Elitt MS, Barbar L, Shick HE, et al. Suppression of proteolipid protein rescues Pelizaeus-Merzbacher disease [published online ahead of print, 2020 Jul 1]. Nature. 2020;10.1038/s41586-020-2494-3. doi:10.1038/s41586-020-2494-3
Mutations in PLP1, the gene that encodes proteolipid protein 1, result in failure of myelination and neurological dysfunction in the X-chromosome linked leukodystrophy Pelizaeus-Merzbacher disease (PMD). Most PLP1 mutations, including point mutations and supernumerary copy variants, lead to severe and fatal disease. Patients who lack PLP1 expression, and Plp1-null mice, can display comparatively mild phenotypes, suggesting that PLP1 suppression might provide a general therapeutic strategy for PMD1. Here we show, using CRISPR-Cas9 to suppress Plp1 expression in the jimpy (Plp1jp) point-mutation mouse model of severe PMD, increased myelination and restored nerve conduction velocity, motor function and lifespan of the mice to wild-type levels. To evaluate the translational potential of this strategy, we identified antisense oligonucleotides that stably decrease the levels of Plp1 mRNA and PLP1 protein throughout the neuraxis in vivo. Administration of a single dose of Plp1-targeting antisense oligonucleotides in postnatal jimpy mice fully restored oligodendrocyte numbers, increased myelination, improved motor performance, normalized respiratory function and extended lifespan up to an eight-month end point. These results suggest that PLP1 suppression could be developed as a treatment for PMD in humans. More broadly, we demonstrate that oligonucleotide-based therapeutic agents can be delivered to oligodendrocytes in vivo to modulate neurological function and lifespan, establishing a new pharmaceutical modality for myelin disorders. ___________________________________________________________________
Using antisense oligonucleotides (ASOs), a team of researchers at Case Western Reserve University (CWRU) targeted abnormal production of a toxic protein in mouse models with Pelizaeus-Merzbacher disease (PMD), which inhibited production of the protein and mitigated PMD symptoms. The study was published in the July 1 online edition of Nature.
In children with PMD, the mutant protein—proteolipid protein 1 (PLP)— interrupts myelin production in the brain, resulting in significant impairment and death. PMD, a rare, progressive, childhood leukodystrophy that begins in infancy, causes deterioration in coordination, motor function, and intellectual function, and is often fatal.
“The preclinical results were profound. PMD mouse models that typically die within a few weeks of birth were able to live a full lifespan after treatment,” senior study author Paul Tesar, PhD, professor of genetics and genome sciences at CWRU School of Medicine, told Neurology Today.
“Our results open the door for the development of the first treatment for PMD as well as a new therapeutic approach for other myelin disorders,” said Dr. Tesar. “We next want to understand how well treatment works after the onset of symptoms, how long it lasts, how often treatment needs to be given, and whether it might be effective for all PMD patients, regardless of their specific mutation. We are working hard on all of these questions.”
ASOs are synthesized segments of DNA or RNA designed to bind to specific molecules of RNA, thus blocking the ability of the RNA to make a protein. In 2016, the US Food and Drug Administration (FDA) approved the first ASO drug for a neurological disorder, spinal muscular atrophy. Multiple ASOs with different clinical applications have entered the market since that time.
Dr. Tesar said that he is optimistic about the potential for the therapy to rapidly advance into clinical testing for PMD patients in the next few years. “This has promise to be truly life-changing for patients and their families,” he said.
He also speculated that ASO treatment could help treat other disorders of myelin, such as multiple sclerosis, cerebral palsy, and even Alzheimer's disease, dementia, and autism spectrum disorders, which show clear aspects of myelin failure.
Study Design, Findings
To explore the utility of this approach, the investigators used jimpy mice, which carry the PLP1 mutation as the model of PMD, in the study. Dr. Tesar and his colleagues first used CRISPR/Cas9 technology to suppress the PLP1 gene in the jimpy embryos.
Before receiving the intervention, the jimpy mice showed severe tremor, ataxia, seizures, and death by the third postnatal week. Post-CRISPR intervention, the mice demonstrated a 21-fold increase in lifespan—while the control group of jimpy mice only lived an average of 23 days, those that received the CRISPR intervention lived an average of 489 days without tremor, ataxia, or seizures through the end of the study, at 18 months. While this provided Dr. Tesar's lab with evidence that suppression of the PLP1 gene could reverse the effects of demyelination, the embryonic injections were not viable in human populations.
With this in mind, Dr. Tesar and his colleagues injected newer generation ASOs targeting the PLP1 gene into another sample of jimpy mice after birth. The jimpy mice that received an injection of PLP1-targeting ASOs at birth demonstrated as much as a 12-fold increase in lifespan compared with ASO control-injected jimpy mice. The mean survival rate for the controls was 20 days, compared with 239 days for the mice that received the ASO drugs.
In addition, oligodendrocytes were restored throughout the neuroaxis at three weeks of age in mice injected with ASO drugs, whereas the oligodendrocytes were mostly depleted in the jimpy mice that were injected with a placebo. Myelinated axons also were significantly increased throughout the corpus callosum and brainstem in the mice that received the ASO drugs.
In other findings, jimpy mice that received the ASO injection experienced markedly reduced tremor and occasional short-duration seizures (less than 15 seconds) and appeared normal while performing daily activities, including the ability to breed. Locomotion partially improved as well.
To assess whether myelin contributed to these improvements, the researchers measured optic nerve speed. At three weeks of age, they observed a modest but significant increase in conduction velocity in the ASO-injected mice compared with the jimpy mice on placebo.
The researchers noted that when the mice were transferred to hypoxic environments, the jimpy control mice exhibited dysfunctional respiratory control compared with those that had received the ASO drugs and the wild-type mice.
Dr. Tesar noted that ASO drugs induce sustained suppression of their target RNAs for many months in both mice and humans, and that at this juncture, it is unknown if or how often repeat ASO treatment would be needed to maintain neurological benefit. Ongoing research seeks to answer that question, he said.
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