Gene therapy using an ortholog of human fragile X mental retardation protein partially rescues behavioral abnormalities and EEG activity

Abstract

Fragile X syndrome (FXS), a neurodevelopmental disorder with no known cure, is caused by a lack of expression of the fragile X mental retardation protein (FMRP). As a singlegene disorder, FXS is an excellent candidate for viral-vectorbased gene therapy, although that is complicated by the existence of multiple isoforms of FMRP, whose individual cellular functions are unknown. We studied the effects of rat and mouse orthologs of human isoform 17, a major expressed isoform of FMRP. Injection of neonatal Fmr1 knockout rats and mice with adeno-associated viral vectors (AAV9 serotype) under the control of an MeCP2 mini-promoter resulted in widespread distribution of the FMRP transgenes throughout the telencephalon and diencephalon. Transgene expression occurred mainly in non-GABAergic neurons, with little expression in glia. Early postnatal treatment resulted in partial rescue of the Fmr1 KO rat phenotype, including improved social dominance in treated Fmr1 KO females and partial rescue of locomotor activity in males. Electro-encephalogram (EEG) recordings showed correction of abnormal slow-wave activity during the sleep-like state in male Fmr1 KO rats. These findings support the use of AAV-based gene therapy as a treatment for FXS and specifically demonstrate the potential therapeutic benefit of human FMRP isoform 17 orthologs.

Gene therapy using human FMRP isoforms driven by the human FMR1 promoter rescues fragile X syndrome mouse deficits

“…9 ) for gene therapy in Fmr1 KO rats. 2 This FMR1 isoform transcript lacks exon 12 and uses a different acceptor site at exon 17 compared with the longest FMR1 isoform 1 transcript, therefore human FMRP isoform 17 lacks amino acids 376–396 and 580–596 compared with human FMRP isoform 1. Hooper et al.…”

“…They found that, at age 1.5–2 months, transgene expression partially rescued social dominance and locomotor activity deficits, and abnormal slow-wave activity during the sleep-like state in Fmr1 KO rats. 2 Nevertheless, so far, all reported AAV gene therapy studies used rodent FMRP and the expression was driven by promoters other than the human FMR1 promoter. To apply AAV gene therapy for FXS clinical tests in the future, the therapeutic efficacy of human FMRP expression in appropriate cell types and at physiological levels, preferably driven by the human FMR1 gene promoter should be determined.…”

“…Thereby, loss of FMRP can result in FXS with clinical manifestations of intellectual disability and autism spectrum disorder (ASD)-related behaviors. 1 , 2 , 3 …”

“…Pharmacotherapeutic approaches, such as stimulants, antidepressants, and antipsychotics, are used for symptom treatment of comorbid behaviors and psychiatric problems, but do not target the underlying cause of FXS. 1 , 2 , 3 As a single-gene disorder, viral vector-based gene therapy using adeno-associated virus (AAV) vectors provides a promising strategy for FXS treatment. 3 , 11 Several studies have explored the potential of using AAV-mediated FMRP expression for FXS treatment in animal models.…”

“…Overall, these studies demonstrate that restoration of FMRP at least partially reversed altered biochemical and physiological features and attenuated behavioral deficits in rodent FXS models. 2 , 12 , 13 , 14 , 15 However, all these studies used rodent FMRP and the expression was driven by promoters other than the human FMR1 promoter. To apply AAV gene therapy for FXS clinical tests in the future, the therapeutic efficacy of human FMRP expression in appropriate cells and at physiological levels, preferably driven by the human FMR1 gene promoter should be determined.…”