Identification of the translocation breakpoints in the Ts65Dn and Ts1Cje mouse lines: relevance for modeling down syndrome

Down syndrome (DS) is the most frequent genetic disorder leading to intellectual disabilities and is caused by three copies of human chromosome 21. Mouse models are widely used to better understand the physiopathology in DS or to test new therapeutic approaches. The older and the most widely used mouse models are the trisomic Ts65Dn and the Ts1Cje mice. They display deficits similar to those observed in DS people, such as those in behavior and cognition or in neuronal abnormalities. The Ts65Dn model is currently used for further therapeutic assessment of candidate drugs. In both models, the trisomy was induced by reciprocal chromosomal translocations that were not further characterized. Using a comparative genomic approach, we have been able to locate precisely the translocation breakpoint in these two models and we took advantage of this finding to derive a new and more efficient Ts65Dn genotyping strategy. Furthermore, we found that the translocations introduce additional aneuploidy in both models, with a monosomy of seven genes in the most telomeric part of mouse chromosome 12 in the Ts1Cje and a trisomy of 60 centromeric genes on mouse chromosome 17 in the Ts65Dn. Finally, we report here the overexpression of the newly found aneuploid genes in the Ts65Dn heart and we discuss their potential impact on the validity of the DS model

Controlled Somatic and Germline Copy Number Variation in the Mouse Model

Changes in the number of chromosomes, but also variations in the copy number of chromosomal regions have been described in various pathological conditions, such as cancer and aneuploidy, but also in normal physiological condition. Our classical view of DNA replication and mitotic preservation of the chromosomal integrity is now challenged as new technologies allow us to observe such mosaic somatic changes in copy number affecting regions of chromosomes with various sizes. In order to go further in the understanding of copy number influence in normal condition we could take advantage of the novel strategy called Targeted Asymmetric Sister Chromatin Event of Recombination (TASCER) to induce recombination during the G2 phase so that we can generate deletions and duplications of regions of interest prior to mitosis. Using this approach in the mouse we could address the effects of copy number variation and segmental aneuploidy in daughter cells and allow us to explore somatic mosaics for large region of interest in the mouse

The App-Runx1 Region Is Critical for Birth Defects and Electrocardiographic Dysfunctions Observed in a Down Syndrome Mouse Model

Down syndrome (DS) leads to complex phenotypes and is the main genetic cause of birth defects and heart diseases. The Ts65Dn DS mouse model is trisomic for the distal part of mouse chromosome 16 and displays similar features with post-natal lethality and cardiovascular defects. In order to better understand these defects, we defined electrocardiogram (ECG) with a precordial set-up, and we found conduction defects and modifications in wave shape, amplitudes, and durations in Ts65Dn mice. By using a genetic approach consisting of crossing Ts65Dn mice with Ms5Yah mice monosomic for the App-Runx1 genetic interval, we showed that the Ts65Dn viability and ECG were improved by this reduction of gene copy number. Whole-genome expression studies confirmed gene dosage effect in Ts65Dn, Ms5Yah, and Ts65Dn/Ms5Yah hearts and showed an overall perturbation of pathways connected to post-natal lethality (Coq7, Dyrk1a, F5, Gabpa, Hmgn1, Pde10a, Morc3, Slc5a3, and Vwf) and heart function (Tfb1m, Adam19, Slc8a1/Ncx1, and Rcan1). In addition cardiac connexins (Cx40, Cx43) and sodium channel sub-units (Scn5a, Scn1b, Scn10a) were found down-regulated in Ts65Dn atria with additional down-regulation of Cx40 in Ts65Dn ventricles and were likely contributing to conduction defects. All these data pinpoint new cardiac phenotypes in the Ts65Dn, mimicking aspects of human DS features and pathways altered in the mouse model. In addition they highlight the role of the App-Runx1 interval, including Sod1 and Tiam1, in the induction of post-natal lethality and of the cardiac conduction defects in Ts65Dn. These results might lead to new therapeutic strategies to improve the care of DS people

Scn2a haploinsufficient mice display a spectrum of phenotypes affecting anxiety, sociability, memory flexibility and ampakine CX516 rescues their hyperactivity

