Implications for Autism Or Just Interesting? “Epigenetic and immune function profiles associated with posttraumatic stress disorder”
Posted June 25, 2010on:
Hello friends -
One of my tangential pubmed alerts notified me of this study the other day: Epigenetic and immune function profiles associated with posttraumatic stress disorder
The biologic underpinnings of posttraumatic stress disorder (PTSD) have not been fully elucidated. Previous work suggests that alterations in the immune system are characteristic of the disorder. Identifying the biologic mechanisms by which such alterations occur could provide fundamental insights into the etiology and treatment of PTSD. Here we identify specific epigenetic profiles underlying immune system changes associated with PTSD. Using blood samples (n = 100) obtained from an ongoing, prospective epidemiologic study in Detroit, the Detroit Neighborhood Health Study, we applied methylation microarrays to assay CpG sites from more than 14,000 genes among 23 PTSD-affected and 77 PTSD-unaffected individuals. We show that immune system functions are significantly overrepresented among the annotations associated with genes uniquely unmethylated among those with PTSD. We further demonstrate that genes whose methylation levels are significantly and negatively correlated with traumatic burden show a similar strong signal of immune function among the PTSD affected. The observed epigenetic variability in immune function by PTSD is corroborated using an independent biologic marker of immune response to infection, CMV—a typically latent herpesvirus whose activity was significantly higher among those with PTSD. This report of peripheral epigenomic and CMV profiles associated with mental illness suggests a biologic model of PTSD etiology in which an externally experienced traumatic event induces downstream alterations in immune function by reducing methylation levels of immune-related genes.
Essentially the authors took a bunch of people that are more likely to experience stressful situations and PTSD, urban Detroit residents, who amazingly report PTSD symptoms at twice the level that previous studies have found in analysis of larger areas. [Apparently, getting physically attacked is more common there, which gives rise to PTSD even more than ‘other traumatic event types’, and was reported by 50% of the participants from a larger study which formed the population pool of this study. (!!)] With this population base, blood was drawn and methylation profiles were analyzed between participants who reported PTSD symptoms (n=23) and those who ‘only’ had ‘potentially traumatic events’ (PTE). PTSD and ‘controls’ where matched by race, age, sex, and blood profiles.
Once methylation levels were identified, a functional annotation clustering analysis was performed, which I believe is similar a pathway analysis; essentially a bioinformatic tool to gain insight into which biological functions were being manipulated as a result of differential methylation of the genome. This is a powerful new tool in discerning what is happening in autism and elsewhere, and I expect it will provide some surprising answers in the future. Here is their text on what they found:
Consistent with previous findings from gene expression (4, 5) and psychoeneuroimmunologic studies (3), each of the top three FACs determined from uniquely unmethylated genes among PTSD-affected individuals shows a strong signature of immune system involvement. This signature includes genes from the innate immune system (e.g.,TLR1 andTLR3), as well as from genes that regulate innate and adaptive immune system processes (e.g., IL8, LTA, and KLRG-1). In contrast, pathways and processes relevant to organismal development in general—and neurogenesis in particular—figure prominently among the genes uniquely unmethylated in the PTSD-unaffected group (e.g., CNTN2 and TUBB2B; Fig. S2). Notably, similar clusters were obtained using an alternative approach based on genes differentially methylated between the two groups at P < 0.01, with annotations in the top five FACs that include signal, cell proliferation, developmental process, neurologic system process, and inflammatory response
Keeping in mind that reduced methylation results in increased gene expression, if we take a look at Table 1, some of the parallels to autism jump out a little more robustly:
In the ‘Uniquely Unmethylated’ (i.e., higher expression), area, we find that participants affected by PTSD had showed greater enrichment in genes related to the immune response, and specifically the inflammatory response and innate immune response. Our evidence for similar immunological profiles in the autism realm is deep, and includes multiple observations of an active immune response in the CNS, highly significant over expression of genes related to immune function in the CNS, several observations of known upregulators of the innate immune response that are associated with inflammatory conditions, and multiple studies finding an exaggerated innate immune response in vitro when compared to controls. The correlations with developmental process and neuron creation are pretty straightforward.
In the ‘Uniquely Methylated’ area (i.e., lower expression), the sensory perception differences hit close to home, and xenobiotic metabolism has been implicated by several studies.
Going further, the researchers attempted to evaluate for correlations between the number of potentially traumatic experiences and the methylation profile, and somewhat unsurprisingly found that as the number of experiences increased, the methylation differentials showed wider variation.
