I love the spectacular symbiosis of my vegetable garden as harvest time approaches. Beanstalks spiral up cornstalks, their tendrils teasing nearby tomato…
The Maui firestorm was so vast and fast that most identification of human remains will come from bits of persisting DNA from mitochondria.
The “Powerhouse of the Cell”
Most people likely last encountered mitochondria in high school biology class. The footprint-shaped “powerhouse of the cell” releases energy from breaking the chemical bonds that hold together nutrient molecules. The energy released in digesting food is held, fleetingly, in molecules of ATP, which serves as an energy debit card of sorts.
Each mitochondrion harbors its own tiny genome, a mere 36 genes compared to the 20,000 or so in a human cell’s nucleus. And mitochondrial genes aren’t just copies of nuclear ones – they’re unique. Most encode enzymes that extract energy from ATP.
Mitochondria likely came from bacteria that single-celled organisms in ancient seas engulfed about 1.5 billion years ago. The idea is famous in biology as the endosymbiont theory. The bacteria in their new cellular homes, over time, retained some genes while surrendering others to the nucleus. And, gradually, the ancient bacteria evolved into mitochondria. Two recent reports in ScienceAdvances describe a contemporary contender for a descendant of the original stowaway bacterial genome that birthed mitochondria.
MtDNA in Forensics and Genealogy
Mitochondrial DNA (mtDNA) is used in forensics to match the tiniest bits of human remains to families. This is possible because mtDNA persists, with distinctive sequences, under conditions that shatter the strings of short repeated DNA sequences from the nucleus (aka nuclear DNA) into meaningless gibberish. Comparing the number of repeats is the basis of using nuclear DNA for identification purposes, and that wouldn’t hold up against a sudden wall of fire.
Another curious fact about mitochondria is that they are inherited from the female parent only. That’s because an oocyte harbors many mitochondria, whereas mitochondria are excluded from all but a sperm cell’s tail, where they form a wrapping that powers the swim by splitting ATP. Almost always, sperm mitochondria drop off before they can enter and transform the oocyte into a fertilized ovum.
Traits conferred through mitochondrial genes are said to be “maternally inherited.” And that’s why ancestry test algorithms use mitochondrial DNA sequences to trace maternal lineages. Y chromosome sequences provide DNA information specific to males.
Mitochondrial DNA is also far more abundant than nuclear DNA. A cell has one nucleus, with two copies of each gene because chromosomes come in pairs. But a cell has hundreds to tens of thousands of mitochondria, depending on activity level of the organ of which it is a part. A relatively quiescent liver cell has about 1,700 mitochondria, but a busy muscle cell may have 10,000 or more. That’s why fatigue is a major symptom of mitochondrial diseases – there’s a different such condition for each of the genes. (Media reports often lump them together as “mitochondrial disease,” which is disturbingly non-specific for this geneticist.)
A Powerful Precedent: 9/11
Tiny and abundant mitochondrial DNA can survive the harshest of conditions. Forensic analyses using mtDNA has impacted criminal cases and identifying human remains from plane crashes, terrorist attacks, wars, and natural disasters like tsunamis, volcanic eruptions, floods, and fires.
Consider the role of mtDNA in the days and weeks following September 11, 2001.
In late September, a company that provides breast cancer tests received three unusual types of DNA samples:
• evidence from the World Trade Center in New York City
• cheek brush scrapings from relatives of missing people
• reference samples from victims’ toothbrushes, razors, and hairbrushes.
Forensic investigators used big pieces of DNA from cell nuclei, including the X and Y chromosomes, for soft tissue. But unfortunately these were rare in the inferno. However, hardier and more abundant mtDNA was collected from tiny bits of bone persisting in the fires that smoldered for weeks.
When a DNA pattern from World Trade Center evidence matched DNA from a victim’s personal item or that of a close relative, identification was fairly certain. DNA profiling provides much more reliable information than traditional forensic identifiers such as dental patterns, scars, and fingerprints, and deducing identity from objects such as jewelry.
But even powerful mtDNA analysis fails in some circumstances. That was the case for the approximately 250,000 bodies that the Indian Ocean tsunami in 2004 scattered about, which rapidly decayed in the hot, wet climate. These conditions, plus the absence of roads and labs, led to identifying 75 percent of the bodies using standard dental record analysis and 10 percent from fingerprints. DNA identified fewer than half of 1 percent of the victims. The tsunami cut a wider swatch of destruction than the toppling towers of 9/11, washing away entire families. That left few or no relatives to provide DNA, as well as evidence such as toothbrushes.
The Grandmothers of the Plaza de Mayo
Genetics intersects politics and sociology when forensic DNA typing is used to reunite separated families. A precedent is the Grandmothers of the Plaza de Mayo project, which began in 1977 in Argentina. I wrote about it in 2018 here, in the context of using DNA evidence to reunite immigrant families separated at US borders.
The eight-year “Dirty War” in Argentina began in 1977. The Argentinean military and police abducted and sold or gave away at least 210 niños desaparecidos (“disappeared children”). Relatives started the Grandmothers project to use conventional evidence, such as documents, to reunite families. Grandmothers were the focus because so many mothers had died.
By 1984, though, conventional evidence hadn’t reunited many families. So a team of geneticists went to Buenos Aires to assist the Grandmothers project. It included Mary Claire King, who would go on to discover the BRCA breast cancer genes.
At the time of the Dirty War, what was then called DNA fingerprinting (now called profiling) was too new to use. So the researchers turned to the older way to match people for organ transplants, variability of the six human leukocyte antigen (HLA) genes. To improve the test, they added analysis of a 1,200-base-long DNA sequence in the mitochondrial genome that is highly variable and does not encode protein.
Like all mtDNA, this tough and abundant marker passed from mothers only. Tracking female relatives was practical, because documentation and conversation focused on women. The mtDNA marker worked so well that Argentina established a voluntary DNA databank.
The Grandmothers project solved 50 cases: 44 of the children had been “gifts” to military leaders. Adoptive parents of the other 6 children were likely unaware of the kidnapping. Children raised by adults who had known of the kidnapping learned that their adoptive parents knew that the biological parents had been murdered.
The hurricane fire that struck Maui is unprecedented. Will it have left behind enough fragments of mtDNA to identify victims? The investigation will go on for many months. Unfortunately, many families will have to rely not on technology but on tragic logic – a missing loved one.