Mitochondrial DNA maternally inherited due to absence in mature sperm

Although sperm cells do contain some mitochondria, scientists have found that these mitochondria do not contain intact mitochondrial DNA (mtDNA).

Mitochondria are organelles which control respiration and energy production. Mutations in mtDNA can result in severe, and even potentially fatal, disorders. Mitochondria are unique as they have their own DNA, which is known to be passed down from the mother, exclusively from egg cells. Until now, it was believed that when the sperm cell fertilises the egg, any mtDNA from the father is eliminated. However, new research has shown that even though mature sperm does contain a small number of mitochondria, they don't have intact mtDNA.

'We found that each sperm cell does bring 100 or so mitochondria as organelles when it fertilises an egg, but there is no mtDNA in them' said Professor Shoukhrat Mitalipov, director of the Centre for Embryonic Cell and Gene Therapy at Ohio Health and Science University, and co-author of the paper.

Publishing their research in Nature Genetics, not only did the scientists find that sperm cells lacked intact mtDNA, they also lacked a protein for mtDNA maintenance, known as mitochondrial transcription factor A (TFAM). This protein is essential in order to protect, maintain and transcribe mtDNA.

It is not known why sperm lack the ability to pass on mtDNA, but Professor Mitalipov hypothesises that it would contain too many mutations from the high amount of mitochondrial energy required when the sperm fertilises the egg. This process would lead to mutations accumulating in the mtDNA. In contrast, egg cells don't get their energy from their mitochondria, they use the surrounding cells, and thus don't accumulate mutations in their mtDNA.

The scientists have also shown that TFAM relocates from the mitochondria to the nucleus of sperm cells during the process of sperm cell development. This results in the elimination of mtDNA and, the authors believe, explains maternal inheritance.

'The discovery of TFAM relocalisation has important implications for the fields of human fertility and germ cell therapy.' wrote the authors in the paper. They believe that sperm that are deficient in nuclear TFAM could account for some cases of unexplained male infertility, and that sperm isoform of TFAM could serve as a new biomarker of male infertility.

'Understanding the role of TFAM during sperm maturation and its function during fertilisation may hold keys to our ability to treat certain infertility disorders and increase the efficiency of assisted reproductive technologies,' said corresponding author Professor Dmitry Temiakov, from Thomas Jefferson University in Philadelphia.