The future of gene-targeted therapy for hereditary tyrosinemia type 1 as a lead indication among the inborn errors of metabolism

Progress in Gene Therapy for Hereditary Tyrosinemia Type 1

Hereditary Tyrosinemia Type-1 (HT1), an inherited error of metabolism caused by a mutation in the fumarylacetoacetate hydrolase gene, is associated with liver disease, severe morbidity, and early mortality. The use of NTBC (2-(2-nitro-4-fluoromethylbenzoyl)-1,3-cyclohexanedione) has almost eradicated the acute HT1 symptoms and childhood mortality. However, patient outcomes remain unsatisfactory due to the neurocognitive effects of NTBC and the requirement for a strict low-protein diet. Gene therapy (GT) offers a potential single-dose cure for HT1, and there is now abundant preclinical data showing how a range of vector-nucleotide payload combinations could be used with curative intent, rather than continued reliance on amelioration. Unfortunately, there have been no HT1-directed clinical trials reported, and so it is unclear which promising pre-clinical approach has the greatest chance of successful translation. Here, to fill this knowledge gap, available HT1 preclinical data and available clinical trial data pertaining to liver-directed GT for other diseases are reviewed. The aim is to establish which vector-payload combination has the most potential as a one-dose HT1 cure. Analysis provides a strong case for progressing lentiviral-based approaches into clinical trials. However, other vector-payload combinations may be more scientifically and commercially viable, but these options require additional investigation.

Focusing on Ischemic Reperfusion Injury in the New Era of Dynamic Machine Perfusion in Liver Transplantation

Liver transplantation is the most effective treatment for end-stage liver disease. Transplant indications have been progressively increasing, with a huge discrepancy between the supply and demand of optimal organs. In this context, the use of extended criteria donor grafts has gained importance, even though these grafts are more susceptible to ischemic reperfusion injury (IRI). Hepatic IRI is an inherent and inevitable consequence of all liver transplants; it involves ischemia-mediated cellular damage exacerbated upon reperfusion and its severity directly affects graft function and post-transplant complications. Strategies for organ preservation have been constantly improving since they first emerged. The current gold standard for preservation is perfusion solutions and static cold storage. However, novel approaches that allow extended preservation times, organ evaluation, and their treatment, which could increase the number of viable organs for transplantation, are currently under investigation. This review discusses the mechanisms associated with IRI, describes existing strategies for liver preservation, and emphasizes novel developments and challenges for effective organ preservation and optimization.

mRNA-Based Approaches to Treating Liver Diseases

Diseases that affect the liver account for approximately 2 million deaths worldwide each year. The increasing prevalence of these diseases and the limited efficacy of current treatments are expected to stimulate substantial growth in the global market for therapeutics that target the liver. Currently, liver transplantation is the only curative option available for many liver diseases. Gene therapy represents a valuable approach to treatment. The liver plays a central role in a myriad of essential metabolic functions, making it an attractive organ for gene therapy; hepatocytes comprise the most relevant target. To date, viral vectors constitute the preferred approach to targeting hepatocytes with genes of therapeutic interest. Alternatively, mRNA-based therapy offers a number of comparative advantages. Clinical and preclinical studies undertaken to treat inherited metabolic diseases affecting the liver, cirrhosis and fibrosis, hepatocellular carcinoma, hepatitis B, and cytomegalovirus using lipid nanoparticle-encapsulated mRNAs that encode the therapeutic or antigenic protein of interest are discussed.

Modifying organs with gene therapy and gene modulation in the age of machine perfusion

PURPOSE OF REVIEW

This review aims to highlight current advances in gene therapy methods, describing advances in CRISPR-Cas9 gene editing and RNA interference in relevance to liver transplantation, and machine perfusion.

RECENT FINDINGS

In order to minimize rejection, increase the donor pool of available organs, and minimize the effects of ischemia-reperfusion injury, gene therapy and gene modification strategies are, thus, required in the context of liver transplantation.

SUMMARY

Gene therapy has been used successfully in a diverse array of diseases, and, more recently, this technique has gained interest in the field of organ transplantation. Biological and logistical challenges reduce the rate of successful procedures, increasing the waiting list even more. We explore the exciting future implications of customized gene therapy in livers using machine perfusion, including its potential to create a future in which organs destined for transplant are individualized to maximize both graft and recipient longevity.

mRNA-based therapy proves superior to the standard of care for treating hereditary tyrosinemia 1 in a mouse model

Hereditary tyrosinemia type 1 is an inborn error of amino acid metabolism characterized by deficiency of fumarylacetoacetate hydrolase (FAH). Only limited treatment options (e.g., oral nitisinone) are available. Patients must adhere to a strict diet and face a life-long risk of complications, including liver cancer and progressive neurocognitive decline. There is a tremendous need for innovative therapies that standardize metabolite levels and promise normal development. Here, we describe an mRNA-based therapeutic approach that rescues Fah-deficient mice, a well-established tyrosinemia model. Repeated intravenous or intramuscular administration of lipid nanoparticle-formulated human FAH mRNA resulted in FAH protein synthesis in deficient mouse livers, stabilized body weight, normalized pathologic increases in metabolites after nitisinone withdrawal, and prevented early death. Dose reduction and extended injection intervals proved therapeutically effective. These results provide proof of concept for an mRNA-based therapeutic approach to treating hereditary tyrosinemia type 1 that is superior to the standard of care.

