Maternal DNA can be inherited from Dad, too

By Linda Carroll

Tiny bits of DNA that scientists thought could only come from a child's mother may sometimes also come from the father, a new study shows.

Usually, mammals inherit mitochondria--small structures known as the "powerhouses" of the cell--from their mothers, Marianne Schwartz, laboratory director of the department of clinical genetics at the University Hospital Rigshospitalet in Copenhagen, Denmark, said in an interview with Reuters Health.

But Schwartz and her colleague found a man who had inherited some of his mitochondria from his father, according to a report published in the New England Journal of Medicine.

The 28-year-old man came into her clinic complaining of severe fatigue after the slightest exercise. Although his lungs and heart were completely normal, the man had never been able to run more than a few steps without tiring, Schwartz said. And everyone in his immediate family was completely healthy.

So the researchers decided to take a closer look at the man's muscles. When they tested a biopsy of muscle tissue, the researchers discovered that the man's muscles had problems taking in oxygen.

This led them to examine the mitochondria, because defective mitochondria can lead to muscles that tire easily, Schwartz said.

When Schwartz and her colleague examined the genetic make-up of the man's muscle mitochondria, they found the reason for the man's exhaustion: a tiny piece of DNA was missing from an important gene.

But there was also a surprise. The muscle sample contained mitochondrial DNA that was from his father, while his blood contained mitochondrial DNA from his mother.

"Even with very sensitive methods, paternal mitochondrial DNA has never been detected in man before," Schwartz said. "There are many examples of family pedigrees that follow mitochondrial diseases through the maternal lines."

Further, Schwartz said, scientists have noticed time and again that even when sperm are directly injected into a woman's egg, in a fertility treatment known as intracytoplasmic sperm injection (ICSI), the paternal mitochondria seem to disappear after only a few cell divisions in the growing embryo.

In an editorial accompanying Schwartz's study, Dr. R. Sanders Williams, a scientist at Duke University Medical Center in Durham, NC, called the new findings "remarkable and unanticipated."

Schwartz still thinks that it would be unusual to find bits of paternal mitochondrial DNA in the general public. "Our theory is that paternal mitochondria will only survive if they harbor a mutation in their DNA," she said.

SOURCE: The New England Journal of Medicine 2002;347:576-579.