Subacute liver injury in two young infants following gene replacement therapy for spinal muscular atrophy.

Spinal muscular atrophy is a neurodegenerative disorder resulting from the irreversible loss of anterior horn cells secondary to homozygous mutations in the survival motor neuron gene SMN1. Gene replacement therapy using a recombinant adeno-associated virus 9 vector containing an SMN1 gene construct, onasemnogene abeparvovec-xioi, was approved by the US Food and Drug Administration in May 2019. Subacute mild elevation of liver function tests following infusion has since been shown to be a common adverse event. Additionally, there have been case reports of liver failure following administration of this therapy and two reported patient deaths. While these adverse events are relatively common, they have not been reported in the youngest treated patients. We present two cases of subacute severe elevation of liver function tests >10-20 times the upper limit of normal, without progression to liver failure, following onasemnogene abeparvovec administration in young infants less than 4 weeks old. Potential mechanisms of injury, management, and implications for future treatment with onasemnogene abeparvovec and other adeno-associated virus vector gene therapies are discussed.