According to a recent study published in the journal Nature Genetics, new research sheds light on the scientific principle that mitochondrial DNA (mtDNA) is exclusively passed on by the mother. This collaborative study between Oregon Health and Science University and other institutions has provided valuable insights into this fundamental genetic process.
Mitochondrial DNA is a unique genetic code found in the organelle known as mitochondria, which acts as the powerhouse of every cell in our body. Scientists have long recognized that mtDNA comes exclusively from human eggs, meaning that only the mother contributes the genetic code carried by thousands of mitochondria necessary for energy production in each cell.
Previously, it was believed that paternal mtDNA was removed shortly after fertilization when a sperm fused with an egg. However, this study discovered that mature sperm not only lack intact mtDNA but also lack a protein essential for mtDNA maintenance called mitochondrial transcription factor A (TFAM). While each sperm contributes around 100 mitochondria as organelles during fertilization, they do not contain mtDNA.
The reason why sperm are not allowed to contribute mtDNA remains unclear. One theory suggests that it may be related to the fact that sperm utilize significant mitochondrial energy during their biological drive to fertilize an egg. Consequently, accumulating mutations in mtDNA could occur. On the other hand, developing eggs known as oocytes draw energy primarily from surrounding cells rather than their own mitochondria, allowing them to maintain relatively pristine mtDNA.
The absence of intact paternal mtDNA highlights the importance of maternal inheritance of mtDNA. Maternal mtDNA alone is thought to confer an evolutionary advantage by limiting the risk of disease-causing mutations in offspring. Mutations in mtDNA can lead to potentially fatal disorders affecting organs with high energy demands such as the heart, muscles, and brain.
To address known mitochondrial DNA disorders transmitted by mothers to their children, researchers have pioneered a technique called mitochondrial replacement therapy. This method involves replacing mutant mtDNA with healthy mtDNA from donor eggs through in vitro fertilization. However, clinical trials for this procedure are being conducted abroad due to restrictions imposed by the US Food and Drug Administration.
The new discovery regarding the absence of intact mitochondrial DNA in mature sperm has significant implications for human fertility and germ cell therapy. Understanding the role of TFAM during sperm maturation and fertilization could be crucial in treating certain infertility disorders and improving assisted reproductive technologies.
As a result, this study provides valuable insights into the exclusive maternal inheritance of mitochondrial DNA. The absence of intact mtDNA in mature sperm emphasizes the importance of maternal contributions to mitochondrial genetics. Further research in this area may lead to advancements in treating infertility disorders and enhancing assisted reproductive technologies.
Reference:
Lee W, Zamudio-Ochoa A, Buchel G, et al. Molecular basis of maternal inheritance of human mitochondrial DNA. Night Genet. 2023:1-8 doi: 10.1038/s41588-023-01505-9
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