Expression of the transcription factor ETS2 in brain of patients with Down syndrome--evidence against the overexpression-gene dosage hypothesis

Generation of a panel of antibodies against proteins encoded on human chromosome 21

BACKGROUND

Down syndrome (DS) is caused by trisomy of all or part of chromosome 21. To further understanding of DS we are working with a mouse model, the Tc1 mouse, which carries most of human chromosome 21 in addition to the normal mouse chromosome complement. This mouse is a model for human DS and recapitulates many of the features of the human syndrome such as specific heart defects, and cerebellar neuronal loss. The Tc1 mouse is mosaic for the human chromosome such that not all cells in the model carry it. Thus to help our investigations we aimed to develop a method to identify cells that carry human chromosome 21 in the Tc1 mouse. To this end, we have generated a panel of antibodies raised against proteins encoded by genes on human chromosome 21 that are known to be expressed in the adult brain of Tc1 mice

RESULTS

We attempted to generate human specific antibodies against proteins encoded by human chromosome 21. We selected proteins that are expressed in the adult brain of Tc1 mice and contain regions of moderate/low homology with the mouse ortholog. We produced antibodies to seven human chromosome 21 encoded proteins. Of these, we successfully generated three antibodies that preferentially recognise human compared with mouse SOD1 and RRP1 proteins on western blots. However, these antibodies did not specifically label cells which carry a freely segregating copy of Hsa21 in the brains of our Tc1 mouse model of DS.

CONCLUSIONS

Although we have successfully isolated new antibodies to SOD1 and RRP1 for use on western blots, in our hands these antibodies have not been successfully used for immunohistochemistry studies. These antibodies are freely available to other researchers. Our data high-light the technical difficulty of producing species-specific antibodies for both western blotting and immunohistochemistry.

Protein levels of genes encoded on chromosome 21 in fetal Down syndrome brain: Challenging the gene dosage effect hypothesis (Part IV)

Down syndrome (DS) is the most frequent genetic disorder with mental retardation and caused by trisomy 21. Although the molecular mechanisms of the various phenotypes of DS could be due to overexpression of gene(s) on chromosome 21, several groups have challenged this gene dosage effect hypothesis. The near completion of the sequencing of human chromosome 21 provides unprecedented opportunities to understand the molecular pathology of DS, however, functional information on gene products is limited so far. We therefore evaluated the levels of six proteins whose genes are encoded on chromosome 21 (trefoil factor 1, trefoil factor 2, trefoil factor 3, coxsackie virus and adenovirus receptor, carbonyl reductase 1 and interferon- alpha receptor) in fetal cerebral cortex from DS and controls at the early second trimester using Western blot analysis. None of the investigated proteins showed overexpression in DS compared to controls suggesting that these proteins are not involved in abnormal development of fetal DS brain and that DS phenotype can not be simply explained by the gene dosage effect hypothesis. We are systematically quantifying all proteins whose genes are encoded on chromosome 21 and these studies may provide a better understanding of genotype-phenotype correlation in DS.

Reduced levels of DEAD-box proteins DBP-RB and p72 in fetal Down syndrome brains

Down syndrome (DS) is characterized by abnormal brain morphology and neurological and behavioral functions. The pivotal role of helicases in brain development, growth, and differentiation made us evaluate three DEAD BOX proteins, DEAD-box protein 1 (DBP-RB), DEAD-box protein 3 (HLP2), DEAD-box protein 72 (P72), and the RuvB-like DNA helicase (TIP49b), in fetal brain of controls and DS subjects, using two-dimensional electrophoresis with subsequent mass spectroscopic (MALDI-MS) identification. HLP2 and TIP49b brain levels were comparable between DS and controls, and protein levels of p72 and DBP-RB were significantly reduced in DS fetal cortex (p72: 2.04+/-1.90 vs. 5.57+/-2.56 in controls, p < 0.01; DBP-RB: 0.58+/-0.94 vs. 1.90+/-0.97 in controls, p < 0.01). Impairment of the helicases p72 and DBP-RB may reflect or lead to deficient growth and differentiation of brain development early in life and can be considered pathogenetic factors along with the reported deficits of transcription, splicing, and elongation factors already described in fetal DS brains.

Deterioration of the transcriptional, splicing and elongation machinery in brain of fetal Down syndrome

Perturbation of brain development i.e. regulation of gene expression, differentiation, growth and migration in Down Syndrome (DS) has been reported to occur early in life pointing to impairment of the complex system of transcription and or translation and indeed, altered expression of transcription factors has been reported in adult DS brain. We therefore decided to compare the transcriptional and translational machinery in cortex of brains of controls and fetuses with Down syndrome in the second trimenon of gestation. We determined a series of transcription/translation factors by 2 D-electrophoresis followed by MALDI--identification and quantification with specific software. The protooncogene C-CRK, CRK-like protein, elongation factor 1-alpha 1, elongation factor 2, elongation factor tu and two out of four spots representing PTB-associated splicing factor PSF were significantly downregulated in brain of fetal DS fetuses as compared to controls. The finding of reduced transcription and translation factors may indicate deranged protein synthesis. The underlying cause for individual reduced transcription, splicing and translation factors may be explained by chromosomal imbalance or by posttranslational modifications as e.g. phosphorylation, known to be aberrant in DS. Reduced expression of transcription factors in fetal DS during early life may be responsible or reflecting impaired brain development and deficient wiring of the brain in DS.