By Eric Mallack, MD
Assistant Professor of Pediatrics and Neurology, Division of Child Neurology, Weill Cornell Medical College; Director, Leukodystrophy Center at Weill Cornell Medicine
SYNOPSIS: A combined adenoviral-mediated gene therapy plus substrate therapy delivered to a mouse model of thymidine kinase 2 (TK2) deficiency, manifested most often as a fatal mitochondrial myopathy in infants and children, rescued TK2 activity and prolonged animal lifespan, thus indicating a promising therapeutic approach for affected patients.
SOURCE: Lopez-Gomez C, Sanchez-Quintero MJ, Lee EJ, et al. Synergistic deoxynucleoside and gene therapies for thymidine kinase 2 deficiency. Ann Neurol 2021;90:640-652.
Thymidine kinase 2 (TK2) phosphorylates mitochondrial pyrimidines are necessary for the replication and maintenance of mitochondrial deoxyribonucleic acid (DNA). Autosomal recessive mutations in the TK2 lead to a progressive, and ultimately fatal, mitochondrial myopathy in infants and children. Treatment with TK2 substrates deoxycytidine and deoxythymidine (dC + dT therapy) has resulted in slowed disease progression and prolonged survival in affected patients; however, it has failed to halt the disease. Given the advancements in gene delivery using adenoviral-associated viral vectors, the authors employed a single adeno-associated virus 9 (AAV9)-mediated gene transfer approach, a serial gene therapy delivery approach using a lower dose AAV9 then AAV2, and a third cohort treated with lower dose AAV9 + AAV2 + substrate therapy (dC + dT) in the treatment of the TK2 mouse model.
Survivorship was superior in the mice who underwent lower dose AAV9 + AAV2 + dC + dT therapy, followed by the combination AAV9 + AAV2, followed by the high dose AAV9. However, survivorship did not approach wild-type (WT) for any of the above treatment paradigms. Early functional assessments indicate that treatment with any of the three paradigms approximated WT. Apart from the kidneys, transfection of the gene was widespread, including in the brain and muscle. End-stage renal disease contributed to early mortality in all TK2-mice, since the vector failed to readily transduce the kidneys in any of the experimental models.
COMMENTARY
The work by Lopez-Gomez et al represents substantial progress in both the treatment of a fatal neurogenetic disorder and in the treatment paradigm to do so; namely, combination gene therapy plus substrate therapy. The effectiveness of substrate therapy alone is restricted by residual enzyme activity (if any) and the effectiveness of alternate compensatory pathways, both of which become the rate limiting steps after the impaired pathway is saturated. In addition, substrates such as dC and dT are readily degraded and lack tissue specificity.
The combined gene therapy approach in this study is novel and reflective of overall progress in the field. One can reasonably expect to see an increase of combined next-generation therapeutic approaches to treat devastating neurological disorders. Combined viral-mediated gene and targeted genetic therapy (e.g., antisense oligonucleotide therapy) presently are being explored to address residual disease and unmet clinical need in the setting of patients with spinal muscular atrophy who previously have received gene therapy or a gene-splicing modifier (RESPOND Study NCT04488133, ASCEND Study NCT05067790).
Despite this innovative therapeutic approach, the challenges highlighted in this work — the most major of which is tissue specificity — are important. Systemic administration of viral-mediated gene therapy requires optimization to minimize off-target effects (e.g., liver toxicity1), maximize transduction of target tissue (muscle in TK2 deficiency), and to target transduction of tissue necessary to maintain survival (in this work, the kidney). Limitations imposed by tissue specificity currently are being addressed, with promising preclinical results.2
Overall, the results from multiple gene therapy trials designed to treat devastating neurological disorders of childhood are promising.1,3,4 Implementation of these therapies has led to novel, post-treatment residual disease natural history and a greater understanding of adverse events. The study of combined gene-based therapeutic approaches to address all of the above is the next step, and in some cases, already has begun.
REFERENCES
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- Tabebordbar M, Lagerborg KA, Stanton A, et al. Directed evolution of a family of AAV capsid variants enabling potent muscle-directed gene delivery across species. Cell 2021;184:4919-4938.e22.
- Eichler F, Duncan C, Musolino PL, et al. Hematopoietic stem-cell gene therapy for cerebral adrenoleukodystrophy. N Engl J Med 2017;377:1630-1638.
- Sessa M, Lorioli L, Fumagalli F, et al. Lentiviral haemopoietic stem-cell gene therapy in early-onset metachromatic leukodystrophy: An ad-hoc analysis of a non-randomised, open-label, phase 1/2 trial. Lancet 2016;388:476-487.
