Opposite phenotypes of muscle strength and locomotor function in mouse models of partial trisomy and monosomy 21 for the proximal Hspa13-App region

Neuronal oscillations in cognition: Down syndrome as a model of mouse to human translation

Down syndrome (DS), a prevalent cognitive disorder resulting from trisomy of human chromosome 21 (Hsa21), poses a significant global health concern. Affecting approximately 1 in 800 live births worldwide, DS is the leading genetic cause of intellectual disability and a major predisposing factor for early-onset Alzheimer's dementia. The estimated global population of individuals with DS is 6 million, with increasing prevalence due to advances in DS health care. Global efforts are dedicated to unraveling the mechanisms behind the varied clinical outcomes in DS. Recent studies on DS mouse models reveal disrupted neuronal circuits, providing insights into DS pathologies. Yet, translating these findings to humans faces challenges due to limited systematic electrophysiological analyses directly comparing human and mouse. Additionally, disparities in experimental procedures between the two species pose hurdles to successful translation. This review provides a concise overview of neuronal oscillations in human and rodent cognition. Focusing on recent DS mouse model studies, we highlight disruptions in associated brain function. We discuss various electrophysiological paradigms and suggest avenues for exploring molecular dysfunctions contributing to DS-related cognitive impairments. Deciphering neuronal oscillation intricacies holds promise for targeted therapies to alleviate cognitive disabilities in DS individuals.

Targeting chromosomal instability in patients with cancer

Chromosomal instability (CIN) is a hallmark of cancer and a driver of metastatic dissemination, therapeutic resistance, and immune evasion. CIN is present in 60-80% of human cancers and poses a formidable therapeutic challenge as evidenced by the lack of clinically approved drugs that directly target CIN. This limitation in part reflects a lack of well-defined druggable targets as well as a dearth of tractable biomarkers enabling direct assessment and quantification of CIN in patients with cancer. Over the past decade, however, our understanding of the cellular mechanisms and consequences of CIN has greatly expanded, revealing novel therapeutic strategies for the treatment of chromosomally unstable tumours as well as new methods of assessing the dynamic nature of chromosome segregation errors that define CIN. In this Review, we describe advances that have shaped our understanding of CIN from a translational perspective, highlighting both challenges and opportunities in the development of therapeutic interventions for patients with chromosomally unstable cancers.

Long-read sequencing reveals the complex structure of extra dic(21;21) chromosome and its biological effects

Complex congenital chromosome abnormalities are rare but often cause severe symptoms. However, the structures and biological impacts of such abnormalities have seldomly been analyzed at the molecular level. Previously, we reported a Japanese female patient with severe developmental defects. The patient had an extra dicentric chromosome 21 (chr21) consisting of two partial chr21 copies fused together within their long arms along with two centromeres and many copy number changes. In this study, we performed whole-genome, transcriptional, and DNA methylation analyses, coupled with novel bioinformatic approaches, to reveal the complex structure of the extra chromosome and its transcriptional and epigenetic changes. Long-read sequencing accurately identified the structures of junctions related to the copy number changes in extra chr21 and suggested the mechanism of the structural changes. Our transcriptome analysis showed the overexpression of genes in extra chr21. Additionally, an allele-specific DNA methylation analysis of the long-read sequencing data suggested that the centromeric region of extra chr21 was hypermethylated, a property associated with the inactivation of one centromere in the extra chromosome. Our comprehensive analysis provides insights into the molecular mechanism underlying the generation of the extra chromosome and its pathogenic roles.

Dysregulated systemic metabolism in a Down syndrome mouse model

OBJECTIVE

Trisomy 21 is one of the most complex genetic perturbations compatible with postnatal survival. Dosage imbalance arising from the triplication of genes on human chromosome 21 (Hsa21) affects multiple organ systems. Much of Down syndrome (DS) research, however, has focused on addressing how aneuploidy dysregulates CNS function leading to cognitive deficit. Although obesity, diabetes, and associated sequelae such as fatty liver and dyslipidemia are well documented in the DS population, only limited studies have been conducted to determine how gene dosage imbalance affects whole-body metabolism. Here, we conduct a comprehensive and systematic analysis of key metabolic parameters across different physiological states in the Ts65Dn trisomic mouse model of DS.

