A mouse model of Down syndrome trisomic for all human chromosome 21 syntenic regions

Abstract

Down syndrome (DS) is caused by the presence of an extra copy of human chromosome 21 (Hsa21) and is the most common genetic cause for developmental cognitive disability. The regions on Hsa21 are syntenically conserved with three regions located on mouse chromosome 10 (Mmu10), Mmu16 and Mmu17. In this report, we describe a new mouse model for DS that carries duplications spanning the entire Hsa21 syntenic regions on all three mouse chromosomes. This mouse mutant exhibits DS-related neurological defects, including impaired cognitive behaviors, reduced hippocampal long-term potentiation and hydrocephalus. These results suggest that when all the mouse orthologs of the Hsa21 genes are triplicated, an abnormal cognitively relevant phenotype is the final outcome of the elevated expressions of these orthologs as well as all the possible functional interactions among themselves and/or with other mouse genes. Because of its desirable genotype and phenotype, this mutant may have the potential to serve as one of the reference models for further understanding the developmental cognitive disability associated with DS and may also be used for developing novel therapeutic interventions for this clinical manifestation of the disorder.

Figure 1.

Schematic representation of Hsa21 and the syntenic regions on Mmu10, Mmu16 and Mmu17. The endpoints of the syntenic regions are indicated. Mrpl39 is the proximal endpoint for the segmental trisomy in Ts65Dn.

Figure 2.

Development of Dp(10)1Yey/+;Dp(16)1Yey/+;Dp(17)1Yey/+ mice. (A) Strategy to generate Dp(10)1Yey. B, BamHI; N, NheI; 5′, 5′HPRT fragment; 3′, 3′HPRT fragment; N, neomycin-resistance gene; P, puromycin-resistance gene; Ty, Tyrosinase transgene; Ag, Agouti transgene; arrowhead, loxP site. (B) Southern blot analysis of NheI-digested mouse-tail DNA using Probe 10B. Lane 1, the wild-type mouse; lane 2, Dp(10)1Yey/+ mouse. (C) Schematic of the genomic locations of BAC probes for FISH analysis. EP1 and EP2, endpoint 1 and endpoint 2, which were targeted with pTV(10)1EP1 and pTV(10)1EP2, respectively. (D) FISH analysis of interphase nuclei prepared from the embryonic fibroblasts carrying Dp(10)1Yey/+. (E) Strategy to generate Dp(17)1Yey. R, EcoRI; Bg, BglII. (F) Southern blot analysis of EcoRI-digested mouse-tail DNA using Probe 17A. Lane 1, the wild-type mouse; lane 2, Dp(17)1Yey/+ mouse. (G) Schematic of the genomic locations of BAC probes for FISH analysis. EP1 and EP2, endpoint 1 and endpoint 2, which were targeted with pTV(17)1EP1 and pTV(17)1EP2, respectively. (H) FISH analysis of interphase nuclei prepared from the embryonic fibroblasts carrying Dp(17)1Yey/+. (I) Whole-genome Agilent microarray CGH profile of DNA isolated from a Dp(10)1Yey/+;Dp(16)1Yey/+;Dp(17)1Yey/+ mouse. (J) CGH profile of the mouse chromosomes 10, 16 and 17 of the Dp(10)1Yey/+;Dp(16)1Yey/+;Dp(17)1Yey/+ mouse. Plotted are log₂-transformed hybridization ratios of Dp(10)1Yey/+;Dp(16)1Yey/+;Dp(17)1Yey/+ mouse DNA versus wild-type mouse DNA.

Figure 3.

Dp(10)1Yey/+;Dp(16)1Yey/+;Dp(17)1Yey/+ mice are impaired in Morris water maze tasks. The mutant mice (n = 15) and the wild-type littermates (n = 15) were examined in the Morris water maze, as described in Materials and Methods. (A) Latency to locate the platform (s, second). (B) Swimming speed during the learning trials (m/s, meter/second). (C) Path length to locate the platform (m, meter). (D) In the probe test on the day after the end of the training trials, the relative amount of time spent in different quadrants.

Figure 4.

Dp(10)1Yey/+;Dp(16)1Yey/+;Dp(17)1Yey/+ mice are impaired in grip strength. The muscle strength of Dp(10)1Yey/+;Dp(16)1Yey/+;Dp(17)1Yey/+ mice (n = 10) and the wild-type littermates (n = 15) was analyzed using a grip-strength meter, as described in Materials and Methods. The grip-strength measurements (gram force/gram of body weight, GF/g) of the forelimbs and all limbs of the mice with different genotypes are shown.

Figure 5.

Dp(10)1Yey/+;Dp(16)1Yey/+;Dp(17)1Yey/+ mice are impaired in the contextual fear-conditioning test. The mutant mice (n = 15) and the wild-type littermates (n = 15) were examined in the contextual fear-conditioning test, as described in Materials and Methods. The percentages of time spent freezing before the foot shock (baseline) as well as during the 24 and 72 h contextual tests are shown.

Figure 6.

Dp(10)1Yey/+;Dp(16)1Yey/+;Dp(17)1Yey/+ mice exhibit normal sensitivity to the electric foot shock. The minimal levels of currents (mA) needed to elicit a response, either flinching or vocalizing, from the mutant mice (n = 15) and the wild-type littermates (n = 15) are shown.

Figure 7.

Dp(10)1Yey/+;Dp(16)1Yey/+;Dp(17)1Yey/+ mice are impaired in hippocampal LTP. The electrophysiological recordings were carried out using hippocampal slices. (A) Input–output curves generated by applying stimuli of increasing intensity and measuring the initial slopes of the fEPSPs for the mutant mice (n = 13) and the wild-type littermates (n = 14). (B) Paired-pulse facilitation was measured by applying two closely spaced stimuli for the mutant mice (n = 18) and the wild-type littermates (n = 17), which was expressed as the ratio of the second synaptic response to the first synaptic response as a function of interpulse interval. (C) For analyzing hippocampal LTP, the fEPSP was induced by TBS. Recordings were carried out before and after the TBS inductions for the mutant mice (n = 7) and the wild-type littermates (n = 7). Evoked potentials were normalized to the fEPSP recorded prior to TBS induction (baseline = 100%). The data are presented as the percentage of fEPSP as a function of time.

Figure 8.

Dp(10)1Yey/+;Dp(16)1Yey/+;Dp(17)Yey/+ causes small reductions in body weight and body length. Body weights (A) and body lengths (nose to anus) (B) of the male Dp(10)1Yey/+;Dp(16)1Yey/+;Dp(17)Yey/+ mice (n = 7) and the male wild-type littermates (n = 9) are shown.

Figure 9.

Dp(10)1Yey/+;Dp(16)1Yey/+;Dp(17)Yey/+ mice exhibit hydrocephalus. The brains of a Dp(10)1Yey/+;Dp(16)1Yey/+;Dp(16)1Yey/+ mouse with a rounded and enlarged cranium at 1 month of age (A and B) and a wild-type littermate (C and D) were analyzed. Coronal views of T2-weighted MR images (A and C) show the lateral ventricles are severely dilated in the mutant mouse (A). Scale bar, 1 mm. The hematoxylin-stained histology sections of the same brains (B and D) reveal aqueductal stenosis in the mutant mouse (B). Scale bar, 0.2 mm.