A mouse model for Down syndrome exhibits learning and behaviour deficits

Applications of the Morris water maze in the study of learning and memory

The Morris water maze (MWM) was described 20 years ago as a device to investigate spatial learning and memory in laboratory rats. In the meanwhile, it has become one of the most frequently used laboratory tools in behavioral neuroscience. Many methodological variations of the MWM task have been and are being used by research groups in many different applications. However, researchers have become increasingly aware that MWM performance is influenced by factors such as apparatus or training procedure as well as by the characteristics of the experimental animals (sex, species/strain, age, nutritional state, exposure to stress or infection). Lesions in distinct brain regions like hippocampus, striatum, basal forebrain, cerebellum and cerebral cortex were shown to impair MWM performance, but disconnecting rather than destroying brain regions relevant for spatial learning may impair MWM performance as well. Spatial learning in general and MWM performance in particular appear to depend upon the coordinated action of different brain regions and neurotransmitter systems constituting a functionally integrated neural network. Finally, the MWM task has often been used in the validation of rodent models for neurocognitive disorders and the evaluation of possible neurocognitive treatments. Through its many applications, MWM testing gained a position at the very core of contemporary neuroscience research.

Chromosome 21: from sequence to applications

Last year we celebrated the sequencing of the entire long arm of human chromosome 21. This achievement now provides unprecedented opportunities to understand the molecular pathophysiology of trisomy 21, elucidate the mechanisms of all monogenic disorders of chromosome 21, and discover genes and functional sequence variations that predispose to common complex disorders. All these steps require the functional analysis of gene products and the determination of the sequence variation of this chromosome.

Use of Comparative Physical and Sequence Mapping to Annotate Mouse Chromosome 16 and Human Chromosome 21

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

On the cause of mental retardation in Down syndrome: extrapolation from full and segmental trisomy 16 mouse models

Down syndrome (DS, trisomy 21, Ts21) is the most common known cause of mental retardation. In vivo structural brain imaging in young DS adults, and post-mortem studies, indicate a normal brain size after correction for height, and the absence of neuropathology. Functional imaging with positron emission tomography (PET) shows normal brain glucose metabolism, but fewer significant correlations between metabolic rates in different brain regions than in controls, suggesting reduced functional connections between brain circuit elements. Cultured neurons from Ts21 fetuses and from fetuses of an animal model for DS, the trisomy 16 (Ts16) mouse, do not differ from controls with regard to passive electrical membrane properties, including resting potential and membrane resistance. On the other hand, the trisomic neurons demonstrate abnormal active electrical and biochemical properties (duration of action potential and its rates of depolarization and repolarization, altered kinetics of active Na(+), Ca(2+) and K(+) currents, altered membrane densities of Na(+) and Ca(2+) channels). Another animal model, the adult segmental trisomy 16 mouse (Ts65Dn), demonstrates reduced long-term potentiation and increased long-term depression (models for learning and memory related to synaptic plasticity) in the CA1 region of the hippocampus. Evidence suggests that the abnormalities in the trisomy mouse models are related to defective signal transduction pathways involving the phosphoinositide cycle, protein kinase A and protein kinase C. The phenotypes of DS and its mouse models do not involve abnormal gene products due to mutations or deletions, but result from altered expression of genes on human chromosome 21 or mouse chromosome 16, respectively. To the extent that the defects in signal transduction and in active electrical properties, including synaptic plasticity, that are found in the Ts16 and Ts65Dn mouse models, are found in the brain of DS subjects, we postulate that mental retardation in DS results from such abnormalities. Changes in timing and synaptic interaction between neurons during development can lead to less than optimal functioning of neural circuitry and signaling then and in later life.

Down syndrome: advances in molecular biology and the neurosciences

The entire DNA sequence for human chromosome 21 is now complete, and it is predicted to contain only about 225 genes, which is approximately three-fold fewer than the number initially predicted just 10 years ago. Despite this remarkable achievement, very little is known about the mechanism(s) whereby increased gene copy number (gene dosage) results in the characteristic phenotype of Down syndrome. Although many of the phenotypic traits show large individual variation, neuromotor dysfunction and cognitive and language impairment are observed in virtually all individuals. Currently, there are no efficacious biomedical treatments for these central nervous system-associated impairments. To develop novel therapeutic strategies, the effects of gene dosage imbalance need to be understood within the framework of those critical biological events that regulate brain organization and function.

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

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

Ts65Dn mice, a model for Down syndrome, have deficits in context discrimination learning suggesting impaired hippocampal function

The Ts65Dn mouse is segmentally trisomic for a part of mouse chromosome 16 and is a genetic model for Down syndrome and Alzheimer's disease. Although many studies have examined the learning and memory processes in Ts65Dn mice, it has yet to be determined if Ts65Dn mice are specifically impaired in learning tasks that require an intact hippocampus. Context discrimination learning is dependent on the dorsal hippocampus in mice. In this task, mice learn to discriminate two similar contexts, one of which is associated with foot shock. In the current study, Ts65Dn mice learned almost identically to what has been reported for mice with dorsal hippocampal lesions, while controls behaved similarly to sham lesioned mice. Therefore, Ts65Dn mice have learning deficits in a hippocampal dependent task that may be related to the loss of cholinergic input to the hippocampus, which occurs after 6 months of age.

Functional genomics of Down syndrome: a multidisciplinary approach

The availability of the DNA sequence of human chromosome 21 (HSA21) is a landmark contribution that will have an immediate impact on the study of the role of specific genes to Down syndrome (DS). Trisomy 21, full or partial, is a major cause of mental retardation and other phenotypic abnormalities, collectively known as Down syndrome (DS), a disorder affecting 1 in 700 births. The identification of genes on HSA21 and the elucidation of the function of the proteins encoded by these genes have been a major challenge for the human genome project and for research in DS. Over 100 of the estimated 300-500 genes of HSA21 have been identified, but the function of most remains largely unknown. It is believed that the overexpression of an unknown number of HSA21 genes is directly or indirectly responsible for the mental retardation and the other clinical features of DS. For this reason, HSA21 genes that are expressed in tissues affected in DS patients are of special interest.