METHODS

Ts65Dn mice and euploid littermates were subjected to comprehensive metabolic phenotyping under basal (chow-fed) state and the pathophysiological state of obesity induced by a high-fat diet (HFD). RNA sequencing of liver, skeletal muscle, and two major fat depots were conducted to determine the impact of aneuploidy on tissue transcriptome. Pathway enrichments, gene-centrality, and key driver estimates were performed to provide insights into tissue autonomous and non-autonomous mechanisms contributing to the dysregulation of systemic metabolism.

RESULTS

Under the basal state, chow-fed Ts65Dn mice of both sexes had elevated locomotor activity and energy expenditure, reduced fasting serum cholesterol levels, and mild glucose intolerance. Sexually dimorphic deterioration in metabolic homeostasis became apparent when mice were challenged with a high-fat diet. While obese Ts65Dn mice of both sexes exhibited dyslipidemia, male mice also showed impaired systemic insulin sensitivity, reduced mitochondrial activity, and elevated fibrotic and inflammatory gene signatures in the liver and adipose tissue. Systems-level analysis highlighted conserved pathways and potential endocrine drivers of adipose-liver crosstalk that contribute to dysregulated glucose and lipid metabolism.

CONCLUSIONS

A combined alteration in the expression of trisomic and disomic genes in peripheral tissues contribute to metabolic dysregulations in Ts65Dn mice. These data lay the groundwork for understanding the impact of aneuploidy on in vivo metabolism.

Comprehensive phenotypic analysis of the Dp1Tyb mouse strain reveals a broad range of Down syndrome-related phenotypes

Down syndrome (DS), trisomy 21, results in many complex phenotypes including cognitive deficits, heart defects and craniofacial alterations. Phenotypes arise from an extra copy of human chromosome 21 (Hsa21) genes. However, these dosage-sensitive causative genes remain unknown. Animal models enable identification of genes and pathological mechanisms. The Dp1Tyb mouse model of DS has an extra copy of 63% of Hsa21-orthologous mouse genes. In order to establish whether this model recapitulates DS phenotypes, we comprehensively phenotyped Dp1Tyb mice using 28 tests of different physiological systems and found that 468 out of 1800 parameters were significantly altered. We show that Dp1Tyb mice have wide-ranging DS-like phenotypes, including aberrant erythropoiesis and megakaryopoiesis, reduced bone density, craniofacial changes, altered cardiac function, a pre-diabetic state, and deficits in memory, locomotion, hearing and sleep. Thus, Dp1Tyb mice are an excellent model for investigating complex DS phenotype-genotype relationships for this common disorder.

Modulation of mitochondrial and inflammatory homeostasis through RIP140 is neuroprotective in an adrenoleukodystrophy mouse model

AIMS

Mitochondrial dysfunction and inflammation are at the core of axonal degeneration in several multifactorial neurodegenerative diseases, including multiple sclerosis, Alzheimer's disease, and Parkinson's disease. The transcriptional coregulator RIP140/NRIP1 (receptor-interacting protein 140) modulates these functions in liver and adipose tissue, but its role in the nervous system remains unexplored. Here, we investigated the impact of RIP140 in the Abcd1^- mouse model of X-linked adrenoleukodystrophy (X-ALD), a genetic model of chronic axonopathy involving the convergence of redox imbalance, bioenergetic failure, and chronic inflammation.

