By David N. Leff
Preeclampsia is a placental disorder of human pregnancies, best abated by additional pregnancies.
It affects about 5 percent of young mothers having their first baby, after the 20th week of pregnancy. Preeclampsia ¿ etiology unknown ¿ is marked by hypertension, proteinuria and edema. It usually responds to bed rest plus mild salts, but if untreated suddenly progresses to eclampsia. This more serious sequel features convulsions, coma and maternal death unless rapidly medicated. Therapy as a rule includes delivery of the fetus if it is viably mature.
¿Infertility, recurrent pregnancy loss, preeclampsia and low birth weight,¿ observed reproductive biologist Thaddeus Golos, ¿are points on a spectrum of placental development disorders.¿ An associate professor of obstetrics/gynecology at the University of Wisconsin in Madison, Golos conducts a scientific program at the affiliated Wisconsin Regional Primate Research Center.
He is senior author of a paper in the current Proceedings of the National Academy of Sciences (PNAS), dated Sept. 11, 2001. Its title: ¿Rhesus monkey placental transgene expression after lentiviral gene transfer into preimplantation embryos.¿
¿Our overall finding,¿ Golos told BioWorld Today, ¿is that gene transfer to primate embryos can allow modification of the genetic program of the primate placenta. This means,¿ he pointed out, ¿that there are new opportunities for scientists, and ultimately clinicians, to learn how specific genes contribute to, or are related to, issues of fertility and pregnancy in maternal and fetal health. That¿s my research focus, and the rhesus program in my primate lab.
¿The novel element in this PNAS paper,¿ Golos added, ¿is that for the first time a gene that has been transferred into a primate embryo has been shown to be functional throughout development to a successful live birth. It also demonstrates that successful pregnancy is possible when genes foreign to the mother are engineered into the embryos of nonhuman primates.¿
Vaulting 25-Million-Year Hiatus
¿The people who study molecular clocks,¿ he observed, ¿say that something like 25 million years separate us Homo sapiens from Old World monkeys, including the rhesus Macaca mulatta. However, across those 25 million years ¿ particularly in terms of early embryonic development, placental structure and function, and the physiology of pregnancy ¿ there¿s been remarkable conservation in a number of areas of maternal and fetal interaction.
¿So the take-home message,¿ he continued, ¿is that, despite the evolutionary distance, the primate reproductive system and pregnancy are very similar to ours. And that makes the nonhuman primate a unique and important nonhuman model for understanding human pregnancy.¿
Golos noted that the brownish rhesus monkey stands about 2 feet tall, weighs in at 7 to 8 kilograms, has a gestational term of 165 days, and may live to 40 years of age in a research facility. ¿That¿s ballpark,¿ he pointed out, ¿it¿s probably far beyond what any rhesus monkey in the wild would survive to.¿
As they report in PNAS, Golos and his co-authors successfully delivered a transgene into two fertilized rhesus macaque embryos. Their feat involved a viral vector and the jellyfish-derived, gene-expressed green fluorescent protein (GFP). This has the property of emitting fluorescence under certain wavelengths of ultraviolet light. ¿We have been using it,¿ he said, ¿to look for success in transfer of living embryos. We flash a few milliseconds of UV light, capture an image and see that the embryo has or has not begun to express the green-glowing transgene.
¿The human tissue-culture cells had received genetic material of the HIV vector,¿ Golos recounted, ¿but broken into separate components. So that there was no opportunity for the actual lentivirus, or infectious structure, to be assembled. The cells were able to produce some components of the HIV virus, but they were separated from the actual genetic material that ultimately was packed into the vector particle.
¿That particle,¿ he went on, ¿which the cell releases, had only about 5 percent of the HIV genome within it, but stripped of the ability to produce any HIV culture. It provided a signal for assembling the viral particle, and then allowed the GFP transgene and a promoter to be integrated into the host cells¿ DNA. The HIV virus itself targets specific cells in its host¿s immune system.¿
From HIV Vector To Surrogate Mom
¿We then injected the gene therapy vector into rhesus monkey embryos that had been produced by in vitro fertilization,¿ Golos related. ¿That¿s essentially identical to the in vitro fertilization technique used in human fertility clinics. When the resulting embryos had developed in vitro to an appropriate stage, we injected the vector into the embryo. This we then transferred to a surrogate mother.
¿Five months after pregnancy was established,¿ Golos went on, ¿the neonatal babies were born, and we looked for evidence of the transgene that had been delivered by the vector. We found a great deal of the green-glowing transgenic protein produced in the placenta, but as of now we don¿t have any evidence that the transgene was transferred to the offspring. However, those progeny are healthy ¿ perfectly fine.¿ The outcome of this birthing experiment was delivery of the two male rhesus monkey neonates, one from each surrogate Mom.
¿The conceptual therapeutic goal would be far in the distance,¿ Golos predicted. ¿Perhaps there would some day be a way to use gene therapy approaches to correct pregnancies at risk. That is to say, to modify placental gene expression, maybe with a gene therapy vector, to rescue or prevent the loss of a pregnancy at risk, and then promote the delivery of a healthy infant. The attractiveness of focusing on the placenta,¿ he allowed, ¿is that in fact this can be done without concerns about modifying the baby¿s genetic components. The placenta of course is an ephemeral organ, and after nine months of pregnancy, your baby no longer needs it.
¿Treating the placenta medically is something that¿s essentially not possible at this point in time,¿ Golos pointed out. ¿But biologists and clinicians in many areas of the science are now thinking about ways they can use genetic approaches and gene therapy to correct defective organs, perhaps even cell-based therapy with all this interest in embryonic stem cells. Although certainly pregnancy and fetal growth are areas that warrant caution,¿ he concluded, ¿the placenta doesn¿t have any inherent reason why it should not be considered a potential target someday for gene therapy and correction of these defects.¿