Abstract

Hemophilia A is a genetic bleeding disorder resulted from a deficiency of blood clotting factor VIII. In order to develop the efficient approach to gene therapy for hemophilia A, we previously explored reporter gene transfer mediated by ultrasound (US) combined with microbubbles (MBs). It was demonstrated that US/MB can significantly enhance gene transfer efficiency and serve as an efficient non-viral physical delivery strategy. In this study, we further delivered a therapeutic FVIII plasmid into the livers of hemophilia A (HA) mice. In consideration of FVIII synthesis from multiple tissues/cell lines, we first explored the distribution of gene expression using a pGL4.13 [luc2/SV40] luciferase plasmid driven by a ubiquitous promoter. One day following gene transfer, hepatocytes and endothelia cells were isolated from treated lobes by liver perfusion and centrifuge method. Evaluation of luciferase levels in two cell populations indicated that luciferase predominantly expressed in hepatocytes (5.35´104 RLU/107 cells vs. 1.46´103 RLU/107 cells in endothelia cells). Furthermore, gene transfer of pGFP (driven by a ubiquitous CMV promoter) mediated by US/MB also showed fluorescence distribution mostly in hepatocytes. These results indicate that hepatocyte is the predominant site of gene expression following US/MB mediated gene transfer into the liver. Based on these results, a hepatocyte-specific human FVIII plasmid (pBS-HCRHP-hFVIII/N6A) was used for US/MB mediated gene transfer in HA mice. In the short-term experiment, FVIII activity levels of treated HA mice ranged from 4-40% of normal FVIII activity. To follow FVIII expression for longer term, HA mice were pretreated with IL-2/IL-2 mAb (JES6-1) complexes on day −5, −4, and −3 to prevent immune response. In addition, the mice were infused with normal mouse plasma and human FVIII protein prior to gene transfer to maintain hemostasis. Subsequently, FVIII plasmids and 5 Vol% NUVOX MBs were injected into the mouse liver under simultaneous US exposure (1.1MHz transducer H158A driven by a pulse generator and high-power radio frequency amplifier capable of generating up to 1000W). Blood and liver samples were collected at serial time points after treatment to determine FVIII activity in plasma and liver damage. Following gene transfer, 10-30% of FVIII activity was achieved on day 4 and persisted in the average level of 20% by day 28. In a separate long-term follow-up experiment (n=3), 2 of 3 mice still maintained 10-30% activity after 120 days. Both transaminase levels (alanine aminotransferase and aspartate aminotransferase) and histological examination showed that the procedure of plasmid/MBs portal-vein injection and pulse-train acoustic exposure produced transiently localized liver damages however the damages were repaired and the liver recovered rapidly. Phenotypic correction of HA mice was further examined by tail clip assay. Blood loss of US/MB treated mice was significantly reduced compared with naive HA mice. Furthermore, a novel plasmid encoding a B domain-deleted FVIII variant containing mutations of 10 amino acids in the A1 domain (BDDFVIII-X10, a kind gift from Weidong Xiao) was constructed. Preliminary results from ongoing study showed that the gene transfer efficiency could be further improved with better plasmid and more efficient immune modulation. Together all the results indicate that US/MB mediated gene transfer is highly promising for efficient and safe gene therapy of hemophilia A.

Disclosures

No relevant conflicts of interest to declare.

Author notes

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