Typically, humans inherit mitochondria and mitochondrial DNA from their mothers only. But using sequencing data from the mitochondrial genomes of members from three unrelated families, researchers have identified 17 individuals who inherited mtDNA from both parents. The results were published last week (November 26) in PNAS.
“This is a very interesting study, bringing compelling evidence that bi-parental inheritance of mitochondrial DNA happens in human,” Sophie Breton, an evolutionary geneticist who studies mitochondrial genome inheritance at the University of Montreal and who was not involved in the work, writes in an email to The Scientist. “The [sequencing] technique used to demonstrate these results is simple but very powerful.”
Taosheng Huang, a pediatrician and medical geneticist who heads the Mitochondrial Diseases Program at the Cincinnati Children’s Hospital Medical Center, stumbled upon the first individual with mtDNA from both his parents by accident. The patient, a four-year-old boy who had some symptoms of a mitochondrial disorder, including fatigue and exercise intolerance, had come to be evaluated by Huang. The doctor sent his blood sample to the hospital’s diagnostic lab for mtDNA sequencing. The results showed that the boy appeared to have two populations of mitochondrial genomes, both at a relatively high level.
“My first instinct was that this was a mistake—even though I’ve never, in six years, seen our diagnostics lab make a mistake like this,” says Huang. He asked the patient to come back, drew a fresh blood sample himself, then sent the blood to the diagnostics lab, an in-house research lab, and an independent laboratory for sequencing. “We saw the same result come back from all three labs. That’s when I said, ‘Ok, this must be real.’”
Family members submitted samples for sequencing, and the mitochondrial genomes of the boy’s siblings, parents, and grandparents revealed that both the patient’s mother and maternal grandfather had a similar bi-parental inheritance pattern of their mtDNA. Huang and his colleagues then went back to their patient database and identified another family that exhibited the same phenomenon, and finally a third family who was evaluated at the Mayo Clinic and whose samples were then sequenced at the Baylor College of Medicine in Texas.
Some children who had mixed mitochondrial genomes appeared to have inherited the mix from their mothers, having none of their father’s mtDNA. For example, the four-year-old boy didn’t have both parents’ mtDNA, but his mother’s mtDNA and a paternal ancestor’s.
In the men with mixed genomes the ability to pass on their mtDNA appeared to be a dominant trait, as they could give their mitochondrial genes to their children. Still, not all of the males with mixed genomes could pass these onto their kids and, according to Huang, men with two populations of bi-parentally inherited mtDNA may just transfer one population onto their offspring.
