Muriel T. Davisson

A mouse model for Down syndrome exhibits learning and behaviour deficits

Trisomy 21 or Down syndrome (DS) is the most frequent genetic cause of mental retardation, affecting one in 800 live born human beings. Mice with segmental trisomy 16 (Ts65Dn mice) are at dosage imbalance for genes corresponding to those on human chromosome 21q21–22.3—which includes the so–called DS ‘critical region’. They do not show early–onset of Alzheimer disease pathology; however, Ts65Dn mice do demonstrate impaired performance in a complex learning task requiring the integration of visual and spatial information. The reproducibility of this phenotype among Ts65Dn mice indicates that dosage imbalance for a gene or genes in this region contributes to this impairment. The corresponding dosage imbalance for the human homologues of these genes may contribute to cognitive deficits in DS.

Genetic variation among 129 substrains and its importance for targeted mutagenesis in mice

Targeted mutagenesis in mice, a powerful tool for the analysis of gene function and human disease, makes extensive use of 129 mouse substrains. Although all are named 129, we document that outcrossing of these substrains, both deliberate and accidental, has lead to extensive genetic variability among substrains and embryonic stem cells derived from them. This clearer understanding of 129 substrain variability allows consideration of its negative impact on targeting technology, including: homologous recombination frequencies, preparation of inbred animals, and availability of appropriate controls. Based on these considerations we suggest a number of recommendations for future experimental design.

Editing-defective tRNA synthetase causes protein misfolding and neurodegeneration

Misfolded proteins are associated with several pathological conditions including neurodegeneration. Although some of these abnormally folded proteins result from mutations in genes encoding disease-associated proteins (for example, repeat-expansion diseases), more general mechanisms that lead to misfolded proteins in neurons remain largely unknown. Here we demonstrate that low levels of mischarged transfer RNAs (tRNAs) can lead to an intracellular accumulation of misfolded proteins in neurons. These accumulations are accompanied by upregulation of cytoplasmic protein chaperones and by induction of the unfolded protein response. We report that the mouse sticky mutation, which causes cerebellar Purkinje cell loss and ataxia, is a missense mutation in the editing domain of the alanyl-tRNA synthetase gene that compromises the proofreading activity of this enzyme during aminoacylation of tRNAs. These findings demonstrate that disruption of translational fidelity in terminally differentiated neurons leads to the accumulation of misfolded proteins and cell death, and provide a novel mechanism underlying neurodegeneration.

Interacting loci cause severe iris atrophy and glaucoma in DBA/2J mice.

Glaucomas are a major cause of blindness. Visual loss typically involves retinal ganglion cell death and optic nerve atrophy subsequent to a pathologic elevation of intraocular pressure (IOP). Some human glaucomas are associated with anterior segment abnormalities such as pigment dispersion syndrome (PDS) and iris atrophy with associated synechiae. The primary causes of these abnormalities are unknown, and their aetiology is poorly understood. We recently characterized a mouse strain (DBA/2J) that develops glaucoma subsequent to anterior segment changes including pigment dispersion and iris atrophy. Using crosses between mouse strains DBA/2J (D2) and C57BL/6J (B6), we now show there are two chromosomal regions that contribute to the anterior segment changes and glaucoma. Progeny homozygous for the D2 allele of one locus on chromosome 6 (called ipd) develop an iris pigment dispersion phenotype similar to human PDS. ipd resides on a region of mouse chromosome 6 with conserved synteny to a region of human chromosome 7q that is associated with human PDS (ref. 4 ). Progeny homozygous for the D2 allele of a different locus on chromosome 4 (called isa) develop an iris stromal atrophy phenotype (ISA). The Tyrp1 gene is a candidate for isa and likely causes ISA via a mechanism involving pigment production. Progeny homozygous for the D2 alleles of both ipd and isa develop an earlier onset and more severe disease involving pigment dispersion and iris stromal atrophy.

