Life-supporting Kidney Xenotransplantation From Genetically Engineered Pigs in Baboons: A Comparison of Two Immunosuppressive Regimens.

BACKGROUND: The aims of this study were to evaluate the efficacy of US Food and Drug Administration-approved drugs in genetically engineered pig-to-baboon kidney xenotransplantation and compare the results with those using an anti-CD40 monoclonal antibody (mAb)-based regimen. METHODS: Ten life-supporting kidney transplants were carried out in baboons using α1,3-galactosyltransferase gene-knockout/CD46 pigs with various other genetic manipulations aimed at controlling coagulation dysregulation. Eight transplants resulted in informative data. Immunosuppressive therapy consisted of induction with antithymocyte globulin and anti-CD20mAb, and maintenance based on either (1) CTLA4-Ig and/or tacrolimus (+rapamycin or mycophenolate mofetil) (GroupA [US Food and Drug Administration-approved regimens], n = 4) or (2) anti-CD40mAb + rapamycin (GroupB, n = 4). All baboons received corticosteroids, interleukin-6R blockade, and tumor necrosis factor-α blockade. Baboons were followed by clinical and laboratory monitoring of kidney function, coagulation, and immune parameters. At euthanasia, morphological and immunohistochemical studies were performed on the kidney grafts. RESULTS: The median survival in GroupB was 186 days (range 90-260), which was significantly longer than in GroupA; median 14 days (range 12-32) (P < 0.01). Only GroupA baboons developed consumptive coagulopathy and the histopathological features of thrombotic microangiopathic glomerulopathy and interstitial arterial vasculitis. CONCLUSIONS: Recognizing that the pig donors in each group differed in some genetic modifications, these data indicate that maintenance immunosuppression including anti-CD40mAb may be important to prevent pig kidney graft failure.

Efficient generation of targeted and controlled mutational events in porcine cells using nuclease-directed homologous recombination.

BACKGROUND: Nuclease-based genome editing has rapidly sped the creation of new models of human disease. These techniques also hold great promise for the future of clinical xenotransplantation and cell-based therapies for cancer or immunodeficient pathology. However, to fully realize the potential of nuclease editing tools, the efficiency and precision of their application must be optimized. The object of this study was to use nonintegrating selection and nuclease-directed homologous recombination to efficiently control the genetic modification of the porcine genome. METHODS: Clustered randomly integrating spaced palindromic repeats and associated Cas9 protein (CRISPR/Cas9)-directed mutagenesis with a single-guide RNA target was designed to target the alpha-1,3-galactosyltransferase locus (GGTA1) of the porcine genome. A vector expressing a single-guide RNA, Cas9 protein, and green fluorescent protein was used to increase plasmid-delivered mutational efficiency when coupled with fluorescence sorting. Single and double-strand DNA oligonucleotides with a restriction site replacing the start codon were created with variable homology lengths surrounding the mutational event site. Finally, a transgene construct was flanked with 50 base pairs of homology directed immediately 5' to a nuclease cut site. These products were introduced to cells with a constant concentration of CRISPR/cas9 vector. Phenotype-specific mutational efficiency was measured by flow cytometer. Controlled homologous insertion was measured by Sanger sequence, restriction enzyme digest and flow cytometry. RESULTS: Expression of a fluorescence protein on the Cas9 vector functioned as a nonintegrating selection marker. Selection by this marker increased phenotype-silencing mutation rates from 3.5% to 82% (P = 0.0002). Insertion or deletion mutation increased from 11% to 96% (P = 0.0007). Co-transfection with homologous DNA oligonucleotides increased the aggregate phenotype-silencing mutation rates up to 22% and increased biallelic events. Single-strand DNA was twice as efficient as double-strand DNA. Furthermore, nuclease-mediated insertion by homology-directed repair successfully drove locus-specific transgene expression in the porcine genome. CONCLUSIONS: A nonintegrating selection strategy based on fluorescence expression can increase the mutational efficiency of the CRISPR/Cas9 system. The precision of this system can be increased by the addition of a very short homologous template sequence and can serve as a method for locus-specific transgene delivery. Together these strategies may be used to efficiently control mutational events. This system may be used to better use the potential of nuclease-mediated genomic editing.

A Novel Approach in Combined Liver and Kidney Transplantation With Long-term Outcomes.

