ECTS-IBMS 2015Mutations in Q20STM1 gene encoding p62 have been associated with Pagets disease of bone (PDB) in up to 50% and 15% of familial and sporadic cases, respectively. Among other functions, p62 is the prototypical cargo receptor for autophagy, a lysosomal recycling process recently implicated in plasma cell ontogenesis and immunoglobulin (Ig) production. Apart from one report of elevated Ig levels in a subset of PDB patients, whether antibody immunity is altered in PDB has not been explored. To address this issue, we measured serum Ig levels in a cohort of 154 patients with active PDB in relation to Q20STM1 mutation status. Compared with age-matched controls (n=60), PDB cases showed higher IgG (p<0.05) and gamma globulin levels by protein electrophoresis (p<0.05). This effect was mainly driven by patients with Q20STM1 mutations (n=27) and became non-significant when patients without Q20STM1 mutation were considered. A similar but not significant trend was observed in IgM levels in Q20STM1 mutated patients versus controls (p=0.09). Moreover, among Q20STM1 mutation carriers, patients with truncating mutations (Y383X and E396X) were associated with the highest serum IgG levels. Since treatment with nitrogen containing bisphosphonates may exert adjuvant, immune-modulating effects, Ig levels were monitored in 10 and 15 patients with and without Q20STM1 mutations, respectively, before and after 1, 3, 6, and 12 months from intravenous treatment with zoledronic acid. In both treatment groups, IgG and IgM levels increased between 1 and 3 months from treatment and then decreased towards pretreatment levels. At each time-point, IgG levels were higher in Q20STM1 mutation carriers than in patients without mutation. To conclusively assess whether Q20STM1 mutations affect plasma cell activity in a cell-autonomous fashion, we lentivirally engineered the IgM-producing B18 plasmocytoma and the IgG-producing OKT3 hybridoma cell lines to stably express wild type and mutant (E396X) p62. Standard 4 hr Ig secretion assays demonstrated increased rates of Ig secretion in cell lines expressing mutant p62, as compared with controls engineered to express wild type p62. In conclusion, our clinical and experimental data indicate that PDB-associated Q20STM1 mutations affect plasma cell activity and humoral immunity. The possible implications of these findings in the pathogenesis of the disorder and the occurrence of comorbidities in mutated PDB patients warrant further investigation.
Disclosure: The authors declared no competing interests. This work was supported by Telethon Grant (GGP11119); Italian Ministery of Health Grant (GR-2011-02352160).