Human and rodent transspecies carcinogens (trans-species carcinogens) often demonstrate similar organotropic patterns of neoplasia and loss of heterozygosity (LOH). We have observed significant chromosome 11 LOH in N5 C57BL/6:129Sv heterozygous p53 mice using simple sequence length polymorphic loci (SSLP). Primers specific for known SSLP loci revealed amplicons consistent with the two genetic strains, C57BL/6 and 129Sv. We hypothesized that carcinogen induced DNA damage in the p53 haploinsufficient mouse results in illegitimate mitotic recombination during repair leading to genomic instability and neoplasia. By exploiting the observed heterozygosity on chromosome 11 in the 12th backcross generation, we learned that LOH was not restricted to the Trp53 locus and exhibits a carcinogen specific patter of LOH. A complete copy of chromosome 11 was apparently lost during carcinogen induced lymphomagenesis to some agents but not others, where LOH pattern suggest dysruption of recombination based repair. This investigation has confirmed the importance of an aneugenic and DNA repair mechanisms of action for a number of environmental carcinogens. For example, chromosome 11 loss also occurred in some, but not all, benzene and p-cresidine induced p53 (+/-) mouse sarcomas (oral, intubation) and thymic lymphomas (inhalation, whole animal) and bladder tumors (dietary). Allelotype data from the benzene and p-cresidine studies showed LOH consistent with DNA breaks and non-homologous sequence directed repair . These results establish microsatellite (SSLP loci) mapping as a useful tool for determination of LOH in carcinogenesis studies using p53 haploinsufficient mice, e.g. (C57BL/6 x 129Sv) or (C57BL/6 x C3H) F1, in which heterozygosity at all loci could be informative in investigating LOH and DNA break repair. In summary, we have shown that in a series of independent cancer biology studies that there is sufficient heterozygosity on chromosome 11 in the heterozygous p53 deficient (+/-) N5 generation mouse to use microsatellite markers at 5 cM intervals to demonstrate whole or partial chromosome loss through non-disjunction and homologous recombination.
We aim to continue to investigate rates of homologous and non-homologous recombination and determine loci specific positive and negative interference with recombination on chromosome 11 under exposure to environmental carcinogens inducing genomic instability. Specifically, this would enhance our scientific understanding of how this genetically altered mouse model responds when exposed to environmental carcinogens. Using this model, we will determine meiotic (parental and progeny germline) and mitotic recombinant genotype patterns (established in normal somatic tissues of progeny during embryogenesis as well as cancers that arise sporadically with different and unique recombinant genotypes). With microsatellite mapping, we will be able to fine map chromosome 11 sites and rates of homologous recombination and the effect on genomic instability. Public Health or Environmental Health Significance: Loss of p53 function is a common feature of human cancer (especially childhood cancer like leukemia and lymphoma). Exposure to environmental agents may be significant cause of LOH and loss of p53 function. If LOH is a common feature of environmental exposure to carcinogens, a mechanistic understanding may be critical to developing methods for prevention (eliminate or minimize carcinogen exposure) and developing biomarkers of exposure for extrapolation between rodent models and humans. Research Accomplishments: We have demonstrated that mice haploinsufficient for p53 (heterozygous p53 null allele mice) show either frequent or infrequent rates of LOH involving the p53 wildtype allele locus that are carcinogen and tissue dependent. By comparing LOH patterns in wildtype and p53 haploinsufficient mouse tumors of mesenchymal origin that show increased LOH of thep53 locus or of chromosome 11, the pattern of allele loss is consistent with either mis-segregation (non-dysjunction) or recombination. In those tumors where LOH is infrequent, mutation of the p53 wildtype allele or the rescued lacIq (B6.129-Trp53 heterozygotes hemizygous for the neutral reporter gene) could be demonstrated in the absence of mutations in the highly conserved region (exons 4-8) (40). Many lines of evidence indicate that the role of p53 in maintaining fidelity in homologous recombination and preventing non-homologous recombination is significant. Based on research conducted to date, we are using the p53 haploinsufficient mouse as an in vivo model for induction of neoplasia with model human carcinogens in order to determine their mechanism of action for allele loss. Use of mice heterozygous at all genetic loci, e.g. (C57BL/6 x 129Sv) or (C57BL/6 x C3H) F1 should increase precision and fine mapping and allow identification of loci reduced to homozygosity. Due to the narrow difference in susceptibility between B6 and C3H mice we now use the DBA/2, which supplies resistance genes. Studies are in progress using F1 and F2 progeny from intercrosses between male C57BL/6-Trp53tm1Brd N12 mice homozygous for the p53 null allele and female inbred 129Sv, C3H/HeN, or DBA/2 mice. The usefulness, although labor intensive, of this approach is based on the use of genomic DNA from pre-neoplastic or neoplastic lesions and does not require tumor explant to culture cells for further cytogenetic analysis. By mapping sites of LOH (and tumor suppressor gene loss), changes in gene copy number and identifying quantitative trait loci, we expect to develop a better understanding of associated mechanisms of cancer. This is important because this research project complements programmatic research on the development and characterization of this genetically altered mouse models for use in carcinogen identification and classification for risk assessment to support alternative cancer bioassays.
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