Myelin protein zero mutations and the unfolded protein response in Charcot Marie Tooth disease type 1B

Abstract

ObjectiveTo determine the prevalence of MPZ mutations that cause Charcot Marie Tooth neuropathy type 1B (CMT1B) and activate the unfolded protein Response (UPR).Background CMT1B is caused by >200 heterozygous mutations in MPZ, the major protein in peripheral nerve myelin. Mutations Ser63del MPZ and Arg98Cys MPZ cause the mutant protein to be retained in the ER and activate the generally adaptive UPR. Treatments that modulate UPR activation have improved cellular and rodent models of CMT1B raising the possibility that other MPZ mutations that activate the UPR would also respond favorably to similar treatment. The prevalence of MPZ mutations that activate the UPR is unknown.MethodsWe developed a dual luciferase reporter assay of Xbp1 splicing using stably transfected RT4 Schwann cells to assay the ability of cDNA constructs bearing 46 distinct MPZ mutations to activate the UPR. Constructs also carried an HA tag to permit detection of ER retention of mutant proteins. UPR activation and ER retention were correlated with clinical phenotypes.ResultsEighteen mutations demonstrated ER retention and UPR activation to a similar degree as Ser63del and Arg98Cys MPZ. Thirty‐five of the mutations activated the UPR > 1.5 fold compared to that of wild‐type MPZ. Correlation was high between firefly and Nano‐luciferase reporters and between both reporters and ER localization. UPR activity did not correlate with clinical onset or severity.ConclusionMany CMT1B causing mutations activate the UPR and may be susceptible to therapeutic efforts to facilitate UPR function.

A charcot‐marie‐tooth type 1B kindred associated with hemifacial spasm and trigeminal neuralgia

“…40 Our kindred's G163R genotype has not been examined for the UPR response, but the similar G167R genotype activates the UPR more robustly than wild-type MPZ. 40 Like G163R, the G167R mutation is expressed in the transmembrane portion of the protein, causing MPZ to be inserted incorrectly in the membrane. 41 Another MPZ mutation, Trp101X, is also predicted to result in inadequate anchoring of the protein into the myelin membrane and been associated with debilitating, chronic neuropathic pain, so perhaps abnormalities in protein anchoring are common mechanisms of pain in CMT1B.…”

“…The pathophysiology of MPZ mutations leading to neuropathy has been theorized to be due to toxic gain‐of‐function mutations that may produce the varied phenotypes of CMT1B . A common underlying mechanism of neuropathy appears to be protein misfolding resulting in retention of the mutant protein in the endoplasmic reticulum triggering the canonical unfolded protein response (UPR) . Our kindred's G163R genotype has not been examined for the UPR response, but the similar G167R genotype activates the UPR more robustly than wild‐type MPZ .…”

“…A common underlying mechanism of neuropathy appears to be protein misfolding resulting in retention of the mutant protein in the endoplasmic reticulum triggering the canonical unfolded protein response (UPR) . Our kindred's G163R genotype has not been examined for the UPR response, but the similar G167R genotype activates the UPR more robustly than wild‐type MPZ . Like G163R, the G167R mutation is expressed in the transmembrane portion of the protein, causing MPZ to be inserted incorrectly in the membrane .…”

Neuropathy-related mutations alter the membrane binding properties of the human myelin protein P0 cytoplasmic tail

“…1a), which have been speculated to be affected by P0ct mutations (Su et al 1993; Xu et al 2001). Many P0 mutations have been suggested to lead to UPR activation (Bai et al 2013; Bai et al 2018; Wrabetz et al 2006), indicating problems with translation rate, folding, and/or membrane insertion. Given the fact that P0ct is known to interact with lipid membrane surfaces (Luo et al 2007; Raasakka et al 2019b; Raasakka et al 2019a), these mutations could also have direct effects on the formation of mature compact myelin at the molecular level.…”

“…In addition, K236E has been linked to dominant axonal CMT type 2I (CMT2I) (Choi et al 2004), and A221T, which was discovered as a co-mutation together with the deletion of Val42 in the Ig-like domain, was identified in a patient with DSS (Planté-Bordeneuve et al 2001). How these mutations relate to CMT/DSS etiology is not known, although P0 mutations have been linked to the unfolded protein response (UPR) (Bai et al 2013; Bai et al 2018; Wrabetz et al 2006), indicating issues in either translation or folding that induce stress within the endoplasmic reticulum (ER).…”