METHODS AND RESULTS

We provide evidence that RIP140 is modulated through a redox-dependent mechanism driven by very long-chain fatty acids (VLCFAs), the levels of which are increased in X-ALD. Genetic inactivation of RIP140 prevented mitochondrial depletion and dysfunction, bioenergetic failure, inflammatory dysregulation, axonal degeneration and associated locomotor disabilities in vivo in X-ALD mouse models.

CONCLUSIONS

Together, these findings show that aberrant overactivation of RIP140 promotes neurodegeneration in X-ALD, underscoring its potential as a therapeutic target for X-ALD and other neurodegenerative disorders that present with metabolic and inflammatory dyshomeostasis.

Down syndrome is an oxidative phosphorylation disorder

Down syndrome is the most common genomic disorder of intellectual disability and is caused by trisomy of chromosome 21. Several genes in this chromosome repress mitochondrial biogenesis. The goal of this study was to evaluate whether early overexpression of these genes may cause a prenatal impairment of oxidative phosphorylation negatively affecting neurogenesis. Reduction in the mitochondrial energy production and a lower mitochondrial function have been reported in diverse tissues or cell types, and also at any age, including early fetuses, suggesting that a defect in oxidative phosphorylation is an early and general event in Down syndrome individuals. Moreover, many of the medical conditions associated with Down syndrome are also frequently found in patients with oxidative phosphorylation disease. Several drugs that enhance mitochondrial biogenesis are nowadays available and some of them have been already tested in mouse models of Down syndrome restoring neurogenesis and cognitive defects. Because neurogenesis relies on a correct mitochondrial function and critical periods of brain development occur mainly in the prenatal and early neonatal stages, therapeutic approaches intended to improve oxidative phosphorylation should be provided in these periods.

Multi-influential genetic interactions alter behaviour and cognition through six main biological cascades in Down syndrome mouse models

Down syndrome (DS) is the most common genetic form of intellectual disability caused by the presence of an additional copy of human chromosome 21 (Hsa21). To provide novel insights into genotype–phenotype correlations, we used standardized behavioural tests, magnetic resonance imaging and hippocampal gene expression to screen several DS mouse models for the mouse chromosome 16 region homologous to Hsa21. First, we unravelled several genetic interactions between different regions of chromosome 16 and how they contribute significantly to altering the outcome of the phenotypes in brain cognition, function and structure. Then, in-depth analysis of misregulated expressed genes involved in synaptic dysfunction highlighted six biological cascades centred around DYRK1A, GSK3β, NPY, SNARE, RHOA and NPAS4. Finally, we provide a novel vision of the existing altered gene–gene crosstalk and molecular mechanisms targeting specific hubs in DS models that should become central to better understanding of DS and improving the development of therapies.

Meta-analysis of metabolites involved in bioenergetic pathways reveals a pseudohypoxic state in Down syndrome

Clinical observations and preclinical studies both suggest that Down syndrome (DS) may be associated with significant metabolic and bioenergetic alterations. However, the relevant scientific literature has not yet been systematically reviewed. The aim of the current study was to conduct a meta-analysis of metabolites involved in bioenergetics pathways in DS to conclusively determine the difference between DS and control subjects. We discuss these findings and their potential relevance in the context of pathogenesis and experimental therapy of DS. Articles published before July 1, 2020, were identified by using the search terms “Down syndrome” and “metabolite name” or “trisomy 21” and “metabolite name”. Moreover, DS-related metabolomics studies and bioenergetics literature were also reviewed. 41 published reports and associated databases were identified, from which the descriptive information and the relevant metabolomic parameters were extracted and analyzed. Mixed effect model revealed the following changes in DS: significantly decreased ATP, CoQ10, homocysteine, serine, arginine and tyrosine; slightly decreased ADP; significantly increased uric acid, succinate, lactate and cysteine; slightly increased phosphate, pyruvate and citrate. However, the concentrations of AMP, 2,3-diphosphoglycerate, glucose, and glutamine were comparable in the DS vs. control populations. We conclude that cells of subjects with DS are in a pseudo-hypoxic state: the cellular metabolic and bio-energetic mechanisms exhibit pathophysiological alterations that resemble the cellular responses associated with hypoxia, even though the supply of the cells with oxygen is not disrupted. This fundamental alteration may be, at least in part, responsible for a variety of functional deficits associated with DS, including reduced exercise difference, impaired neurocognitive status and neurodegeneration.

