Down syndrome gene dosage imbalance on cerebellum development

doi:10.1016/j.pneurobio.2007.02.006

Progress in Neurobiology

Volume 82, Issue 2, June 2007, Pages 87-94

https://doi.org/10.1016/j.pneurobio.2007.02.006 Get rights and content

Abstract

Down syndrome (DS) is a chromosomal disorder whereby genes on chromosome 21 are present in three copies. This gene copy imbalance is thought to be responsible for a number of debilitating conditions experienced by individuals with DS. Amongst these is a reduced cerebellar volume, or cerebellar hypoplasia, which is believed to contribute to the perturbation of fine motor control. Mouse models of DS (such as Ts65Dn, Ts1Cje, Tc1) exhibit a cerebellar phenotype similar to that of individuals with DS and which primarily manifests as a disruption of the density of the granule cell layer. Dissecting which of the three-copy genes are responsible for this phenotype (the primary gene dosage effect) has been a task undertaken by researchers working with various segmental trisomies and transgenic mice. It is generally agreed that, when expressed, three-copy genes of trisomic mice are expressed at around 1.5 times that of the same genes in euploid (wild-type) mice. However, amongst these studies there does not appear to be a consensus on the nature and extent of differential expression of two-copy genes in trisomic mice—the secondary dosage effect. Much of this variation may have to do with the stage of development investigated and the nature and complexity of the tissue (i.e. whole brain versus the cerebellum). The recent discovery that trisomic granule cell precursors are less sensitive to sonic hedgehog-induced proliferation has opened up another avenue for the identification of three-copy genes responsible for the cerebellar phenotype. It is hoped that further investigation of this phenomenon, together with new mouse segmental trisomies and transgenics, will reveal the cause of the proliferation deficit and allow for potential treatment.

Introduction

Down syndrome (DS) results from trisomy of human chromosome 21 (Hsa21) and is the most frequent genetic cause of congenital mental retardation (OMIM 190685). The risk of DS at birth up to maternal age 30 is around 1 in 1000 and at maternal age 40 is around 1 in 200 (Hook et al., 1983). Worldwide, about 220,000 individuals are born with DS each year (Christianson et al., 2006).

One hundred years after the first description by Langdon Down, Jérôme Lejeune found that an extra small chromosome was present in individuals with DS (Lejeune et al., 1959a, Lejeune et al., 1959b). Trisomy for Hsa21 usually results from chromosomal non-disjunction during meiosis of the maturing egg. Most individuals with DS have full trisomy 21 whereby all Hsa21 genes (about 352, NCBI Genome database entry NC000021) are present in three copies. Around 1–2% of those with DS have segmental trisomy 21 where only a subset of Hsa21 genes are present in 3 copies, or are mosaic, containing both trisomic and euploid cells.

Individuals with DS may experience a wide range of medical conditions (Roizen and Patterson, 2003) including gastrointestinal problems, hearing loss, immune deficiencies, craniofacial dysmorphism, ophthalmic disorders and speech impairment. The most debilitating and frequent problems are congenital heart disease, early cognitive impairment and cognitive decline with aging due to an early onset of Alzheimer's disease-like symptoms.

Structural deficits in the DS brain have been described elsewhere (e.g. Aylward et al., 1997, Becker and Joost, 1999, Pinter et al., 2001). Of these, the cerebellar hypoplasia phenotype is mimicked in varying degrees in most murine models of DS, and thus permits a more exhaustive comparison between gene dosage effects and phenotype.

The human cerebellum develops over a long time, from early embryonic period until the first postnatal years. Consequently, the cerebellum is vulnerable to a broad spectrum of developmental disorders due to numerous genetic, epigenetic, metabolic and environmental effects. For example, hypoplasia of the cerebellar hemispheres and vermis is seen in Dandy–Walker syndrome (OMIM 220200) and is caused by disruption of a segment of Hsa3q2, wherein the genes ZIC1 and ZIC4 are found. Their heterozygous deletion in a mouse model mimics the Dandy-Walker cerebellar hypoplasia (Grinberg et al., 2004). Cerebellar hypoplasia has also been associated with X-linked mental retardation (OMIM 300486)—caused by mutation in the oligophrenin-1 gene (Philip et al., 2003).

