The Genscher Crispr / Cas9 is considered one of the most promising new tools of genetic engineering. Because of this procedure, simpler and more targeted interventions in the genetic material are possible than before. Using three simple components, the general-purpose tool finds any desired DNA site. Even point mutations can accurately correct the gene scissors - with significantly fewer undesirable side effects such as the insertion of gene pieces outside the target region. Researchers have already successfully used Crispr / Cas9 to repair the genetic defect responsible for sickle cell anemia in human blood cells, as well as correcting an Alzheimer's disease-causing mutation and the gene defect of Duchenne muscular dystrophy. However: So far, these gene repair usually took place either in animals or on adult human cells.
Intervention in the germline
However, gene repair on fertilized oocytes or embryos is banned in many countries, including Germany. The reason for this: If something is changed in the genetic material of these cells, then this affects not only all the cells and tissues of this person, but also all his offspring. Because of the high risks, but also the possibility of abuse, for example, for "designer babies" such keiming interventions are ethically highly controversial. But on the other hand, such sustained interventions could also have a beneficial effect: one could prevent genetic disorders, which are caused by the defect in only one dominant gene, from generation to generation in the affected families. Among other things, interventions in the germline in some countries such as China and under certain conditions in the United States are allowed. In China, researchers carried out a first gene repair on human embryos in 2016, albeit with an extremely low success rate.
An international research team led by Hong Ma at Oregon Health & Science University, Portland, has been much more successful. For the first time, they corrected the genetic defect in human embryos, which causes the so-called hypertrophic cardiomyopathy (HCM). This dominant inherited heart disease affects one in 500 people and leads to cardiac arrhythmia at an early age and in extreme cases to sudden heart failure and death. It is not curable, only the symptoms can be treated. In about 40 percent of cases, the hereditary disease is caused by a mutation in the MYBPC3 gene on the eleventh chromosome. It is sufficient if the person in question inherits a defective gene copy from one of his parents in order to trigger the disease. display
Sperm plus gene scissors
Looking for a way to cure this hereditary heart disease using gene therapy, Ma and his colleagues start at the earliest stage of life: they combine oocytes from healthy donors with the sperm from donors, each containing one of two copies of the MYBPC3 gene was defective. Together with the sperm, they injected the gene scissors Crispr / Cas9 into the egg cell, as well as some copies of the correct, non-mutated DNA code of the MYBPC3 gene - as a kind of repair template. Thus, the gene scissors was already active and present in the phase in which the genetic material of sperm and egg merge together. "In previous experiments, in which the gene repair constructs were first introduced into the embryos in the single-cell stage, descendants with mosaic cells, " the researchers explain. Some cells of the embryo carry the repaired gene, while others do not.
The new gene therapy has another peculiarity: In contrast to most common gene repair, the gene scissors should cut out only the defective four letter letters in the chromosome of the sperm, but not actively insert a correct version. Instead, the researchers left this to the cell's DNA repair mechanisms. These are inherently designed to mend defects in the DNA. To the surprise of the scientists, this was much more efficient and different from what they had expected. Because the freshly fertilized egg cell simply used its own intact gene copy as a model for the newly inserted in the DNA strand of the sperm genome gene letters.
Surprisingly effective and accurate
The result: The gene scissors had excised accurately the defective DNA sequence in all embryos their hit rate was 100 percent, as the researchers report. The cell's own repair mechanisms had then repaired the gene defect correctly in 42 of the 58 embryos, which corresponds to 72.2 percent. Without gene treatment, half of the embryos would have gotten the defective gene. DNA analysis also showed that there were no further, undesirable gene changes in these cells. In the remaining embryos, the DNA strings were also repaired, but because the cell used a different repair mechanism, it left unwanted implications or omissions at the repair site.
"Our results thus demonstrate the great potential of gene therapy in the embryo, " says co-author Juan Carlos Izpisua Belmonte of the Salk Institute for Biological Studies. "Thanks to advances in genetic engineering and stem cell technology, we can finally begin to address pathogenic mutations that affect millions of people." However, despite all the optimism, researchers also stress that there is still a likelihood of such therapies being applied to patients way is. For the safety of the method must first be checked in further studies with other mutations. "Gene therapy is still in its infancy, although this tentative effort has proven safe and effective, " said Belmonte. "Nevertheless, it is crucial that we continue with the greatest caution, paying close attention to ethical considerations."
- Hong Ma (Oregon Health and Science University, Portland) et al, Nature, doi: 10.1038 / nature23305