Bilirubin glucuronidation by intact Gunn rat fibroblasts expressing bilirubin UDP-glucuronosyltransferase

A Quantitative In Vitro Potency Assay for Adeno-Associated Virus Vectors Encoding for the UGT1A1 Transgene

Potency assessment of clinical-grade vector lots is crucial to support adeno-associated virus (AAV) vector release and is required for future marketing authorization. We have developed and validated a cell-based, quantitative potency assay that detects both transgenic expression and activity of an AAV8-hUGT1A1 vector, which is currently under clinical evaluation for the treatment of Crigler-Najjar syndrome. Potency of AAV8-hUGT1A1 was evaluated in vitro. After transduction of human hepatoma 7 (Huh7) cells, transgene-positive cells were quantified using flow cytometry and transgenic activity by a bilirubin conjugation assay. The in vitro potency of various AAV8-hUGT1A1 batches was compared with their potency in vivo. After AAV8-hUGT1A1 transduction, quantification of UGT1A1-expressing cells shows a linear dose-response relation (R² = 0.98) with adequate intra-assay and inter-day reproducibility (coefficient of variation [CV] = 11.0% and 22.6%, respectively). In accordance, bilirubin conjugation shows a linear dose-response relation (R² = 0.99) with adequate intra- and inter-day reproducibility in the low dose range (CV = 15.7% and 19.7%, respectively). Both in vitro potency assays reliably translate to in vivo efficacy of AAV8-hUGT1A1 vector lots. The described cell-based potency assay for AAV8-hUGT1A1 adequately determines transgenic UGT1A1 expression and activity, which is consistent with in vivo efficacy. This novel approach is suited for the determination of vector lot potency to support clinical-grade vector release.

Generation of Ugt1-deficient murine liver cell lines using TALEN technology

The Crigler-Najjar Syndrome Type I (CNSI) is a rare genetic disorder caused by mutations in the Ugt1a1 gene. It is characterized by unconjugated hyperbilirubinemia that may result in severe neurologic damage and death if untreated. To date, liver transplantation is the only curative treatment. With the aim of generating mutant cell lines of the Ugt1 gene, we utilized the TALEN technology to introduce site-specific mutations in Ugt1 exon 4. We report a fast and efficient method to perform gene knockout in tissue culture cells, based on the use of TALEN pairs targeting restriction enzyme (RE) sites in the region of interest. This strategy overcame the presence of allele-specific single nucleotide polymorphisms (SNPs) and pseudogenes, conditions that limit INDELs' detection by Surveyor. We obtained liver-derived murine N-Muli cell clones having INDELs with efficiency close to 40%, depending on the TALEN pair and RE target site. Sequencing of the target locus and WB analysis of the isolated cell clones showed a high proportion of biallelic mutations in cells treated with the most efficient TALEN pair. Ugt glucuronidation activity was reduced basal levels in the biallelic mutant clones. These mutant liver-derived cell lines could be a very useful tool to study biochemical aspects of Ugt1 enzyme activity in a more natural context, such as substrate specificity, requirement of specific co-factors, the study of inhibitors and other pharmacological aspects, and to correlate enzyme activity to the presence of specific mutations in the gene, by adding back to the mutant cell clones specific variants of the Ugt1 gene. In addition, since genome editing has recently emerged as a potential therapeutic approach to cure genetic diseases, the definition of the most efficient TALEN pair could be an important step towards setting up a platform to perform genome editing in CNSI.

Polyinosinic acid blocks adeno-associated virus macrophage endocytosis in vitro and enhances adeno-associated virus liver-directed gene therapy in vivo

Adeno-associated virus serotype 8 (AAV8) has been demonstrated to be effective for liver-directed gene therapy in humans. Although hepatocytes are the main target cell for AAV8, there is a loss of the viral vector because of uptake by macrophages and Kupffer cells. Reducing this loss would increase the efficacy of viral gene therapy and allow a dose reduction. The receptor mediating this uptake has not been identified; a potential candidate seems the macrophage scavenger receptor A (SR-A) that is involved in the endocytosis of, for instance, adenovirus. In this study we show that SR-A can mediate scAAV8 endocytosis and that blocking it with polyinosinic acid (poly[i]) reduces endocytosis significantly in vitro. Subsequently, we demonstrate that blocking this receptor improves scAAV-mediated liver-directed gene therapy in a model for inherited hyperbilirubinemia, the uridine diphospho-glucuronyl transferase 1A1-deficient Gunn rat. In male rats, preadministration of poly[i] increases the efficacy of a low dose (1×10¹¹ gc/kg) but not of a higher dose (3×10¹¹ gc/kg) scAAV8-LP1-UT1A1. Administration of poly[i] just before the vector significantly increases the correction of serum bilirubin in female rats. In these, the effect of poly[i] is seen by both doses but is more pronounced in the females receiving the low vector, where it also results in a significant increase of bilirubin glucuronides in bile. In conclusion, this study shows that SR-A mediates the endocytosis of AAV8 in vitro and in vivo and that blocking this receptor can improve the efficacy of AAV-mediated liver-directed gene therapy.