While it has been postulated that the presence of an additional chromosome confers developmental instability resulting in the DS phenotype (Shapiro, 2001), another, testable hypothesis, foresees specific genes whose deregulated protein function/presence is the cause of the phenotype. Much of the recent research has been directed at identifying the genes responsible. Evidence for this latter approach will be reviewed here.

Section snippets

DS mouse models

The DNA sequences which have emerged from human and mouse genome projects has assisted research into the expression and function of specific genes on Hsa21 that may play a role in the DS phenotype. Additionally, the question of the potential impact of this gene dosage imbalance on the transcription and function of thousands of two-copy genes and their proteins can also be addressed. A systematic study of gene expression in brain during development is only possible in animal models. The distal

Cerebellum phenotype in mouse models

Granule cells are glutamatergic neurons that regulate the output of the cerebellum by controlling the activity of Purkinje cells. Unlike all other neurons, postnatal granule cells are not generated in the ventricular zone but at the cerebellar surface, in the external germinal layer. For the first three weeks after birth, in mice, precursor cells from this layer proliferate extensively to generate a large number of granule cell precursors (GCPs). As these GCPs exit the cell cycle, their cell

Cerebellar transcriptome profiles of mouse models

In order to find molecular targets and define affected developmental pathways, gene expression profiles have been established in cerebellum of DS mouse models. In adult Ts65Dn and Ts1Cje mice cerebella, microarrays detect an elevated expression of Hsa21 orthologs with a mean trisomic/euploid (differential) expression ratio of around 1.5 (Dauphinot et al., 2005, Saran et al., 2003). This is considered the primary dosage effect and is an accurate reflection of the gene dosage-dependent increase

Genes at dosage imbalance involved in the DS cerebellum phenotype

From murine model studies described above, genes present on the chromosome segment Mrpl39 to Sod1 inclusive contribute to an exacerbated cerebellar phenotype only when they are over-expressed with other triploid genes (those found in the segment characterizing the Ts1Cje mouse). This is highlighted by the fact that mice trisomic uniquely for the Mrpl39-Sod1 segment (Ms1Cje/Ts65Dn) do not display a loss in Purkinje neurons yet their GC density resembles that of Ts1Cje mice containing the Sod1-

Molecular pathways important for GC proliferation involved in DS

The large number of studies on postnatal development of the cerebellum and medulloblastoma cerebellar tumors point to several molecular pathways involved in the control of granule cell proliferation (for review see Sotelo, 2004). Fig. 2 gives a summary of some of these main pathways.

Roper et al. (2006) recently demonstrated that poor proliferation was the cause of decreased GC density and not cell death by observing reduced mitotic cell counts in Ts65Dn cerebella, rather than increased TUNEL

Conclusion

The facts that the internal granule cell layer in individuals with DS comes to be significantly reduced in thickness and that this is associated with a reduction in density of granule cells have been known for some time. With respect to understanding the DS cerebellar phenotype as a result of gene dosage imbalance there have been two recent discoveries that have had a significant impact on how current research is approached. The first has been the discovery that a failure of granule cells to

Acknowledgements

The authors would like to thank Prof. R.H. Reeves for his invaluable contribution in the drafting of this review. Grants from the CEE (00816) and Fondation Jérôme Lejeune (France) made this work possible. RXM is a CJ Martin Fellow of the NHMRC (Australia).

