p-cresol and autism continued

In a post a while back (see here) I talked about the important, but sometimes confusing, research on a possible role for p-cresol (4-methylphenol) in relation to autism.

This potentially important aromatic compound has cropped up more than once in the autism research literature [1] although not necessarily in a universal direction. In my previous post, I called for 'further research' into the possibility of a relationship between p-cresol and autism and lo and behold, today I'm going to be discussing some further work...

The work in question is that published by Stefano Gabriele and colleagues [2] who sought to "investigate the mechanism producing elevated urinary p-cresol in ASD [autism spectrum disorder]." To do this and alongside measuring levels of urinary p-cresol, researchers "assessed for (a) presence of Clostridium spp. strains in the gut by means of an in vitro fecal stool test and of Clostridium difficile-derived toxin A/B in the feces, (b) intestinal permeability using the lactulose/mannitol (LA/MA) test, (c) frequent use of antibiotics due to recurrent infections during the first 2 years of postnatal life, and (d) stool habits with the Bristol Stool Form Scale" in 53 children with ASD. The Clostridium difficile bit to their study is based on the idea that this bacterium can convert the aromatic amino acid tyrosine into p-cresol under certain conditions [3].

Gabriele et al reported that: "Chronic constipation was the only variable significantly associated with total urinary p-cresol concentration" and that: "No association was found with presence of Clostridium spp. in the gut flora (P = 0.92), augmented intestinal permeability (P = 0.18), or frequent use of antibiotics in early infancy (P = 0.47)." Some readers might note that the correlation between urinary p-cresol levels and "augmented intestinal permeability" came closest to significance although with no statistical significance cigar.

I was rather interested in how chronic constipation and by virtue, slow intestinal transit, might figure in some relationship between p-cresol and autism. I've previously discussed the paper by Ridha and colleagues [4] (see here) and how: "Neuropsychiatric disorders, in particular autism, are useful predictors of STC and FFR [Slow transit constipation and functional fecal retention] in children." This building on a sizeable volume of work suggesting that functional bowel complaints (see here) and more pathological bowel disease (see here) are over-represented when it comes to autism.

The idea that the longer the intestinal transit time, the longer food and other material spend inside the body and hence available for longer for certain types of gut bacteria to 'go to work on them' is an interesting one. Although a fan of how our gut bacteria may be doing so much more than just helping digest food (see here) I can see the downside to this when it comes to the 'wrong' type of bacteria (or other components) perhaps having access to a food supply longer than they normally should or without the required competition for said food. The idea that the appearance of a compound like p-cresol might have some of its roots in how long intestinal transit time typically occurs offers not only a roadmap for further, more detailed study but also a potential way in which elevated levels of this "uremic toxin" might be lessened assuming a more typical intestinal transit time can be affected. Indeed, if one considers that certain probiotics might do more to aid gut transit time [5] than anything else, one gets a flavour for where such studies with autism in mind might end up heading including with other related uremic compounds and autism in mind (see here).

Music: Bonfire night here in the UK tonight. So what else but V (for Vendettta) - 'Good evening London'.

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[1] Persico AM. & Napolioni V. Urinary p-cresol in autism spectrum disorder. Neurotoxicol Teratol. 2013 Mar-Apr;36:82-90.

[2] Gabriele S. et al. Slow intestinal transit contributes to elevate urinary p-cresol level in Italian autistic children. Autism Res. 2015. October 6.

[3] Dawson LF. et al. The analysis of para-cresol production and tolerance in Clostridium difficile 027 and 012 strains. BMC Microbiol. 2011 Apr 28;11:86.

[4] Ridha Z. et al. Predictors of slow colonic transit in children. Pediatr Surg Int. 2015 Feb;31(2):137-42.

[5] Dimidi E. et al. The effect of probiotics on functional constipation in adults: a systematic review and meta-analysis of randomized controlled trials. Am J Clin Nutr. 2014 Oct;100(4):1075-84.

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Gabriele S, Sacco R, Altieri L, Neri C, Urbani A, Bravaccio C, Riccio MP, Iovene MR, Bombace F, De Magistris L, & Persico AM (2015). Slow intestinal transit contributes to elevate urinary p-cresol level in Italian autistic children. Autism research : official journal of the International Society for Autism Research PMID: 26437875

p-cresol and autism: in need of further research

"These results confirm the elevation of urinary p-cresol in a sizable set of small autistic children and spur interest into biomarker roles for p-cresol and p-cresylsulfate in autism".

The peasant dance @ Wikipedia 

That was the primary conclusion from the paper by Gabriele and colleagues [1] looking at "three components of urinary p-cresol, namely p-cresylsulfate, p-cresylglucuronate and free p-cresol" in samples from 33 participants diagnosed with an autism spectrum disorder (ASD) compared with matched asymptomatic controls. The confirmation bit of that quote refers to the fact that members of this authorship group have previously reported on elevated urinary p-cresol in cases of autism [2] which was talked about in the very first proper research-based post on this blog (see here).

