203 SINGLE-STEP GENE EDITING OF 3 XENOANTIGENS IN PORCINE FIBROBLASTS USING PROGRAMMABLE NUCLEASES

Abstract

Programmable nucleases (ZFN, Tal Effector Nucleases, and CRISPR) opened a new era for mammal genome editing, in particular for the pigs used for xenotransplantation. Multiple gene editing events are required both for knockout (KO) of xenoantigens and for targeted integration of human protective genes (Perota et al. 2016 J. Genet. Genomics 43, 233–23). The objective of the present work was to edit selected pig lines to KO the enzymes coding for the most relevant xenoantigens (i.e. GGTA1, CMAH, and B4GalNT2), combining Talens and CRISPR/Cas9 technologies to magnetic beads selection (Li et al. 2013 Xenotransplantation 22, 20–31). Primary porcine adult fibroblasts were transfected using Nucleofector (V-024 program). In a single reaction 2 × 10⁶ fibroblasts were co-transfected using 2 different sets of TALENS (4 μg/set) specific for CMAH (Conchon et al., 2013) and GGTA1 (Perota et al., 2015) genes together with B4GalNT2-specific CRISPR/Cas9 expression vector (2 μg; pX330-B4GalNT2; Estrada et al., 2015). Eight days post-transfection (DPT), Gal–/– cells were selected initially using biotin-conjugated IB4 lectin (Sigma, St. Louis, MO, USA) and magnetic beads (Dynabeads M-280, Thermo Fisher Scientific, Waltham, MA, USA). The selected cells were then plated on 150-mm Petri dishes (200 cells/dish) and cultured for 10 days. Selected colonies were expanded for PCR analysis and cryopreserved for somatic cell nuclear transfer (SCNT). All colonies were analysed by PCR for CMAH gene and their resulting products were digested with HindIII (HindIII-RFLP). Colonies that lost wild-type HindIII as a consequence of Talens effected deletion were PCR characterised for GGTA1, selecting those that had detectable Indels after gel electrophoresis and finally analysed by PCR for B4GalNT2. All PCR products were validated by sequencing for all the 3 genes of interest (TopoTA, Thermo Fisher Scientific). Selected colonies were used as nuclear donors for SCNT (Lagutina et al., 2006). Eight DPT we obtained 3.45 ×10⁶ cells. About 6.0 × 10³ Gal-negative cells (0.17%) were collected from the supernatant after magnetic beads separation. Eighteen DPT, 120 colonies were picked up and their HindIII-RFLP analyses on CMAH gene revealed that 22 colonies (18.3%) were KO for both CMAH alleles. Of these 22 colonies following electrophoretic analyses of GGTA1-PCR products, 13 colonies had detectable Indels. These 13 colonies were finally PCR analysed and sequenced for B4GalNT2 and sequenced. Final sequencing results confirmed that 2 colonies (1.6%) resulted in KO for the 3 genes. Three different zona-free SCNT experiments were done and 579 reconstructed embryos were obtained. On Day 7, 322 morulae or blastocysts (56%) were transferred in 3 synchronised sows and 2 (66%) became pregnant. In conclusion, after gene editing with programmable nucleases, combining beads-mediated selection with well-designed molecular analyses, we developed a multistep assay that can be used efficiently to detect desired gene edited events in cell colonies suitable for the SCNT. Embryos generated after SCNT were able to establish pregnancies at a high rate.

This work is supported by European FP7 grants Translink (n° 603049) and Xenoislet (n° 601827).