title: LoxP-FRT Trap (LOFT): a simple and flexible system for conventional and reversible gene targeting.
abstract: BACKGROUND
  Conditional gene knockout (cKO) mediated by the Cre/LoxP system is indispensable for exploring gene functions in mice. However, a major limitation of this method is that gene KO is not reversible. A number of methods have been developed to overcome this, but each method has its own limitations.
  
  
  RESULTS
  We describe a simple method we have named LOFT [LoxP-flippase (FLP) recognition target (FRT) Trap], which is capable of reversible cKO and free of the limitations associated with existing techniques. This method involves two alleles of a target gene: a standard floxed allele, and a multi-functional allele bearing an FRT-flanked gene-trap cassette, which inactivates the target gene while reporting its expression with green fluorescent protein (GFP); the trapped allele is thus a null and GFP reporter by default, but is convertible into a wild-type allele. The floxed and trapped alleles can typically be generated using a single construct bearing a gene-trap cassette doubly flanked by LoxP and FRT sites, and can be used independently to achieve conditional and constitutive gene KO, respectively. More importantly, in mice bearing both alleles and also expressing the Cre and FLP recombinases, sequential function of the two enzymes should lead to deletion of the target gene, followed by restoration of its expression, thus achieving reversible cKO. LOFT should be generally applicable to mouse genes, including the growing numbers of genes already floxed; in the latter case, only the trapped alleles need to be generated to confer reversibility to the pre-existing cKO models. LOFT has other applications, including the creation and reversal of hypomorphic mutations. In this study we proved the principle of LOFT in the context of T-cell development, at a hypomorphic allele of Baf57/Smarce1 encoding a subunit of the chromatin-remodeling Brg/Brahma-associated factor (BAF) complex. Interestingly, the FLP used in the current work caused efficient reversal in peripheral T cells but not thymocytes, which is advantageous for studying developmental epigenetic programming of T-cell functions, a fundamental issue in immunology.
  
  
  CONCLUSIONS
  LOFT combines well-established basic genetic methods into a simple and reliable method for reversible gene targeting, with the flexibility of achieving traditional constitutive and conditional KO.
authors.affiliation: Department of Immunobiology, Yale University Medical School, 300 Cedar Street, New Haven, CT 06520, USA.
authors.forename: Barbara H
authors.initials: BH
authors.lastname: Chaiyachati
authors.forename: Ravinder K
authors.initials: RK
authors.lastname: Kaundal
authors.forename: Jiugang
authors.initials: J
authors.lastname: Zhao
authors.forename: Jie
authors.initials: J
authors.lastname: Wu
authors.forename: Richard
authors.initials: R
authors.lastname: Flavell
authors.forename: Tian
authors.initials: T
authors.lastname: Chi
mesh_terms: D000483:Alleles; D000818:Animals; D002868:Chromosomal Proteins, Non-Histone / Q000235:genetics / Q000378:metabolism; D005786:Gene Expression Regulation / Q000502:physiology; D055786:Gene Knockout Techniques; D018390:Gene Targeting; D005818:Genetic Engineering / Q000379:methods*; D005822:Genetic Vectors; D019426:Integrases; D051379:Mice; D018345:Mice, Knockout; D021122:Protein Subunits
chemical_list: D002868:Chromosomal Proteins, Non-Histone; D021122:Protein Subunits; C465207:Smarce1 protein, mouse; C045073:Cre recombinase; D019426:Integrases