Another day, another PR barrage centered on a scientific paper and a real danger that people will be tempted to treat a serious human disease with untried, ineffective, and potentially harmful “cures.” The paper, published in the prestigious magazine Cell, reports that some autism-like symptoms in mice, generated by treating pregnant animals with a compound that mimics viral infection, can be ameliorated by treating their offspring with the bacteria Bacterioides fragilis (1). Rob Knight, a major player in the “American Gut” project, claims that, “The broader potential of this research is obviously an analogous probiotic that could treat subsets of individuals with autism spectrum disorder.”(2) This is a truly remarkable claim given the multiple inherent and substantial limitations of the original study.
So what are these limitations and why do they lead to significant doubts about whether their “promise” is either misguided or likely to be fulfilled? The first and most obvious issue is whether mice, no matter how experimentally manipulated, can actually be autistic or, better put, whether the symptoms such manipulated mice display are related in any useful way to developing an understanding of autism or the more general “autism spectrum disorder” (ASD) in humans, yet alone alleviate the symptoms displayed by people diagnosed with ASD.
Let us begin with what autism is currently defined to be. According to the National Institute of Neurological Disorders and Stroke, “The hallmark feature of ASD is impaired social interaction.”(3) Because people are so deeply and inherently social, ASD is a serious condition. That said, it remains unclear whether ASD is one or a number of distinct diseases that produce similar symptoms.
Mice, and in fact the vast majority of animals, are very, very much less social than humans, and in so far as they are social, they are social in dramatically different ways. Moreover, mice often differ quite dramatically from humans in their responses to various physiological insults and how diseases arise and progress. Unless carefully taken into account, these differences can make observations in mice more or less irrelevant to humans. This has been revealed most recently in the course of comparative studies on sepsis, the life threatening consequence of a wide-spread bacterial infection, where “… researchers report evidence that the mouse has been totally misleading as a model system to investigate at least three major killers – sepsis, burns, and trauma. As a result, years and billions of dollars have been wasted following false leads, they say.” (Kolata, 2013).
Let us consider the mouse as a model for autism/ASD used in the Cell study. The investigators used an inbred line called C57BL/6N. To induce autism/ASD-like symptoms, pregnant mice were injected on embryonic day 12.5 (birth occurs between 19-21 days) with the compound poly(I:C); this treatment mimics an acute viral infection. Administration of this compound hyper-stimulates the mother’s immune system, leading to a condition known as “maternal immune activation” (MIA) associated with elevated levels of inflammatory factors in the maternal blood, placenta, and amniotic fluid. These changes have dramatic effects on fetal development and the affected offspring often display behaviors seemingly analogous to some of those displayed by people with ASD. Of course, since mice never display the types of social behaviors that normal people do, whether the symptoms mice from MIA mothers display are relevant to individuals diagnosed with ASD is speculative at best.
What the authors of the Cell paper focused on were changes in the behavior of the gut, that is primarily changes in cellular behavior affecting the gut’s permeability in MIA-derived mice and the possibility of repairing these behaviors through treatment of afflicted mice with bacteria. There have been some reports that humans with autism/ASD have gastrointestinal abnormalities, but the data is unclear. A small study, published in the British Medical Journal, found no relationship between an ASD diagnosis and gastrointestinal symptoms (4) while a larger study by the Autism Microbiome Consortium reported “significant enrichment of bowel symptoms and disorders in patients with ASD (11.74% vs. 4.5%, p<0.0001 by chi-square test)”, perhaps not a completely unexpected result from a project centered on the relationship between autism and gut bacteria, but these authors note that their conclusions “may well be affected by the limitations of our study,” which relate to the specific population of patients examined. Here it is worth noting that only a minority (~12%) of individuals with ASD appeared to display gastrointestinal symptoms.
The authors of the Cell paper pursued the hypothesis that autistic/ASD-like symptoms displayed by mice might well arise directly from the defects in their gastrointestinal tract associated with MIA treatment. They found changes in the relative abundance of intestinal microbes between normal and MIA mice. We know that the interaction between intestinal microbes and gut is critical for the normal “development, maintenance, and repair of the intestinal epithelium.” Following on they tested whether feeding mice B. fragilis, a microbe also found in humans and previously shown to ameliorate experimental colitis, could influence the effects of MIA. Their observation was a striking yes. This treatment also corrected some, but not all of the autism/ASD-linked behavioral effects found in MIA offspring mice. Most interestingly, however, from the perspective of the defining social aspects of autism and ASD, “they (the B. fragilis treated MIA mice) retain deficits in sociability and social preference” (emphasis added). Now given that autism/ASD is primarily a disease of social interactions, together with the rather dramatic differences between human and mouse immune systems brought to our attention by the sepsis studies, these observations suggest that this study’s results may well not be relevant to ASD. Moreover, from a practical perspective “B. fragilis, which accounts for only 0.5% of the human colonic flora, is the most commonly isolated anaerobic pathogen due, in part, to its potent virulence factors. Species of the genus Bacteroides have the most antibiotic resistance mechanisms and the highest resistance rates of all anaerobic pathogens. Clinically, Bacteroides species have exhibited increasing resistance to many antibiotics.” (6) This suggests that the use of B. fragilis as a “probiotic” treatment in humans might be actively dangerous.
Based on the lack of compelling evidence i) that mice really can, in any meaningful sense, be made to be autistic, ii) that there are substantial differences between mice and humans, particularly with respect to their immune systems, which is critical to these studies, and iii) that treatment of mice with B. fragilis fails to reverse the social symptoms of MIA mice, one might well have expected that objective, disinterested scientists would refrain from excessive hyperbole until more relevant and DEFINITIVE data is available. This would not be of great concern if confined to the scientific community but the danger is that such sanguine interpretations in the public press will lead parents or caretakers of people with ASD to “medicate” affected children with unproven, ineffective, and potentially hazardous “probiotic” treatments. This concern is not a theoretical one when one considers how ubiquitous self-medication is in our society. Any trip to the grocery store or Costco will reveal the widespread availability of dietary supplements including probiotics and other “alternative” medicines that claimed to prevent or treat a wide array of disorders, real or imagined, including gastrointestinal ailments. In the absence of a sober evaluation of the soundness of the observations made in mice and, more importantly, their relevance to humans, the essential question is whether it is socially irresponsible not to include clear and appropriate caveats directed to a scientifically-naïve population who may rightly assume the experiments described represent established fact and therefore, give license to self-directed probiotic treatment of affected individuals, or included in diets of unaffected children as a prophylactic measure.
1. “Microbiota Modulate Behavioral and Physiological Abnormalities Associated with Neurodevelopmental Disorders,” http://www.ncbi.nlm.nih.gov/pubmed/24315484.
5. Kohane et al., 2012. PLoS One DOI: 10.1371/journal.pone.0033224
6. Wexler, 2007: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2176045/
Engber, D. 2011. The Mouse Trap. Slate: http://www.slate.com/articles/health_and_science/the_mouse_trap/2011/11/lab_mice_are_they_limiting_our_understanding_of_human_disease_.html
Kolata, G. 2013. Mice Fall Short as Test Subjects for Some of Humans’ Deadly Ills. New York Times. http://www.nytimes.com/2013/02/12/science/testing-of-some-deadly-diseases-on-mice-mislead-report-says.html?_r=1&