How do dominant PMP2 mutations cause demyelinating neuropathy?

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Peripheral neuropathy is one of the most common neurodegenerative diseases in America, with an overall prevalence of 1.66%, and 6.6% in persons older than 60 years. In spite of its prevalence, a cause cannot be identified in a substantial fraction of patients, and unless a treatable cause is identified, treatment is limited to symptoms. These sobering statistics have hung over the field for decades. However, if one accepts the idea that the genetic causes of neuropathy (which are being found at an astounding rate) will likely be informative, then exploiting this rich source of information should provide the keys to unlocking the causes and thus finding new treatments of neuropathies, not only the specific genetic causes, but also the pathways involved in the more common cause, such as diabetic neuropathy. In this view, each new cause of CMT helps to complete the “puzzle” of how molecular defects cause neuropathy, with many unanticipated surprises. We recently described a new genetic cause of dominantly inherited demyelinating neuropathy in humans – de novo, dominant mutations in PMP2. PMP2 encodes the myelin protein P2, which is a cytosolic protein that binds to fatty acids and cholesterol. It is minor component of compact myelin; more abundant in the PNS myelin than in CNS myelin. The disease-associated mutations described to date - p.Ile43Asn, p.Thr51Pro, and p.Ile52Thr – are found in a similar region of the protein, giving rise to the idea that the mutants share a common toxic gain of function in myelinating Schwann cells that results in demyelination. We have made one mutation in mice, p.Ile52Thr, using CRISPR-Cas9, and propose to analyze this mouse model in the following specific aims. Aim 1: Do p.Ile52Thr mutant mice develop a demyelinating neuropathy? We generated four, independent lines of mice harboring the p.Ile52Thr mutation, and have germ line transmission in three of these lines. We will perform pathological and electrophysiological analysis on each of the three lines, comparing homozygous (p.Ile52Thr/p.Ile52Thr), heterozygous (p.Ile52Thr/+), and wild type (WT) (+/+) mice. Although heterozygous mice more accurately model the human disease, we included homozygous mice because they might have a more pronounced phenotype. Aim 2. Are PNS myelin sheaths altered in p.Ile52Thr mutant mice? Because P2 binds to fatty acids and cholesterol, we will investigate the possibility that the p.Ile43Asn mutation alters the lipid composition of myelin, by analyzing nerves by lipidomics and by performing electron microscopy of compact myelin.