Identification of novel carcinogen-mediated mammary tumor susceptibility loci in the rat using the chromosome substitution technique

Abstract

We here report the genetic basis for susceptibility and resistance to carcinogen-mediated [7,12-dimethylbenz[a]anthracene (DMBA)] mammary tumorigenesis using the full panel of SS/BN consomic rat strains, in which substitutions of individual chromosomes from the resistant BN strain onto the genomic background of the susceptible SS strain were made. Analysis of 252 consomic females identified rat mammary Quantitative Trait Loci (QTLs) affecting tumor incidence on chromosomes 3 and 5, latency on chromosomes 3, 9, 14, and 19, and multiplicity on chromosomes 13, 16, and 19. In addition, we unexpectedly identified a novel QTL on chromosome 6 controlling a lethal toxic phenotype in response to DMBA. Upon further investigation with chromosomes 6 and 13 congenic lines, in which an additional 114 rats were investigated, we mapped (1) a novel mammary tumor QTL to a region of 27.1 Mbp in the distal part of RNO6, a region that is entirely separated from the toxicity phenotype, and (2) a novel and powerful mammary tumor susceptibility locus of 4.5 Mbp that mapped to the proximal q-arm of RNO13. Comparison of genetic strain differences using existing rat genome databases enabled us to further construct priority lists containing single breast cancer candidate genes within the defined QTLs, serving as potential functional variants for future testing.

Figure 1

Illustration of phenotypes from 20 consomic strains. a) The tumor incidence at week 15 after carcinogen treatment was calculated to 29% (4/14) and 54% (7/13) in the SS-3^BN and SS-5^BN consomic rat, respectively, which is significantly reduced compared with the SS parental strain (93%; 37/40) (SS vs. SS-3^BN, P<0.0001; SS vs. SS-5^BN, P=0.0317). b) Comparison of tumor latency showed a significantly different value for the SS-3^BN (black line, filled circles in the figure), d), SS-9^BN (black dashed line, open diamonds), SS-14^BN (green dotted line, open stars) and SS-19^BN consomic strains (purple dotted line, open triangles) vs. the SS (blue dotted line, stars) (SS vs. SS-3^BN, P=0.0043; SS vs. SS-9^BN, P=0.0018; SS vs. SS-14^BN, P<0.0001; SS vs. SS-19^BN, P<0.0001). c) A significant difference in tumor multiplicity was observed in the SS-13^BN, SS-16^BN and SS-19^BN rat compared to the SS (SS vs. SS-13^BN, P=0.0024; SS vs. SS-16^BN, P=0.0002; SS vs. SS-19^BN, P<0.0001; standard deviation (s.d.) is given). Due to a high death incidence (+) of the SS-6^BN consomic females early after carcinogen treatment, a true value of tumor incidence and multiplicity was not possible to obtain.

Figure 2

Schematic representation of the congenic lines derived from the SS-6^BN (a) and SS-13^BN-consomic rat (b). The markers displayed on the right side of each chromosome were used to define the differential segments. Each markers cytogenetic and Mb position is indicated. The short vertical lines flanking the differential segments represent the intervals of recombination between the SS and BN parental genomes.

Figure 3

Illustration of phenotypes from congenic lines derived from the SS-6^BN consomic rat. a) Comparison of tumor latency showed a significantly different value for the SS.BN-(D6Rat119-D6RatArb3) strain compared to the SS (SS vs. SS.BN-(D6Rat119-D6RatArb3), P<0.0001). SS: black line, filled circles; SS.BN-(D6Rat149-D6Rat18): purple dotted line, open triangles; SS-6^BN: dashed black line, open diamonds; SS.BN-(D6Rat149-D6RatHF8): red dotted line, open circles; SS.BN-(D6Rat149-D6Arb3): green dotted line, open stars; SS.BN-(D6Rat119-D6Arb3): blue dotted line, stars. b) Also a significant difference in tumor multiplicity was observed in the SS.BN-(D6Rat119-D6RatArb3) rat compared to the SS (SS vs. SS.BN-(D6Rat119-D6RatArb3), P=0.0003; s.d. is given). c) The death rates were evaluated to 57% (8/14) for the SS.BN-(D6Rat149-D6RatArb3), 80% (12/15) for the SS.BN-(D6Rat149-D6Rat18) and 40% (6/15) for the SS.BN-(D6Rat149-D6RatHF8) congenic by the end of week 15, all of which showed a significant phenotypic difference compared with the SS (SS vs. SS.BN-(D6Rat149-D6RatArb3), P<0.0001; SS vs. SS.BN-(D6Rat149-D6Rat18), P<0.0001; SS vs. SS.BN-(D6Rat149-D6RatHF8), P=0.0044). Due to a high death incidence (+) of the SS.BN-(D6Rat149-D6RatArb3), SS.BN-(D6Rat149-D6Rat18) and SS.BN-(D6Rat149-D6RatHF8) females (similarly to what was observed in the SS-6^BN consomic females), a true value of the tumor phenotypes were not possible to obtain. SS: black line, filled circles; SS.BN-(D6Rat149-D6Rat18): purple dotted line, open triangles; SS-6^BN, black dashed line, open diamonds; SS.BN-(D6Rat149-D6RatHF8): red dotted line, open circles; SS.BN-(D6Rat149-D6RatArb3): green dotted line, open stars; SS.BN-(D6Rat119-D6RatArb3): blue dotted line, stars.

Figure 4

Illustration of phenotypes from congenic lines derived from the SS-13^BN consomic rat. a) Comparison of tumor latency showed a significantly different value for the SS.BN-(D13Rat123-D13Rat101) strain compared to the SS (SS vs. SS.BN-(D13Rat123-D13Rat101), P<0.0001). SS.BN-(D13Rat7-D13Rat60): red dotted line, open circles; SS.BN-(D13Rat123-D13Rat101): purple dotted line, open triangle; SS.BN-(D13Rat151-D13Rat197): blue dotted line, stars; SS: black line, filled circles; SS.BN-(D13Rat111-D13Got22): green dotted line, open stars; SS-13^BN: black dashed line, open diamonds. b) Also a significant difference in tumor multiplicity was observed in the SS.BN-(D13Rat123-D13Rat101) rat compared to the SS (SS vs. SS.BN-(D13Rat123-D13Rat101), P=0.0343; s.d. is given).