Multiple sclerosis (MS) is the most common demyelinating disease of human adults. Its therapies have limited effectiveness in decreasing relapse frequencies without affecting disease progression

Multiple sclerosis (MS) is the most common demyelinating disease of human adults. Its therapies have limited effectiveness in decreasing relapse frequencies without affecting disease progression (Steinman and Zamvil, 2006). Treatments are essentially immunosuppressive (Wingerchuk, 2008; Comi and Martino, 2006; Hemmer et al., 2005), reflecting the broad consensus of autoimmune effector mechanisms in MS. However, disease heterogeneity provides evidence for non-autoimmune, biochemical and epigenetic MS abnormalities, whose role in the complex hierarchy of pathogenesis remains unclear. In a study of 286 MS cases (mostly biopsies) (Lucchinetti et al., 2000), MS has been categorized into four different patterns of pathogenesis, which we have recognized in our animal models, which combined show most of the features of MS. In early studies we showed that myelin basic protein (MBP) isolated from normal human brain contained about 20% of the citrullinated MBP (Moscarello et al., 1994). In chronic MS white matter, the citrullinated protein was 45% and in fulminating MS (Marburg’s Disease) it accounted for 90% of the MBP (Wood et al., 1996). This less cationic MBP was unable to compact lipid bilayers in model systems, as revealed with several techniques including X-ray diffraction (Brady et al., 1981a; Brady et al., 1981b) electron spin resonance (Boggs et al., 1982) circular dichroism (Epand et al., 1974) and NMR (Deber et al., 1986). We postulated that the decreased protein-lipid interactions resulted in destabilization of the myelin, which could then be more readily degraded. Citrulline in proteins is generated by a family of enzymes, the peptidylarginine deiminases (PADs) of which five are known to be all localized to a single locus on chromosome 1 (lp36.1). Of the five isozymes, PAD2 and PAD4 are found in brain, localized in myelin and oligodendrocytes. PAD4 is unique because it is the only PAD to carry a nuclear localization signal. We have shown that PAD4 can be translocated to the nucleus in oligodendrocytes in culture in the presence of TNFα where it deiminates histone H3, suggesting a role in apoptosis (Mastronardi et al., 2006). PAD2 was found in myelin, the axons, and the periaxonal space at the contact between myelin and the oligodendrocyte (Wood et al., 2008). Both processes, the decreased myelin compaction and apoptosis of the oligodendrocytes, represent important pathways in the pathomechanism of demyelination. Further support for an important role of PAD enzymes and hypercitrullination of proteins in MS was obtained with two transgenic mouse lines that demyelinate spontaneously. In the ND4 line, which contains 70 copies of the cDNA for DM20 (a myelin proteolipid), an increase in PAD enzymes was observed at 2 months of age, 1 month prior to the onset of both clinical signs of demyelination and increased protein citrullination at 3 months of age, suggesting a causative role of PAD enzymes (Moscarello et al., 2002). In the other model, generated by the addition of 30 copies of cDNA for PAD2, signs of demyelination occurred at 6 months