In vivo liver targeted genome editing as therapeutic approach: progresses and challenges.

The liver is an essential organ of the body that performs several vital functions, including the metabolism of biomolecules, foreign substances, and toxins, and the production of plasma proteins, such as coagulation factors. There are hundreds of genetic disorders affecting liver functions and, for many of them, the only curative option is orthotopic liver transplantation, which nevertheless entails many risks and long-term complications. Some peculiar features of the liver, such as its large blood flow supply and the tolerogenic immune environment, make it an attractive target for in vivo gene therapy approaches. In recent years, several genome-editing tools mainly based on the clustered regularly interspaced short palindromic repeats associated protein 9 (CRISPR-Cas9) system have been successfully exploited in the context of liver-directed preclinical or clinical therapeutic applications. These include gene knock-out, knock-in, activation, interference, or base and prime editing approaches. Despite many achievements, important challenges still need to be addressed to broaden clinical applications, such as the optimization of the delivery methods, the improvement of the editing efficiency, and the risk of on-target or off-target unwanted effects and chromosomal rearrangements. In this review, we highlight the latest progress in the development of in vivo liver-targeted genome editing approaches for the treatment of genetic disorders. We describe the technological advancements that are currently under investigation, the challenges to overcome for clinical applicability, and the future perspectives of this technology.

Formation of an invasion-permissive matrix requires TGFß/SNAIL1-regulated alternative splicing of fibronectin.

BACKGROUND: As in most solid cancers, the emergence of cells with oncogenic mutations in the mammary epithelium alters the tissue homeostasis. Some soluble factors, such as TGFß, potently modify the behavior of healthy stromal cells. A subpopulation of cancer-associated fibroblasts expressing a TGFß target, the SNAIL1 transcription factor, display myofibroblastic abilities that rearrange the stromal architecture. Breast tumors with the presence of SNAIL1 in the stromal compartment, and with aligned extracellular fiber, are associated with poor survival prognoses. METHODS: We used deep RNA sequencing and biochemical techniques to study alternative splicing and human tumor databases to test for associations (correlation t-test) between SNAIL1 and fibronectin isoforms. Three-dimensional extracellular matrices generated from fibroblasts were used to study the mechanical properties and actions of the extracellular matrices on tumor cell and fibroblast behaviors. A metastatic mouse model of breast cancer was used to test the action of fibronectin isoforms on lung metastasis. RESULTS: In silico studies showed that SNAIL1 correlates with the expression of the extra domain A (EDA)-containing (EDA+) fibronectin in advanced human breast cancer and other types of epithelial cancers. In TGFß-activated fibroblasts, alternative splicing of fibronectin as well as of 500 other genes was modified by eliminating SNAIL1. Biochemical analyses demonstrated that SNAIL1 favors the inclusion of the EDA exon by modulating the activity of the SRSF1 splicing factor. Similar to Snai1 knockout fibroblasts, EDA- fibronectin fibroblasts produce an extracellular matrix  that does not sustain TGFß-induced fiber organization, rigidity, fibroblast activation, or tumor cell invasion. The presence of EDA+ fibronectin changes the action of metalloproteinases on fibronectin fibers. Critically, in an mouse orthotopic breast cancer model, the absence of the fibronectin EDA domain completely prevents lung metastasis. CONCLUSIONS: Our results support the requirement of EDA+ fibronectin in the generation of a metastasis permissive stromal architecture in breast cancers and its molecular control by SNAIL1. From a pharmacological point of view, specifically blocking EDA+ fibronectin deposition could be included in studies to reduce the formation of a pro-metastatic environment.

Lethality rescue and long-term amelioration of a citrullinemia type I mouse model by neonatal gene-targeting combined to SaCRISPR-Cas9.

