Mice containing a human chromosome 21 model behavioral impairment and cardiac anomalies of Down's syndrome

Modeling specific aneuploidies: from karyotype manipulations to biological insights

An abnormal chromosome number, or aneuploidy, underlies developmental disorders and is a common feature of cancer, with different cancer types exhibiting distinct patterns of chromosomal gains and losses. To understand how specific aneuploidies emerge in certain tissues and how they contribute to disease development, various methods have been developed to alter the karyotype of mammalian cells and mice. In this review, we provide an overview of both classic and novel strategies for inducing or selecting specific chromosomal gains and losses in human and murine cell systems. We highlight how these customized aneuploidy models helped expanding our knowledge of the consequences of specific aneuploidies to (cancer) cell physiology.

Panel of human cell lines with human/mouse artificial chromosomes

Human artificial chromosomes (HACs) and mouse artificial chromosomes (MACs) are non-integrating chromosomal gene delivery vectors for molecular biology research. Recently, microcell-mediated chromosome transfer (MMCT) of HACs/MACs has been achieved in various human cells that include human immortalised mesenchymal stem cells (hiMSCs) and human induced pluripotent stem cells (hiPSCs). However, the conventional strategy of gene introduction with HACs/MACs requires laborious and time-consuming stepwise isolation of clones for gene loading into HACs/MACs in donor cell lines (CHO and A9) and then transferring the HAC/MAC into cells via MMCT. To overcome these limitations and accelerate chromosome vector-based functional assays in human cells, we established various human cell lines (HEK293, HT1080, hiMSCs, and hiPSCs) with HACs/MACs that harbour a gene-loading site via MMCT. Model genes, such as tdTomato, TagBFP2, and ELuc, were introduced into these preprepared HAC/MAC-introduced cell lines via the Cre-loxP system or simultaneous insertion of multiple gene-loading vectors. The model genes on the HACs/MACs were stably expressed and the HACs/MACs were stably maintained in the cell lines. Thus, our strategy using this HAC/MAC-containing cell line panel has dramatically simplified and accelerated gene introduction via HACs/MACs.

A transchromosomic rat model with human chromosome 21 shows robust Down syndrome features

Progress in earlier detection and clinical management has increased life expectancy and quality of life in people with Down syndrome (DS). However, no drug has been approved to help individuals with DS live independently and fully. Although rat models could support more robust physiological, behavioral, and toxicology analysis than mouse models during preclinical validation, no DS rat model is available as a result of technical challenges. We developed a transchromosomic rat model of DS, TcHSA21rat, which contains a freely segregating, EGFP-inserted, human chromosome 21 (HSA21) with >93% of its protein-coding genes. RNA-seq of neonatal forebrains demonstrates that TcHSA21rat expresses HSA21 genes and has an imbalance in global gene expression. Using EGFP as a marker for trisomic cells, flow cytometry analyses of peripheral blood cells from 361 adult TcHSA21rat animals show that 81% of animals retain HSA21 in >80% of cells, the criterion for a "Down syndrome karyotype" in people. TcHSA21rat exhibits learning and memory deficits and shows increased anxiety and hyperactivity. TcHSA21rat recapitulates well-characterized DS brain morphology, including smaller brain volume and reduced cerebellar size. In addition, the rat model shows reduced cerebellar foliation, which is not observed in DS mouse models. Moreover, TcHSA21rat exhibits anomalies in craniofacial morphology, heart development, husbandry, and stature. TcHSA21rat is a robust DS animal model that can facilitate DS basic research and provide a unique tool for preclinical validation to accelerate DS drug development.

Engineering of human induced pluripotent stem cells via human artificial chromosome vectors for cell therapy and disease modeling

Genetic engineering of induced pluripotent stem cells (iPSCs) holds great promise for gene and cell therapy as well as drug discovery. However, there are potential concerns regarding the safety and control of gene expression using conventional vectors such as viruses and plasmids. Although human artificial chromosome (HAC) vectors have several advantages as a gene delivery vector, including stable episomal maintenance and the ability to carry large gene inserts, the full potential of HAC transfer into iPSCs still needs to be explored. Here, we provide evidence of a HAC transfer into human iPSCs by microcell-mediated chromosome transfer via measles virus envelope proteins for various applications, including gene and cell therapy, establishment of versatile human iPSCs capable of gene loading and differentiation into T cells, and disease modeling for aneuploidy syndrome. Thus, engineering of human iPSCs via desired HAC vectors is expected to be widely applied in biomedical research.

Centromere identity and function put to use: construction and transfer of mammalian artificial chromosomes to animal models

Mammalian artificial chromosomes (MACs) are widely used as gene expression vectors and have various advantages over conventional expression vectors. We review and discuss breakthroughs in MAC construction, initiation of functional centromeres allowing their faithful inheritance, and transfer from cell culture to animal model systems. These advances have contributed to advancements in synthetic biology, biomedical research, and applications in industry and in the clinic.

Establishment of FXS-A9 panel with a single human X chromosome from fragile X syndrome-associated individual

Fragile X syndrome (FXS) is the most common inheritable form of intellectual disability. FMR1, the gene responsible for FXS, is located on human chromosome Xq27.3 and contains a stretch of CGG trinucleotide repeats in its 5' untranslated region. FXS is caused by CGG repeats that expand beyond 200, resulting in FMR1 silencing via promoter hypermethylation. The molecular mechanism underlying CGG repeat expansion, a fundamental cause of FXS, remains poorly understood, partly due to a lack of experimental systems. Accumulated evidence indicates that the large chromosomal region flanking a CGG repeat is critical for repeat dynamics. In the present study, we isolated and introduced whole human X chromosomes from healthy, FXS premutation carriers, or FXS patients who carried disease condition-associated CGG repeat lengths, into mouse A9 cells via microcell-mediated chromosome transfer. The CGG repeat length-associated methylation status and human FMR1 expression in these monochromosomal hybrid cells mimicked those in humans. Thus, this set of A9 cells containing CGG repeats from three different origins (FXS-A9 panel) may provide a valuable resource for investigating a series of genetic and epigenetic CGG repeat dynamics during FXS pathogenesis.

