Gene targeting in mammalian cells has become a routine technique and is currently used to study gene function, create biomedical models, and generate potential tissue sources for xenotransplantation. Severe combined immunodeficiency (SCID) is a condition characterised by the absence of T cells and a lack of B cell function. Severe combined immunodeficiency affects ~1 out of every 100 000 infants. Autosomal recessive SCID can occur due to a mutation within the recombination activating genes (RAG-1/RAG-2) that play a role in recombination of immunoglobulins and T-cell receptors. Gene targeting has been used to create mouse models to study the effects of a RAG-1 or RAG-2 deficiency on the immune system. In 1992, Mombaerts et al. generated a homozygous mouse model of RAG-1 deficiency, whereas Shinkai et al. generated a homozygous mouse model of RAG-2. Both models resulted in the absence of mature T or B lymphocytes; which was concluded to be due to the lack of the ability to initiate the V(D)J recombination process. Because of the anatomical and physiological similarities between humans and pigs, a swine model of both RAG-1 and RAG-2 deficiency would have utility. We hypothesise that disruption of RAG-1, RAG-2, or both in swine will result in a SCID phenotype. A first step in the creation of a swine SCID model is to assemble targeting vectors. The objective of this work was to construct targeting vectors. To accomplish this goal, genomic DNA from porcine fetal fibroblasts was used to amplify a 6840-bp PCR product including the porcine RAG-1 gene. This fragment was cloned into TOPO pCR-XL (Invitrogen, Carlsbad, CA, USA). So that a mammalian G418 resistance cassette could be used for selection of targeting events, this plasmid was modified to remove the endogenous AphII gene (provides G418 resistance). The pKW4 contains LoxP (locus of X-ing over) sites that flank a G418 resistance cassette (based on mammalian codon usage), which is driven by a phosphoglycerate kinase (PGK) promoter (Lorson et al. 2011). This cassette was inserted into the RAG-1 gene to create the targeting construct pAB6. For RAG2, a 9466-bp PCR product i ncluding the RAG-2 gene was amplified and cloned into TOPO pCR-Blunt II (Invitrogen). The LoxP flanked G418 resistance cassette from pKW4 was inserted into the second exon of the RAG-2 gene sequence, creating the targeting construct pAB13. Further, diagnostic screening strategies were developed and validated to discriminate gene-targeting events from random integration. We report here 2 targeting vectors and validated screening methods for gene targeting in porcine fetal fibroblasts that have been validated for cloning. These vectors will be applied toward an effort to create a porcine SCID model. The implications of such a model include evaluation of basic immune function, evaluation of the innate immune system in vaccine efficacy, and organ transplantation.
Functional analysis of the immune system by gene targeting