Citrullinemia type I is a rare autosomal-recessive disorder caused by deficiency of argininosuccinate synthetase (ASS1). The clinical presentation includes the acute neonatal form, characterized by ammonia and citrulline accumulation in blood, which may lead to encephalopathy, coma, and death, and the milder late-onset form. Current treatments are unsatisfactory, and the only curative treatment is liver transplantation. We permanently modified the hepatocyte genome in lethal citrullinemia mice (Ass1fold/fold) by inserting the ASS1 cDNA into the albumin locus through the delivery of two AAV8 vectors carrying the donor DNA and the CRISPR-Cas9 platform. The neonatal treatment completely rescued mortality ensuring survival up to 5 months of age, with plasma citrulline levels significantly decreased, while plasma ammonia levels remained unchanged. In contrast, neonatal treatment with a liver-directed non-integrative AAV8-AAT-hASS1 vector failed to improve disease parameters. To model late-onset citrullinemia, we dosed postnatal day (P) 30 juvenile animals using the integrative approach, resulting in lifespan improvement and a minor reduction in disease markers. Conversely, treatment with the non-integrative vector completely rescued mortality, reducing plasma ammonia and citrulline to wild-type values. In summary, the integrative approach in neonates is effective, although further improvements are required to fully correct the phenotype. Non-integrative gene therapy application to juvenile mice ensures a stable and very efficient therapeutic effect.

Gene Therapy in Patients with the Crigler-Najjar Syndrome.

BACKGROUND: Patients with the Crigler-Najjar syndrome lack the enzyme uridine diphosphoglucuronate glucuronosyltransferase 1A1 (UGT1A1), the absence of which leads to severe unconjugated hyperbilirubinemia that can cause irreversible neurologic injury and death. Prolonged, daily phototherapy partially controls the jaundice, but the only definitive cure is liver transplantation. METHODS: We report the results of the dose-escalation portion of a phase 1-2 study evaluating the safety and efficacy of a single intravenous infusion of an adeno-associated virus serotype 8 vector encoding UGT1A1 in patients with the Crigler-Najjar syndrome that was being treated with phototherapy. Five patients received a single infusion of the gene construct (GNT0003): two received 2×1012 vector genomes (vg) per kilogram of body weight, and three received 5×1012 vg per kilogram. The primary end points were measures of safety and efficacy; efficacy was defined as a serum bilirubin level of 300 µmol per liter or lower measured at 17 weeks, 1 week after discontinuation of phototherapy. RESULTS: No serious adverse events were reported. The most common adverse events were headache and alterations in liver-enzyme levels. Alanine aminotransferase increased to levels above the upper limit of the normal range in four patients, a finding potentially related to an immune response against the infused vector; these patients were treated with a course of glucocorticoids. By week 16, serum bilirubin levels in patients who received the lower dose of GNT0003 exceeded 300 µmol per liter. The patients who received the higher dose had bilirubin levels below 300 µmol per liter in the absence of phototherapy at the end of follow-up (mean [±SD] baseline bilirubin level, 351±56 µmol per liter; mean level at the final follow-up visit [week 78 in two patients and week 80 in the other], 149±33 µmol per liter). CONCLUSIONS: No serious adverse events were reported in patients treated with the gene-therapy vector GNT0003 in this small study. Patients who received the higher dose had a decrease in bilirubin levels and were not receiving phototherapy at least 78 weeks after vector administration. (Funded by Genethon and others; ClinicalTrials.gov number, NCT03466463.).

Activation of Alternative Bilirubin Clearance Pathways Partially Reduces Hyperbilirubinemia in a Mouse Model Lacking Functional Ugt1a1 Activity.

Bilirubin is a heme catabolite and Ugt1a1 is the only enzyme involved in the biological elimination of bilirubin. Partially functional or non-functional Ugt1a1 may result in neuronal damage and death due to the accumulation of unconjugated bilirubin in the brain. The understanding of the role of alternative bilirubin detoxification mechanisms that can reduce bilirubin toxicity risk is crucial for developing novel therapeutic strategies. To provide a proof-of-principle showing whether activation of alternative detoxification pathways could lead to life-compatible bilirubin levels in the absence of Ugt1a1 activity, we used Ugt1-/- hyperbilirubinemic mice devoid of bilirubin glucuronidation activity. We treated adult Ugt1-/- mice with TCPOBOP, a strong agonist of the constitutive androstane receptor (CAR). TCPOBOP treatment decreased plasma and liver tissue bilirubin levels by about 38%, and resulted in the transcriptional activation of a vast array of genes involved in bilirubin transport and metabolism. However, brain bilirubin level was unaltered. We observed ~40% degradation of bilirubin in the liver microsomes from TCPOBOP treated Ugt1-/- mice. Our findings suggest that, in the absence of Ugt1a1, the activation of alternative bilirubin clearance pathways can partially improve hyperbilirubinemic conditions. This therapeutic approach may only be considered in a combinatorial manner along with other treatments.