All Creatures Great and Small: New Approaches for Understanding Down Syndrome Genetics

Human chromosome 21 (Hsa21) contains more than 500 genes, making trisomy 21 one of the most complex genetic perturbations compatible with life. The ultimate goal of Down syndrome (DS) research is to design therapies that improve quality of life for individuals with DS by understanding which subsets of Hsa21 genes contribute to DS-associated phenotypes throughout the lifetime. However, the complexity of DS pathogenesis has made developing appropriate animal models an ongoing challenge. Here, we examine lessons learned from a variety of model systems, including yeast, nematode, fruit fly, and zebrafish, and discuss emerging methods for creating murine models that better reflect the genetic basis of trisomy 21.

A non-mosaic transchromosomic mouse model of down syndrome carrying the long arm of human chromosome 21

Animal models of Down syndrome (DS), trisomic for human chromosome 21 (HSA21) genes or orthologs, provide insights into better understanding and treatment options. The only existing transchromosomic (Tc) mouse DS model, Tc1, carries a HSA21 with over 50 protein coding genes (PCGs) disrupted. Tc1 is mosaic, compromising interpretation of results. Here, we “clone” the 34 MB long arm of HSA21 (HSA21q) as a mouse artificial chromosome (MAC). Through multiple steps of microcell-mediated chromosome transfer, we created a new Tc DS mouse model, Tc(HSA21q;MAC)1Yakaz (“TcMAC21”). TcMAC21 is not mosaic and contains 93% of HSA21q PCGs that are expressed and regulatable. TcMAC21 recapitulates many DS phenotypes including anomalies in heart, craniofacial skeleton and brain, molecular/cellular pathologies, and impairments in learning, memory and synaptic plasticity. TcMAC21 is the most complete genetic mouse model of DS extant and has potential for supporting a wide range of basic and preclinical research.

Transchromosomic technology for genomically humanized animals

"Genomically" humanized animals are invaluable tools for generating human disease models and for biomedical research. Humanized animal models have generally been developed via conventional transgenic technologies; however, conventional gene delivery vectors such as viruses, plasmids, bacterial artificial chromosomes, P1 phase-derived artificial chromosomes, and yeast artificial chromosomes have limitations for transgenic animal creation as their loading gene capacity is restricted, and the expression of transgenes is unstable. Transchromosomic (Tc) techniques using mammalian artificial chromosomes, including human chromosome fragments, human artificial chromosomes, and mouse artificial chromosomes, have overcome these limitations. These tools can carry multiple genes or Mb-sized genomic loci and their associated regulatory elements, which has facilitated the creation of more useful and complex transgenic models for human disease, drug development, and humanized animal research. This review describes the history of Tc animal development, the applications of Tc animals, and future prospects.

Challenges and Opportunities for Translation of Therapies to Improve Cognition in Down Syndrome

While preclinical studies have reported improvement of behavioral deficits in the Ts65Dn mouse model of Down syndrome (DS), translation to human clinical trials to improve cognition in individuals with DS has had a poor success record. Timing of the intervention, choice of animal models, strategy for drug selection, and lack of translational endpoints between animals and humans contributed to prior failures of human clinical trials. Here, we focus on in vitro cell models from humans with DS to identify the molecular mechanisms underlying the brain phenotype associated with DS. We emphasize the importance of using these cell models to screen for therapeutic molecules, followed by validating them in the most suitable animal models prior to initiating human clinical trials.

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.

Humanising the mouse genome piece by piece

To better understand human health and disease, researchers create a wide variety of mouse models that carry human DNA. With recent advances in genome engineering, the targeted replacement of mouse genomic regions with orthologous human sequences has become increasingly viable, ranging from finely tuned humanisation of individual nucleotides and amino acids to the incorporation of many megabases of human DNA. Here, we examine emerging technologies for targeted genomic humanisation, we review the spectrum of existing genomically humanised mouse models and the insights such models have provided, and consider the lessons learned for designing such models in the future.

Generation of transgenic mice has become routine in studying gene function and disease mechanisms, but often this is not enough to fully understand human biology. Here, the authors review the current state of the art of targeted genomic humanisation strategies and their advantages over classic approaches.

Humanized UGT2 and CYP3A transchromosomic rats for improved prediction of human drug metabolism

Genomically humanized animals overcoming species differences are invaluable for biomedical research. Although rats would be preferred over mice for several applications, generation of a humanized model is restricted to mice due to the difficulty of complex genetic manipulations in rats. In this study, we successfully generated humanized rats with megabase-sized gene clusters via combination of chromosome transfer using mouse artificial chromosome vector and genome editing technologies. In the humanized UGT2 and CYP3A transchromosomic rats described in this paper, the expression of the human genes, as well as the pharmacokinetics and metabolism of relevant probe substrates, accurately mimic the situation in humans. Thus, the advanced technologies can be used to generate fully humanized rats useful for biomedical research.

Although “genomically” humanized animals are invaluable tools for generating human disease models as well as for biomedical research, their development has been mainly restricted to mice via established transgenic-based and embryonic stem cell-based technologies. Since rats are widely used for studying human disease and for drug efficacy and toxicity testing, humanized rat models would be preferred over mice for several applications. However, the development of sophisticated humanized rat models has been hampered by the difficulty of complex genetic manipulations in rats. Additionally, several genes and gene clusters, which are megabase range in size, were difficult to introduce into rats with conventional technologies. As a proof of concept, we herein report the generation of genomically humanized rats expressing key human drug-metabolizing enzymes in the absence of their orthologous rat counterparts via the combination of chromosome transfer using mouse artificial chromosome (MAC) and genome editing technologies. About 1.5 Mb and 700 kb of the entire UDP glucuronosyltransferase family 2 and cytochrome P450 family 3 subfamily A genomic regions, respectively, were successfully introduced via the MACs into rats. The transchromosomic rats were combined with rats carrying deletions of the endogenous orthologous genes, achieved by genome editing. In the “transchromosomic humanized” rat strains, the gene expression, pharmacokinetics, and metabolism observed in humans were well reproduced. Thus, the combination of chromosome transfer and genome editing technologies can be used to generate fully humanized rats for improved prediction of the pharmacokinetics and drug–drug interactions in humans, and for basic research, drug discovery, and development.

