Mitochondria are biological dynamos. Damage to genetic material in these cellular powerhouses does more than sap energy—it can cause neurological disorders and derail central body functions such as motor control and sight. The mutations that cause the damage are rare and passed down exclusively from mother to child. But their catastrophic effects can be difficult to detect and prevent when scrutinizing an embryo in the laboratory because the genetic abnormalities are often inconsistent across cells. One cell may contain a hundred copies of bad mitochondria whereas the next contains none at all.
Scientists have not yet come up with an effective cure that would fix the damaged DNA and enable affected mothers to have disease-free offspring. Now, researchers are reporting the first success finding and removing such mutations in mice and in human cells via a controversial new gene-editing technique.
The approach taps enzymes called transcription activator-like effector nucleases, or TALENs, that work in pairs to search for and cut away specific damaged mitochondrial DNA sequences in maternal eggs while leaving aside other genetic material. “They work like scissors and go out and cut out the problematic mitochondria,” says Juan Carlos Izpisua Belmonte, a developmental biologist at the Salk Institute for Biological Studies and senior author of a new study using the method, published in Cell.
Previous research on TALENs has shown promise modifying genes in frogs, rats and pigs as well as human somatic and pluripotent stem cells. Those successes, alongside the method’s relative simplicity, have spurred interest in using it to overcome mitochondrial mutations that would be passed down from mother to offspring. The new study, however, comes even as a cadre of scientists has called for a moratorium on genome editing in human embryos because of the potential long-term effect on germ line cells.
Germ line modification refers to changes that are made to the DNA found in the nucleus of a sperm or egg before fertilization or to the nuclear DNA of undifferentiated cells in early embryos that could then be passed to future generations. Changing a mitochondrion (which contains 37 genes) is not the same as altering a sperm or an egg, of course. Nevertheless, because mitochondrial changes are passed from mother to offspring and could potentially affect multiple generations, they can get mired in similar controversies.
In the latest paper on TALENs researchers apparently prevented the transmission of mitochondrial defects in two generations of mice that would otherwise carry disease-causing mutations. The mice appear to be asymptomatic and healthy, and acted normally in behavioral tests. Moreover, the offspring only have low levels of inherited mitochondrial mutations, suggesting that future generations may not have many problematic defects either. Separately, the research team also managed to reduce the number of disease-linked mitochondrial mutations in human cells that were put into mice eggs.
The technique hinged on first auditioning hundreds of different nucleases until the team found the right combination that would uniquely search for and snip away certain damaged mitochondrial DNA. Because this gene-editing approach does not catch every single mutated mitochondrion it had to catch enough of them to prevent disease. But for this work to move forward in humans it would need to prove it only affects mutated mitochondrial DNA without harming other parts of the genome in both the child resulting from the embryo and future generations of kids.
The procedure is attractive for its relative simplicity: The nucleases would only need to be injected into the mother’s egg once, potentially at the same time that sperm are introduced in the laboratory during a standard in vitro fertilization procedure, Belmonte says. But even if this approach proves safe in humans and receives regulatory approval, that still does not mean an average fertility clinic would be equipped to do it, says Evan Snyder, director of the Center for Stem Cell Biology and Regenerative Medicine at the Sanford–Burnham Medical Research Institute. Even though the actual injection may not be hard, “this is a pretty heavily sophisticated intervention that could probably only be done in a few academic centers,” says Snyder, who was not involved in the recent study.
Right now Belmonte’s team is doing research with human embryos and plans to conduct genome analysis in the lab to begin to answer initial safety questions. If changes are occurring elsewhere on the genome or the cells are not healthy (heart cells are not demonstrating good contractions, for example) those would be early red flags.
Yet editing the genome of embryos—even if they are not implanted into women and thus will not become fetuses—has concerned some researchers who have publicly called for a pause on such work until more specific discussions take place about its potential ramifications. Recently a group of five researchers that use genome-editing technologies for research that would not require altering sperm or eggs called for a moratorium on gene-editing research on human embryos in a comment published March 12 in Nature. “Such research could be exploited for nontherapeutic modifications. We are concerned that a public outcry about such an ethical breach could hinder a promising area of therapeutic development,” they cautioned. (Scientific American is part of Nature Publishing Group.)
“While there are important differences between germ line engineering and mitochondrial engineering my goal isn’t to parse those differences. Our goal is to call for a meeting so we can have this broad discussion,” says Edward Lanphier, president and CEO of Sangamo BioSciences, Inc., and first author of the Nature comment. “The question around genome editing around mitochondria adds urgency to the discussion.”
“I personally don’t have difficulty with very specific and accepted modifications of a very specific disease-causing problem,” Snyder says, but a scientific evaluative body should be set up to assess each proposed germ line modification on a case-by-case, or gene-by-gene, basis. The model would be reminiscent of how recombinant DNA research was assessed in the 1970s. The process would likely be long, he says, but without it scientists may not get a full picture of potential unintended effects from such procedures on areas like life span, cognitive function and exercise tolerance.
Short of using TALENs or a similar genome-editing technology to prevent the transmission of mutated mitochondrial DNA, scientists are currently considering one other option to help moms with mitochondrial mutations birth disease-free, biologically related offspring: mitochondrial replacement therapy. The method, nicknamed “three parent babies” is also controversial and requires another woman to donate an egg with mutation-free mitochondrial DNA. The nucleus of that egg, however, is replaced with the mom’s nuclear DNA. The egg is inseminated with the father’s sperm in a laboratory setting and the resulting embryo is transferred into the prospective mother’s uterus.
Mitochondrial replacement therapy has not been cleared for use in the U.S. or U.K. and there are only a few scientists in the world currently capable of performing it. But one hurdle for both TALENs and mitochondrial replacement therapy is similar: the need to study the genome and look at multiple generations of test animals and how their organs function to make sure these methods do not cause any unforeseen problems.