Inhibition of Moloney murine leukemia virus-induced leukemia in transgenic mice expressing antisense RNA complementary to the retroviral packaging sequences.

Recombinant plasmids pLP psi as and pCP psi as were constructed by positioning the Moloney murine leukemia virus (M-MuLV) proviral packaging (psi) sequences in reverse orientation under the transcriptional regulation of lymphotropic promoter/regulatory elements from the M-MuLV long terminal repeat or the cytomegalovirus immediate-early region. Linear fragments containing the antisense psi and the appropriate transcriptional regulatory sequences from these plasmids were introduced into the mouse germ line by zygote microinjection. The chromosomal integration, germ-line transmission, and lymphocyte-directed expression of the antisense psi RNA were confirmed. Control (nontransgenic) and transgenic mice containing either the pLP psi as or the pCP psi as sequences were infected with M-MuLV on the day of birth and assayed for signs of leukemia between 12 and 14 weeks of age with standard assay procedures. While 31% (11 of 36) of the control, nontransgenic, mice developed leukemia, none of the antisense psi transgenic mice developed any symptoms of leukemia. The pCP psi as sequences were also introduced into mouse NIH 3T3 cells and stably transformed cell lines were isolated. When infected with M-MuLV these cells were shown to produce virus devoid of packaged viral RNA.