Fludarabine increases nuclease-free AAV- and CRISPR/Cas9-mediated homologous recombination in mice.

Homologous recombination (HR)-based gene therapy using adeno-associated viruses (AAV-HR) without nucleases has several advantages over classic gene therapy, especially the potential for permanent transgene expression. However, the low efficiency of AAV-HR remains a major limitation. Here, we tested a series of small-molecule compounds and found that ribonucleotide reductase (RNR) inhibitors substantially enhance AAV-HR efficiency in mouse and human liver cell lines approximately threefold. Short-term administration of the RNR inhibitor fludarabine increased the in vivo efficiency of both non-nuclease- and CRISPR/Cas9-mediated AAV-HR two- to sevenfold in the murine liver, without causing overt toxicity. Fludarabine administration induced transient DNA damage signaling in both proliferating and quiescent hepatocytes. Notably, the majority of AAV-HR events occurred in non-proliferating hepatocytes in both fludarabine-treated and control mice, suggesting that the induction of transient DNA repair signaling in non-dividing hepatocytes was responsible for enhancing AAV-HR efficiency in mice. These results suggest that use of a clinically approved RNR inhibitor can potentiate AAV-HR-based genome-editing therapeutics.

Promoterless Gene Targeting Approach Combined to CRISPR/Cas9 Efficiently Corrects Hemophilia B Phenotype in Neonatal Mice.

Many inborn errors of metabolism require life-long treatments and, in severe conditions involving the liver, organ transplantation remains the only curative treatment. Non-integrative AAV-mediated gene therapy has shown efficacy in adult patients. However, treatment in pediatric or juvenile settings, or in conditions associated with hepatocyte proliferation, may result in rapid loss of episomal viral DNA and thus therapeutic efficacy. Re-administration of the therapeutic vector later in time may not be possible due to the presence of anti-AAV neutralizing antibodies. We have previously shown the permanent rescue of the neonatal lethality of a Crigler-Najjar mouse model by applying an integrative gene-therapy based approach. Here, we targeted the human coagulation factor IX (hFIX) cDNA into a hemophilia B mouse model. Two AAV8 vectors were used: a promoterless vector with two arms of homology for the albumin locus, and a vector carrying the CRISPR/SaCas9 and the sgRNA. Treatment of neonatal P2 wild-type mice resulted in supraphysiological levels of hFIX being stable 10 months after dosing. A single injection of the AAV vectors into neonatal FIX KO mice also resulted in the stable expression of above-normal levels of hFIX, reaching up to 150% of the human levels. Mice subjected to tail clip analysis showed a clotting capacity comparable to wild-type animals, thus demonstrating the rescue of the disease phenotype. Immunohistological analysis revealed clusters of hFIX-positive hepatocytes. When we tested the approach in adult FIX KO mice, we detected hFIX in plasma by ELISA and in the liver by western blot. However, the hFIX levels were not sufficient to significantly ameliorate the bleeding phenotype upon tail clip assay. Experiments conducted using a AAV donor vectors containing the eGFP or the hFIX cDNAs showed a higher recombination rate in P2 mice compared to adult animals. With this study, we demonstrate an alternative gene targeting strategy exploiting the use of the CRISPR/SaCas9 platform that can be potentially applied in the treatment of pediatric patients suffering from hemophilia, also supporting its application to other liver monogenic diseases. For the treatment of adult patients, further studies for the improvement of targeting efficiency are still required.

Long-Term Effects of Biliverdin Reductase a Deficiency in Ugt1-/- Mice: Impact on Redox Status and Metabolism.

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.

Low efficacy of recombinant SV40 in Ugt1a1-/- mice with severe inherited hyperbilirubinemia.

In contrast to AAV, Simian Virus 40 (rSV40) not inducing neutralizing antibodies (NAbs) allowing re-treatment seems a promising vector for neonatal treatment of inherited liver disorders. Several studies have reported efficacy of rSV40 in animal models for inherited liver diseases. In all studies the ubiquitous endogenous early promoter controlled transgene expression establishing expression in all transduced tissues. Restricting this expression to the target tissues reduces the risk of immune response to the therapeutic gene. In this study a liver specific rSV40 vector was generated by inserting a hepatocyte specific promoter. This increased the specificity of the expression of hUGT1A1 in vitro. However, in vivo the efficacy of rSV40 appeared too low to demonstrate tissue specificity while increasing the vector dose was not possible because of toxicity. In contrast to earlier studies, neutralizing antibodies were induced. Overall, the lack of a platform to produce high titered and pure rSV40 particles and the induction of NAbs, renders it a poor candidate for in vivo gene therapy.

