The power of comparative and developmental studies for mouse models of Down syndrome

Since the genetic basis for Down syndrome (DS) was described, understanding the causative relationship between genes at dosage imbalance and phenotypes associated with DS has been a principal goal of researchers studying trisomy 21 (Ts21). Though inferences to the gene-phenotype relationship in humans have been made, evidence linking a specific gene or region to a particular congenital phenotype has been limited. To further understand the genetic basis for DS phenotypes, mouse models with three copies of human chromosome 21 (Hsa21) orthologs have been developed. Mouse models offer access to every tissue at each stage of development, opportunity to manipulate genetic content, and ability to precisely quantify phenotypes. Numerous approaches to recreate trisomic composition and analyze phenotypes similar to DS have resulted in diverse trisomic mouse models. A murine intraspecies comparative analysis of different genetic models of Ts21 and specific DS phenotypes reveals the complexity of trisomy and important considerations to understand the etiology of and strategies for amelioration or prevention of trisomic phenotypes. By analyzing individual phenotypes in different mouse models throughout development, such as neurologic, craniofacial, and cardiovascular abnormalities, greater insight into the gene-phenotype relationship has been demonstrated. In this review we discuss how phenotype-based comparisons between DS mouse models have been useful in analyzing the relationship of trisomy and DS phenotypes.     

Trisomy 21 and Down syndrome: a short review

Even though the molecular mechanisms underlying the Down syndrome (DS) phenotypes remain obscure, the characterization of the genes and conserved non-genic sequences of HSA21 together with large-scale gene expression studies in DS tissues are enhancing our understanding of this complex disorder. Also, mouse models of DS provide invaluable tools to correlate genes or chromosome segments to specific phenotypes. Here we discuss the possible contribution of HSA21 genes to DS and data from global gene expression studies of trisomic samples.     

Down syndrome: searching for the genetic culprits

Down syndrome (DS) is caused by trisomy of human chromosome 21 (Hsa21) and results in a large number of phenotypes, including learning difficulties, cardiac defects, distinguishing facial features and leukaemia. These are likely to result from an increased dosage of one or more of the ∼310 genes present on Hsa21. The identification of these dosage-sensitive genes has become a major focus in DS research because it is essential for a full understanding of the molecular mechanisms underlying pathology, and might eventually lead to more effective therapy. The search for these dosage-sensitive genes is being carried out using both human and mouse genetics. Studies of humans with partial trisomy of Hsa21 have identified regions of this chromosome that contribute to different phenotypes. In addition, novel engineered mouse models are being used to map the location of dosage-sensitive genes, which, in a few cases, has led to the identification of individual genes that are causative for certain phenotypes. These studies have revealed a complex genetic interplay, showing that the diverse DS phenotypes are likely to be caused by increased copies of many genes, with individual genes contributing in different proportions to the variance in different aspects of the pathology.     

唐氏综合征小鼠模型的遗传背景和应用

唐氏综合征(Down syndrome, DS)是最常见的常染色体异常疾病,由人类21号染色体(human chromosome 21, Hsa21)的重复引起。由于Hsa21的直系同源基因分散于小鼠16、17和10号染色体上,所以用小鼠模拟人类唐氏综合征并不容易。早期的Ts65Dn小鼠虽然具有DS表型特征,但其重复片段由电离辐射产生,未包含所有Hsa21直系同源基因。2004年,Cre/LoxP重组酶系统介导的染色体编辑技术在Ts1Rhr小鼠中的成功应用,解决了特定片段重复化的难题,使DS小鼠模型在基因重复和表型模拟方面实现了精准化。本文从同源基因重复和DS表型模拟两方面简要介绍了不同时期DS小鼠模型的优势和局限,为科研人员在DS研究中对不同小鼠模型的选用提供了参考。     

Use of comparative physical and sequence mapping to annotate mouse chromosome 16 and human chromosome 21

Distal mouse chromosome 16 (MMU16) shares conserved linkage with human chromosome 21 (HSA21), trisomy for which causes Down syndrome (DS). A 4.5-Mb physical map extending from Cbr1 to Tmprss2 on MMU16 provides a minimal tiling path of P1 artificial chromosomes (PACs) for comparative mapping and genomic sequencing. Thirty-four expressed sequences were positioned on the mouse map, including 19 that were not physically mapped previously. This region of the mouse:human comparative map shows a high degree of evolutionary conservation of gene order and content, which differs only by insertion of one gene (in mouse) and a small inversion involving two adjacent genes. "Low-pass" (2.2x) mouse sequence from a portion of the contig was ordered and oriented along 510 kb of finished HSA21 sequence. In combination with 68 kb of unique PAC end sequence, the comparison provided confirmation of genes predicted by comparative mapping, indicated gene predictions that are likely to be incorrect, and identified three candidate genes in mouse and human that were not observed in the initial HSA21 sequence annotation. This comparative map and sequence derived from it are powerful tools for identifying genes and regulatory regions, information that will in turn provide insights into the genetic mechanisms by which trisomy 21 results in DS.     

