“Very degraded samples like skeletal remains that have been sitting out exposed to the environment for 20 or 30 years may have very little nuclear DNA remaining, but because of the high copy number of mitochondria, you may actually be able to get results with the mitochondrial DNA.”
Dr. Michael Coble, Associate Professor and the Executive Director of the Center for Human Identification at The University of North Texas Health Science Center at Fort Worth
Investigators rely on evidence to help them solve crimes. Much of this evidence is forensic, meaning investigators rely on scientists to evaluate, test, or assess it. This evidence can range from ballistics to blood tests, hair samples, and fingerprints. Sometimes, evidence isn’t readily available or has been degraded due to time or exposure to the elements. When this happens, alternative forensic research methods can be applied to extract results.
One such method is the use of mitochondrial DNA (mtDNA) typing. This type of DNA was first discovered in 1963 by Margit Nass and Sylvan Nass. However, the first sequence wasn’t published until 1981. This is a relatively recent discovery compared to nuclear DNA (what we typically think of when we hear the word DNA).
While mtDNA has many uses, it is primarily used in unsolved cases. Because of where mtDNA is stored, there is more of it in each cell which makes the likelihood of extracting it much higher than with nuclear DNA. Evidence that may not have nuclear DNA, such as a strand of hair or an older bone, can still have mtDNA which, given the right tools, can be extracted by forensic scientists to help solve crimes.
To learn more about mtDNA and its use, we talked to Dr. Michael Coble, associate professor at The University of North Texas Health Science Center at Fort Worth. Notably, he is an expert in mtDNA and has also been involved in solving cold cases using it.
Dr. Michael Coble is an associate professor and the executive director of the Center for Human Identification at The University of North Texas Health Science Center at Fort Worth. His research is centered on DNA mixture interpretation and the use of software analysis for probabilistic methods of interpretation.
He holds a PhD in genetics and a master’s in forensic science in forensic molecular biology, both from George Washington University. He is a fellow of the American Academy of Forensic Sciences and a member of the International Society for Forensic Genetics and the Texas Forensic Science Commission.
Dr. Coble is a prolific author with over 70 peer-reviewed articles in publications such as the Journal of Forensic Sciences and Forensic Science International.
Before understanding how mtDNA is used, it is essential to understand what it is: “When we talk about the human genome, we usually think about the DNA that you find in the nucleus of the cell. We call that nuclear DNA,” explains Dr. Coble.
“Humans actually have two genomes in our cells: we have the genome in the nucleus and a genome in these little structures called mitochondria. The mitochondria provide many functions for the cell. One of the more important things they do is provide energy for the cell. You can think of the mitochondria like the battery.”
Dr. Coble continues, “Mitochondria have their own separate DNA from the nucleus. The reason why mitochondrial DNA is so useful there is so much of it. When you look inside a cell, you see one nucleus with two copies of your chromosomes, one from your mother and one from your father. However, each mitochondrion may have dozens of circular DNA, and within each cell, you may have hundreds of mitochondria. So you can have tens of thousands of mitochondrial genomes in one cell.”
Because there is so much mtDNA in cells compared to nuclear DNA, there is a better chance of extracting it from older evidence.”Very degraded samples like skeletal remains that have been sitting out exposed to the environment for 20 or 30 years may have very little nuclear DNA remaining, but because of the high copy number of mitochondria, you may actually be able to get results with the mitochondrial DNA,” shares Dr. Coble. Because of this, mtDNA is often used in cold cases or on older evidence.
MtDNA differs from nuclear DNA in several ways, but the most important is in how it is inherited: “It’s what we call a lineage marker. The mitochondria are passed along the maternal line. So all the mitochondria that you have in your body all came from your mother. Because of that, you, your mother, sisters, aunts or uncles from your mother, and grandmother will have the same mitochondria,” explains Dr. Coble.
So if all of the mitochondria along a maternal line are the same, how are mtDNA results used? “MtDNA is not a unique marker because it’s being passed from mother to child. So you don’t get that powerful statistic like one in a million like you do with the nuclear DNA because you’re not adding new information with each generation,” says Dr. Coble.
Despite the mtDNA not being unique, it can still be very useful in forensic science: “It’s a great marker for elimination,” he says. “If you have a bone with a particular mitochondrial type and you have a reference sample, and they don’t match, well, they don’t match.”
However, sometimes there is no reference sample. In these cases, detectives can still use mtDNA to aid their investigations. “For example, in a case with a missing person, we may find some skeletal remains that we have not been able to extract a nuclear profile because of the degradation. So we can go to the mitochondria and extract them. Then, we look into that family and search for maternal references to that person.”
If a familial reference can be located, and there is a match, investigators must determine how common that particular type of mtDNA is. “We compare that mitochondrial DNA to those of random people in the population whose DNA has been collected to establish a database. We just simply ask, ‘How rare is this type?’ Then the probability statistics are based on the size of the database. So if you’ve got 10,000 people, and you’ve never seen this before, you know it’s not very common in the population. Sure it is one in 10,000 is not one in a quadrillion like for nuclear DNA, but it may be enough information to make an identification,” he says.
Unfortunately, mtDNA can’t always give forensic scientists the answers they are looking for: “There are some mitochondrial types that are quite common. In fact, there’s one type for European Caucasians that’s found in about 7 percent of that population,” says Dr. Coble. “In those cases, the mitochondrial DNA evidence might be weak, and we may have to rely on non-DNA evidence. We have to find other things that could be useful in identification like maybe previous bone breaks or dental records.”
“Not all laboratories in the US do mitochondrial testing. It is a specialized kind of testing,” shares Dr. Coble. Despite it not being very common, there are some cases where it can be extremely useful.
“One of the more common pieces of evidence that you find at a crime scene are hairs,” explains Dr. Coble. “People are constantly shedding hairs, and where there is contact between two people, such as a struggle or sexual assault, those hairs could be transferred from the perpetrator to the victim. Now, if those hairs have a root attached, then you can do the nuclear DNA testing, but if you only have a fragment of a hair, then mitochondrial DNA becomes a critical piece of the puzzle.”
“Mitochondrial DNA has its niche. But if you have one of those cases where you need mitochondrial DNA based upon the type of sample you’re trying to test, it can become an important marker for the investigation,” he says.
In addition to current investigations, mitochondrial DNA can be essential in cold cases or with degraded and weathered samples. Dr. Coble has worked on some interesting cases over the years: “I was involved in identifying the two missing Romanov children,” he shares. “The Romanovs were the last royal family of Russia, and they were executed in 1918 by the Bolsheviks, who later became the Soviets. When the family’s remains were discovered, two of the children were missing.” The original family remains were discovered in the 1990s, but the suspected bones of the two missing children were not found until 2007.
“The first thing we did was mitochondrial testing because all we had were skeletal remains. Once we saw that mitochondrial match, we were pretty sure that these were the missing children. It was very exciting because we were dealing with a piece of history.”
Kimmy Gustafson is a freelance writer and researcher with a passion for sharing stories of bravery. Her love for world-traveling began when her family moved to Spain when she was six and since then, she has lived overseas extensively, visited six continents, and traveled to over 25 countries. She is fluent in Spanish and conversational in French. When not writing or parenting she can be found kiteboarding, hiking, or cooking.