Before proceeding, perhaps it might be worth my while going through a few descriptive details. p-cresol (para-cresol) otherwise known as 4-methylphenol is a compound of some note in terms of the various ways and means one arrives at this organic aromatic compound. The solvent toluene is eventually metabolised into p-cresol, as is the amino acid tyrosine in the presence of strains of the anaerobic bacterium Clostridium difficile [3] for example. That being said, there are quite a few other ways in which one might come into contact with this compound.

According to the paper by Vanholder and colleagues [4] there is quite a bit of evidence to suggest that whilst p-cresol and its metabolites are compounds found in some quantity in just about everyone, under certain circumstances, elevations in amount may not be particularly desirable [5] particularly when it comes to renal functions. Indeed, quite a bit of the focus has been on the conjugated derivative p-cresylsulfate (formed through O-sulfonation) when it comes to toxicity [6]. I'll come back to this issue shortly.

A few points on the Gabriele paper might be useful:

• Based on a small participant group comprising 33 participants of various ages on the autism spectrum and 33 sex- and age-matched asymptomatic controls, levels of free p-cresol and it's two metabolites were measured via HPLC with fluorescence detection.
• All metabolites were "significantly elevated" in ASD cases compared to controls.
• "This increase was limited to ASD children ≤8 [less than or equal to 8] years". Also: "Urinary levels of p-cresol and p-cresylsulfate were associated with stereotypic, compulsive/repetitive behaviors (p < 0.05), although not with overall autism severity".

I probably don't need to say it, but when it comes to talk about biomarkers and autism, I do tend to be a little restrained about the promise of any results. Think back to my recent post on organic acids as biomarkers for autism (see here) and just about all the caveats talked about then in terms of heterogeneity and comorbidity come into play here too. That also this and other results from this group are based on HPLC with either UV (ultraviolet) or fluorescence detection could also be considered problematic as a function of the many and varied components found in urine and how without mass spectrometry or NMR, assigning labels to compounds is slightly problematic. Think casomorphins as another example...

Elevated levels of urinary p-cresol are also not a feature of every metabolomic study looking at autism. In their review of all-things p-cresol and autism, Persico & Napolioni [7] talked about how the results from Yap and colleagues [8] reported "blunted and not increased levels of p-cresylsulfate in autistic patients". The Yap study did utilise (1)H NMR spectroscopy and so did not suffer the same analytical shortcomings as the more recent trials. That all being said, I don't want to come down too hard on the latest results from Gabriele and colleagues. They got what they got and now put their results out for further inspection and hopefully, independent verification.

I am also wondering whether the paper by Clayton and colleagues [9] might also be relevant in this case. Dr Clayton, who some might remember from other work talked about on this blog (see here), discussed how "in individuals with high bacterially mediated p-cresol generation, competitive O-sulfonation of p-cresol reduces the effective systemic capacity to sulfonate acetaminophen [paracetamol]". Sulphation capacity when it comes to autism is already something of a research interest (see here) which when added to a growing body of work looking at paracetamol use during pregnancy and possible links to offspring development (see here) might indicate some other interesting investigations to be done. I wonder if perhaps even the sulphation depleting metabolism of something like p-cresol might actually be the more important part of such investigations to autism research?

Music to close, and are you a troublemaker?

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[1] Gabriele S. et al. Urinary p-cresol is elevated in young French children with autism spectrum disorder: a replication study. Biomarkers. 2014 Jul 10:1-8.

[2] Altieri L. et al. Urinary p-cresol is elevated in small children with severe autism spectrum disorder. Biomarkers. 2011 May;16(3):252-60.

[3] Dawson LF. et al. The analysis of para-cresol production and tolerance in Clostridium difficile 027 and 012 strains. BMC Microbiology 2011, 11:86

[4] Vanholder R. et al. p-cresol: a toxin revealing many neglected but relevant aspects of uraemic toxicity. Nephrol Dial Transplant. 1999 Dec;14(12):2813-5.

[5] Liabeuf S. et al. Free p-cresylsulphate is a predictor of mortality in patients at different stages of chronic kidney disease. Nephrol Dial Transplant. 2010 Apr;25(4):1183-91.

[6] Vanholder R. et al. The Uremic Toxicity of Indoxyl Sulfate and p-Cresyl Sulfate: A Systematic Review. J Am Soc Nephrol. 2014 May 8. [Epub ahead of print]

[7] Persico AM. & Napolioni V. Urinary p-cresol in autism spectrum disorder. Neurotoxicol Teratol. 2013 Mar-Apr;36:82-90.

[8] Yap IK. et al. Urinary metabolic phenotyping differentiates children with autism from their unaffected siblings and age-matched controls. J Proteome Res. 2010 Jun 4;9(6):2996-3004.

[9] Clayton TA. et al. Pharmacometabonomic identification of a significant host-microbiome metabolic interaction affecting human drug metabolism. Proc Natl Acad Sci U S A. 2009 Aug 25;106(34):14728-33.



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Gabriele S, Sacco R, Cerullo S, Neri C, Urbani A, Tripi G, Malvy J, Barthelemy C, Bonnet-Brihault F, & Persico AM (2014). Urinary p-cresol is elevated in young French children with autism spectrum disorder: a replication study. Biomarkers : biochemical indicators of exposure, response, and susceptibility to chemicals, 1-8 PMID: 25010144