Motor learning in Ts65Dn mice, a model for Down syndrome

Ts65Dn mice are a genetic model for Down syndrome. Both individuals with Down syndrome and Ts65Dn mice have reduced cerebellar volumes and the cerebellum is involved in motor learning. Conflicting results have been reported on the motor learning abilities of Ts65Dn mice, which may be related to procedural differences between the motor learning tasks used in different laboratories and/or variability in phenotype because of the segregating background on which the mice are maintained. In this study, we examined learning in three types of motor tasks (peg running, accelerating rotorod, and rotating rod) which were initially easy for mice and gradually increased in difficulty. Ts65Dn mice learned the peg running task as well as controls, and learned the accelerating rotorod and rotating rod tasks as well as, and even better than, controls. These data indicate that Ts65Dn mice are not impaired in motor learning.

The Mouse Brain Transcriptome by SAGE: Differences in Gene Expression between P30 Brains of the Partial Trisomy 16 Mouse Model of Down Syndrome (Ts65Dn) and Normals

Trisomy 21, or Down syndrome (DS), is the most common genetic cause of mental retardation. Changes in the neuropathology, neurochemistry, neurophysiology, and neuropharmacology of DS patients' brains indicate that there is probably abnormal development and maintenance of central nervous system structure and function. The segmental trisomy mouse (Ts65Dn) is a model of DS that shows analogous neurobehavioral defects. We have studied the global gene expression profiles of normal and Ts65Dn male and normal female mice brains (P30) using the serial analysis of gene expression (SAGE) technique. From the combined sample we collected a total of 152,791 RNA tags and observed 45,856 unique tags in the mouse brain transcriptome. There are 14 ribosomal protein genes (nine underexpressed) among the 330 statistically significant differences between normal male and Ts65Dn male brains, which possibly implies abnormal ribosomal biogenesis in the development and maintenance of DS phenotypes. This study contributes to the establishment of a mouse brain transcriptome and provides the first overall analysis of the differences in gene expression in aneuploid versus normal mammalian brain cells.

The mouse brain transcriptome by SAGE: differences in gene expression between P30 brains of the partial trisomy 16 mouse model of Down syndrome (Ts65Dn) and normals

Trisomy 21, or Down syndrome (DS), is the most common genetic cause of mental retardation. Changes in the neuropathology, neurochemistry, neurophysiology, and neuropharmacology of DS patients' brains indicate that there is probably abnormal development and maintenance of central nervous system structure and function. The segmental trisomy mouse (Ts65Dn) is a model of DS that shows analogous neurobehavioral defects. We have studied the global gene expression profiles of normal and Ts65Dn male and normal female mice brains (P30) using the serial analysis of gene expression (SAGE) technique. From the combined sample we collected a total of 152,791 RNA tags and observed 45,856 unique tags in the mouse brain transcriptome. There are 14 ribosomal protein genes (nine under expressed) among the 330 statistically significant differences between normal male and Ts65Dn male brains, which possibly implies abnormal ribosomal biogenesis in the development and maintenance of DS phenotypes. This study contributes to the establishment of a mouse brain transcriptome and provides the first overall analysis of the differences in gene expression in aneuploid versus normal mammalian brain cells.

Cloning and characterization of a putative human glycerol 3-phosphate permease gene (SLC37A1 or G3PP) on 21q22.3: mutation analysis in two candidate phenotypes, DFNB10 and a glycerol kinase deficiency

Using multiple exons trapped from human chromosome 21 (HC21)-specific cosmids with homology to a putative Arabidopsis thaliana glycerol 3-phosphate permease, we have determined the full-length cDNA sequence of a novel HC21 gene encoding a putative sugar-phosphate transporter (HGMW-approved symbol SLC37A1, aka G3PP). The predicted protein has 12 putative transmembrane domains and is also highly homologous to bacterial glpT proteins. The transcript was precisely mapped to 21q22.3 between D21S49 and D21S113. Comparison of the SLC37A1 cDNA to genomic sequence revealed that the gene encompasses 82 kb, and it is split into 19 coding exons and 7 untranslated exons, which are alternatively spliced in a complex and tissue-specific manner. Glycerol 3-phosphate (G3P) is produced by glycerol kinase (GK) and is found in several biochemical pathways in different cellular compartments, such as the glycerol phosphate shuttle and glycerophospholipid synthesis. Thus SLC37A1 mutations may cause a phenotype similar to GK deficiency. Mutational analyses of SLC37A1 in seven patients with no mutations in the GK gene and low GK activity revealed only nonpathogenetic sequence variants, excluding SLC37A1 as the gene for the phenotype in these patients. SLC37A1 maps in the refined critical region of the autosomal recessive deafness locus, DFNB10, on 21q22.3. Mutation analyses also excluded SLC37A1 as the gene for DFNB10.

Genetic dissection of region associated with behavioral abnormalities in mouse models for Down syndrome

Two animal models of Down syndrome (human trisomy 21) with segmental trisomy for all (Ts65Dn) or part (Ts1Cje) of human chromosome 21-homologous region of mouse chromosome 16 have cognitive and behavioral abnormalities. To compare these trisomies directly and to assess the phenotypic contribution of the region of difference between them, Ts65Dn, Ts1Cje, and a new segmental trisomic (Ms1Ts65) for the region of difference (APP: to Sod1) have been generated as littermates and tested in parallel. Although the performance of Ts1Cje mice in the Morris water maze is similar to that of Ts65Dn mice, the reverse probe tests indicate that Ts65Dn is more severely affected. By contrast, the deficits of Ms1Ts65 mice are significantly less severe than those of Ts65Dn. Therefore, whereas triplication of Sod1 to Mx1 plays the major role in causing the abnormalities of Ts65Dn in the Morris water maze, imbalance of APP: to Sod1 also contributes to the poor performance. Ts65Dn mice are hyperactive and Ts1Cje mice are hypoactive; the activity of Ms1Ts65 mice is not significantly above normal. These findings indicate that genes in the Ms1Ts65 trisomic region must interact with others in the Ts1Cje region to produce hyperactivity in Ts65Dn mice.

