Success rates for reducing the inheritance of mitochondrial diseases using assisted reproduction could be vastly improved through the application of three key changes, research indicates.
Using a finer micropipette and a more precise method significantly reduced the transfer of cytoplasm potentially containing affected mitochondria in mouse and human eggs.
The technique, reported in PLOS Biology, resulted in less than a twentieth of the mitochondrial (mt)DNA transfer to a donor human egg compared with traditional means.
Importantly, this occurred without signs of damage to chromosomes and with a high likelihood of normal early embryonic development.
“This novel protocol should reduce mitochondrial DNA carryover to the lowest level currently possible,” said researcher Qifeng Lyu, PhD, from Shanghai JiaoTong University School of Medicine.
“However, we have not been able to conclude yet that this procedure is adequately safe in the clinic for preventing the transmission of inherited mitochondrial DNA diseases,” he acknowledged.
Mitochondria are the energy powerhouses of cells and are exclusively inherited through the maternal line, coming exclusively from the egg not sperm.
Despite only carrying 37 genes, mutations in their DNA can lead to neurometabolic disorders such as Leigh syndrome and also deafness, blindness, diabetes, muscle weakness, and liver failure.
As treatment options are limited, interventions to prevent the maternal transmission of these diseases are encouraged, such as mitochondrial replacement between gametes or embryos.
One of the techniques used for this is spindle-chromosomal complex transfer (SCCT). This involves the transfer of genetic material, specifically the spindle with maternally derived chromosomes attached, from one oocyte at the metaphase (M)II stage of maturation to another that has had its nuclear material removed.
However, the researchers note there may be transferral of cytoplasm in oocytes during micromanipulation, which could result in the inclusion of affected mtDNA and a less successful technique due to genetic drift.
They therefore developed a novel strategy called maximal residue removal (MRR) for the extreme removal of transferred mtDNA along with spindles in mouse and human MII oocytes.
Firstly, they transferred sperm into the egg cell prior to removing the spindle-chromosomal complex rather than waiting until after its removal, as previous research has shown that the act of manipulation can prematurely activate the meiotic process, which is normally halted until fertilization.
After removing the spindle-chromosomal complex from the egg, it was then transferred to even finer micropipettes of 12 micrometers for mice, 10 micrometers for humans.
This squeezed further amounts of cytoplasm potentially containing mtDNA from the complex.
Lastly, the team “swung away” the carried cytoplasm by aspirating the cytoplasm/complex into a viscous chemical medium and then manipulating the pipette to further separate the complex, which was then transferred into another egg cell lacking a nucleus.
Mouse embryos underwent normal development and resulted in healthy babies, with no signs of expansion of the donor-egg mitochondrial population in tissues from the next generation.
In human studies, mitochondria carried over from the donor egg was only about four percent that of the standard protocol and the procedure occurred without apparent chromosomal damage.
The researchers conclude that “this procedure might represent a reliable therapy approach to prevent inherited mtDNA diseases.”
