Gene therapy for urea cycle defects: An update from historical perspectives to future prospects

Abstract

Urea cycle defects (UCDs) are severe inherited metabolic diseases with high unmet needs which present a permanent risk of hyperammonaemic decompensation and subsequent acute death or neurological sequelae, when treated with conventional dietetic and medical therapies. Liver transplantation is currently the only curative option, but has the potential to be supplanted by highly effective gene therapy interventions without the attendant need for life-long immunosuppression or limitations imposed by donor liver supply. Over the last three decades, pioneering genetic technologies have been explored to circumvent the consequences of UCDs, improve quality of life and long-term outcomes: adenoviral vectors, adeno-associated viral vectors, gene editing, genome integration and non-viral technology with messenger RNA. In this review, we present a summarised view of this historical path, which includes some seminal milestones of the gene therapy's epic. We provide an update about the state of the art of gene therapy technologies for UCDs and the current advantages and pitfalls driving future directions for research and development.

Keywords: ammonia; argininaemia; argininosuccinic aciduria; citrullinaemia; ornithine transcarbamylase; urea cycle; urea cycle defect.

FIGURE 1

Preclinical and clinical development of gene therapy for urea cycle defects overtime. ARG1, arginase 1; ASL, argininosuccinate lyase; ASS, argininosuccinate synthase; CPSI, carbamoyl‐phosphate synthase I; OTC, ornithine transcarbamylase.

FIGURE 2

Transduction pathways of lentiviral, adeno‐associated viral vectors and messenger RNA (mRNA) lipid nanoparticles: cellular uptake and in‐cell processing. AAV, adeno‐associated virus.

FIGURE 3

Non‐targeted and targeted integrations. Non‐targeted integration is mediated by integrative viral vector systems like lentiviral vectors or nucleases such as piggybac transposase. Targeted integration can use homologous recombination only but usually combines the action of a nuclease and homologous recombination, a strategy which shows a higher insertional rate. Homology‐independent targeted integration (HITI) does not use homology arms but a Cas9 nuclease, which cuts the genomic target sequence and the donor plasmid DNA, thereby creating complementary blunt ends between both target and donor sequences. The donor DNA plasmid is used for repair by the non‐homologous end‐joining pathway and integrates at the genome double‐strand break site.