Peroxiredoxin2 Deficiency Aggravates Aging-Induced Insulin Resistance and Declines Muscle Strength

This study examined the role of peroxiredoxin2 (Prx2) in aging-induced insulin resistance and reduction in skeletal muscle function in young (2-month-old) and old (24-month-old) Prx2 knockout (KO) and wild-type mice. Plasma insulin levels increased with aging in Prx2 KO mice but not in wild-type mice. Insulin sensitivity in the whole-body and skeletal muscle as assessed with the hyperinsulinemic-euglycemic clamp was lower in Prx2 KO mice than in wild-type mice in the old group but was not significantly different between the two genotypes in the young group. Insulin-induced activation of intracellular signaling molecules was also suppressed in old Prx2 KO mice compared to their wild-type littermates. Oxidative stress, inflammation, and p53 expression levels in skeletal muscle were higher in Prx2 KO mice than in wild-type mice in the old group but were not different between the two genotypes in the young group. p53 expression was negatively correlated with skeletal muscle insulin sensitivity in old mice. Skeletal muscle mass was similar between the two genotypes but grip strength was reduced in old Prx2 KO mice compared to old wild-type mice. These results suggest that Prx2 plays a protective role in aging-induced insulin resistance and declines in muscle strength by suppressing oxidative stress.

Modeling Down syndrome in animals from the early stage to the 4.0 models and next

The genotype-phenotype relationship and the physiopathology of Down Syndrome (DS) have been explored in the last 20 years with more and more relevant mouse models. From the early age of transgenesis to the new CRISPR/CAS9-derived chromosomal engineering and the transchromosomic technologies, mouse models have been key to identify homologous genes or entire regions homologous to the human chromosome 21 that are necessary or sufficient to induce DS features, to investigate the complexity of the genetic interactions that are involved in DS and to explore therapeutic strategies. In this review we report the new developments made, how genomic data and new genetic tools have deeply changed our way of making models, extended our panel of animal models, and increased our understanding of the neurobiology of the disease. But even if we have made an incredible progress which promises to make DS a curable condition, we are facing new research challenges to nurture our knowledge of DS pathophysiology as a neurodevelopmental disorder with many comorbidities during ageing.

Co-Operation between Aneuploidy and Metabolic Changes in Driving Tumorigenesis

Alterations from the normal set of chromosomes are extremely common as cells progress toward tumourigenesis. Similarly, we expect to see disruption of normal cellular metabolism, particularly in the use of glucose. In this review, we discuss the connections between these two processes: how chromosomal aberrations lead to metabolic disruption, and vice versa. Both processes typically result in the production of elevated levels of reactive oxygen species, so we particularly focus on their role in mediating oncogenic changes.

The impact of aneuploidy on cellular homeostasis

Genomic instability is a common feature of tumours that has a wide range of disruptive effects on cellular homeostasis. In this review we briefly discuss how instability comes about, then focus on the impact of gain or loss of DNA (aneuploidy) on oxidative stress. We discuss several mechanisms that lead from aneuploidy to the production of reactive oxygen species, including the effects on protein complex stoichiometry, endoplasmic reticulum stress and metabolic disruption. Each of these are involved in positive feedback loops that amplify relatively minor genetic changes into major cellular disruption or cell death, depending on the capacity of the cell to induce antioxidants or processes such as mitophagy that can moderate the disruption. Finally we examine the direct effects of reactive oxygen species on mitosis and how oxidative stress can compromise centrosome number, cytoskeletal integrity and signalling processes that are vital for mitotic fidelity.