The murine DSCR1-like (Down syndrome candidate region 1) gene family: conserved synteny with the human orthologous genes

A recently recognized gene family, conserved from yeast to humans, includes Down syndrome candidate region 1 gene (DSCR1), Adapt78 (recognized as the hamster ortholog of the DSCR1 isoform 4), ZAKI-4 (renamed DSCR1-like 1, DSCR1L1) and DSCR1L2 (a novel gene on human chromosome 1), along with yeast and C. elegans single members (Strippoli P., Lenzi L., Petrini M., Carinci P., Zannotti M., 2000. A new gene family including DSCR1 (Down Syndrome Candidate Region 1) and ZAKI-4: characterization from yeast to human and identification of DSCR1-like 2, a novel human member. Genomics 64, 252-263). The proposed family labels were a putative single-strand nucleic acid binding domain similar to the RNA recognition motif, and a unique, highly-conserved serine-proline motif. We have used a bioinformatics-driven molecular biology approach to characterize the murine members of DSCR1-like gene family. Systematic expressed-sequence-tags (EST) database search and reverse-transcription polymerase chain rection (RT-PCR) product sequencing allowed identification of the murine DSCR1, DSCR1L1 and DSCR1L2. The sequences of the respective protein products are of 198, 197 and 241 amino acids, respectively, and are very similar to the corresponding human proteins. The very broad expression pattern of the murine DSCR1 genes is similar to that of the human genes. Using a radiation hybrid panel, we mapped the murine DSCR1-like family members. The murine DSCR1 ortholog is located on the chromosome 16, in a region corresponding to that on human chromosome 21 just upstream of the Down syndrome candidate region. DSCR1L1 and DSCR1L2 murine genes are also located in chromosomal segments of chromosome 17 and 4, respectively, exactly corresponding to those containing the respective human homologs on chromosomes 6 and 1. Description of the mouse orthologs for DSCR1-like genes will allow knockout mice to be obtained for specific family members.

Isolation and characterization of a human chromosome 21q22.3 gene (WDR4) and its mouse homologue that code for a WD-repeat protein

To identify candidate genes for Down syndrome phenotypes or disorders that map to human chromosome 21q22.3, trapped exons are being used to isolate full-length transcripts. We isolated a full-length cDNA (WDR4) encoding a novel WD-repeat protein and its mouse homologue. Two RNA species of 1.5 and 2.1 kb were observed in human, with the 1.5-kb transcript being produced by a splicing event after the stop codon, and thus both transcripts encode the same putative 412-amino-acid protein containing four guanine nucleotide-binding WD repeats. The more highly expressed 1.5-kb transcript was expressed mainly in fetal tissues while the 2.1-kb transcript showed a faint expression in most tissues. Two additional alternative splicing events of 270 and 52 nt within the coding region were observed. The WDR4 gene spans 37 kb and is divided into 11 coding exons. WDR4 maps between PDE9A and NDUFV3, a region where several genetic disorders, including a form of manic-depressive psychosis, also map, and seven sequence variants observed in the WDR4 gene could be used in association studies.

C21orf5, a novel human chromosome 21 gene, has a Caenorhabditis elegans ortholog (pad-1) required for embryonic patterning

To contribute to the development of the transcription map of human chromosome 21 (HC21), we isolated a new transcript, C21orf5 (chromosome 21 open reading frame 5), encoding a predicted 2298-amino-acid protein. Analysis of the genomic DNA sequence revealed that C21orf5 consists of 37 exons that extend over 130 kb and maps between the CBR3 (carbonyl reductase 3) and the KIAA0136 genes. Northern blot analyses showed a ubiquitously expressed RNA species of 8.5 kb. RNA in situ hybridization on brain sections of normal human embryos revealed a strong labeling in restricted areas of the cerebral cortex. In silico analysis of the deduced C21orf5 protein revealed several highly probable transmembrane segments but no known protein domains or homology with known proteins. However, there were significant homologies to several hypothetical Caenorhabditis elegans proteins and Drosophila melanogaster genomic sequences. To investigate the function of C21orf5, we isolated the cDNA of the C. elegans ortholog and performed double-stranded RNA-mediated genetic interference experiments. The major phenotype observed in the progeny of injected animals was embryonic lethality. Most of the tissues of the embryo failed to undergo proper patterning during gastrulation, and morphogenesis did not occur; thus we termed the ortholog pad-1, for patterning defective 1. These results indicated that pad-1 is essential for the development and the survival of C. elegans. This study provides the first example of the use of C. elegans as a model to study the function of genes on human chromosome 21 that might be involved in Down syndrome.

Specific impairment of cardiogenesis in mouse ES cells containing a human chromosome 21

Down syndrome (DS) leads to cardiac defects which are common and significant in babies with DS. We recently generated chimeric mice carrying a human chromosome (hChr) 21. The contribution ratio of embryonic stem (ES) cells containing a hChr 21 was specifically low in the heart, compared to other organs, and cardiovascular malformations were observed, suggesting that an additional copy of hChr 21 also disrupts the normal development of heart in mice. Here we describe that the presence of hChr 21 in ES cells delays the appearance of beating cardiomyocyte during differentiation, whereas differentiation into other cell types is not disrupted. Furthermore, the defect in cardiogenesis was restored following the deletion of a specific region of hChr 21. Therefore, we conclude that the imbalance of specific gene(s) on hChr 21 may lead to the disturbance of cardiogenesis and that this may be a useful system to model and investigate the cardiac defects of human DS.

A síndrome de Down e sua patogênese: considerações sobre o determinismo genético

O trabalho apresenta uma análise de fatores causais da síndrome de Down e sua patogênese. Faz também uma revisão da história natural dessa síndrome e dos efeitos da trissomia da banda cromossômica 21q22, considerada crítica para o distúrbio. Embora esse desequilíbrio cromossômico esteja necessariamente presente na síndrome de Down, a relevância do determinismo genético é questionada a partir da observação da possibilidade de desenvolvimento do potencial cognitivo em sujeitos afetados pela síndrome, após a aplicação de programas de estimulação neuromotora e psicopedagógicos.

Molecular abnormalities of the brain in Down syndrome: relevance to Alzheimer's neurodegeneration

Down syndrome is caused by over-expression of genes located within a segment of chromosome 21, termed the Down locus. Down syndrome is associated with developmental abnormalities of the central nervous system that result in mental retardation and age-dependent Alzheimer-type neurodegeneration. Some of the neurodegenerative lesions, including A beta amyloid deposition, apoptotic cell death, and aberrant dendritic arborization, are in part due to constitutively increased expression of genes that encode the amyloid precursor protein, superoxide dismutase I, and S100-beta, and located within the Down locus. However, neurodegeneration in Down syndrome is also associated with aberrant expression of genes that are not linked to the Down locus, including the growth associated protein, GAP-43, nitric oxide synthase 3, neuronal thread protein, and pro-apoptosis genes such as p53, Bax, and interleukin-1 beta-converting enzyme. Increased expression of these non-Down locus genes correlates with proliferation of dystrophic neurites and apoptotic cell death, two important correlates of cognitive impairment in Alzheimer's disease. This article reviews the functional importance of abnormal gene expression in relation to Alzheimer-type neurodegeneration in brains of individuals with Down syndrome.

