Integration by design

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Abstract
As goes history, so goes research: this year, activity in areas of retrovirus research related only indirectly have provoked events that are notable when considered together. Last summer it was reported that a patient in one X-linked severe combined immunodeficiency retroviral vector gene therapy trial had developed leukemia. Now disquietingly, there has been a second such event, and a third patient is reported to have a vector insertion near the same gene (LMO2) as observed in the other two individuals (1). Meanwhile, in a basic research laboratory, experiments have moved us another step closer to understanding the mechanics of insertion specificity for retrovirus-type integrases (IN). As reported in this issue of PNAS, investigators have produced active retroviruslike elements with synthetic insertion specificities (2). Dan Voytas and colleagues at Iowa State University (Ames) study the Saccharomyces long terminal repeat (LTR)-retrotransposon Ty5, which targets heterochromatic regions (3). Now, in an elegant adaptation of the two-hybrid system, the 6-aa Ty5 targeting domain (TD) was exchanged for two heterologous domains shown to mediate interaction of their respective proteins with protein partners. When domains from those partners were produced fused to the LexA DNA-binding domain, targeting to LexA-binding sites was observed. Although integration specificity in the system was by no means absolute, these results are of interest to genetic engineers and future gene therapists. Interest in the integration patterns of retroviruses is longstanding. Despite the potential danger of deleterious activating or even inactivating insertions, retroviruses present compelling advantages as therapy vectors (reviewed in ref. 4). Early investigations of oncogenic retrovirus insertion sites in transformed cells showed that insertions were linked to activation of flanking oncogenes or DNaseI hypersensitive sites, leading to the notion that insertion into open chromatin was favored (reviewed in ref. 5; see also refs. 6 and 7). The potential for deleterious …