Segmentally Trisomic Mouse Models
Ts65Dn is currently the most widely used mouse model for DS. This strain is the chromosomally unbalanced progeny of a mouse mutant carrying a balanced translocation; the genetic alteration was randomly induced by irradiation at Muriel Davisson's laboratory [
Davisson et al., 1990;
Reeves et al., 1995]. The unbalanced derivative chromosome in Ts65Dn consists of a genomic fragment of approximately 13 Mb extending from
Mrpl39 to the telomere on Mmu16 with approximately 49.2% of the syntenic regions and approximately 55% of the Hsa21 gene orthologs triplicated (fig. ). Ts65Dn mice also carry a trisomic segment for a >5.8-Mb subcentromeric region of Mmu17 that is not syntenic to a region on Hsa21 [
Davisson et al., 1990;
Akeson et al., 2001;
Li et al., 2007]. Similar to individuals with DS, the density of dendritic spines was decreased and spine heads were enlarged in Ts65Dn mice [
Belichenko et al., 2004,
2007,
2009b;
Popov et al., 2011]. Ts65Dn mice consistently exhibit impairments of hippocampal-mediated behaviors. Significantly severe impairment of hippocampal long-term potentiation (LTP), particularly in the dentate gyrus, was observed in Ts65Dn mice [
Siarey et al., 1997;
Kleschevnikov et al., 2004;
Belichenko et al., 2007,
2009b]. Excess inhibition has been proposed to explain this abnormal synaptic plasticity, which is supported by the following evidence. (1) Immunocytochemical analysis of synapse-associated proteins showed no change in the overall density of inhibitory and excitatory synapses in Ts65Dn mice in the dentate gyrus. However, there was a marked increase in colocalization of synaptophysin with the inhibitory presynaptic proteins GAD65 and VGAT (table ) [
Belichenko et al., 2007,
2009b]. (2) At the electron microscopic level, the synaptic apposition length of inhibitory synapses (i.e., symmetric synapses) was significantly increased, with no change in excitatory (i.e., asymmetric) synapses [
Belichenko et al., 2009b]. (3) Blocking GABA(A) receptor-mediated inhibitory neurotransmission with picrotoxin restored LTP in the dentate gyrus [
Kleschevnikov et al., 2004;
Belichenko et al., 2007]. Interestingly, the application of another GABAergic inhibitor, PTZ, not only has restored hippocampal LTP but also has enhanced hippocampus-mediated learning in Ts65Dn mice [
Fernandez et al., 2007;
Heller et al., 2009].
| Table 1Alterations of the protein markers in hippocampal synapses in Ts65Dn mice |
Ts1Cje is the unbalanced progeny of a mouse mutant carrying a balanced translocation, t(12;16), which was induced by gene-targeting in Charles Epstein's laboratory [
Sago et al., 1998,
2000]. The unbalanced derivative chromosome in Ts1Cje consists of a genomic fragment of approximately 8.1 Mb from
Sod1 to the telomere on Mmu16, with the
Sod1 gene inactivated [
Sago et al., 1998,
2000] (fig. ). Ts1Cje mice harbor 3 copies of approximately 67% of the Hsa21 gene orthologs triplicated in Ts65Dn mice [
Olson et al., 2004b]. A 2-Mb heterozygous deletion on Mmu12 was reported in Ts1Cje mice in a recent study [
Laffaire et al., 2009]. There are important similarities between Ts1Cje and Ts65Dn mice with regard to synaptic and cognitive phenotypes [
Olson et al., 2004b;
Belichenko et al., 2007]. For instance, widespread enlargement of dendritic spines and decreased density of spines in the dentate gyrus were found in both strains [
Belichenko et al., 2007]. Ms1Ts65Dn mice are produced by crossing Ts65Dn mice with t(12;16)1Cje mice [
Sago et al., 1998;
Sago et al., 2000]; these mice are segmentally trisomic for the genetic segment from
Mrpl39 to
Sod1. No significant cognitive impairment was found in the Morris water maze test for this mutant [
Sago et al., 2000].
Engineered in Roger Reeves’ laboratory, Ts1Rhr is trisomic for the
Cbr1-
Fam3b region [
Olson et al., 2004a;
Aldridge et al., 2007] and syntenic to the so-called DS critical chromosomal region on Hsa21 (fig. ) [
Delabar et al., 1993;
Sinet et al., 1993;
Korenberg et al., 1994]. Cognitively relevant phenotypes of this mutant have been extensively characterized and significant abnormalities in synaptic structure and functions as well as cognitive behaviors were detected [
Belichenko et al., 2009a]. Ms1Ts1Rhr mice were generated by crossing Ts65Dn mice with a mutant carrying a deletion in the
Cbr1-
Fam3b region [
Olson et al., 2004a], thus resulting in the reduction of the
Cbr1-
Fam3b segment to 2 copies in Ts65Dn mice. A reported phenotype of Ms1Ts1Rhr mice was an approximately 18% reduction in the brain volume [
Aldridge et al., 2007].
Ts1Yah mice that carry a 0.59-Mb duplication between
Abcg1 and
U2af1 in the Hsa21 syntenic region on Mmu17 were generated recently in Yann Herault's laboratory (fig. ) [
Pereira et al., 2009]. Interestingly, this duplication apparently led to increased hippocampal LTP in the mutant mice, providing the first evidence of possible genetic interaction between different mouse syntenic regions underlying altered synaptic plasticity associated with DS.