Cbs overdosage is necessary and sufficient to induce cognitive phenotypes in mouse models of Down syndrome and interacts genetically with Dyrk1a

Identifying dosage-sensitive genes is a key to understand the mechanisms underlying intellectual disability in Down syndrome (DS). The Dp(17Abcg1-Cbs)1Yah DS mouse model (Dp1Yah) shows cognitive phenotypes that need to be investigated to identify the main genetic driver. Here, we report that three copies of the cystathionine-beta-synthase gene (Cbs) in the Dp1Yah mice are necessary to observe a deficit in the novel object recognition (NOR) paradigm. Moreover, the overexpression of Cbs alone is sufficient to induce deficits in the NOR test. Accordingly, overexpressing human CBS specifically in Camk2a-expressing neurons leads to impaired objects discrimination. Altogether, this shows that Cbs overdosage is involved in DS learning and memory phenotypes. To go further, we identified compounds that interfere with the phenotypical consequence of CBS overdosage in yeast. Pharmacological intervention in Tg(CBS) mice with one selected compound restored memory in the NOR test. In addition, using a genetic approach, we demonstrated an epistatic interaction between Cbs and Dyrk1a, another human chromosome 21-located gene (which encodes the dual-specificity tyrosine phosphorylation-regulated kinase 1a) and an already known target for DS therapeutic intervention. Further analysis using proteomic approaches highlighted several molecular pathways, including synaptic transmission, cell projection morphogenesis and actin cytoskeleton, that are affected by DYRK1A and CBS overexpression. Overall, we demonstrated that CBS overdosage underpins the DS-related recognition memory deficit and that both CBS and DYRK1A interact to control accurate memory processes in DS. In addition, our study establishes CBS as an intervention point for treating intellectual deficiencies linked to DS.

Rodent models in Down syndrome research: impact and future opportunities

Down syndrome is caused by trisomy of chromosome 21. To date, a multiplicity of mouse models with Down-syndrome-related features has been developed to understand this complex human chromosomal disorder. These mouse models have been important for determining genotype-phenotype relationships and identification of dosage-sensitive genes involved in the pathophysiology of the condition, and in exploring the impact of the additional chromosome on the whole genome. Mouse models of Down syndrome have also been used to test therapeutic strategies. Here, we provide an overview of research in the last 15 years dedicated to the development and application of rodent models for Down syndrome. We also speculate on possible and probable future directions of research in this fast-moving field. As our understanding of the syndrome improves and genome engineering technologies evolve, it is necessary to coordinate efforts to make all Down syndrome models available to the community, to test therapeutics in models that replicate the whole trisomy and design new animal models to promote further discovery of potential therapeutic targets.

Summary: Mouse models have boosted therapeutic options for Down syndrome, and improved models are being developed to better understand the pathophysiology of this genetic condition.

A Dynamic Simulation of Musculoskeletal Function in the Mouse Hindlimb During Trotting Locomotion

Mice are often used as animal models of various human neuromuscular diseases, and analysis of these models often requires detailed gait analysis. However, little is known of the dynamics of the mouse musculoskeletal system during locomotion. In this study, we used computer optimization procedures to create a simulation of trotting in a mouse, using a previously developed mouse hindlimb musculoskeletal model in conjunction with new experimental data, allowing muscle forces, activation patterns, and levels of mechanical work to be estimated. Analyzing musculotendon unit (MTU) mechanical work throughout the stride allowed a deeper understanding of their respective functions, with the rectus femoris MTU dominating the generation of positive and negative mechanical work during the swing and stance phases. This analysis also tested previous functional inferences of the mouse hindlimb made from anatomical data alone, such as the existence of a proximo-distal gradient of muscle function, thought to reflect adaptations for energy-efficient locomotion. The results do not strongly support the presence of this gradient within the mouse musculoskeletal system, particularly given relatively high negative net work output from the ankle plantarflexor MTUs, although more detailed simulations could test this further. This modeling analysis lays a foundation for future studies of the control of vertebrate movement through the development of neuromechanical simulations.

