心は移行しつつある。この傾向に拍車をかけたのがここ数年話題になることの多いヒトゲノムプロジェクトであり、またヒトの遺伝病の原因遺伝子の解析と遺伝子診断、遺伝子治療の進展である。農学や畜産学の分野でも、分子育種という概念が確立しつつあり、本論文ではヒト、家畜および実験動物を対象とした哺乳類の分子遺伝学の最近の動向について、我々の研究結果を含めて概説したい

Characterization of the chromosomal inversion associated with the Koa mutation in the mouse revealed the cause of skeletal abnormalities

<p>Abstract</p> <p>Background</p> <p>Koala (<it>Koa</it>) is a dominant mutation in mice causing bushy muzzle and pinna, and is associated with a chromosomal inversion on the distal half of chromosome 15. To identify the gene responsible for the <it>Koa </it>phenotypes, we investigated phenotypes of <it>Koa </it>homozygous mice and determined the breakpoints of the inversion with a genetic method using recombination between two different chromosomal inversions.</p> <p>Results</p> <p>Skeletal preparation of <it>Koa </it>homozygotes showed marked deformity of the ribs and a wider skull with extended zygomatic arches, in addition to a general reduction in the lengths of long bones. They also had open eyelids at birth caused by a defect in the extension of eyelid anlagen during the embryonic stages. The proximal and distal breakpoints of the <it>Koa </it>inversion were determined to be 0.8-Mb distal to the <it>Trsps1 </it>gene and to 0.1-Mb distal to the <it>Hoxc4 </it>gene, respectively, as previously reported. The phenotypes of mice with the recombinant inverted chromosomes revealed the localization of the gene responsible the <it>Koa </it>phenotype in the vicinity of the proximal recombinant breakpoint. Expression of the <it>Trsps1 </it>gene in this region was significantly reduced in the <it>Koa </it>homozygous and heterozygous embryos.</p> <p>Conclusion</p> <p>While no gene was disrupted by the chromosomal inversion, an association between the <it>Koa </it>phenotype and the proximal recombinant breakpoint, phenotypic similarities with <it>Trps1</it>-deficient mice or human patients with <it>TRSP1 </it>mutations, and the reduced expression of the <it>Trsps1 </it>gene in <it>Koa </it>mice, indicated that the phenotypes of the <it>Koa </it>mice are caused by the altered expression of the <it>Trps1 </it>gene.</p

Elevated fibroblast growth factor signaling is critical for the pathogenesis of the dwarfism in Evc2/Limbin mutant mice

Ellis-van Creveld (EvC) syndrome is a skeletal dysplasia, characterized by short limbs, postaxial polydactyly, and dental abnormalities. EvC syndrome is also categorized as a ciliopathy because of ciliary localization of proteins encoded by the two causative genes, EVC and EVC2 (aka LIMBIN). While recent studies demonstrated important roles for EVC/EVC2 in Hedgehog signaling, there is still little known about the pathophysiological mechanisms underlying the skeletal dysplasia features of EvC patients, and in particular why limb development is affected, but not other aspects of organogenesis that also require Hedgehog signaling. In this report, we comprehensively analyze limb skeletogenesis in Evc2 mutant mice and in cell and tissue cultures derived from these mice. Both in vivo and in vitro data demonstrate elevated Fibroblast Growth Factor (FGF) signaling in Evc2 mutant growth plates, in addition to compromised but not abrogated Hedgehog-PTHrP feedback loop. Elevation of FGF signaling, mainly due to increased Fgf18 expression upon inactivation of Evc2 in the perichondrium, critically contributes to the pathogenesis of limb dwarfism. The limb dwarfism phenotype is partially rescued by inactivation of one allele of Fgf18 in the Evc2 mutant mice. Taken together, our data uncover a novel pathogenic mechanism to understand limb dwarfism in patients with Ellis-van Creveld syndrome

Positional cloning of the gene LIMBIN responsible for bovine chondrodysplastic dwarfism

Chondrodysplastic dwarfism in Japanese brown cattle is an autosomal recessive disorder characterized by short limbs. Previously, we mapped the locus responsible for the disease on the distal end of bovine chromosome 6. Here, we narrowed the critical region to ≈2 cM by using linkage analysis, constructed a BAC and YAC contig covering this region, and identified a gene, LIMBIN (LBN), that possessed disease-specific mutations in the affected calves. One mutation was a single nucleotide substitution leading to an activation of a cryptic splicing donor site and the other was a one-base deletion resulting in a frameshift mutation. Strong expression of the Lbn gene was observed in limb buds of developing mouse embryos and in proliferating chondrocytes and bone-forming osteoblasts in long bones. These findings indicate that LBN is responsible for bovine chondrodysplastic dwarfism and has a critical role in a skeletal development

Refining the evolutionary tree of the horse Y chromosome

The Y chromosome carries information about the demography of paternal lineages, and thus, can prove invaluable for retracing both the evolutionary trajectory of wild animals and the breeding history of domesticates. In horses, the Y chromosome shows a limited, but highly informative, sequence diversity, supporting the increasing breeding influence of Oriental lineages during the last 1500 years. Here, we augment the primary horse Y-phylogeny, which is currently mainly based on modern horse breeds of economic interest, with haplotypes (HT) segregating in remote horse populations around the world. We analyze target enriched sequencing data of 5 Mb of the Y chromosome from 76 domestic males, together with 89 whole genome sequenced domestic males and five Przewalski's horses from previous studies. The resulting phylogeny comprises 153 HTs defined by 2966 variants and offers unprecedented resolution into the history of horse paternal lineages. It reveals the presence of a remarkable number of previously unknown haplogroups in Mongolian horses and insular populations. Phylogenetic placement of HTs retrieved from 163 archaeological specimens further indicates that most of the present-day Y-chromosomal variation evolved after the domestication process that started around 4200 years ago in the Western Eurasian steppes. Our comprehensive phylogeny significantly reduces ascertainment bias and constitutes a robust evolutionary framework for analyzing horse population dynamics and diversity