Mouse models of Down syndrome: how useful can they be? Comparison of the gene content of human chromosome 21 with orthologous mouse genomic regions.

With an incidence of approximately 1 in 700 live births, Down syndrome (DS) remains the most common genetic cause of mental retardation. The phenotype is assumed to be due to overexpression of some number of the >300 genes encoded by human chromosome 21. Mouse models, in particular the chromosome 16 segmental trisomies, Ts65Dn and Ts1Cje, are indispensable for DS-related studies of gene-phenotype correlations. Here we compare the updated gene content of the finished sequence of human chromosome 21 (364 genes and putative genes) with the gene content of the homologous mouse genomic regions (291 genes and putative genes) obtained from annotation of the public sector C57Bl/6 draft sequence. Annotated genes fall into one of three classes. First, there are 170 highly conserved, human/mouse orthologues. Second, there are 83 minimally conserved, possible orthologues. Included among the conserved and minimally conserved genes are 31 antisense transcripts. Third, there are species-specific genes: 111 spliced human transcripts show no orthologues in the syntenic mouse regions although 13 have homologous sequences elsewhere in the mouse genomic sequence, and 38 spliced mouse transcripts show no identifiable human orthologues. While these species-specific genes are largely based solely on spliced EST data, a majority can be verified in RNA expression experiments. In addition, preliminary data suggest that many human-specific transcripts may represent a novel class of primate-specific genes. Lastly, updated functional annotation of orthologous genes indicates genes encoding components of several cellular pathways are dispersed throughout the orthologous mouse chromosomal regions and are not completely represented in the Down syndrome segmental mouse models. Together, these data point out the potential for existing mouse models to produce extraneous phenotypes and to fail to produce DS-relevant phenotypes.

Stargazer: a new neurological mutant on chromosome 15 in the mouse with prolonged cortical seizures.

We report here the initial description of the inheritance pattern, linkage mapping, and electroclinical phenotype of a recessive mutation on mouse Chromosome 15, stargazer (stg), that produces epilepsy. The salient epileptic phenotype is a syndrome of spontaneous, prolonged, generalized spike-wave cortical discharges with behavioral arrest. A second, complex, seizure pattern featuring movements during the discharge can also appear. The stg/stg mutant phenotype confirms the general principal that inherited epilepsies sharing similar cortical excitability patterns can be transmitted by single gene loci residing on different chromosomes and provides new evidence that the severity of seizure expression depends on the specific mutant gene affected.

The nob2 mouse, a null mutation in Cacna1f : Anatomical and functional abnormalities in the outer retina and their consequences on ganglion cell visual responses

Glutamate release from photoreceptor terminals is controlled by voltage-dependent calcium channels (VDCCs). In humans, mutations in the Cacna1f gene, encoding the alpha1F subunit of VDCCs, underlie the incomplete form of X-linked congenital stationary night blindness (CSNB2). These mutations impair synaptic transmission from rod and cone photoreceptors to bipolar cells. Here, we report anatomical and functional characterizations of the retina in the nob2 (no b-wave 2) mouse, a naturally occurring mutant caused by a null mutation in Cacna1f. Not surprisingly, the b-waves of both the light- and dark-adapted electroretinogram are abnormal in nob2 mice. The outer plexiform layer (OPL) is disorganized, with extension of ectopic neurites through the outer nuclear layer that originate from rod bipolar and horizontal cells, but not from hyperpolarizing bipolar cells. These ectopic neurites continue to express mGluR6, which is frequently associated with profiles that label with the presynaptic marker Ribeye, indicating potential points of ectopic synapse formation. However, the morphology of the presynaptic Ribeye-positive profiles is abnormal. While cone pedicles are present their morphology also appears compromised. Characterizations of visual responses in retinal ganglion cells in vivo, under photopic conditions, demonstrate that ON-center cells have a reduced dynamic range, although their basic center-surround organization is retained; no alteration in the responses of OFF-center cells was evident. These results indicate that nob2 mice are a valuable model in which to explore the pathophysiological mechanisms associated with Cacna1f mutations causing CSNB2, and the subsequent effects on visual information processing. Further, the nob2 mouse represents a model system in which to define the signals that guide synapse formation and/or maintenance in the OPL.

