Missense UROS mutations causing congenital erythropoietic porphyria reduce UROS homeostasis that can be rescued by proteasome inhibition.

Congenital erythropoietic porphyria (CEP) is an inborn error of heme biosynthesis characterized by uroporphyrinogen III synthase (UROS) deficiency resulting in deleterious porphyrin accumulation in blood cells responsible for hemolytic anemia and cutaneous photosensitivity. We analyzed here the molecular basis of UROS impairment associated with twenty nine UROS missense mutations actually described in CEP patients. Using a computational and biophysical joint approach we predicted that most disease-causing mutations would affect UROS folding and stability. Through the analysis of enhanced green fluorescent protein-tagged versions of UROS enzyme we experimentally confirmed these data and showed that thermodynamic instability and premature protein degradation is a major mechanism accounting for the enzymatic deficiency associated with twenty UROS mutants in human cells. Since the intracellular loss in protein homeostasis is in excellent agreement with the in vitro destabilization, we used molecular dynamic simulation to rely structural 3D modification with UROS disability. We found that destabilizing mutations could be clustered within three types of mechanism according to side chain rearrangements or contact alterations within the pathogenic UROS enzyme so that the severity degree correlated with cellular protein instability. Furthermore, proteasome inhibition using bortezomib, a clinically available drug, significantly enhanced proteostasis of each unstable UROS mutant. Finally, we show evidence that abnormal protein homeostasis is a prevalent mechanism responsible for UROS deficiency and that modulators of UROS proteolysis such as proteasome inhibitors or chemical chaperones may represent an attractive therapeutic option to reduce porphyrin accumulation and prevent skin photosensitivity in CEP patients when the genotype includes a missense variant.

Targeted gene therapy in human-induced pluripotent stem cells from a patient with primary hyperoxaluria type 1 using CRISPR/Cas9 technology.

Primary hyperoxaluria type 1 (PH1) is an inherited metabolic disorder caused by a deficiency of the peroxisomal enzyme alanine-glyoxylate aminotransferase (AGT), which leads to overproduction of oxalate by the liver and results in urolithiasis, nephrocalcinosis and renal failure. The only curative treatment for PH1 is combined liver and kidney transplantation, which is limited by the lack of suitable organs, significant complications, and the life-long requirement for immunosuppressive agents to maintain organ tolerance. Hepatocyte-like cells (HLCs) generated from CRISPR/Cas9 genome-edited human-induced pluripotent stem cells would offer an attractive unlimited source of autologous gene-corrected liver cells as an alternative to orthotopic liver transplantation (OLT). Here we report the CRISPR/Cas9 nuclease-mediated gene targeting of a single-copy AGXT therapeutic minigene into the safe harbour AAVS1 locus in PH1-induced pluripotent stem cells (PH1-iPSCs) without off-target inserts. We obtained a robust expression of a codon-optimized AGT in HLCs derived from AAVS1 locus-edited PH1-iPSCs. Our study provides the proof of concept that CRISPR/Cas9-mediated integration of an AGXT minigene into the AAVS1 safe harbour locus in patient-specific iPSCs is an efficient strategy to generate functionally corrected hepatocytes, which in the future may serve as a source for an autologous cell-based gene therapy for the treatment of PH1.

Generation of induced pluripotent stem cells-derived hepatocyte-like cells for ex vivo gene therapy of primary hyperoxaluria type 1.

Primary hyperoxaluria type 1 (PH1) is a rare autosomal recessive disorder of the liver metabolism due to functional deficiency of the peroxisomal enzyme alanine:glyoxylate aminotransferase (AGT). AGT deficiency results in overproduction of oxalate which complexes with calcium to form insoluble calcium-oxalate salts in urinary tracts, ultimately leading to end-stage renal disease. Currently, the only curative treatment for PH1 is combined liver-kidney transplantation, which is limited by donor organ shortage and lifelong requirement for immunosuppression. Transplantation of genetically modified autologous hepatocytes is an attractive therapeutic option for PH1. However, the use of fresh primary hepatocytes suffers from limitations such as organ availability, insufficient cell proliferation, loss of function, and the risk of immune rejection. We developed patient-specific induced pluripotent stem cells (PH1-iPSCs) free of reprogramming factors as a source of renewable and genetically defined autologous PH1-hepatocytes. We then investigated additive gene therapy using a lentiviral vector encoding wild-type AGT under the control of the liver-specific transthyretin promoter. Genetically modified PH1-iPSCs successfully provided hepatocyte-like cells (HLCs) that exhibited significant AGT expression at both RNA and protein levels after liver-specific differentiation process. These results pave the way for cell-based therapy of PH1 by transplantation of genetically modified autologous HLCs derived from patient-specific iPSCs.