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Antibodies to myelin‐associated glycoprotein (anti‐Mag) in IgM amyloidosis may influence expression of neuropathy in rare patients

We have examined whether antibodies to myelin‐associated glycoprotein (anti‐MAG) influence neuropathy occurrence and phenotype in primary (AL IgM) amyloidosis. Anti‐MAG and the cross‐reacted sulfoglucuronyl paragloboside antibodies (SGPG) were studied in 46 patients with IgM amyloidosis (21 with polyneuropathy), and 21 matched IgM MGUS (monoclonal gammopathies of undetermined significance) controls without neuropathy. We assessed the occurrence, phenotype of neuropathy, and attributes of nerve conduction and their relation to antibody activity. Twenty of 46 patients with IgM amyloidosis (7 with and 13 without polyneuropathy) had elevation of anti‐MAG or SGPG by enzyme‐linked immunosorbent assay (ELISA). Two of the polyneuropathy patients with IgM amyloidosis had antibodies to MAG based on Western blot (WB) positivity. One of these patients, with the highest anti‐MAG titer, had a painful sensory ataxia, with prominent demyelination, and amyloid deposition in sural nerve. The other anti‐MAG WB‐positive amyloid patient had an axonal neuropathy and dysautonomia. Low levels of anti‐MAG antibodies were found in 12 of 21 IgM MGUS controls without neuropathy (mean follow‐up, 11 years). We conclude that finding serum anti‐MAG antibodies does not exclude the diagnosis of primary amyloidosis. They do not appear to affect the occurrence or expression of polyneuropathy, except possibly in occasional cases with WB positivity. Muscle Nerve, 2007