You probably learned two things about mitochondria in high school biology. First, it's the cell's power plants. Secondly, you can only inherit them from your mother. But a new study seems to darken this second point.
A team of researchers from the United States, China and Taiwan identified three unrelated families whose mitochondria contained both parents' DNA. While this discovery may revive the debate about the nature of mitochondrial DNA inheritance, the researchers hope this will open up new ways to treat diseases.
The story began when a four-year-old patient with fatigue and muscle pain was referred to doctor-researcher Taosheng Huang of the Cincinnati Children's Hospital Medical Center. Others had suggested that the boy was showing signs of a mitochondrial disorder. Huang therefore sequenced and analyzed the boy's mitochondrial DNA.
Mystified, he sequenced the mitochondrial DNA of the other members of the boy's family, including his sisters, parents, grandparents, and the siblings of his parents and grandparents. It appeared that the boy had, as expected, received his strange mitochondria from his mother, who shared the high level of heteroplasmy. But further analysis revealed that his mother, grandfather and two of his great aunts had inherited mitochondrial DNA from both parents.
Huang went to other laboratories, looking for families whose members were also suffering from heteroplasmy. He found two with similar patterns, where some individuals seemed to inherit mitochondrial DNA from both parents, which mothers would then pass on to their own children. And to dispel any doubt, Huang appealed to Baylor College of Medicine to take independent blood samples and sequence the mitochondrial DNA of the three families by himself. Their results confirmed those of Huang.
These results are naturally provocative, as several other attempts to show the mitochondrial DNA transmitted by both parents or from father to child have been negative. Gizmodo has contacted several external sources to comment, and will update the post as soon as we hear. It is not known how the male mitochondria would make an egg, because it destroys the mitochondria of sperm after fertilization. The new article, published in the Proceedings of the National Academies of Science, may suggest a mutation that disables the mitochondrial elimination capabilities of the egg.
This study is not likely to have huge implications for the time scales of evolution, where the maternal inheritance remains "absolutely dominant," write the authors, an exception to the rule. But Huang was more interested in what the result could mean to treat a disease.
You may know another project, in which researchers produced a baby of three parents: a donor egg, a mother's nucleus and a father's sperm. This was to prevent the mother from transmitting Leigh's syndrome. "It's a very complicated and expensive procedure," Huang told Gizmodo. Perhaps if they can find the mechanism to transfer the father's mitochondria instead of the mother's, they can offer these options without the three-parent procedure.
It sometimes seems difficult to find firm rules in biology, even with respect to the most accepted facts.[PNAS]