Selective loss of myelin-associated glycoprotein from myelin correlates with anti-MAG antibody titre in demyelinating paraproteinaemic polyneuropathy

The IGM monoclonal autoantibodies of patients with demyelinating paraproteinaemic polyneuropathy recognize a carbohydrate structure present on both myelin-associated glycoprotein (MAG) and protein zero (P-0). These auto-antibodies are sufficient to cause the disease but the mechanism of demyelination remains unclear. We have analysed nerve biopsies from eight patients with polyneuropathy and anti-MAG antibodies by quantitative immunohistochemistry and find a concordant pattern of reduced expression of myelin markers with the loss of myelinated fibres. We report here novel features of this disease, in particular a selective lack of detectable MAG in a large proportion of myelinated fibres containing P-0, myelin basic protein (MBP) and periaxin. There is also an inverse correlation of the distribution of MAG in peripheral nerve myelin with the serum anti-MAG antibody titres but no correlation of these titres with the loss of myelinated fibres. Double immunofluorescence staining of paraproteinaemic polyneuropathy (PPN) nerves shows anti-MAG IgM deposited on the periphery of myelinated fibres associated with or lacking MAG staining. These data suggest that the binding of anti-MAG antibodies to MAG and/or other myelin component(s) results in MAG downregulation and may have an essential role in the molecular mechanisms leading to demyelination adn partial regeneration in this disease.