Adeno-associated viral vector serotype 5 poorly transduces liver in rat models

Preclinical studies in mice and non-human primates showed that AAV serotype 5 provides efficient liver transduction and as such seems a promising vector for liver directed gene therapy. An advantage of AAV5 compared to serotype 8 already shown to provide efficient correction in a phase 1 trial in patients suffering from hemophilia B, is its lower seroprevalence in the general population. Our goal is liver directed gene therapy for Crigler-Najjar syndrome type I, inherited severe unconjugated hyperbilirubinemia caused by UGT1A1 deficiency. In a relevant animal model, the Gunn rat, we compared the efficacy of AAV 5 and 8 to that of AAV1 previously shown to be effective. Ferrying a construct driving hepatocyte specific expression of UGT1A1, both AAV8 and AAV1 provided an efficient correction of hyperbilirubinemia. In contrast to these two and to other animal models AAV5 failed to provide any correction. To clarify whether this unexpected finding was due to the rat model used or due to a problem with AAV5, the efficacy of this serotype was compared in a mouse and two additional rat strains. Administration of an AAV5 vector expressing luciferase under the control of a liver specific promoter confirmed that this serotype poorly performed in rat liver, rendering it not suitable for proof of concept studies in this species.

Long-term reduction of jaundice in Gunn rats by nonviral liver-targeted delivery of Sleeping Beauty transposon

Asialoglycoprotein receptor (ASGPR)-mediated endocytosis has been used to target genes to hepatocytes in vivo. However, the level and duration of transgene expression have been low because of lysosomal translocation and degradation of the DNA and lack of its integration into the host genome. In this study we packaged the DNA of interest in proteoliposomes containing the fusogenic galactose-terminated F-glycoprotein of the Sendai virus (FPL) for targeted delivery to hepatocytes. After the FPL binds to ASGPR on the hepatocyte surface, fusogenic activity of the F-protein delivers the DNA into the cytosol, bypassing the endosomal pathway. For transgene integration we designed plasmids containing one transcription unit expressing the Sleeping Beauty transposase (SB) and another expressing human uridinediphosphoglucuronate glucuronosyltransferase-1A1 (pSB-hUGT1A1). The latter was flanked by inverted/direct repeats that are substrates of SB. In cell culture, FPL-mediated delivery of the E. coli beta-galactosidase gene (LacZ) resulted in transduction of ASGPR-positive cells (rat hepatocytes or Hepa1 cell line), but not of ASGPR-negative 293 cells. Intravenous injection of the FPL-entrapped pSB-hUGT1A1 (4-8 microg/day, 1-4 doses) into UGT1A1-deficient hyperbilirubinemic Gunn rats (model of Crigler-Najjar syndrome type 1) resulted in hUGT1A1 expression in 5%-10% of hepatocytes, but not in other cell types. Serum bilirubin levels declined by 30% +/- 4% in 2 weeks and remained at that level throughout the 7-month study duration. With histidine containing FPL, serum bilirubin was reduced by 40% +/- 5%, and bilirubin glucuronides were excreted into bile. No antibodies were detectable in the recipient rats against the F-protein or human UGT1A1.

CONCLUSION

FPL is an efficient hepatocyte-targeted gene delivery platform in vivo that warrants further exploration toward clinical application.

