Interchromosomal recombination is suppressed in mammalian somatic cells.

Abstract

Homologous recombination occurs intrachromosomally as well as interchromosomally, both in mitotic (somatic) cells as well as meiotically in the germline. These different processes can serve very different purposes in maintaining the integrity of the organism and in enhancing diversity in the species. As shown here, comparison of the frequencies of intra‐ and interchromosomal recombination in meiotic and mitotic cells of both mouse and yeast argues that interchromosomal recombination is particularly low in mitotic cells of metazoan organisms. This result in turn suggests that the recombination machinery of metazoa might be organized to avoid the deleterious effects of homozygotization in somatic cells while still deriving the benefits of species diversification and of DNA repair.

Requirements for Ectopic Homologous Recombination in Mammalian Somatic Cells

“…Our results reveal a very high frequency of ectopic recombination in cell lines in which the recombining C sequences are separated on the chromosome by about 1 to 2 Mb. The cis preference for recombination is striking, being as much as 10 4 -fold higher than recombination between the same homologous C sequences present on nonhomologous chromosomes or in their normal position on homologous chromosomes (53). A similar preference for intra-over interchromosomal recombination was also observed by us (3,7,53) and by others (19,39) when homologous repeats were closely linked in the mammalian genome.…”

“…In contrast to the high transfection Table 1. Transformants 1, 2, and 3 derived from transfer of the XbaI fragment are the transformants E29, E15, and E26 described previously (53).…”

“…Thus, it might be argued that increased accessibility of the chromosomal locus favors intra-over interchromosomal recombination. However, as discussed previously (53), the chromosomal gene segments involved in interchromosomal allelic recombination are expected to be at least as accessible as those involved in ectopic and intrachromosomal recombination. This suggests that the preference for intra-over interchromosomal recombination is not the result of increased accessibility of the chromosomal locus to recombination.…”

“…Thus, in transformants E9, E26, and E29, the transferred vector integrated in a single genomic site, whereas in transformant E69, there were two sites of vector integration. Since E69 appears to contain a single copy of the full-length wild-type C region (53), only one of the vector integration sites contains the functional C donor. The same conclusion regarding the number of vector integration sites was reached following digestion of transformant genomic DNA with the enzyme XbaI, which also does not cut within the integrated pTC vector (data not shown).…”

A Strand Invasion 3′ Polymerization Intermediate of Mammalian Homologous Recombination

“…While formation of noncrossover (i.e., SDSA-like) recombinants has not been measured in this study, other investigators have reported a bias in noncrossover events during intra-and interchromosomal homologous recombination in yeast (Esposito 1978;Klein and Petes 1981;Haber and Hearn 1985;Christman et al 1988;Kupiec and Petes 1988;Pâques and Haber 1999;Ira et al 2003) and mammalian cells (Liskay et al 1984;Baker 1989;Shulman et al 1995;Baker et al 1996;Richardson et al 1998;Johnson and Jasin 2000). This mechanism acting at the level of recombination between chromosomal sequences would provide the means of suppressing potentially deleterious rearrangements that may arise from crossover between chromosomal sequences (Shulman et al 1995;Richardson et al 1998). To test whether a residual fraction of uncut pT Cμ vector remains in the BstEII-digested preparation, primers ampR20 and neoF20 were used to amplify the Cμ region in a dilution series prepared from BstEII-cleaved pT Cμ vector (lanes 14-20) and the level of product compared to that generated from a series of uncut pT Cμ standards (lanes 2-13).…”