Genetic Mapping in Mice Identifies DMBT1 as a Candidate Modifier of Mammary Tumors and Breast Cancer Risk
2007; Elsevier BV; Volume: 170; Issue: 6 Linguagem: Inglês
10.2353/ajpath.2007.060512
ISSN1525-2191
AutoresAnneke C. Blackburn, Linda Zhai Hill, Amy L. Roberts, Jun Wang, Dee Aud, Jimmy Jung, Tania Nikolcheva, John Allard, Gary Peltz, Christopher N. Otis, Qing Cao, Reva St. J. Ricketts, Stephen P. Naber, Jan Mollenhauer, Annemarie Poustka, Daniel Malamud, D. Joseph Jerry,
Tópico(s)BRCA gene mutations in cancer
ResumoLow-penetrance breast cancer susceptibility alleles seem to play a significant role in breast cancer risk but are difficult to identify in human cohorts. A genetic screen of 176 N2 backcross progeny of two Trp53+/− strains, BALB/c and C57BL/6, which differ in their susceptibility to mammary tumors, identified a modifier of mammary tumor susceptibility in an ∼25-Mb interval on mouse chromosome 7 (designated SuprMam1). Relative to heterozygotes, homozygosity for BALB/c alleles of SuprMam1 significantly decreased mammary tumor latency from 70.7 to 61.1 weeks and increased risk twofold (P = 0.002). Dmbt1 (deleted in malignant brain tumors 1) was identified as a candidate modifier gene within the SuprMam1 interval because it was differentially expressed in mammary tissues from BALB/c-Trp53+/− and C57BL/6-Trp53+/− mice. Dmbt1 mRNA and protein was reduced in mammary glands of the susceptible BALB/c mice. Immunohistochemical staining demonstrated that DMBT1 protein expression was also significantly reduced in normal breast tissue from women with breast cancer (staining score, 1.8; n = 46) compared with cancer-free controls (staining score, 3.9; n = 53; P < 0.0001). These experiments demonstrate the use of Trp53+/− mice as a sensitized background to screen for low-penetrance modifiers of cancer. The results identify a novel mammary tumor susceptibility locus in mice and support a role for DMBT1 in suppression of mammary tumors in both mice and women. Low-penetrance breast cancer susceptibility alleles seem to play a significant role in breast cancer risk but are difficult to identify in human cohorts. A genetic screen of 176 N2 backcross progeny of two Trp53+/− strains, BALB/c and C57BL/6, which differ in their susceptibility to mammary tumors, identified a modifier of mammary tumor susceptibility in an ∼25-Mb interval on mouse chromosome 7 (designated SuprMam1). Relative to heterozygotes, homozygosity for BALB/c alleles of SuprMam1 significantly decreased mammary tumor latency from 70.7 to 61.1 weeks and increased risk twofold (P = 0.002). Dmbt1 (deleted in malignant brain tumors 1) was identified as a candidate modifier gene within the SuprMam1 interval because it was differentially expressed in mammary tissues from BALB/c-Trp53+/− and C57BL/6-Trp53+/− mice. Dmbt1 mRNA and protein was reduced in mammary glands of the susceptible BALB/c mice. Immunohistochemical staining demonstrated that DMBT1 protein expression was also significantly reduced in normal breast tissue from women with breast cancer (staining score, 1.8; n = 46) compared with cancer-free controls (staining score, 3.9; n = 53; P < 0.0001). These experiments demonstrate the use of Trp53+/− mice as a sensitized background to screen for low-penetrance modifiers of cancer. The results identify a novel mammary tumor susceptibility locus in mice and support a role for DMBT1 in suppression of mammary tumors in both mice and women. Approximately 27% of breast cancer risk has been attributed to genetic factors.1Lichtenstein P Holm NV Verkasalo PK Iliadou A Kaprio J Koskenvuo M Pukkala E Skytthe A Hemminki K Environmental and heritable factors in