Ex vivo gene transfer into hepatocytes

Ex vivo gene transfer into hepatocytes could serve several purposes in the context of gene therapy or cell transplantation: (1) isolated hepatocytes can be transduced in culture with therapeutic genes and then transplanted into the recipient; (2) marker genes can be introduced for subsequent identification of transplanted cells and their progeny; (3) gene transfer can be used for conditional immortalization of hepatocytes for expansion in culture; (4) immunomodulatory genes can be transferred into hepatocytes to prevent allograft rejection. Gene transfer into cultured hepatocytes can be achieved using DNA that is not incorporated into recombinant viruses. In such systems, transgene integration into the host cell genome can be enhanced using transposon systems, such as "sleeping beauty." In addition to using the conventional reagents, such as cationic liposomes, DNA transfer into hepatocytes can be achieved by Nucleofection or special hepatocyte-targeted carriers such as proteoliposomes containing galactose-terminated glycoproteins (e.g. the F protein of the Sendai virus). Alternatively, genes can be transferred using recombinant viruses, such as adenoviral vectors that are episomal or retroviral vectors (including lentiviruses) that permit integration of the transgene into the host genome. Gene transfer using lentiviral vectors has been achieved in both attached and suspended hepatocytes. Transduction efficiency of lentiviral vectors can be enhanced using magnetic nanoparticles (Magnetofection).

Hepatic repopulation with stably transduced conditionally immortalized hepatocytes in the Gunn rat

BACKGROUND/AIMS

Conditionally immortalized hepatocytes offer a renewable source of hepatocytes, but although preparative maneuvers have been developed for hepatic repopulation with primary hepatocytes, extensive proliferation of transplanted immortalized hepatocytes has not been accomplished heretofore. Our aim was to achieve ex vivo gene therapy of uridinediphosphoglucuronate glucuronosyltransferase-1A1 (UGT1A1)-deficient jaundiced Gunn rats (model of Crigler-Najjar syndrome type-1) by hepatic repopulation with genetically modified and conditionally immortalized hepatocytes.

METHODS

Gunn rat hepatocytes were conditionally immortalized by stable transduction with a thermolabile mutant simian virus 40 T-antigen ((ts)Tag(A58)) and further transduced with UGT1A1. These hepatocytes proliferate at 33 degrees C, but at 37 degrees C the (ts)Tag(A58) is degraded and the cells become quiescent. The cells were transplanted into Gunn rat livers after preparative hepatic irradiation (50 Gy) and 66% hepatectomy.

RESULTS

The engrafted UGT1A1-positive immortalized hepatocytes replaced approximately 80% of the host hepatocytes in 20 weeks, leading to normalization of hyperbilirubinemia. Liver histology, and serum albumin and alanine aminotransferase levels remained normal.

CONCLUSIONS

We achieved complete cure of hyperbilirubinemia in Gunn rats by ex vivo gene therapy via genetically modified and conditionally immortalized hepatocytes.

Role of cysteine residues in the function of human UDP glucuronosyltransferase isoform 1A1 (UGT1A1)

Bilirubin glucuronidation, catalysed by UGT1A1 [UGT (UDP glucuronosyltransferase) isoform 1A1, EC 2.4.1.17], is critical for biliary elimination of bilirubin. UGT1A1 deficiency causes CN-1 (Crigler-Najjar syndrome type 1), which is characterized by potentially lethal unconjugated hyperbilirubinaemia. Nucleotide sequence analysis of UGT1A1 in two CN-1 patients revealed that patient A was homozygous for a nt 530 G-->A (where nt 530 G-->A means guanine to adenine transition at nucleotide 530) mutation, predicting a C177Y substitution, and patient B had a nt 466 T-->C mutation on one allele and a nt 1070 A-->G mutation on the other, predicting a C156R and a Q357R substitution respectively. All 11 cysteine residues of mature human UGT1A1 are highly conserved in other human UGT isoforms and in rat, mouse and Rhesus monkey UGT1A1, suggesting their functional importance. Expression of mutagenized UGT1A1 plasmids showed that substitution of any of the seven cysteine residues located within the endoplasmic reticulum cisternae (including those mutated in patients A and B) abolished UGT1A1 activity or markedly increased its apparent K(m) for bilirubin. Substitution of the three cysteine residues within the C-terminal cytosolic tail had minimal effect on basal UGT1A1 activity, but prevented UGT1A1 activation by UDP-GlcNAc. N-Ethylmaleimide did not inhibit UGT1A1 activity in native microsomes, but prevented UGT1A1 activation by UDP-GlcNAc and inhibited the activity in digitonin-permeabilized microsomes. Dithiothreitol did not affect UGT1A1 activity in human liver microsomes. Together, the results suggested that free thiol groups, but not disulphide bonding, of seven cysteine residues within the intracisternal region of human UGT1A1 are important for its catalytic activity, while cysteine residues in the cytosolic domain may be involved in its physiological activation by UDP-GlcNAc.

