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The recent scandal around the South Korean stem cell researcher Woo-Suk Hwang that has tainted his report describing the generation of 11 patient-specific embryonic stem cell lines by somatic cell nuclear transfer highlights a crucial point: this technique remains extremely challenging. It involves the transfer of a nucleus from an adult cell to an enucleated egg, which is then induced to undergo early embryonic differentiation. It is this reprogramming of adult nuclei for early development that most often fails. One possible explanation is that differentiated nuclei cannot keep up with the speed of cell division required in the initial phases of development. Their DNA replication process is too slow, and consequently the genetic material is not distributed evenly and the cells die.

Now Marcel Méchali and Jean-Marc Lemaitre from the Institute of Human Genetics at CNRS in France describe a platform of cellular and biochemical techniques, published in a recent issue of Cell, to systematically check which factors are needed for differentiated nuclei to undergo rapid DNA replication.

The Méchali laboratory has a long history of investigating DNA replication during development, and they first wanted to test if there is a connection between chromatin structure and speed of DNA replication. The most dramatic remodeling of chromatin happens during mitosis, and thus Méchali and his team reasoned that a passage through mitosis may be necessary for subsequent rapid DNA replication. They exposed the nuclei of differentiated cells to extracts from Xenopus oocytes undergoing mitosis and found the rate of DNA replication in these nuclei to be equivalent to that in sperm nuclei and significantly faster than that in differentiated nuclei that had been exposed to extracts from oocytes in S phase or were left untreated (Fig. 1). They concluded that the passage through mitotic phase and the formation of metaphase chromosomes were essential to increase replication speed, and next investigated the mechanism behind this rapid replication.

Figure 1: When differentiated somatic nuclei are exposed to cell extract from mitotic oocytes, they replicate at a rapid rate.
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Because this fast DNA replication also occurs in early embryos, the authors suggest that the formation of metaphase chromosomes before somatic nuclear transfer may increase the efficiency of this procedure.

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By performing DNA combing, a technique that allows the measurement of DNA length between origins of replication, the Méchali team found that somatic nuclei contained more replication origins when the nuclei had undergone mitosis. This explained the increase in DNA replication speed, because a cell that simultaneously uses more origins of replications will duplicate its genetic material faster. The researchers also found that the increase in replication sites went hand in hand with a remodeling of chromatin that increased the number of DNA attachment sites to the nuclear membrane, typical for sperm nuclei or the nuclei of early embryos.

These experiments suggested to Méchali that the formation of mitotic chromosomes is important for genetic reprogramming of somatic nuclei used in nuclear transfer because it resets the organization of replication origins and thus ensures that the nuclei can keep up with the frantic pace of DNA replication needed in the first cell divisions of early embryonic development. It will be interesting to see if the success rate of nuclear transfer is indeed increased if the somatic nucleus can be induced to form metaphase chromosomes before its transfer. If this is the case, researchers can then focus on the next challenges in therapeutic cloning, for example, obtaining stem cells from the cloned embryos, or generating specific tissue—potential highlights for another day.