the causation of cancer—analyses of cohorts of twins from Sweden, Denmark, and Finland.N Engl J Med. 2000; 343: 78-85Crossref PubMed Scopus (3189) Google Scholar Highly penetrant, dominant alleles of genes such as BRCA1, BRCA2, and TP53 confer a high risk of developing breast cancer; however, mutant alleles such as these occur at a low frequency accounting for less than one-third of hereditary breast cancer and fewer than 5% of breast cancer cases in the general population.2Anglian Breast Cancer Study Group Prevalence and penetrance of BRCA1 and BRCA2 mutations in a population-based series of breast cancer cases. Anglian Breast Cancer Study Group.Br J Cancer. 2000; 83: 1301-1308Crossref PubMed Scopus (544) Google Scholar, 3Ford D Easton DF Stratton M Narod S Goldgar D Devilee P Bishop DT Weber B Lenoir G Chang-Claude J Sobol H Teare MD Struewing J Arason A Scherneck S Peto J Rebbeck TR Tonin P Neu-hausen S Barkardottir R Eyfjord J Lynch H Ponder BA Gayther SA Zelada-Hedman M The Breast Cancer Linkage Consortiu Genetic heterogeneity and penetrance analysis of the BRCA1 and BRCA2 genes in breast cancer families.Am J Hum Genet. 1998; 62: 676-689Abstract Full Text Full Text PDF PubMed Scopus (2489) Google Scholar, 4Newman B Mu H Butler LM Millikan RC Moorman PG King MC Frequency of breast cancer attributable to BRCA1 in a population-based series of American women.JAMA. 1998; 279: 915-921Crossref PubMed Scopus (321) Google Scholar, 5Whittemore AS Gong G Itnyre J Prevalence and contribution of BRCA1 mutations in breast cancer and ovarian cancer: results from three U.S. population-based case-control studies of ovarian cancer.Am J Hum Genet. 1997; 60: 496-504PubMed Google Scholar Therefore, the susceptibility alleles underlying the majority of heritable breast cancer remain to be identified. Intensive efforts during the past decade have sought additional highly penetrant genes; however, the search remains primarily unfulfilled. This has stimulated interest in the role of low-penetrance modifier alleles in heritable breast cancers. Low-penetrance alleles may be very frequent in the population, but breast cancer would occur in only a small fraction of individuals carrying these alleles. In this situation, it would be difficult to recognize familial clustering. Therefore, low-penetrance alleles are likely to contribute to a significant fraction of what is presently recognized as sporadic breast cancer.6Nathanson KL Weber BL “Other” breast cancer susceptibility genes: searching for more holy grail.Hum Mol Genet. 2001; 10: 715-720Crossref PubMed Scopus (82) Google Scholar, 7Ponder BA Cancer genetics.Nature. 2001; 411: 336-341Crossref PubMed Scopus (466) Google Scholar Modeling of the genetic basis of breast cancer risk indicated that more than 50% of breast cancers may originate from the 12% of the population that are highly susceptible to cancer,8Pharoah PD Antoniou A Bobrow M Zimmern RL Easton DF Ponder BA Polygenic susceptibility to breast cancer and implications for prevention.Nat Genet. 2002; 31: 33-36Crossref PubMed Scopus (642) Google Scholar suggesting that most of the genetic predisposition to breast cancer might be connected to these low-penetrance risk alleles that are common among the general population. The significance of low-penetrance alleles is also evident in breast cancer families with known mutations. Although mutations in BRCA1 and BRCA2 seem to cause strikingly similar clinical disease in twins,9Delgado L Fernandez G Gonzalez A Bressac-de Paillerets B Gualco G Bombled J Cataldi S Sabini G Roca R Muse IM Hereditary breast cancer associated with a germline BRCA2 mutation in identical female twins with similar disease expression.Cancer Genet Cytogenet. 