Introduction

Of all the cytogenetic abnormalities, Down syndrome (DS) is the one that most frequently comes to term with an incidence of 1 in 700-1000 live births within the general population. Individuals with DS suffer from a wide range of defects that affects almost every major organ system in the body, although the penetrance and severity varies between DS individuals (1). DS is responsible for the greatest genesis of individuals with mental retardation, and in all DS cases investigated this is accompanied by early-onset Alzheimer-type brain pathology (2,3). In addition, the DS brain is associated with decreased neuronal number and abnormal neuronal differentiation (4). Other abnormalities include congenital heart defects (5); in utero growth retardation resulting in approximately 10% reduction in the birth weight; increased susceptibility to infections (6); a 20-50-fold higher incidence of leukemia (7); eye lens defects (1) and premature aging (8).

Approximately 95% of DS individuals present with an extra copy of human chromosome 21 (HSA21) in its entirety (trisomy 21), while the remainder are either mosaic (a mixture of normal and trisomy 21 cells) or have karyotypes with partial trisomy of HSA21 (1). Research has therefore focused on understanding how extra chromosomal material contributes to the syndrome. The "gene dosage effect" hypothesis, which proposes that increased expression of HSA21 genes directly contributes to the syndrome, has largely been accepted as the modus operandi for the abnormalities associated with the DS phenotype (9,10). An opposing view, the "developmental instability" or "quantitative" hypothesis proposes that it is the amount of additional chromosomal material that causes the DS phenotype (11,12). Most available data (the analyses of HSA21 gene products in DS tissues (13-17) and the genotype-phenotype relationship of transgenic mice over-expressing HSA21-specific genes (18,19)) support the "gene dosage effect" hypothesis. However, not all genes are up-regulated in strict accordance with gene dosage in DS tissues (20) and some authors have interpreted this as an evidence against the "gene dosage effect" hypothesis (21-23).

The research in our laboratory has focused on gaining a better understanding of the functional role played by a number of genes located on HSA21 (19,24-29). In particular, we have focused on the HSA21-specific antioxidant Cu/Zn-superoxide dismutase (SOD1) for two reasons. First, due to its location on HSA21, this antioxidant can be used as a marker to investigate the "gene dosage effect hypothesis" and second, as part of the

Oxidative Stress and Neurodegenerative Disorders Edited by G. Ali Qureshi and S. Hassan Parvez

© 2007 Elsevier B.V. All rights reserved.

antioxidant pathway, this gene product has important implications for the regulation of redox flux within the cell. This chapter will highlight some of our most relevant findings with respect to cellular antioxidant balance and the consequences of an altered antioxidant ratio. This chapter will also show that an altered antioxidant ratio exists in all DS fetal organs investigated thus far, and will link this perturbation with known phenotypic changes that occur as part of the DS phenotype.

ANALYSIS OF THE HSA21 GENE, CU/ZN-SUPEROXIDE DISMUTASE-1 (SOD1) AND ITS RELATIONSHIP TO THE ANTIOXIDANT PATHWAY

In initial studies, we investigated the expression of SOD1, which is located at 21q22.1 (30), in various tissues of control and DS-aborted conceptuses (31,32). Previous studies had focused on a number of cell types primarily isolated from adult DS blood (33-35) and adult brain (36). Given the congenital nature of DS, an understanding of aberrant antioxidant expression during fetal development in a range of organs would be of extreme significance. Prior to our study, SOD1 expression had only been investigated in fetal brain with conflicting results (13,37), and furthermore, a SOD1 expression profile in a broader range of DS fetal tissue was not available. In addition, it seemed incorrect to assume that SOD1 expression is elevated 1.5-fold in all DS fetal organs since not all genes investigated in DS tissues were found to be expressed at the predicted gene dosage increase of 1.5-fold (20).

For our study, DS conceptuses were obtained from patients who were screened because of advanced maternal age (i.e. 37 years and more) and control conceptuses were obtained from patients who had other chromosomal (i.e. non-DS) and/or morphological abnormalities. Ethical approval for the study was obtained from the Monash Medical Centre Ethics Committee, Melbourne, Australia and from patients participating in the study (31,32).

Our study demonstrated that expression of SOD1 was elevated in four of the five fetal organs investigated (namely, brain, heart, thymus and lung) in agreement with a 1.5-fold gene dosage increase for three of the four organs (namely brain, heart and lung) (31,32). Our results were largely in agreement with previous studies that showed gene dosage-elevated SOD1 levels (13,33,35,38), despite one study to the contrary (37). Importantly, in our study, analysis of DS thymus and liver did not adhere to strict gene dosage (a 3-fold increase and 1.8-fold decrease in expression, respectively). Departure from strict gene dosage in DS organs is not unprecedented (20,39,40). For example, Stefani et al. (20) reported decreased expression of one of four HSA21-specific sequences in DS fetal liver, while three other sequences were expressed at the same level in both control and DS livers.

A lack of increase in the level of HSA21 gene expression in DS tissue has previously been interpreted as an evidence against the "gene dosage effect" hypothesis (21,23,41). The results of our study are in agreement with the notion that not all HSA21 gene products are elevated 1.5-fold. However, we feel this should not be interpreted as an evidence against the gene dosage hypothesis. In our opinion, it is the qualitative change of specific HSA21 gene products that contribute to the DS phenotype, such that elevated levels of some HSA21 gene products may increase or reduce the expression of other genes in a common pathway, whether or not they are HSA21-derived genes. It is therefore more important to investigate genomic and proteomic pathways involved in DS to obtain a clearer understanding of the DS phenotype.

In the light of our interpretation of the "gene dosage effect" hypothesis, and the knowledge that SOD1 is a key enzyme involved in the antioxidant pathway (see Fig. 1), we felt it significant to investigate the expression of glutathione peroxidase-1 (GPx1), an important downstream antioxidant of this pathway. An understanding of the interaction between SOD1 and GPx1 within the antioxidant pathway is essential in understanding the function of these two antioxidant enzymes in the removal of reactive oxygen species (ROS), and is briefly summarized here. SOD1 is a key enzyme in the conversion of superoxide radicals (O-*) to hydrogen peroxide (H2O2), which constitutes the first step of the antioxidant pathway and the cell's natural defense against oxidative stress (42). A build-up of H2O2 is prevented by two further antioxidant enzymes, namely GPx and catalase, in the second step in which H2O2 is neutralized to water (Fig. 1). Thus a delicate balance exists in cells and perturbations of this balance (as may be predicted from a gene-dosage increase in SOD1 with respect to second step antioxidant enzymes which are not HSA21 genes) give rise to noxious hydroxyl radicals (*OH) through Fenton-type reactions of H2O2 with transition metals (43). It is these highly reactive *OH species that damage DNA (43), protein (44) and lipid molecules (42), and initiate many rounds of peroxidative damage to

1st step SOD1

2nd step GPxl

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