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Atrioventricular septal defects (AVSDs) are a common severe form of congenital heart disease (CHD). In this study we identified deleterious non-synonymous mutations in two cilia genes, Dnah11 and Mks1, in independent N-ethyl-N-nitrosourea-induced mouse mutant lines with heritable recessive AVSDs by whole-exome sequencing. Cilia are required for left/right body axis determination and second heart field (SHF) Hedgehog (Hh) signaling, and we find that cilia mutations affect these requirements differentially. Dnah11 avc 4 did not disrupt SHF Hh signaling and caused AVSDs only concurrently with heterotaxy, a left/right axis abnormality. In contrast, Mks1 avc 6 disrupted SHF Hh signaling and caused AVSDs without heterotaxy. We performed unbiased whole-genome SHF transcriptional profiling and found that cilia motility genes were not expressed in the SHF whereas cilia structural and signaling genes were highly expressed. SHF cilia gene expression predicted the phenotypic concordance between AVSDs and heterotaxy in mice and humans with cilia gene mutations. A two-step model of cilia action accurately predicted the AVSD/heterotaxyu phenotypic expression pattern caused by cilia gene mutations. We speculate that cilia gene mutations contribute to both syndromic and non-syndromic AVSDs in humans and provide a model that predicts the phenotypic consequences of specific cilia gene mutations., (© The Author 2016. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Trisomic Ts65Dn mice show direct parallels with many phenotypes of Down syndrome (DS), including effects on the structure of cerebellum and hippocampus. A small segment of Hsa21 known as the 'DS critical region' (DSCR) has been held to contain a gene or genes sufficient to cause impairment in learning and memory tasks involving the hippocampus. To test this hypothesis, we developed Ts1Rhr and Ms1Rhr mouse models that are, respectively, trisomic and monosomic for this region. Here, we show that trisomy for the DSCR alone is not sufficient to produce the structural and functional features of hippocampal impairment that are seen in the Ts65Dn mouse and DS. However, when the critical region is returned to normal dosage in trisomic Ms1Rhr/Ts65Dn mice, performance in the Morris water maze is identical to euploid, demonstrating that this region is necessary for the phenotype. Thus, although the prediction of the critical region hypothesis was disproved, novel gene dosage effects were identified, which help to define how trisomy for this segment of the chromosome contributes to phenotypes of DS.

Trisomy 21 (Down syndrome) results in cerebellar dysmorphology with direct parallels in the Ts65Dn mouse. Despite pronounced changes in morphology, cerebellar function is not markedly different. As a first test of whether those cerebellar cells that have survived to adulthood in trisomic mice are equivalent to euploid cells, we used microarrays to assess the trisomic and euploid cerebella. Trisomic and euploid transcriptomes were robustly distinguished. Changes in expression of individual genes were very subtle, but the differences in respective transcriptome phenotypes extended deeply into the set of nearly 7000 probes (genes) located throughout the genome. In contrast to deterministic models of gene action in trisomy, examination of the discriminating genes in two independent experiments suggests that the global perturbation includes a significant stochastic component. Thus, dosage imbalance of 124 genes in Ts65Dn mice alters the expression of thousands of genes to create a variable trisomic transcriptome. This global destabilization has important implications for approaches to ameliorative therapies in Down syndrome.

Trisomy 21 (Ts21) is the most common live-born human aneuploidy; it results in a constellation of features known as Down's syndrome (DS). Ts21 is the most frequent cause of congenital heart defects and the leading genetic cause of mental retardation. To investigate the gene dosage effects of an extra copy of human chromosome 21 (Chr 21) on various phenotypes, we used microcell-mediated chromosome transfer to create embryonic stem (ES) cells containing Chr 21. ES cell lines retaining Chr 21 as an independent chromosome were used to produce chimeric mice with a substantial contribution from Chr 21-containing cells. Fluorescence in situ hybridization and PCR-based DNA analysis revealed that Chr 21 was substationally intact but had sustained a small deletion. The freely segregating Chr 21 was lost during development in some tissues, resulting in a panel of chimeric mice with various mosaicism as regards retention of the Chr 21. These chimeric mice showed a high correlation between retention of Chr 21 in the brain and impairment in learning or emotional behavior by open-field, contextual fear conditioning and forced swim tests. Hypoplastic thymus and cardiac defects, i.e. double outlet right ventricle and riding aorta, were observed in a considerable number of chimeric mouse fetuses with a high contribution of Chr 21. These chimeric mice mimic a wide variety of phenotypic traits of DS, revealing the utility of mice containing Chr 21 as unique models for DS and for the identification of genes responsible for DS.

Down syndrome (DS) is the most common genetic cause of mental retardation and affects many aspects of brain development. DS individuals exhibit an overall reduction in brain size with a disproportionately greater reduction in cerebellar volume. The Ts65Dn mouse is segmentally trisomic for the distal 12-15 Mb of mouse chromosome 16, a region that shows perfect conserved linkage with human chromosome 21, and therefore provides a genetic model for DS. In this study, high resolution magnetic resonance imaging and histological analysis demonstrate precise neuro- anatomical parallels between the DS and the Ts65Dn cerebellum. Cerebellar volume is significantly reduced in Ts65Dn mice due to reduction of both the internal granule layer and the molecular layer of the cerebellum. Granule cell number is further reduced by a decrease in cell density in the internal granule layer. Despite these changes in Ts65Dn cerebellar structure, motor deficits have not been detected in several tests. Reduction in granule cell density in Ts65Dn mice correctly predicts an analogous pathology in humans; a significant reduction in granule cell density in the DS cerebellum is reported here for the first time. The candidate region of genes on chromosome 21 affecting cerebellar development in DS is therefore delimited to the subset of genes whose orthologs are at dosage imbalance in Ts65Dn mice, providing the first localization of genes affecting a neuroanatomical phenotype in DS. The application of this model for analysis of developmental perturbations is extended by the accurate prediction of DS cerebellar phenotypes.