Abstract Background Mutations of the SCN2A gene encoding a voltage-gated sodium channel alpha-II subunit Nav1.2 are associated with neurological disorders such as epilepsy, autism spectrum disorders, intellectual disability, and schizophrenia. However, causal relationships and pathogenic mechanisms underlying these neurological defects, especially social and psychiatric features, remain to be elucidated. Methods We investigated the behavior of mice with a conventional or conditional deletion of Scn2a in a comprehensive test battery including open field, elevated plus maze, light-dark box, three chambers, social dominance tube, resident-intruder, ultrasonic vocalization, and fear conditioning tests. We further monitored the effects of the positive allosteric modulator of AMPA receptors CX516 on these model mice. Results Conventional heterozygous Scn2a knockout mice (Scn2a KO/+) displayed novelty-induced exploratory hyperactivity and increased rearing. The increased vertical activity was reproduced by heterozygous inactivation of Scn2a in dorsal-telencephalic excitatory neurons but not in inhibitory neurons. Moreover, these phenotypes were rescued by treating Scn2a KO/+ mice with CX516. Additionally, Scn2a KO/+ mice displayed mild social behavior impairment, enhanced fear conditioning, and deficient fear extinction. Neuronal activity was intensified in the medial prefrontal cortex of Scn2a KO/+ mice, with an increase in the gamma band. Conclusions Scn2a KO/+ mice exhibit a spectrum of phenotypes commonly observed in models of schizophrenia and autism spectrum disorder. Treatment with the CX516 ampakine, which ameliorates hyperactivity in these mice, could be a potential therapeutic strategy to rescue some of the disease phenotypes

Changes in cardiovascular morphology observed in the Ts65Dn model.

<p>Posterior view of a wt individual illustrates the normal location of the aorta and efferent vessels (A). Posterior view of a Ts65Dn individual shows an aberrant right subclavian artery (RSA) arising from the distal part of the aortic arch and describing a retro-oesophagian loop (B). Haematoxylin-eosin staining of histological cross-sections illustrates full septation of the ventricles with individualized valves in wt (C) while Ts65Dn heart cross-section shows an upper communication of the ventricles (D; arrow). This cardiovascular anomaly was observed in 1 in 18 Ts65Dn dead newborns. LSA, left subclavian artery; RSA, right subclavian artery; DA, descending aorta; RA, right atrium; LA, left atrium; RV, right ventricle; LV, left ventricle; Sept, septum; OT, outflow tract. Scale bars represent 1 mm in A, B and 400 µm in C,</p

Representation of the DS mouse models displaying lethality and cardiac features.

<p>The position of the trisomic segment with the homologous regions to Hsa21, or the Hsa21 regions found in Tc1, is indicated in grey black blocks. While the disomic segments in compound mutants Dp(16)1Yey/Df(16)2Yey or in Ts65Dn/Ms5Yah or the deleted segments in Tc1 are indicated in dashed boxes. “+” indicates the presence and “−” the absence of phenotypes whereas ND indicates a non-determined state for presence or absence of CHD in Ts1Cje. References are given in the text.</p

Atrial and ventricular connexins and sodium channel genes expression levels in wt and Ts65Dn mice.

<p>(A) Atrial; (B) ventricular. Connexins and sodium channels genes expression levels were assessed by qRT-PCR using RNA extracted from adult mice cardiac atria and ventricle separately. Ts65Dn show an overall decrease in connexin and sodium channel mRNA expression except for <i>Cx45</i> and <i>Cx30.2</i> in the atria. <i>Cx40</i> expression level is reduced in the ventricles. No amplification was obtained for <i>Cx30.2</i> in ventricle extracts confirming its atria specific expression pattern. Mean±sem with Student t-test <i>P</i>-values, N = 7 and 5 for Ts65Dn and wt respectively.</p

Genes deregulated in the heart of Ts65Dn, Ms5Yah, and Ts65Dn/Ms5Yah compound mice.

<p>Only the genes for which the Anova test is below 0.1 and which are not triplicated on the Ts65Dn minichromosome are listed here. (<i>P</i> values: ***: <i>P</i><0.001; **: <i>P</i><0.01; *: <i>P</i><0.05).</p

Ratio of the expression level of 5 genes located on Mmu16 in the heart of mutant compared to disomic control mice.

<p>The analysis was done by qRT-PCR. Significance is given with <i>P</i> values of 0.05 (*). 0.01 (**).</p