Generation of a novel isogenic trisomy panel in human embryonic stem cells via microcell-mediated chromosome transfer

Aneuploidy is the gain or loss of a chromosome. Down syndrome or trisomy (Ts) 21 is the most frequent live-born aneuploidy syndrome in humans and extensively studied using model mice. However, there is no available model mouse for other congenital Ts syndromes, possibly because of the lethality of Ts in vivo, resulting in the lack of studies to identify the responsible gene(s) for aneuploid syndromes. Although induced pluripotent stem cells derived from patients are useful to analyse aneuploidy syndromes, there are concerns about differences in the genetic background for comparative studies and clonal variations. Therefore, a model cell line panel with the same genetic background has been strongly desired for sophisticated comparative analyses. In this study, we established isogenic human embryonic stem (hES) cells of Ts8, Ts13, and Ts18 in addition to previously established Ts21 by transferring each single chromosome into parental hES cells via microcell-mediated chromosome transfer. Genes on each trisomic chromosome were globally overexpressed in each established cell line, and all Ts cell lines differentiated into all three embryonic germ layers. This cell line panel is expected to be a useful resource to elucidate molecular and epigenetic mechanisms of genetic imbalance and determine how aneuploidy is involved in various abnormal phenotypes including tumourigenesis and impaired neurogenesis.

Triplications of human chromosome 21 orthologous regions in mice result in expansion of megakaryocyte-erythroid progenitors and reduction of granulocyte-macrophage progenitors

Individuals with Down syndrome (DS) frequently have hematopoietic abnormalities, including transient myeloproliferative disorder and acute megakaryoblastic leukemia which are often accompanied by acquired GATA1 mutations that produce a truncated protein, GATA1s. The mouse has been used for modeling DS based on the syntenic conservation between human chromosome 21 (Hsa21) and three regions in the mouse genome located on mouse chromosome 10 (Mmu10), Mmu16 and Mmu17. To assess the impact of the dosage increase of Hsa21 gene orthologs on the hematopoietic system, we characterized the related phenotype in the Dp(10)1Yey/+;Dp(16)1Yey/+;Dp(17)1Yey/+ model which carries duplications spanning the entire Hsa21 orthologous regions on Mmu10, Mmu16 and Mmu17, and the Dp(10)1Yey/+;Dp(16)1Yey/+;Dp(17)1Yey/+;Gata1^Yeym2 model which carries a Gata1s mutation we engineered. Both models exhibited anemia, macrocytosis, and myeloproliferative disorder. Similar to human DS, the megakaryocyte-erythrocyte progenitors (MEPs) and granulocyte-monocyte progenitors (GMPs) were significantly increased and reduced, respectively, in both models. The subsequent identification of all the aforementioned phenotypes in the Dp(16)1Yey/+ model suggests that the causative dosage sensitive gene(s) are in the Hsa21 orthologous region on Mmu16. Therefore, we reveal here for the first time that the human trisomy 21-associated major segmental chromosomal alterations in mice can lead to expanded MEP and reduced GMP populations, mimicking the dynamics of these myeloid progenitors in DS. These models will provide the critical systems for unraveling the molecular and cellular mechanism of DS-associated myeloproliferative disorder, and particularly for determining how human trisomy 21 leads to expansion of MEPs as well as how such an alteration leads to myeloproliferative disorder.

Combinations of chromosome transfer and genome editing for the development of cell/animal models of human disease and humanized animal models

Chromosome transfer technology, including chromosome modification, enables the introduction of Mb-sized or multiple genes to desired cells or animals. This technology has allowed innovative developments to be made for models of human disease and humanized animals, including Down syndrome model mice and humanized transchromosomic (Tc) immunoglobulin mice. Genome editing techniques are developing rapidly, and permit modifications such as gene knockout and knockin to be performed in various cell lines and animals. This review summarizes chromosome transfer-related technologies and the combined technologies of chromosome transfer and genome editing mainly for the production of cell/animal models of human disease and humanized animal models. Specifically, these include: (1) chromosome modification with genome editing in Chinese hamster ovary cells and mouse A9 cells for efficient transfer to desired cell types; (2) single-nucleotide polymorphism modification in humanized Tc mice with genome editing; and (3) generation of a disease model of Down syndrome-associated hematopoiesis abnormalities by the transfer of human chromosome 21 to normal human embryonic stem cells and the induction of mutation(s) in the endogenous gene(s) with genome editing. These combinations of chromosome transfer and genome editing open up new avenues for drug development and therapy as well as for basic research.

CRISPR/Cas9-induced transgene insertion and telomere-associated truncation of a single human chromosome for chromosome engineering in CHO and A9 cells

Chromosome engineering techniques including gene insertion, telomere-associated truncation and microcell-mediated chromosome transfer (MMCT) are powerful tools for generation of humanised model animal, containing megabase-sized genomic fragments. However, these techniques require two cell lines: homologous recombination (HR)-proficient DT40 cells for chromosome modification, and CHO cells for transfer to recipient cells. Here we show an improved technique using a combination of CRISPR/Cas9-induced HR in CHO and mouse A9 cells without DT40 cells following MMCT to recipient cells. Transgene insertion was performed in CHO cells with the insertion of enhanced green fluorescence protein (EGFP) using CRISPR/Cas9 and a circular targeting vector containing two 3 kb HR arms. Telomere-associated truncation was performed in CHO cells using CRISPR/Cas9 and a linearised truncation vector containing a single 7 kb HR arm at the 5′ end, a 1 kb artificial telomere at the 3′ end. At least 11% and 6% of the targeting efficiency were achieved for transgene insertion and telomere-associated truncation, respectively. The transgene insertion was also confirmed in A9 cells (29%). The modified chromosomes were transferrable to other cells. Thus, this CHO and A9 cell-mediated chromosome engineering using the CRISPR/Cas9 for direct transfer of the modified chromosome is a rapid technique that will facilitate chromosome manipulation.

Transfer of a Mouse Artificial Chromosome into Spermatogonial Stem Cells Generates Transchromosomic Mice

The introduction of megabase-sized large DNA fragments into the germline has been a difficult task. Although microcell-mediated chromosome transfer into mouse embryonic stem cells (ESCs) allows the production of transchromosomic mice, ESCs have unstable karyotypes and germline transmission is unreliable by chimera formation. As spermatogonial stem cells (SSCs) are the only stem cells in the germline, they represent an attractive target for germline modification. Here, we report successful transfer of a mouse artificial chromosome (MAC) into mouse germline stem cells (GSCs), cultured spermatogonia enriched for SSCs. MAC-transferred GSCs maintained the host karyotype and MAC more stably than ESCs, which have significant variation in chromosome number. Moreover, MAC-transferred GSCs produced transchromosomic mice following microinjection into the seminiferous tubules of infertile recipients. Successful transfer of MACs to GSCs overcomes the problems associated with ESC-mediated germline transmission and provides new possibilities in germline modification.

