Sperm cells carry traces of childhood stress, epigenetic study finds

Fathers may carry traces of their childhood trauma in their sperm cells, a new study finds.

The new research, published Jan. 3 in the journal Molecular Psychiatry, looked at the "epigenetics" of the sperm cells of fathers who had been exposed to high stress in childhood.

Epigenetics involves how DNA — the blueprint used to build the proteins and molecules that make up our bodies — is read. Epigenetics doesn't change DNA's underlying code but rather alters which genes can be switched on. Research suggests that people’s life experiences and environments can leave these "epigenetic changes" on DNA, which can then modify gene activity.

"Epigenetics are basically saying which genes are active," said lead study author Dr. Jetro Tuulari, an associate professor in the Department of Clinical Medicine at the University of Turku in Finland. The work adds to a growing body of research investigating whether parents' life experiences can be passed on to future generations through these epigenetic changes.

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"Understanding inheritance through genes and DNA has been one of the most crucial elements of our understanding of biology," Tuulari told Live Science. "We are now doing research that questions whether or not we have a complete picture."

The new study analyzed sperm cells from 58 individuals, looking at two types of epigenetic markers: DNA methylation and small noncoding RNA.

DNA methylation is a chemical reaction that adds a tag to DNA. When DNA is methylated, the body can read this as a signal to change how a gene is read — turning it off, for instance. Small noncoding RNAs have a similar effect on genes, except that instead of tagging the DNA molecule itself, they can interfere with the way the body reads RNA, a genetic cousin of DNA that shuttles instructions out of the nucleus and into the cell.

The fathers, the majority of whom were in their late 30s to early 40s, were recruited through the FinnBrain Birth Cohort, a University of Turku study of more than 4,000 families that looks at both environmental and genetic factors that might influence child development.

To measure participants' childhood stress, the team used the Trauma and Distress Scale (TADS), an established questionnaire that queries people about their memories of emotional or physical neglect, as well as emotional, physical or sexual abuse. These TADS scores were then categorized as either low (0 to 10), meaning they remembered relatively few childhood stressors, or high (over 39), meaning they remembered many traumatic events.

The analysis revealed that the sperm of men who had high scores had a different epigenetic profile compared with the sperm from men who reported less trauma. This pattern held even after the researchers checked if the differences could be attributed to other factors, such as drinking or smoking behaviors, which are also known to affect the "epigenome."

Uncovering any associations between these stressors and epigenetics is "absolutely fascinating," Tuulari said, because the stressors happened early in the men's lives. This suggests that the epigenetic changes persisted over time, despite decades passing since the events that would have initially triggered them.

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Notably — as has been reported in previous studies from other research teams — the researchers found that one specific small noncoding RNA molecule was expressed differently in those who experienced high stress as children. This molecule, known as hsa-mir-34c-5p, drew their attention because it had previously been shown to alter the development of mice's brains early in development.

The researchers also noted different DNA methylation profiles around two genes, called CRTC1 and GBX2. This raised some eyebrows among the team, as these genes have also been implicated in early brain development in other studies, mostly done in animals.

Together, these findings hint that these epigenetic changes may alter early development, provided that they are passed down from parent to offspring, Tuulari speculated.

However, it's important to note that this field of research is still in its infancy. Seeing epigenetic changes in sperm does not necessarily mean these changes are passed on to children. In fact, researchers are working very hard to answer that question, Tuulari said.

It has not been confirmed that epigenetic changes can be passed from a human parent to their children, he said. "It has been shown, however, in experimental animal models, including worms and mice."

Therefore, it is too soon to draw any conclusions about the influence of paternal stress on a child's health, Tuulari said. It is also not clear whether the epigenetic changes observed would ultimately have a positive, negative or neutral effect, he added.

Nonetheless, the findings "add to the weight of evidence" that life experiences can change the epigenetics of human sperm, said Richard Jenner, a professor of molecular biology at University College London who was not involved in the study.

Whether these epigenetic changes are important remains to be seen, Jenner noted. It would be good for the study to be repeated with a bigger sample. In the current study, some factors, like the men's range of ages and diets, could have muddled the results, so these factors could be further studied too, he said.

In the meantime, we can only speculate about whether this information could, down the line, be used to improve the health of a child whose parent was exposed to childhood stress, he concluded.

"Time will tell," Jenner said.