Gene expression relevant to Down syndrome: problems and approaches

The long arm of human chromosome 21 likely contains several hundred genes. To determine which of these are responsible for specific aspects of the Down Syndrome phenotype, protein functional analysis coupled to phenotypic analysis of transgenic mice will be required. Because such experiments are both time consuming and expensive, prioritizing 21q genes for further studies would be advantageous. Here, we discuss expression analysis, specifically the use of Northern analysis, cDNA array screening and RNA tissue in situ hybridization to assess place and time of expression of forty-two genes. For a subset of these, over expression in normal versus trisomy cell lines and mouse tissues is discussed. Lastly, several examples of alternative processing and their potential for generation of brain specific proteins are described. Together, these experiments give information on time, place and level of expression of a number of 21q genes and suggest some interesting candidates worth further investigation for relevance to Down Syndrome. These data also illustrate the complexities and ambiguities inherent in interpretation and use of expression information.

Neuronal cell death in Down's syndrome

Down's syndrome (DS), occurring in 0.8 out of 1,000 live births, is a genetic disorder in which an extra portion of chromosome 21 leads to several abnormalities. With respect to the nervous system, it causes mental retardation. It is conceived that abnormal neuronal cell death in development is involved, but there is no direct evidence yet. In addition to developmental brain abnormalities, almost all DS brains over 40 years old manifest a similar pathology to Alzheimer's disease (AD), including the presence of senile plaques (SP) and neurofibrillary tangles (NFT). Although there was a debate to segregate dementia from underlying mental retardation, at least some portion of DS patients exhibit deteriorated mental status with aging. The mechanism underlying these abnormalities at the molecular level remains to be elucidated. Recently there have been several reports suggesting abnormalities reflecting increased risk to apoptosis in DS brains. Increased expression of several apoptosis-related genes (p53, fas, ratio of bax to bcl-2, GAPDH) in DS brains has been reported. Cultured neurons from both patients and model animals are reportedly more vulnerable to apoptosis. Overproduction of reactive oxygen species and its causative roles for increased apoptosis in DS tissues are suggested. One possible hypothesis is an increased susceptibility to apoptosis due to p53 overactivation in DS brains. A beta 42, a critical peptide for AD pathology from amyloid precursor protein (APP), can be detected in DS brains. A beta 42 is deposited in SP from an early stage, suggesting common molecular mechanisms in DS and AD. Animal models for DS are important in the search of molecular mechanisms. Several types of models are now available. Future DS studies are expected to integrate information from animal models and human tissues.

Ts65Dn mouse, a Down syndrome model, exhibits elevated myo-inositol in selected brain regions and peripheral tissues

myo-Inositol is elevated in the Down syndrome (DS; trisomy 21) brain and may play a role in mental retardation. In the present study, we examined brain regions and peripheral tissues of Ts65Dn mouse, a recently characterized genetic model of DS, for abnormal myo-inositol accumulation. A GC/MS technique was used to quantitate myo-inositol and other polyol species (ribitol, arabitol, xylitol, and 1,5-anhydrosorbitol) in tissues from the Ts65Dn mice and control diploid mice. myo-Inositol was found to be elevated in frontal cortex, hippocampus, and brain stem but not in cerebellum of the Ts65Dn mouse. Among peripheral organs examined, liver and skeletal muscle were found to excessively accumulate myo-inositol. In all tissues, concentrations of polyol internal controls were normal. The Ts65Dn mouse is useful to study the possible effect of elevated myo-inositol on cellular processes.

Oestrogen and nerve growth factor - neuroprotection and repair in Alzheimer's disease

The neurogenetics and neuropathology of Alzheimer's disease (AD) are still largely unknown, even though recent work has clarified some genetic components in this common and devastating neurodegenerative disease. Most of the genetic mutations have been shown to be, at least in the early onset type of AD, related to the function of a large transmembrane protein, amyloid precursor protein (APP). This protein is cleaved into various smaller fragments that are either soluble or aggregating. It is thought that this processing of APP is inherently important for the initiation and progression of AD. Recent animal models have suggested that it is not the formation of beta-amyloid plaques per se, but the altered processing of APP and the subsequent loss of soluble APP, that sets the stage for the massive neuronal cell loss which occurs in AD. We would like to propose a three-way relationship between oestrogen, APP and nerve growth factor (NGF) in the neural pathways of the brain which are involved in learning and memory - the limbic system. The degeneration of the cholinergic innervation from the basal forebrain to the hippocampal formation in the temporal lobe is thought to be one of the factors determining the progression of memory decay, both during normal ageing and AD. Oestrogen and NGF are among the neuroprotective agents that have shown some potential for the treatment of AD. Previous results of treatment with these two agents and their relationship to the amyloid proteins, will be discussed in this review.

The t(14;21)(q11.2;q22) chromosomal translocation associated with T-cell acute lymphoblastic leukemia activates the BHLHB1 gene

We have cloned the genomic breakpoints for a balanced t(14;21)(q11. 2;q22) chromosomal translocation associated with T-cell acute lymphoblastic leukemia. Sequence analysis of the genomic breakpoints indicated that the translocation had been mediated by an illegitimate V(D)J recombination event that disrupted the T-cell receptor (TCR) alpha locus and placed the TCR alpha locus enhancer on the derivative 21 chromosome. We identified a previously unreported transcript, designated BHLHB1 (for basic domain, helix-loop-helix protein, class B, 1) that had been activated by the translocation. BHLHB1 mapped to the region of chromosome 21 that has been proposed to be responsible, at least in part, for the learning deficits seen in children with Down's syndrome. Although BHLHB1 expression normally is restricted to neural tissues, T-cell lymphoblasts with the t(14;21)(q11.2;q22) also expressed high levels of BHLHB1 mRNA. Expression of BHLHB1 dramatically inhibited E2A-mediated transcription activation in NIH 3T3 fibroblasts and Jurkat T cells. This observation suggests that BHLHB1, similar to SCL/TAL1, may exert a leukemogenic effect through a functional inactivation of E2A or related proteins.

