Environment, dysbiosis, immunity and sex-specific susceptibility: A translational hypothesis for regressive autism pathogenesis

Figures & data

Figure 1. The gut microbial composition and development in healthy and autistic children. Asterisks refer to a higher (**) or low abundance (*) of microbial flora in an infant healthy gut. Arrows, instead, indicate the increase (↑) or decrease (↓) of gut microbial components according to C-section, breast or formula feed, ASD. ASD, autism spectrum disorder; C-section, cesarean section. ¹Data by Finegold et al.²⁷.

Figure 2. Differences between eubiosis (a healthy normal microflora) and dysbiosis establishment and consequences.

Figure 3. The critical window for immune vulnerability of children. At about 12 months the maternal protection is disappeared as well as antibodies from breastfeeding (especially in early weaning). On the other hand, the functionality and stability of gut microbiota as well as child's own antibodies formation are not yet reached.

Table 1. Review analysis of literature regarding the most relevant environmental factors or stressors, associated to ASD

Environmental factors/stressors	Relevant literature	Study type	Association with:		Main findings	Comments
			ASD	ASD + GI disorders
Antibiotics	Sandler et al.^Citation69	Clinical	Yes	Yes	Short-term improvement after low doses of vancomycin and probiotic therapy in a sub-group of regressive ASD children	Small sample size. The reported improvements waned at follow-up. Further investigations are required
	Atladóttir et al.^Citation59	Clinical	No	–	Population-based cohort study reporting no associations for ASD and mild infections, febrile episodes, or the use of antibiotics during pregnancy	Methodological limitations due to self-reported data/incomplete information
Diet and food xenobiotics	Batista et al.^Citation129	Clinical	No	No	No relevant associations among ASD, celiac disease, and glutein sensitivity	Poor diagnosis for ASD Because of the age of celiac disorder's onset is variable, it is possible that ASD patients would still develop this disease in the future
	Pennesi and Klein^Citation125	Clinical	Yes	Yes	Effectiveness of glutein/casein free diet in a sub-group of ASD children	Retrospective study Potential diet errors
	Reichelt et al.^Citation106	Clinical	Yes	–	Finding of exorphins in urines of ASD children	Further studies are required to strengthen the role of bio-active peptides in ASD
	Critchfield et al.^Citation71	Review	–	–	By comparing GI disorders in ASD and IBD, the supply of probiotics for ameliorating ASD symptoms is strongly encouraged	Need for pre-clinical/clinical trials
	Williams et al.^Citation42	Clinical	Yes	Yes	Insights between human gene expression and gut bacterial community composition.	Small sample size.
	Genuis et al.^Citation128	Clinical	–	Yes	Glutein-free diet effectiveness in a 5-years-old child with ASD and suspected celiac disease	Case report
	Cass et al.^Citation126	Clinical	No	–	Lack of evidence for opioid peptides in urine of male ASD children. No effectiveness for opioids in predicting or monitoring the effects of casein/glutein-free diet	No clinical evaluation for GI disorders in the samples. Further studies are required to validate the effectiveness of casein/glutein-free diet in ASD
Microbial metabolites/end-products	Frye and MacFabe^Citation36	Clinical	Yes	–	Short-chain fatty acids and acetyl-carnitine abnormalities in ASD patients versus controls	Further studies are required to strength the similarity between animal models and ASD and to evaluate the role of microbiota as potential PPA source
	Mavel et al.^Citation94	Clinical	Yes	–	Aberrant urine metabolic profiling in ASD children versus controls by ^Citation1H^Citation13C NMR	Needs for larger sample size Needs for testing the methodological specificity
	Kuwabara et al.^Citation90	Clinical	Yes	–	CE-TOFMS revealed aberrant metabolites, associated with oxidative stress and mitochondrial dysfunction, in the plasma of ASD adult males	Only adult males taking into account Needs for testing the methodological specificity and sensitivity
	Ming et al.^Citation93	Clinical	Yes	Yes	Altered gut microbial metabolites in urines of ASD children, with a stronger association in patients with GI disorders, suggesting a link between gut-dysbiosis and metabolic perturbances	No differences between ASD patients and controls have been found, by considering gender, diet, and vitamins supplementations. No evaluation for microbial gut-composition in ASD versus control children
