Towards clinical application of pronuclear transfer to prevent mitochondrial DNA disease

Abstract

Preclinical evaluation and optimization of mitochondrial replacement therapy reveals that a modified form of pronuclear transfer is likely to give rise to normal pregnancies with a reduced risk of mitochondrial DNA disease, but may need further modification to eradicate the disease in all cases. These authors report a preclinical evaluation and optimization of mitochondria replacement therapy using pronuclear transfer. The influence of different parameters and manipulations is evaluated in normally fertilized human embryos, and the improved protocol is shown to produce blastocysts with a low level of mitochondrial carryover. The results highlight the need for continued optimization and monitoring of mitochondria replacement therapy techniques for future human application. Mitochondrial DNA (mtDNA) mutations are maternally inherited and are associated with a broad range of debilitating and fatal diseases1. Reproductive technologies designed to uncouple the inheritance of mtDNA from nuclear DNA may enable affected women to have a genetically related child with a greatly reduced risk of mtDNA disease. Here we report the first preclinical studies on pronuclear transplantation (PNT). Surprisingly, techniques used in proof-of-concept studies involving abnormally fertilized human zygotes2 were not well tolerated by normally fertilized zygotes. We have therefore developed an alternative approach based on transplanting pronuclei shortly after completion of meiosis rather than shortly before the first mitotic division. This promotes efficient development to the blastocyst stage with no detectable effect on aneuploidy or gene expression. After optimization, mtDNA carryover was reduced to <2% in the majority (79%) of PNT blastocysts. The importance of reducing carryover to the lowest possible levels is highlighted by a progressive increase in heteroplasmy in a stem cell line derived from a PNT blastocyst with 4% mtDNA carryover. We conclude that PNT has the potential to reduce the risk of mtDNA disease, but it may not guarantee prevention.