Amplification of exons 4 and 5 of Androgen Receptor gene by testosterone in aged female mouse brain cortex

We have investigated the effect of testosterone on the amplification of androgen receptor (AR) gene in the brain cortex of aging female mice. For this purpose, high molecular weight (HMW) DNA purified from the brain cortex of intact, gonadectomized, testosterone- and estradiol-treated adult and old female mice was digested with different restriction enzymes and used for Southern hybridization with ³²P-labeled AR cDNA fragments representing different domains of AR. The results reveal that only exons 4 and 5 corresponding to amino-terminal part of the hormone binding domain of AR are amplified in testosterone-treated old female but not in adult mice. Densitometric analysis further shows that testosterone increases the copy number of exons 4 and 5 of mouse AR gene by four-fold. Reprobing of slot blots with estrogen receptor and cathepsin D cDNA as probes supports the observation that amplification occurs only in AR gene. The tissue specificity is also confirmed when the slot blot hybridization of mouse liver HMW DNA with AR cDNA fails to show similar amplification. As the restriction map analysis of Southern blots does not show restriction fragment length polymorphism, the possibility of structural rearrangement leading to amplification of AR gene is ruled out. Thus our results suggest that the in vivo induction of mouse AR gene amplification by testosterone is tissue- and age-specific, and might contribute to the progress of genetic instability in the brain of aged female mice.