	Kalużna-Czaplińska and Blaszczyk^Citation39	Clinical	–	Yes	High levels of arabinose found in autistic children and positive effects of probiotics in reducing them	No controls evaluated. A possible involvement of Candida is suggested Preliminary study
	MacFabe et al.^Citation35	Pre-clinical	Yes	–	Effects of propionic acid (PPA) in inducing autistic-like behaviors in male adolescent rats	These findings support further evidences on the effects of PPA in young rodents. Further works are required to validate this model in clinical studies
	Shaw^Citation92	Clinical	Yes	–	Higher concentrations of HPHPA in the urine of ASD children, in comparison to aged-matched controls, and in one adult affected by C. difficile infection	No clinical evaluation for GI disorders in the samples. No analysis on the ASD stool samples
	Yap et al., 2010^102	Clinical	Yes	–	Urinary metabolic alterations in ASD children versus siblings and controls identified by ^Citation1H NMR, and potentially associated with gut disorders	No evaluation for gut microbiota in ASD samples. Needs for larger sample of early onset patients to clarify the role of these metabolic differences in the autism etiology
	Shaw et al.^Citation38	Clinical	Yes	–	Positive effect of the antifungal therapy in reducing fungal metabolites in ASD patients	No data reported about GI disorders or the analysis of stool samples
Gut microbiota	Gondalia et al.^Citation24	Clinical	No	No	Pyrosequencing analysis on ASD patients with and without GI disorders and siblings. No evidences for the involvement of gut dysbiosis in ASD	No evaluation for eukaryotic gut microbiota, viruses, or protozoa as well as for dietary differences among participants. Microbial end-products/metabolites should also be studied in the future
	Adams et al.^Citation6	Clinical	–	Yes	Strong correlation with GI disorders and autism severity; lower levels of Bifidobacteria have been found in ASD patients versus healthy controls	No differences for yeasts was reported. Cultural and biochemical tests for microbiota profiling
	Finegold et al.^Citation22	Clinical	–	Yes	Positive correlation with the highest levels of Bacteroides in ASD children and autism severity. Significantly differences have been also found for Actinobacterium and Proteobacterium	No differences between ASD patients and healthy siblings. No evaluation for the potential diet effects on gut microbiota
	Parracho et al.^Citation25	Clinical	Yes	Yes	Association between the high levels of Clostridium spp. and GI disorders in ASD patients versus healthy, unrelated, controls	No differences found between ASD patients and healthy siblings
	Song et al., 2004^23	Clinical-methodol	Yes	–	Identification with Real-Time PCR of Clostridiales spp. in ASD stools	Small sample size. A comparison group of ASD patients with GI disorders was not included
	Finegold et al.^Citation40	Clinical	–	Yes	Higher levels of Clostridium spp. in ASD feces versus healthy controls	Analysis of both fecal flora and gastric and small-bowel specimens. Small sample size

ASD, autism spectrum disorder; GI, gastrointestinal; IBD, intestinal bowel disease; PPA, propionic acid; HPHPA, 3-(3-hydroxyphenyl)-3-hydroxypropionic acid; NMR, nuclear magnetic resonance spectroscopy; CE-TOF-MS, capillary electrophoresis time-of-flight mass spectrometry.

Figure 4. Suggested pathogenesis for autism. Genetic/immunological susceptibility and environmental risk factors could enhance gut dysbiosis, leading to an aberrant inflammatory response, to an abnormal production of microbial end-products, and to leaky gut. The latter can enhance mal-absorption of both microbial and exogenous xenobiotics derived from diet. Once absorbed in the bloodstream, all these compounds can affect the normal brain development and function both directly and impairing the immune system: the latter creates a loop, of aberrant gut–brain axis communication that contributes to enhance these aberrant physiological responses. Finally, endogenous or exogenous stressors might have an impact in the development of senses, language, and higher cognitive functions developing and integrating in the first period of life.

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