letter nature genetics ⢠volume 28 ⢠june 2001. 169. Mitotic recombination is suppressed by chromosomal divergence in hybrids of distantly related mouse strains.
© 2001 Nature Publishing Group http://genetics.nature.com
letter
Mitotic recombination is suppressed by chromosomal divergence in hybrids of distantly related mouse strains
© 2001 Nature Publishing Group http://genetics.nature.com
Changshun Shao1, Peter J. Stambrook2 & Jay A. Tischfield1
Mitotic recombination occurs with high frequency in humans1,2 and mice3. It leads to loss of heterozygosity (LOH) at important gene loci and can cause disease4–7. However, the genetic modulators of mitotic recombination are not well understood. As recombination depends on a high level of nucleotide sequence homology8–12, we postulate that the frequency of somatic variants derived from mitotic recombination should be diminished in progeny from crosses between strains of mice in which nucleotide sequences have diverged. Here we report that mitotic recombination is suppressed, to various degrees in different tissues, in hybrids of distantly related mouse strains. Reintroduction of greater chromosomal homology by backcrossing restores mitotic recombination in offspring. Thus, chromosomal divergence inhibits mitotic recombination and, consequently, may act as a modifier of cancer susceptibility by limiting the rate of LOH. The suppression of mitotic recombination in some F1 hybrids in which meiotic recombination persists indicates that these processes are differentially affected by chromosomal divergence.
LOH, or recessive expression of a defective tumor suppressor gene, is a rate-limiting event in tumorigenesis5,7. Mice heterozygous for the adenine phosphoribosyltransferase (Aprt) gene are a useful model for studying the frequency and mechanisms of in vivo LOH (refs. 3,13). Cells that have undergone in vivo LOH at Aprt are Aprt deficient and recoverable because of their ability to form colonies during subsequent in vitro culture in medium containing the adenine analog 2,6–diaminopurine (DAP). In hybrid progeny of the two inbred strains 129S2 and C3H/HeJ, skin fibroblasts and splenic T cells from Aprt+/– mice lose the wild-type allele at a high frequency (10–5–10–4) in vivo. Most (70–80%) Aprt-deficient colonies arise as a result of mitotic recombination, which can initiate anywhere along the length of chromosome 8 (refs. 3, 13). We show here that mitotic recombination is also the predominant mechanism leading to LOH in hybrids of strains 129S2×C57BL/6J and
BALB/cJ × C57BL/6J (Table 1), which indicates that, in hybrids of these and other classic inbred strains14,15, LOH is mediated primarily by mitotic recombination. To determine whether evolutionary divergence affects the rate of mitotic recombination-mediated LOH, we analyzed hybrids between two distantly related strains, 129S2 and CAST/Ei (Mus musculus castaneus), or chromosome 8 LOH with Aprt as a reporter. None of the 16 DAP-resistant (DAPr) fibroblast clones was produced by mitotic recombination, the absence of which probably contributes to the reduction in the observed frequency of DAPr colonies. Unlike fibroblasts, however, mitotic recombination in splenic T cells appears to be less affected. Mitotic recombination produced eight T-cell clones (44%) in two mice (Table 1). To confirm that reduced mitotic recombination is associated with genetic distance between the two parental strains, we crossed 129S2 mice with SPRET/Ei (Mus spretus) mice, a strain that is more distantly related to classic inbred strains than CAST/Ei. In the F1 mice of this cross, not only is mitotic recombination in the fibroblasts suppressed, but none of 24 DAPr T cell clones is a result of mitotic recombination (Table 1). Thus, suppression extends to more tissues as evolutionary distance between the parental strains increases. We hypothesized that the suppression of mitotic recombination in F1 hybrids of evolutionarily distant strains is caused by a reduction in DNA sequence homology, which is consistent with observations that decreased DNA sequence homology reduces recombination in lower organisms8–10 and in cultured mammalian cells11,12. Classic inbred strains are much more similar to each other than they are to feral-derived strains such as CAST/Ei and SPRET/Ei, based on divergence of simple sequence length polymorphisms (SSLP) (ref. 16) and single nucleotide polymorphisms17. Approximately 49% of the SSLP markers differ between any two classic inbred strains, whereas 94% of the markers differ between a classic inbred strain and CAST/Ei or SPRET/Ei (ref. 16). Similarly,
Table 1 • Suppression of mitotic recombination in hybrids of distantly related strains Number of mice
% CFE (SD)
MF (range) ×10–6
Fibroblast
129 × C3H 129 × C57 BALB × C57 129 × CAST 129 × SPRET
7 12 4 12 6
1.9 (0.5) 1.9 (0.6) n.d. 1.9 (0.3) 2.2 (0.7)
169 (0–1,875) 92 (0–2,420) n.d. 21 (0–246)* 29 (15–113)**
25 34 21 16 16
60 85 71 0 0
T cell
129 × C3H 129 × C57 129 × CAST 129 × SPRET
13 13 6 9
5.6 (1.2) 5.8 (1.3) 2.9 (1.3) 4.7 (3.6)
18 (3–51) 17 (0–131) 18 (0–74) 15 (0–62)
59 53 18 24
66 51 44 0
Cell type
Hybrid
Number of DAPr colonies
% colonies due to MR
CFE, colony forming efficiency. SD, standard deviation. MF, the median mutant frequencies, in individual ears for fibroblasts and in individual spleens for T cells. *P
