Fragments were taken from separate parts of hormone-dependent (HD) primary GR mouse mammary tumors and serially transplanted in estrone plus progesterone treated or hormonally untreated castrated mice. The transplants were examined with respect to int-1 DNA rearrangement, proviral integrations of the murine mammary tumor virus (MMTV), and estrogen and progesterone receptor content. One of the fragments (b) taken from the primary tumor of line TSI 96 produced transplants that showed int-1 rearrangement in one allele and also MMTV proviral integrations not at the int-1 gene, whereas transplants from another fragment (a) only had the normal germ-line int-1 arrangement and no extra MMTV provirus. These respective genotypes were retained when the tumors became hormonally independent during further transplantations. The results indicate that int-1 rearrangement was not present in the originally transformed cell but occurred in a HD cell during growth of the tumor. Furthermore they indicate that loss of hormonal dependence in GR mammary tumors is due to a mutational event, unrelated to int-1 rearrangement. © 1988.
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