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Journal of Virology, July 1999, p. 5490-5496, Vol. 73, No. 7
Programs in Developmental Biology and Blood
and Cancer Research, Hospital for Sick Children, Toronto, Ontario,
Canada M5G 1X81; Department of Molecular
and Medical Genetics, University of Toronto, Toronto, Ontario,
Canada4; Department of Medicine,
Division of Medical Oncology, University of Minnesota, Minneapolis,
Minnesota 55455-03622; and Massey Cancer
Center, Virginia Commonwealth University, Richmond, Virginia
23298-00373
Received 17 December 1998/Accepted 9 April 1999
Retroviral vectors are transcriptionally silenced in hematopoietic
stem cells, and this phenomenon must be overcome for effective gene
therapy of blood diseases. The murine stem cell virus (MSCV) vector
completely silences
0022-538X/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
Amelioration of Retroviral Vector Silencing in
Locus Control Region
-Globin-Transgenic Mice and Transduced F9
Embryonic Cells

-globin reporter genes regulated by locus
control region (LCR) elements 5'HS2 to 5'HS4 in seven of eight
transgenic mice. Here, we show that no single known MSCV silencer
element is sufficient for complete LCR
-globin transgene silencing.
However, partial silencing of high-copy transgenes is conveyed by the
MSCV direct repeat and promoter elements. The CpG methylation pattern
of silenced and expressed MSCV promoter transgenes is virtually
identical, demonstrating that silencing does not absolutely correlate
with methylation status. Combined mutations in all four MSCV silencer
elements leads to expression of
-globin in 6 of 10 transgenic mice.
The same mutations incorporated into the HSC1 retrovirus vector direct
neo gene expression in 71% of transduced F9 embryonic
carcinoma cells. These studies demonstrate that combined mutation of
four retroviral silencer elements relieves complete silencing in most
transgenic mice and transduced F9 cells and suggests that novel
silencer elements remain. Enhanced expression of the HSC1 vector in
primitive stem cells is well suited for blood gene therapy applications.
*
Corresponding author. Mailing address: Program in
Developmental Biology, Hospital for Sick Children, 555 University Ave., Toronto, Ontario, Canada M5G 1X8. Phone: (416) 813-7295. Fax: (416)
813-8883. E-mail: jellis{at}sickkids.on.ca.
Present address: Section of Hematology and Medical Oncology, LSU
Medical Center, Shreveport, LA 71130-3932.
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