EL PROCESO DE ENSEÑANZA DE LOS PIP

Autistic pathology results most likely from the interplay between genetic predisposi- tion and the action of environmental triggers. Several environmental toxins, such as heavy metals (Bokara et al., 2008) including mercury (Palmer et al., 2009; Windham et al., 2006) and pesticides (D’Amelio et al., 2005), polychlorinated biphenyls (PCBs) (Kimura-Kuroda et al., 2007), as well as maternal infection (Brown et al., 2008) have been implicated in autism; these in turn share the ability to increase oxidative stress.

Oxidative stress occurs in the course of normal physiological processes. The dam- aging action of reactive oxygen species (ROS) targets key cellular macromolecules, including lipids, carbohydrates, proteins, DNA, and RNA. This action is kept in check by the body’s natural antioxidant defense system that includes metal-binding metallo- proteins and other antioxidants. In response to exposure to environmental toxins and in a number of disease conditions, this defense system appears to be overburdened, lead- ing to unchecked oxidative stress damage. Specifi cally, oxidative damage to proteins is mediated by peroxynitrate formed by the reaction of superoxide with nitric oxide (NO), reacting with tyrosine residues (Beckman and Koppenol, 1996), and resulting in the formation of 3-nitrotyrosine (3-NT, a specifi c marker of oxidative damage to proteins). Modifi cation of tyrosine residues in proteins can alter their conformation and function, which in turn has a profound effect on the developing brain.

There is growing evidence supporting the role of oxidative stress and specifi cally modifi cation of proteins, as evidenced by increased 3-nitrotyrosine in over 50 pathol- ogies including Alzheimer disease (AD), Parkinson disease (PD), and cancer (Beal, 2002; Neumann et al., 2008). Increased oxidative stress is also being implicated in autistic pathology. We have recently reported an increase in 3-nitrotyrosine in autis- tic cerebella (Sajdel-Sulkowska et al., 2008). Furthermore, it appears that tyrosine residues likely to be modifi ed under increased oxidative stress are specifi cally related to the particular pathology and are genetically determined (Neumann et al., 2008). A proteonomic approach has identifi ed specifi c target proteins of nitration in the Alzheimer’s hippocampus (Sultana et al., 2006); others have shown that more than half of the modifi ed proteins in AD and PD were related to the respective pathologies

(Sacksteder et al., 2006). It is possible that the genetic predisposition implicated in autism involves genes involved in oxidative defense systems that render the individu- als more sensitive to oxidative stress triggers. Such a possibility is very intriguing since early screening could identify individuals “allergic” to environmental toxins. Once identifi ed, the families may be able to implement dietary and other measures to reduce the risk of autism and/or control the extent of symptoms.

Indeed, there is evidence for the disruption of antioxidant defense mechanisms in autism manifested by lower than control levels of glutathione peroxidase (GSPHx) (Yorbik et al., 2002), by lower levels of plasma glutathione levels and higher ratios of oxidized glutathione to reduced glutathione (James et al., 2004, 2006) and by lower levels of two major serum antioxidant metalloproteins ceruloplasmin (copper- binding protein) and transferrin (iron-binding protein (Chauhan and Chauhan, 2006; Chauhan et al., 2004).

Concomitant with a decrease in the oxidative defense in autism, there is an increased oxidative stress in autism indicated by observations of increased lipid oxidation markers in blood (Chauhan et al., 2004; Zoroglu et al., 2004) and in urine (Ming et al., 2005; Yao et al., 2006), increased levels of NO in red blood cells (Sogut et al., 2003), and plasma levels of NO metabolites (Sweeten et al., 2004).

We have previously reported a 65% increase in cerebellar 3-nitrotyrosine (Figure 3.1; Sajdel-Sulkowska et al., 2008). More recently, we have observed an increase in the DNA oxidation marker, 8-OH-dG, in the autistic cerebellum (Sajdel-Sulkowska et al., 2009). Our study showed an overall 63.4% increase in the level of 8-OH-dG in the autistic cerebellum with four out of fi ve autistic samples higher than mean control level. However, one of the autistic cases fell within the control values, sug- gesting that there may be a subset of autistic cases with little or no DNA damage. A similar conclusion was recently arrived at with respect to decrease in Purkinje cell number in a subset of autistic cases (Whitney et al., 2008); it would be of inter- est to compare directly Purkinje cell number and the levels of 8-OH-dG directly in the same cerebellar samples. A trend toward increased urinary 8-OH-dG levels in

FIGURE 3.1 Increased 3-nitrotyrosine levels in autistic cerebella. 3-NT, 3-nitrotyrosine. (From Sajdel-Sulkowska, E. M. et al., Am. J. Biochem. Biotechnol. (Special Issue on Autism Spectrum Disorders), 4, 73, 2008. With permission.)

0 10 20 30 40 50 60 70 Control Autism 3 -NT (p m o l/ g)

children has been reported in autism (Ming et al., 2005). Increased 8-OH-dG levels have been also observed in lymphocytes (Mecocci et al., 2002), the cerebrospinal fl uid (CSF)-DNA (Lovell et al., 1999), and in the urine of patients with AD (Lee et al., 2007) and PD (Sato et al., 2005). The children with brain damage showed increased urinary 8-OH-dG levels (Fukuda et al., 2008). An immunohistochemi- cal approach showed 10 times higher brain levels of 8-OH-dG in schizophrenia (Nishioka and Arnold, 2004).

The oxidative stress in autism could result from (1) the exposure to high levels of environmental pro-oxidants such as pesticides (Abdollahi et al., 2004; D’Amelio et al., 2005) and mercury (Hg) (Mutter et al., 2005a,b; Palmer et al., 2008; Windham et al., 2006); (2) inability to metabolize and clear the toxicant, such as heavy metals, from the system (Bradstreet et al., 2003; McGinnis 2004; Serajee et al., 2004); (3) the decreased internal antioxidant defense mechanisms (James et al., 2004; Yorbik et al., 2002, 2006; Zoroglu et al., 2004); or (4) increased sensitivity to oxidative stress (Buyske et al., 2006; Yang et al., 2008). It is possible that all four mechanisms may be involved in precipitating autistic pathology.

3.2 NEUROTROPHINS AND BRAIN DEVELOPMENT: COULD

THE IMBALANCE IN BRAIN NEUROTROPHIN EXPRESSION