CCR3 gene overexpression in patients with Down syndrome

Molecular Biology Reports - Chromosome 21 trisomy or Down syndrome (DS) is the most common genetic cause of intellectual disability (ID). DS is also associated with hypotonia, muscle weakness,...     

An additional human chromosome 21 causes suppression of neural fate of pluripotent mouse embryonic stem cells in a teratoma model

Background

Down syndrome (DS), caused by trisomy of human chromosome 21 (HSA21), is the most common genetic cause of mental retardation in humans. Among complex phenotypes, it displays a number of neural pathologies including smaller brain size, reduced numbers of neurons, reduced dendritic spine density and plasticity, and early Alzheimer-like neurodegeneration. Mouse models for DS show behavioural and cognitive defects, synaptic plasticity defects, and reduced hippocampal and cerebellar neuron numbers. Early postnatal development of both human and mouse-model DS shows the reduced capability of neuronal precursor cells to generate neurons. The exact molecular cause of this reduction, and the role played by increased dosage of individual HSA21 genes, remain unknown.

Results

We have subcutaneously injected mouse pluripotent ES cells containing a single freely segregating supernumerary human chromosome 21 (HSA21) into syngeneic mice, to generate transchromosomic teratomas. Transchromosomic cells and parental control cells were injected into opposite flanks of thirty mice in three independent experiments. Tumours were grown for 30 days, a time-span equivalent to combined intra-uterine, and early post-natal mouse development. When paired teratomas from the same animals were compared, transchromosomic tumours showed a three-fold lower percentage of neuroectodermal tissue, as well as significantly reduced mRNA levels for neuron specific (Tubb3) and glia specific (Gfap) genes, relative to euploid controls. Two thirds of transchromosomic tumours also showed a lack of PCR amplification with multiple primers specific for HSA21, which were present in the ES cells at the point of injection, thus restricting a commonly retained trisomy to less than a third of HSA21 genes.

Conclusion

We demonstrate that a supernumerary chromosome 21 causes Inhibition of Neuroectodermal DIfferentiation (INDI) of pluripotent ES cells. The data suggest that trisomy of less than a third of HSA21 genes, in two chromosomal regions, might be sufficient to cause this effect.     

Synaptic Vesicle Recycling Is Unaffected in the Ts65Dn Mouse Model of Down Syndrome

Down syndrome (DS) is the most common genetic cause of intellectual disability, and arises from trisomy of human chromosome 21. Accumulating evidence from studies of both DS patient tissue and mouse models has suggested that synaptic dysfunction is a key factor in the disorder. The presence of several genes within the DS trisomy that are either directly or indirectly linked to synaptic vesicle (SV) endocytosis suggested that presynaptic dysfunction could underlie some of these synaptic defects. Therefore we determined whether SV recycling was altered in neurons from the Ts65Dn mouse, the best characterised model of DS to date. We found that SV exocytosis, the size of the SV recycling pool, clathrin-mediated endocytosis, activity-dependent bulk endocytosis and SV generation from bulk endosomes were all unaffected by the presence of the Ts65Dn trisomy. These results were obtained using battery of complementary assays employing genetically-encoded fluorescent reporters of SV cargo trafficking, and fluorescent and morphological assays of fluid-phase uptake in primary neuronal culture. The absence of presynaptic dysfunction in central nerve terminals of the Ts65Dn mouse suggests that future research should focus on the established alterations in excitatory / inhibitory balance as a potential route for future pharmacotherapy.     

