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Advances in Brief |
Division of Molecular Genetics, The Netherlands Cancer Institute, Plesmanlaan 121, 1066 CX Amsterdam, the Netherlands
| ABSTRACT |
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| Introduction |
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Mouse models are powerful tools for identifying susceptibility genes because inbred strains differ widely in their susceptibility to DMH-induced colon adenomas (7) . We used the CcS series of the recombinant congenic strains (8) to study colon cancer susceptibility (9) . The CcS strains are derived from the mouse strains BALB/c and STS which are resistant and susceptible, respectively, to chemically induced colon adenomas. When treated with DMH, STS mice develop a large number of tumors, and BALB/c mice develop only a few tumors. This difference is caused by multiple genes (7) . Each individual CcS strain has obtained a random subset of 12.5% genes from strain STS on the genetic background of mouse strain BALB/c. In this way, the STS alleles of different loci involved in the susceptibility to colon cancer are divided between the 20 CcS strains, thus converting a multigenic difference into oligogenic or monogenic differences (10) .
Previously, five colon cancer susceptibility loci, Scc1, Scc2, Scc3, Scc4, and Scc5, were identified using the RC strains CcS-16, CcS-17, and CcS-19, which are highly susceptible to colon cancer (11 , 12) . One of these loci, Scc1, has been mapped to a small region on chromosome 2 (13) .
The CcS strains CcS-3, CcS-5, and CcS-11 are also more susceptible to colon cancer than the BALB/c strain(9) . However, loci Scc1, Scc2, Scc4, and Scc5 cannot be responsible for this difference because these strains carry the BALB/c allele for these loci.
The CcS-11 strain has the resistant STS allele of Scc3, which cannot be the cause of its susceptibility. Therefore, the susceptibility of these strains is caused by loci other than the known loci Scc1 Scc5. In the present study, we used backcrosses of CcS-3, CcS-5, and CcS-11 to the resistant strain BALB/c to search for additional susceptibility loci. This approach led to the detection of four new colon cancer susceptibility loci: (a) Scc6 on chromosome 11; (b) Scc7 on chromosome 3; (c) Scc8 on chromosome 8; and (d) Scc9 on chromosome 10. Scc7 and Scc8 show a genetic interaction, a phenomenon we also observed for Scc4 and Scc5 (12) and for several lung cancer susceptibility loci (14 , 15) .
| Materials and Methods |
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DNA Preparation and Genotyping.
DNA was prepared from mouse tails using a standard proteinase K procedure. The backcross mice were genotyped as described previously (17)
using microsatellite markers (Mouse MapPairs TM; Research Genetics, Huntsville AL). The CcS-3 backcross used microsatellite markers D3Mit18, D3Mit162, D3Mit163, D4Mit7, D4Mit17, D6Mit14, D6Mit15, D6Mit48, D6Mit58, D6Mit158, D7Mit9, D7Mit10, D7Mit14, D7Mit47, D7Mit67, D7Mit117, D8Mit12, D8Mit17, D10Mit14, D10Mit46, D10Mit47, D11Mit4, D11Mit20, D11Mit21, D11Mit139, D11Mit164, D11Nds9, D11Nds10, D16Mit19, D16Mit56, D16Mit73, D16Mit81, D16Mit134, D16Nds2, and D18Mit40. The CcS-5 backcross was genotyped with D1Mit129, D3Mit46, D5Mit27, D5Mit112, D5Mit164, D6Mit10, D6Mit23, D6Mit35, D6Mit173, D7Mit137, D8Mit17, D8Mit58, D8Mit155, D10Mit12, D10Mit14, D10Mit24, D10Mit25, D10Mit46, D10Mit47, D10Mit103, D10Mit133, D11Mit1, D11Mit2, D11Mit62, D11Mit71, D17Mit10, D17Mit13, D17Mit18, D17Mit19, D17Mit22, D17Mit35, D17Mit46, D17Nds3, D17Nds4, and D18Mit19. The CcS-11 backcross was genotyped with D1Mit15, D1Mit36, D1Mit205, D1Mit208, D3Mit18, D3Mit162, D3Mit163, D7Mit7, D7Mit8, D7Mit9, D7Mit10, D7Mit14, D7Mit15, D7Mit26, D7Mit47, D7Mit54, D7Mit55, D7Mit67, D7Nds1, D7Nds2, D7Nds5, D7Nds4, D8Mit40, D8Mit85, D10Mit12, D10Mit14, D10Mit24, D10Mit46, D10Mit47, D10Mit133, D10Mit150, D12Mit37, D16Mit34, D16Mit73, D19Mit12, D19Mit41, D19Mit56, D19Mit60, D19Mit61, and D19Mit62. These markers cover the known STS-derived segments from strains CcS-3, CcS-5, and CcS-11 with a spacing of approximately 5 cM (16)
Statistical Analysis.