Analysis of motor dysfunction in Down Syndrome reveals motor neuron degeneration

Down Syndrome (DS) is caused by trisomy of chromosome 21 (Hsa21) and results in a spectrum of phenotypes including learning and memory deficits, and motor dysfunction. It has been hypothesized that an additional copy of a few Hsa21 dosage-sensitive genes causes these phenotypes, but this has been challenged by observations that aneuploidy can cause phenotypes by the mass action of large numbers of genes, with undetectable contributions from individual sequences. The motor abnormalities in DS are relatively understudied-the identity of causative dosage-sensitive genes and the mechanism underpinning the phenotypes are unknown. Using a panel of mouse strains with duplications of regions of mouse chromosomes orthologous to Hsa21 we show that increased dosage of small numbers of genes causes locomotor dysfunction and, moreover, that the Dyrk1a gene is required in three copies to cause the phenotype. Furthermore, we show for the first time a new DS phenotype: loss of motor neurons both in mouse models and, importantly, in humans with DS, that may contribute to locomotor dysfunction.

Systematic proteome and proteostasis profiling in human Trisomy 21 fibroblast cells

Down syndrome (DS) is mostly caused by a trisomy of the entire Chromosome 21 (Trisomy 21, T21). Here, we use SWATH mass spectrometry to quantify protein abundance and protein turnover in fibroblasts from a monozygotic twin pair discordant for T21, and to profile protein expression in 11 unrelated DS individuals and matched controls. The integration of the steady-state and turnover proteomic data indicates that protein-specific degradation of members of stoichiometric complexes is a major determinant of T21 gene dosage outcome, both within and between individuals. This effect is not apparent from genomic and transcriptomic data. The data also reveal that T21 results in extensive proteome remodeling, affecting proteins encoded by all chromosomes. Finally, we find broad, organelle-specific post-transcriptional effects such as significant downregulation of the mitochondrial proteome contributing to T21 hallmarks. Overall, we provide a valuable proteomic resource to understand the origin of DS phenotypic manifestations.

Trisomy 21 (T21) is a major cause of Down syndrome but little is known about its impact on the cellular proteome. Here, the authors define the proteome of T21 fibroblasts and its turnover and also map proteomic differences in monozygotic T21-discordant twins, revealing extensive, organelle-specific changes caused by T21.

Efficient and rapid generation of large genomic variants in rats and mice using CRISMERE

Modelling Down syndrome (DS) in mouse has been crucial for the understanding of the disease and the evaluation of therapeutic targets. Nevertheless, the modelling so far has been limited to the mouse and, even in this model, generating duplication of genomic regions has been labour intensive and time consuming. We developed the CRISpr MEdiated REarrangement (CRISMERE) strategy, which takes advantage of the CRISPR/Cas9 system, to generate most of the desired rearrangements from a single experiment at much lower expenses and in less than 9 months. Deletions, duplications, and inversions of genomic regions as large as 24.4 Mb in rat and mouse founders were observed and germ line transmission was confirmed for fragment as large as 3.6 Mb. Interestingly we have been able to recover duplicated regions from founders in which we only detected deletions. CRISMERE is even more powerful than anticipated it allows the scientific community to manipulate the rodent and probably other genomes in a fast and efficient manner which was not possible before.

Down syndrome and the complexity of genome dosage imbalance

Down syndrome (also known as trisomy 21) is the model human phenotype for all genomic gain dosage imbalances, including microduplications. The functional genomic exploration of the post-sequencing years of chromosome 21, and the generation of numerous cellular and mouse models, have provided an unprecedented opportunity to decipher the molecular consequences of genome dosage imbalance. Studies of Down syndrome could provide knowledge far beyond the well-known characteristics of intellectual disability and dysmorphic features, as several other important features, including congenital heart defects, early ageing, Alzheimer disease and childhood leukaemia, are also part of the Down syndrome phenotypic spectrum. The elucidation of the molecular mechanisms that cause or modify the risk for different Down syndrome phenotypes could lead to the introduction of previously unimaginable therapeutic options.

