An evolutionary characteristic of humans and most animals is that they inherit this mitochondrial DNA exclusively from the mother, although the father’s sperm have mitochondria. Then why don’t they transfer mitochondrial DNA?
The answer is that they don’t have it, says the study published in Natural genetics Researchers from the Barcelona Biomedical Research Institute (IIBB-CSIC-IDIBAPS) and various centers in the United States.
After several years of research, the authors were able to demonstrate that sperm lack this type of DNA and, moreover, their mitochondria do not have a mitochondrial transcription factor A (either TFAMin English Mitochondrial transcription factor A), which is essential for the replication of human mitochondrial DNA.
“In many species, including humans, sperm mitochondria are introduced into the egg during fertilization, so one of the existing hypotheses was that mitochondrial DNA reached the egg but was eliminated in the fertilization process,” explains he -author Ramon TrullasResearcher at IIBB and at the Network Biomedical Research Center for Neurodegenerative Diseases (CIBERNED).
Spanish technique and Oregon examples
To find out whether sperm have mitochondrial DNA, scientists at the Barcelona Biomedical Research Institute have developed a technique digital PCR, more precise than traditional PCR and allows quantification of the absolute number of DNA in different cell types in the same sample. The analyzes were carried out using samples from clinics in Oregon (USA).
The results show that sperm do not have a single complete molecule of mitochondrial DNA and that the remaining mitochondrial DNA Probably not even sperm will be found.
“Each sperm contains between 50 and 70 mitochondria, which corresponds to less than 0.01 molecules of mitochondrial DNA. Although this value is extremely low, it can probably be explained by a few cells contaminating the samples (particularly leukocytes),” the authors explain in the paper.
The team found that the absence of DNA in sperm mitochondria is due to a process of relocalization of the transcription factor TFAM, a molecule that enters mitochondria to replicate mitochondrial DNA.
During the SpermatogenesisThe sperm modifies the TFAM localization signal and does not allow it to enter the mitochondria, but directs it to the sperm cell nucleus, preventing the replication of mitochondrial DNA.
“Our work shows that modification of TFAM during spermatogenesis leads to the elimination of mitochondrial DNA and explains its maternal inheritance. “A fascinating process, a product of evolution, that prevents the inheritance of paternal DNA,” he adds. Trullas.
Our work shows that modification of TFAM during spermatogenesis leads to the elimination of mitochondrial DNA and explains its maternal inheritance
This discovery of TFAm relocalization has important implications for the field human fertility and that Germ cell therapyas this could explain some cases of oligospermia and oligoasthenospermia, a condition that results in low sperm count and motility.
In fact, high levels of mitochondrial DNA have been found in the sperm of infertile men with severe oligoasthenospermia, the authors recall, mainly from Thomas Jefferson University and Oregon Health & Science University in the United States.
Reference:
William Lee et al. “Molecular basis for maternal inheritance of human mitochondrial DNA”. Natural genetics2023.