Towards germline gene therapy of inherited mitochondrial diseases

Mutations in mitochondrial DNA (mtDNA) are associated with serious human diseases and inherited from mother's eggs. Here we investigated the feasibility of mtDNA replacement in human oocytes by spindle transfer (ST). Of 106 human oocytes donated for research, 65 were subjected to reciprocal ST and 33 served as controls. Fertilization rate in ST oocytes (73%) was similar to controls (75%). However, a significant portion of ST zygotes (52%) displayed abnormal fertilization as determined by irregular number of pronuclei. Among normally fertilized ST zygotes, blastocyst development (62%) and embryonic stem cell (ESC) isolation (38%) rates were comparable to controls. All ESC lines derived from ST zygotes displayed normal euploid karyotypes and contained exclusively donor mtDNA. The mtDNA can be efficiently replaced in human oocytes. Although some ST oocytes displayed abnormal fertilization, remaining embryos were capable of developing to blastocysts and producing ESCs similar to controls.