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Synthetic particles as vectors
Synthetic particles may combine useful attributes of different classes of vectors and avoid viral pathogenicity. These synthetic vectors include liposomes or molecular conjugate vectors or a combination of both. These synthetic vectors include liposomes or molecular conjugate vectors or a De combination of both.

These synthetic vectors can accommodate genes, more than 8-kb long and can be engineered for entry into the cell through receptor mediated endocytosis (RME). They are so engineered that after entry into the cell, they leave endosome and enter the nucleus and not the lysosome.

For this purpose, a receptor binding moiety (e.g. asialo-orosomucoid, which is a ligand for the asialoglycoprotein receptor of hepatocytes) and a DNA binding moiety are incorporated. Microtubule inhibitors may block a v uptake by Iysosomes, and nuclear localization which signals (NLS) may facilitate entry into the nucleus. In contrast to vector-mediated gene transfer, transfection is though safe and efficient but involves no mechanism for integration or autonomous replication. It allows only transient expression. In view of this, efforts are also being made to combine transduction and transfection for efficient and safe gene transfer providing long-term expression.

Selectivity and long-term expression
The delivery system and the gene used for gene therapy should also provide selectivity at the following levels; (i)it should enter non-germline cells; (ii) it should selectively integrate at a specific site within a genome and (iii) it should be selectively expressed (over time and space). Even if this selectivity' is achieved, long-term expression may still remain a problem.

Stable integration may partly solve the problem and may be achieved through the use of retroviral reverse transcriptase, retroviral integrase, transposase and parvovirus 'rep' proteins. The enzymes may be used with the vector, or else DNA encoding these enzymes may be included in non-integrable form for transient expression to facilitate stable integration. Autonomous replicating units like mammalian artificial chromosomes (MACs) or autonomous replicating
circular minichromosomes (found in some cancer cell lines) may also be used for achieving long-term expression.

 
 

 

 
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