There are a little over 37 trillion cells in a standard human. They make up the skin, the brain, the liver, the muscles—everything. They make up what you think of as, well, you.
But when you die, all those cells don’t instantly die with you. Though you may be gone, many of your cells are still kicking in the hours and days after death, and some even show increased activity, finds a study in Nature Communications published last week. Understanding exactly what cells do after death gives insight into the basics of cell behavior, but it also has some practical applications, like better predicting a corpse’s time of death for use in criminal investigations.
Roderic Guigó, a computational biologist and co-author on the paper, didn’t set out to develop a new tool for forensic science. He spends his days wondering what makes cells act in different ways. Why is a skin cell a skin cell, when a liver cell has all the same genetic material in it? To answer that, he looks at gene expression. Even though each cell carries an organism’s whole genome, only certain genes are “turned on” or expressed in each cell, which is how they can be so different from one another.
One way researchers can tell when a gene is turned on is by looking for signs of gene transcription, which is the first step in gene expression. Segments of DNA are copied to a molecule called RNA, which will go on to make a protein—which actually does something in the body and expresses the gene it came from.
But Guigó says that in studying gene expression, scientists rely heavily on post-mortem tissue samples. “This is a large part of biology,” he says. “It’s based on analysis of samples from dead people. Our main interest isn’t gene expression in death, but gene expression in the living. We measured the expression that came from post-mortem samples initially to see if they even could be a good estimation for gene expression in people that are alive.”
Guigó and his team’s new study examined more than 7,000 samples of 36 types of human tissue from 540 donors. By looking for levels of RNA, they could see if and how much expression was taking place. "We found that each tissue has a specific pattern of changes after death," he says. "It’s not the same what happens in the muscle, what happens in the skin, what happens in the brain." They also found, a bit unexpectedly, that the body doesn’t uniformly quiet down; in many places, a tissue's specific pattern involves several genes becoming more expressed.
Peter Noble, a microbiologist at The University of Washington, was not involved in the new study, but his previous research has shown similar findings in animals. He too didn’t initially set his sights on examining gene expression after death. He and his co-workers came up with a new method to calculate gene expression, and they wanted a way to test it out.
A colleague at the Max Planck Institute for Evolutionary Biology in Germany told Noble that they were in the process of moving from zebra fish to mice as an animal model. “We said, If you’re going to kill all these fish maybe we should do an experiment them,” Noble tells me.
What they found in 2017, just like in Guigó’s human tissues, was that there are certain genes in cells that become more expressed after an organism dies. “We thought, just by common sense, that gene transcripts should go away really fast after something dies,” he says. “And it’s true that about 99 percent of the genes do go away. But there is a certain 1 to 2 percent that actually increase in transcript abundance through time.”
Last year, he published his results: 1,063 genes in the zebra fish and mice were significantly more abundant even up to 96 hours after death. Most of the genes that became more active did so in the first half hour after death, but some waited 24 hours, and others up to 48 hours. “I certainly was very shocked that we got this result,” he says. “It was not anticipated. When I told people about it they rolled their eyes and thought I was crazy.”
Noble and his team called it the “twilight of death”—when an organism is neither a living body or a decomposed corpse, but in transition between the two.
Some of the genes Noble found made sense: They were involved in wound healing, stress responses, or ways of shutting the cell down. But other genes were a surprise.“What was really interesting was the fact that some of the genes were involved in development,” Noble says. “Specifically, development of the embryo.”
When Noble ran their findings through a cancer database, he also found that many of the genes that increased in activity were associated with cancer, either with suppressing it or causing it. He thinks that this could have implications for transplant recipients who have been shown to have higher chances of getting cancer. Previously, this was thought to be an immunological issue, but now Noble thinks that these turned-on cancer genes could be playing a role.
In the new study, Guigó didn’t find expression in development genes, though he looked for them. Still, there were some surprises. One was that in many tissues they saw an increase in a gene that makes a protein that carries oxygen. After death, you might expect to see less of this since, after all, there’s less oxygen in the body. But Guigó guesses that the cells could be sensing the lack of oxygen and producing more of this protein “with a hope of recruiting more oxygen,” he says. They also saw an increase in an enzyme that’s involved in the production of RNAs, the molecule that helps gene expression take place.
Guigó says that while each tissue behaved differently, there was consistency within each for potential forensic applications. Forensic science has many ways to try and guess time of death, from a body’s temperature, gauging rigor mortis, measuring various chemical levels, or even seeing what bugs have colonized the body. But there’s always room for more accurate methods.
Noble has published a paper that used his findings to show he could calculate time of death using the pattern of gene transcriptions that he saw in the zebrafish and mice. Guigó's team created software that, based on the death patterns of 399 people, was able to predict the death times of 129 others. Based on known gene activity changes, it looked for similar increases or decreases to make its guesses.
“The software discovered, for example, that in blood, decreased activity of genes involved in DNA production, immune response, and metabolism—but an increase in those involved with stress responses—signaled the person had died about 6 hours before preservation,” reported Science.
And what about Guigó’s original question: With all we’ve uncovered about how the body changes after death, can we still use dead tissues to study living ones?
“If we know the genes that are changing after death, we can control for them and still actually get a lot of useful information from the post-mortem samples,” he says. “By correcting for these factors, we can actually get a very good representation of the gene expression in the living.”
In general, it's not groundbreaking to know that your body can continue on after you die, Guigó says—it’s why we can take out hearts and livers from a dead person and transplant them into someone else. But still, it can be an eerie feeling to know that your cells keep churning along after “you” have left them, and even can be more alive without you, at least for a little while.
“There are parts of the body that are still alive when the body itself dies,” Guigó says. “What does is it mean? That’s a question that maybe isn’t for science. What is the difference between the death of an individual and the death of the parts that constitute the individual? Am I my liver, my stomach, my kidney? All these things still work when I am dead. They are alive, but I am dead. Essentially, the idea that comes through is that life is just a coordinated action of all these parts, and death of the individual is the lack of that coordination.”
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