Friday, 25 July 2014

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?


[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.

---------- 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