Improving single injection CSF delivery of AAV9-mediated gene therapy for SMA: a dose-response study in mice and nonhuman primates

Abstract

Spinal muscular atrophy (SMA) is the most frequent lethal genetic neurodegenerative disorder in infants. The disease is caused by low abundance of the survival of motor neuron (SMN) protein leading to motor neuron degeneration and progressive paralysis. We previously demonstrated that a single intravenous injection (IV) of self-complementary adeno-associated virus-9 carrying the human SMN cDNA (scAAV9-SMN) resulted in widespread transgene expression in spinal cord motor neurons in SMA mice as well as nonhuman primates and complete rescue of the disease phenotype in mice. Here, we evaluated the dosing and efficacy of scAAV9-SMN delivered directly to the cerebral spinal fluid (CSF) via single injection. We found widespread transgene expression throughout the spinal cord in mice and nonhuman primates when using a 10 times lower dose compared to the IV application. Interestingly, in nonhuman primates, lower doses than in mice can be used for similar motor neuron targeting efficiency. Moreover, the transduction efficacy is further improved when subjects are kept in the Trendelenburg position to facilitate spreading of the vector. We present a detailed analysis of transduction levels throughout the brain, brainstem, and spinal cord of nonhuman primates, providing new guidance for translation toward therapy for a wide range of neurodegenerative disorders.

Figure 1

Single intracerebroventricular injections of scAAV9.CBA.SMN improve survival and motor performances of SMNΔ7 mice. (a) Five different doses of scAAV9.CBA.SMN were tested in SMNΔ7 mice. The first and second dose (2.7e12 and 1e13 vg/kg) led to no and very mild increase in survival respectively. With 1.8e13 vg/kg, survival significantly increased from 17.5 days in untreated mice to a median of 165 days (purple line, P value = 0.05). The two higher doses (2.6e13 and 3.3e13 vg/kg) led to a further increase in survival to a median of 274 and 282 days respectively (blue and green line, P values = 0.001 and 0.0001). (b–e) The increase in survival correlated with increase in weight and rescue of motor performances compared to wild type mice. Error bars = SEM; n = 10–15 per group.

Figure 2

Single intracerebroventricular injections of scAAV9.CBA.GFP reveal that targeting 20–40% of the spinal motor neurons is required to achieve significant improvement in survival and motor performances of SMNΔ7 mice. (a) SMNΔ7 mice were injected with scAAV9.CBA.GFP using the same dosing regime as for SMN, immunofluorescence revealed increasing GFP levels with higher doses. (b) ChAT⁺/GFP⁺ cell counts revealed that the first dose leading to significant increase in survival corresponds to targeting of about 20–40% of spinal motor neurons. The highest most effective dose results in the targeting of about 46–72% of motor neurons. (c) GFP and SMN mRNA levels were measured in injected heterozygote littermates via digital droplet polymerase chain reaction. All samples were normalized to cyclophilin. The expression patterns of the two transcripts overlap and correlate with the increase in viral dose and motor neuron counts in all spinal cord segments. Error bars = SD; n = 3 per group.

Figure 3

Single intrathecal sacral injection of 1e13 vg/kg scAAV9.CBA.GFP in nonhuman primates is sufficient to target >50% motor neurons in all spinal cord segments. (a,b) Seven cynomolgus macaques were injected with 1e13 vg/kg scAAV9.CBA.GFP via intrathecal sacral infusion. ChAT/GFP staining and cell counts of double positive cells reveal that keeping the subjects in the Trendelenburg position for 5 (n = 2) or 10 (n = 3) minutes significantly improves motor neuron transduction in both cervical and thoracic spinal cord compared to subjects that received standard procedure (n = 2). This procedure leads to targeting more than 50% motor neurons in all spinal cord segments. (c) GFP mRNA levels correlated with the increase in cell transduction and confirmed the positive effects of adopting the Trendelenburg position to improve virus distribution. Error bars = SD; n = 2–3 per group; *P < 0.05.

Figure 4

Wide cell transduction is achieved in the brain of nonhuman primates. (a,b) Wide cell transduction was achieved in several areas of the brain as shown by DAB staining (a) compared to the negative control (b). (c,d) Among the regions with the highest transduction were hippocampus and motor cortex. (e,f) High-magnification pictures of CA1 neurons in the hippocampus and upper motor neurons in layer V of the motor cortex that are stained with GFP (green) and NeuN (red).

Figure 5

Wide cell transduction is achieved in the brain stem and cerebellum of nonhuman primates that were kept in the Trendelenburg position for 5 or 10 minutes. (a–f) DAB staining showed that several areas of the brainstem were transduced by scAAV9.CBA.GFP compared to the negative control (a–c). In particular, the XII and the V cranial nerve nuclei, the hypoglossal (b–e) and the trigeminal (c–f) nucleus respectively, were highly targeted. (d) Upper motor neuron counts revealed that about 50% motor neurons were transduced in the V layer of the motor cortex, and more than 60 and 75% motor neurons were GFP⁺ in the hypoglossal and trigeminal nucleus respectively; GFP (green) and ChAT (red). (g–i) High transduction levels were also achieved in the cerebellum, in particular the Purkinje cells and the nuclear layer. Error bars = SD; n = 5.

Figure 6

DNA and RNA biodistribution in nonhuman primates 2 weeks postinjection. DNA and RNA were isolated from 1 mg tissue biopsies and tested for the presence of viral genome as well as GFP mRNA molecules. Biodistribution comparison was achieved by interpolating Taqman quantitative polymerase chain reaction (qPCR) values for viral DNA or RNA to the same standard curve obtained with serial dilutions of the same plasmid carrying GFP and its CBA promoter. DNA biodistribution showed the presence of scAAV9.CBA.GFP (expressed as viral genome molecules/µg of total extracted DNA) at high levels in the targeted tissues, brain and spinal cord, as well as muscles and peripheral organs. RNA biodistribution revealed that GFP expression (expressed as GFP mRNA molecules/µg of total extracted RNA) differs significantly among different organs and tissues. The comparison shows that a close correlation exists between DNA and RNA levels in the central nervous system and in muscles; however, RNA levels are significantly lower in internal organs, apart from liver and adrenal glands.