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Retro-MMCT method allows transfer of a mouse artificial chromosome into GSCs

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GSCs maintained exogenous chromosomes more stably than ESCs

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Transchromosomic mice were born from GSCs following germ cell transplantation

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Unlike ESCs, transchromosomic mice were born directly in F1 generation

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.

Moving toward a higher efficiency of microcell-mediated chromosome transfer

Microcell-mediated chromosome transfer (MMCT) technology enables individual mammalian chromosomes, megabase-sized chromosome fragments, or mammalian artificial chromosomes that include human artificial chromosomes (HACs) and mouse artificial chromosomes (MACs) to be transferred from donor to recipient cells. In the past few decades, MMCT has been applied to various studies, including mapping the genes, analysis of chromosome status such as aneuploidy and epigenetics. Recently, MMCT was applied to transfer MACs/HACs carrying entire chromosomal copies of genes for genes function studies and has potential for regenerative medicine. However, a safe and efficient MMCT technique remains an important challenge. The original MMCT protocol includes treatment of donor cells by Colcemid to induce micronucleation, where each chromosome becomes surrounded with a nuclear membrane, followed by disarrangement of the actin cytoskeleton using Cytochalasin B to help induce microcells formation. In this study, we modified the protocol and demonstrated that replacing Colcemid and Cytochalasin B with TN-16 + Griseofulvin and Latrunculin B in combination with a Collage/Laminin surface coating increases the efficiency of HAC transfer to recipient cells by almost sixfold and is possibly less damaging to HAC than the standard MMCT method. We tested the improved MMCT protocol on four recipient cell lines, including human mesenchymal stem cells and mouse embryonic stem cells that could facilitate the cell engineering by HACs.

A pathway from chromosome transfer to engineering resulting in human and mouse artificial chromosomes for a variety of applications to bio-medical challenges

Microcell-mediated chromosome transfer (MMCT) is a technique to transfer a chromosome from defined donor cells into recipient cells and to manipulate chromosomes as gene delivery vectors and open a new avenue in somatic cell genetics. However, it is difficult to uncover the function of a single specific gene via the transfer of an entire chromosome or fragment, because each chromosome or fragment contains a set of numerous genes. Thus, alternative tools are human artificial chromosome (HAC) and mouse artificial chromosome (MAC) vectors, which can carry a gene or genes of interest. HACs/MACs have been generated mainly by either a "top-down approach" (engineered creation) or a "bottom-up approach" (de novo creation). HACs/MACs with one or more acceptor sites exhibit several characteristics required by an ideal gene delivery vector, including stable episomal maintenance and the capacity to carry large genomic loci plus their regulatory elements, thus allowing the physiological regulation of the introduced gene in a manner similar to that of native chromosomes. The MMCT technique is also applied for manipulating HACs and MACs in donor cells and delivering them to recipient cells. This review describes the lessons learned and prospects identified from studies on the construction of HACs and MACs, and their ability to drive exogenous gene expression in cultured cells and transgenic animals via MMCT. New avenues for a variety of applications to bio-medical challenges are also proposed.

Retargeting of microcell fusion towards recipient cell-oriented transfer of human artificial chromosome

Background

Human artificial chromosome (HAC) vectors have some unique characteristics as compared with conventional vectors, carrying large transgenes without size limitation, showing persistent expression of transgenes, and existing independently from host genome in cells. With these features, HACs are expected to be promising vectors for modifications of a variety of cell types. However, the method of introduction of HACs into target cells is confined to microcell-mediated chromosome transfer (MMCT), which is less efficient than other methods of vector introduction. Application of Measles Virus (MV) fusogenic proteins to MMCT instead of polyethylene glycol (PEG) has partly solved this drawback, whereas the tropism of MV fusogenic proteins is restricted to human CD46- or SLAM-positive cells.

Results

Here, we show that retargeting of microcell fusion by adding anti-Transferrin receptor (TfR) single chain antibodies (scFvs) to the extracellular C-terminus of the MV-H protein improves the efficiency of MV-MMCT to human fibroblasts which originally barely express both native MV receptors, and are therefore resistant to MV-MMCT. Efficacy of chimeric fusogenic proteins was evaluated by the evidence that the HAC, tagged with a drug-resistant gene and an EGFP gene, was transferred from CHO donor cells into human fibroblasts. Furthermore, it was demonstrated that no perturbation of either the HAC status or the functions of transgenes was observed on account of retargeted MV-MMCT when another HAC carrying four reprogramming factors (iHAC) was transferred into human fibroblasts.

Conclusions

Retargeted MV-MMCT using chimeric H protein with scFvs succeeded in extending the cell spectrum for gene transfer via HAC vectors. Therefore, this technology could facilitate the systematic cell engineering by HACs.

Electronic supplementary material

The online version of this article (doi:10.1186/s12896-015-0142-z) contains supplementary material, which is available to authorized users.

Down syndrome-associated haematopoiesis abnormalities created by chromosome transfer and genome editing technologies

Infants with Down syndrome (DS) are at a high risk of developing transient abnormal myelopoiesis (TAM). A GATA1 mutation leading to the production of N-terminally truncated GATA1 (GATA1s) in early megakaryocyte/erythroid progenitors is linked to the onset of TAM and cooperated with the effect of trisomy 21 (Ts21). To gain insights into the underlying mechanisms of the progression to TAM in DS patients, we generated human pluripotent stem cells harbouring Ts21 and/or GATA1s by combining microcell-mediated chromosome transfer and genome editing technologies. In vitro haematopoietic differentiation assays showed that the GATA1s mutation blocked erythropoiesis irrespective of an extra chromosome 21, while Ts21 and the GATA1s mutation independently perturbed megakaryopoiesis and the combination of Ts21 and the GATA1s mutation synergistically contributed to an aberrant accumulation of skewed megakaryocytes. Thus, the DS model cells generated by these two technologies are useful in assessing how GATA1s mutation is involved in the onset of TAM in patients with DS.

