First human pregnancy from nuclear transfer: lasted until 29 weeks' gestation.
His announcement, made at the annual meeting of the American Society for Reproductive Medicine, immediately generated controversy, in part because the procedure closely resembles the technique used for cloning.
"Even though it involves a similar technique, this is nothing close to cloning," said Dr. Zhang, a fellow in reproductive medicine at New York University.
The work, pioneered by Dr. Zhang and Dr. Jamie Grifo, head of the division of reproductive endocrinology and infertility at NYU, was the culmination of 6 years of research, first in New York and then (with the local ethics board's approval) at Sun Yat-Sen University of Medical Sciences in Guangzhou, China.
The pregnancy, in a 30-year-old Chinese woman, did not continue past 29 weeks' gestation. Fetal reduction from a triplet to a twin pregnancy was performed 33 days after embryo transfer. The second fetus was delivered at 24 weeks and died of respiratory distress after premature rupture of membranes. The third fetus was delivered at 29 weeks after intrauterine fetal demise caused by cord prolapse.
Dr. Zhang said nuclear transfer holds therapeutic potential for enabling women with mitochondrial disorders and certain oocyte defects to have children.
"Current genetic tests and counseling cannot help these patients, but oocyte genetic diagnosis and, potentially, nuclear transfer could," he said.
In their presentation, the researchers reported treating five women with nuclear transfer. The one patient who got pregnant had gone through two failed in-vitro fertilization cycles characterized by embryo arrest at the two-cell stage and "probably had some kind of cytoplasmic defect," Dr. Zhang said in an interview.
The procedure involved fertilization of both the patient's oocytes and the donor's oocytes using intracytoplasmic injection of the husband's sperm. The resulting pronuclei (karyoplasts) in the donor oocytes were microsurgically removed and discarded, and the pronuclei from the patient's oocytes were transferred into the donor's enucleated cells (cytoplasts).
Electrofusion of these oocytes, made of the patient's karyoplasts and the donor's cytoplasts, resulted in five "reconstructed" zygotes that were transferred into the patient's uterus.
Dr. Grifo stressed that the technique is different from cloning in that cloning involves the transfer of an adult cell without sexual reproduction, and the nucleus of that cell needs to be reprogrammed to reinitiate totipotentiality. Experts have speculated that it is this reprogramming that may be at the root of the health problems reported in cloned animals. But in the technique reported by Dr. Zhang and Dr. Grifo, the pronuclei of the reconstructed zygote are already developing naturally and do not need reprogramming.
Concerns about this technique have centered around the potential for heteroplasmy--mixing more than one source of egg-derived mitochondria (sperm mitochondria disappear shortly after fertilization), explained Gerald Schatten, Ph.D.; professor of obstetrics, gynecology and reproductive sciences and of cell biology and physiology at the University of Pittsburgh.
Nuclear transfer involves transferring not just the nucleus of a cell, but also the centrosome and some cytoplasm. And within the cytoplasm are membranes and mitochondria, thus the potential for mixing patient and donor mitochondria, he said.
"There are human diseases caused by inherited or de novo mitochondrial mutations with tragic outcomes, including certain blindnesses, muscle wasting, and mental retardation.... I remain concerned about having children with mitochondria coming from more than one source, particularly if there are mismatches between the nuclear source and the mitochondrial source," Dr. Schatten told this newspaper.
"We also had concerns, but we listen to data," Dr. Grifo said. "Fifty mice [that were created using the same procedure] have been born over three generations in our lab, and they all appear healthy and have normal fertility. In addition, 30 humans have been born from a very similar technique called cytoplasmic transfer and they are also apparently healthy. [Dr. Schatten] is allowed to be concerned about the possibility of mitochondrial heteroplasmy, but we did not see it in the three fetuses from this procedure," Dr. Grifo said.
Indeed, Dr. Zhang said "extensive molecular-biological investigations to evaluate the mitochondrial DNA makeup of the human fetuses in this case could detect no carryover of mitochondrial DNA from the patient into the donor cytoplast."
Additionally, ultrasound and chromosomal results on the fetuses, as well as one autopsy, were all normal, he said.
In order to perform nuclear transfer in this country, U.S. researchers are required to have the Food and Drug Administration's Investigational New Drug approval, which the NYU team does not have.
The team has been criticized by some experts for bypassing the FDA and working in China--but this points to "the perils of prohibiting, rather than funding and regulating, fertility research," commented Sean Tipton, who is a spokesman for the American Society for Reproductive Medicine.
China is in the process of setting up new reproductive technology guidelines that are expected to be even more restrictive in certain aspects than the United States', said Dr. Zhang.
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|Publication:||OB GYN News|
|Date:||Dec 15, 2003|
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