CRISPR-Cas9 is a promising technology for genome editing. Here we use Cas9 nuclease-induced double-strand break DNA (DSB) at the UROS locus to model and correct congenital erythropoietic porphyria. We demonstrate that homology-directed repair is rare compared with NHEJ pathway leading to on-target indels and causing unwanted dysfunctional protein. Moreover, we describe unexpected chromosomal truncations resulting from only one Cas9 nuclease-induced DSB in cell lines and primary cells by a p53-dependent mechanism. Altogether, these side effects may limit the promising perspectives of the CRISPR-Cas9 nuclease system for disease modeling and gene therapy. We show that the single nickase approach could be safer since it prevents on- and off-target indels and chromosomal truncations. These results demonstrate that the single nickase and not the nuclease approach is preferable, not only for modeling disease but also and more importantly for the safe management of future CRISPR-Cas9-mediated gene therapies.

No abstract

Congenital erythropoietic porphyria (CEP) is a recessive autosomal disorder characterized by a deficiency in uroporphyrinogen III synthase (UROS), the fourth enzyme of the heme biosynthetic pathway. The severity of the disease, the lack of specific treatment except for allogeneic bone marrow transplantation, and the knowledge of the molecular lesions are strong arguments for gene therapy. An animal model of CEP has been designed to evaluate the feasibility of retroviral gene transfer in hematopoietic stem cells. We have previously demonstrated that the knockout of the Uros gene is lethal in mice (Uros(del) model). This work describes the achievement of a knock-in model, which reproduces a mutation of the UROS gene responsible for a severe UROS deficiency in humans (P248Q missense mutant). Homozygous mice display erythrodontia, moderate photosensitivity, hepatosplenomegaly, and hemolytic anemia. Uroporphyrin (99% type I isomer) accumulates in urine. Total porphyrins are increased in erythrocytes and feces, while Uros enzymatic activity is below 1% of the normal level in the different tissues analyzed. These pathological findings closely mimic the CEP disease in humans and demonstrate that the Uros(mut248) mouse represents a suitable model of the human disease for pathophysiological, pharmaceutical, and therapeutic purposes.

Congenital erythropoietic porphyria (CEP) is a rare autosomal recessive disorder characterized by uroporphyrinogen III synthase (UROS) deficiency resulting in massive porphyrin accumulation in blood cells, which is responsible for hemolytic anemia and skin photosensitivity. Among the missense mutations actually described up to now in CEP patients, the C73R and the P248Q mutations lead to a profound UROS deficiency and are usually associated with a severe clinical phenotype. We previously demonstrated that the UROS C73R mutant protein conserves intrinsic enzymatic activity but triggers premature degradation in cellular systems that could be prevented by proteasome inhibitors. We show evidence that the reduced kinetic stability of the UROS P248Q mutant is also responsible for increased protein turnover in human erythroid cells. Through the analysis of EGFP-tagged versions of UROS enzyme, we demonstrate that both UROS C73R and UROS P248Q are equally destabilized in mammalian cells and targeted to the proteasomal pathway for degradation. We show that a treatment with proteasomal inhibitors, but not with lysosomal inhibitors, could rescue the expression of both EGFP-UROS mutants. Finally, in CEP mice (Uros P248Q/P248Q ) treated with bortezomib (Velcade), a clinically approved proteasome inhibitor, we observed reduced porphyrin accumulation in circulating RBCs and urine, as well as reversion of skin photosensitivity on bortezomib treatment. These results of medical importance pave the way for pharmacologic treatment of CEP disease by preventing certain enzymatically active UROS mutants from early degradation by using proteasome inhibitors or chemical chaperones.protein misfolding | Günther's disease | enzyme instability | heme biosynthetic pathway | pharmacological therapy