Last week newspapers were abuzz with reports that the worlds first genetically modified human babies had been born. Most accounts gave only a sketchy description of what had occurred, but managed to convey a real sense of alarm. The St. Louis Post Dispatch called for federal guidelines that must have teeth to prevent free-wheeling fertility experiments that may alter humanity.
Anything that alarming is worth trying to understand. Here is what really happened.
Five years ago, Dr. Jacques Cohen, scientific director of a fertility clinic in New Jersey, set out to help an infertile woman have a child. This unfortunate woman suffered from a rare form of infertility in which her eggs are unable to carry out genetic imprinting. The DNA in an egg undergoes chemical conditioning before fertilization, so that early development may proceed properly.
This woman lacked the ability to properly condition the DNA of her eggs. As a result, her eggs can be fertilized, but the embryos fall apart, dying before they can implant in the uterus.
Dr. Cohen suspected that the womans conditioning enzymes were defective, a defect she had probably inherited. If that was indeed the case, he saw a way to help her have a child with her husband. All that was needed, he reasoned, was to introduce healthy enzymes into her eggs. The added enzymes would condition the egg DNA, which when fertilized would then be able to develop normally.
While not a simple procedure, this sort of fertility experiment is not rocket science either. Taking a very fine needle, he and his coworkers sucked a tiny bit of the interior fluid called cytoplasm from a normal human egg. They injected this bit of cytoplasm, containing a sample of the eggs enzymes, into one of the infertile womans eggs. The egg was fertilized in vitro (in a test tube) with her husbands sperm, then implanted in the infertile woman. Nine months later, she gave birth to a healthy baby.
This remarkable advance in treating infertility is called ooplasmic transfer (transfer of egg stuff). In the five years since this success, Cohens clinic has attempted the procedure for 30 infertile women. Seventeen failed to become pregnant and one became pregnant but had a miscarriage. The remaining 12 women delivered babies, with three of the women having twins. All 15 babies are doing fine. The work was privately funded, and used no government money.
Other clinics around the world, following Dr. Cohens lead, have allowed women with this disorder to have 15 additional babies.
So what is the uproar all about?
The cytoplasm of a human cell is not a simple uniform mixture. It contains a variety of tiny inclusions, sort of like fruit suspended in Jello. Among the many kinds in a human egg are some 1000 bacteria-sized objects called mitochondria.
It comes as no surprise that Dr. Cohens needle sucked up a few mitochondria in the course of ooplasmic transfer. Why has this fact created alarm in the press?
Mitochondria are relict bacteria, taken up in the distant past by the early ancestors of todays animal cells. Over more than a billion years, most of the mitochontrial genes have passed to the cells nucleus.
Not quite all of them, however. Thirteen genes remain in mitochondria, all involved in the final stages of extracting energy from food molecules.
That is what has so alarmed the Post Dispatch: the mitochondria transferred by Dr. Cohen contain DNA.
How much? Human mitochondrial DNA uses exactly 16,569 nucleotides to encode 13 genes. Thats one two-millionth of the 3.2 billion nucleotides that the nuclear DNA uses to encode some 30,000 genes. To get a sense of the minute amount of DNA in a mitochondrion, image selecting one number from Southwestern Bells Greater Saint Louis phone book, or one word from 17 copies of Websters New World Dictionary.
When examined, two of the 30 ooplasmic transfer babies proved to have received some of their mitochondria from the cytoplasm donor.
Are these two babies reason for alarm? Not in my judgement. There is no evidence that the donated mitochondrias 13 genes differ in any way from normal. Far greater genetic modification occurs when someone smokes a cigarette whose chemicals mutate their DNA. While I would like to see some laws with teeth to prevent this cigarette-driven form of gene modification, I am delighted that Dr. Cohen was able to help 30 women have much-wanted children. I dont think we need be afraid of them.
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