Art by Alondra Moreno Santana
Traumatic events are often said to leave invisible scars on a person. Adverse childhood experiences (ACEs), defined as traumatic events that occur during childhood with lasting negative impacts, leave these scars on nearly two-thirds of all adults. Developmental researchers study ACEs because they tend to confer an enhanced likelihood of substance abuse, personality disorders, and anxiety disorders. Such trauma tends to manifest itself across several generations, inspiring two primary foci of research. One side delves into sociological factors, including how generational trauma is transmitted by passing down learned behaviors, disrupts healthy attachment patterns, and inherits legacies of trauma within socioeconomic groups. The other side hones in on biological factors, specifically exploring how ACEs change gene expression in lasting ways from parent to child.
Jetro Tuulari, principal investigator of the FinnBrain Neuroimaging Lab at the University of Turku in Finland, is interested in the latter avenue of exploration. He and his team published a study in Molecular Psychiatry investigating the impacts of one type of ACE called childhood maltreatment exposure (CME) on long-term transmissible genetic expression, or germline changes.
One key mechanism of transmission involves modifications to DNA. The epigenome consists of the collection of these “epigenetic” DNA modifications, which effectively control which genes are expressed in humans. One such modification is DNA methylation, where a small chemical group called a methyl group attaches to the building blocks of DNA. Depending on the location, DNA methylation can have variable impacts on the way the methylated gene is read, increasing or decreasing the gene’s expression.
Another mechanism that changes how DNA is read uses a class of RNA sequences called small non-coding RNAs (sncRNAs). RNA is synthesized from DNA and serves a variety of biological functions. SncRNA is a catch-all term for a group of regulatory RNAs transcribed by our genetic code. These single-stranded molecules contain codes that alter DNA expression or the function of other coding complexes in our cells. One class of sncRNA is microRNAs (miRNAs), which are involved in gene regulation. Though every cell in the body contains the entire genome, miRNAs can change the way different parts of the body express the genetic information encoded within; this is what differentiates cell types. Genes that express eye color are transcribed in the eyes but not in the brain. Likewise, genes that are expressed exclusively in brain cells are not transcribed in the eyes. MiRNAs do this by making different parts of the DNA difficult to access, inhibiting their function and preventing them from being processed into RNA and protein. Modifying the genes encoding sncRNAs changes the cellular profile of these regulatory molecules.
Germline changes have previously been shown to link parental cigarette smoking, stress, and exposure to environmental toxins to physiological changes in the offspring. However, there are no prior hypotheses involving the strength or direction of association between CME and sperm DNA methylation and sncRNA profiles. Hence, Tuulari’s goal was to explore these novel associations.
From 2011 to 2015, data was collected on the fathers of seventy-five families who were evaluated eight months before and nine years after their child’s birth. In 2024, the study remeasured the parents’ CME using an improved analysis: the Trauma and Distress Scale (TADS) questionnaire. This exam reviews the amount of emotional neglect, emotional abuse, physical neglect, physical abuse, and sexual abuse that the parents were exposed to by the time they were eighteen years of age. A higher TAD score indicates greater exposure at an early age. Their sperm samples were also evaluated for biomarkers of epigenetic change, including changes in DNA methylation patterns and expression of sncRNA sequences.
The sncRNA of sperm samples from fathers with low versus high TADs scores were sequenced using sncRNA-Seq processing and compared, and DNA methylation was analyzed using Reduced Representation Bisulfite Sequencing (RRBS). Both methods allow for analyses across an entire genome, or the set of genes in an organism, by looking at the DNA’s building blocks. The expression of sncRNA and DNA methylation can, in turn, change the expression of the genome.
Tuulari and his collaborators found differential expression of a particular miRNA important for brain development between the groups with different TAD scores. This miRNA had lower expression levels in the sperm samples of fathers who experienced difficult childhoods. Not only were they able to identify this association in their study, but they replicated the genetic inheritance pattern caused by this miRNA in mice. Replicable data is as good as gold in the scientific community. Now, this miRNA can be used as a biomarker in other stress-related studies.
Tuulari suspects that there are also other contributors at play. “I don’t believe it’s [just] that one, but maybe something together with that in humans,” Tuulari said. “We can use this as an anchor molecule and look at what happens around it.”
Tuulari also found significant decreases in methylation in regions of three genes in the high-TADS group, including two genes that are necessary for brain development. One of these regions was the tail of the CREB-regulated Transcription Coactivator 1 (CRTC1) gene, which, among other roles, controls the formation of the hippocampus, the region in the brain responsible for capturing memory and associations. This corroborates prior work Tuulari’s group had performed showing the correlation between CME and differences in brain region structure and size via magnetic resonance imaging (MRI). Increased expression of CRTC1 has previously been implicated in mood-related disorders including depression. Interestingly, decreased methylation of this gene in the high-TADS group caused increased CRTC1 expression, which suggests germline epigenetic changes in this gene may affect the likelihood of parental-conferred mood disorders. “[This work] is a very important stepping stone for us, because this means we will be able to base our future studies on this,” Tuulari said.
Tuulari’s work has crucial implications in the field of developmental epidemiology, most importantly showing that early life stress can cause distinct, stable epigenetic signatures that can be passed down across generations. His study identified novel biomarkers of childhood CME, including three methylated regions and a signature sncRNA profile. His work has also shown that human sncRNA profiles are similar to that of rodent models, and this similarity across species may suggest evolutionarily significant signatures of stress. “There must be something around this [miRNA] molecule. We were able to replicate it [in humans, which is] so rare,” Tuulari said. Identifying individuals with such epigenetic signatures might better help target existing therapies for stress management and trauma to individuals who will most benefit from them.
Beyond simply identifying biomarkers of CME, this work opens the door to begin imagining interventions for intergenerational adverse health effects. After all, CME is the single most preventable risk factor for future mental health in an individual’s life. Now, we know that CME matters for the offspring of these individuals as well. Sociological researchers have long emphasized the behavioral mechanisms by which parents confer stressors to their children, often placing blame on individuals’ actions in perpetuating generational trauma. Identifying biological markers of trauma offers another perspective and provides hope for a medical solution to passed-down trauma.
Nonetheless, researchers must still be conscious about the double-edged sword of medicalizing mental health issues, ensuring that identifying potential molecular targets for intervention doesn’t institutionalize such biological endowments as out of a person’s control. In fact, healthy lifestyle strategies, such as exercise and a healthy diet, have been shown to confer positive epigenetic germline changes, highlighting how a combination of controllable healthy behavior and biological changes goes against those resulting from CME. “Maybe there could be a public health policy recommendation to, let’s say, exercise more before thinking about getting children,” Tuulari said. This perspective underscores a crucial shift: rather than resigning to biological determinism, we should empower individuals with actionable steps—emphasizing lifestyle choices as a first-line defense in shaping not only their own well-being but also the health of future generations.