OBJECTIVE: The aim of this study was to compare the outcomes of simultaneous and delayed implantation of kidney grafts in combined liver-kidney transplantation (CLKT). BACKGROUND DATA: Delayed function of the renal graft (DGF), which can result from hypotension and pressor use related to the liver transplantation (LT), may cause worse outcomes in CLKT. METHODS: A total of 130 CLKTs were performed at Indiana University between 2002 and 2015 and studied in an observational cohort study. All kidneys underwent continuous hypothermic pulsatile machine perfusion until transplant: 69 with simultaneous kidney transplantation (KT) (at time of LT, group 1) and 61 with delayed KT (performed at a later time as a second operation, group 2). All patients received continuous veno-venous hemodialysis during the LT. Propensity score match analysis in a 1:1 case-match was performed. RESULTS: Mean kidney cold ischemia time was 10 ± 3 and 50 ± 15 hours, for groups 1 and 2 (P < 0.0001), respectively. The rate of DGF was 7.3% in group 1, but no DGF was seen in group 2 (P = 0.0600). Kidney function was significantly better in group 2, if the implantation of kidneys was delayed >48 hours (P < 0.01). Patient survival was greater in group 2 at 1 year (91%), and 5 year (87%) post-transplantation (P = 0.0019). On multivariate analysis, DGF [hazard ratio (HR), 165.7; 95% confidence interval (CI), 9.4-2926], extended criteria donor kidneys (HR, 15.9; 95% CI 1.8-145.2), and recipient hepatitis C (HR, 5.5; 95% CI 1.7-17.8) were significant independent risk factors for patient survival. CONCLUSIONS: Delayed KT in CLKT (especially if delayed >48 h) is associated with improved kidney function with no DGF post-KT, and improved patient and graft survival.

Intestinal Graft Failure: Should We Perform the Allograft Enterectomy Before or With Retransplantation?

BACKGROUND: Intestinal graft dysfunction is sometimes irreversible and requires allograft enterectomy with or without retransplantation. There is no comprehensive assessment of allograft enterectomy regarding indications and outcomes. The aim of this study was to evaluate management of patients with intestinal graft failure with special reference to indications and outcomes of allograft enterectomy and the procedure's validity as a bridge to retransplantation. METHODS: Graft and patient survivals, reason for graft failure, and rejection episodes were evaluated in 221 intestinal recipients (primary transplantation [n = 201], retransplantation [n = 20]). Indications, surgical factors, and outcomes of allograft enterectomy were investigated. RESULTS: Reasons for isolated enterectomy included systemic infection in 11, gastrointestinal bleeding in 1, and severe electrolyte imbalance in 1, all of which were associated with rejection. One isolated intestinal transplantation patient underwent isolated enterectomy due to cytomegalovirus enteritis. One multivisceral transplantation patient underwent isolated allograft enterectomy due to bowel necrosis. Of these 15 patients, 3 died from persistent infection postoperatively, whereas 8 underwent retransplantation with median interval of 74 days (42-252 days). Allosensitization occurred between isolated enterectomy and retransplantation in 2, one of whom lost the second graft due to rejection. Simultaneous allograft enterectomy and retransplantation was performed in 3 isolated intestinal transplantation and 9 multivisceral transplantation patients. Patient survival after retransplantation was similar between patients who underwent isolated allograft enterectomy and those who did simultaneous enterectomy with retransplantation (P = 0.82). CONCLUSIONS: In cases of irreversible intestinal graft dysfunction, isolated allograft enterectomy successfully provides recovery from comorbidities as a lifesaving procedure and does not compromise outcomes of retransplantation.

Prognosis after recurrence of hepatocellular carcinoma in liver transplantation: predictors for successful treatment and survival.

There are no established prognostic factors or standardized therapies for hepatocellular carcinoma (HCC) recurrence in liver transplantation (LT). The aim of this study was to investigate impact of underlying patient condition on treatment and outcomes of recurrence of HCC after LT. The medical records of 268 LT patients with HCC were evaluated. Potential prognostic factors for survival after recurrence were evaluated, including recurrent tumor characteristics, medical/radiological/surgical therapies for recurrence, and an inflammatory marker (neutrophil/lymphocyte ratio). Laboratory tests at recurrence, including albumin, absolute lymphocyte count (ALC), prognostic nutritional index (PNI: ALC(/µL) × 0.005 + Albumin(g/dL) × 10), were evaluated as surrogate markers for underlying patient conditions. A total of 51 (19%) patients developed HCC recurrence. The use of sirolimus and sorafenib significantly improved outcome (p = 0.007 and 0.04), and better nutritional status (PNI ≥ 40) enhanced their efficacy. On multivariate analysis, low ALC (<500/µL) and albumin (<2.8 g/L) remained independent prognostic factors (p = 0.03 and 0.02; hazard ratio = 3.61 [Ref. >1000/µL] and 4.97 [Ref. >3.5 g/dL], respectively). Low PNI (<40) showed significantly lower survival rate after adjusting the risk (p = 0.006, hazard ratio = 3.29). Underlying patient conditions and nutritional status, represented by ALC and albumin, are important to successful cancer treatment and strong prognostic markers for survival after HCC recurrence.

