Unraveling the Mysteries of Meiosis.

STANFORD, Calif.--(BW HealthWire)--Aug. 6, 1998--Egg and sperm cells form through meiosis, a delicate process in which mistakes can later cause birth defects.

Stanford researchers have now found that a key event in meiosis -- a genetic shuffling called recombination -- is started by the same protein in two very different organisms, yeast and worms.

Given the evolutionary distance between yeast and worms, it seems likely that humans also use this mechanism, said the study's senior author, Anne Villeneuve, an assistant professor of developmental biology at Stanford University School of Medicine.

The critical protein, Spo11, cuts DNA strands to allow the genetic shuffling to take place. This process of recombination is also essential for maintaining temporary links between pairs of chromosomes so that they can later be distributed correctly, with one of each pair going to each of two "daughter" cells. Eggs or sperm with the wrong numbers of chromosomes will form embryos that either miscarry or develop disorders such as Down syndrome.

Ultimately, basic research into meiosis could help clarify the root causes of Down syndrome and several other serious birth defects.

"To understand what might be going wrong in these cases, we first have to understand the order of the different events of meiosis and their dependence on each other," Villeneuve explained.

The newly published study represents a significant step in that direction, she believes. "We found out about some of the essential features of meiosis -- the core or the guts of the operation -- as well as how these different features can be modified in different animals to deal with different physiologies."

Villeneuve and her colleagues report their findings in the Aug. 7 issue of the journal Cell. The paper's lead author is Abby Dernburg, a postdoctoral fellow in Villeneuve's laboratory.

Error-Prone Phase

Sexual reproduction combines an egg and a sperm. The new organism has two copies of each chromosome: one from the mother and one from the father. But the first stage of meiosis, when recombination occurs, is the opposite of this process. Here the number of chromosomes in a cell is halved, so each egg or sperm has only one copy of each. This first phase of meiosis is when a developing egg or sperm cell is most likely to make mistakes in distributing one of each chromosome to its two daughter cells.

In humans, these errors usually result in a spontaneous abortion (miscarriage) because the developing embryo cannot survive. But some errors are not lethal. For example, if both of the mother's copies of chromosome 21 go to one egg, the fertilized embryo will survive with its three copies of the chromosome (the third one coming from the sperm). The resulting child will have Down syndrome.

To begin piecing together the underlying chain of events, Villeneuve and Dernburg chose to study the relationship between two major steps in meiosis: synapsis and recombination.

Aligning, Recombining

Meiosis starts with the two copies of each chromosome lining up side-by -side. In synapsis, a structure called the synaptonemal complex solidifies this pairing, forming a scaffold between the two chromosomes.

Recombination takes advantage of the chromosome alignment. With the DNA of one chromosome lined up next to the similar DNA of its partner, recombination proteins are able to recognize the similarity and do some cutting and pasting.

The result is a chromosome that is a mixture of DNA from both parents, so any child resulting from the egg or sperm will carry traits from all grandparents. After the synaptonemal complex dissolves, recombination also serves to link the chromosome pairs together, so the cell can select one chromosome from each pair for the two daughter cells.

From Yeast to Worms

Harvard University researchers studying meiosis in yeast found last year that a protein called Spo11 starts the recombination process by chopping into the DNA.

In the new work, the researchers found a gene from the worm Caenorhabditis elegans that looked like the yeast gene for Spo11 and then deleted it to test whether things worked the same way in the worm.

Villeneuve's group found that mutant worms lacking the gene could survive, but that fewer than 1 percent of their offspring survived. A look at the chromosomes in the egg cells of the mutant worms made it clear why: there had been no recombination, so each egg cell ended up with 12 single chromosomes instead of six linked pairs. The cell had no basis for distributing the chromosomes to the two daughter cells, and the distribution mistakes resulted in eggs that, when fertilized, couldn't survive.

Thus, both worms and yeast need Spo11 to get recombination going. The way Spo11 does its job also appears to be consistent between the two diverse organisms. Yeast uses this protein to cut the DNA; other proteins then connect the cut DNA strands to the DNA of the partner chromosome.

In an observation suggesting a similar function in worms, the Stanford researchers found they could restore fertility to mutant worms lacking Spo11 by treating them with radiation, which is known to damage cells by cutting DNA. In this case the radiation did just enough cutting to give the other recombination proteins something to work on. The resulting recombination linked the chromosome pairs and increased the number of living offspring more than fifty-fold.

Dependent Relationship?

Not all events of meiosis are identical in the two organisms, however. In yeast, synapsis of partner chromosomes appears to be dependent on recombination: it doesn't start unless recombination is already under way, and getting rid of Spo11 prevents synapsis. But in worms lacking Spo11, pairing and synapsis occurred right on schedule despite the absence of recombination, the Stanford researchers found.

"We found that the same pathways for meiotic recombination are used in highly divergent organisms. Divergent organisms nevertheless have significant differences in the ways they use the same players," Villeneuve said.

She hypothesizes that the worm situation -- in which synapsis does not depend on recombination -- may be the more typical scenario for other animals, including humans. Yeast meiosis takes a matter of hours, so recombination may act as a trigger to make sure that synapsis gets started on time. But meiosis is more leisurely in worms and humans, so they may have no need for a carefully regulated trigger for synapsis, Villeneuve said.

Study participants in addition to Villeneuve and Dernburg included Kent McDonald, director of the Electron Microscope Laboratory at the University of California, Berkeley; and three collaborators from the Oklahoma Medical Research Foundation: Gary Moulder, Robert Barstead and Dr. Michael Dresser.

Funding for the research came from the National Institute of General Medical Sciences, the Searle Scholars Program of the Chicago Community Trust, and the Cancer Research Fund of the Damon Runyon-Walter Winchell Foundation.

Note to Editors: A diagram with caption is available online at http://www-med.stanford.edu/center/communiations/pressrel/august98/ meiosis_graph.html

    CONTACT: Stanford University Medical Center
              Media contact:
              Rosanne Spector, 650/725-5374 or 723-6911
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              For comment:
              Anne Villeneuve, 650/725-5505
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