Mitochondria are small organelles present within every nucleated mammalian cell. They are the principal source of intracellular energy, in the form of adenosine triphosphate (ATP), which is generated by oxidative phosphorylation (OXPHOS). Mitochondria contain their own DNA- mtDNA - which codes for 13 essential components of the OXPHOS system, along with 22 RNAs required for intra-mitochondrial protein synthesis. In mammals, mtDNA is generally considered to be exclusively transmitted down the maternal line. This important feature has underpinned the use of mtDNA in population genetic studies of human migration and disease, along with widespread forensic use. However, several strands of evidence suggest that this might not actually be the case.

Extensive paternal transmission of mtDNA has been observed in other species, including the marine mussel (Mytillus edulis), and rarely in the fruit fly (Drosophila melanogaster), Lepidopteran insects and the honey bee (Apis mellifera). In mammals, the "leakage" of paternal mtDNA during transmission has only been observed in atypical situations such as inter-species breeding in mice (Mus musculus), or in vitro embryo manipulation in cattle (Bos taurus), or in abnormal embryos in humans. However, the recent description of a paternally-derived 2 base pair pathogenic deletion in the mtDNA ND2 (MTDN2) in a 28-year-old man with a mitochondrial myopathy, coupled with evidence of paternally transmitted mtDNA in healthy sheep (Ovis aries), questions the accepted dogma of exclusive maternal transmission in mammals.

In humans, the debate hinges on the analysis of mtDNA sequences at the population level. By studying partial or complete mtDNA sequences from individuals across the globe, some have argued that the co-occurrence of phylogenetically unrelated genetic variants indicates inter-molecular recombination between paternal and maternal mitochondrial genomes, but others have argued that the high mtDNA mutation rate confounds this analysis through the generation of homoplasy which can reach ~20%. The issue is unlikely to be resolved without direct experimental evidence showing paternal transmission of mtDNA in human pedigrees.

We therefore set out to study the potential transmission of paternal mtDNA in 10 trios where the mother and the father were known to have different mtDNA signatures (haplogroup backgrounds). Using sensitive PCR/RFLP-based techniques, our pilot work has shown that ~1 in 10,000 mtDNAs within the child is likely to be of paternal origin in ~50% of cases. We aim to confirm these findings in a larger cohort of trios using a different methodological approach before we consider published what would be potentially radical findings.

We are therefore seeking access to small aliquots of anonymised DNA from 100 ALSPAC trios. Using existing GWAS data on the mothers, we will be able to identify trios where the mother and the father have a different mtDNA haplotype, and thus study the child's DNA to determine whether any paternal mtDNA has "leaked through".

We will use ultra high depth next generation sequencing (IlluminaHiSeq 2000) to carry out this experiment, allowing the detection of 1 in 100,000 mtDNA molecules. We have already established the methodology to do this for mtDNA.