Accordingly, the committee recommends that any initial MRT clinical investigations focus on minimizing the future child’s exposure to risk while ascertaining the safety and efficacy of the techniques. The recommended restrictions and conditions for initial clinical investigations include

• limiting clinical investigations to women who are otherwise at risk of transmitting a serious mtDNA disease, where the mutation’s pathogenicity is undisputed, and the clinical presentation of the disease is predicted to be severe, as characterized by early mortality or substantial impairment of basic function; and
• transferring only male embryos for gestation to avoid introducing heritable genetic modification during initial clinical investigations.

Following successful initial investigations of MRT in males, the committee recommends that FDA could consider extending MRT research to include the transfer of female embryos if clear evidence of safety and efficacy from male cohorts, using identical MRT procedures, were available, regardless of how long it took to collect this evidence; preclinical research in animals had shown evidence of intergenerational safety and efficacy; and FDA’s decisions were consistent with the outcomes of public and scientific deliberations to establish a shared framework concerning the acceptability of and moral limits on heritable genetic modification.

Earlier this month, a study published in Nature by Shoukhrat Mitalipov, head of the Center for Embryonic Cell and Gene Therapy at the Oregon Health and Science University in Portland, suggested that in roughly 15 percent of cases, the mitochondrial replacement could fail and allow fatal defects to return, or even increase a child’s vulnerability to new ailments.

The findings confirmed the suspicions of many researchers, and the conclusions drawn by Mitalipov and his team were unequivocal: The potential for conflicts between transplanted and original mitochondrial genomes is real, and more sophisticated matching of donor and recipient eggs — pairing mothers whose mitochondria share genetic similarities, for example — is needed to avoid potential tragedies.

“This study shows the potential as well as the risks of gene therapy in the germline,” Mitalipov says. This is especially true of mitochondria, because its genomes are so different than the genomes in the nucleus of cells. Slight variations between mitochondrial genomes, he adds, “turn out to matter a great deal.”