THE IMMORTAL STRAND HYPOTHESIS:
Older DNA strands are preferentially sorted into stem cells when they self-renew (A). This segregates the DNA most recently replicated, and therefore most error-prone, into cells destined for differentiation – which, in highly renewable tissues, are soon shed. (SSC indicates somatic stem cell). (B) If adult stem cells underwent random chromosome segregation, all chromosomes would assort with equal frequency to either stem cells or their differentiating sisters (squares) (B).
Matthew Meselson and Franklin Stahl's 1957 demonstration of DNA replication is considered "the most beautiful experiment in biology." Their density-shift demonstrations not only have stood the test of time, but also have renewed relevance for stem cell biology.
Watson and Crick's classic 1953 paper closed with the tantalizing clue "that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material."1 Their follow-up paper explained the probable semi-conservative route: The double helix parts and each half assembles new nucleotides.2
Experimental evidence came just a few years later. In 1954, Meselson met Stahl at a summer course taught by Watson and Crick at Woods Hole. Meselson returned to Cal Tech to complete his doctorate, Stahl went as a postdoc, and the rest, as they say, is history. Using an idea suggested by English geneticist J.B.S. Haldane in 1941, they labeled newly replicating DNA in bacteria with a heavy isotope of nitrogen. The results clearly showed that yes, DNA replication is semiconservative, and no, it is not conservative (the double helix somehow spawning an entire new double helix) or dispersive (the replicated molecule a hodgepodge of old and new).3
But the Meselson-Stahl experiments didn't tell the full story. Complexity emerged when cells were considered.
In 1966 and 1967, experiments on cultured cells from mouse embryos and Chinese hamster ovaries revealed that the distribution of "old" versus "new" DNA strands among daughter cells is not random: Newer strands tend to wind up together. In 1975, John Cairns, now at the Radcliffe Infirmary in Oxford, UK, and who bestowed the "most beautiful experiment" title, presented the "immortal-strand hypothesis" to explain why this might happen.4 And that's where stem cells come in.
It is a stunningly logical idea: If a stem cell grabs the oldest DNA strands, it minimizes replication errors, which can cause cancer. It doesn't matter that the daughter cells destined for differentiation receive the newly replicated, more error-prone DNA, for they exit or perish, especially in highly renewable tissues.
Experimental evidence for the immortal-strand hypothesis came and is still coming. The basic approach is to use two labels to distinguish old from new DNA.
In 1978, Cairns and Christopher Potten, now at the Paterson Institute for Cancer Research in Manchester, UK, and colleagues demonstrated skewed segregation in epithelium.5 Potten's group did so for intestinal crypts more recently.6 And James Sherley's team at MIT uses a cell culture system that reveals immortal DNA strands and the cosegregation of the chromosomes that bear them into stem cells.7 The next step is discovering the mechanism for the exquisite control of many chromosomes at once, Sherley hints.
As a graduate student I was entranced with Cairn's 1975 paper on the natural history of cancer, cited so often that it is a literary immortal strand of sorts. But it was a short talk at the Society for Neuroscience meeting on October 24 that brought home the importance of the hypothesis in light of the recently reawakened interest in stem cells.
The scene: a "hot topic stem cell biology datablitz" sponsored by the Southern California Stem Cell Consortium and Invitrogen, in honor of Christopher Reeve. Each group had 15 minutes, the principal investigator introducing a junior team member, who spoke. Some 90 minutes into the blitz, Phillip Karpowicz, from Derek van der Kooy's lab at the University of Toronto, described their neurosphere system. They labeled, plated, and imaged cells in real time, capturing over and over the sequestering of the eldest molecules of heredity into the cells that maintained the lineage. "After 30 years of the immortal-strand hypothesis, there's never been real strong evidence at the level of single dividing cells. I think with this imaging system we've shown that," he said.
Given the clinical promise of stem cells, it's comforting to know that they have a built-in mechanism to avoid mutation and, presumably, cancer. Yet at the same time, it is unnerving to realize that we still have much to learn from these cells that make multicellular life possible.
Ricki Lewis (