Severe offtarget effects following intravenous delivery of AAV9-MECP2 in a female mouse model of Rett syndrome

Rett syndrome (RTT) is a severe X-linked neurodevelopmental disorder that primarily caused by mutations in the methyl CpG binding protein 2 gene (MECP2). RTT second most prevalent genetic cause of intellectual disability girls, and there currently no cure for disease. We have previously shown therapy using self-complementary AAV9 viral vector expressing codon-optimized Mecp2 version (AAV9-MCO) significantly improved symptoms increased survival male Mecp2-deficient mice. Here, we pursued our studies investigated safety efficacy long-term genetically relevant mouse model: heterozygous (HET) deficient female mouse. These mice were injected with AAV9-MCO through tail vein an array behavioral tests was performed. At 16- 30-weeks post-injection, this treatment able to rescue apneas spontaneous locomotor deficits circadian activity HET treated at dose 5 × 1011 vg/mouse. To examine whether higher could result improvements, MCO (1012 vg/mouse), which resulted some severe, sometimes lethal, side effects. In order confirm these effects, new cohort administered increasing doses (1011, 1012 vg/mouse). Again, two weeks after administration, found dead while others displayed effects had be euthanized. deleterious not observed high AAV9-GFP directly proportionate dosage (0, 23 or 54% mortality 1011, vg/mouse, respectively), such lethality wild-type (WT) medium doses, blood chemistry analysis post-mortem histology showed liver damage drastically elevated levels transaminases disorganized architecture. Apoptosis confirmed presence TUNEL- cleaved-caspase 3-positive cells AAV9-MCO. then studied involvement unfolded response (UPR) triggering apoptosis since it can activated AAV vectors. Increased expression C/EBP homologous (CHOP), one UPR downstream effectors, administration. The toxic reaction seen indicates that, although breathing mice, further are needed better understand underlying mechanisms caution must exercised before similar attempts undertaken patients.