Efficacy of AAV8-hUGT1A1 with Rapamycin in neonatal, suckling, and juvenile rats to model treatment in pediatric CNs patients.

A clinical trial using adeno-associated virus serotype 8 (AAV8)-human uridine diphosphate glucuronosyltransferase 1A1 (hUGT1A1) to treat inherited severe unconjugated hyperbilirubinemia (Crigler-Najjar syndrome) is ongoing, but preclinical data suggest that long-term efficacy in children is impaired due to loss of transgene expression upon hepatocyte proliferation in a growing liver. This study aims to determine at what age long-term efficacy can be obtained in the relevant animal model and whether immune modulation allows re-treatment using the same AAV vector. Neonatal, suckling, and juvenile Ugt1a1-deficient rats received a clinically relevant dose of AAV8-hUGT1A1, and serum bilirubin levels and anti-AAV8 neutralizing antibodies (NAbs) in serum were monitored. The possibility of preventing the immune response toward the vector was investigated using a rapamycin-based regimen with daily intraperitoneal (i.p.) injections starting 2 days before and ending 21 days after vector administration. In rats treated at postnatal day 1 (P1) or P14, the correction was (partially) lost after 12 weeks, whereas the correction was stable in rats injected at P28. Combining initial vector administration with the immune-suppressive regimen prevented induction of NAbs in female rats, allowing at least partially effective re-administration. Induction of NAbs upon re-injection could not be prevented, suggesting that this strategy will be ineffective in patients with low levels of preexisting anti-AAV NAbs.

Long-term correction of ornithine transcarbamylase deficiency in Spf-Ash mice with a translationally optimized AAV vector.

Ornithine transcarbamylase deficiency (OTCD) is an X-linked liver disorder caused by partial or total loss of OTC enzyme activity. It is characterized by elevated plasma ammonia, leading to neurological impairments, coma, and death in the most severe cases. OTCD is managed by combining dietary restrictions, essential amino acids, and ammonia scavengers. However, to date, liver transplantation provides the best therapeutic outcome. AAV-mediated gene-replacement therapy represents a promising curative strategy. Here, we generated an AAV2/8 vector expressing a codon-optimized human OTC cDNA by the α1-AAT liver-specific promoter. Unlike standard codon-optimization approaches, we performed multiple codon-optimization rounds via common algorithms and ortholog sequence analysis that significantly improved mRNA translatability and therapeutic efficacy. AAV8-hOTC-CO (codon optimized) vector injection into adult OTCSpf-Ash mice (5.0E11 vg/kg) mediated long-term complete correction of the phenotype. Adeno-Associated viral (AAV) vector treatment restored the physiological ammonia detoxification liver function, as indicated by urinary orotic acid normalization and by conferring full protection against an ammonia challenge. Removal of liver-specific transcription factor binding sites from the AAV backbone did not affect gene expression levels, with a potential improvement in safety. These results demonstrate that AAV8-hOTC-CO gene transfer is safe and results in sustained correction of OTCD in mice, supporting the translation of this approach to the clinic.

Beclin-1-mediated activation of autophagy improves proximal and distal urea cycle disorders.

Urea cycle disorders (UCD) are inherited defects in clearance of waste nitrogen with high morbidity and mortality. Novel and more effective therapies for UCD are needed. Studies in mice with constitutive activation of autophagy unravelled Beclin-1 as druggable candidate for therapy of hyperammonemia. Next, we investigated efficacy of cell-penetrating autophagy-inducing Tat-Beclin-1 (TB-1) peptide for therapy of the two most common UCD, namely ornithine transcarbamylase (OTC) and argininosuccinate lyase (ASL) deficiencies. TB-1 reduced urinary orotic acid and improved survival under protein-rich diet in spf-ash mice, a model of OTC deficiency (proximal UCD). In AslNeo/Neo mice, a model of ASL deficiency (distal UCD), TB-1 increased ureagenesis, reduced argininosuccinate, and improved survival. Moreover, it alleviated hepatocellular injury and decreased both cytoplasmic and nuclear glycogen accumulation in AslNeo/Neo mice. In conclusion, Beclin-1-dependent activation of autophagy improved biochemical and clinical phenotypes of proximal and distal defects of the urea cycle.