VAC14 nucleates a protein complex essential for the acute interconversion of PI3P and PI(3,5)P2 in yeast and mouse

The signalling lipid PI(3,5)P2 is generated on endosomes and regulates retrograde traffic to the trans‐Golgi network. Physiological signals regulate rapid, transient changes in PI(3,5)P2 levels. Mutations that lower PI(3,5)P2 cause neurodegeneration in human patients and mice. The function of Vac14 in the regulation of PI(3,5)P2 was uncharacterized previously. Here, we predict that yeast and mammalian Vac14 are composed entirely of HEAT repeats and demonstrate that Vac14 exerts an effect as a scaffold for the PI(3,5)P2 regulatory complex by direct contact with the known regulators of PI(3,5)P2: Fig4, Fab1, Vac7 and Atg18. We also report that the mouse mutant ingls (infantile gliosis) results from a missense mutation in Vac14 that prevents the association of Vac14 with Fab1, generating a partial complex. Analysis of ingls and two additional mutants provides insight into the organization of the PI(3,5)P2 regulatory complex and indicates that Vac14 mediates three distinct mechanisms for the rapid interconversion of PI3P and PI(3,5)P2. Moreover, these studies show that the association of Fab1 with the complex is essential for viability in the mouse.

Scrambler, a new neurological mutation of the mouse with abnormalities of neuronal migration.

A novel spontaneous neurological mutation, scrambler (scm), appeared in the inbred mouse strain DC/Le (dancer) in 1991. Mice homozygous for this recessive mutation are recognized by an unstable gait and whole-body tremor. The cerebella of 30-day-old scrambler homozygotes are hypoplastic and devoid of folia; however, neither seizures nor abnormal brain wave patterns have been observed. Homozygous scrambler mutants have an ataxic gait which in the male may be a contributory factor in the failure to mate. Female homozygotes mate and breed. Life span is not reduced in either sex. Scrambler is similar to the reeler mutation in phenotype and pathology and, like reeler, probably results from defective neuronal migration. We mapped the scrambler mutation to Chromosome (Chr) 4, proving that it is distinct from the recently cloned reeler gene on Chr 5. We also determined the map position of the agrin gene, Agrn, on Chr 4, and on this basis eliminated it as a candidate for scm. Currently there is no known homology of scrambler with human lissencephalies or other human disorders caused by abnormal neuronal migration.

Motor dysfunction in a mouse model for Down syndrome

Motor deficits are among the most frequently occurring features of Down syndrome (DS). Individuals with DS exhibit disturbances in the dynamics of movement production and postural control that are thought to have a significant impact in delaying their acquisition of motor skills. The origin of these deficits has been hypothesized to be cerebellar. The Ts65Dn mouse is the most robust and genetically sound animal model for DS currently available. Ts65Dn mice show many DS-like features, including significant learning deficits in different behavioral tasks and neurodegeneration of cholinergic neurons. In the present study, we investigate the motor function of these animals. We have analyzed hind paw print patterns during walking, running speeds, rotarod performance, grip force production, swim paths, and swimming speeds. Our results indicate that Ts65Dn mice present mild to severe dysfunction according to all of the above assessments. The most evident impairments presented by these mice were related to equilibrium and motor coordination, which agrees with reported clinical observations made on individuals with DS. Because none of these findings were readily apparent by simple inspection of these animals, these findings reiterate the need for a careful evaluation of any mutant mouse strain for which there is reason to suspect motor deficits. The identification of motor dysfunction in Ts65Dn mice may have important consequences for the interpretation of some previous assessments of learning and memory of these animals that assumed intact motor function, and further strengthens the use of this aneuploid mouse strain as a model for DS.