Glycosylationof Immunoglobulins Determine Bone Loss in Multiple Myeloma

About 80% of patients with multiple myeloma develop a severe osteolytic bone disease causing pain and fractures. The myeloma cells secrete immunoglobulins and the presence of monoclonal immunoglobulins in serum is a hallmark of the disease. Immunoglobulins play a role in bone loss, but have not been linked with the bone disease in multiple myeloma.

In this work, we isolated immunoglobulins from serum of myeloma patients using protein G coupled magnetic beads and columns. We found that immunoglobulins from patients with bone disease (n=16) promoted osteoclast differentiation when added to human monocyte-derived pre-osteoclasts (p=0.006). We next fractionated the immunoglobulin samples by size-exclusion chromatography and found that the "osteoclast promoting activity" was in the high-molecular weight fractions, suggesting that they are in complexes. Since extent of complex formation may be determined by glycosylation, we examined whether there is a difference in immunoglobulin glycosylation between healthy controls and patients, and whether it changes during disease progression. To this end we analysed IgG glycosylation in serum samples from patients (n=72) and age and sex matched controls (n=51). These analyses showed that patient IgG was less galactosylated (p=0.02) and less sialylated (p=0.04) compared with control IgG. Moreover, patients with bone disease (n=43) had significantly less galactose on IgG compared with patients without bone disease (p=0.02, n=33). Supporting this data, we found that galactosidase treatment of immunoglobulins from patients without bone disease induced osteoclastogenesis (p=0.03), whereas addition of galactose to immunoglobulins of patients with bone disease removed their pro-osteoclastogenic effect (p=0.01). Further, the glycosyltransferases ST6GAL1 and B4GALT11, which add sialic acid and galactose to the sugar chain, respectively, are less expressed in plasma cells obtained from patients with bone disease (n=137) compared with those without (n=36, p<0.002, p<0.001, GSE755). Importantly, we observed a significant reduction of IgG glycosylation (p=0.02, n=8) in serum samples obtained from individual patients before and after the onset of bone disease.

Taken together, our data support that immunoglobulins promote bone loss in multiple myeloma.