Variable efficiency of retroviral-mediated gene transfer into early-passage cultures of fetal lamb epithelial, mesenchymal, and neuroectodermal tissues

Animal models for prenatal gene therapy: the sheep model

Large animal experiments are vital in the field of prenatal gene therapy, to allow translation from small animals into man. Sheep provide many advantages for such experiments. They have been widely used in research into fetal physiology and pregnancy and the sheep fetus is a similar size to that in the human. Sheep are tolerant to in utero manipulations such as fetoscopy or even hysterotomy, and they are cheaper and easier to maintain than non-human primates. In this chapter, we describe the animal husbandry involved in generating time-mated sheep pregnancies, the large number of injection routes in the fetus that can be achieved using ultrasound or fetoscopic-guided injection, and laparotomy when these more minimally invasive routes of injection are not feasible.

Fetal muscle gene therapy/gene delivery in large animals

Gene delivery to the fetal muscles is a potential strategy for the early treatment of muscular dystrophies. In utero muscle gene therapy can also be used to treat other genetic disorders such as hemophilia, where the missing clotting proteins may be secreted from the treated muscle. In the past few years, studies in small animal models have raised the hopes that a phenotypic cure can be obtained after fetal application of gene therapy. Studies of efficacy and safety in large animals are, however, essential before clinical application can be considered in the human fetus. For this reason, the development of clinically applicable strategies for the delivery of gene therapy to the fetal muscles is of prime importance. In this chapter, we describe the protocols for in utero ultrasound-guided gene delivery to the ovine fetal muscle in early gestation. In particular, procedures to inject skeletal muscle groups such as the thigh and thoracic musculature and targeting the diaphragm in the fetus are described in detail.

Identification of adjacent binding sites for the YY1 and E4BP4 transcription factors in the ovine PrP (Prion) gene promoter

The PrP gene encodes the cellular isoform of the prion protein (PrP(c)) which has been shown to be crucial to the development of transmissible spongiform encephalopathies (TSEs). PrP knock-out mice, which do not express endogenous PrP(c), exhibit resistance to TSE disease. The regulation of PrP gene expression represents, therefore, a crucial factor in the development of TSEs. Two sequence motifs in the PrP promoter (positions -287 to -263 from transcriptional start) were previously reported as being highly conserved, and it was suggested that they represent binding sites for as yet unidentified transcription factors. To test this hypothesis, binding of nuclear proteins was analyzed by electrophoretic mobility shift assays using ovine or murine cells and tissues with radiolabeled DNA probes containing the conserved motif sequences. Specific binding was observed to both motifs, and polymorphic variants of these motifs exhibited differential binding. Two proteins bound to these motifs were identified as the Yin Yang 1 (YY1) (motif 1) and E4BP4 (motif 2) transcription factors. Functional promoter analysis of four different promoter variants revealed that motif 1 (YY1) was associated with inhibitory activity in the context of the PrP promoter, whereas motif 2 (E4BP4) was linked to a slight enhancing activity. This represents the first demonstration of binding of nuclear factors to two highly conserved DNA sequence motifs within mammalian PrP promoters. The action of these factors on the PrP promoter is haplotype-specific, leading us to propose that the prion protein expression pattern and, with it, the distribution of TSE infectivity may be associated with PrP promoter genotype.

Inefficient transduction of sheep in utero after intra-amniotic injection of retroviral producer cells

OBJECTIVE

Our purpose was to test the hypothesis that the intra-amniotic injection of a retroviral vector producer cell line into pregnant sheep will result in retrovirus-mediated transduction and stable gene transfer to the ovine fetus.

STUDY DESIGN

Thirteen pregnant ewes at various gestational ages underwent amniocentesis and injection of cells producing the retrovirus vector LIRESgeo, which is derived from Maloney murine leukemia virus and encodes Escherichia coli LacZ (beta-galactosidase) as a marker gene. Pregnant ewes and fetuses were killed, and amniotic fluid, placenta, and fetal tissues were collected and assayed for transgene expression 7 to 77 days after intraamniotic injection. In addition, serum was collected and analyzed for evidence of specific immune responses against the producer cells, and amniotic fluid was collected and analyzed for deleterious effects on producer cell viability, vector production, and vector transduction.

RESULTS

Only 1 of 10 fetuses exposed to the retroviral producer cells demonstrated beta-galactosidase activity that correlated with positive immunohistochemistry for LacZ in lung, trachea, pancreas, and small intestine. However, the presence of the LacZ gene could not be confirmed by polymerase chain reaction. Thus, we could not confirm transduction after any of the injections. The retroviral producer cells survived well in amniotic fluid and continued to produce high levels of retroviral vector after intra-amniotic injection, although amniotic fluid inhibited the transducing activity of the vector in a manner dependent on gestational age.

CONCLUSIONS

Intra-amniotic retroviral gene transfer with the use of these amphotropic producer cells does not result in reproducible gene transfer in the ovine fetus although amniotic fluid sustains producer cell viability and vector production. Possible reasons for the inefficient transduction are discussed.