Gene-dosage effects in Down syndrome and trisomic mouse models

The abnormalities found in human Down syndrome (trisomy 21) have been thought to result from increased expression of genes on chromosome 21 because of their higher gene dosage. Now, several groups have shown this to be generally the case, but some inter-individual variability and other exceptions were found.     

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

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

The use of mouse models to understand and improve cognitive deficits in Down syndrome

Remarkable advances have been made in recent years towards therapeutics for cognitive impairment in individuals with Down syndrome (DS) by using mouse models. In this review, we briefly describe the phenotypes of mouse models that represent outcome targets for drug testing, the behavioral tests used to assess impairments in cognition and the known mechanisms of action of several drugs that are being used in preclinical studies or are likely to be tested in clinical trials. Overlaps in the distribution of targets and in the pathways that are affected by these diverse drugs in the trisomic brain suggest new avenues for DS research and drug development.     

Biomarkers of oxidative stress are significantly elevated in Down syndrome

There is convincing epidemiological and in vitro evidence of chronic oxidative stress in individuals with Down syndrome (DS). These individuals develop Alzheimer like changes in the brain in their 30s and 40s. The incidence of autoimmune diseases and cataracts is significantly increased, and the overall ageing process is accelerated. In vitro studies show that impaired viability of DS neurons may be amended by simple chemical antioxidants, such as vitamin E, BHT and propyl gallate, clearly indicative of oxyl radical involvement. However, because of the lack of in vivo experiments, the role of oxidative stress in DS remains controversial. We report here on the results of the chemical analyses of urine samples of 166 individuals, where DS subjects were matched by their siblings. The levels of 8-hydroxy-2′-deoxyguanosine (2.35 ± 1.69 in DS vs. 1.35 ± 1.04 in controls, P = 0.00011), a biomarker of oxidative damage to DNA, and malondialdehyde (0.255 ± 0.158 in DS vs. 0.204 ± 0.128 in controls, P = 0.033), a biomarker of lipid peroxidation, are significantly elevated in individuals with DS. Dietary influences failed to show any significant correlation with the oxidative stress biomarkers. These results provide direct evidence for increased oxidative stress in individuals with DS.     

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.     

Increased lipid peroxidation in Down's syndrome mouse models

Elevated oxidative stress has been suggested to be associated with the features of Down's syndrome (DS). We previously reported increased oxidative stress in cultured cells from the embryonic brain of Ts1Cje, a mouse genetic DS model. However, since in vivo evidence for increased oxidative stress is lacking, we here examined lipid peroxidation, a typical marker of oxidative stress, in the brains of Ts1Cje and another DS mouse model Ts2Cje with an overlapping but larger trisomic segment. Accumulations of proteins modified with the lipid peroxidation-derived products, 13-hydroperoxy-9Z,11E-octadecadienoic acid and 4-hydroxy-2-nonenal were markedly increased in Ts1Cje and Ts2Cje brains. Analysis with oxidation-sensitive fluorescent probe also showed that reactive oxygen species themselves were increased in Ts1Cje brain. However, electron spin resonance analysis of microdialysate from the hippocampus of Ts1Cje showed that antioxidant activity remained unaffected, suggesting that the reactive oxygen species production was accelerated in Ts1Cje. Proteomics approaches with mass spectrometry identified the proteins modified with 13-hydroperoxy-9Z,11E-octadecadienoic acid and/or 4-hydroxy-2-nonenal to be involved in either ATP generation, the neuronal cytoskeleton or antioxidant activity. Structural or functional impairments of these proteins by such modifications may contribute to the DS features such as cognitive impairment that are present in the Ts1Cje mouse.     

