Down's syndrome development: the case for oxidative stress

Down's syndrome risk factors: the lack of a knowledge base

Down's syndrome is the most common cause of mental retardation in humans and is caused by one of three abnormalities in chromosome 21 that develops in babies in the womb. It is a lifelong condition that inhibits the normal physical development of the child and inflicts learning difficulties to varying degrees.

In the UK, about one in every 1,000 live births is affected by Down's syndrome. There is no cure but, just as worryingly, there is limited understanding of the risk factors associated with the diseases. The maternal age is the only clear risk factor identified to date although environmental factors have also been suspected.

The use of oral contraception, smoking during pregnancy, and exposure to pesticides, radiation and infectious agents during pregnancy have all been implicated without much hard evidence to support them.

At the molecular level, oxidative stress is thought to be involved in the pathogenesis of Down's syndrome but a cause-and effect relationship has yet to be established. The increased activity of copper/zinc superoxide dismutase has been proposed as the cause of many neurological symptoms of Down's syndrome but this is still in dispute.

Of the few published studies of oxidative stress in the early stages of the disease, most have been carried out on foetal brain tissue. However, amniotic fluid would give a clearer indication of the true state of the foetus, with the protein composition reflecting any physiological and pathological changes taking place.

This prospect prompted a team of researchers to look for various signs of oxidative stress in amniotic fluid of pregnant women with a positive diagnosis for Down's syndrome in the foetus.

Oxidative stress increased in amniotic fluid

Allan Butterfield from the University of Kentucky and colleagues from the Roman organisations La Sapienza University, the CNR Institute of Molecular Biology and Pathology, and the Artemisia Fetal Maternal Medical Centre compared the oxidative state of amniotic fluid from ten women carrying Down's syndrome foetuses with that for women with normal births.

The first signs of increased oxidative stress were raised levels of protein carbonyl groups and lipid peroxidation products. In addition, the researchers observed a decrease in the levels of reduced thioredoxin and total glutathione and a subtle increase in levels of oxidised glutathione.

A further indication of elevated oxidative stress was supplied by three specific heat shock proteins, all of which were more than 2-fold more abundant in Down's syndrome amniotic fluid. This correlates with published data on heat shock protein levels in the brain tissue of patients and foetuses with Down's syndrome. Butterfield stated that heat shock protein activation is most probably indicative of a cytoprotective response towards oxidative stress.

The team also searched for oxidised proteins in the amniotic fluid using proteomics techniques. Following 2D gel electrophoresis and image analysis, proteins with altered abundances in Down's syndrome amniotic fluid compared with normal fluid were chosen for further analysis. They were digested with trypsin for tandem mass spectrometric analysis and identified by database searching.

The comparison revealed ten proteins in the Down's syndrome samples that were more oxidised than those in the control samples, displaying increased levels of carbonylation. They were associated with iron homeostasis, lipid metabolism and inflammatory response.

Damaged ceruloplasmin, a multicopper enzyme, and transferrin, which regulates iron haemostasis, could affect iron storage and induce free radical damage to other proteins during oxidative stress. This would contribute to the development of damaging Down's syndrome phenotypes.

Increased carbonyl levels in zinc alpha2-glycoprotein and retinol-binding protein 4 are consistent with altered fat metabolism in Down's syndrome patients, which can lead to obesity. The observed increase in oxidised apolipoprotein A1 agreed with reports that apolipoprotein A dysfunction could be linked to an increased risk of cognitive impairment.

Complement C9, alpha-1B-glycoprotein and collagen alpha-1(V) chain are all involved in inflammation and the acute-phase response. Increased oxidation of these three suggests that their protection of the foetus is compromised, which is consistent with the greater susceptibility of Down's syndrome subjects to autoimmune diseases and acquired immunodeficiency.

The researchers suggested that the changes induced by oxidation will affect various cellular pathways, which, in turn, will produce different clinical outcomes.

The increased occurrence of early ageing, atherosclerosis, obesity, dementia and neurodegenerative problems might be a direct response to oxidative stress during foetal development.

So, oxidative damage has been confirmed as an early event in Down's syndrome development and these oxidised proteins might be viable biomarkers of the disease. They could also help to design new nutritional or pharmacological treatments based on antioxidant compounds to prevent or slow down the onset of these side effects.

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.