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.

Experimental models assessing bilirubin neurotoxicity.

The molecular and cellular events leading to bilirubin-induced neurotoxicity, the mechanisms regulating liver and intestine expression in neonates, and alternative pathways of bilirubin catabolism remain incompletely defined. To answer these questions, researchers have developed a number of model systems to closely recapitulate the main characteristics of the disease, ranging from tissue cultures to engineered mouse models. In the present review we describe in vitro, ex vivo, and in vivo models developed to study bilirubin metabolism and neurotoxicity, with a special focus on the use of engineered animal models. In addition, we discussed the most recent studies related to potential therapeutic approaches to treat neonatal hyperbilirubinemia, ranging from anti-inflammatory drugs, activation of nuclear receptor pathways, blockade of bilirubin catabolism, and stimulation of alternative bilirubin-disposal pathways.

Coupling AAV-mediated promoterless gene targeting to SaCas9 nuclease to efficiently correct liver metabolic diseases.

Non-integrative AAV-mediated gene therapy in the liver is effective in adult patients, but faces limitations in pediatric settings due to episomal DNA loss during hepatocyte proliferation. Gene targeting is a promising approach by permanently modifying 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 recombination rate and therapeutic efficacy, here we used CRISPR/SaCas9. Neonatal mice were transduced with two AAVs: one expressing the SaCas9 and sgRNA, and one containing a promoterless cDNA flanked by albumin homology regions. Targeting efficiency increased ~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 ~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.

Preclinical Development of an AAV8-hUGT1A1 Vector for the Treatment of Crigler-Najjar Syndrome.

Adeno-associated viruses (AAVs) are among the most efficient vectors for liver gene therapy. Results obtained in the first hemophilia clinical trials demonstrated the long-term efficacy of this approach in humans, showing efficient targeting of hepatocytes with both self-complementary (sc) and single-stranded (ss) AAV vectors. However, to support clinical development of AAV-based gene therapies, efficient and scalable production processes are needed. In an effort to translate to the clinic an approach of AAV-mediated liver gene transfer to treat Crigler-Najjar (CN) syndrome, we developed an (ss)AAV8 vector carrying the human UDP-glucuronosyltransferase family 1-member A1 (hUGT1A1) transgene under the control of a liver-specific promoter. We compared our construct with similar (sc)AAV8 vectors expressing hUGT1A1, showing comparable potency in vitro and in vivo. Conversely, (ss)AAV8-hUGT1A1 vectors showed superior yields and product homogeneity compared with their (sc) counterpart. We then focused our efforts in the scale-up of a manufacturing process of the clinical product (ss)AAV8-hUGT1A1 based on the triple transfection of HEK293 cells grown in suspension. Large-scale production of this vector had characteristics identical to those of small-scale vectors produced in adherent cells. Preclinical studies in animal models of the disease and a good laboratory practice (GLP) toxicology-biodistribution study were also conducted using large-scale preparations of vectors. These studies demonstrated long-term safety and efficacy of gene transfer with (ss)AAV8-hUGT1A1 in relevant animal models of the disease, thus supporting the clinical translation of this gene therapy approach for the treatment of CN syndrome.

EDA fibronectin-TLR4 axis sustains megakaryocyte expansion and inflammation in bone marrow fibrosis.

The fibronectin EDA isoform (EDA FN) is instrumental in fibrogenesis but, to date, its expression and function in bone marrow (BM) fibrosis have not been explored. We found that mice constitutively expressing the EDA domain (EIIIA+/+), but not EDA knockout mice, are more prone to develop BM fibrosis upon treatment with the thrombopoietin (TPO) mimetic romiplostim (TPOhigh). Mechanistically, EDA FN binds to TLR4 and sustains progenitor cell proliferation and megakaryopoiesis in a TPO-independent fashion, inducing LPS-like responses, such as NF-κB activation and release of profibrotic IL-6. Pharmacological inhibition of TLR4 or TLR4 deletion in TPOhigh mice abrogated Mk hyperplasia, BM fibrosis, IL-6 release, extramedullary hematopoiesis, and splenomegaly. Finally, developing a novel ELISA assay, we analyzed samples from patients affected by primary myelofibrosis (PMF), a well-known pathological situation caused by altered TPO signaling, and found that the EDA FN is increased in plasma and BM biopsies of PMF patients as compared with healthy controls, correlating with fibrotic phase.