Unraveling the complexity of neurodegeneration in brains of subjects with Down syndrome: insights from proteomics

Down syndrome (DS) is one of the most common genetic causes of intellectual disability characterized by multiple pathological phenotypes, among which neurodegeneration is a key feature. The neuropathology of DS is complex and likely results from impaired mitochondrial function, increased oxidative stress, and altered proteostasis. After the age of 40 years, many (most) DS individuals develop a type of dementia that closely resembles that of Alzheimer's disease with deposition of senile plaques and neurofibrillary tangles. A number of studies demonstrated that increased oxidative damage, accumulation of damaged/misfolded protein aggregates, and dysfunction of intracellular degradative systems are critical events in the neurodegenerative processes. This review summarizes the current knowledge that demonstrates a “chronic” condition of oxidative stress in DS pointing to the putative molecular pathways that could contribute to accelerate cognition and memory decline. Proteomics and redox proteomics studies are powerful tools to unravel the complexity of DS phenotypes, by allowing to identifying protein expression changes and oxidative PTMs that are proved to be detrimental for protein function. It is reasonable to suggest that changes in the cellular redox status in DS neurons, early from the fetal period, could provide a fertile environment upon which increased aging favors neurodegeneration. Thus, after a critical age, DS neuropathology can be considered a human model of early Alzheimer's disease and could contribute to understanding the overlapping mechanisms that lead from normal aging to development of dementia.

Congenital heart disease protein 5 associates with CASZ1 to maintain myocardial tissue integrity

The identification and characterization of the cellular and molecular pathways involved in the differentiation and morphogenesis of specific cell types of the developing heart are crucial to understanding the process of cardiac development and the pathology associated with human congenital heart disease. Here, we show that the cardiac transcription factor CASTOR (CASZ1) directly interacts with congenital heart disease 5 protein (CHD5), which is also known as tryptophan-rich basic protein (WRB), a gene located on chromosome 21 in the proposed region responsible for congenital heart disease in individuals with Down's syndrome. We demonstrate that loss of CHD5 in Xenopus leads to compromised myocardial integrity, improper deposition of basement membrane, and a resultant failure of hearts to undergo cell movements associated with cardiac formation. We further report that CHD5 is essential for CASZ1 function and that the CHD5-CASZ1 interaction is necessary for cardiac morphogenesis. Collectively, these results establish a role for CHD5 and CASZ1 in the early stages of vertebrate cardiac development.

A novel transchromosomic system: stable maintenance of an engineered Mb-sized human genomic fragment translocated to a mouse chromosome terminal region

Transchromosomic (Tc) technology using human chromosome fragments (hCFs), or human artificial chromosomes (HACs), has been used for generating mice containing Mb-sized segments of the human genome. The most significant problem with freely segregating chromosomes with human centromeres has been mosaicism, possibly due to the instability of hCFs or HACs in mice. We report a system for the stable maintenance of Mb-sized human chromosomal fragments following translocation to mouse chromosome 10 (mChr.10). The approach utilizes microcell-mediated chromosome transfer and a combination of site-specific loxP insertion, telomere-directed chromosome truncation, and precise reciprocal translocation for the generation of Tc mice. Human chromosome 21 (hChr.21) was modified with a loxP site and truncated in homologous recombination-proficient chicken DT40 cells. Following transfer to mouse embryonic stem cells harboring a loxP site at the distal region of mChr.10, a ~4 Mb segment of hChr.21 was translocated to the distal region of mChr.10 by transient expression of Cre recombinase. The residual hChr.21/mChr.10ter fragment was reduced by antibiotic negative selection. Tc mice harboring the translocated ~4 Mb fragment were generated by chimera formation and germ line transmission. The hChr.21-derived Mb fragment was maintained stably in tissues in vivo and expression profiles of genes on hChr.21 were consistent with those seen in humans. Thus, Tc technology that enables translocation of human chromosomal regions onto host mouse chromosomes will be useful for studying in vivo functions of the human genome, and generating humanized model mice.

A novel mouse model for Down syndrome that harbor a single copy of human artificial chromosome (HAC) carrying a limited number of genes from human chromosome 21

Down syndrome (DS), also known as Trisomy 21, is the most common chromosome aneuploidy in live-born children and displays a complicated symptom. To date, several kinds of mouse models have been generated to understand the molecular pathology of DS, yet the gene dosage effects and gene(s)-phenotype(s) correlation are not well understood. In this study, we established a novel method to generate a partial trisomy mice using the mouse ES cells that harbor a single copy of human artificial chromosome (HAC), into which a small human DNA segment containing human chromosome 21 genes cloned in a bacterial artificial chromosome (BAC) was recombined. The produced mice were found to maintain the HAC carrying human genes as a mini-chromosome, hence termed as a Trans-Mini-Chromosomal (TMC) mouse, and HAC was transmitted for more than twenty generations independent from endogenous mouse chromosomes. The three human transgenes including cystathionine β-synthase, U2 auxiliary factor and crystalline alpha A were expressed in several mouse tissues with various expression levels relative to mouse endogenous genes. The novel system is applicable to any of human and/or mouse BAC clones. Thus, the TMC mouse carrying a HAC with a limited number of genes would provide a novel tool for studying gene dosage effects involved in the DS molecular pathogenesis and the gene(s)-phenotype(s) correlation.

Highly stable maintenance of a mouse artificial chromosome in human cells and mice

Human artificial chromosomes (HACs) and mouse artificial chromosomes (MACs) display several advantages as gene delivery vectors, such as stable episomal maintenance that avoids insertional mutations and the ability to carry large gene inserts including the regulatory elements. Previously, we showed that a MAC vector developed from a natural mouse chromosome by chromosome engineering was more stably maintained in adult tissues and hematopoietic cells in mice than HAC vectors. In this study, to expand the utility for a gene delivery vector in human cells and mice, we investigated the long-term stability of the MACs in cultured human cells and transchromosomic mice. We also investigated the chromosomal copy number-dependent expression of genes on the MACs in mice. The MAC was stably maintained in human HT1080 cells in vitro during long-term culture. The MAC was stably maintained at least to the F8 and F4 generations in ICR and C57BL/6 backgrounds, respectively. The MAC was also stably maintained in hematopoietic cells and tissues derived from old mice. Transchromosomic mice containing two or four copies of the MAC were generated by breeding. The DNA contents were comparable to the copy number of the MACs in each tissue examined, and the expression of the EGFP gene on the MAC was dependent on the chromosomal copy number. Therefore, the MAC vector may be useful not only for gene delivery in mammalian cells but also for animal transgenesis.

