With great fanfare, the first purported human clones derived from somatic cell nuclear transfer were announced by Korean researcher Woo Suk Hwang and colleagues at Seoul University in February 2004 (see ARF related news story). What followed was a bitter disappointment to scientists worldwide. Accusations of data fabrication led to an inquiry, culminating in the retraction of two seminal papers (see ARF related news story). But while an investigation committee set up by the Seoul National University concluded that data was fabricated, it failed to resolve provenance of SCNT-hES-1, the stem cell line supposedly derived by human cloning. Now, in today’s Cell Stem Cell, researchers led by George Daley at Children’s Hospital and the Harvard Stem Cell Institute, Boston, report that the SCNT-hES-1 line was actually derived by a process called parthenogenesis. Ironically, deriving stem cells in this fashion may be almost as useful as doing it by nuclear transfer.
Parthenogenesis, where an unfertilized egg develops into an embryo, is an extremely rare phenomenon in primates (see ARF related news story). It requires a duplication of the haploid set of chromosomes in the oocyte, or egg. The Seoul National University committee considered parthenogenesis as a likely explanation for Hwang’s cells. However, analysis of 40 genetic markers in the SCNT-hES-1 line showed that 32 of them were heterozygous, an unexpected result if the DNA was derived from oocyte DNA alone, leaving the parthenogenetic origin in doubt.
That doubt has now been erased. First author Kitai Kim and colleagues report that, contrary to expectation, heterozygosity does arise during parthenogenesis—through genetic recombination. What’s more, this recombination occurs predominantly at the distal ends of chromosomes, while the regions around the centromeres remain undisturbed. What this means is that there is a recombination signature in parthenogenetically derived embryonic stem cells (pES) that distinguishes them from stem cells derived by somatic cell nuclear transfer (ntES).
Armed with this knowledge, Kim and colleagues re-analyzed the SCNT-hES-1 cells. The parthenogenetic pattern clearly emerged. Chromosome by chromosome the researchers found that near the centromeres homozygosity dominated, but toward the end of the chromosomes heterozygosity was rampant. The X and seventh chromosomes were the only exceptions, showing complete homozygosity. This may be due to chromosome loss followed by reduplication.
“The evidence indicates that SCNT-hES-1 represented the first reported isolation of a human pES cell,” conclude the authors. They also note that though there are problems with these cells—mouse parthenogenetic embryos fail to develop properly, for example—researchers have been able to improve the quality of pES by genetic manipulation, and a successful hematopoietic engraftment of pES-derived cells has recently been reported in mice (see Eckardt et al., 2007).
There may be a deeper lesson to be learned here beyond ethics. Hwang and colleagues tried to carry out somatic cell nuclear transfer using egg and nuclei from the same donor. “If the somatic cell nucleus and the recipient oocytes come from different donors, the genomic DNA of any resulting ntES cells can be readily distinguished from parthenogenetic derivatives that might mistakenly arise,” write the authors. “However, if nuclear transfer is performed using autologous oocytes from the somatic-cell donor, as in the case of SCNT-hES-1, all genetic markers will be shared, and selection of a small number of markers could mistakenly lead to the conclusion of genetic identity.” Using separate donors for eggs and somatic nuclei would seem to be the logical choice.—Tom Fagan
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- Eckardt S, Leu NA, Bradley HL, Kato H, Bunting KD, McLaughlin KJ. Hematopoietic reconstitution with androgenetic and gynogenetic stem cells. Genes Dev. 2007 Feb 15;21(4):409-19. PubMed.
- Kim K, Ng K, Rugg-Gunn PJ, Shieh JH, Kirak O, Jaenisch R, Wakayama T, Moore MA, Pedersen RA, Daley GQ. Recombination signatures distinguish embryonic stem cells derived by parthenogenesis and somatic cell nuclear transfer. Cell Stem Cell. 2007 Sep 13;1(3):346-52. Epub 2007 Aug 2 PubMed.