Human trisomy 21 fibroblasts rescue methotrexate toxic effect after treatment with 5-methyl-tetrahydrofolate and 5-formyl-tetrahydrofolate

Trisomy 21 causes Down syndrome (DS), the most common human genetic disorder and the leading genetic cause of intellectual disability. The alteration of one-carbon metabolism was described as the possible metabolic cause of the intellectual disability development in subjects with DS. One of the biochemical pathways involved in the one-carbon group transfer is the folate cycle. The cytotoxic drug methotrexate (MTX) is a folic acid (FA) analogue which inhibits the activity of dihydrofolate reductase enzyme involved in the one-carbon metabolic cycle. Trisomy 21 cells are more sensitive to the MTX effect than euploid cells, and in 1986 Jérôme Lejeune and Coll. demonstrated that MTX was twice as toxic in trisomy 21 lymphocytes than in control cells. In the present work, the rescue effect on MTX toxicity mediated by FA and some of its derivatives, tetrahydrofolate (THF), 5-formyl-THF, and 5-methyl-THF, in both normal and trisomy 21 skin fibroblast cells, was evaluated. A statistically significant rescue effect was obtained by 5-formyl-THF, 5-methyl-THF, and their combination, administered together with MTX. In conclusion, trisomy 21 fibroblast cell lines showed a good response to the rescue effects of 5-formyl-THF and 5-methyl-THF on the MTX toxicity almost as normal cell lines.     

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.     

Surveying the Down syndrome mouse model resource identifies critical regions responsible for chronic otitis media

Chronic otitis media (OM) is common in Down syndrome (DS), but underlying aetiology is unclear. We analysed the entire available mouse resource of partial trisomy models of DS looking for histological evidence of chronic middle-ear inflammation. We found a highly penetrant OM in the Dp(16)1Yey mouse, which carries a complete trisomy of MMU16. No OM was found in the Dp(17)1Yey mouse or the Dp(10)1Yey mouse, suggesting disease loci are located only on MMU16. The Ts1Cje, Ts1RhR, Ts2Yah, and Ts65Dn trisomies and the transchomosomic Tc1 mouse did not develop OM. On the basis of these findings, we propose a two-locus model for chronic middle-ear inflammation in DS, based upon epistasis of the regions of HSA21 not in trisomy in the Tc1 mouse. We also conclude that environmental factors likely play an important role in disease onset.     

Trisomic and allelic differences influence phenotypic variability during development of Down syndrome mice

Individuals with full or partial Trisomy 21 (Ts21) present with clinical features collectively referred to as Down syndrome (DS), although DS phenotypes vary in incidence and severity between individuals. Differing genetic and phenotypic content in individuals with DS as well as mouse models of DS facilitate the understanding of the correlation between specific genes and phenotypes associated with Ts21. The Ts1Rhr mouse model is trisomic for 33 genes (the "Down syndrome critical region" or DSCR) hypothesized to be responsible for many clinical DS features, including craniofacial dysmorphology with a small mandible. Experiments with Ts1Rhr mice showed that the DSCR was not sufficient to cause all DS phenotypes by identifying uncharacteristic craniofacial abnormalities not found in individuals with DS or other DS mouse models. We hypothesized that the origins of the larger, dysmorphic mandible observed in adult Ts1Rhr mice develop from larger embryonic craniofacial precursors. Because of phenotypic variability seen in subsequent studies with Ts1Rhr mice, we also hypothesized that genetic background differences would alter Ts1Rhr developmental phenotypes. Using Ts1Rhr offspring from two genetic backgrounds, we found differences in mandibular precursor volume as well as total embryonic volume and postnatal body size of Ts1Rhr and nontrisomic littermates. Additionally, we observed increased relative expression of Dyrk1a and differential expression of Ets2 on the basis of the genetic background in the Ts1Rhr mandibular precursor. Our results suggest that trisomic gene content and allelic differences in trisomic or nontrisomic genes influence variability in gene expression and developmental phenotypes associated with DS.     

Early Lineage Priming by Trisomy of Erg Leads to Myeloproliferation in a Down Syndrome Model

Down syndrome (DS), with trisomy of chromosome 21 (HSA21), is the commonest human aneuploidy. Pre-leukemic myeloproliferative changes in DS foetal livers precede the acquisition of GATA1 mutations, transient myeloproliferative disorder (DS-TMD) and acute megakaryocytic leukemia (DS-AMKL). Trisomy of the Erg gene is required for myeloproliferation in the Ts(17¹⁶)65Dn DS mouse model. We demonstrate here that genetic changes specifically attributable to trisomy of Erg lead to lineage priming of primitive and early multipotential progenitor cells in Ts(17¹⁶)65Dn mice, excess megakaryocyte-erythroid progenitors, and malignant myeloproliferation. Gene expression changes dependent on trisomy of Erg in Ts(17¹⁶)65Dn multilineage progenitor cells were correlated with those associated with trisomy of HSA21 in human DS hematopoietic stem and primitive progenitor cells. These data suggest a role for ERG as a regulator of hematopoietic lineage potential, and that trisomy of ERG in the context of DS foetal liver hemopoiesis drives the pre-leukemic changes that predispose to subsequent DS-TMD and DS-AMKL.     

