When a four-year-old boy was brought to the Cincinnati Children’s Hospital Medical Center with fatigue, muscle pain and muscle weakness, doctors suspected there might be something wrong with his mitochondria—specialized structures within the cell that are responsible for a number of vital functions, like breaking down carbohydrates and fatty acids. The child’s mitochondrial DNA was sequenced, and no pathogenic mutations were detected. But as Lacy Schley reports for Discover, doctors did find something that shocked them: the boy had mitochondrial DNA not only from his mother, but also from his father.
Scientists have long believed that unlike DNA in the nucleus of the cell, which is inherited from both parents, mitochondrial DNA (mtDNA) comes exclusively from the mother. It is not entirely clear why the father’s mtDNA does not endure, but previous research has indicated that sperm cells have a gene that triggers the destruction of paternal mitochondria when fertilization occurs.
A remarkable study published recently in PNAS, however, suggests that paternal mtDNA can coexist with maternal mtDNA on rare occasions. Researchers repeated the four-year-old boy’s genetic analysis to make sure the original results were correct, and then tested the mtDNA of other family members. They found that at least four people across multiple generations in the boy’s family have significant levels of biparental mtDNA.
Next, the researchers looked at the families of two other patients suspected of having mitochondrial disorders: a 35-year-old male who had also been evaluated at the Cincinnati Children’s Hospital Medical Center, and a 46-year-old women who had been seen at the Mayo Clinic in Jacksonville, Florida. Both these patients were found to have mtDNA from their mothers and their fathers, as did other members of their families.
In total, the researchers identified 17 people from three different families who had inherited a high degree of mtDNA from their dads—between 24 and 76 percent, according to Tina Hesman Saey of Science News.
“We thought, ‘What on earth is going on here?’” study co-author Paldeep Atwal tells Hesman Saey.
In truth, the researchers cannot yet say why paternal mtDNA does not get destroyed in some individuals. But the new study seems to settle a long-standing debate over whether it was even possible for paternal and maternal mtDNA to coexist in humans. The phenomenon has been documented in plants and a small number of animals, but prior to the recent study, biparental mtDNA in humans had only been attested to in a 2002 report, which described a single man who was found to have mtDNA from both his mother and father in his skeletal muscle cells. This report was, however, controversial. The mtDNA in the man’s other tissues was exclusively maternal, and experts questioned whether the findings were a fluke, or the result of contamination.
The new study is more expansive, and presents a “real and a very interesting discovery,” Sophie Breton, an evolutionary biologist at the University of Montreal who was not involved in the research, tells Hesman Saey. And according to the study authors, having a more nuanced understanding of how mtDNA is inherited may one day help medical professionals develop new treatments for devastating mitochondrial disorders, which currently cannot be cured.