In general, Aβ (1-40) is less neurotoxic, less common in the neuritic plaques of AD, and less likely to be involved in the neuropathology of AD than Aβ (1-42). However, Aβ (1-42) is more difficult to study than Aβ (1-40) because of polymerization. As is the case for any peptide, the concentrations of Aβ are a balance between its rate of synthesis and its rate of degradation [26
]. Moreover, it has been reported that the concentrations of Aβ in brain and blood are in equilibrium, through the blood-brain-barrier (BBB), and that peripheral sequestration of Aβ may shift this equilibrium toward the blood, eventually drawing out the excess from the brain ("sink" effect) [27
In the present study, the concentrations of both Aβ (1-40) and Aβ (1-42) were lower in autistic children than in age-matched control subjects (Table Figure 1). This finding could be attributed to loss of Aβ equilibrium between the brain and blood, which may lead to the failure to draw out Aβ from the brain, i.e., increased blood-to-brain influx and decreased brain-to-blood efflux across the BBB. The observed low plasma concentrations of Aβ (1-40) and (1-42) in the autistic Saudi children, together with the LPS hypothesis of Jaeger et al. [28
], could be easily supported by the findings of many studies showing that children with autism have an overload of gram-negative bacteria that contain LPS as a causative agent of mitochondrial dysfunction, a biochemical aspect recorded in a high percentage of autistic patients [16
Proteolytic cleavage of amyloid precursor protein (APP) by the sequential actions of β- and γ-secretases form the neurotoxic Aβ peptide, which typically consists of 40 or 42 amino acid residues (the amyloidogenic pathway). This could help to suggest the impairment of β- and γ- secretase's levels and/or kinetics in autistic patients showing lower plasma concentrations of Aβ (1-40) and Aβ (1-42). This suggestion could be supported by the work of Sokol et al. [33
] and Bailey et al. [34
], who reported higher plasma concentrations of secreted APPα in autistic patients than in aged-matched control subjects and their recommendation to measure sAPP-α concentrations in serum and human umbilical cord blood as a potential tool for the early diagnosis of autism.
The pathogenesis of many neurological disorders is also believed to be associated with oxidative stress, which may be responsible for the dysfunction or death of neurons. Aβ can serve as a metalloenzyme to catalyze the generation of neurotoxic H2
through binding and reduction of Cu (II) [35
]. Fang et al. (2010) reported that oligomer and the fibril form of Aβ (1-42) can promote the generation of H2
when the concentration of co-incubated Cu (II) is below a critical level [36
] and the amount of TBARS reactivity is greatest when generated by Aβ (1-42) ˃˃ Aβ (1-40) [37
At normal physiological conditions, SOD1 is known to increase cellular resistance to oxidative stress [38
]. However, when the SOD enzyme is overexpressed at levels that are much higher than those of other antioxidant enzymes, such as GPx and CAT, or higher than the ability of cells to supply reducing equivalents, increased oxidative stress is observed [39
]. Oxidative damage is likely because of the generation of ˑOH from the interaction of accumulating H2
with redox cycling proteins via Fenton-like chemistry [40
]. The lower Aβ (1-42) and Aβ (1-40) plasma concentrations reported in the present study, together with the proposed higher brain concentrations of both peptides, could be easily related to the findings of previous reports by Al-Gadani et al. (2009), which demonstrated that autistic Saudi children are under H2
stress because of overexpression of SOD and normal CAT activity [16
Recent evidence suggests that the low-density lipoprotein receptor-related protein 1 (LRP1) transcytoses Aβ out of the brain across the blood-brain barrier (BBB) [41
]. Deane et al. [42
] reported that in RAP knockout mice the expression of LRP-1 is reduced in the brain and that Aβ (1-40) elimination from the brain to blood is also reduced. These findings provide evidence for a direct protein-protein interaction between LRP and Aβ and demonstrate that this interaction takes place in an isoform-specific manner. This finding shows that Aβ isoforms are differentially transcytosed or endocytosed through the BBB and that LRP at the BBB favors the clearance of Aβ isoforms relative to high β sheet content.