Synaptic deficit in the temporal cortex of partial trisomy 16 (Ts65Dn) mice

Down syndrome results from triplication of human chromosome 21. The distal end of mouse chromosome 16 shares a large region of genetic homology with the Down syndrome 'critical region' of human chromosome 21. Therefore, a partially trisomic mouse (Ts65Dn) that possesses a triplication of the distal region of chromosome 16 has been developed as a putative model for Down syndrome. Ts65Dn mice display learning and memory deficits. However, despite the importance of preserved synaptic integrity for learning and memory, the ultrastructure of neural connectivity has not yet been studied in Ts65Dn mice. Therefore, the density and apposition zone length of synapses in the temporal cortex of aged Ts65Dn mice (n=4) were compared with those in diploid controls (n=4), using quantitative electron microscopy. There were significantly less (30%) asymmetric synapses in the temporal cortex of Ts65Dn mice than in controls (t=-5.067; p=0.023). However, there was no significant difference between the mean density of symmetric synapses in Ts65Dn mice and control mice. In addition, the mean synaptic apposition lengths of both asymmetric (15%; t=9.812, p<0.0001) and symmetric (11%; t=5. 582; p<0.0001) synapses were significantly larger in Ts65Dn mice than in controls. These results suggest that excitatory synapses are preferentially affected in Ts65Dn mice and that there is an attempt to compensate for the deficit of asymmetric synapses by increasing the contact zone area of existing synapses. The results may also reveal the morphological basis for the learning and memory deficits observed in Ts65Dn mice and have a bearing on the cognitive deficits in Down syndrome in old age.

Loss of cholinergic phenotype in basal forebrain coincides with cognitive decline in a mouse model of Down's syndrome

Mice with segmental trisomy of chromosome 16 (Ts65Dn) have been used as a model for Down's syndrome. These mice are born with a normal density of basal forebrain cholinergic neurons but, like patients with Down's syndrome, undergo a significant deterioration of these neurons later in life. The time course for this degeneration of cholinergic neurons has not been studied, nor is it known if it correlates with the progressive memory and learning deficits described. Ts65Dn mice that were 4, 6, 8, and 10 months old were sacrificed for evaluation of basal forebrain morphology. Separate groups of mice were tested on visual or spatial discrimination learning and reversal. We found no alterations in cholinergic markers in 4-month-old Ts65Dn mice, but thereafter a progressive decline in density of cholinergic neurons, as well as significant shrinkage of cell body size, was seen. A parallel loss of staining for the high-affinity nerve growth factor receptor, trkA, was observed at all time points, suggesting a biological mechanism for the cell loss involving this growth factor. Other than transient difficulty in learning the task requirements, there was no impairment of trisomic mice on visual discrimination learning and reversal, whereas spatial learning and reversal showed significant deficits, particularly in the mice over 6 months of age. Thus, the loss of ChAT-immunoreactive neurons in the basal forebrain was coupled with simultaneous deficits in behavioral flexibility on a spatial task occurring for the first time around 6 months of age. These findings suggest that the loss of cholinergic function and the simultaneous decrease in trkA immunoreactivity in basal forebrain may directly correlate with cognitive impairment in the Ts65Dn mouse

Parallels of craniofacial maldevelopment in Down syndrome and Ts65Dn mice

Mouse genetic models can be used to dissect molecular mechanisms that result in human disease. This approach requires detection and demonstration of compelling parallels between phenotypes in mouse and human. Ts65Dn mice are at dosage imbalance for many of the same genes duplicated in trisomy 21 or Down syndrome (DS), the most common live-born human aneuploidy. Analysis of the craniofacial skeleton of Ts65Dn mice using three-dimensional morphometric methods demonstrates an absolute correspondence between Ts65Dn and DS craniofacial dysmorphology, a distinctive and completely penetrant DS phenotype. The genes at dosage imbalance in Ts65Dn are localized to a small region of mouse chromosome 16 and, by comparative mapping, to the corresponding region of human Chromosome 21, providing independent experimental data supporting the contribution of genes in this region to this characteristic DS phenotype. This analysis establishes precise parallels in human and mouse skull phenotypes resulting from dosage imbalance for the same genes, revealing strong conservation of the evolved developmental genetic program that underlies mammalian skull morphology and validating the use of this mouse model in the analysis of this important DS phenotype. This evolutionary conservation further establishes the mouse as a valid model for a wide range of syndromes producing craniofacial maldevelopment.

Increased synaptic depression in the Ts65Dn mouse, a model for mental retardation in Down syndrome

Long-term potentiation (LTP) and depression (LTD) were investigated in hippocampus of a genetic model of Down syndrome, the segmental trisomy (Ts65Dn) mouse. Field excitatory postsynaptic potentials were recorded from hippocampal slices and LTP and LTD evoked sequentially. LTP decreased whereas LTD increased significantly in Ts65Dn compared with control hippocampus.

Perfect conserved linkage across the entire mouse chromosome 10 region homologous to human chromosome 21

The distal end of human Chromosome (HSA) 21 from PDXK to the telomere shows perfect conserved linkage with mouse Chromosome (MMU) 10. This region is bounded on the proximal side by a segment of homology to HSA22q11.2, and on the distal side by a region of homology with HSA19p13.1. A high-resolution PAC-based physical map is described that spans 2.8 Mb, including the entire 2.1 Mb from Pdxk to Prmt2 corresponding to HSA21. Thirty-four expressed sequences are mapped, three of which were not mapped previously in any species and nine more that are mapped in mouse for the first time. These genes confirm and extend the conserved linkage between MMU10 and HSA21. The ordered PACs and dense STS map provide a clone resource for biological experiments, for rapid and accurate mapping, and for genomic sequencing. The new genes identified here may be involved in Down syndrome (DS) or in several genetic diseases that map to this conserved region of HSA21.

