I recommend the Primary Papers

The paper by Mill et al. is one of the first comprehensive attempts to examine changes in methylation across the entire genome in patients with various diagnoses of mental illness. The study is well designed, extensive, and uses fairly new technology to examine changes in methylation profiles across the genome. In the frontal cortex, the authors provide evidence for psychosis-associated differences in DNA methylation in numerous loci, including those involved in glutamatergic and GABAergic transmission, brain development, and other processes linked with disease etiology. Methylation in the frontal cortex of the BDNF gene is correlated with a non-synonymous SNP previously associated with major psychosis. These data provide further support for an epigenetic origin of major psychosis, as evidenced by DNA methylation-induced changes likely important to gene expression.

In many ways, this seems reminiscent of the trend in genetics several years ago when the inclination was to move from single gene loci association and linkage studies to genomewide scans. The only downside of the approach is that what one gains in information, one (at least initially) loses in biology. That is given the wealth of new findings uncovered; we now need to go back and examine these results in light of what we know regarding gene function in neurobiology and cognition. Of course, this is the trend, now that microarrays have increased our capacity to look at all things at the same time. The flipside is that it will take several large-scale studies of this sort to better understand which findings are replicable and which are not. That is, do the results of the Mill paper agree with data obtained and carried out by laboratories using the methyl DIP or MeCP2 ChIP assays coupled with microarrays. While these experiments ask different questions, the implication is that there may be some degree of overlap in comparing these different methodologies. While this may be premature, there is a sense that this information will be available shortly.

Finally, I would like to focus on recent findings regarding the methylation of the reelin promoter. These authors (Mill et al.) and Tochigi and colleagues (Tochigi et al., 2008) have found that the reelin promoter is not hypermethylated in patients with schizophrenia. In fact, Tochigi et al., 2008, found that the reelin promoter is not methylated at all using pyrosequencing. However, several groups (Grayson et al., 2005; Abdolmaleky et al., 2005; Tamura et al., 2007; Sato et al., 2006) have shown that the human reelin promoter is methylated in different circumstances. Interestingly, there is little consensus in the precise bases that are methylated in these latter studies. Our group (Grayson et al., 2005) performed bisulfite treatment of genomic DNA and sequencing of individual clones. Moreover, we analyzed two distinct patient populations. The clones were sequenced at a separate facility. What was intriguing was that the baseline methylation patterns in the two populations was different, and yet several sites stood out as being relevant in both. We mapped methylation to the somewhat rare CpNpG sites proximal to the promoter. Interestingly, these bases were located in a transcription factor-rich portion (Chen et al., 2007) of the promoter and in a region that shows 100 percent identity with the mouse promoter over a 45 bp stretch. We have also been able to show that changing one of these two bases to something other than cytosine reduces activity 50 percent in a transient transfection assay. So the question becomes, How do we reconcile these disparate findings regarding methylation? As suggested by Dr. McCaffrey, the answer may lie in regional differences that arise due to the nature of the material available for each study. We have found a degree of reproducibility by using human neuronal precursor (NT2) cells for many of our studies. At the same time, this cell line is somewhat artificial and cannot be used to reconcile differences found in human tissue. Perhaps it might be prudent to examine material taken by using laser capture microdissection to enrich in more homogenous populations of neurons/glia. In moving ahead, it might be best to now focus on the mechanism for these differences in methylation patterns and try to understand the biology associated with the new findings (Mill et al., 2008) as a starting point.

References:

Abdolmaleky HM, Cheng KH, Russo A, Smith CL, Faraone SV, Wilcox M, Shafa R, Glatt SJ, Nguyen G, Ponte JF, Thiagalingam S, Tsuang MT. Hypermethylation of the reelin (RELN) promoter in the brain of schizophrenic patients: a preliminary report. Am J Med Genet B Neuropsychiatr Genet. 2005 Apr 5;134(1):60-6. Abstract

Chen Y, Kundakovic M, Agis-Balboa RC, Pinna G, Grayson DR. Induction of the reelin promoter by retinoic acid is mediated by Sp1. J Neurochem. 2007 Oct 1;103(2):650-65. Abstract

Grayson DR, Jia X, Chen Y, Sharma RP, Mitchell CP, Guidotti A, Costa E. Reelin promoter hypermethylation in schizophrenia. Proc Natl Acad Sci U S A. 2005 Jun 28;102(26):9341-6. Abstract

Mill J, Tang T, Kaminsky Z, Khare T, Yazdanpanah S, Bouchard L, Jia P, Assadzadeh A, Flanagan J, Schumacher A, Wang SC, Petronis A. Epigenomic profiling reveals DNA-methylation changes associated with major psychosis. Am J Hum Genet. 2008 Mar 1;82(3):696-711. Abstract

Sato N, Fukushima N, Chang R, Matsubayashi H, Goggins M. Differential and epigenetic gene expression profiling identifies frequent disruption of the RELN pathway in pancreatic cancers. Gastroenterology. 2006 Feb 1;130(2):548-65. Abstract

Tamura Y, Kunugi H, Ohashi J, Hohjoh H. Epigenetic aberration of the human REELIN gene in psychiatric disorders. Mol Psychiatry. 2007 Jun 1;12(6):519, 593-600. Abstract

Tochigi M, Iwamoto K, Bundo M, Komori A, Sasaki T, Kato N, Kato T. Methylation status of the reelin promoter region in the brain of schizophrenic patients. Biol Psychiatry. 2008 Mar 1;63(5):530-3. Abstract

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