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GENE EDITING

The possibility of gene editing (actually physically fixing a disease-causing mutation) in an embryo or human tissue is quickly becoming a reality. Several different technologies permit gene editing, but currently the most promising is based on CRISPR/Cas9. The Cas9 enzyme is part of a rudimentary bacterial immune system that binds to a guide RNA, finds the complementary DNA sequence, and cuts it. Bacteria use the CRISPR system to defend themselves against invading viruses, but the same technology can be harnessed to make a cut almost anywhere in the human genome. Additional DNA can then be added to replace the DNA around the cut site. Any alteration in an embryo or in a germ cell (predecessor to an egg or sperm) would be passed on indefinitely in future generations. This technology is not yet highly efficient, but progress has been stunning and major advancements are inevitable.

For the purpose of preventing recessive disease, when compared with traditional PGD, this approach does not offer any benefit because if multiple embryos are formed with traditional PGD (see Chapter 1), statistically some of them will not be affected and can be implanted. Gene editing offers the possibility of creating, repairing, and implanting only one embryo, which might be more palatable to parents who consider life to begin at conception and would not consider fertilizing several eggs and discarding some. Gene editing would also enhance selection against multiple conditions simultaneously because one would not need to find the improbable embryo that was free of all the traits under consideration, but rather could select the embryo with the fewest conditions and repair the rest.

CRISPR technology could be used to treat a wide variety of diseases and is an area of breakneck research. Some approaches seek to edit cancer cells, others alter the immune system, and some insert missing genes into organs such as the liver or retina. The endless possibilities have certainly not been fully studied to date and this or related technologies are likely to stay at the forefront of biomedicine for years to come.

The most ethically concerning use of CRISPR technology would be the editing of an embryo to possess alleles or genes not present in either parent. Such technology is already available, and if or how such genetic manipulation will be regulated is difficult to predict. Society will need to come to grips with the breakdown of the immutability of the genome and place appropriate safeguards that prevent harm while permitting the treatment of disease.

MITOCHONDRIAL TRANSFER (3-PARENT BABIES)

Another recent reproductive technology offers hope to mothers with mutations in their mitochondrial DNA. All of a child's mitochondrial DNA is inherited from the mother, so a woman with a mutation in the mitochondrial DNA in her eggs could pass this disease to 100% of her biological children.

Avoiding this seemingly certain fate ...

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