Reactive Oxygen Intermediates During Programmed Cell Death Induced in the Thymus of the Ts(1716)65Dn Mouse, a Murine Model for Human Down’s Syndrome

Down’s syndrome (DS) is one of the most frequent genetic disorders in humans. It has been suggested that overexpression of copper-zinc superoxide dismutase (SOD-1) in DS may be involved in some of the abnormalities observed, mainly neurodegenerative and immunopathological processes. One of the consequences is early thymic involution. Recently, Ts(1716)65Dn mice (Ts65Dn mice), made segmentally trisomic for a chromosome 16 segment, fulfill the criteria for a DS model. To study the possible role of SOD-1 overexpression in thymocyte biology, we analyzed the role of reactive oxygen intermediates during in vivo and in vitro programmed cell death (PCD) induced in the thymus of Ts65Dn mice. Our main findings can be summarized as follows. Ts65Dn thymuses exhibit greater PCD activity than controls, as ascertained by a combination of morphological, histochemical, and ultrastructural procedures. Ts65Dn thymocytes were highly susceptible to PCD induced by both LPS (in vivo) and dexamethasone, a synthetic glucocorticoid agonist (both in vivo and in vitro). Thymus abnormalities were probably caused by SOD-1 hyperexpression in Ts65Dn cells, in that reactive oxygen intermediate generation (specifically H2O2 production) is enhanced in thymocytes and clearly correlates with apoptosis. Similarly, oxidative injury correlated with the formation of lipid peroxidation by-products and antioxidants which partly inhibit PCD in thymocytes.

The promise of comparative genomics in mammals

Dense genetic maps of human, mouse, and rat genomes that are based on coding genes and on microsatellite and single-nucleotide polymorphism markers have been complemented by precise gene homolog alignment with moderate-resolution maps of livestock, companion animals, and additional mammal species. Comparative genetic assessment expands the utility of these maps in gene discovery, in functional genomics, and in tracking the evolutionary forces that sculpted the genome organization of modern mammalian species.

Recent advances in the genetics of epilepsy: insights from human and animal studies

Progress in understanding the genetics of epilepsy is proceeding at a dizzying pace. Due in large part to rapid progress in molecular genetics, gene defects underlying many of the inherited epilepsies have been mapped, and several more are likely to be added each year. In this review, we summarize the available information on the genetic basis of human epilepsies and epilepsy syndromes, and correlate these advances with rapidly expanding information about the mechanisms of epilepsy gained from both spontaneous and transgenic animal models. We also provide practical suggestions for clinicians confronted with families in which multiple members are afflicted with epilepsy.

Mild impairment of learning and memory in mice overexpressing the mSim2 gene located on chromosome 16: an animal model of Down's syndrome

Human Sim2 is a product of one of the genes located on human chromosome 21q22 and is a homolog of Drosophila single-minded ( sim ) which is a critical player in midline development of the central nervous system of the fly. Since Sim2 mRNA is expressed in facial, skull, palate and vertebra primordia in human and rodent embryos, features that are associated with phenotypes of Down's syndrome (DS), its trisomic state is suspected to contribute to the symptoms of DS. Here we describe that mSim2 mRNA is expressed in hippocampus and amygdala of adult mice, and that while mice overexpressing mSim2 under the control of the beta-actin promoter are viable and fertile and have superficially normal skeletal, brain and heart structures, they exhibit a moderate defect in context-dependent fear conditioning and a mild defect in the Morris water maze test. Taken together, our data show that overdosage of Sim2 may be important for the pathogenesis of Down's syndrome, especially mental retardation.

Integration of cytogenetic with recombinational and physical maps of mouse chromosome 16

To link the cytogenetic map for mouse chromosome 16 (Chr 16) to the more detailed recombinational and physical maps, multiple probes were mapped by fluorescence in situ hybridization (FISH). Sixteen large insert clones (YACs, BACs, and PACs) containing markers that have been assigned to the Chr 16 recombinational map were localized to a cytogenetic band or subband by high-resolution FISH. This linkage of the cytogenetic and recombinational maps provides a useful tool for assigning new probe locations and for defining breakpoints in mice with chromosomal rearrangements. A direct application of these probes is demonstrated by identifying mice trisomic for distal Chr 16 using FISH analysis of interphase nuclei.

Mouse autosomal trisomy: two's company, three's a crowd

Autosomal trisomy causes a large proportion of all human pregnancy loss and so is a significant source of lethality in the human population. The autosomal trisomy syndromes each have a different phenotype and are probably caused by the effects of specific genes that are present in three copies, rather than the normal two. Identifying these genes will require the application of classical genetic and new genome-manipulation approaches. Recent advances in chromosome engineering are now allowing us to create precisely defined autosomal trisomies in the mouse, and so provide new routes to identifying the critical, dosage-sensitive genes that are responsible for these highly deleterious, yet very common, syndromes.

Transchromosomal mouse embryonic stem cell lines and chimeric mice that contain freely segregating segments of human chromosome 21

At least 8% of all human conceptions have major chromosome abnormalities and the frequency of chromosomal syndromes in newborns is >0.5%. Despite these disorders making a large contribution to human morbidity and mortality, we have little understanding of their aetiology and little molecular data on the importance of gene dosage to mammalian cells. Trisomy 21, which results in Down syndrome (DS), is the most frequent aneuploidy in humans (1 in 600 live births, up to 1 in 150 pregnancies world-wide) and is the most common known genetic cause of mental retardation. To investigate the molecular genetics of DS, we report here the creation of mice that carry different human chromosome 21 (Hsa21) fragments as a freely segregating extra chromosome. To produce these 'transchromosomal' animals, we placed a selectable marker into Hsa21 and transferred the chromosome from a human somatic cell line into mouse embryonic stem (ES) cells using irradiation microcell-mediated chromosome transfer (XMMCT). 'Transchromosomal' ES cells containing different Hsa21 regions ranging in size from approximately 50 to approximately 0.2 Mb have been used to create chimeric mice. These mice maintain Hsa21 sequences and express Hsa21 genes in multiple tissues. This novel use of the XMMCT protocol is applicable to investigations requiring the transfer of large chromosomal regions into ES or other cells and, in particular, the modelling of DS and other human aneuploidy syndromes.