2002; 133: 24-28Abstract Full Text Full Text PDF PubMed Scopus (12) Google Scholar the penetrance of breast cancer varies between 28% and 85% among different populations carrying BRCA1 susceptibility alleles.6Nathanson KL Weber BL “Other” breast cancer susceptibility genes: searching for more holy grail.Hum Mol Genet. 2001; 10: 715-720Crossref PubMed Scopus (82) Google Scholar, 10Fodor FH Weston A Bleiweiss IJ McCurdy LD Walsh MM Tartter PI Brower ST Eng CM Frequency and carrier risk associated with common BRCA1 and BRCA2 mutations in Ashkenazi Jewish breast cancer patients.Am J Hum Genet. 1998; 63: 45-51Abstract Full Text Full Text PDF PubMed Scopus (174) Google Scholar, 11Warner E Foulkes W Goodwin P Meschino W Blondal J Paterson C Ozcelik H Goss P Allingham-Hawkins D Hamel N Di Prospero L Contiga V Serruya C Klein M Moslehi R Honeyford J Liede A Glendon G Brunet JS Narod S Prevalence and penetrance of BRCA1 and BRCA2 gene mutations in unselected Ashkenazi Jewish women with breast cancer.J Natl Cancer Inst. 1999; 91: 1241-1247Crossref PubMed Scopus (329) Google Scholar Therefore, genetic background can alter latency and clinical course of breast cancer. Patients carrying germline mutations in TP53 suffer from Li-Fraumeni syndrome and are at high risk of developing multiple tumor types, with early-onset breast cancer being the most common cancer in females. Penetrance of breast cancer also varies considerably among women with identical TP53 mutations12Varley JM McGown G Thorncroft M Santibanez-Koref MF Kelsey AM Tricker KJ Evans DG Birch JM Germ-line mutations of TP53 in Li-Fraumeni families: an extended study of 39 families.Cancer Res. 1997; 57: 3245-3252PubMed Google Scholar, 13Varley JM Thorncroft M McGown G Appleby J Kelsey AM Tricker KJ Evans DG Birch JM A detailed study of loss of heterozygosity on chromosome 17 in tumours from Li-Fraumeni patients carrying a mutation to the TP53 gene.Oncogene. 1997; 14: 865-871Crossref PubMed Scopus (95) Google Scholar that may be caused by low-penetrance alleles that modify cancer phenotypes. Indeed, a single nucleotide polymorphism (SNP) in the promoter of MDM2, an inhibitor of p53 function, decreased the age-at-onset by 10 years of hereditary breast cancer in Li-Fraumeni syndrome patients in one study.14Bond GL Hu W Bond EE Robins H Lutzker SG Arva NC Bargonetti J Bartel F Taubert H Wuerl P Onel K Yip L Hwang SJ Strong LC Lozano G Levine AJ A single nucleotide polymorphism in the MDM2 promoter attenuates the p53 tumor suppressor pathway and accelerates tumor formation in humans.Cell. 2004; 119: 591-602Abstract Full Text Full Text PDF PubMed Scopus (1056) Google Scholar Although evidence suggests a strong role for low-penetrance modifiers of breast cancer risk, genetic mapping in human populations has limited power. In contrast, inbred mouse strains bearing genetic deletion of a tumor suppressor gene or with transgenic oncogene expression can be used to identify genetic loci that modify cancer risk. Mice bearing a mutation in the Apc gene (Min) have been used to discover a modifier of Min (Mom1 or Pla2g2a) that affects tumor multiplicity and size of intestinal tumors.15Cormier RT Hong KH Halberg RB Hawkins TL Richardson P Mulherkar R Dove WF Lander ES Secretory phospholipase Pla2g2a confers resistance to intestinal tumorigenesis.Nat Genet. 