Gene transfer limitations

Transduction and transformation are generally limited to the same or related speeies and are therefore not effective as a means of antibiotie resistance transfer across species boundaries. However, our knowledge of transformation in nature is limited, and the significance of this meehanism of gene transfer is unknown. 

CNTs with different characteristics, which will lead to differences in the mechanism of CNT metabolism, degradation or dissolution, clearance and bioaccumulation. On the other hand, most non-viral gene delivery systems today suffer from both limited levels of gene expression and an unfavourable toxicity profile due to their highly cationic surface character. Therefore, opportunities for CNT-based gene transfer systems are still ample. 

The strategy for development of /3-lactamase-resistant /3-lactams has some limitations. Indeed, it has often been found that the more-resistant compounds are less-efficient antibiotics. Furthermore, the natural weapons wielded by bacteria mutation, gene transfer, and natural selection, combine to counter /3-lactamase resistance. Thus, /3-lactamase mutants have emerged that efficiently hydrolyze compounds that were previously considered /3-lactamase-resistant [37-41], The overproduction of enzymes - either PBPs or the original /3-lactamases - as well as a decrease in the permeability of the bacterial membrane to antibiotics - are other defense strategies of the bacteria [42] [43],... 

Therefore alternative techniques for gene transfer are necessary. While progress has been made in electroporation of Desulfovibrio (Rousset et al. 1991, 1998), the development of reliable and efficient procedures deserves further attention. Evidence has been obtained for the presence of a restriction barrier that limits transformation in D. vulgaris (Fu and Voordouw 1997). The efficiency of electroporation of plasmids into D. vulgaris was increased from an undetectable level to lO transformants... 

Some investigators have used electrical current (electroporation) to improve DNA (or drug) entry into tumor cells with some preliminary success. Liposomes are attractive vehicles for gene delivery, since they can carry plasmid, antisense, or viral DNA. Compared with viral approaches, however, liposomes remain relatively inefficient at facilitating gene transfer, although their safety profile remains more desirable. Some of the attributes and limitations of the nonviral methods are listed in Table 58.1. 

Early gene therapy protocols were principally of the ex vivo type and relied on recombinant retroviruses as gene transfer vehicles. Retroviruses can accomplish effective gene transfer to target cells despite being rendered replication-incompetent by genomic deletions. Flowever, a variety of limitations have restricted their use to achieve in vivo gene transfer. 

Pioneering studies have demonstrated the potential of gene therapy for the treatment of inherited hematopoietic diseases [440300], and particularly for ADA-SCID [470017], [470024], [666662], [666664], [666665]. However, vector design, gene transfer protocols and inadequate engraftment and expansion of genetically engineered cells limited the success of earlier studies [206054], [657269], [657273], [668669]. 

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