Researchers in New York have succeeded where the disgraced South Korean Hwang Woo-suk failed. They have successfully derived the world’s first stem cells from a cloned human embryo. But, notably, the new stem cells are not suitable for therapeutic use because they contain three sets of chromosomes, instead of the regular two.
In work published today in Nature, a team led by Dieter Egli at the New York Stem Cell Foundation took skin cells from healthy and diabetic volunteers and inserted the cells’ nuclei into unfertilized human eggs. Within a few days cloned embryos developed allowing researchers to pluck out and propagate stem cells. These cells displayed all the hallmarks of embryonic stem (ES) cells, including characteristic gene activity and the ability to differentiate into all three germ layers in the teratoma test — the gold standard of pluripotency.
“It is the first demonstration that human oocytes have the ability to reprogram a specialized adult cell to the state of a pluripotent stem cell,” Egli told Nature Medicine.
However, the cells are not your typical ES cells: in addition to the two sets of chromosomes derived from the skin cells, the stem cells contained a full set from the egg as well, making them triploid. These new cells have been dubbed “somatic cell genome, oocyte genome pluripotent stem cells”, or soPS (rhymes with ‘hops’) cells for short. As triploid cells, they are not true donor-matched, patient-specific cells, and have a higher chance of immune system rejection when transplanted. This is likely to prove a sticking point, according to Robert Lanza, chief scientific officer of Advanced Cell Technology, a Santa Monica, California-based stem cell biotechnology company, who points out that personalized cell-therapy “is the whole purpose of this technology.”
This failure to create ES cells with only the donor cell’s genome was not for lack of trying. In early experiments, Egli’s team attempted to create normal cloned embryos through a process known as ‘somatic cell nuclear transfer’ (SCNT), which involves removing the egg’s haploid nucleus before inserting the adult diploid nucleus. This technique has worked in many mammalian species, ranging from mice to nonhuman primates, and, indeed, it’s the method that scientists used to create Dolly the sheep. But when Egli’s group swapped the egg and skin cell nuclei in their unfertilized eggs, they found that all the resulting embryos stopped dividing after just 6 to 10 cells. Some research groups have used SCNT to create later-stage human embryos with better success. Yet, barring Hwang’s fraudulent claims in 2004, none have reported any success at creating stem cell lines from these clones.
In a clever workaround, Egli and his colleagues left the egg’s nucleus in place, and fused it with the nucleus taken from the skin cell. Around 20% of the triploid eggs developed to the blastocyst stage (a mass of 70–100 cells), demonstrating the viability of the manipulated embryos. From the 13 blastocysts the researchers created, they obtained two stem cell lines — one containing the DNA of a man with type 1 diabetes and another from a healthy adult male donor.
According to Shoukhrat Mitalipov, a developmental biologist at the Oregon National Primate Research Center who, in 2007, derived cloned ESCs from rhesus macaques, the paper confirms what many researchers had suspected all along: “The human oocyte, just like in any other species, has the ability to reprogram,” he says. But the study doesn’t resolve the bigger issue plaguing the field. “The question is why it doesn’t work, and I don’t think this paper addresses that.”
“This paper has only a marginal contribution to therapeutic cloning,” notes Samuel Wood, chief executive of Stemagen, a La Jolla, California-based company. “In fact, it would be very surprising if this didn’t work. We thought about doing it, but it’s not useful therapeutically and you don’t want to us precious eggs to do a study where the result is rather obvious and doesn’t have any clinical utility.” He points to a 2005 study from researchers at the Harvard Stem Cell Institute in Cambridge, Massachusetts showing that fusing ESCs with skin cells led to stable tetraploid cells that passed all the tests of pluripotency. The newest study, he argues, does little more than replicate those older findings but starting with a haploid egg instead of diploid stem cells.
Get with the reprogram
Most researchers think it’s only a matter of time before researchers derive cloned ESCs with only two copies of the genome. Yet “the problem is simply not enough oocytes,” asserts Wood, whose company has managed to derive cloned blastocysts — although not stable stem cell lines — using just 57 eggs. In Egli’s team study, the researchers obtained a whopping 270 eggs from 16 donors at the Columbia University Center for Women’s Reproductive Care in New York. The young women, all of whom were healthy, employed and between the ages of 22 and 33, voluntarily chose to donate their eggs for research instead of for reproductive purposes and received $8,000 per donation in compensation, as permitted under New York state law. Elsewhere, such as in California, financial reimbursement is generally banned.
Egli’s team is currently investigating whether starting with adult cell types other than skin — for example, blood cells — leads to better cloning yields. But in the meantime, George Daley, a stem-cell expert at Children’s Hospital Boston who wrote an accompanying commentary in Nature, argues that comparing soPS cells with induced pluripotent stem (iPS) cells should reveal whether the latter have fundamental abnormalities that preclude their use in the clinic. This has been a particularly contentious issue in the stem cell community after recent studies showed that iPS cells retain epigenetic memories of their cells of origin, and can even trigger immune responses in mice not seen with embryonic stem cells. The new stem cells “will allow us to start making the comparisons between the fidelity of reprogramming by nuclear transfer and direct factors,” Daley says.
Image of soPS cells courtesy of NYSCF