To further understand the impact of the different syntenic regions on developmental cognitive disabilities, the laboratory of Eugene Yu has recently generated the mouse mutants
Dp(10)1Yey/+,
Dp(16)1Yey/+ and
Dp(17)1Yey/+, carrying the duplications spanning the entire Hsa21 syntenic regions on Mmu10, Mmu16 and Mmu17, respectively (fig. ) [
Li et al., 2007;
Yu et al., 2010b]. The phenotypic results show that, while the genotype of
Dp(17)1Yey/+ results in increased hippocampal LTP, the genotype of
Dp(16)1Yey/+ leads to decreased hippocampal LTP and impaired cognitive behaviors. Surprisingly, no significant abnormalities have been detected in
Dp(10)1Yey/+ mice based on Morris water maze tests, contextual fear conditioning test and hippocampal LTP analysis even though some genes in the duplicated region have been implicated in neurological disorder, such as
S100b [
Yu et al., 2010c]. This is consistent with a recent report from Yann Herault's laboratory that the 2.2-Mb heterozygous deletion of the Hsa21 syntenic region on Mmu10 did not alter cognitive deficiency in Tc1 mice [
Duchon et al., 2010]. However, it remains possible that some cognitively relevant phenotypes could be detected in
Dp(10)1Yey/+ mice if different phenotyping approaches are used. To assess the effect of the simultaneous presence of all the segmental trisomies,
Dp(10)1Yey/+;
Dp(16)1Yey/+;
Dp(17)1Yey/+ mice were generated by crossing the mutants carrying individual duplications, which represent all the evolutionarily conserved genetic alterations and interactions of DS in mice (fig. ). We showed that these mutant mice exhibited abnormal cognitively relevant phenotypes: a significant decrease in hippocampal LTP and a significant impairment in cognitive behaviors that are based on the Morris water maze and contextual fear conditioning tests [
Yu et al., 2010b]. Because of their desirable genotypes and phenotypes, these mouse models offer a new platform for further understanding DS. Unlike Ts65Dn and Tc1 models,
Dp(1)1Yey/+,
Dp(16)1Yey/+ and
Dp(17)1Yey/+ mice can be maintained in 129S5 background and they are also viable and fertile after backcrossing to C57BL/6J mice for five generations. Compound mutants can also be generated in these backgrounds. Therefore, these new models can be used to alleviate the effects of heterogeneous strain backgrounds. In addition, the inbred and congenic backgrounds of the models can facilitate the analysis to identify the genetic modifiers for mutant phenotypes, including cognitively relevant phenotypes.
Transchromosomic Mouse Models
Extensive efforts to develop transchromosomic mice by introducing Hsa21 into mouse ES cells using microcell-mediated chromosome transfer [
Hernandez et al., 1999;
Inoue et al., 2000;
Shinohara et al., 2001;
O'Doherty et al., 2005] have led to the establishment of the Tc1 mouse model, which is a major achievement in DS modeling. The genotyping result has shown that the transchromosome contains almost the entire Hsa21 with only an approximately 4.9-Mb deletion that contains approximately 19 genes [
O'Doherty et al., 2005]. Tc1 mice have been characterized by a number of laboratories and several major DS-related phenotypes were observed in this model, including abnormal developmental cognitive phenotypes, despite a subset of cells in Tc1 mice that did not carry the transchromosome [
O'Doherty et al., 2005]. The behavioral experiments, including novel object recognition and Morris water maze tests, showed that Tc1 mice are impaired in learning and memory. Analysis of the dentate gyrus of hippocampal slices isolated from Tc1 mice showed decreased LTP within 60 min after induction [
O'Doherty et al., 2005;
Morice et al., 2008]. However, additional behavioral experiments and analysis of the dentate gyrus of freely moving mice showed that long-term memory and synaptic plasticity are preserved in Tc1 mice [
Morice et al., 2008].
Transgenic Models
Transgenic mice are powerful tools in the functional characterizations of genes. However, many transgenic mouse mutants may not be appropriate for analyzing the impact of overexpression of Hsa21 orthologous genes on DS-related phenotypes because the regulatory elements of the transgenes were not derived from the endogenous loci so the expression levels as well as the spatial and temporal expression patterns might be different from those of the endogenous genes. Nevertheless, BAC or YAC transgenic mice may be useful for unraveling the consequences of the dosage increase for Hsa21 gene orthologs because these types of transgenes may retain all the endogenous regulatory elements for the associated genes [
Smith et al., 1995,
1997;
Chabert et al., 2004;
Roubertoux and Carlier, 2010]. As an interesting example of such BAC transgenic mice, mice harboring a single copy of
DYRK1A BAC showed impaired cognitive behaviors but, surprisingly, showed increasedhippocampal LTP [
Ahn et al., 2006]. In contrast, the mouse models of DS carrying 3 copies of large segments of Mmu16 syntenic to Hsa21 (i.e., Ts65Dn, Ts1Cje and
Dp(16)1Yey/+, all of which contain 3 copies of the mouse
Dryk1a gene) exhibited decreased hippocampal LTP [
Siarey et al., 1997;
Kleschevnikov et al., 2004;
Belichenko et al., 2007;
Yu et al., 2010c]. One interpretation of this discrepancy is that the triplication(s) of (an)other Hsa21 gene ortholog(s) in Ts65Dn, Ts1Cje and
Dp(16)1Yey/+ mice is/are responsible for the decrease in hippocampal LTP and that triplication of the
Dyrk1a ortholog may actually help to reduce the impact of the causative genes [
Ahn et al., 2006]. The more compelling conclusion is that analyses confined solely to examining the impact of an individual gene without the context of an optimal reference model trisomic for Hsa21 syntenic regions may be insufficient to unravel the details of the contribution of the gene to a DS phenotype.