Segregation of mtDNA Throughout Human Embryofetal Development: m.3243A > G as a Model System

Mitochondrial DNA (mtDNA) mutations cause a wide range of serious diseases with high transmission risk and maternal inheritance. Tissue heterogeneity of the heteroplasmy rate (“mutant load”) accounts for the wide phenotypic spectrum observed in carriers. Owing to the absence of therapy, couples at risk to transmit such disorders commonly ask for prenatal (PND) or preimplantation diagnosis (PGD). The lack of data regarding heteroplasmy distribution throughout intrauterine development, however, hampers the implementation of such procedures. We tracked the segregation of the m.3243A > G mutation (MT-TL1 gene) responsible for the MELAS syndrome in the developing embryo/fetus, using tissues and cells from eight carrier females, their 38 embryos and 12 fetuses. Mutant mtDNA segregation was found to be governed by random genetic drift, during oogenesis and somatic tissue development. The size of the bottleneck operating for m.3243A > G during oogenesis was shown to be individual-dependent. Comparison with data we achieved for the m.8993T > G mutation (MT-ATP6 gene), responsible for the NARP/Leigh syndrome, indicates that these mutations differentially influence mtDNA segregation during oogenesis, while their impact is similar in developing somatic tissues. These data have major consequences for PND and PGD procedures in mtDNA inherited disorders. Hum Mutat 32:116–125, 2011. © 2010 Wiley-Liss, Inc

Mutations of the SCO1 Gene in Mitochondrial Cytochrome c Oxidase Deficiency with Neonatal-Onset Hepatic Failure and Encephalopathy

Cytochrome c oxidase (COX) catalyzes both electron transfer from cytochrome c to molecular oxygen and the concomitant vectorial proton pumping across the inner mitochondrial membrane. Studying a large family with multiple cases of neonatal ketoacidotic comas and isolated COX deficiency, we have mapped the disease locus to chromosome 17p13.1, in a region encompassing two candidate genes involved in COX assembly—namely, SCO1 and COX10. Mutation screening revealed compound heterozygosity for SCO1 gene mutations in the patients. The mutated allele, inherited from the father, harbored a 2-bp frameshift deletion (ΔGA; nt 363–364) resulting in both a premature stop codon and a highly unstable mRNA. The maternally inherited mutation (C520T) changed a highly conserved proline into a leucine in the protein (P174L). This proline, adjacent to the CxxxC copper-binding domain of SCO1, is likely to play a crucial role in the tridimentional structure of the domain. Interestingly, the clinical presentation of SCO1-deficient patients markedly differs from that of patients harboring mutations in other COX assembly and/or maturation genes

Poor Correlations in the Levels of Pathogenic Mitochondrial DNA Mutations in Polar Bodies versus Oocytes and Blastomeres in Humans

Because the mtDNA amount remains stable in the early embryo until uterine implantation, early human development is completely dependent on the mtDNA pool of the mature oocyte. Both quantitative and qualitative mtDNA defects therefore may negatively impact oocyte competence or early embryonic development. However, nothing is known about segregation of mutant and wild-type mtDNA molecules during human meiosis. To investigate this point, we compared the mutant levels in 51 first polar bodies (PBs) and their counterpart (oocytes, blastomeres, or whole embryos), at risk of having (1) the “MELAS” m.3243A>G mutation in MT-TL1 (n = 30), (2) the “MERRF” m.8344A>G mutation in MT-TK (n = 15), and (3) the m.9185T>G mutation located in MT-ATP6 (n = 6). Seven out of 51 of the PBs were mutation free and had homoplasmic wild-type counterparts. In the heteroplasmic PBs, measurement of the mutant load was a rough estimate of the counterpart mutation level (R2 = 0.52), and high mutant-load differentials between the two populations were occasionally observed (ranging from –34% to +34%). The mutant-load differentials between the PB and its counterpart were higher in highly mutated PBs, suggestive of a selection process acting against highly mutated cells during gametogenesis or early embryonic development. Finally, individual discrepancies in mutant loads between PBs and their counterparts make PB-based preconception diagnosis unreliable for the prevention of mtDNA disorder transmission. Such differences were not observed in animal models, and they emphasize the need to conduct thorough studies on mtDNA segregation in humans