Generation of a Twist1 conditional null allele in the mouse

Twist1 is the mouse ortholog of TWIST1, the human gene mutated in Saethre-Chotzen syndrome. Previously, a Twist1 null allele was generated by gene targeting in mouse embryonic stem cells. Twist1 heterozygous mice develop polydactyly and a craniofacial phenotype similar to Saethre-Chotzen patients. Mice homozygous for the Twist1 null allele die around embryonic day 11.5 (E11.5) with cranial neural tube closure and vascular defects, hindering in vivo studies of Twist1 function at later stages of development. Here, we report the generation of a Twist1 conditional null allele in mice that functions like a wild-type allele but can be converted to a null allele upon Cre-mediated recombination.     

Generation of a conditional null allele for Dmp1 in mouse

Dentin matrix protein1 (DMP1), highly conserved in humans and mice, is highly expressed in teeth, the skeleton, and to a lesser extent in nonskeletal tissues such as brain, kidney, and salivary gland. Pathologically, DMP1 is associated with several forms of cancers and with tumor-induced osteomalacia. Conventional disruption of the murine Dmp1 gene results in defects in dentin in teeth and in the skeleton, including hypophosphatemic rickets, and abnormalities in phosphate homeostasis. Human DMP1 mutations are responsible for the condition known as autosomal recessive hypophosphatemic rickets. For better understanding of the roles of DMP1 in different tissues at different stages of development and in pathological conditions, we generated Dmp1 floxed mice in which loxP sites flank exon 6 that encodes for over 80% of DMP1 protein. We demonstrate that Cre-mediated recombination using Sox2-Cre, a Cre line expressed in epiblast during early embryogenesis, results in early deletion of the gene and protein. These homozygous Cre-recombined null mice display an identical phenotype to conventional null mice. This animal model will be useful to reveal distinct roles of DMP1 in different tissues at different ages.     

Generation of mice with a conditional Stat1 null allele

Interferons (IFNs) are key cytokines in the innate immune response that also bridge the gap to adaptive immunity. Signaling upon stimulation by IFN type I, II and III is mediated by the Jak-Stat pathway. STAT1 is activated by all three IFN receptor complexes and absence of STAT1 from mice increases their susceptibility to pathogens. In addition, depending on the setting, STAT1 can act as tumor suppressor or oncogene. Here we report the generation and detailed functional characterization of a conditional Stat1 knockout mouse. We show the integrity of the conditional Stat1 locus and report successful in vivo deletion by means of a ubiquitous and a tissue-specific Cre recombinase. The conditional Stat1 null allele represents an important tool for identifying novel and cell-autonomous STAT1 functions in infection and cancer.     

New ataxic tottering-6j mouse allele containing a cacna1a gene mutation

Voltage-gated Ca(2+) (Ca(v)) channels control neuronal functions including neurotransmitter release and gene expression. The Cacna1a gene encodes the α1 subunit of the pore-forming Ca(v)2.1 channel. Mice with mutations in this gene form useful tools for defining channel functions. The recessive ataxic tottering-6j strain that was generated in the Neuroscience Mutagenesis Facility at The Jackson Laboratory has a mutation in the Cacna1a gene. However, the effect of this mutation has not been investigated in detail. In this study, mutation analysis shows a base substitution (C-to-A) in the consensus splice acceptor sequence linked to exon 5, which results in the skipping of exon 5 and the splicing of exon 4 directly to exon 6. The effect of this mutation is expected to be severe as the expressed α1 subunit protein lacks a significant part of the S4-S5 linker, S5, and part of S5-S6 linker in domain I. Tottering-6j mice display motor dysfunctions in the footprint, rotating rod, and hind-limb extension tests. Although cytoarchitecture of the mutant brains appears normal, tyrosine hydroxylase was persistently expressed in cerebellar Purkinje cells in the adult mutant mice. These results indicate that tottering-6j is a useful model for functional studies of the Ca(v)2.1 channel.     

PGM1 null allele detected in a Caucasian mother-son pair

The presence of a PGM1 null allele in a mother and her son was deduced from their inconsistent phenotypes. Quantitation studies were done to confirm the half-normal enzyme activity. Phenotype analysis of 29 additional genetic markers gave no indication of non-parentage, making non-maternity a very unlikely explanation for the discrepancy.     

A null mutation at the mouse Phosphoglucomutase-1 locus and a new locus,Pgm-3

A null mutation at the phosphoglucomutase locus (Pgm-1) was discovered by electrophoretic analysis of the inbred mouse strain C57 BL/6J. The null allele (Pgm-1 ⁿ) was shown to segregate as a Mendelian unit alternative to the Pgm-1 ^a and Pgm-1 ^b alleles. Mice expressing the Pgm-1 ⁿ allele, either in the heterozygous or homozygous state, are viable, healthy, and fertile. The occurrence of the Pgm-1 ⁿ mutant revealed a previously unreported genetic locus (Pgm-3) that controls the expression of a third phosphoglucomutase. Two electrophoretically expressed alleles of Pgm-3 (inherited without dominance) are found in the inbred mouse strains C57 BL/6J and DBA/2J. Linkage observed between the Pgm-3 locus, the dilute locus (d) and the cytoplasmic malic enzyme locus (Mod-1) has allowed assignment of the Pgm-3 locus to chromosome 9. A striking tissue specific expression of Pgm-1 and Pgm-3 was observed. Products of the Pgm-3 locus were detected in kidney, testes, brain, and heart. In contrast, Pgm-1 controlled isozymes were present in kidney, spleen, ovaries, and erythrocytes.Financial support for this work was provided in part by Contract #263-78-C-0393 from the National Institute of Environmental Health Sciences to the Research Triangle Institute.     

