PLP1 mutations in patients with multiple sclerosis: Identification of a new mutation and analysis of in vitro effects of PLP1 mutations

Abstract

PLP1 is located on the X-chromosome and encodes myelin proteolipid protein (PLP), which is produced by oligodendrocytes and is the most abundant protein of central nervous system (CNS) myelin. Generally, mutations in PLP1 result in the X-linked dysmyelinating disorders Pelizaeus–Merzbacher disease (PMD) or spastic paraplegia type 2. However, 2 case studies (Warshawsky et al., 2005; Gorman et al., 2007) identified 2 patients with clinical symptoms and signs indistinguishable from multiple sclerosis (MS) who had missense point mutations in PLP1 (resulting in changes Leu30Arg and Arg136Trp in the protein). To investigate whether PLP1 mutations might occur relatively frequently in MS, we sequenced the coding regions of PLP1 in 22 female MS patients who had developed MS after the age of 40 and in 51 healthy females, and identified a missense mutation in exon 2 of PLP1 (resulting in a Leu30Val mutation in the protein) in one of the MS patients. mCherry-tagged plasmids containing wild type or mutant PLP1 sequences encoding the 7 exons of PLP (including two known PMD-related mutations as positive controls) were constructed and transfected into COS-7 cells. In the cells transfected with wild type PLP1, PLP was expressed throughout the cells and at the cell surface by 12 h post-transfection. In contrast, the two PMD-related mutations and one of the 3 MS-related PLP1 mutations (Leu30Arg) caused accumulation of PLP in the endoplasmic reticulum of the cells and induction of the unfolded protein response, leading to apoptosis of the cells. While the Leu30Val mutation identified in the current study did not adversely affect the distribution of PLP in the COS-7 cells or induce the unfolded protein response, in silico analysis of HLA binding for the HLA molecules carried by the patient harbouring this mutation suggested that the mutation would produce a novel immunogenic PLP epitope in this patient. These results suggest that PLP1 mutations can occur in patients who meet clinical criteria for MS, and that some of these mutations have the potential to induce oligodendrocyte apoptosis. Other PLP1 mutations may produce novel epitopes of PLP that could become targets of autoreactivity in MS. The results support the idea that mutations in myelin-related genes could underlie development of MS in some patients.