To obtain a normal distribution, the numbers of colon tumors in the CcS-3 backcross were elog-transformed. In the CcS-5 and CcS-11 backcrosses, the exponent 0.2 normalized the colon tumor numbers. Linkage between the number of colon tumors and the markers in the three different backcrosses was determined by advanced ANOVA (NCSS, Kaysville, UT) using gender and marker(s) as fixed factors. For CcS-5 and CcS-11, the experimental group was used as a random factor.
All single markers and all pairs of nonlinked markers were tested. Markers and interactions with P < 0.05 were combined in one model, and, subsequently, markers and interactions with a P > 0.05 were eliminated one by one, starting with the one with the highest P. Final Ps were corrected for multiple comparisons using the following formula:
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(T) is the observed P, the F ratio from ANOVA is used for T2, the genome length in Morgans G = 2 (the length of the segregating part of the donor genome; 12.5% of 16 M), the constant
= 1 (crossover rate) for a backcross, and C (the number of chromosomes segregating in the cross) is 8 for CcS-11 and 9 for CcS-3 and CcS-5 (16
, 18)
. | Results and Discussion |
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In the CcS-5 cross, the susceptibility locus Scc6 was detected. Linkage of susceptibility was found to the STS allele of D11Mit2 (P = 0.0043; Table 1
). Scc6 maps to a 6.6-cM region near the centromere on chromosome 11 between markers D11Mit71 and D11Mit162 (Fig. 1)
. A suggestive linkage (P = 0.074) in this cross was found to D1Mit129. The marker D1Mit129 is in fact located on mouse chromosome 5, closely linked to D5Mit304.6
Additional independent experiments are needed to confirm linkage at this locus. No significant or suggestive linkages have been found in the CcS-11 cross.
STS alleles of the markers to which Scc7, Scc8, and Scc9 were mapped are also present in some of the other backcrosses (Scc7 in CcS-11, Scc8 in CcS-5, and Scc9 in both CcS-5 and CcS-11). However, only Scc9 was detected in another cross (CcS-5; Table 1
). The failure to detect these loci in all crosses does not disprove them. As pointed out by Lander and Kruglyak (18)
, the initial, significant linkages can be overestimates of the effect of the loci; due to random fluctuations, the detected linkage will be above the threshold of significance. In other experiments, random fluctuations can push the effect of these genes below the threshold of detection. In addition, some of the loci are detected by virtue of their interactions with other loci (12
, 14
, 15)
. Therefore they can probably only be detected in a particular genetic background and are not observed in mice with a different genetic make-up. Paradoxically, in the present experiments, most loci were detected in the cross with the smallest number of animals. However, the detection of four loci in a total of 196 mice is comparable to the number of loci detected in similar experiments (12
, 14
, 15
, 19)
. The distribution of the linkages between the crosses has a stochastic component and is influenced by interlocus interactions. These interactions, which are presently poorly defined, probably play a much larger role in the genetics of quantitative traits than is generally recognized (15)
.
In conclusion, we have found four novel loci involved in the complex genetic trait of colon cancer susceptibility. This extends the number of colon tumor susceptibility loci we have detected using the recombinant congenic strain system to nine. Recombinants for the regions on chromosomes 10 and 11 containing Scc9 and Scc6, respectively, have already been produced and will be tested for colon tumor susceptibility to confirm the linkage and map these loci more precisely. Subsequent cloning of these genes could elucidate the underlying mechanisms of cancer susceptibility and of the genetic interactions. The small initial segment to which the loci are mapped, together with the physical maps and the increasing density of the available Expressed Sequence Tag maps, will speed up the identification of candidate genes for these Scc loci.
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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1 Supported by Dutch Cancer Society Grants NKI-97-1463 and NKI-98-1832 (to P. D.). ![]()
2 Present address: Department of Human Genetics, Leiden University Medical Center, Wassenaarseweg 72, 2333 AL Leiden, the Netherlands. ![]()
3 To whom requests for reprints should be addressed. ![]()
4 The abbreviations used are: FAP, familial adenomatous polyposis; DMH, 1,2-dimethylhydrazine; CcS, CcS/Dem; BALB/c, BALB/cHeA; STS, STS/A; HNPCC, hereditary nonpolyposis colorectal cancer. ![]()
5 In addition to Ref. 16
, see also http://www.informatics.jax.org/rcset.html. ![]()
6 H. Havelkova, personal communication. ![]()
Received 3/25/99. Accepted 7/19/99.
| REFERENCES |
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