Generation of mice with a conditional null fraser syndrome 1 (Fras1) allele

Summary: Fraser syndrome (FS) is an autosomal recessive disease characterized by skin lesions and kidney and upper airway malformations. Fraser syndrome 1 (FRAS1) is an extracellular matrix protein, and FRAS1 homozygous mutations occur in some FS individuals. FRAS1is expressed at the epithelial‐mesenchymal interface in embryonic skin and kidney. blebbed mice have a null Fras1 mutation and phenocopy human FS. Like humans with FS, they exhibit a high fetal and neonatal mortality, precluding studies of FRAS1 functions in later life. We generated conditional Fras1 null allele mice. Cre‐mediated generalized deletion of this allele generated embryonic skin blisters and renal agenesis characteristic of blebbed mice and human FS. Targeted deletion of Fras1 in kidney podocytes circumvented skin blistering, renal agenesis, and early death. FRAS1 expression was downregulated in maturing glomeruli which then became sclerotic. The data are consistent with the hypothesis that locally produced FRAS1 has roles in glomerular maturation and integrity. This conditional allele will facilitate study of possible role for FRAS1 in other tissues such as the skin. genesis 50:892–898, 2012. © 2012 Wiley Periodicals, Inc.     

The muscular dystrophy with myositis (mdm) mouse mutation disrupts a skeletal muscle-specific domain of titin

Muscular dystrophy with myositis (mdm) is a recessive mouse mutation that causes severe and progressive muscular degeneration. Here we report the identification of the mdm mutation as a complex rearrangement that includes a deletion and a LINE insertion in the titin (Ttn) gene. Mutant allele-specific splicing results in the deletion of 83 amino acids from the N2A region of TTN, a domain thought to bind calpain-3 (CAPN3) the product of the human limb-girdle muscular dystrophy type 2A (LGMD2A) gene. The Ttn(mdm) mutant mouse may serve as a model for human tibial muscular dystrophy, which maps to the TTN locus at 2q31 and shows a secondary reduction of CAPN3 similar to that observed in mdm skeletal muscle. This is the first demonstration that a mutation in Ttn is associated with muscular dystrophy and provides a novel animal model to test for functional interactions between TTN and CAPN3.     

The mouse genomic instability mutation chaos1 is an allele of Polq that exhibits genetic interaction with Atm

chaos1 (for chromosome aberrations occurring spontaneously 1) is a recessive mutation that was originally identified in a phenotype-based screen for chromosome instability mutants in mice. Mutant animals exhibit significantly higher frequencies of spontaneous and radiation- or mitomycin C-induced micronucleated erythrocytes, indicating a potential defect in homologous recombination or interstrand cross-link repair. The chaos1 allele was genetically associated with a missense mutation in Polq, which encodes DNA polymerase theta;. We demonstrate here that chaos1 is a mutant allele of Polq by using two genetic approaches: chaos1 mutant phenotype correction by a bacterial artificial chromosome carrying wild-type Polq and a failed complementation test between chaos1 and a Polq-disrupted allele generated by gene targeting. To investigate the potential involvement of Polq in DNA double-strand break repair, we introduced chaos1 into an Atm (for ataxia telangiectasia mutated)-deficient background. The majority ( approximately 90%) of double-homozygous mice died during the neonatal period. Surviving double mutants exhibited synergistic phenotypes such as severe growth retardation and enhanced chromosome instability. However, remarkably, double mutants had delayed onset of thymic lymphoma, significantly increasing life span. These data suggest a unique role of Polq in maintaining genomic integrity, which is probably distinctive from the major homologous recombination pathway regulated by ATM.     

Generation of a conditionally null allele of the laminin alpha1 gene

Laminins are heterotrimeric glycoproteins of the basement membranes. Laminin 1 (alpha1, beta1, gamma1) is the major laminin expressed during early mouse embryogenesis. To gain access to the physiological function of laminin alpha1 chain, we developed a conditionally null allele of its encoding gene (Lama1) using the cre/loxP system. Floxed-allele-carrying mice (Lama1(flox/flox)) display no overt phenotype. Lama1(flox/flox) mice were crossed with transgenic deleter mice (CMV-Cre) to generate Lama1-deficient mice (Lama1(Delta/Delta)). Lama1(Delta/Delta) embryos die during the early postimplantation period after embryonic day 6.5. They lack Reichert's membrane, an extraembryonic basement membrane in which laminin alpha1 is normally highly expressed. In parallel, Lama1(Delta/Delta) embryos display 1) parietal and visceral endoderm differentiation defects with altered expression of cytokeratin 19 and GATA4, respectively, and 2) an induction of apoptosis. This new mouse model is of particular interest as it will allow time- and tissue-specific inactivation of the Lama1 gene in various organs.