This is the first genome-wide scan for QTLs affecting radiation-induced mammary tumorigenesis in rodents. We detected two significant Mts
-QTLs on chromosomes 9 and 1 and two suggestive Mts
-QTLs on chromosomes 5 and 18. The chromosome 9 Mts-1
, chromosome 5 -Mts
and chromosome 18 Mts-4
QTLs represent novel QTL regions associated with mammary tumorigenesis not previously observed in other rat intercrosses of DMBA-induced and estrogen-induced mammary tumorigenesis 
QTL spanning chromosome1 125–145 Mbp region overlaps with Mcs3
QTL that maps to chromosome1 109.1–138.8 Mbp region in a COP x WF intercross rat population phenotyped for DMBA-induced mammary carcinogenesis 
. Interesting, both QTLs decrease susceptibility to tumorigenesis suggesting that the same gene might underlie both QTLs effects on mammary tumorigenesis. Mts-3
mapping to chromosome 5 147–167 Mbp region partially overlaps with Emca1
spanning chromosome 5 107–159 Mbp region detected in an F2 (COP x ACI)-intercross rat population characterized for estrogen-induced mammary carcinogenesis 
. However, having different modes of inheritance, recessive for Emca1
and co-dominant or additive for Mts-3
, data suggest that different genes might account for these two QTL effects on rat mammary tumorigenesis. Finally, Mts-4
at chromosome 18 66–86 Mbp region partially overlaps with Mcsta2
that peaks at 68 Mbp in a SPRD-Cu3 x WKY backcross rat population characterized for DMBA-induced mammary carcinogenesis 
. However, Mts-4
have opposite effects on susceptibility with Mcsta
decreasing and Mts-4
increasing susceptibility to mammary tumorigenesis suggesting that different genetic determinants underlie these QTLs effects on tumorigenesis. Altogether, the data suggest that distinct genetic determinants exist that confer susceptibility to irradiation-induced mammary tumorigenesis from those loci affecting chemically-induced and estrogen-induced mammary tumorigenesis.
As described by Cronkite et al 1960 
, radiation-induced mammary tumor models exhibits both adenocarcinoma and fibroadenoma in Sprague Dawley rats. Notably, both adenocarcinoma and fibroadenoma were also detected in the F2[Dahl S x R] breast cancer cohort studied here concordant with the fact that both Dahl S and Dahl R rats were derived from Sprague Dawley rats selected for salt-sensitive and salt-resistant hypertension respectively. Given the same environmental insult, the spectrum of pathologies from malignant to benign, and the detection of multiple QTLs suggest that susceptibility to mammary tumorigenesis is a complex multifactorial event likely involving multiple genetic determinants and genetic modifiers. As spectrum of susceptibility, the data suggest that genetic analysis for sporadic breast cancer and fibroadenoma can be analyzed as one pathogenic event with a spectrum much like other diseases.
Although further studies are needed to identify causal genes in respective Mts-
QTLs, the panel of candidate genes with reported gene expression changes or polymorphisms in breast cancer patients, Prlh
for Mts-1, Anpep
for Mts-2, Ece1
, and Smad2
_validate the hypotheses that these genes should be studied further in different experimental systems and in humans as potential susceptibility genes for mammary tumorigenesis. Although no statistically significant genetic interaction was detected among the QTLs, we note all Mts
4 candidate genes are associated with key aspects of breast tumor progression and malignancy, which collectively could increase tumorigenesis susceptibility. Increased Prlh
leading to higher prolactin levels is associated with increased risk for breast cancer, increased metastasis, disease progression, lower response to tamoxifen and worse clinical prognosis 
. Furthermore, Prlh
as a candidate gene for Mts-
1 QTL is concordant with the observation that prolactin accelerated mammary tumorigenesis initiated by radiation 
. Similarly, the candidate genes for Mts-2
are both implicated in breast cancer. Anpep is increased in breast cancer effusions 
, and its down regulation is associated with invasive colorectal cancer 
and prostate cancer 
, while over expression of Anpep has been linked to Barrett’s adenocarcinomas 
, the gene for Bloom syndrome, is a DNA repair gene which may play a role in breast cancer occurrence as its loss may contribute to somatic mutations and loss of heterozygosis, chromosomal instability, aneuploidy, and sensitivity to DNA damaging agents 
. For Mts-3
QTL candidate gene, Ece1
, cumulative studies point to its role in breast cancer invasiveness and more frequent recurrence 
. Lastly, Mts-4
QTL candidate gene, Smad2,
underlies Smad2-dependent epithelial mesenchymal transition of breast cancer cells 
, a key step in invasiveness and subsequent metastasis. Intriguingly, the deduced synergisms from these breast cancer roles for each Mts
QTL candidate gene suggest the hypothesis that QTL-burden could increase risk, reiterating the need for further study.
Further inspection of the Rat Genome Database (RGD) reveals additional candidate genes within Mts-1, Mts-2, Mts-3 and Mts-4 QTLs that have been associated with different types of cancers (). However, more studies involving fine mapping through sub-strain construction will be necessary to identify the genes underlying these QTLs.
Genes associated with cancer within mammary tumorigenesis susceptibility QTLs detected in F2 (Dahl S x R)-intercross female rats.
In summary, the genome-wide scan for QTLs influencing breast cancer susceptibility identified genetic linkage to chromosomes 9, 1, 5 and 18 with radiation-induced mammary tumorigenesis in Dahl rats. The chromosomes 9, 5 and 18 QTLs were unique to this F2 (Dahl S x R)-intercross rat population suggesting that genetic mechanisms underlying radiation-induced mammary tumorigenesis differs substantially from mechanisms involved in chemically-induced and hormone-induced mammary tumorigenesis. Histopathology observations of a spectrum from poorly differentiated adenocarcinoma to benign fibroadenoma despite exposure to identical radiation dose in our F2[Dahl S x R]-intercross rat mammary tumor cohort suggest the importance of genetic susceptibility and/or gene modifiers in this gene-environment interaction paradigm.
Altogether, the detection of Mts- QTLs spanning fibroadenoma to poorly differentiated adenocarcinoma suggest the paradigm that genetic analysis of tumorigenesis-QTLs in humans could help elucidate breast cancer susceptibility genes which have remained elusive to date. Alternatively, the detection of candidate genes within Mts-QTLs associated with breast cancer in humans validates their analysis as causal susceptibility genes in human cohorts respectively. Elucidation of variants accounting for mammary tumorigenesis in Dahl rats will give insights into gene pathways important for gene-environment interactions, tumor initiation and progression.