| First Author | Liddell RA | Year | 1994 |
| Journal | Mouse Genome | Volume | 92 |
| Issue | 3 | Pages | 520-22 |
| Mgi Jnum | J:20791 | Mgi Id | MGI:68861 |
| Citation | Liddell RA, et al. (1994) Chromosomal localization and evolution of topoisomerase I sequences in the Mus genome. Mouse Genome 92(3):520-22 |
| abstractText | Full text of Mouse Genome contribution: CHROMOSOMAL LOCALIZATION AND EVOLUTION OF TOPOISOMERASE I SEQUENCES IN THE Mus GENOME. Rebecca A. Liddelll, Linda S. Cleveland2, Hai-Young Wu3, Leroy F. Liu4, Debra J. Gilbert2, Neal G. Copeland2, Nancy A. Jenkins2, and Linda D. Siracusa1; 1Jefferson Cancer Institute, 233 South 10th Street, Philadelphia, PA 19107, 2ABL-Basic Research Program, NCI-Frederick Cancer Research and Development Center, Frederick, MD 21702, 3Wayne State University School of Medicine, Detroit, MI 48201, and 4University of Medicine and Dentistry of the New Jersey-Robert Wood Johnson Medical School, Piscataway, NJ 08854. Introduction The double helical nature of the DNA molecule combined with the formation of chromatin in eukaryotes imposes biochemical and mechanical restraints upon the replication, transcription, and condensation of the DNA strand. DNA topoisomerase I is a ubiquitously expressed eukaryotic protein which helps alleviate these restraints by regulating and altering the topological states of DNA. Specifically, DNA topoisomerase I catalyzes the transient breakage of one strand of a supercoiled DNA helix at a time, to permit the passage of another strand of DNA through the break (reviewed in 1-3). Previous studies identified the human DNA topoisomerase I mRNA (4, 5) and mapped the structural gene to Chr 20ql2-q13.1 (5, 6). Additional studies located two pseudogenes on human Chr lq23-q24 and 22q11.2-q13.1 (6). The murine DNA topoisomerase I cDNA has been cloned and previous mapping studies identified two loci on Chr 2 and 14 (7, 8). We used two interspecific backcrosses to refine the map locations of topoisomerase I-related sequences in the mouse genome. Materials and Methods Mice: The interspecific backcrosses are (C57BL/6J x Mus spretus)Fl x C57BL/6J (9) and (AEJ/Gn -a bpH/a bpH x M. spretus)Fl x AEJ/Gn -a bpH/a bpH (10). Topoisomerase I probes: The mouse topoisomerase I probe used for mapping in both interspecific backcrosses was a 1.2 kb mouse cDNA cloned into the EcoRI site of pBluescript that corresponds to the C-terminal portion of the mRNA (H-Y. Wu and L.F. Liu, unpublished data). Southern and PCR analyses: Genomic DNA isolations, restriction endonuclease digestions, agarose gel electrophoresis and Southern transfers were as described (9). The probes, hybridization or PCR conditions, and segregation patterns for Ada (adenosine deaminase), Ctlal (cytotoxic T-lymphocyte -associated protein-1, Emv15 (endogenous ecotropic MuLV-15), Pckl (phosphoenolpyruvate carboxykinase -1), Plau (plasminogen activator, urokinase), Plcgl (phospholipase C-gamma 1), Psp (parotid secretory protein), and Src (Rous sarcoma protooncogene) were as described (9, 11, 12, 13). Hybridization conditions for the mouse topoisomerase I probe on (C57BL/6J x M. spretus)Fl x C57BL/6J DNA were as described (9) and (AEJ/Gn x M. spretus)Fl x AEJ/Gn DNA were as described (10). Results Restriction fragment length polymorphisms corresponding to Top1 sequences were identified by digesting genomic DNA from AEJ/Gn, C57BL/6J and M. spretus progenitor mice with 14 restriction endonucleases. The mouse Top1 probe detected EcoRI fragments of 7.8 kb, 3.7 kb*, 3.0 kb**, 1.6 kb** and 1.0 kb in M. spretus and 7.8 kb, 4.0 kb**, 3.6 kb*, 1.7 kb, 1.2 kb, and 0.8 kb* in AEJ/Gn and C57BL/6J. The asterisks indicate the relative intensity of each fragment; fragments with two asterisks gave the strongest autoradiographic signals. In addition, KpnI digestions showed common fragments of ~23.0 kb* and 7.0 kb in M. spretus and C57BL/6J, and fragments of 2.5 kb** in M. spretus and 5.2 kb** in C57BL/6J. The EcoRI 3.0 kb and 1.0 kb M. spretus-specific fragments co-segregated and showed a pattern distinct from the EcoRI 1.6 kb and KpnI 2.5 kb M. spretus-specific fragments which also co-segregated. Both segregation patterns were compared to marker loci across the genome and localized the two Top1 loci to Chr 2 and 14. Marker loci used to refine the positions of the Topl loci and corresponding map distances are listed in Table 1. Table 1 Linkage of the Top1 and Top1-rs2 loci with marker loci