Recent advances in genome editing and creation of genetically modified pigs.

The field of xenotransplantation is benefiting greatly from recent advances in genetic engineering. The efficiency and pace with which new model animals are being created has dramatically sped progress towards clinical relevance. Endonuclease-driven genome editing now allows for the efficient generation of targeted genetic alterations. Herein we review the available methods of genetic engineering that have been successfully employed to create genetically modified pigs.

Current status of pig liver xenotransplantation.

The shortage of organs from deceased human donors is a major problem limiting the number of organs transplanted each year and results in the death of thousands of patients on the waiting list. Pigs are currently the preferred species for clinical organ xenotransplantation. Progress in genetically-engineered (GE) pig liver xenotransplantation increased graft and recipient survival from hours with unmodified pig livers to up to 9 days with normal to near-normal liver function. Deletion of genes such as GGTA1 (Gal-knockout pigs) or adding genes such as human complement regulatory proteins (hCD55, hCD46 expressing pigs) enabled hyperacute rejection to be overcome. Although survival up to 9 days was recorded, extended pig graft survival was not achieved due to lethal thrombocytopenia. The current status of GE pig liver xenotransplantation with world experience, potential factors causing thrombocytopenia, new targets on pig endothelial cells, and novel GE pigs with more genes deletion to avoid remaining antibody response, such as beta1,4-N-acetyl galactosaminyl transferase 2 (ß4GalNT2), are discussed.

The need for xenotransplantation as a source of organs and cells for clinical transplantation.

The limited availability of deceased human organs and cells for the purposes of clinical transplantation remains critical worldwide. Despite the increasing utilization of 'high-risk', 'marginal', or 'extended criteria' deceased donors, in the U.S. each day 30 patients either die or are removed from the waiting list because they become too sick to undergo organ transplantation. In certain other countries, where there is cultural resistance to deceased donation, e.g., Japan, the increased utilization of living donors, e.g., of a single kidney or partial liver, only very partially addresses the organ shortage. For transplants of tissues and cells, e.g., pancreatic islet transplantation for patients with diabetes, and corneal transplantation for patients with corneal blindness (whose numbers worldwide are potentially in the millions), allotransplantation will never prove a sufficient source. There is an urgent need for an alternative source of organs and cells. The pig could prove to be a satisfactory source, and clinical xenotransplantation using pig organs or cells, particularly with the advantages provided by genetic engineering to provide resistance to the human immune response, may resolve the organ shortage. The physiologic compatibilities and incompatibilities of the pig and the human are briefly reviewed.

Immunobiological barriers to xenotransplantation.

Binding of natural anti-pig antibodies in humans and nonhuman primates to carbohydrate antigens expressed on the transplanted pig organ, the most important of which is galactose-α1,3-galactose (Gal), activate the complement cascade, which results in destruction of the graft within minutes or hours, known as hyperacute rejection. Even if antibody is removed from the recipient's blood by plasmapheresis, recovery of antibody is associated with acute humoral xenograft rejection. If immunosuppressive therapy is inadequate, the development of high levels of T cell-dependent elicited anti-pig IgG similarly results in graft destruction, though classical acute cellular rejection is rarely seen. Vascular endothelial activation by low levels of anti-nonGal antibody, coupled with dysregulation of the coagulation-anticoagulation systems between pigs and primates, leads to a thrombotic microangiopathy in the graft that may be associated with a consumptive coagulopathy in the recipient. The most successful approach to overcoming these barriers is by genetically-engineering the pig to provide it with resistance to the human humoral and cellular immune responses and to correct the coagulation discrepancies between the two species. Organs and cells from pigs that (i) do not express the important Gal antigen, (ii) express a human complement-regulatory protein, and (iii) express a human coagulation-regulatory protein, when combined with an effective immunosuppressive regimen, have been associated with prolonged pig graft survival in nonhuman primates.

