14 15 Mutations in proteolipid protein 1 (PLP1) result in failure of myelination and severe 16 neurological dysfunction in the X-linked pediatric leukodystrophy Pelizaeus-Merzbacher 17 disease (PMD). The majority of PLP1 variants, including supernumerary copies and various 18 point mutations, lead to early mortality. However, PLP1-null patients and mice display 19 comparatively mild phenotypes, suggesting that reduction of aberrant PLP1 expression 20 might provide a therapeutic strategy across PMD genotypes. Here we show, CRISPR-Cas9 21 mediated germline knockdown of Plp1 in the severe jimpy (Plp1 jp ) point mutation mouse 22 model of PMD rescued myelinating oligodendrocytes, nerve conduction velocity, motor 23 function, and lifespan to wild-type levels, thereby validating PLP1 suppression as a 24 therapeutic approach. To evaluate the therapeutic potential of Plp1 suppression in postnatal 25 PMD mice, we tested antisense oligonucleotides (ASOs) that stably decrease mouse Plp1 26 mRNA and protein in vivo. Administration of a single intraventricular dose of Plp1-targeted 27 ASOs to postnatal jimpy mice increased myelination, improved motor behavior, and 28 extended lifespan through an 8-month endpoint. Collectively, these results support the 29 development of PLP1 suppression as a disease-modifying therapy for most PMD patients. 30 More broadly, we demonstrate that RNA therapeutics can be delivered to oligodendrocytes 31 in vivo to modulate neurological function and lifespan, opening a new treatment modality for 32 myelin disorders. 33 2 Main text: 34 Pelizaeus-Merzbacher disease (PMD; OMIM 312080) is an X-linked leukodystrophy 35 typified by extensive hypomyelination of the central nervous system (CNS). Symptoms typically 36 present early in childhood with a constellation of nystagmus, spasticity, hypotonia, and cognitive 37 dysfunction, leading to mortality in early adulthood. In severe forms, symptoms present connatally 38 and patients succumb to their disease in early childhood. Despite intense interest in PMD 39 therapeutic development, including several clinical trials, no disease modifying therapies have 40 proven efficacious in patients 1-6 . 41 Mutations in the proteolipid protein 1 (PLP1; OMIM 300401) gene underlie the 42 pathogenesis of PMD, with variability in disease onset and progression dictated by the severity of 43 the particular mutation 7-9 . PLP1 codes for the tetraspan protein PLP as well as its shorter splice 44 isoform DM20, which lacks 35 amino acids in the cytosolic loop region 10 . PLP/DM20 is highly 45 conserved across species, with human, mouse, and rat sharing identical amino acid sequence 11 . 46 Expression of PLP1 is largely restricted to myelinating oligodendrocytes, where it is responsible 47 53 15 . Null patients can live until 40-60 years old 8,16 , do not develop lower body spastic paraparesis 54 until the second or third decade of life, and do not demonstrate cognitive regression until the third 55 or fourth decade of life (see Extended Data Table 1 for detailed clinical present...