Recently, Gu et al. [43
] reported that exposure to lead (Pb2+
) increases the concentrations of Aβ in the brain and inhibits LRP1 expression; this finding could explain the suggested Aβ accumulation in the brains of the autistic Saudi children in the present study. This explanation could find support in the work of El-Ansary et al. [31
], who found that Pb2+
concentrations were significantly higher in the red blood cells (RBC) of 12 of 14 autistic Saudi children than in those of control subjects; this finding indicates that autistic children are more vulnerable to Pb2+
toxicity and hence are more likely to accumulate Aβ (1-40) and (1-42) in their brains. This could be supported through considering the lower Aβ 40/42 ratios recorded in the present study in autistic patients compared to control subjects. It is well known that clearance and transport from brain to blood is facilitated by an increased Aβ 40/42 ratio present at young ages [44
]. Moreover, young mouse model harboring a mutation favoring generation of Aβ 1-42 over Aβ 1-40 had a low Aβ 40/42 ratio, was shifted to plaque deposition [45
Our speculated explanation could find a support in the most recent experimental study of Frackowiak et al. [46
] in which they used immunoblotting to prove that frozen autopsy brain samples of 9 autistic patients show accumulation of Aβ 40 and 42 in the cerebellum and cortex. Moreover, the explained association between chronic Pb toxicity previously recorded in 15/15 autistic patients of Saudi Arabia and the speculated Aβ accumulation of the present study is in good agreement with the finding of Garcidue˜nas [47
] which show that Children's exposure to urban air pollution increases their risk for auditory and vestibular impairment through the accumulation of Aβ 42 in their brainstems. To better understand changes in Aβ production, accumulation, and clearance in autistic patients, it will be necessary to continue studying the normal and disease-related metabolism of Aβ in various body fluids and in the brains of rodents used in animal models of autism.
Nutrition plays a vital role in the methylation of DNA, specifically the homocysteine (HCY)/S-adenosylmethionine (SAM) cycle. This cycle requires the presence of folate and B12, which facilitate the conversion of HCY to methionine, which is then converted to SAM. SAM then serves as a source of methyl groups for multiple methylation reactions, including the methylation of DNA. The increased concentrations of Aβ in the brains of autistic Saudi children could be easily explained by the hypothesis recently proposed by Lahiri and Maloney [49
]. They proposed that most AD cases follow an etiology based on Latent Early-life Associated Regulation or "LEARn" as a two-hit model [50
]. They reported that exposure to metals, nutritional imbalance (low B12), and other environmental stressors modify potential expression levels of AD-associated genes (e.g., Aβ peptide precursor protein) in a latent fashion. Autistic patients are known to exhibit oxidative stress [16
], high RBC lead concentrations [31
], and impaired DNA methylation because of a remarkably lower concentration of S-adenosylmethionine (SAM) [52
]. On the basis of this information, the two-hit hypothesis of Lahiri and Maloney [50
] could explain the impaired Aβ concentrations in the plasma of autistic Saudi children, as reported in the present study.
The Pearson correlations presented in Table and Figure 2 show that while there was only an acceptable level of correlation between Aβ (1-40) and Aβ (1-42) (correlation coefficient less than 0.5), a very good level of association was found between Aβ (1-40) and Aβ (40/42) ratio (correlation coefficient of 0.859). This could be helpful to suggest that lower values of Aβ (1-40) and Ab (40/42) ratio must be recorded together as biomarker in a patient diagnosed as autistic while an association between Aβ (1-40) and Aβ (1-42) is not a must.
Table and Figure 3 illustrate the results of ROC analyses of the two measured Aβ peptides. Although Aβ 40/42 ratio reported low value of sensitivity and specificity, absolute values of Aβ (1-42) and Aβ (1-40) reported satisfactory figures of sensitivity and specificity to be considered as potential biomarkers for autism.