Generation and utility of genetically humanized mouse models

Identifying in vivo models that are naturally predictive for particular areas of study in humans can be challenging due to the divergence that has occurred during speciation. One solution to this challenge that is gaining increasing traction is the use of genetic engineering to introduce human genes into mice to generate superior models for predicting human responses. This review describes the state-of-the-art for generating such models, provides an overview of the types of genetically humanized mouse models described to date and their applications in basic research, drug discovery and development and to understand clinical drug toxicity. We discuss limitations and explore promising future directions for the use of genetically humanized mice to further improve translational research.

The complex SNP and CNV genetic architecture of the increased risk of congenital heart defects in Down syndrome

Congenital heart defect (CHD) occurs in 40% of Down syndrome (DS) cases. While carrying three copies of chromosome 21 increases the risk for CHD, trisomy 21 itself is not sufficient to cause CHD. Thus, additional genetic variation and/or environmental factors could contribute to the CHD risk. Here we report genomic variations that in concert with trisomy 21, determine the risk for CHD in DS. This case-control GWAS includes 187 DS with CHD (AVSD = 69, ASD = 53, VSD = 65) as cases, and 151 DS without CHD as controls. Chromosome 21–specific association studies revealed rs2832616 and rs1943950 as CHD risk alleles (adjusted genotypic P-values <0.05). These signals were confirmed in a replication cohort of 92 DS-CHD cases and 80 DS-without CHD (nominal P-value 0.0022). Furthermore, CNV analyses using a customized chromosome 21 aCGH of 135K probes in 55 DS-AVSD and 53 DS-without CHD revealed three CNV regions associated with AVSD risk (FDR ≤ 0.05). Two of these regions that are located within the previously identified CHD region on chromosome 21 were further confirmed in a replication study of 49 DS-AVSD and 45 DS- without CHD (FDR ≤ 0.05). One of these CNVs maps near the RIPK4 gene, and the second includes the ZBTB21 (previously ZNF295) gene, highlighting the potential role of these genes in the pathogenesis of CHD in DS. We propose that the genetic architecture of the CHD risk of DS is complex and includes trisomy 21, and SNP and CNV variations in chromosome 21. In addition, a yet-unidentified genetic variation in the rest of the genome may contribute to this complex genetic architecture.

The transfer of human artificial chromosomes via cryopreserved microcells

Microcell-mediated chromosome transfer (MMCT) technology enables a single and intact mammalian chromosome or megabase-sized chromosome fragments to be transferred from donor to recipient cells. The conventional MMCT method is performed immediately after the purification of microcells. The timing of the isolation of microcells and the preparation of recipient cells is very important. Thus, ready-made microcells can improve and simplify the process of MMCT. Here, we established a cryopreservation method to store microcells at -80 °C, and compared these cells with conventionally- (immediately-) prepared cells with respect to the efficiency of MMCT and the stability of a human artificial chromosome (HAC) transferred to human HT1080 cells. The HAC transfer in microcell hybrids was confirmed by FISH analysis. There was no significant difference between the two methods regarding chromosome transfer efficiency and the retention rate of HAC. Thus, cryopreservation of ready-to-use microcells provides an improved and simplified protocol for MMCT.

Trans-chromosomic mice containing a human CYP3A cluster for prediction of xenobiotic metabolism in humans

Human CYP3A is the most abundant P450 isozyme present in the human liver and small intestine, and metabolizes around 50% of medical drugs on the market. The human CYP3A subfamily comprises four members (CYP3A4, CYP3A5, CYP3A7, CYP3A43) encoded on human chromosome 7. However, transgenic mouse lines carrying the entire human CYP3A cluster have not been constructed because of limitations in conventional cloning techniques. Here, we show that the introduction of a human artificial chromosome (HAC) containing the entire genomic human CYP3A locus recapitulates tissue- and stage-specific expression of human CYP3A genes and xenobiotic metabolism in mice. About 700 kb of the entire CYP3A genomic segment was cloned into a HAC (CYP3A-HAC), and trans-chromosomic (Tc) mice carrying a single copy of germline-transmittable CYP3A-HAC were generated via a chromosome-engineering technique. The tissue- and stage-specific expression profiles of CYP3A genes were consistent with those seen in humans. We further generated mice carrying the CYP3A-HAC in the background homozygous for targeted deletion of most endogenous Cyp3a genes. In this mouse strain with 'fully humanized' CYP3A genes, the kinetics of triazolam metabolism, CYP3A-mediated mechanism-based inactivation effects and formation of fetal-specific metabolites of dehydroepiandrosterone observed in humans were well reproduced. Thus, these mice are likely to be valuable in evaluating novel drugs metabolized by CYP3A enzymes and in studying the regulation of human CYP3A gene expression. Furthermore, this system can also be used for generating Tc mice carrying other human metabolic genes.

Behavior genetics and postgenomics

The science of genetics is undergoing a paradigm shift. Recent discoveries, including the activity of retrotransposons, the extent of copy number variations, somatic and chromosomal mosaicism, and the nature of the epigenome as a regulator of DNA expressivity, are challenging a series of dogmas concerning the nature of the genome and the relationship between genotype and phenotype. According to three widely held dogmas, DNA is the unchanging template of heredity, is identical in all the cells and tissues of the body, and is the sole agent of inheritance. Rather than being an unchanging template, DNA appears subject to a good deal of environmentally induced change. Instead of identical DNA in all the cells of the body, somatic mosaicism appears to be the normal human condition. And DNA can no longer be considered the sole agent of inheritance. We now know that the epigenome, which regulates gene expressivity, can be inherited via the germline. These developments are particularly significant for behavior genetics for at least three reasons: First, epigenetic regulation, DNA variability, and somatic mosaicism appear to be particularly prevalent in the human brain and probably are involved in much of human behavior; second, they have important implications for the validity of heritability and gene association studies, the methodologies that largely define the discipline of behavior genetics; and third, they appear to play a critical role in development during the perinatal period and, in particular, in enabling phenotypic plasticity in offspring. I examine one of the central claims to emerge from the use of heritability studies in the behavioral sciences, the principle of minimal shared maternal effects, in light of the growing awareness that the maternal perinatal environment is a critical venue for the exercise of adaptive phenotypic plasticity. This consideration has important implications for both developmental and evolutionary biology.