Liver transplantation for metastatic neuroendocrine tumors: Outcomes and prognostic variables.

BACKGROUND: Patient selection for liver transplantation for metastatic neuroendocrine tumors remains a topic of debate. There is no established MELD exception, making it difficult to obtain donor organs. METHODS: A multicenter database was created assessing outcomes for liver and multivisceral transplantation for metastatic neuroendocrine tumors and identifying prognostic factors for survival. Demographic, transplant, primary tumor site and management, pathology, recurrent disease and survival data were collected and analyzed. Survival probabilities were calculated using the Kaplan-Meier method. RESULTS: Analysis included 85 patients who underwent liver transplantation November 1988-January 2012 at 28 centers. One, three, and five-year patient survival rates were 83%, 60%, and 52%, respectively; 40 of 85 patients died, with 20 of 40 deaths due to recurrent disease. In univariate analyses, the following were predictors of poor prognosis: large vessel invasion (P < 0.001), extent of extrahepatic resection at liver transplant (P = 0.007), and tumor differentiation (P = 0.003). In multivariable analysis, predictors of poor overall survival included large vessel invasion (P = 0.001), and extent of extrahepatic resection at liver transplant (P = 0.015). CONCLUSION: In the absence of poor prognostic factors, metastatic neuroendocrine tumor is an acceptable indication for liver transplantation. Identification of favorable prognostic factors should allow assignment of a MELD exception similar to hepatocellular carcinoma.

A brief history of clinical xenotransplantation.

Between the 17th and 20th centuries, blood was transfused from various animal species into patients with a variety of pathological conditions. Skin grafts were carried out in the 19th century, with grafts from a variety of animals, with frogs being the most popular. In the 1920s, Voronoff advocated the transplantation of slices of chimpanzee testis into elderly men, believing that the hormones produced by the testis would rejuvenate his patients. In 1963-4, when human organs were not available and dialysis was not yet in use, Reemtsma transplanted chimpanzee kidneys into 13 patients, one of whom returned to work for almost 9 months before suddenly dying from what was believed to be an electrolyte disturbance. The first heart transplant in a human ever performed was by Hardy in 1964, using a chimpanzee heart, but the patient died within 2 h. Starzl carried out the first chimpanzee-to-human liver transplantation in 1966; in 1992 he obtained patient survival for 70 days following a baboon liver transplant. The first clinical pig islet transplant was carried out by Groth in 1993. Today, genetically-modified pigs offer hope of a limitless supply of organs and cells for those in need of a transplant.

Pre-transplant antibody screening and anti-CD154 costimulation blockade promote long-term xenograft survival in a pig-to-primate kidney transplant model.

Xenotransplantation has the potential to alleviate the organ shortage that prevents many patients with end-stage renal disease from enjoying the benefits of kidney transplantation. Despite significant advances in other models, pig-to-primate kidney xenotransplantation has met limited success. Preformed anti-pig antibodies are an important component of the xenogeneic immune response. To address this, we screened a cohort of 34 rhesus macaques for anti-pig antibody levels. We then selected animals with both low and high titers of anti-pig antibodies to proceed with kidney transplant from galactose-α1,3-galactose knockout/CD55 transgenic pig donors. All animals received T-cell depletion followed by maintenance therapy with costimulation blockade (either anti-CD154 mAb or belatacept), mycophenolate mofetil, and steroid. The animal with the high titer of anti-pig antibody rejected the kidney xenograft within the first week. Low-titer animals treated with anti-CD154 antibody, but not belatacept exhibited prolonged kidney xenograft survival (>133 and >126 vs. 14 and 21 days, respectively). Long-term surviving animals treated with the anti-CD154-based regimen continue to have normal kidney function and preserved renal architecture without evidence of rejection on biopsies sampled at day 100. This description of the longest reported survival of pig-to-non-human primate kidney xenotransplantation, now >125 days, provides promise for further study and potential clinical translation.

Prospective Monitoring of Donor-specific Anti-HLA Antibodies After Intestine/Multivisceral Transplantation: Significance of De Novo Antibodies.