1997; 17: 88-91Crossref PubMed Scopus (291) Google Scholar, 16Dietrich WF Lander ES Smith JS Moser AR Gould KA Luongo C Borenstein N Dove W Genetic identification of Mom-1, a major modifier locus affecting Min-induced intestinal neoplasia in the mouse.Cell. 1993; 75: 631-639Abstract Full Text PDF PubMed Scopus (608) Google Scholar Expression of the polyoma middle T-antigen oncogene has also been used to identify tyrosine kinase signaling pathways that alter mammary tumor latency.17Hunter K Welch DR Liu ET Genetic background is an important determinant of metastatic potential.Nat Genet. 2003; 34: 23-24Crossref PubMed Scopus (85) Google Scholar, 18Le Voyer T Lu Z Babb J Lifsted T Williams M Hunter K An epistatic interaction controls the latency of a transgene-induced mammary tumor.Mamm Genome. 2000; 11: 883-889Crossref PubMed Scopus (48) Google Scholar, 19Le Voyer T Rouse J Lu Z Lifsted T Williams M Hunter KW Three loci modify growth of a transgene-induced mammary tumor: suppression of proliferation associated with decreased microvessel density.Genomics. 2001; 74: 253-261Crossref PubMed Scopus (37) Google Scholar, 20Rose-Hellekant TA Gilchrist K Sandgren EP Strain background alters mammary gland lesion phenotype in transforming growth factor-alpha transgenic mice.Am J Pathol. 2002; 161: 1439-1447Abstract Full Text Full Text PDF PubMed Scopus (23) Google Scholar, 21Lifsted T Le VT Williams M Muller W Klein-Szanto A Buetow KH Hunter KW Identification of inbred mouse strains harboring genetic modifiers of mammary tumor age of onset and metastatic progression.Int J Cancer. 1998; 77: 640-644Crossref PubMed Scopus (187) Google Scholar In addition to predisposing women to breast cancer when mutated in the germline, somatic mutation or loss of the p53 tumor suppressor gene is a common feature of breast cancer. Therefore, we used Trp53+/− mice as a model system to identify genes that modify breast cancer susceptibility and may be relevant to both hereditary and sporadic breast cancer. The frequency of mammary tumors in Trp53+/− mice is highly strain-dependent, with a high frequency (55%) occurring only on a BALB/c background,22Kuperwasser C Hurlbut GD Kittrell FS Dickinson ES Laucirica R Medina D Naber SP Jerry DJ Development of spontaneous mammary tumors in BALB/c p53 heterozygous mice: a model for Li-Fraumeni syndrome.Am J Pathol. 2000; 157: 2151-2159Abstract Full Text Full Text PDF PubMed Scopus (157) Google Scholar whereas occurrence of mammary tumors was 3.3 was used as described previously.28Lander E Kruglyak L Genetic dissection of complex traits: guidelines for interpreting and reporting linkage results.Nat Genet. 1995; 11: 241-247Crossref PubMed Scopus (4471) Google Scholar In regions of significant association (−logP >3.3), markers were genotyped in individual mice for confirmation, and finer mapping with additional markers (Table 1) was performed. Using the genotype at the marker with the highest LOD score, Kaplan-Meier estimates of the tumor-free survival curves were calculated and plotted for homozygotes and heterozygotes. The entire group of animals was used for survival analysis The median time to tumor was used for comparison of latencies and the significance of differences in latency (tumor-free survival times) and were analyzed by the log-rank test.Table 1Locations of Markers Used for Localizing SuprMam1 LocusChrPosition (Mb)SNP ID−log P valueHuman Chr714.1S46665_10.19819728.7M-09441_10.28019760.3M-10051_30.26615772.9Mfge8_3_JC5236_20.87415774.3Blm_1_JC5359_10.86715785.2M-00295_11.156117107.9M-09671_12.921117109.2M-08779_32.509117113.2M-10217_12.456167119.6M64879_201_13.523167120.5X67140_BS2_12.796167137.1U71085_JC2185_21.47111Data based on Ensembl (build 34). Open table in a new tab Data based on Ensembl (build 34). Mammary glands from C57BL/6- and BALB/c-Trp53+/− mice were collected from 12-week-old virgins and snap-frozen in liquid nitrogen. Total RNA was reverse-transcribed