The in vivo Down syndrome genomic library in mouse

Mouse models are key elements to better understand the genotype-phenotype relationship and the physiopathology of Down syndrome (DS). Even though the mouse will never recapitulate the whole spectrum of intellectual disabilities observed in the DS, mouse models have been developed over the recent decades and have been used extensively to identify homologous genes or entire regions homologous to the human chromosome 21 (Hsa21) that are necessary or sufficient to induce DS cognitive features. In this chapter, we review the principal mouse DS models which have been selected and engineered over the years either for large genomic regions or for a few or a single gene of interest. Their analyses highlight the complexity of the genetic interactions that are involved in DS cognitive phenotypes and also strengthen the hypothesis on the multigenic nature of DS. This review also addresses future research challenges relative to the making of new models and their combination to go further in the characterization of candidates and modifier of the DS features.

Genetic analysis of Down syndrome facilitated by mouse chromosome engineering

Human trisomy 21 is the most frequent live-born human aneuploidy and causes a constellation of disease phenotypes classified as Down syndrome, which include heart defects, myeloproliferative disorder, cognitive disabilities and Alzheimer-type neurodegeneration. Because these phenotypes are associated with an extra copy of a human chromosome, the genetic analysis of Down syndrome has been a major challenge. To complement human genetic approaches, mouse models have been generated and analyzed based on evolutionary conservation between the human and mouse genomes. These efforts have been greatly facilitated by Cre/loxP-mediated mouse chromosome engineering, which may result in the establishment of minimal critical genomic regions and eventually new dosage-sensitive genes associated with Down syndrome phenotypes. The success in genetic analysis of Down syndrome will further enhance our understanding of this disorder and lead to better strategies in developing effective therapeutic interventions.

Genomically humanized mice: technologies and promises

Mouse models have become an invaluable tool for understanding human health and disease owing to our ability to manipulate the mouse genome exquisitely. Recent progress in genomic analysis has led to an increase in the number and type of disease-causing mutations detected and has also highlighted the importance of non-coding regions. As a result, there is increasing interest in creating 'genomically' humanized mouse models, in which entire human genomic loci are transferred into the mouse genome. The technical challenges towards achieving this aim are large but are starting to be tackled with success.

Aneuploid human embryonic stem cells: origins and potential for modeling chromosomal disorders

Chromosomal aneuploidies are widely recognized genetic disorders in humans that often lead to spontaneous abortion. Aneuploid fetuses that survive to term commonly exhibit impaired developmental growth and mental retardation in addition to multiple congenital malformations. Preimplantation genetic screening is used to detect chromosomal aneuploidies in early embryos. Human embryonic stem cell (ESC) cell lines generated from aneuploid embryos created a unique repository of cell lines. The spectrum of aneuploidies in these ESC lines reflects the range of common embryonic chromosomal aberrations and significantly differs from the spectrum of aneuploid human ESC lines generated by cell adaptation in culture. The aneuploid human ESC lines represent an excellent model to study human chromosomal abnormalities especially in the early stages of development.

Mouse models for Down syndrome-associated developmental cognitive disabilities

Down syndrome (DS) is mainly caused by the presence of an extra copy of human chromosome 21 (Hsa21) and is a leading genetic cause for developmental cognitive disabilities in humans. The mouse is a premier model organism for DS because the regions on Hsa21 are syntenically conserved with three regions in the mouse genome, which are located on mouse chromosome 10 (Mmu10), Mmu16 and Mmu17. With the advance of chromosomal manipulation technologies, new mouse mutants have been generated to mimic DS at both the genotypic and phenotypic levels. Further mouse-based molecular genetic studies in the future may lead to the unraveling of the mechanisms underlying DS-associated developmental cognitive disabilities, which would lay the groundwork for developing effective treatments for this phenotypic manifestation. In this review, we will discuss recent progress and future challenges in modeling DS-associated developmental cognitive disability in mice with an emphasis on hippocampus-related phenotypes.

Human artificial chromosomes for gene delivery and the development of animal models

Random integration of conventional gene delivery vectors such as viruses, plasmids, P1 phage-derived artificial chromosomes, bacterial artificial chromosomes and yeast artificial chromosomes can be associated with transgene silencing. Furthermore, integrated viral sequences can activate oncogenes adjacent to the insertion site resulting in cancer. Various human artificial chromosomes (HACs) exhibit several potential characteristics desired for an ideal gene delivery vector, including stable episomal maintenance and the capacity to carry large genomic loci with their regulatory elements, thus allowing the physiological regulation of the introduced gene in a manner similar to that of native chromosomes. HACs have been generated mainly using either a "top-down approach" (engineered chromosomes), or a "bottom-up approach" (de novo artificial chromosomes). The recent emergence of stem cell-based tissue engineering has opened up new avenues for gene and cell therapies. This review describes the lessons learned and prospects identified mainly from studies in the construction of HACs and HAC-mediated gene expression systems in cultured cells, as well as in animals.

Gene expression profiling in a mouse model identifies fetal liver- and placenta-derived potential biomarkers for Down Syndrome screening

Background

As a first step to identify novel potential biomarkers for prenatal Down Syndrome screening, we analyzed gene expression in embryos of wild type mice and the Down Syndrome model Ts1Cje. Since current Down Syndrome screening markers are derived from placenta and fetal liver, these tissues were chosen as target.

Methodology/Principal Findings

Placenta and fetal liver at 15.5 days gestation were analyzed by microarray profiling. We confirmed increased expression of genes located at the trisomic chromosomal region. Overall, between the two genotypes more differentially expressed genes were found in fetal liver than in placenta. Furthermore, the fetal liver data are in line with the hematological aberrations found in humans with Down Syndrome as well as Ts1Cje mice. Together, we found 25 targets that are predicted (by Gene Ontology, UniProt, or the Human Plasma Proteome project) to be detectable in human serum.

Conclusions/Significance

Fetal liver might harbor more promising targets for Down Syndrome screening studies. We expect these new targets will help focus further experimental studies on identifying and validating human maternal serum biomarkers for Down Syndrome screening.

