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Published on: July 22, 2025
Replication Errors Made During Oogenesis Lead to Detectable De Novo mtDNA Mutations in Zebrafish Oocytes with a Low
Auke B C Otten1, Alphons P M Stassen1, Michiel Adriaens2
1Department of Genetics and Cell Biology, Clinical Genomics Unit, School for Oncology and Developmental Biology, Maastricht University, 6200 MD, The Netherlands.
Abstract:
Of all pathogenic mitochondrial DNA (mtDNA) mutations in humans, ∼25% is de novo, although the occurrence in oocytes has never been directly assessed. We used next-generation sequencing to detect point mutations directly in the mtDNA of 3-15 individual mature oocytes and three somatic tissues from eight zebrafish females. Various statistical and biological filters allowed reliable detection of de novo variants with heteroplasmy ≥1.5%. In total, we detected 38 de novo base substitutions, but no insertions or deletions. These 38 de novo mutations were present in 19 of 103 mature oocytes, indicating that ∼20% of the mature oocytes carry at least one de novo mutation with heteroplasmy ≥1.5%. This frequency of de novo mutations is close to that deducted from the reported error rate of polymerase gamma, the mitochondrial replication enzyme, implying that mtDNA replication errors made during oogenesis are a likely explanation. Substantial variation in the mutation prevalence among mature oocytes can be explained by the highly variable mtDNA copy number, since we previously reported that ∼20% of the primordial germ cells have a mtDNA copy number of ≤73 and would lead to detectable mutation loads. In conclusion, replication errors made during oogenesis are an important source of de novo mtDNA base substitutions and their location and heteroplasmy level determine their significance.
Insights
Approximately 20% of mature oocytes carry de novo mitochondrial DNA (mtDNA) mutations. These mutations, likely arising from replication errors during oogenesis, contribute significantly to genetic variation.
Area of Science:
- Genetics
- Developmental Biology
- Mitochondrial Biology
Background:
- Mitochondrial DNA (mtDNA) mutations contribute to human disease, with ~25% arising de novo.
- The occurrence of de novo mutations specifically within oocytes has not been directly evaluated.
Purpose of the Study:
- To directly assess the frequency and characteristics of de novo mutations in mature zebrafish oocytes.
- To investigate the potential role of oogenesis in generating de novo mtDNA mutations.
Main Methods:
- Next-generation sequencing was employed to detect point mutations in mtDNA from individual mature oocytes and somatic tissues of zebrafish.
- Statistical and biological filters were utilized to reliably identify de novo variants with heteroplasmy levels of 1.5% or greater.
Main Results:
- Thirty-eight de novo base substitutions were detected, with no insertions or deletions observed.
- Approximately 20% of mature oocytes (19 out of 103) harbored at least one de novo mutation at ≥1.5% heteroplasmy.
- The observed mutation frequency aligns with the known error rate of polymerase gamma, suggesting replication errors during oogenesis.
Conclusions:
- Replication errors during oogenesis are a significant source of de novo mtDNA base substitutions.
- The variability in mutation prevalence among oocytes may be linked to fluctuations in mtDNA copy number.
- The significance of these de novo mutations is determined by their location and heteroplasmy level.
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