Chromosome 2 (AEJ/Gn x Mus spretus)F1 x AEJ/Gn Pairwise loci combinationsa: Emv15 �� Src; Recombinants/Total: 1/186; Distance (cM +/- s.e.): 0.5 +/- 0.5. Pairwise loci combinationsa: Src �� Plcg1; Recombinants/Total: 3/179; Distance (cM +/- s.e.): 1.7 +/- 1.0. Pairwise loci combinationsa: Plcg1 �� Top1; Recombinants/Total: 0/192; Distance (cM +/- s.e.): <1.6b. Pairwise loci combinationsa: Top1 �� Pck1; Recombinants/Total: 18/195; Distance (cM +/- s.e.): 9.2 +/- 2.1. (C57BL/6J) x Mus spretus)F1 x C57BL/6J Pairwise loci combinationsa: Psp - Src; Recombinants/Total: 5/150; Distance (cM +/- s.e.): 3.3 +/- 1.5. Pairwise loci combinationsa: Src �� Top1; Recombinants/Total: 2/93; Distance (cM +/- s.e.): 2.2 +/- 1.5. Pairwise loci combinationsa: Top1 - Ada; Recombinants/Total: 2/93; Distance (cM +/- s.e.): 2.2 +/- 1.5. Chromosome 14 (AEJ/Gn x Mus spretus)F1 x AEJ/Gn Pairwise loci combinationsa: Plau - Top1-rs2; Recombinants/Total: 11/95; Distance (cM +/- s.e.): 11.6 +/- 3.3. Pairwise loci combinationsa: Top1-rs2 �� Psp-rs1; Recombinants/Total: 6/152; Distance (cM +/- s.e.): 3.9 +/- 1.6. (C57BL/6J) x Mus spretus)F1 x C57BL/6J Pairwise loci combinationsa: Plau - Top1-rs2; Recombinants/Total: 24/158; Distance (cM +/- s.e.): 15.2 +/- 2.8. Pairwise loci combinationsa: Top1-rs2 �� Psp-rs1; Recombinants/Total: 5/162; Distance (cM +/- s.e.): 3.1 +/- 1.4. Pairwise loci combinationsa: Psp-rs1 �� Ctla1; Recombinants/Total: 3/196; Distance (cM +/- s.e.): 1.5 +/- 0.9. aThe most proximal locus is listed first. Loci were ordered by arranging to obtain the least number of multiple crossovers; no double crossovers were found with the loci order shown. bThis number represents the upper 95% confidence limit. Discussion Figure 1 shows the location of Topl loci derived from each of the interspecific backcrosses compared to the consensus linkage maps of Chr 2 and 14 (7, 8). These results are consistent with previous data (7, 8) that placed Topl on Chr 2 and refine the map location of Topl to within 0-1.6 cM from Plcg1. However, our data place Topl-rs2 in the interval between Plau and Psp-rs1 on Chr 14; this interval does not overlap with the position previously defined for Topl-related sequences which placed Top1-rs1 7.0 cM distal to Ctlal and 10.5 cM proximal to Rbl on Chr 14 (8). Thus, there are two Topl-related loci, Top1-rs1 and Top1-rs2, on Chr 14. The human TOP1 structural gene was localized to Chr 20q12-q13.1 (5, 6). Multiple genes on human Chr 20 have homologous genes that map only to the distal end of Chr 2 in the mouse. Therefore, we agree with the conclusions of Koiwai et al. (7) and Kingsmore et al. (8) that the Top1 locus on Chr 2 represents the structural gene whereas the Chr 14 loci represent related or pseudogene sequences. In addition, Top1 on Chr 2 is the only common locus reported in all three papers. Figure 1. (Legend) Genetic linkage maps showing the location of theTop1 and Topl-rs2 loci. The maps are derived from results of A) the (AEJ/Gn x M. spretus)Fl x AEJ/Gn cross, B) the (C57BL/6J x M. spretus) Fl x C57BL/6J cross, and compared to C) partial versions of the Chr 2 and 14 consensus linkage maps (14, 15). Dotted lines indicate marker loci used for alignment. Loci mapped in humans are underlined and their locations in the human genome are shown between B) and C). It is interesting that both the Psp and Top1 genes have related sequences that map to Chr 14 in the mouse. We have found that the Psp and Top1 genes on Chr 2 reside ~6 cM apart whereas their related sequences Psp-rsl and Topl-rs2 on Chr 14 reside ~4 cM apart and are inverted with respect to the centromere (Figure 1). We also estimate that Psp-rs1 and Topl-rsl (8) map ~9 cM apart on Chr 14. These observations suggest that these regions may have evolved through duplication, although related sequences for genes that map between the Psp and Top1 genes have not yet been found on Chr 14. Therefore, these relationships may be coincidental. However, the identified mouse Topl-related sequences and human TOP1 pseudogenes appear to have arisen after the radiation between humans and mice, since these loci do not map to syntenic regions. Acknowledgements and Notes All symbols were approved by the International Committee for Standardized Genetic Nomenclature in Mice. Psp-rs1 was formerly Psp-2, Topl was formerly Topl and Top-1, Topl-rs1 was formerly Top-lrs, and Topl-rs2 is new. 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