BACKGROUND: Presence of circulating donor-specific antibodies (DSA) may be associated with worse clinical outcomes after intestine/multivisceral transplantation. METHODS: In 79 intestine/multivisceral recipients, sera were prospectively screened for DSA by Luminex Single antigen test at 1, 3, 6, 9, 12, 18, 24, and 36 months after transplantation. Standard immunosuppression included thymoglobulin-rituximab induction and tacrolimus-prednisone maintenance. C4d staining was performed retrospectively on biopsies in patients that developed acute rejection (AR). RESULTS: Twenty-two (28%) patients developed de novo DSA at a median posttransplant period of 3 (1-36) months. De novo DSA were observed in 10 of 40 liver-including and 12 of 39 liver-excluding transplants (P = 0.57). Occurrence of AR was slightly higher in patients with de novo DSA (45% vs 33%, respectively; P = 0.41). Similarly, chronic rejection (14% vs 5%; P = 0.21) and graft loss due to AR (18% vs 7%; P = 0.14) were numerically higher in patients with de novo DSA. Only 35% patients experiencing AR had circulating de novo DSA at the time of AR. Antibody-mediated rejection was diagnosed in 6 patients based on C4d staining, of these 2 patients had circulating de novo DSA at the time of biopsy. CONCLUSIONS: De novo DSA formation, particularly early in the posttransplant course may be associated with trends toward worse outcomes. However, its significance in the pathophysiology of AR remains uncertain. Studies focusing mechanisms of DSA-related graft injury and intragraft DSA detection might provide further insight into this issue.

Intestine and multivisceral transplantation: current status and future directions.

Intestinal failure and associated parenteral nutrition-induced liver failure cause significant morbidity, mortality, and health care burden. Intestine transplantation is now considered to be the standard of care in patients with intestinal failure who fail intestinal rehabilitation. Intestinal failure-associated liver disease is an important sequela of intestinal failure, caused by parenteral lipids, requiring simultaneous liver-intestine transplant. Lipid minimization and, in recent years, the emergence of fish oil-based lipid emulsions have been shown to reverse parenteral nutrition-associated hyperbilirubinemia, but not fibrosis. Significant progress in surgical techniques and immunosuppression has led to improved outcomes after intestine transplantation. Intestine in varying combination with liver, stomach, and pancreas, also referred to as multivisceral transplantation, is performed for patients with intestinal failure along with liver disease, surgical abdominal catastrophes, neuroendocrine and slow-growing tumors, and complete portomesenteric thrombosis with cirrhosis of the liver. Although acute and chronic rejection are major problems, long-term survivors have excellent quality of life and remain free of parenteral nutrition.

Utility of pre-procurement bedside liver biopsy in the deceased extended-criteria liver donor.

INTRODUCTION: The Indiana Organ Procurement Organization (IOPO) utilizes preoperative bedside liver biopsies in certain extended-criteria donors (ECDs), obtained by the on-site coordinator, to determine the utility of pursuing donation. This study reports the clinical and financial outcomes for this management strategy. METHODS: All bedside liver biopsies obtained in ECDs over a five-yr period were reviewed. Study variables included the following: indication for biopsy, biopsy results, taking the case to the operating room, transplantation of the donor liver, and graft survival. All biopsies were processed at a single university center. RESULTS: There were 110 donors biopsied. Primary indications included the following: old age (29%), extensive/current alcohol abuse (26%), hepatitis C-positive serology (21%), obesity (25%), and severely elevated liver function enzymes (18%). Biopsy results demonstrated a potentially transplantable liver in 73 cases (66%), all of whom were taken to the OR (while 37 ruled out for donation based upon liver biopsy [34%]). Of all biopsied livers, 49 ultimately were transplanted (45%). Intra-operative decisions included the following: transplant 51/73 (70%), surgeon decision to exclude 20/73 (27%), nonuse due to finding of malignancy two (3%). CONCLUSIONS: Bedside liver biopsy may be a valuable tool to determine the utility in pursuing donation in ECDs, particularly with liver-only donors.

Crossmatch-positive liver transplantation in patients receiving thymoglobulin-rituximab induction.

