The integrase (Int) encoded by the lambdoid coliphage HK022 targets in its host chromosome a 21 base pair (bp) recombination site termed attB or BOB'. attB comprises two 7 bp partially inverted (palindromic) Int-binding sites of 7 bp each termed B and B'. B and B' flank a central 7 bp crossover site or 'overlap' (O). We show that replacing O with a random 7 bp sequence supports Int-mediated site-specific recombination as long as the cognate and larger phage recombination site attP features an identical O sequence. This promiscuity allowed us to identify on the human genome several native active secondary attB sites ('attB') with random overlaps that flank human deleterious mutations, raising the prospect of using such sites to cure the 'attB'-flanked mutations by Int-catalyzed RMCE (recombinase-mediated cassette exchange) reactions. An analysis of such active and inactive 'attB's suggested a minimal 14-15 bp attB consensus sequence (instead of the 21 bp) with a reduced 3 bp palindrome.

Since the publication of this article, the authors have noticed an error in the text. In the Discussion section, the last line of the third paragraph should say 'This shorter palindrome exists also in the core of the wild-type attP (Figure 3a) and only in attP of λ'.The author names were also published incorrectly in the online PDF. The correct author names are listed above.The html and online versions have been amended. The authors apologise for any inconvenience caused.Gene Therapy (2015) 22, 602

HK022 coliphage site-specific recombinase Integrase (Int) can catalyze integrative site-specific recombination and recombinase-mediated cassette exchange (RMCE) reactions in mammalian cell cultures. Owing to the promiscuity of the 7 bp overlap sequence in its att sites, active ‘attB’ sites flanking human deleterious mutations were previously identified that may serve as substrates for RMCE reactions for future potential gene therapy. However, the wild type Int proved inefficient in catalyzing such RMCE reactions. To address this low efficiency, variants of Int were constructed and examined by integrative site-specific recombination and RMCE assays in human cells using native ‘attB’ sites. As a proof of concept, various Int derivatives have demonstrated successful RMCE reactions using a pair of native ‘attB’ sites that were inserted as a substrate into the human genome. Moreover, successful RMCE reactions were demonstrated in native locations of the human CTNS and DMD genes whose mutations are responsible for Cystinosis and Duchene Muscular Dystrophy diseases, respectively. This work provides a steppingstone for potential downstream therapeutic applications.

The binary system presented in this work is based on the bacteriophage HK022 integrase recombinase that activates the expression of a silenced Diphtheria toxin gene, both controlled by the cancer specific hTERT promoter. Using a lung cancer mice model, assays of different apoptotic and anti-apoptotic factors have demonstrated that the Integrase based binary system is highly specific towards cancer cells and more efficient compared to the conventional mono system whose toxin is directly expressed under hTERT. In a mice survival test, this binary system demonstrated longer persistence compared to the untreated and the mono treated ones. The reason underlying the advantage of this binary system over the mono system seems to be an overexpression of various hTERT suppressing factors induced by the mono system.

Cancer gene therapy is a great promising tool for cancer therapeutics due to the specific targeting based on the cancerous gene expression background. Binary systems based on sitespecific recombination are one of the most effective potential approaches for cancer gene therapy. In these systems, a cancer specific promoter expresses a site-specific recombinase/integrase that in turn controls the expression of a toxin gene. In the current study, we have developed a new HK022 bacteriophage Integrase (Int) based binary system activating a Diphtheria toxin (DTA) gene expression specifically in cancer cells. We have demonstrated the efficiency, and the high specificity of the system in vitro in cell cultures and in vivo in a lung cancer mouse model. Strikingly, different apoptotic and anti-apoptotic factors demonstrated a remarkable efficacy killing capability of the Int-based binary system compared to the conventional hTERT-DTA mono system in the LLC-Kat lung cancer mice model; we observed that the active hTERT promoter down regulation by the transcription factors Mad-1 is the cornerstone of this phenomenon. The new Int-based binary system offers advantages over already known counterparts and may therefore be developed into a safer and efficient cancer treatment technology.