using a T7-promoter-coupled oligo(d)T primer (GeneChip T7-Oligo(d)T promoter primer kit; Affymetrix, Santa Clara, CA). After the second-strand cDNA synthesis, an in vitro transcription reaction was performed using Enzo BioArray high-yield RNA transcript labeling kit (Affymetrix). The labeled samples were hybridized to the murine genome U74v2 set that contains probe sets for ∼36,000 full-length mouse genes and expressed sequence tag clusters from the UniGene database (Affymetrix). GeneChips were scanned using the GS2500 scanner, and images were analyzed by Affymetrix software (Microarray Analysis Suite version 5.0). Four mice of each strain were analyzed with pairwise comparisons. Genes showing at least twofold expression differences, with intensity values >150 in one strain, and a P value <0.05 were considered to be differentially expressed. Mammary glands and small intestines from C57BL/6- and BALB/c-Trp53+/− mice were collected from 8-week-old virgin females as described above. Total RNA was extracted from these tissues using QIAzol reagent (Qiagen, Valencia, CA) following the manufacturer's manual. One μg of each tissue total RNA was reverse-transcribed using AMV reverse transcriptase (Seikagaku America, East Falmouth, MA) in a 20-μl reaction mix. A 5-μl aliquot of the cDNA products was then amplified using two sets of forward and reverse primers using CUB5 (5′-AGCACAAGTCTCCATCACCCAAACA-3′) and ZP3 (5′-GATTGGTGGTGTTATTGTCAAAGTC-3′) as well as TM5 (5′-ATCTTTGGCGGAGTCTTCCTGG-3′) and mUTR3 (5′-GTTGGCTATACATGGGGAAAAGGG-3′). The annealing temperature for the primer pairs was 60°C, and product sizes were 761 and 418 bp, respectively. Mouse β-actin was also amplified as a control using primers Actin5F (5′-TGCTGTCCCTGTATGCCTCT-3′) and Actin3R (5′-TGCCACAGGATTCCATACCC-3′), which anneal to the cDNA template at 60°C and produce a 405-bp product. PCR was performed with 300 pmol/ml of each primer in a 20-μl reaction volume containing 1× PCR buffer (Sigma, St. Louis, MO), 2 mmol/L MgCl2 (Sigma), 250 μmol/L dNTPs (Sigma), and 0.5 U of Taq polymerase (Sigma) and amplified 30 cycles (94°C for 1 minute, 60°C for 1 minute, 72°C for 1 minute). The products were run on agarose gels and viewed by ethidium bromide staining. RNA from normal human tissues was purchased from Clontech (Palo Alto, CA). RNA was isolated from the 76N breast epithelial cell line series, normal cells immortalized with telomerase, mutant p53, or adenovirus E6 (76N + hTERT, 76N + p53mt, and 76N + E6, respectively; a generous gift from Dr. Vimla Band, Northwestern University, Chicago, IL)29Cao Y Gao Q Wazer DE Band V Abrogation of wild-type p53-mediated transactivation is insufficient for mutant p53-induced immortalization of normal human mammary epithelial cells.Cancer Res. 1997; 57: 5584-5589PubMed Google Scholar, 30Gao Q Hauser SH Liu XL Wazer DE Madoc-Jones H Band V Mutant p53-induced immortalization of primary human mammary epithelial cells.Cancer Res. 1996; 56: 3129-3133PubMed Google Scholar using QIAzol reagent (Qiagen). Human breast cancer RNA samples were from a panel of infiltrating ductal carcinomas (grades I to III) collected previously from the frozen tissue bank at Baystate Medical Center, Springfield, MA.31Pinkas J Naber SP Butel JS Medina D Jerry DJ Expression of MDM2 during mammary tumorigenesis.Int J Cancer. 