Down syndrome cell adhesion molecule is conserved in mouse and highly expressed in the adult mouse brain

Down Syndrome (DS) is a major cause of mental retardation and is associated with characteristic well-defined although subtle brain abnormalities, many of which arise after birth, with particular defects in the cortex, hippocampus and cerebellum. The neural cell adhesion molecule DSCAM (Down syndrome cell adhesion molecule) maps to 21q22.2-->q22.3, a region associated with DS mental retardation, and is expressed largely in the neurons of the central and peripheral nervous systems during development. In order to evaluate the contribution of DSCAM to postnatal morphogenetic and cognitive processes, we have analyzed the expression of the mouse DSCAM homolog, Dscam, in the adult mouse brain from 1 through 21 months of age. We have found that Dscam is widely expressed in the brain throughout adult life, with strongest levels in the cortex, the mitral and granular layers of the olfactory bulb, the granule cells of the dentate gyrus and the pyramidal cells of the CA1, CA2 and CA3 regions, the ventroposterior lateral nuclei of the thalamus, and in the Purkinje cells of the cerebellum. Dscam is also expressed ventrally in the adult spinal cord. Given the homology of DSCAM to cell adhesion molecules involved in development and synaptic plasticity, and its demonstrated role in axon guidance, we propose that DSCAM overexpression contributes not only to the structural defects seen in these regions of the DS brain, but also to the defects of learning and memory seen in adults with DS.     

Trisomy 21 Down syndrome

Summary In a series of 374 families with Down syndrome progeny, structural chromosome rearrangements were detected in the parents of six children with regular trisomy. The aberrations were reciprocal translocations and inversions. In all three informative families, the parent who transmitted the extra chromosome was not the one with the structural rearrangement. Among the three non-informative families there was one in which both parents carried different reciprocal translocations. In two other families a chromosome aberration was detected: a triple X mother and a father with a Philadelphia chromosome. Omitting the four parents with possible biased asccrtainment, 0.4% had a chromosome rearrangement. When the parents with constitutional chromosome aberrations and those with mosaicism, described previously, are combined, the frequency of chromosomally abnormal parents lies between 1.9% and 3.2%. When correlated with parental transmission of the extra chromosome, mosaicism rather than structural rearrangements appears to be of ctiologic significance.     

Antioxidants in Down syndrome

Individuals with Down syndrome (DS) have high levels of oxidative stress throughout the lifespan. Mouse models of DS share some structural and functional abnormalities that parallel findings seen in the human phenotype. Several of the mouse models show evidence of cellular oxidative stress and have provided a platform for antioxidant intervention. Genes that are overexpressed on chromosome 21 are associated with oxidative stress and neuronal apoptosis. The lack of balance in the metabolism of free radicals generated during processes related to oxidative stress may have a direct role in producing the neuropathology of DS including the tendency to Alzheimer disease (AD). Mitochondria are often a target for oxidative stress and are considered to be a trigger for the onset of the AD process in DS. Biomarkers for oxidative stress have been described in DS and in AD in the general population. However, intervention trials using standard antioxidant supplements or diets have failed to produce uniform therapeutic effect. This chapter will examine the biological role of oxidative stress in DS and its relationship to abnormalities in both development and aging within the disorder. This article is part of a Special Issue entitled: Antioxidants and Antioxidant Treatment in Disease.     

Antioxidants in Down syndrome

Individuals with Down syndrome (DS) have high levels of oxidative stress throughout the lifespan. Mouse models of DS share some structural and functional abnormalities that parallel findings seen in the human phenotype. Several of the mouse models show evidence of cellular oxidative stress and have provided a platform for antioxidant intervention. Genes that are overexpressed on chromosome 21 are associated with oxidative stress and neuronal apoptosis. The lack of balance in the metabolism of free radicals generated during processes related to oxidative stress may have a direct role in producing the neuropathology of DS including the tendency to Alzheimer disease (AD). Mitochondria are often a target for oxidative stress and are considered to be a trigger for the onset of the AD process in DS. Biomarkers for oxidative stress have been described in DS and in AD in the general population. However, intervention trials using standard antioxidant supplements or diets have failed to produce uniform therapeutic effect. This chapter will examine the biological role of oxidative stress in DS and its relationship to abnormalities in both development and aging within the disorder. This article is part of a Special Issue entitled: Antioxidants and Antioxidant Treatment in Disease.     

