Exploring new gene integration sites for gene knock-in by gene-trapping strategy

Prediction of drug-drug interaction risk of P-glycoprotein substrate in drug discovery

We aimed at predicting the drug-drug interaction (DDI) risk of P-glycoprotein (P-gp) substrates by using P-gp expressing LLC-PK1 cells and its knockout mice (KO). The area under the curve (AUC) of 16 marketed drugs and plasma concentration (Cplasma) of 207 screening compounds, with corrected efflux ratio (CER) ≥ 2, were compared between P-gp KO mice and wild type mice (WT). At permeability (Papp) ≥ 10 × 10-6 cm/s in parent LLC-PK1 cells, AUC ratios (KO/WT) and Cplasma ratios (KO/WT) of these compounds were within 3-fold. AUC ratios (KO/WT) of clinical P-gp substrates, with human AUC ratios with and without P-gp inhibitor administration ≥2, were higher than 8.7. These observations led us to establish a work-flow of P-gp substrate assessment with the threshold AUC ratio (KO/WT) ≥ 9 leading to a DDI risk of AUC ratio (human) ≥ 2. A screening compound showing high CER (=57.6) was found, but its AUC ratio (KO/WT) was 3.7, had been presumed to be a weak risk and its AUC ratio (human) was 1.2 in a later clinical DDI study. Our proposed workflow should be useful for predicting the DDI risk of P-gp substrates in drug discovery.

Evaluation of hepatic function using dynamic contrast-enhanced magnetic resonance imaging in melanocortin 4 receptor-deficient mice as a model of nonalcoholic steatohepatitis

INTRODUCTION

Melanocortin 4 receptor-deficient (MC4R-KO) mice fed a high-fat diet (HFD) develop liver pathology similar to human nonalcoholic steatohepatitis (NASH). However, although liver histology and blood biochemistry have been reported, hepatic function has not been evaluated. In the present study, we evaluated hepatic function in MC4R-KO mice fed an HFD using dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) with gadolinium‑ethoxybenzyl‑diethylenetriamine pentaacetic acid (Gd-EOB-DTPA).

MATERIALS AND METHODS

Wild type (WT) mice and MC4R-KO mice were fed a standard diet (SD) or an HFD for 20 weeks. The hepatic signal intensity was obtained from DCE-MRI images, and relative enhancement (RE), the time to maximum RE (T_max), and the half-life of RE elimination (T_1/2) were calculated. Histopathological analysis was then performed.

RESULTS

Histological analysis with nonalcoholic fatty liver disease activity score (NAS) revealed that MC4R-KO mice fed an HFD achieved the NAS of 5. There was moderate fibrosis in MC4R-KO mice fed an HFD. DCE-MRI with Gd-EOB-DTPA showed that T_max and T_1/2 were significantly longer in MC4R-KO mice fed an HFD compared with wild type (WT) mice (T_max, WT, 3.9 ± 0.4 min; MC4R-KO, 7.4 ± 1.5 min; T_1/2, WT, 23.7 ± 1.9 min; MC4R-KO, 62.5 ± 18.5 min). T_max and T_1/2 were significantly correlated with histopathologic score (steatosis vs. Tmax, rho = 0.48, P = 0.04; steatosis vs. T_1/2, rho = 0.50, P = 0.03; inflammation vs. Tmax, rho = 0.55, P = 0.02; inflammation vs. T_1/2, rho = 0.61, P < 0.01; ballooning vs. T_1/2, rho = 0.51, P = 0.03;fibrosis vs T_max, rho = 0.72, P < 0.01; fibrosis vs T_1/2, rho = 0.75, P < 0.01).

CONCLUSIONS

MC4R-KO mice fed an HFD developed obesity and NASH. The liver kinetics of Gd-EOB-DTPA were significantly different in MC4R-KO mice fed an HFD from WT mice, and correlated with the histopathologic score. These results suggest that MC4R-KO mice fed an HFD mimic the hepatic pathology and liver function of human NASH, and therefore might be useful for the study of hepatic dysfunction during the fibrotic stage of NASH.

Identification and characterization of MYH9 locus for high efficient gene knock-in and stable expression in mouse embryonic stem cells

Targeted integration of exogenous genes into so-called safe harbors/friend sites, offers the advantages of expressing normal levels of target genes and preventing potentially adverse effects on endogenous genes. However, the ideal genomic loci for this purpose remain limited. Additionally, due to the inherent and unresolved issues with the current genome editing tools, traditional embryonic stem (ES) cell-based targeted transgenesis technology is still preferred in practical applications. Here, we report that a high and repeatable homologous recombination (HR) frequency (>95%) is achieved when an approximate 6kb DNA sequence flanking the MYH9 gene exon 2 site is used to create the homology arms for the knockout/knock-in of diverse nonmuscle myosin II (NM II) isoforms in mouse ES cells. The easily obtained ES clones greatly facilitated the generation of multiple NM II genetic replacement mouse models, as characterized previously. Further investigation demonstrated that though the targeted integration site for exogenous genes is shifted to MYH9 intron 2 (about 500bp downstream exon 2), the high HR efficiency and the endogenous MYH9 gene integrity are not only preserved, but the expected expression of the inserted gene(s) is observed in a pre-designed set of experiments conducted in mouse ES cells. Importantly, we confirmed that the expression and normal function of the endogenous MYH9 gene is not affected by the insertion of the exogenous gene in these cases. Therefore, these findings suggest that like the commonly used ROSA26 site, the MYH9 gene locus may be considered a new safe harbor for high-efficiency targeted transgenesis and for biomedical applications.

Multiple expression cassette exchange via TP901-1, R4, and Bxb1 integrase systems on a mouse artificial chromosome

The site-specific excision of a target DNA sequence for genetic knockout or lineage tracing is a powerful tool for investigating biological systems. Currently, site-specific recombinases (SSRs), such as Cre or Flp recombination target cassettes, have been successfully excised or inverted by a single SSR to regulate transgene expression. However, the use of a single SSR might restrict the complex control of gene expression. This study investigated the potential for expanding the multiple regulation of transgenes using three different integrase systems (TP901-1, R4, and Bxb1). We designed three excision cassettes that expressed luciferase, where the luciferase expression could be exchanged to a fluorescent protein by site-specific recombination. Individual cassettes that could be regulated independently by a different integrase were connected in tandem and inserted into a mouse artificial chromosome (MAC) vector in Chinese hamster ovary cells. The transient expression of an integrase caused the targeted luciferase activity to be lost and fluorescence was activated. Additionally, the integrase system enabled the specific excision of targeted DNA sequences without cross-reaction with the other recombination targets. These results suggest that the combined use of these integrase systems in a defined locus on a MAC vector permits the multiple regulation of transgene expression and might contribute to genomic or cell engineering.