Chromosome X loci and spontaneous granulosa cell tumor development in SWR mice: Epigenetics and epistasis at work for an ovarian phenotype

Females of the SWR/Bm (SWR) inbred mouse strain possess a unique susceptibility to juvenile-onset tumors originating from the granulosa cells (GC) of the ovarian follicles. Tumor susceptibility is an inherited, polygenic trait in SWR females, minimally involving an oncogenic Granulosa cell tumor susceptibility (Gct) locus on chromosome (Chr) 4 (Gct1), and two GC tumor susceptibility modifier genes mapped to distinct regions of Chr X (Gct4 and Gct6). Shifts in the frequency of GC tumor initiation in the SWR female population from low penetrance to moderate penetrance, or phenotype switching between GC tumor-susceptible and GC tumor-resistant, is strongly influenced by the allelic contributions at Gct4 and Gct6. In addition to the allele-specific effects, GC tumor susceptibility is controlled by the mode of X-linked transmission with a dominant, paternal parent-of-origin effect. We took advantage of the robust paternal effect with a recombinant male progeny testing strategy to resolve the Gct4 locus interval to 1.345 million base (Mb) pairs. Based on the mapping resolution and the phenotype sensitivity to endogenous and exogenous androgen exposure, a promising candidate for Gct4 identity is the androgen receptor (Ar) gene. We explored the mechanism of allelic variation for Ar between SWR (low penetrance allele) and SJL/Bm (SJL) (moderate penetrance allele) using an SWR.SJL-X congenic strain resource and a quantitative gene expression method. We report the low GC tumor penetrance allele of the SWR strain correlates with significantly reduced Ar transcript levels in the female ovary at the pubertal transition.