Isolation and characterization of the mouse Aire gene

Autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy (APECED) is a rare autosomal recessive disorder characterized by Addison's disease and/or hypoparathyroidism and/or chronic mucocutaneous candidiasis. Patients may also have other clinical symptoms both within and outside the endocrine system, mainly as a result of autoimmunity against organ-specific autoantigens. The gene for APECED has recently been identified and termed AIRE (for AutoImmune REgulator). APECED is a model of organ-specific autoimmunity and isolation and characterization of the homologous mouse gene, Aire, will provide tools for dissection of the mechanisms underlying this human disorder and defining molecular pathways involved in organ-specific autoimmunity. We have isolated and completely sequenced the mouse Aire gene which is split into 14 exons over 13 kb and encodes a predicted protein of 552 amino acids. The predicted mouse and human AIRE proteins are 71% identical and contain motifs suggestive of a transcriptional regulator. Additional conserved motifs are emerging in the AIRE/Aire proteins including a nuclear localization signal, an "ASS" domain, and a "SAND" domain. The human and mouse AIRE promoters have conserved sites for several thymus-specific transcription factors and others important in hematopoesis, consistent with its expression in rare cells of the thymus medulla, lymph nodes, and fetal liver. We have mapped mouse Aire to mouse chromosome 10 by FISH, to the same region as Pwp2 and Pfkl, confirming synteny to the corresponding region of human chromosome 21.

Transgenic animals relevant to Alzheimer's disease

This article reviews the functional studies that have been carried out on transgenic and knockout animals that are relevant to Alzheimer's disease (AD). The discussion focuses upon the functional characterisation of these strains, particularly upon factors that affect synaptic processes that are thought to contribute to memory formation, including hippocampal long-term potentiation. We examine the use of transgenes associated with amyloid precursor protein and presenilin-1, their mutations linked to early onset familial AD, and the recent attempts to establish double transgenic strains that have an AD-like pathology which occurs with a more rapid onset. The development of new transgenic strains relevant to Alzheimer's disease has rapidly outpaced their characterisation for functional deficits in synaptic plasticity. To date most studies have focused on those transgenes linked to the minority of familial early onset rather than late-onset sporadic AD cases, and have focused on those changes linked to the induction of the early-phase of hippocampal long-term potentiation. Future studies will need to address the question of whether the development of AD pathology can be reversed or at least halted and this will be aided by the use of conditional transgenics in which genes linked to AD can either be switched on or off later in development. Furthermore, it remains to be resolved whether the deficits in synaptic function are specific to the hippocampus and whether deficits affect late-phase long-term potentiation. Nonetheless, the recent advances in genome sciences and the development of transgenic technology have provided a unique opportunity to study how genes associated with human cognitive dysfunction alter synaptic transmission between neurones in the mammalian brain.

Long-term basal forebrain cholinergic-rich grafts derived from trisomy 16 mice do not develop beta-amyloid pathology and neurodegeneration but demonstrate neuroinflammatory responses

Patients with Down syndrome (human trisomy 21) develop neuropathological and cholinergic functional defects characteristic of Alzheimer's disease, which has been attributed to the location of the Alzheimer beta-amyloid precursor protein on chromosome 21. Due to the partial genetic homology between mouse chromosome 16 and human chromosome 21, murine trisomy 16 was used as a model to study functional links between increased expression of the amyloid precursor protein, neurodegeneration and neuroinflammatory responses. Basal forebrain cholinergic-rich tissue derived from trisomy 16 mice at embryonic age of day 16 was transplanted into the lateral ventricle of adult normal mice. At 1, 3, 6, 9 and 12 months after transplantation, the grafts were characterized by immunocytochemistry, molecular biological analysis, and stereological methods. Grafts survived up to one year and still demonstrated immunoreactivity for cholinergic, GABAergic and astroglial cells. Though a 1.5-fold neuronal over-expression of amyloid precursor protein was detected in brains from trisomy 16 embryos by Northern analysis, beta-amyloid deposits were found neither in control nor trisomic grafts. Detailed stereological analysis of trisomic grafts did not reveal any neurodegeneration or morphological changes of cholinergic and GABAergic neurons during the course of graft maturation up to one year, as compared to grafts derived from euploid tissue. However, both euploid and trisomic grafts demonstrated a strong infiltration with T- and B-lymphocytes and a significant micro- and astroglial activation (hypertrophic astrocytes) within and around the grafts. These observations further suggest that the trisomy 16-induced neurodegeneration is seemingly due to a lack of neuron supporting factors which are provided by either the metabolic interaction of trisomic graft with surrounding healthy host tissue or by cells of the immune system infiltrating the graft.

Two isoforms of a human intersectin (ITSN) protein are produced by brain-specific alternative splicing in a stop codon

Using selected trapped exons with homology to specific protein domains, we identified a new full-length cDNA encoding a protein containing many motifs for protein-protein interactions. There are two major mRNA transcripts, a ubiquitously expressed mRNA of 5.3 kb and a brain-specific transcript of approximately 15 kb, encoding proteins of 1220 and 1721 amino acids, respectively. The stop codon of the ORF of the shorter transcript is split between adjacent exons. In brain tissues the last exon of the short transcript is skipped, and an alternative downstream exon, the first of several additional, is used to produce the 15-kb mRNA. The putative human protein is highly homologous to Xenopus intersectin (81% identical) and to Drosophila dynamin-associated protein, Dap160 (31% identical) and was termed intersectin (ITSN). Both human proteins contain five SH3 (Src homology 3) domains, two EH (Eps15 homology) domains, and an alpha-helix-forming region. The brain-specific long transcript encodes for three additional domains: a GEF (guanine-nucleotide exchange factors), a PH (pleckstrin homology), and a C2 domain. The Drosophila homologue is associated with dynamin, a protein family involved in the endocytic pathway and/or synaptic vesicle recycling. The structure of the human ITSN protein is consistent with its involvement in membrane-associated molecular trafficking and signal transduction pathways. The human ITSN gene has been mapped to 21q22. 1-q22.2 between markers D21S319 and D21S65, and its importance in Down syndrome and monogenic disorders is currently unknown.