Novel Gene-Correction-Based Therapeutic Modalities for Monogenic Liver Disorders

The majority of monogenic liver diseases are autosomal recessive disorders, with few being sex-related or co-dominant. Although orthotopic liver transplantation (LT) is currently the sole therapeutic option for end-stage patients, such an invasive surgical approach is severely restricted by the lack of donors and post-transplant complications, mainly associated with life-long immunosuppressive regimens. Therefore, the last decade has witnessed efforts for innovative cellular or gene-based therapeutic strategies. Gene therapy is a promising approach for treatment of many hereditary disorders, such as monogenic inborn errors. The liver is an organ characterized by unique features, making it an attractive target for in vivo and ex vivo gene transfer. The current genetic approaches for hereditary liver diseases are mediated by viral or non-viral vectors, with promising results generated by gene-editing tools, such as CRISPR-Cas9 technology. Despite massive progress in experimental gene-correction technologies, limitations in validated approaches for monogenic liver disorders have encouraged researchers to refine promising gene therapy protocols. Herein, we highlighted the most common monogenetic liver disorders, followed by proposed genetic engineering approaches, offered as promising therapeutic modalities.

Use of an adeno-associated virus serotype Anc80 to provide durable cure of phenylketonuria in a mouse model

Phenylketonuria (PKU) is the most common inborn error of metabolism of the liver, and results from mutations of both alleles of the phenylalanine hydroxylase gene (PAH). As such, it is a suitable target for gene therapy via gene delivery with a recombinant adeno-associated virus (AAV) vector. Here we use the synthetic AAV vector Anc80 via systemic administration to deliver a functional copy of a codon-optimized human PAH gene, with or without an intron spacer, to the Pah^enu2 mouse model of PKU. Dose-dependent transduction of the liver and expression of PAH mRNA were present with both vectors, resulting in significant and durable reduction of circulating phenylalanine, reaching near control levels in males. Coat color of treated Pah^enu2 mice reflected an increase in pigmentation from brown to the black color of control animals, further indicating functional restoration of phenylalanine metabolism and its byproduct melanin. There were no adverse effects associated with administration of AAV up to 5 × 10¹² VG/kg, the highest dose tested. Only minor and/or transient variations in some liver enzymes were observed in some of the AAV-dosed animals which were not associated with pathology findings in the liver. Finally, there was no impact on cell turnover or apoptosis as evaluated by Ki-67 and TUNEL staining, further supporting the safety of this approach. This study demonstrates the therapeutic potential of AAV Anc80 to safely and durably cure PKU in a mouse model, supporting development for clinical consideration.

Hereditary tyrosinemia type Ⅰ: newborn screening, diagnosis and treatment

Hereditary tyrosinemia type Ⅰ (HT-1) is a severe autosomal recessive inherited metabolic disease. Due to the deficiency of fumarylacetoacetase hydrolase (FAH), the toxic metabolites are accumulated in the body, resulting in severe liver dysfunction, renal tubular dysfunctions, neurological crises, and the increased risk of hepatocellular carcinoma. Clinical symptoms typically begin at after the birth; the prognosis of patients is poor if they are not treated timely. Succinylacetone is a specific and sensitive marker for HT-1, and the screening in newborns can make early diagnosis of HT-1 at the asymptomatic stage. The diagnosis of HT-1 can be confirmed based on the characteristic biochemical findings and molecular testing of mutations in both alleles of gene. Combined treatment with nitisinone and a low tyrosine diet may significantly improve outcomes for patients. Liver transplantation is an effective treatment in cases where nitisinone is not available. Some novel HT-1 treatments are in clinical trials, including enzyme replacement therapy, hepatocyte transplantation and gene-targeted therapy.

Diagnostic and Therapeutic Challenges of Hereditary Tyrosinemia Type 1 in Lebanon: A 12-Year Retrospective Review

Background: Hereditary tyrosinemia type 1 is a rare genetic disorder leading to liver cirrhosis and hepatocellular carcinoma. Few decades ago, dietary measures and ultimately liver transplant constituted the only treatment modalities. Nowadays, early diagnosis and therapy with nitisinone can reverse the clinical picture. In developing countries, diagnostic and therapeutic challenges may affect the outcome of this disease. The choice of the treatment modality may depend on the economic status of each country. Few reports on the long-term outcome of hereditary tyrosinemia type 1 are available from developing and Arab countries. Methods: A retrospective study of charts of Lebanese patients diagnosed with tyrosinemia type 1 and followed, at the American University of Beirut, during a 12-year period was performed. Clinical presentation and liver biochemical profile at diagnosis were analyzed, along with therapeutic modalities and long-term outcome. Results: Twenty-two children were diagnosed and followed during the study period. Median age at diagnosis was 7 months (range: one day to 35 months). Most of the patients presented with hepatomegaly and jaundice. Four patients were referred for atypical presentations with developmental delay and seizures, secondary to undiagnosed hypoglycemia episodes. Around half of the patients presented with failure to thrive. Transaminitis, cholestasis and increased α-fetoprotein level were variably present at diagnosis (36% to 50%). All patients had elevated plasma tyrosine and urinary succinylacetone levels. Genetic testing was performed in 9%. Only one third could be treated with nitisinone. Liver transplant was electively performed in 9% of cases, to overcome the long-term cost of nitisinone. One third of the patients died between the age of 1 month and 11 years. Surviving patients are still candidates for liver transplant. Conclusion: Our experience reflects the challenges of diagnosis and treatment of hereditary tyrosinemia type 1 in a developing country. In the absence of specific neonatal screening, early diagnosis relies mostly on the clinical awareness of the physician. Long-term nitisinone use may be deterred by its high cost and liver transplantation carries risks of surgical complications. New, effective, and less expensive treatments are needed, especially for developing countries.