1999; 81: 292-298Crossref PubMed Scopus (19) Google Scholar For DMBT1 Q-PCR, the primer pair 5′-ATTGTGCTGCACCTGGTCAT-3′ and 5′-AGCGGGAAGAGGGGTCATA-3′ was used to amplify a 263-bp product. Total RNA (90 ng) was reverse-transcribed and amplified using the Applied Biosystems (Foster City, CA) rTth DNA polymerase in a single tube. Reactions were run on GeneAmp 5700 sequence detection system (Applied Biosystems) with an RT cycle of 50°C for 2 minutes, 95°C for 1 minute, 60°C for 30 minutes, followed by 45 cycles of 95°C for 20 seconds, 60°C for 20 seconds, and 72°C for 10 minutes. The products were measured using SYBR green. Human β-actin was amplified as a loading control. For RT-PCR, total RNAs were used as a template to amplify a 780-bp fragment of DMBT1 mRNA with the forward primer ZP5 (5′-TTCTTGTATCCTGTGACCAG-3′) and reverse primer hUTR3 (5′-GCAGTTTCACCAAAATTCCTTTATG-3′). Human GAPDH was also amplified using primers 5′-TTCACCACCATGGAGAAGGC-3′ (forward) and 5′-TGCATGGACTGTGGTCATGA-3′ (reverse) as the loading control. The reaction conditions were 30 PCR cycles at 94°C for 45 seconds, 51°C for 45 seconds, 72°C for 1 minute. Northern blot analysis was performed as described previously.26Jerry DJ Kuperwasser C Downing SR Pinkas J He C Dickinson ES Marconi S Naber SP Delayed involution of the mammary epithelium in BALB/c-p53null mice.Oncogene. 1998; 17: 2305-2312Crossref PubMed Scopus (75) Google Scholar Total RNA (10 μg) from each tissue were subjected to electrophoresis on a 1.2% agarose-formaldehyde gel and then immobilized onto nylon membranes (CUNO Laboratory Products, Meridan, CT). The membranes were hybridized to cDNA probes labeled with [32P]dCTP. The probe covers mouse Dmbt1 nucleotides 4479 to 5250 in the coding region, which was obtained through RT-PCR using CUB5 + ZP3 primer set. The hybridization signals were visualized by Cyclone phosphor imager (Packard Bioscience, Downers Grove, IL). The results were quantified using Optiquant image analysis software (Packard Bioscience). Band intensities for Dmbt1 were standardized for loading by comparison to bands hybridized with a probe for Gapdh, and the significance of differences was determined by t-tests. Polyclonal antibodies raised against the sodium dodecyl sulfate-denatured human salivary agglutinin/gp340 isoform (anti-SAG1529, provided by D.M.) were used for immunohistochemical staining of mouse tissues at a dilution of 1:100 with DAKO (Carpinteria, CA) anti-rabbit polymer for detection. The immunohistochemical expression of DMBT1 was evaluated in benign breast glandular epithelium from 53 patients without a history of carcinoma [46 reduction mammoplasties and seven excisional breast biopsies for ectopic (axillary) mass or calcifications found to be benign] and in benign breast glandular epithelium from 46 patients with a history of breast carcinoma (six ductal carcinoma in situ, six infiltrating lobular carcinomas, and 34 infiltrating ductal carcinomas). All cases were formalin-fixed, paraffin-embedded specimens retrieved from the surgical pathology archives of Department of Pathology, Baystate Medical Center, Springfield, MA. The specimens were stripped of identifiers in accordance with procedures approved by the Institutional Review Board at Baystate Medical Center and the University of Massachusetts. The primary diagnoses were confirmed by review of the original H&E-stained slides. The DMBT1 protein expression was evaluated by immunohistochemistry with anti-DMBT1h12 monoclonal antibody32Mollenhauer J Herbertz S Holmskov U Tolnay M Krebs I Merlo A Schroder HD Maier D Breitling F Wiemann S Grone HJ Poustka A DMBT1 encodes a protein involved in the immune defense and in epithelial differentiation and is highly unstable in cancer.Cancer Res. 2000; 60: 1704-1710PubMed Google Scholar using standard protocols on a DAKO autostainer. Intracytoplasmic granular staining in the epithelium was the most common pattern of staining and was regarded as positive. Immunoreactivity in inflammatory cells