Retrovirus-mediated gene transfer into hematopoietic cells may provide a means 
of treating both inherited and acquired diseases involving hematopoietic cells. 
Implementation of this approach for disorders resulting from mutations affecting 
the beta-globin gene (e.g., beta-thalassemia and sickle cell anemia), however, 
has been hampered by the inability to generate recombinant viruses able to 
efficiently and faithfully transmit the necessary sequences for appropriate gene 
expression. We have addressed this problem by carefully examining the 
interactions between retroviral and beta-globin gene sequences which affect 
vector transmission, stability, and expression. First, we examined the 
transmission properties of a large number of different recombinant proviral 
genomes which vary both in the precise nature of vector, beta-globin structural 
gene, and locus control region (LCR) core sequences incorporated and in the 
placement and orientation of those sequences. Through this analysis, we 
identified one specific vector, termed M beta 6L, which carries both the human 
beta-globin gene and core elements HS2, HS3, and HS4 from the LCR and faithfully 
transmits recombinant proviral sequences to cells with titers greater than 10(6) 
per ml. Populations of murine erythroleukemia (MEL) cells transduced by this 
virus expressed levels of human beta-globin transcript which, on a per gene copy 
basis, were 78% of the levels detected in an MEL-derived cell line, Hu11, which 
carries human chromosome 11, the site of the beta-globin locus. Analysis of 
individual transduced MEL cell clones, however, indicated that, while expression 
was detected in every clone tested (n = 17), the levels of human beta-globin 
treatment varied between 4% and 146% of the levels in Hu11. This clonal 
variation in expression levels suggests that small beta-globin LCR sequences may 
not provide for as strict chromosomal position-independent expression of 
beta-globin as previously suspected, at least in the context of 
retrovirus-mediated gene transfer.