Zeaxanthin dipalmitate from Lycium chinense fruit reduces experimentally induced hepatic fibrosis in rats

We previously reported that zeaxanthin dipalmitate (ZD), a carotenoid from Lycium chinense fruit, reduces myofibroblast-like cell proliferation and collagen synthesis in vitro. To determine whether ZD might reduce the severity of hepatic fibrosis in an animal model, hepatic fibrosis was induced in rats by bile duct ligation/scission (BDL) for a period of 6 weeks. Treatment of BDL rats with ZD at a dose of 25 mg/kg body weight significantly reduced the activities of aspartate transaminase (p<0.05) and alkaline phosphatase (p<0.001) in serum. Furthermore, collagen deposition was significantly reduced as assessed by the Sirius Red binding assay in BDL rats administered ZD at the dose of 25 mg/kg body weight (p<0.01). In addition, the levels of thiobarbituric acid-reactive substances and 4-hydroxyproline were reduced when BDL rats received ZD at the dose of 25 mg/kg body weight. These results showed that ZD effectively inhibited hepatic fibrosis in BDL rats, at least in part via its antioxidative activity.

Homodimerization of human bilirubin-uridine-diphosphoglucuronate glucuronosyltransferase-1 (UGT1A1) and its functional implications

Genetic lesions of bilirubin-uridine-diphosphoglucuronate glucuronosyltransferase-1 (UGT1A1) completely or partially abolish hepatic bilirubin glucuronidation, causing Crigler-Najjar syndrome type 1 or 2, respectively. Clinical observations indicate that some mutant forms of human UGT1A1 (hUGT1A1) may be dominant-negative, suggesting their interaction with the wild-type enzyme. To evaluate intermolecular interaction of hUGT1A1, Gunn rat fibroblasts were stably transduced with hUGT1A1 cDNA. Gel permeation chromatography of solubilized microsomes suggested dimerization of hUGT1A1 in solution. Nearest-neighbor cross-linking analysis indicated that, within microsomal membranes, hUGT1A1 dimerized more efficiently at pH 7.4 than at pH 9. Two-hybrid analysis in yeast and mammalian systems demonstrated positive interaction of hUGT1A1 with itself, but not with another UGT isoform, human UGT1A6, which differs only in the N-terminal domain. Dimerization was abolished by deletion of the membrane-embedded helix from the N-terminal domain of hUGT1A1, but not by substitution of several individual amino acid residues or partial deletion of the C-terminal domain. A C127Y substitution abolished UGT1A1 activity, but not its dimerization. Coexpression of mutagenized and wild-type hUGT1A1 in COS-7 cells showed that the mutant form markedly suppressed the catalytic activity of wild-type hUGT1A1. Homodimerization of hUGT1A1 may explain the dominant-negative effect of some mutant forms of the enzyme.

Mechanism of indinavir-induced hyperbilirubinemia

Indinavir is a viral protease inhibitor used for the treatment of HIV infection. Unconjugated hyperbilirubinemia develops in up to 25% of patients receiving indinavir, prompting drug discontinuation and further clinical evaluation in some instances. We postulated that this side-effect is due to indinavir-mediated impairment of bilirubin UDP-glucuronosyltransferase (UGT) activity and would be most pronounced in individuals with reduced hepatic enzyme levels, as occurs in approximately 10% of the population manifesting Gilbert's syndrome. This hypothesis was tested in vitro, in the Gunn rat model of UGT deficiency, and in HIV-infected patients with and without the Gilbert's polymorphism. Indinavir was found to competitively inhibit UGT enzymatic activity (K(I) = 183 microM) while concomitantly inducing hepatic bilirubin UGT mRNA and protein expression. Although oral indinavir increased plasma bilirubin levels in wild-type and heterozygous Gunn rats, the mean rise was significantly greater in the latter group of animals. Similarly, serum bilirubin increased by a mean of 0.34 mg/dl in indinavir-treated HIV patients lacking the Gilbert's polymorphism versus 1.45 mg/dl in those who were either heterozygous or homozygous for the mutant allele. Whereas saquinavir also competitively inhibits UGT activity, this drug has not been associated with hyperbilirubinemia, most likely because of the higher K(I) (360 microM) and substantially lower therapeutic levels as compared with indinavir. Taken together, these findings indicate that elevations in serum-unconjugated bilirubin associated with indinavir treatment result from direct inhibition of bilirubin-conjugating activity.