Opposite Phenotypes of Muscle Strength and Locomotor Function in Mouse Models of Partial Trisomy and Monosomy 21 for the Proximal Hspa13-App Region

The trisomy of human chromosome 21 (Hsa21), which causes Down syndrome (DS), is the most common viable human aneuploidy. In contrast to trisomy, the complete monosomy (M21) of Hsa21 is lethal, and only partial monosomy or mosaic monosomy of Hsa21 is seen. Both conditions lead to variable physiological abnormalities with constant intellectual disability, locomotor deficits, and altered muscle tone. To search for dosage-sensitive genes involved in DS and M21 phenotypes, we created two new mouse models: the Ts3Yah carrying a tandem duplication and the Ms3Yah carrying a deletion of the Hspa13-App interval syntenic with 21q11.2-q21.3. Here we report that the trisomy and the monosomy of this region alter locomotion, muscle strength, mass, and energetic balance. The expression profiling of skeletal muscles revealed global changes in the regulation of genes implicated in energetic metabolism, mitochondrial activity, and biogenesis. These genes are downregulated in Ts3Yah mice and upregulated in Ms3Yah mice. The shift in skeletal muscle metabolism correlates with a change in mitochondrial proliferation without an alteration in the respiratory function. However, the reactive oxygen species (ROS) production from mitochondrial complex I decreased in Ms3Yah mice, while the membrane permeability of Ts3Yah mitochondria slightly increased. Thus, we demonstrated how the Hspa13-App interval controls metabolic and mitochondrial phenotypes in muscles certainly as a consequence of change in dose of Gabpa, Nrip1, and Atp5j. Our results indicate that the copy number variation in the Hspa13-App region has a peripheral impact on locomotor activity by altering muscle function.     

Too much of a good thing: mechanisms of gene action in Down syndrome

The molecular mechanisms underlying the specific traits in individuals with Down syndrome (DS) have been postulated to derive either from nonspecific perturbation of balanced genetic programs, or from the simple, mendelian-like influence of a small subset of genes on chromosome 21. However, these models do not provide a comprehensive explanation for experimental or clinical observations of the effects of trisomy 21. DS is best viewed as a complex genetic disorder, where the specific phenotypic manifestations in a given individual are products of genetic, environmental and stochastic influences. Mouse models that recapitulate both the genetic basis for and the phenotypic consequences of trisomy provide an experimental system to define these contributions.     

Down syndrome mouse models are looking up

A new mouse model of Down syndrome (DS) carries a copy of human chromosome 21 (Hsa21), in addition to a full complement of mouse chromosomes. In terms of the number of trisomic genes represented, this model, known as 'Tc1', is closer to the genetic background of DS than any previous model. The Tc1 model not only recapitulates several of the DS features present in other mouse models but also exhibits heart defects that are similar to those that make trisomy 21 the leading cause of congenital heart disease in humans. Many cells in adult Tc1 mice show mosaicism - that is, the Hsa21 is lost from some cells during development - increasing the complexity of analyses using this model. Tc1 mice provide a powerful tool for investigation of the pathogenesis of trisomy 21, and a platform for analysis of similarities and differences in the evolution of gene regulation.     

CASP3 protein expression by flow cytometry in Down’s syndrome subjects

Down’s syndrome (DS), the most common chromosomal disorder, is caused by 21 trisomy and is featured by intellectual disability. Subjects with DS can develop some traits of Alzheimer disease (AD) at an earlier age than subjects without trisomy 21. Apoptosis is a programmed cell death process under both normal physiological and pathological conditions. Caspase-3 (CASP3) plays an important role in neuronal death during nervous system development and under certain pathological conditions. Furthermore, in vitro and in vivo studies report elevated expression and activation of CASP3 in models of AD. On this account, the expression of CASP3 gene was evaluated in cultures of fibroblasts of DS and normal subjects by flow cytometry. CASP3 protein was up-regulated in fibroblasts of DS. The data obtained from this study strengthen the hypothesis that the over-expression of CASP3 gene could have a role in the activation of the apoptotic pathways acting in the neurodegenerative processes in DS.