Optimizing gene expression in BPV-transformed cells: effects of cell type on enhancer/promoter interaction

Enhanced growth of canine bone marrow stromal cell cultures in the presence of acidic fibroblast growth factor and heparin

The ex vivo establishment, expansion, transduction, and reintroduction of autologous bone marrow stromal cells offers a potential efficacious system for somatic cell gene therapy. It is likely that any ex vivo system will require the use of large numbers of cells which express high levels of transgene products. We present a method for routine expansion of canine bone marrow stromal cells, established from initial 10-20 ml marrow aspirates, to greater than 10(9) cells. This high level expansion of cell cultures uses the stimulatory effect of acidic fibroblast growth factor (aFGF) and heparin. In the absence of these factors, stromal cell cultures grow actively for only 1 to 2 passages, become flattened in morphology, and expand to only 10(8) cells. In the presence of heparin (5 U/ml), aFGF exerts its effect over a wide range of concentrations (0.1-10 ng/ml) in a dose-dependent manner. The stimulatory effect is dependent on the presence of both aFGF and heparin. Immunocytochemical and cytochemical analyses phenotypically characterize these stromal cells as bone marrow stromal myofibroblasts. Stromal cells grown in the presence of aFGF and heparin grow actively and maintain a fibroblast-like morphology for a number of passages, transduce efficiently with a human growth hormone (hGH) expression vector, and express and secrete high levels of hGH. Human marrow stromal cells were also established and expanded by the same culture method. This culture method should be of great value in somatic cell gene therapy for the delivery of secreted gene products to the plasma of large mammals.

Systemic delivery of human growth hormone or human factor IX in dogs by reintroduced genetically modified autologous bone marrow stromal cells

Canine bone marrow stromal cells were expanded to numbers in excess of 10(9) cells from the initial 10-20 ml of marrow aspirates and transfected to express high levels of human growth hormone (hGH) in vitro. Ex vivo-modified marrow stromal cells were used in a gene therapy model system for the systemic delivery of transgene products in dogs. Adherent bone marrow stromal cell cultures, established and expanded from iliac crest marrow aspirates from each of 8 dogs, were transfected with a hGH gene plasmid expression vector and shown to express from 0.54-3.84 micrograms/10(6) cells per 24 hr hGH in vitro. The transfected plasmid vector does not possess a eukaryotic origin of replication nor does it possess sequences required for efficient integration into the host cell genome. As such, expression was expected to be transient. Transfected cells were autologously reintroduced into each dog by either infusion into a foreleg vein or directly into iliac crest marrow. In two cases, the stromal cells were cryopreserved following transfection, and subsequently thawed and infused. In one case, the expanded stromal cells were first cryopreserved, and then thawed, recultured, transfected, and infused. Reintroduced cell numbers ranged from 2.2 x 10(7) to 2.6 x 10(9), with total hGH expression capacities ranging from 62 to 1,400 micrograms/24 hr. Plasma of each of the dogs contained detectable hGH for a mean of 3.1 days (SD +/- 0.8 day) ranging from 2 to 5 days following reinfusion of cells. Peak plasma levels ranged from 0.10 to 1.76 ng/ml. Similar hGH expression values, based upon total expression capacity of the cells infused and dog body weight, were obtained for all dogs. Vector-modified stromal cells were detectable, by polymerase chain reaction (PCR) analysis, in the peripheral circulation following reinfusion in all 4 dogs analyzed. In 3 of the dogs, modified stromal cells were detected for 8.5-15 weeks. In addition, modified stromal cells were detected in iliac crest marrow of 2 dogs for 9 and 13 weeks, respectively, following reinfusion. In another experiment, cultured bone marrow stromal cells were transfected with a human factor IX (hFIX) plasmid vector. Modified cells (5.57 x 10(8)), with a total hFIX expression capacity of 281 micrograms/24 hr, were reinfused, resulting in detectable hFIX in plasma continuously for 9 days with a peak level of 8 ng/ml on day 1. These results demonstrate that the ex vivo bone marrow stromal cell system is a potentially powerful method by which to deliver secreted transgene product to the systemic circulation of large animals.

