I have to thank Dr Malav Trivedi for bringing my attention to some recent findings reported by Yiting Zhang and colleagues (including Malav)  (open-access) suggesting that: "levels of vitamin B12, especially its MeCbl [methylcobalamin] form, decrease with age in frontal cortex of control human subjects."
Further, researchers reported: "abnormally lower total Cbl [cobalamin] and MeCbl levels in subjects with autism and schizophrenia, as compared to age-matched controls." Some media on the findings can also be read here.
Working from the lab of Dr Richard Deth (quite a familiar name to this blog), researchers initially analysed a most precious sample medium (postmortem brain samples) obtained from various biobanks and including various patient groups. So alongside samples from 12 children with autism were samples from 9 people diagnosed with schizophrenia and some 43 'controls' with ages ranging between 19 weeks old and 80 years old. "Changes in Cbl species were compared with the status of methylation and antioxidant pathway metabolites" accompanied by data derived from a knock-out mouse model: "the influence of decreased GSH [glutathione] production on brain Cbl levels was evaluated in glutamate-cysteine ligase modulatory subunit knockout (GCLM-KO) mice in which GSH synthesis was impaired, leading to a brain GSH level decrease of 60–70%."
Looking at postmortem frontal cortex brain samples, researchers reported that finding on levels of vitamin B12 - particularly the MeCbl vitamer - decreasing with age. Bearing in mind the relatively small participant numbers included, the idea that lower brain tissue levels of total cobalamin and methylcobalamin were also present (almost unanimously) in the autism and schizophrenia groups could be important. I might at this point direct readers to previous discussions on vitamin B12 and autism on this blog (see here) including the research idea of supplementing (see here) with no medical advice given or intended.
There are a few other details worth pointing out from the Zhang findings. Analysis of thiols in brain samples across the autism vs control group revealed some potentially interesting data. So, methionine levels were quite a bit lower in the autism group [significantly lower] as were levels of "the methyl donor S-adenosylmethionine (SAM)." Both these compounds form an important part of the whole 'methylation of DNA' process (see here) among other things.
Glutathione, a compound that has seen its fair share of speculation with autism in mind (see here), was also on the research menu in the Zhang study. Interestingly and again bearing mind the small participant numbers, brain levels of this stuff were lower in the autism group as a whole but not significantly so when compared to controls. This finding might map on to other brain studies with autism in mind (see here). Likewise, cysteine (another potentially relevant compound to some autism) produced a similar finding.
I would encourage readers to take some time looking at the Zhang paper. In conjunction with other results reporting on some important elements to the emerging story (see here) I believe there are further studies to be done applicable to the notion that: "impaired methylation may be a critical pathological component" for at least some autism (see here). Indeed, other research papers have also discussed this issue . The idea that studies about human ageing may likewise be informative to autism (and schizophrenia) research also carries quite a lot of traction too.
 Zhang Y. et al. Decreased Brain Levels of Vitamin B12 in Aging, Autism and Schizophrenia. PLoS One. 2016 Jan 22;11(1):e0146797.
 Keil KP. & Lein PJ. DNA methylation: a mechanism linking environmental chemical exposures to risk of autism spectrum disorders? Environmental Epigenetics. 2016; 1-15.
Zhang Y, Hodgson NW, Trivedi MS, Abdolmaleky HM, Fournier M, Cuenod M, Do KQ, & Deth RC (2016). Decreased Brain Levels of Vitamin B12 in Aging, Autism and Schizophrenia. PloS one, 11 (1) PMID: 26799654