Genetic analysis of Down syndrome-associated heart defects in mice

Human trisomy 21, the chromosomal basis of Down syndrome (DS), is the most common genetic cause of heart defects. Regions on human chromosome 21 (Hsa21) are syntenically conserved with three regions located on mouse chromosome 10 (Mmu10), Mmu16 and Mmu17. In this study, we have analyzed the impact of duplications of each syntenic region on cardiovascular development in mice and have found that only the duplication on Mmu16, i.e., Dp(16)1Yey, is associated with heart defects. Furthermore, we generated two novel mouse models carrying a 5.43-Mb duplication and a reciprocal deletion between Tiam1 and Kcnj6 using chromosome engineering, Dp(16Tiam1-Kcnj6)Yey/+ and Df(16Tiam1-Kcnj6)Yey/+, respectively, within the 22.9-Mb syntenic region on Mmu16. We found that Dp(16Tiam1-Kcnj6)Yey/+, but not Dp(16)1Yey/Df(16Tiam1-Kcnj6)Yey, resulted in heart defects, indicating that triplication of the Tiam1-Knj6 region is necessary and sufficient to cause DS-associated heart defects. Our transcriptional analysis of Dp(16Tiam1-Kcnj6)Yey/+ embryos confirmed elevated expression levels for the genes located in the Tiam-Kcnj6 region. Therefore, we established the smallest critical genomic region for DS-associated heart defects to lay the foundation for identifying the causative gene(s) for this phenotype.

Evaluation of Pax6 mutant rat as a model for autism

Autism is a highly variable brain developmental disorder and has a strong genetic basis. Pax6 is a pivotal player in brain development and maintenance. It is expressed in embryonic and adult neural stem cells, in astrocytes in the entire central nervous system, and in neurons in the olfactory bulb, amygdala, thalamus, and cerebellum, functioning in highly context-dependent manners. We have recently reported that Pax6 heterozygous mutant (rSey²/+) rats with a spontaneous mutation in the Pax6 gene, show impaired prepulse inhibition (PPI). In the present study, we further examined behaviors of rSey²/+ rats and revealed that they exhibited abnormality in social interaction (more aggression and withdrawal) in addition to impairment in rearing activity and in fear-conditioned memory. Ultrasonic vocalization (USV) in rSey²+ rat pups was normal in male but abnormal in female. Moreover, treatment with clozapine successfully recovered the defects in sensorimotor gating function, but not in fear-conditioned memory. Taken together with our prior human genetic data and results in other literatures, rSey²/+ rats likely have some phenotypic components of autism.

Effects of individual segmental trisomies of human chromosome 21 syntenic regions on hippocampal long-term potentiation and cognitive behaviors in mice

As the genomic basis for Down syndrome (DS), human trisomy 21 is the most common genetic cause of intellectual disability in children and young people. The genomic regions on human chromosome 21 (Hsa21) are syntenic to three regions in the mouse genome, located on mouse chromosome 10 (Mmu10), Mmu16, and Mmu17. Recently, we have developed three new mouse models using chromosome engineering carrying the genotypes of Dp(10)1Yey/+, Dp(16)1Yey/+, or Dp(17)1Yey/+, which harbor a duplication spanning the entire Hsa21 syntenic region on Mmu10, Mmu16, or Mmu17, respectively. In this study, we analyzed the hippocampal long-term potentiation (LTP) and cognitive behaviors of these models. Our results show that, while the genotype of Dp(17)1Yey/+ results in abnormal hippocampal LTP, the genotype of Dp(16)1Yey/+ leads to both abnormal hippocampal LTP and impaired learning/memory. Therefore, these mutant mice can serve as powerful tools for further understanding the mechanism underlying cognitively relevant phenotypes associated with DS, particularly the impacts of different syntenic regions on these phenotypes.

[Trisomy 21: fifty years between medicine and science]

Fifty years after the discovery of the etiology of Down syndrome, trisomy 21 remains the model of choice for studying human diseases resulting from the presence of a chromosome or a chromosome segment in excess. In this review, mechanisms of aneuploidy occurrence and consequences of genomic imbalances will be mainly discussed. The study of genetic markers showed that trisomy 21 results in 90% of cases from an error during maternal meiosis. Approximately 8% of cases result from an error during paternal meiosis and in 2% of cases there is a postzygotic mitotic nondisjunction. The biological basis of the effect of maternal age remains largely unknown. The absence of genetic recombination between homologous chromosomes or the presence of an exchange in telomeric position are two risk factors of non-disjunction observed in young women. Non-disjunctions associated with pericentromeric exchanges are observed with an increase in maternal age. The study of mouse models and patients with partial trisomy 21, combined with advances in knowledge of the physical map and the transcriptome, identified genes directly or indirectly involved in the pathogenesis of Down syndrome. The recent description of metabolic pathways controlled by RCAN1 and DYRK1A genes which may be involved in many biological processes and phenotypes associated with trisomy 21 allows to consider new therapeutic strategies.

Exploitation of the interaction of measles virus fusogenic envelope proteins with the surface receptor CD46 on human cells for microcell-mediated chromosome transfer

Background

Microcell-mediated chromosome transfer (MMCT) is a technique by which a chromosome(s) is moved from donor to recipient cells by microcell fusion. Polyethylene glycol (PEG) has conventionally been used as a fusogen, and has been very successful in various genetic studies. However, PEG is not applicable for all types of recipient cells, because of its cell type-dependent toxicity. The cytotoxicity of PEG limits the yield of microcell hybrids to low level (10^-6 to 10^-5 per recipient cells). To harness the full potential of MMCT, a less toxic and more efficient fusion protocol that can be easily manipulated needs to be developed.

Results

Microcell donor CHO cells carrying a human artificial chromosome (HAC) were transfected with genes encoding hemagglutinin (H) and fusion (F) proteins of an attenuated Measles Virus (MV) Edmonston strain. Mixed culture of the CHO transfectants and MV infection-competent human fibrosarcoma cells (HT1080) formed multinucleated syncytia, suggesting the functional expression of the MV-H/F in the CHO cells. Microcells were prepared and applied to HT1080 cells, human immortalized mesenchymal stem cells (hiMSC), and primary fibroblasts. Drug-resistant cells appeared after selection in culture with Blasticidin targeted against the tagged selection marker gene on the HAC. The fusion efficiency was determined by counting the total number of stable clones obtained in each experiment. Retention of the HAC in the microcell hybrids was confirmed by FISH analyses. The three recipient cell lines displayed distinct fusion efficiencies that depended on the cell-surface expression level of CD46, which acts as a receptor for MV. In HT1080 and hiMSC, the maximum efficiency observed was 50 and 100 times greater than that using conventional PEG fusion, respectively. However, the low efficiency of PEG-induced fusion with HFL1 was not improved by the MV fusogen.

Conclusions

Ectopic expression of MV envelope proteins provides an efficient recipient cell-oriented MMCT protocol, facilitating extensive applications for studies of gene function and genetic corrections.