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Citation Excerpt :
It would be helpful to know if the electrical properties of cerebellar GCs are similarly altered in the Tc1 mouse model of DS, which carries much of human chromosome 21 and shows both impaired motor performance and a marked decrease in GC density (Galante et al., 2009; O'Doherty et al., 2005). Motor performance of the TsC1je mouse model of DS, which shows a smaller decrease in GC density and contains a smaller number of triplicated genes, has not been described (Moldrich et al., 2007). The cerebellum is also important for the production of fluent speech (Ackermann, 2008) and people with DS have difficulty in producing clear and ordered speech (Barnes et al., 2006) but this is one characteristic that cannot be assessed in mouse models of DS.
Down syndrome (DS) is characterized by intellectual disability and impaired motor control. Lack of coordinated movement, poor balance, and unclear speech imply dysfunction of the cerebellum, which is known to be reduced in volume in DS. The principal cause of the smaller cerebellum is a diminished number of granule cells (GCs). These neurons form the ‘input layer’ of the cerebellar cortex, where sensorimotor information carried by incoming mossy fibers is transformed before it is conveyed to Purkinje cells and inhibitory interneurons. However, it is not known how processing of this information is affected in the hypogranular cerebellum that characterizes DS. Here we explore the possibility that the electrical properties of the surviving GCs are changed. We find that in the Ts65Dn mouse model of DS, GCs have a higher input resistance at voltages approaching the threshold for firing, which causes them to be more excitable. In addition, they fire narrower and larger amplitude action potentials. These subtly modified electrical properties may result in atypical transfer of information at the input layer of the cerebellum.

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Down syndrome gene dosage imbalance on cerebellum development

Abstract

Introduction

Section snippets

DS mouse models

Cerebellum phenotype in mouse models

Cerebellar transcriptome profiles of mouse models

Genes at dosage imbalance involved in the DS cerebellum phenotype

Molecular pathways important for GC proliferation involved in DS

Conclusion

Acknowledgements

Exp. Neurol.

J. Biol. Chem.

Trends Mol. Med.

Neurosci. Lett.

Gene

Biochem. Biophys. Res. Commun.

Brain Res.

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Lancet

Trends Genet.

Neuron

Neuron

Prog. Neurobiol.

Biochem. Biophys. Res. Commun.

Neurodevelopmental delay, motor abnormalities and cognitive deficits in transgenic mice overexpressing Dyrk1A (minibrain), a murine model of Down's syndrome

Hum. Mol. Genet.

Dosage-dependent over-expression of genes in the trisomic region of Ts1Cje mouse model for Down syndrome

Hum. Mol. Genet.

Chromosome 21 and down syndrome: from genomics to pathophysiology

Nat. Rev. Genet.

Hedgehog antagonist REN(KCTD11) regulates proliferation and apoptosis of developing granule cell progenitors

J. Neurosci.

NFAT dysregulation by increased dosage of DSCR1 and DYRK1A on chromosome 21

Nature

Cerebellar volume in adults with Down syndrome

Arch. Neurol.

Discovery and genetic localization of Down syndrome cerebellar phenotypes using the Ts65Dn mouse

Hum. Mol. Genet.

Structural and functional characteristics of Dyrk, a novel subfamily of protein kinases with dual specificity

Prog. Nucleic Acid Res. Mol. Biol.

Beta1-integrins are critical for cerebellar granule cell precursor proliferation

J. Neurosci.

The human GARS-AIRS-GART gene encodes two proteins which are differentially expressed during human brain development and temporally overexpressed in cerebellum of individuals with Down syndrome

Hum. Mol. Genet.

Characterization of transgenic mice with an increased content of chromosomal protein HMG-14 in their chromatin

DNA Cell Biol.

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

Genome Res.

Global Report on birth defects: The hidden toll of dying and disabled children

The cerebellar transcriptome during postnatal development of the Ts1Cje mouse, a segmental trisomy model for Down syndrome

Hum. Mol. Genet.

Segmental trisomy as a mouse model for Down syndrome

Prog. Clin. Biol. Res.

Down's syndrome: critical genes in a critical region

Nature

The DSCR1 (Adapt78) isoform 1 protein calcipressin 1 inhibits calcineurin and protects against acute calcium-mediated stress damage, including transient oxidative stress

Faseb J.

DSCR1, overexpressed in Down syndrome, is an inhibitor of calcineurin-mediated signaling pathways

Hum. Mol. Genet.