This is an archive from the old blog, originally written in 2007.
The “Thursday Paper” column on the blog is for reporting on a recently published peer-reviewed research. Apologies if this one isn’t so polished, but I have a train to catch.
It would appear to be medical genetics week on the blog. On Tuesday I discussed the types of genetic aberration that can lead to developmental syndromes. The subject of this week’s Thursday paper will hopefully shed a little light on why those genetic aberrations occur. Sexually reproducing eukaryotes (plants, animals, fungi; not bacteria) produce gametes — sperm and egg — by a specialised version of cell division called meiosis. Meiosis is a tricky process that can go catastrophically wrong, and many of the genetic aberrations described in the Sunday syndrome column arise from meiotic failures. During meiosis homologous chromosomes pair up and undergo the process of “recombination”. At several sites along their length, the pair join together to form “chiasmata”, and then “cross over”: sections are swapped. Each of those pairs consists of one chromosome each from your parents; because of recombination, the single copy of the chromosome that you pass on to a child consists of a mixture, alternating between the two that you in turn inherited. This shuffling of genes is a mechanism of evolution, creating novel combinations of genes, and preventing good mutations from being lost simply because they share a chromosome with bad ones, and vice versa. In order to swap material, the DNA molecules must be deliberately cut to form a double-strand break (DSB) — something cells normally spend considerable effort trying to avoid. DSBs are liable to lead to loss of the broken section of chromosome, or reattachment of the fragment in the wrong place, which could lead to the types of problems described in Tuesday’s post, and in the Sunday syndrome column.
In “C. elegans Germ Cells Switch between Distinct Modes of Double-Strand Break Repair During Meiotic Prophase Progression“, Hayashi, Chin and Villeneuve look at how recombination is able to proceed without causing widespread damage to the genome, focusing particularly on the mechanisms of double-strand break repair (DSBR). DSBs can occur at any time, not just during production of gametes. In somatic cells (all cells except the sperm, eggs, and their precursors), DSBs may lead to diseases, including cancer, if they are not repaired properly, so DSBR mechanisms are required in all cells, and at all times. One particular mechanism relevant to this story involves the enzyme Rad51, which helps rapair DSBs caused by radiation, and is also known to be recruited to DSBs during meiosis. However, since recombination is an event specific to meiosis, Hayashi et al made the hypothesis that the mechanism of DSBR used in meiosis will also be unique. To investigate this hypothesis they used C. elegans, a small worm, whose genome has been sequenced, and whose development is relatively simple and very well documented. C. elegans was a particularly good species for this study because its gametes are arranged in a line according to their age/development status, meaning that in a single experiment one can look at each of the stages of meiosis.
Recombination is already known to be specific to the pachytene (third of six) sub-stage of prophase (first of four) in the first of two cell divisions that occur in meiosis. If those words mean nothing to you, don’t worry: the main point is that we already know when recombination happens, and we can therefore place it amongst other knowledge we have about the molecular state of the cell at that time. Hayashi, et al, looked speficially at the Mre11/Rad50 enzyme complex, which has previously been shown to have a role in recombination; and because Mre11 is a “nuclease”, meaning that it is capable of cutting DNA. They looked at what happens during meiosis in worms that have had their Rad50 switched off (“Rad50 knockouts”), and discovered that in these individuals, though the chromosomes pair up normally, the chiasmata do not form, and therefore recombination does not occur. When they looked at Rad51, they found that it was no longer being recruited to DSB sites during the first three sub-stages of prophase I, suggesting that in meiotic prophase, Rad50 is required for the recruitment of Rad51 to DSBs. Also, in the Rad50 knockouts, an increased number of cells were destroyed by their built-in self-destruct mechanism (apoptosis) after they had completed the pachytene — something that is seen in other situations where DNA damage repair mechanisms have failed. To show that these results were not just showing the previously unknown fact that Rad50 is always required for the recruitment of Rad51 to DSBs, they induced DSBs throughout the gonads of normal, and Rad50 knockout worms. In the normal worms, cells in all stages of meiosis showed recruitment of Rad51 to DSBs, indicating no requirement for Rad50. In Rad50 knockouts, a window between the onset of prophase 1 and the end of the pachytene showed a break in Rad51 recruitment to the breaks, indicating a requirement for Rad50.
These results suggest that during the production of gametes, animals temporarily drop their guard against mutation in order to enable recombination. Hayashi, et al, also catalogued a series of other enzymes and signals which interact with Rad50, to build up a better picture of its role in this mechanism. Rad50 is not required to induce DSBs, but may well promote their formation. Rad50 is required for the formation of chiasmata, crossing-over, and subsequent repair of breaks and restoration of chromosome integrity. While this is occurring, normal mechanisms of DSBR are suppressed during a window that lasts for the first three sub-stages of prophase, ending in the late pachytene. This specialised meiotic DSBR mechanism reflects a balance that must be struck between allowing recombination to occur, and preventing DNA damage that could lead to mutations and chromosomal aberrations, and subsequently to problems in development; and it helps to explain why genetic disorders occur so frequently (taking into account those that miscarry), and why the gonads are so vulnerable to physical and chemical insults.