Nonintegrative AAV-mediated gene therapy in the liver is effective in adult patients but faces limitations in pediatric settings because of episomal DNA loss during hepatocyte proliferation. Gene targeting is a promising approach as it results in the permanent modification of the genome. We previously rescued neonatal lethality in Crigler-Najjar mice by inserting a promoterless human uridine glucuronosyl transferase A1 ( UGT1A1 ) cDNA in exon 14 of the albumin gene, without the use of nucleases. To increase the recombination rate and therapeutic efficacy, we used CRISPR/ Sa Cas9. Neonatal mice were transduced with 2 AAVs: one expressing the Sa Cas9 and sgRNA and one containing a promoterless cDNA flanked by albumin homology regions. Targeting efficiency increased approximately 26-fold with an EGFP reporter cDNA, reaching up to 24% of EGFP-positive hepatocytes. Next, we fully corrected the diseased phenotype of Crigler-Najjar mice by targeting the hUGT1A1 cDNA. Treated mice had normal plasma bilirubin up to 10 months after administration, hUGT1A1 protein levels were approximately 6-fold higher than in WT liver, with a 90-fold increase in recombination rate. Liver histology, inflammatory markers, and plasma albumin were normal in treated mice, with no off-targets in predicted sites. Thus, the improved efficacy and reassuring safety profile support the potential application of the proposed approach to other liver diseases.

Accumulation of neurotoxic bilirubin due to a transient neonatal or persistent inherited deficiency of bilirubin glucuronidation activity can cause irreversible brain damage and death. Strategies to inhibit bilirubin production and prevent neurotoxicity in neonatal and adult settings seem promising. We evaluated the impact of Bvra deficiency in neonatal and aged mice, in a background of unconjugated hyperbilirubinemia, by abolishing bilirubin production. We also investigated the disposal of biliverdin during fetal development. In Ugt1^−/− mice, Bvra deficiency appeared sufficient to prevent lethality and to normalize bilirubin level in adults. Although biliverdin accumulated in Bvra-deficient fetuses, both Bvra^−/− and Bvra^−/−Ugt1^−/− pups were healthy and reached adulthood having normal liver, brain, and spleen histology, albeit with increased iron levels in the latter. During aging, both Bvra^−/− and Bvra^−/−Ugt1^−/− mice presented normal levels of relevant hematological and metabolic parameters. Interestingly, the oxidative status in erythrocytes from 9-months-old Bvra^−/− and Bvra^−/−Ugt1^−/− mice was significantly reduced. In addition, triglycerides levels in these 9-months-old Bvra^−/− mice were significantly higher than WT controls, while Bvra^−/−Ugt1^−/− tested normal. The normal parameters observed in Bvra^−/−Ugt1^−/− mice fed chow diet indicate that Bvra inhibition to treat unconjugated hyperbilirubinemia seems safe and effective.

Hepatocytes are the liver's main functional cells and are key targets for in vivo gene therapy to treat monogenic diseases. Integrating the transgene into the genome is critical for long-term expression from a single early-life dose, which is achievable via integrating vectors or genome editing. To ensure persistence through liver growth and cell turnover, it is also necessary to target the hepatocytes driving these processes. While liver regeneration and homeostasis have been studied extensively, hepatocyte growth and maturation remain less well understood. Here, we investigate how hepatocyte heterogeneity evolves during liver growth and its implications for in vivo gene engineering. We performed clonal tracing, as well as single-cell and spatial transcriptomics, on mouse livers of various ages. We evaluated the efficiency, stability, and lobule distribution of lentiviral gene transfer and targeted transgene integration. We found that a subset of clonogenic hepatocytes (15-20%) in the newborn liver generates >90% of the adult tissue and co-localizes with hematopoietic islands within a spatial niche. Preferential gene editing of these clonogenic hepatocytes resulted in an increased proportion of the gene-engineered liver area, supporting their role in liver growth. Age-dependent hepatocellular heterogeneity affected the efficiency of lentiviral gene delivery in vivo and its distribution throughout the hepatic lobule. The gradual establishment of metabolic zonation after weaning and elevated proteasome activity in the peri-central area in adults influenced the observed age-related outcomes. These insights into spatiotemporal hepatocyte dynamics enhance our understanding of liver biology and have important implications for therapeutic strategies. We provide new insights into the spatiotemporal dynamics of the mouse liver during postnatal growth, highlighting both proliferative and transcriptomic heterogeneity among hepatocytes and their impact on the efficiency and distribution of in vivo lentiviral gene delivery and targeted gene editing. Understanding and manipulating the biological processes behind this heterogeneity can enhance gene transfer outcomes. We report that not all hepatocytes contribute equally to liver growth, indicating that effectively targeting clonogenic hepatocytes in the newborn liver is crucial for the long-term maintenance of therapeutic genetic modifications. Furthermore, this phenomenon can be leveraged to expand the pool of genetically corrected cells, as illustrated here by a targeted gene editing strategy. Finally, we reveal the existence of a tissue niche that supports the proliferation of both clonogenic hepatocytes and hematopoietic progenitors in neonatal livers. Gaining a deeper understanding of this niche and its signals may be beneficial for regenerative purposes.