Mouse-based genetic modeling and analysis of Down syndrome

INTRODUCTION

Down syndrome (DS), caused by human trisomy 21 (Ts21), can be considered as a prototypical model for understanding the effects of chromosomal aneuploidies in other diseases. Human chromosome 21 (Hsa21) is syntenically conserved with three regions in the mouse genome.

SOURCES OF DATA

A review of recent advances in genetic modeling and analysis of DS. Using Cre/loxP-mediated chromosome engineering, a substantial number of new mouse models of DS have recently been generated, which facilitates better understanding of disease mechanisms in DS.

AREAS OF AGREEMENT

Based on evolutionary conservation, Ts21 can be modeled by engineered triplication of Hsa21 syntenic regions in mice. The validity of the models is supported by the exhibition of DS-related phenotypes.

AREAS OF CONTROVERSY

Although substantial progress has been made, it remains a challenge to unravel the relative importance of specific candidate genes and molecular mechanisms underlying the various clinical phenotypes.

GROWING POINTS

Further understanding of mechanisms based on data from mouse models, in parallel with human studies, may lead to novel therapies for clinical manifestations of Ts21 and insights to the roles of aneuploidies in other developmental disorders and cancers.

Mouse models of Down syndrome: gene content and consequences

Down syndrome (DS), trisomy of human chromosome 21 (Hsa21), is challenging to model in mice. Not only is it a contiguous gene syndrome spanning 35 Mb of the long arm of Hsa21, but orthologs of Hsa21 genes map to segments of three mouse chromosomes, Mmu16, Mmu17, and Mmu10. The Ts65Dn was the first viable segmental trisomy mouse model for DS; it is a partial trisomy currently popular in preclinical evaluations of drugs for cognition in DS. Limitations of the Ts65Dn are as follows: (i) it is trisomic for 125 human protein-coding orthologs, but only 90 of these are Hsa21 orthologs and (ii) it lacks trisomy for ~75 Hsa21 orthologs. In recent years, several additional mouse models of DS have been generated, each trisomic for a different subset of Hsa21 genes or their orthologs. To best exploit these models and interpret the results obtained with them, prior to proposing clinical trials, an understanding of their trisomic gene content, relative to full trisomy 21, is necessary. Here we first review the functional information on Hsa21 protein-coding genes and the more recent annotation of a large number of functional RNA genes. We then discuss the conservation and genomic distribution of Hsa21 orthologs in the mouse genome and the distribution of mouse-specific genes. Lastly, we consider the strengths and weaknesses of mouse models of DS based on the number and nature of the Hsa21 orthologs that are, and are not, trisomic in each, and discuss their validity for use in preclinical evaluations of drug responses.

Muscle moment arms and sensitivity analysis of a mouse hindlimb musculoskeletal model