Hippocampal volume and neuronal number in Ts65Dn mice: a murine model of Down syndrome

Ts65Dn mouse displays a partial triplication of chromosome 16 and is adopted as a model for Down syndrome (DS). It is known that Ts65Dn mice present memory deficiencies. In order to gain insight into the cause of these deficiencies, we studied the possibility of changes in volumes and neuronal numbers in different regions of the hippocampus (dentate gyrus, CA3, CA2 and CA1) in trisomic mice as compared to control littermates using stereological methods. The mean hippocampal volumes of Ts65Dn mice did not show significant differences as compared to controls, except in CA2 where there was a barely significant decrease. However, mean neuron number was significantly lower in Ts65Dn mice than in controls in dentate gyrus (43.7 x 10(4), CV 21%, n = 5, vs. 30.4 x 10(4), CV 18.1%, n = 4) and higher in CA3 (23.1 x 10(4), CV 18.9% vs. 33.3 x 10(4), CV 14.9%). These quantitative changes may account for the memory deficiencies observed in Ts65Dn mice.

Physical and comparative mapping of distal mouse chromosome 16. 5 p5

Distal mouse Chromosome 16 (Chr. 16) includes a region of conserved linkage with human Chromosome 21 (Chr. 21). Mouse models of Down syndrome based on trisomy of distal Chr. 16 have several phenotypes similar to those seen in human patients and have proven useful for correlating dosage imbalance of specific genes with specific developmental anomalies. The degree to which such findings can be related to Down syndrome depends on how well the conserved synteny is maintained. Twenty-four genes have been mapped in both species and there are no discordancies, but the region could carry hundreds of genes. Comparative sequence represents the ultimate comparative map and will aid in identification of genes and their regulatory sequences. A physical map of the distal 4.5 Mb of Chr. 16 has been assembled as an essential step toward a map of sequence-ready templates. The map consists of 51 YACs and 15 BACs and includes 18 transcripts, 9 of which are mapped for the first time in mouse, and 3 of which are, for the first time, described in either species. YAC fragmentation was used to precisely localize the 49 markers on the map. Comparison of this physical map with that of the corresponding region on Chr. 21 shows conservation not only of gene order but of size in the 3 Mb from Cbr1 to Ets2; distal to Ets2, the human map is expanded.

10 years of Genomics, chromosome 21, and Down syndrome

During the first 10 years of Genomics (1987-1997), the molecular structure of human chromosome 21 (HC21) has been intensively investigated. Due to its small size and involvement in Down syndrome, it continues to serve as a model in the development of "genomics technologies." Increasingly more detailed genetic, radiation hybrid, physical, and transcription maps, in addition to NotI restriction and chromosomal breakpoint maps, of HC21 have been developed, and approximately 10% of its genes have been cloned. These maps have been vital in the localization of loci for 15 monogenic disorders to HC21, and 10 of these genes have been identified and characterized. The genetic maps have aided in the detailed elucidation of the origin of the supernumerary HC21 in trisomy 21 from investigations of recombination and nondisjunction events. Mouse models of Down syndrome, with partial trisomy 16, the mouse chromosome principally syntenic to HC21, have been created and initially characterized. A substantial number of the above studies related to the molecular mapping, gene cloning, and infrastructure of HC21 were published in Genomics (e.g., approximately 30% of papers describing HC21 maps were published here). The future goals of genomic analysis of HC21 will be the determination of its complete nucleotide sequence and the identification and functional analysis of all of its genes. These advances will help to provide a molecular explanation of the pathophysiology of Down syndrome and aid in the identification of genes for monogenic and polygenic disorders that map on this chromosome. Novel therapeutic interventions for Down syndrome and the monogenic and polygenic disorders that map to HC21 will be designed and tried based on the knowledge of the disease pathogenesis resulting from the genomic analysis.

Event-related brain potentials during an extended visual recognition memory task depict delayed development of cerebral inhibitory processes among 6-month-old infants with Down syndrome

Development of cerebral inhibitory processes among individuals with Down syndrome (DS) may be delayed at an early age. In support of this hypothesis, sensory-evoked potentials (EPs) and event-related brain potentials (ERPs) have previously delineated altered habituation to stimuli among infants with DS. The purpose of the current study was to provide extended experience with visual stimuli among 6-month-old infants with and without DS (nDS) to determine if altered ERP and behavioral response decrements would be evident even after repeated presentations of stimuli. An 80/20% oddball paradigm was employed. Infants with DS and nDS were matched according to age and gender. Infants with DS demonstrated significantly larger Nc areas, Nc peak amplitudes, Nc2 areas and, inversely, significantly smaller peak Pb amplitudes when compared to infants nDS. Contrasts of the two study groups were most robust within ERP measures from frontal (Fz) and parietal (Pz) recording sites. Infants with DS also demonstrated a significantly slower decrement of most ERP components with repetitive stimulus experience. Most noteworthy was the observation of little or no decrement of ERP components at Fz among infants with DS. Both infants with DS and nDS demonstrated significantly larger Nc peak amplitudes, Nc areas, Nc2 areas, Pb peak amplitudes and NSW areas to rare stimuli. While significant probability and experiential trends were observed in visual fixation measures across both study groups, there were no significant differences of visual attention between infants with DS or nDS. These data demonstrate the value of ERPs within the study of atypical cognitive development during infancy and support the concept of altered inhibitory processes in the brain of infants with DS.

Down syndrome and mouse models

The past year has seen major advancements in the characterisation of the Ts65Dn mouse model (which is now known to display many features of Down syndrome). A newer model that is trisomic for the region 21 q22.2--previously called 'Down syndrome' region--has been generated and these mice display behavioural and learning defects. Mutations in the genes Minibrain and SOD1 have been implicated in the development of learning defects in Down syndrome and many new genes from human chromosome 21 are being cloned, which should result in the genesis of other models that phenocopy one or more pathologies of the syndrome.

Ts1Cje, a partial trisomy 16 mouse model for Down syndrome, exhibits learning and behavioral abnormalities

A mouse model for Down syndrome, Ts1Cje, has been developed. This model has made possible a step in the genetic dissection of the learning, behavioral, and neurological abnormalities associated with segmental trisomy for the region of mouse chromosome 16 homologous with the so-called "Down syndrome region" of human chromosome segment 21q22. Tests of learning in the Morris water maze and assessment of spontaneous locomotor activity reveal distinct learning and behavioral abnormalities, some of which are indicative of hippocampal dysfunction. The triplicated region in Ts1Cje, from Sod1 to Mx1, is smaller than that in Ts65Dn, another segmental trisomy 16 mouse, and the learning deficits in Ts1Cje are less severe than those in Ts65Dn. In addition, degeneration of basal forebrain cholinergic neurons, which was observed in Ts65Dn, was absent in Ts1Cje.