Here again we see a distinct signature of immune-related methylation profiles among the PTSD-affected group only. More specifically, we see methylation profiles that are suggestive of immune activation among persons with more PTE exposure in the genes that are significantly negatively correlated with increasing number of PTEs—a pattern reflective of that observed for the uniquely unmethylated genes in this same group (Table 1).
From the discussion section:
Among the many analyses performed in this work, the immune related functions identified in the PTSD-affected group were consistently identified only among gene sets with relatively lower levels of methylation (Tables 1 and 2). Demethylation has previously been shown to correlate with increased expression in several immune system–related genes (reviewed in ref. 22), including some identified here [e.g., IL8 (23)]. In contrast, methylation profiles among the PTSD-unaffected are distinguished by neurogenesis-related functional annotations. Neural progenitor cells have previously been identified in the adult human hippocampus (24); however, stress can inhibit cell proliferation and neurogenesis in this brain region (reviewed in ref. 25), and recent work suggests that adult neurogenesis may be regulated by components of the immune system (reviewed in ref. 26). Thus, immune dysfunction among persons with PTSD may be influenced by epigenetic profiles that are suggestive of immune activation or enhancement and also by an absence of epigenetic profiles that would be consistent with the development of normal neural-immune interactions (27).
Among the genes uniquely methylated in the PTSD-affected group, it is striking that the second most enriched cluster—sensory perception of sound—directly reflects one of the three major symptom clusters that define the disorder (Fig. 3B). Genes in this FAC thatmay be particularly salient to this symptom domain include otospiralin (OTOS),which shows decreased expression in guinea pigs after acoustic stress (28) and otoferlin (OTOF), mutations in which have been linked to nonsyndromic hearing loss in humans (29). Exaggerated acoustic startle responses, often measured via heart rate or skin conductance after exposure to a sudden, loud tone, have been well documented among the PTSD affected (30) and are indicative of a hyperarousal state that characterizes this symptom domain. Notably, prospective studies have demonstrated that an elevated startle response is a consequence of having PTSD, because the response was not present immediately after exposure to trauma but developed with time among trauma survivors who developed the disorder (30, 31).
My son had some very severe auditory related problems earlier in his life, and still occasionally struggles with either sudden loud noises, or some very specific noises, such as some dog barks, or the sound of an infant crying. Previously the only physiology based attempt at an explanation I’d heard of for this type of response involved fine grained brain architecture and consequent filtering and/or overexcitation problems. The idea that sound sensitivities in particular can be obtained environmentally is of particular interest to the autism community.
From the common sense angle, I find this completely fascinating; we’ve known for a long time that living with consistent stress is bad for you with a variety of nasty endpoints, but this type of finding narrows down the means by which this happens. In the far off future, perhaps targeted methyl affecting drugs could be considered for people who experience extremely stressful events, as sort of a ‘PTSD vaccine’ [hehe] could be developed.
From an ASD perspective, increased feeling of anxiety, or just generally being ‘stressed out’ is a consistent finding both in research and from what I’ve read of readings from people with autism on the Internet. I’ve seen several explanations, with sensory based problems being mentioned several times. From a biological standpoint we seem to have a growing body of evidence of an abnormally regulated stress response in the autism cohort. An internet friend of mine, Loftmatt, has written extensively on his thoughts concerning the increase in stress in modern society and the mechanisms by which this could be contributing to our apparent observations of an increase in autism. This study would seem to provide insight towards a possible mechanism by which a frequent state of stress could lead to some of our immunological findings in the autism realm; a possibility I hadn’t considered previously when trying to detangle a means by which our observations of immune activation were not participating in autistic behavior. The thought of a feedback loop also strikes me looking at this, something causes a feeling of extreme stress, which leads to abnormal methylation levels and genetic expression, which leads to increased physiological (and behavioral?) alterations, and even more stress.
I may try to poke through the supplementary materials to see if any specific genes or pathways found to be differentially regulated have parallels in some of the other studies we’ve seen recently such as Garbett or Hu, although this may be somewhat of a crapshoot unless I could figure out how to actually submit gene lists to GSEA and read the responses.
And we may need to consider the possibility that these types of effects can be trans-generational. One of the most fascinating studies I’ve seen on epigentics involved exactly that, a multi-generational effect of famine in Holland, wherein the grandchildren of women who were pregnant during a time of famine bore striking differences in a variety of endpoints compared to children whose grandmothers were not pregnant during that time.
The more we learn, the more complicated the world becomes.