Attenuation of neuro-inflammation improves survival and neurodegeneration in a mouse model of severe neonatal hyperbilirubinemia.

All pre-term newborns and a high proportion of term newborns develop neonatal jaundice. Neonatal jaundice is usually a benign condition and self-resolves within few days after birth. However, a combination of unfavorable complications may lead to acute hyperbilirubinemia. Excessive hyperbilirubinemia may be toxic for the developing nervous system leading to severe neurological damage and death by kernicterus. Survivors show irreversible neurological deficits such as motor, sensitive and cognitive abnormalities. Current therapies rely on the use of phototherapy and, in unresponsive cases, exchange transfusion, which is performed only in specialized centers. During bilirubin-induced neurotoxicity different molecular pathways are activated, ranging from oxidative stress to endoplasmic reticulum (ER) stress response and inflammation, but the contribution of each pathway in the development of the disease still requires further investigation. Thus, to increase our understanding of the pathophysiology of bilirubin neurotoxicity, encephalopathy and kernicterus, we pharmacologically modulated neurodegeneration and neuroinflammation in a lethal mouse model of neonatal hyperbilirubinemia. Treatment of mutant mice with minocycline, a second-generation tetracycline with anti-inflammatory and neuroprotective properties, resulted in a dose-dependent rescue of lethality, due to reduction of neurodegeneration and neuroinflammation, without affecting plasma bilirubin levels. In particular, rescued mice showed normal motor-coordination capabilities and behavior, as determined by the accelerating rotarod and open field tests, respectively. From the molecular point of view, rescued mice showed a dose-dependent reduction in apoptosis of cerebellar neurons and improvement of dendritic arborization of Purkinje cells. Moreover, we observed a decrease of bilirubin-induced M1 microglia activation at the sites of damage with a reduction in oxidative and ER stress markers in these cells. Collectively, these data indicate that neurodegeneration and neuro-inflammation are key factors of bilirubin-induced neonatal lethality and neuro-behavioral abnormalities. We propose that the application of pharmacological treatments having anti-inflammatory and neuroprotective effects, to be used in combination with the current treatments, may significantly improve the management of acute neonatal hyperbilirubinemia, protecting from bilirubin-induced neurological damage and death.

Bilirubin-Induced Oxidative Stress Leads to DNA Damage in the Cerebellum of Hyperbilirubinemic Neonatal Mice and Activates DNA Double-Strand Break Repair Pathways in Human Cells.

Unconjugated bilirubin is considered a potent antioxidant when present at moderate levels. However, at high concentrations, it produces severe neurological damage and death associated with kernicterus due to oxidative stress and other mechanisms. While it is widely recognized that oxidative stress by different toxic insults results in severe damage to cellular macromolecules, especially to DNA, no data are available either on DNA damage in the brain triggered by hyperbilirubinemia during the neonatal period or on the activation of DNA repair mechanisms. Here, using a mouse model of neonatal hyperbilirubinemia, we demonstrated that DNA damage occurs in vivo in the cerebellum, the brain region most affected by bilirubin toxicity. We studied the mechanisms associated with potential toxic action of bilirubin on DNA in in vitro models, which showed significant increases in DNA damage when neuronal and nonneuronal cells were treated with 140 nM of free bilirubin (Bf), as determined by γH2AX Western blot and immunofluorescence analyses. Cotreatment of cells with N-acetyl-cysteine, a potent oxidative-stress inhibitor, prevented DNA damage by bilirubin, supporting the concept that DNA damage was caused by bilirubin-induced oxidative stress. Bilirubin treatment also activated the main DNA repair pathways through homologous recombination (HR) and nonhomologous end joining (NHEJ), which may be adaptive responses to repair bilirubin-induced DNA damage. Since DNA damage may be another important factor contributing to neuronal death and bilirubin encephalopathy, these results contribute to the understanding of the mechanisms associated with bilirubin toxicity and may be of relevance in neonates affected with severe hyperbilirubinemia.