Elements within the beta-lactoglobulin gene inhibit expression of human serum albumin cDNA and minigenes in transfected cells but rescue their expression in the mammary gland of transgenic mice

Two new beta-lactoglobulin (BLG)/human serum albumin (HSA) hybrid gene vectors were constructed and tested for expression in COS-7 cells and in transgenic mice. The HSA sequences were inserted between the second and sixth BLG exons. Transient transfection experiments with these vectors as well as a series of additional vectors with either the BLG 5'- or 3'- intragenic sequences revealed that sequences within BLG exon 1/intron 1/exon 2 abrogated BLG- directed HSA expression in vitro, regardless of the presence of HSA introns or the origin of the 3' polyadenylation signal. In contrast, the same BLG expression cassette enabled the efficient expression of HSA cDNA or minigene in the mammary gland of transgenic mice with subsequent secretion of the corresponding protein into the milk of 56 and 82%, respectively of the mouse strains at levels up to 0.3 mg/ml. Previous attempts to express HSA cDNA inserted into exon 1 of the BLG gene had failed [Shani,M., Barash,I., Nathan,M., Ricca,G., Searfoss,G.H., Dekel,I., Faerman,A., Givol,D. and Hurwitz,D.R. (1992) Transgenic Res. 1, 195- 208]. The new BLG expression cassette conferred more stringent tissue specific expression than previously described BLG/HSA constructs [Barash,I, Faerman,A., Ratovitsky,T, Puzis,R., Nathan,M., Hurwitz,D.R. and Shani, M. (1994) Transgenic Res. 3, 141-151]. However, it was not able to insulate the transgenes from the surrounding host DNA sequences and did not result in copy number dependent expression in transgenics. Together, the in vitro and in vivo results suggest both positive and negative regulatory elements within the BLG intragenic sequences evaluated. The new BLG construct represents an extremely valuable vector for the efficient expression of cDNAs in the mammary gland of transgenic animals.

Expression of human serum albumin in the milk of transgenic mice

We have tested the feasibility of producing large quantities of human serum albumin (HSA) in the milk of transgenic livestock by generating transgenic mice as a model system. The sheep beta-lactoglobulin (BLG) 5'-regulatory promoter sequences were used to support expression of BLG or HSA in transgenic mice. Transgenic animals generated from the entire BLG gene including 3, 5.5 or 10.8 kb of 5'-sequences demonstrated that 3 kb of 5'-sequences were sufficient to support high levels of expression of BLG, and that the longer 5'-sequences did not improve upon the levels of expression. As such, the 3 kb 5'-sequences were used to drive expression of HSA in BLG-HSA constructs. HSA was not detectably expressed in eight transgenic lines generated from a BLG-HSA construct containing the HSA cDNA. Two transgenic lines of 26 generated, using five different constructs, with an HSA minigene possessing the first intron expressed HSA in their milk. One of these expressed HSA at high levels (2.5 mg ml-1) and has stably transmitted this ability to its progeny. A high percentage of transgenic mouse lines (four of six) generated from a vector containing an HSA minigene possessing introns 1 and 2 expressed HSA in their milk at levels which ranged from 1 to 35 micrograms ml-1. In a similar trend, levels of expression of HSA by transfected tissue culture cells from BLG-HSA vectors containing an introduced SV40 enhancer were low with the HSA cDNA, increased with the HSA minigene with intron 1 and increased further with the minigene containing introns 1 and 2. This study demonstrates that high levels of HSA can be expressed in the milk of transgenic animals, that introns of the HSA gene play a role in its expression and that transfected cell lines may be used to quickly evaluate the relative expression efficiencies of various vector constructs intended for future transgenic evaluation.