Transgene loss and changes in the promoter methylation status as determinants for expression duration in nonviral...

Advances in delivery techniques and in expression construct design have renewed interest in nonviral gene transfer. Here, we test plasmid or bacterial backbone free minicircle vectors for factor IX (FIX) expression by hydrodynamic liver-directed delivery. Both constructs are driven by a hepatic control region, the human α(1)-antitrypsin promoter, which results in long-term expression in FIX knockout mice. However, levels of expression were higher and expression loss over time was reduced when using minicircles. Even at the highest expression levels (>700% of normal) FIX was fully functional. Transgene loss was the main determinant for expression loss over time for both vector types. A significant effect of gene silencing was observed only for the plasmid, not for the minicircle vector. To determine the influence of promoter methylation, we performed bisulfite-mediated conversion and sequencing of vector DNA on days 14 and 100 after gene transfer. We determined a higher frequency of methyl-protected cytosines in CpGs and a lower degree of demethylation at bacterial Dcm methylation sequences near transcription factor-binding sites in the α(1)-antitrypsin promoter in plasmid compared with minicircle mice on day 100. Therefore, the methylation status might reflect differences in the levels and durability of expression. Judging from the high levels of functional FIX obtained, small fractions of liver or single liver segments should be sufficient to reach therapeutic efficacy in translating hydrodynamic delivery to humans. However, transgene loss remains to be addressed to further guarantee sustained expression over time.