The role of alterations in free radical metabolism in mediating cognitive impairments in Down's syndrome

The Oxygen Paradox, the French Paradox, and age-related diseases

A paradox is a seemingly absurd or impossible concept, proposition, or theory that is often difficult to understand or explain, sometimes apparently self-contradictory, and yet ultimately correct or true. How is it possible, for example, that oxygen "a toxic environmental poison" could be also indispensable for life (Beckman and Ames Physiol Rev 78(2):547-81, 1998; Stadtman and Berlett Chem Res Toxicol 10(5):485-94, 1997)?: the so-called Oxygen Paradox (Davies and Ursini 1995; Davies Biochem Soc Symp 61:1-31, 1995). How can French people apparently disregard the rule that high dietary intakes of cholesterol and saturated fats (e.g., cheese and paté) will result in an early death from cardiovascular diseases (Renaud and de Lorgeril Lancet 339(8808):1523-6, 1992; Catalgol et al. Front Pharmacol 3:141, 2012; Eisenberg et al. Nat Med 22(12):1428-1438, 2016)?: the so-called, French Paradox. Doubtless, the truth is not a duality and epistemological bias probably generates apparently self-contradictory conclusions. Perhaps nowhere in biology are there so many apparently contradictory views, and even experimental results, affecting human physiology and pathology as in the fields of free radicals and oxidative stress, antioxidants, foods and drinks, and dietary recommendations; this is particularly true when issues such as disease-susceptibility or avoidance, "healthspan," "lifespan," and ageing are involved. Consider, for example, the apparently paradoxical observation that treatment with low doses of a substance that is toxic at high concentrations may actually induce transient adaptations that protect against a subsequent exposure to the same (or similar) toxin. This particular paradox is now mechanistically explained as "Adaptive Homeostasis" (Davies Mol Asp Med 49:1-7, 2016; Pomatto et al. 2017a; Lomeli et al. Clin Sci (Lond) 131(21):2573-2599, 2017; Pomatto and Davies 2017); the non-damaging process by which an apparent toxicant can activate biological signal transduction pathways to increase expression of protective genes, by mechanisms that are completely different from those by which the same agent induces toxicity at high concentrations. In this review, we explore the influences and effects of paradoxes such as the Oxygen Paradox and the French Paradox on the etiology, progression, and outcomes of many of the major human age-related diseases, as well as the basic biological phenomenon of ageing itself.

Epigenetic dysregulation in the developing Down syndrome cortex

Using Illumina 450K arrays, 1.85% of all analyzed CpG sites were significantly hypermethylated and 0.31% hypomethylated in fetal Down syndrome (DS) cortex throughout the genome. The methylation changes on chromosome 21 appeared to be balanced between hypo- and hyper-methylation, whereas, consistent with prior reports, all other chromosomes showed 3-11 times more hyper- than hypo-methylated sites. Reduced NRSF/REST expression due to upregulation of DYRK1A (on chromosome 21q22.13) and methylation of REST binding sites during early developmental stages may contribute to this genome-wide excess of hypermethylated sites. Upregulation of DNMT3L (on chromosome 21q22.4) could lead to de novo methylation in neuroprogenitors, which then persists in the fetal DS brain where DNMT3A and DNMT3B become downregulated. The vast majority of differentially methylated promoters and genes was hypermethylated in DS and located outside chromosome 21, including the protocadherin gamma (PCDHG) cluster on chromosome 5q31, which is crucial for neural circuit formation in the developing brain. Bisulfite pyrosequencing and targeted RNA sequencing showed that several genes of PCDHG subfamilies A and B are hypermethylated and transcriptionally downregulated in fetal DS cortex. Decreased PCDHG expression is expected to reduce dendrite arborization and growth in cortical neurons. Since constitutive hypermethylation of PCDHG and other genes affects multiple tissues, including blood, it may provide useful biomarkers for DS brain development and pharmacologic targets for therapeutic interventions.

Down's syndrome and myocardial reperfusion injury

Down syndrome is known to be an independent risk factor for mortality after surgical repair of congenital heart anomalies. It is also associated with neurodegenerative disease and accelerated aging. The mechanism of the latter features has been attributed to abnormal handling of oxygen-free radicals as well as mitochondrial dysfunction. These properties also place the child with Down syndrome at a risk of an exaggerated myocardial ischemia/reperfusion injury. A 6 month old child with Down syndrome is reported who suffered from obvious clinical ischemia/reperfusion injury following an uncomplicated repair of complete AV canal. Both intraoperative as well as postoperative echocardiography documented a satisfactory technical repair. After resting the heart on ECMO the child's myocardial function returned to normal. The mechanisms by which patients with Down syndrome are at risk of ischemia/reperfusion injury are reviewed. Future studies should focus on specific approaches for myocardial protection in the child with Down syndrome undergoing cardiac surgery.

Coronary artery disease, hypertension, ApoE, and cholesterol: a link to Alzheimer's disease?

The premature presence of senile plaques (SP) in coronary artery disease (CAD), and neurofibrillary tangles (NFT) as well as SP in hypertension (HyperT), suggest a neuropathologic link between CAD, HyperT, and AD. Previous MI, CAD and HyperT often occur in and may increase the risk of AD. Expression of Apo-E4 likely increases risk of CAD by elevating blood cholesterol and the risk of AD via proposed interactions with beta-amyloid and/or free radicals (FRs). Any Apo-E4 effect is vague, but FRs probably mediate vascular damage in HyperT. Increasing FR content in the blood is related to increasing CAD severity, while the severity of elevated FR level correlates with how deep into a blood vessel there is activation of the FR scavenger enzyme, superoxide dismutase (SOD). The ApoE genotype and SP/NFT areal densities were determined in a large population of non-demented CAD, HyperT and non-heart disease (non-HD) control subjects, and compared to findings in a similar number of AD patients. ApoE immunoreactivity was determined in many individuals. Cholesterol content in cortex was determined by HPLC in a small, loosely age-matched group of Apo-E4 genotype-matched AD, CAD and non-HD subjects. SOD immunoreactivity was also assessed in a number of subjects. The Apo-E4 genotype frequency was increased in CAD, HyperT and AD compared to non-HD controls. Dose of Apo-E4 correlated with SP densities, but not NFT, and only in the non-demented groups. Essentially all SP in CAD, HyperT and non-HD subjects were ApoE-immunoreactive. Cortical cholesterol was increased in CAD and AD compared to controls. SOD immunoreactivity was similar in HyperT and AD; SP were immunodecorated in both. AD, CAD and HyperT may be linked, while CAD and HyperT subjects may die of heart disease before showing cognitive change.