Single-dose gene-replacement therapy for spinal muscular atrophy

Parts used

Key findings

• Spinal muscular atrophy type 1 (SMA1) results from biallelic loss-of-function mutations in the SMN1 gene, which encodes the survival motor neuron protein. The paralogous SMN2 gene preferentially excludes exon 7 during splicing and produces only a small fraction of functional SMN protein compared with SMN1; infants with biallelic SMN1 exon 7 deletions who carry exactly two copies of SMN2 have a 97% risk of developing SMA1, the most severe SMA subtype and the most common genetic cause of death among infants.
• In the SMNΔ7 mouse model of spinal muscular atrophy, a single intravenous dose of self-complementary AAV9 (scAAV9) carrying SMN complementary DNA extends average survival from 15 days in untreated animals to 28.5 days at a low dose (6.7×10¹³ vg per kilogram) and to more than 250 days at higher doses (2.0×10¹⁴ or 3.3×10¹⁴ vg per kilogram), establishing the dose range later tested in SMA1 infants.
• Fifteen infants with genetically confirmed SMA1, homozygous SMN1 exon 7 deletions, and two copies of SMN2 received a single intravenous infusion of self-complementary AAV9 carrying SMN complementary DNA under a hybrid cytomegalovirus enhancer–chicken beta-actin promoter (scAAV9.CB.hSMN, AVXS-101) — three at a low dose (6.7×10¹³ vg per kilogram) and twelve at a high dose (2.0×10¹⁴ vg per kilogram). As of a data cutoff of August 7, 2017, all 15 treated infants were alive and free of permanent ventilatory support at 20 months of age, compared with only 8% event-free survival at the same age in a historical cohort of untreated SMA1 infants with two SMN2 copies.
• In the high-dose cohort (2.0×10¹⁴ vg per kilogram; 12 patients), CHOP INTEND motor-function scores (0–64 scale, higher indicating better function) rose by a mean of 9.8 points at 1 month and 15.4 points at 3 months after infusion (P<0.001 for both comparisons), and 11 of the 12 patients attained and sustained scores above 40 points — a threshold essentially never reached by untreated SMA1 infants. A separate historical cohort of SMA1 infants with two SMN2 copies showed a mean decline of more than 10 points on the same scale between 6 and 12 months of age.
• Among the 12 high-dose recipients, 11 sat unassisted for at least 5 seconds, 9 rolled over, 11 achieved head control, 11 attained the ability to speak, and 2 crawled, pulled to stand, stood independently, and walked independently — milestones not observed in historical natural-history cohorts of untreated SMA1 infants. None of the three low-dose recipients (6.7×10¹³ vg per kilogram) attained any motor milestone, even though their CHOP INTEND scores rose by a mean of 7.7 points from a baseline mean of 16.3 points by the data cutoff, indicating that milestone attainment tracked specifically with the higher of the two tested doses.
• At the last follow-up visit, 11 of the 12 high-dose patients had achieved or retained the ability to swallow independently and 4 fed orally without nutritional support. Among high-dose patients who required no ventilatory or nutritional support at enrollment, 7 of 10 remained free of ventilatory support and 6 of 7 remained free of nutritional support at last follow-up.
• Serious adverse events after single-dose intravenous scAAV9.CB.hSMN infusion occurred in 13 of 15 treated infants (56 events total), but only 2 were adjudicated as treatment-related grade 4 events, both consisting of elevated serum aminotransferase levels without other liver-function abnormalities. Elevated aminotransferases occurred in 4 patients overall and were attenuated by oral prednisolone (1 mg per kilogram per day for approximately 30 days, starting 24 hours before vector infusion) — a regimen adopted for patients 2 through 15 after the first-treated patient developed marked transaminase elevations (alanine aminotransferase 31 times and aspartate aminotransferase 14 times the upper limit of normal) prior to its introduction.
• Pre-existing anti-AAV9 antibodies exclude infants from receiving this AAV9-based gene therapy: of 16 SMA1 infants screened for the trial, 1 was excluded because of a persistently elevated anti-AAV9 antibody titer (>1:50), consistent with population data showing low anti-AAV9 seropositivity among children and young adults and increasing seropositivity after age 40.

Abstract

BACKGROUND: Spinal muscular atrophy type 1 (SMA1) is a progressive, monogenic motor neuron disease with an onset during infancy that results in failure to achieve motor milestones and in death or the need for mechanical ventilation by 2 years of age. We studied functional replacement of the mutated gene encoding survival motor neuron 1 (SMN1) in this disease. METHODS: vg per kilogram). The primary outcome was safety. The secondary outcome was the time until death or the need for permanent ventilatory assistance. In exploratory analyses, we compared scores on the CHOP INTEND (Children's Hospital of Philadelphia Infant Test of Neuromuscular Disorders) scale of motor function (ranging from 0 to 64, with higher scores indicating better function) in the two cohorts and motor milestones in the high-dose cohort with scores in studies of the natural history of the disease (historical cohorts). RESULTS: As of the data cutoff on August 7, 2017, all 15 patients were alive and event-free at 20 months of age, as compared with a rate of survival of 8% in a historical cohort. In the high-dose cohort, a rapid increase from baseline in the score on the CHOP INTEND scale followed gene delivery, with an increase of 9.8 points at 1 month and 15.4 points at 3 months, as compared with a decline in this score in a historical cohort. Of the 12 patients who had received the high dose, 11 sat unassisted, 9 rolled over, 11 fed orally and could speak, and 2 walked independently. Elevated serum aminotransferase levels occurred in 4 patients and were attenuated by prednisolone. CONCLUSIONS: In patients with SMA1, a single intravenous infusion of adeno-associated viral vector containing DNA coding for SMN resulted in longer survival, superior achievement of motor milestones, and better motor function than in historical cohorts. Further studies are necessary to confirm the safety and efficacy of this gene therapy. (Funded by AveXis and others; ClinicalTrials.gov number, NCT02122952 .).