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 and rogen 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 So uthern hybridization with P-32-labeled AR cDNA fragments representing diffe rent domains of AR. The results reveal that only exons 4 and 5 correspondin g to amino-terminal part of the hormone binding domain of AR are amplified in testosterone-treated old female but not in adult mice. Densitometric ana lysis 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 ampli fication occurs only in AR gene. The tissue specificity is also confirmed w hen the slot blot hybridization of mouse liver HMW DNA with AR cDNA fails t o show similar amplification. As the restriction map analysis of Southern b lots does not show restriction fragment length polymorphism, the possibilit y 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 a mplification by testosterone is tissue- and age-specific, and might contrib ute to the progress of genetic instability in the brain of aged female mice .