and stroma was infrequent and was not considered in this study. The immunohistochemical expression of DMBT1 within the glandular epithelium was quantified using percentage of staining as well as staining intensity. Staining percentage was quantified as follows: 0,= no staining; 1, 50% of cells with positive staining. A separate score was given for staining intensity; 0, no staining; 1, weak staining; 2, moderate staining; 3, strong staining (see Figure 6C). The scores for staining percentage and intensity were combined for an overall score. The staining score was assessed by two pathologists independently (J.C. and R.S.J.R.) with consensus recorded. Cases with combined scores of 0 to 2 were considered negative. Cases with a score ≥4 (with 6 being the maximum) were considered significantly positive. Comparisons of the occurrence of significant positive staining (score ≥4) and the average staining score in women with breast cancer versus cancer-free controls were made using Fisher's exact tests and Wilcoxon rank sum tests, respectively. Adjusted analyses to control for age were performed in addition to analyses on the whole set of results, and the association of age with staining was examined by Fisher's exact test on subgroups of the samples. The variance of each group was compared using variance ratio tests. All analyses were performed using the Stata statistical software package (StataCorp, College Station, TX). To identify loci contributing to mammary tumor susceptibility, 224 ([C57BL/6 × BALB/c] × BALB/c)-N2-Trp53+/− female mice were palpated weekly for mammary tumor development for 18 months. After histological confirmation of the mammary gland phenotypes, two groups of mice were defined—mice with mammary tumors (n = 85) and mice without mammary tumors (n = 91). Individual DNA samples were prepared and then pooled within each group. A genome scan was performed on the two pools of DNA to identify mammary tumor modifier loci. The results indicated a single genomic region on chromosome 7 that had a significant association with occurrence of mammary tumors in Trp53+/− mice (Figure 1A). Analysis of the DNAs from individual mice using additional SNP markers confirmed a locus on chromosome 7. The association was greatest (−logP = 3.5) with marker M64879_201_1 (Figure 1B) with significant linkage extending between markers 107.9 (−logP = 2.92; −09671_1) and 120.5 Mb (−logP = 2.79; X67140_BS2_1). This chromosome 7 locus was designated SuprMam1 for suppressor of mammary tumors. Of note, there were potentially two peaks within this linkage region. This raises the possibility that there may be more than one gene within the interval affecting susceptibility. The genotype of N2 mice at M64879_201_1 significantly altered the prevalence of mammary tumors during the observation period. Of the mice homozygous for BALB/c alleles, 63 of 116 (54%) developed mammary tumors compared with 26 of 92 (28%) among those heterozygous for C57BL/6 and BALB/c alleles (P < 0.0001, χ2 test). Mice that did not succumb to mammary tumors developed other tumor types typical of Trp53+/− mice,24Harvey M McArthur MJ Montgomery CA Bradley A Donehower LA Genetic background alters the spectrum of tumors that develop in p53-deficient mice.FASEB J. 1993; 7: 938-943Crossref PubMed Scopus (237) Google Scholar, 25Blackburn AC Brown JS Naber SP Otis CN Wood JT Jerry DJ BALB/c alleles for prkdc and cdkn2a interact to modify tumor susceptibility in trp53+/− mice.Cancer Res. 2003; 63: 2364-2368PubMed Google Scholar primarily lymphoma, adrenal gland tumors, and osteosarcomas. However, in contrast to mammary
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