Musculoskeletal modelling has become a valuable tool with which to understand how neural, muscular, skeletal and other tissues are integrated to produce movement. Most musculoskeletal modelling work has to date focused on humans or their close relatives, with few examples of quadrupedal animal limb models. A musculoskeletal model of the mouse hindlimb could have broad utility for questions in medicine, genetics, locomotion and neuroscience. This is due to this species’ position as a premier model of human disease, having an array of genetic tools for manipulation of the animal in vivo, and being a small quadruped, a category for which few models exist. Here, the methods used to develop the first three‐dimensional (3D) model of a mouse hindlimb and pelvis are described. The model, which represents bones, joints and 39 musculotendon units, was created through a combination of previously gathered muscle architecture data from microdissections, contrast‐enhanced micro‐computed tomography (CT) scanning and digital segmentation. The model allowed muscle moment arms as well as muscle forces to be estimated for each musculotendon unit throughout a range of joint rotations. Moment arm analysis supported the reliability of musculotendon unit placement within the model, and comparison to a previously published rat hindlimb model further supported the model's reliability. A sensitivity analysis performed on both the force‐generating parameters and muscle's attachment points of the model indicated that the maximal isometric muscle moment is generally most sensitive to changes in either tendon slack length or the coordinates of insertion, although the degree to which the moment is affected depends on several factors. This model represents the first step in the creation of a fully dynamic 3D computer model of the mouse hindlimb and pelvis that has application to neuromuscular disease, comparative biomechanics and the neuromechanical basis of movement. Capturing the morphology and dynamics of the limb, it enables future dissection of the complex interactions between the nervous and musculoskeletal systems as well as the environment.

Genetic dissection of Down syndrome-associated congenital heart defects using a new mouse mapping panel

Down syndrome (DS), caused by trisomy of human chromosome 21 (Hsa21), is the most common cause of congenital heart defects (CHD), yet the genetic and mechanistic causes of these defects remain unknown. To identify dosage-sensitive genes that cause DS phenotypes, including CHD, we used chromosome engineering to generate a mapping panel of 7 mouse strains with partial trisomies of regions of mouse chromosome 16 orthologous to Hsa21. Using high-resolution episcopic microscopy and three-dimensional modeling we show that these strains accurately model DS CHD. Systematic analysis of the 7 strains identified a minimal critical region sufficient to cause CHD when present in 3 copies, and showed that it contained at least two dosage-sensitive loci. Furthermore, two of these new strains model a specific subtype of atrio-ventricular septal defects with exclusive ventricular shunting and demonstrate that, contrary to current hypotheses, these CHD are not due to failure in formation of the dorsal mesenchymal protrusion.

Down syndrome is a condition caused by having an extra copy of one of the 46 chromosomes found inside human cells. Specifically, instead of two copies, people with Down syndrome are born with three copies of chromosome 21. This results in many different effects, including learning and memory problems, heart defects and Alzheimer’s disease. Each of these different effects is caused by having a third copy of one or more of the approximately 230 genes found on chromosome 21. However, it is not known which of these genes cause any of these effects, and how an extra copy of the genes results in such changes.

Now, Lana-Elola et al. have investigated which genes on chromosome 21 cause the heart defects seen in Down syndrome, and how those heart defects come about. This involved engineering a new strain of mouse that has an extra copy of 148 mouse genes that are very similar to 148 genes found on chromosome 21 in humans. Like people with Down syndrome, this mouse strain developed heart defects when it was an embryo.

Using a series of six further mouse strains, Lana-Elola et al. then narrowed down the potential genes that, when in three copies, are needed to cause the heart defects, to a list of just 39 genes. Further experiments then showed that at least two genes within these 39 genes were required in three copies to cause the heart defects.

The next step will be to identify the specific genes that actually cause the heart defects, and then work out how a third copy of these genes causes the developmental problems.

Dissecting Alzheimer disease in Down syndrome using mouse models

Down syndrome (DS) is a common genetic condition caused by the presence of three copies of chromosome 21 (trisomy 21). This greatly increases the risk of Alzheimer disease (AD), but although virtually all people with DS have AD neuropathology by 40 years of age, not all develop dementia. To dissect the genetic contribution of trisomy 21 to DS phenotypes including those relevant to AD, a range of DS mouse models has been generated which are trisomic for chromosome segments syntenic to human chromosome 21. Here, we consider key characteristics of human AD in DS (AD-DS), and our current state of knowledge on related phenotypes in AD and DS mouse models. We go on to review important features needed in future models of AD-DS, to understand this type of dementia and so highlight pathogenic mechanisms relevant to all populations at risk of AD.