Trisomy 21 Down syndrome

Summary The lymphocyte chromosomes of trisomy 21 Down syndrome patients and their parents in a random series of 374 families were analyzed, the objective being the identification of parental mosaicism. The numbers of parents in whom at least two trisomy 21 cells were detected were seven mothers and three fathers, a frequency of 2.7% of families. Confirmation of mosaicism was by identification of parental transmission of the extra chromosome to the progeny, by repeat chromosome analysis, and/or by the presence of more than one affected child. If to these are added six others in whom only one trisomic cell was detected, but with no other supporting evidence, the frequency could be as high as 4.3%. Differences in parental age at the birth of Down syndrome progeny may be accounted for by differences in frequencies of mosaicism in germ cells and somatic tissue. Mosaicism was found more frequently in the mothers than in the fathers, but more data are required for confirmation of a real difference.     

Neuronal dysfunction in Down syndrome: contribution of neuronal models in cell culture.

Down syndrome (DS) in humans, or trisomy of autosome 21, represents the hyperdiploidy that most frequently survives gestation, reaching an incidence of 1 in 700 live births. The condition is associated with multisystemic anomalies, including those affecting the central nervous system (CNS), determining a characteristic mental retardation. At a neuronal level, our group and others have shown that the condition determines marked alterations of action potential and ionic current kinetics, which may underlie abnormal processing of information by the CNS. Since the use of human tissue presents both practical and ethical problems, animal models of the human condition have been sought. Murine trisomy 16 (Ts16) is a model of the human condition, due to the great homology between human autosome 21 and murine 16. Both conditions share the same alterations of electrical membrane properties. However, the murine Ts16 condition is unviable (animals die in utero), thus limiting the quantity of tissue procurable. To overcome this obstacle, we have established immortal cell lines from normal and Ts16 mice with a method developed by our group that allows the stable in vitro immortalization of mammalian tissue, yielding cell lines which retain the characteristics of the originating cells. Cell lines derived from cerebral cortex, hippocampus, spinal cord and dorsal root ganglion of Ts16 animals show alterations of intracellular Ca2+ signals in response to several neurotransmitters (glutamate, acetylcholine, and GABA). Gene overdose most likely underlies these alterations in cell function, and the identification of the relative contribution of DS associated genes on such specific neuronal dysfunction should be investigated. This could enlighten our understanding on the contribution of these genes in DS, and identify new therapeutic targets.     

Survival and lung pathology of mouse models of Hermansky-Pudlak syndrome and Chediak-Higashi syndrome

Hermansky-Pudlak Syndrome (HPS), a recessively inherited disease in humans, affects the biosynthesis/processing of the related intracellular organelles: lysosomes, melanosomes, and platelet dense granules. The disease is multigenic in both humans and mice where 14 separate genes have been demonstrated to be causative. Patients often die prematurely with severe lung abnormalities. Patients with the related Chediak-Higashi Syndrome (CHS) likewise have significantly reduced life spans. Long-term survival and lung histomorphology were analyzed in a pilot experiment involving several genetically defined singly and doubly mutant mouse HPS mutants and the beige CHS mutant to determine whether these parameters are altered in the mouse models. The mutants differed widely in both longevity and lung architecture. Mice doubly homozygous for the pale ear and ruby eye or for the muted and pearl genes had the shortest life spans with none surviving the two-year experimental duration. Life spans were similarly severely reduced in the beige and gunmetal mutants. Intermediate life spans were apparent in the pearl, pallid, and cocoa mutants whereas minimal effects were noted in ruby eye, muted, light ear, and cocoa mutants. Enlarged air spaces were noted in histologic sections of lungs of several of the mutants. For the most part, the severity of lung abnormalities was inversely proportional to the long-term survival of these various mutants, suggesting that lung pathology may contribute to mortality, as has been suggested for human HPS patients.