Glybera (alipogene tipovarvec) is the first human gene therapy which received market approval. According to the distributor Glybera should be given only once in a lifetime.
The LPL expression is driven by the CMV promotor. Can anybody comment on the long term stability of CMV driven expression in human cells? From my own experiments with stable HepG2 and HEK293 cell lines I know that after several cell passages the expression is going down, gene silencing/deletions or whatsoever.
CMV promoter is easy to be silenced in rapidly proliferating cells and stem cells, but much more stable in non-proliferating cells, its effectiveness and persistence in vivo depend on cell and tissue types. CMV promoters are still in use in some recent clinical trials of gene therapy, but many researchers also use CAG promoters or tissue-specific promoters.
Hello Michael,
You are correct, the CMV promoter is known for its tendency to be silenced, usually by methylation of the promoter. As far as I am aware, Uniqure has not shown long term gene expression data from their patients, so it is currently unknown as to whether or not the CMV promoter is silenced in this case. Thus it is impossible to say whether or not the gene expression will last a lifetime. What IS clear is that after administration of Glybera, the patients will make neutralizing antibodies to the AAV capsid, thus preventing any future re-administration. This could be why the distributor states that it should only be given once in a lifetime.
John
I agree with the response above and would like to add that the CMV promoter allows strong expression in transfected cells. Therefore, if ectopic expression of a gene results in growth disadvantage to cells, cells that express lower levels are selected with time. Therefore, multiple factors may contribute to the time-dependent reduced expression of transfected gene.
IJust to add to the responses above, we too have successfully expressed many genes in cell lines using CMV promoter and have observed no silencing. However, in some cases where the stable clones have extremely high expression of the recombinant gene, we have lost its expression over passages. Typically if we go back to early passage cells, we can recover cells with high expression, suggesting that the gene maybe silenced due to toxicity rather than CMV methylation. We did not observe any methylation in most cases. Routinely in my lab, we now select clones that have 'medium' levels of recombinant gene expression rather than selecting only the clones that have tremendously high expression.
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