BACKGROUND: Positive crossmatch (CM) in liver transplantation (LT) is associated with worse outcomes. Role of induction immunosuppression in this setting remains to be studied. METHODS: One thousand consecutive LT patients receiving rabbit antithymocyte globulin±rituximab induction were studied. Pretransplantation sera of 55 CM-positive (CM) patients were tested for C1q-fixing donor-specific antibodies (DSA). Diagnosis of antibody-mediated rejection required presence of diffuse vascular C4d expression on liver biopsies. RESULTS: CM was positive in 112 (11%) recipients. Antibody-mediated rejection was observed in 3 (0.03%) patients, whereas acute cellular rejection (ACR) occurred in 31 (3%) patients. CM status was associated with a higher incidence of ACR (9% in CM vs. 2% in CM-negative [CM]; P<0.01) and chronic rejection (4% in CM vs. 1% in CM; P<0.01). Graft survival was slightly lower in CM patients (at 1 year; 85% in CM vs. 89% in CM; P=0.26). Patients with autoimmune hepatitis, primary sclerosing cholangitis, and primary biliary cirrhosis as a group had a tendency toward CM status as well as developing ACR. Upon multivariate analysis, CM status was the strongest predictor of ACR (B=1.14; P=0.02). Only half of CM patients harbored C1q-fixing DSA. Presence of C1q-fixing DSA was not associated with increased incidence of ACR. CONCLUSIONS: In LT, CM status is associated with an increased incidence of acute rejection, chronic rejection, and slightly worse graft survival. With the use of rabbit antithymocyte globulin±rituximab induction, overall low rejection rates can be achieved in CM LT.

De novo malignancy post-liver transplantation: a single center, population controlled study.

BACKGROUND: With the growing numbers of liver transplant recipients, it is increasingly important to understand the risks of de novo malignancy after liver transplantation. AIM: To characterize the incidence of de novo malignancy after liver transplantation compared with a control non-transplant population. METHODS: We studied 534 Indiana state residents undergoing liver transplantation at our center between 1997 and 2004, followed through August 2010. The incidence and predictors of malignancy were determined. The standardized incidence ratio (SIR) of cancer in our cohort was compared with age-, gender-, and period-matched state population using the Indiana State Cancer Registry. RESULTS: After a mean follow-up of 5.7 ± 3.2 yr, 73 patients (13.7%) developed 80 cancers, with five- and 10-yr incidence rates of 11.7% and 24.8%, respectively. These included 24 (30%) skin, 16 (20%) hematologic, and 40 (50%) solid tumors. The most common solid cancers were aerodigestive. Compared with matched state population, liver transplant recipients had significantly higher incidence of all cancers (SIR: 3.1, 95% CI [Confidence interval]: 2.9-3.2), skin (melanoma) (SIR: 5.8, 95% CI: 4.7-7.0), hematologic (SIR: 7.1, 95% CI: 6.3-8.0), and solid (SIR: 2.7, 95% CI: 2.5-2.8) tumors. CONCLUSION: There is a significantly increased risk of de novo malignancies after liver transplantation, highlighting the need for surveillance strategies in this population.

Biallelic knockout of the α-1,3 galactosyltransferase gene in porcine liver-derived cells using zinc finger nucleases.

BACKGROUND: Genetic modification of the pig has been hampered by inefficiency of homologous recombination and unavailability of pig embryonic stem cells. Engineered zinc finger nuclease (ZFN)-mediated genetic modification in somatic cells combined with somatic cell nuclear transfer (SCNT) technology provides a new approach for targeted modification in pig genome. In this study, we used a ZFN pair to disrupt porcine α-1,3, galactosyltransferase (GGTA1) gene in liver-derived cells (LDC). ZFN-treated LDC were used as nuclear donors to produce fetuses and piglets via SCNT. All cloned fetuses and piglets showed biallelic knockout of GGTA1 gene. MATERIALS AND METHODS: A ZFN pair was designed to target exon 8 of pig GGTA1 gene. LDC were transfected with GGTA1 ZFN plasmids. SURVEYOR assay was used to evaluate the ZFN activity in LDC. GGTA1 gene knockout cells (GTKO) were obtained by counter-selection and used as nuclear donors for SCNT. The cloned fetuses and piglets were characterized by DNA sequencing. Expression of α-Gal epitope was further examined by flow cytometry and confocal microscopy. RESULTS: SURVEYOR assay revealed 6.48% ZFN activity in LDC. GTKO cells were obtained by counter-selection 10 d after ZFN transfection. A total of six fetuses and 13 piglets were produced by SCNT. All fetuses and piglets had biallelic mutations in the ZFN targeted region and were negative for α-Gal epitope. CONCLUSIONS: Biallelic GGTA1 gene disruption in LDC was generated efficiently by ZFN. GTKO fetuses were produced from ZFN-treated LDC by SCNT. GTKO piglets were obtained by SCNT of ZFN-treated LDC or recloning of fetal fibroblasts from GTKO fetuses. With longer lifespan and robust growth rate, LDC has the potential to endure multiple genetic modifications in vitro without going to SCNT, which could accelerate the production of genetically modified pig organs for xenotransplantation.