Bilirubin and bile acids may modulate their own metabolism via regulating uridine diphosphate-glucuronosyltransferase expression in the rat

BACKGROUND AND AIMS

Uridine diphosphate (UDP)-glucuronosyltransferase (UGT) is a critical enzyme in the elimination of bilirubin and it also plays a role in the metabolism of bile acids. The aim of this study was to determine whether bilirubin and bile acids could modulate their own metabolism by regulating UGT levels in cultured rat hepatocytes.

METHODS AND RESULTS

Incubation of hepatocytes with bilirubin (48 micromol/L) for 24 h significantly increased the mRNA expression of UGT1A1 and UGT1A5, two UGT isoforms responsible for the conjugation of bilirubin. The induction of UGT1A1 and UGT1A5 by bilirubin was concentration and time dependent. Treatment with chenodeoxycholic acid, cholic acid, deoxycholic acid, hyodeoxycholic acid and lithocholic acid at a concentration of 100 micromol/L for 48 h significantly enhanced the mRNA expression of UGT2B1, a UGT isoform responsible for the glucuronidation of bile acids. The UGT2B3 mRNA level was also increased by hyodeoxycholic acid. The regulation of UGT2B1 mRNA by chenodeoxycholic acid and hyodeoxycholic acid was dose and time dependent.

CONCLUSION

Our results suggest that bilirubin and bile acids can induce UGT expression and as a result, these compounds may modulate their own metabolism. Such regulation could play a compensatory role in the pathological increased concentrations of these compounds in some hepatobiliary diseases.

Mechanism of hepatocellular uptake of albumin-bound bilirubin

We previously demonstrated that unconjugated bilirubin spontaneously diffuses through phospholipid bilayers at a rate which exceeds albumin dissociation, suggesting that solvation from albumin represents the rate-limiting step in hepatic bilirubin clearance. To further examine this hypothesis, we studied the uptake of bovine serum albumin (BSA)-bound bilirubin by cultured hepatoblastoma (HepG2) cells. Uptake of bilirubin was saturable, with a K(m) and V(max) of 4.2+/-0.5 microM (+/-S.E.M.) and 469+/-41 pmol min(-1) mg(-1) at 25 degrees C. Substantial bilirubin uptake also was observed at 4 degrees C (K(m)=7.0+/-0.8 microM, V(max)=282+/-26 pmol min(-1) mg(-1)), supporting a diffusional transport mechanism. Consistent with reported solvation rates, the cellular uptake of bilirubin bound to human serum albumin was more rapid than for BSA-bound bilirubin, indicative of dissociation-limited uptake. Counterintuitively, an inverse correlation between pH and the rate of bilirubin flip-flop was observed, due to pH effects on the rate of dissociation of bilirubin from albumin and from the membrane bilayer. The identification of an inflection point at pH 8.1 is indicative of a pK(a) value for bilirubin in this range. Taken together, our data suggest that hepatocellular uptake of bilirubin is dissociation-limited and occurs principally by a mechanism involving spontaneous transmembrane diffusion.

Cloning and gene structure of rat phosphatidylcholine transfer protein, Pctp

Phosphatidylcholine transfer protein (PC-TP) is a cytosolic lipid transfer protein that promotes intermembrane transfer of phosphatidylcholines but no other phospholipids. Although its physiological function remains unknown, phosphatidylcholine transfer protein is enriched in liver and evidence from model systems suggests a role in hepatocellular selection and transport of biliary phospholipids. To facilitate in vivo studies, a cDNA encoding rat PC-TP was cloned by library screening and 5'-rapid amplification of cDNA ends. Genomic cloning demonstrated the rat Pctp gene spans 10. 8kb and is comprised of six exons. The putative transcription initiation site was identified 50bp upstream of the translation initiation site. Nucleotide sequence analysis of the 5'-flanking region revealed a CAAT- but no TATA-box. Transient transfection of a series of 5'-deleted Pctp-promoter-firefly luciferase constructs into Reuber H35 rat hepatoma cells, which express Pctp mRNA, and Gunn rat fibroblasts, which do not, suggest that cis-acting elements in a 637bp promoter region contribute to enhanced expression of PC-TP in liver.

Unconjugated bilirubin exhibits spontaneous diffusion through model lipid bilayers and native hepatocyte membranes

The liver is responsible for the clearance and metabolism of unconjugated bilirubin, the hydrophobic end-product of heme catabolism. Although several putative bilirubin transporters have been described, it has been alternatively proposed that bilirubin enters the hepatocyte by passive diffusion through the plasma membrane. In order to elucidate the mechanism of bilirubin uptake, we measured the rate of bilirubin transmembrane diffusion (flip-flop) using stopped-flow fluorescence techniques. Unconjugated bilirubin rapidly diffuses through model phosphatidylcholine vesicles, with a first-order rate constant of 5.3 s-1 (t(1)/(2) = 130 ms). The flip-flop rate is independent of membrane cholesterol content, phospholipid acyl saturation, and lipid packing, consistent with thermodynamic analyses demonstrating minimal steric constraint to bilirubin transmembrane diffusion. The coincident decrease in pH of the entrapped vesicle volume supports a mechanism whereby the bilirubin molecule crosses the lipid bilayer as the uncharged diacid. Transport of bilirubin by native rat hepatocyte membranes exhibits kinetics comparable with that in model vesicles, suggesting that unconjugated bilirubin crosses cellular membranes by passive diffusion through the hydrophobic lipid core. In contrast, there is no demonstrable flip-flop of bilirubin diglucuronide or bilirubin ditaurate in phospholipid vesicles, yet these compounds rapidly traverse isolated rat hepatocyte membranes, confirming the presence of a facilitated uptake system(s) for hydrophilic bilirubin conjugates.

Long-term reduction of serum bilirubin levels in Gunn rats by retroviral gene transfer in vivo

Conjugation with glucuronic acid, mediated by bilirubin-uridinediphosphoglucuronate glucuronosyltransferase (bilirubin-UGT), is essential for efficient biliary excretion of bilirubin. Inherited absence of this enzyme activity results in the potentially lethal Crigler-Najjar syndrome type I in humans and lifelong hyperbilirubinemia in Gunn rats. To develop a gene therapy for bilirubin-UGT deficiency, we constructed a high-titer replication-deficient amphotropic recombinant retrovirus (MFG-S hB-UGT1) capable of transferring the gene encoding bilirubin-UGT1, the principal bilirubin-UGT isoform in human liver. To stimulate hepatocyte proliferation, Gunn rats were subjected to 66% hepatectomy. After 24 hours, the portal vein, the hepatic artery, and the inferior vena cava above and below the hepatic vein were clamped, and the portal vein and the isolated segment of the vena cava were cannulated. The liver was perfused with the MFG-S hB-UGT1 preparation through the portal vein at 5 ml/min for 10 minutes, then circulation was restored. Control rat livers were perfused with a recombinant retrovirus expressing Escherichia coli beta-galactosidase. In MFG-S hB-UGT1-perfused rats, but not in controls, expression of human bilirubin-UGT1 was shown by immunotransblotting, bilirubin-UGT assay of liver homogenates, and biliary excretion of bilirubin diglucuronide and monoglucuronide. Mean serum bilirubin levels decreased by 20% to 25% in 3 weeks and remained at that level throughout the study period (18 months). This is the first report of long-term amelioration of inherited jaundice by retrovirus-directed gene therapy in an animal model for Crigler-Najjar syndrome.

Transplantation of Gunn rats with autologous fibroblasts expressing bilirubin UDP-glucuronosyltransferase: correction of genetic deficiency and tumor formation

The end product of the breakdown of the heme group of hemoglobin and other heme-containing proteins is bilirubin. Bilirubin is hydrophobic and cannot be excreted as such. Therefore, mammals have a liver enzyme bilirubin UDP-glucuronosyltransferase (B-UGT), which conjugates bilirubin with glucuronic acid, thereby making the molecule much more water soluble. Bilirubin glucuronides are secreted into bile. Patients with Crigler-Najjar (CN) disease have a deficiency in bilirubin UDP-glucuronosyltransferase and accumulate high serum levels of bilirubin. An animal model for CN disease is the Gunn rat. The obvious target for gene therapy for CN disease is the liver, but because liver cells do only divide infrequently, they are difficult to transduce. To investigate whether cells that are easily transduced can be used to develop gene therapy for CN disease, we have transduced Gunn rat fibroblasts with B-UGT, using a recombinant retrovirus. Gunn rat fibroblasts expressing B-UGT were able to glucuronidate bilirubin present in cell culture media. In this study, we describe the intraperitoneal transplantation of Gunn rats with Gunn rat fibroblasts expressing B-UGT. Transplantation of the fibroblasts corrected the genetic deficiency of the Gunn rats, serum bilirubin concentrations of the transplanted Gunn rats were reduced to normal, and bilirubin glucuronides appeared in bile. However, due to the prolonged period of cell culture, the transplanted fibroblasts were transformed, and the experimental animals developed tumors after transplantation.