Hyperphosphorylation of Antigenic Targets of Paraproteins In MGUS and Multiple Myeloma (MM) Is a Consistent Finding: Implications for the Pathogenesis of MGUS/MM

Hyperphosphorylated paratarg-7 (pP-7) is the target of paraproteins in 15% of patients with MGUS/MM (Preuss et al. Int J Cancer 2009; 125:656-61). Carriers of pP-7 have an 8-times increased risk to develop IgA/IgG-MGUS and multiple myeloma (Grass et al., Lancet Oncology 2009; 10:950-956) and a 6.5 times increased risk to develop IgM-MGUS and Waldenström′s macroglobulinemia (Grass et al., Blood 2009;114:1513s). Analysis of affected families showed that pP-7 is inherited in a dominant fashion. Thus, pP-7 is the first molecularly defined autosomal-dominant risk factor for any hematological neoplasm. Objective: Since paratarg-7, a frequent target of paraproteins in patients with MGUS/MM is hyperphosphorylated and inherited in a dominant fashion we extended our studies to investigate the prevalence of further antigenic targets and their phosphorylation state.

Sera of MGUS/MM patients and relatives of affected families were tested for antibody reactivity against antigens represented in a fetal brain derived protein macroarray using a modified SEREX approach (Preuss et al. International J. Cancer 2009;125, 656–661). Lysates of peripheral blood from patients, family-members and controls were tested by gel electrophoresis and isoelectric focusing before and after phosphatase treatment.

Analysis of 3 families with multiple members affected by MGUS and MM revealed that the antigenic targets of paraproteins from affected members of a given family are identical: 2 families shared paratarg-7 and 1 family shared paratarg-8 as the family-typic paraprotein antigen. Paratarg-8 is encoded by the ATG13 gene, a member of the „autophagy regulatory complex“ family of genes. Paratarg-8 showed no mutations or polymorphisms in patients compared to controls. However, IEF before and after phosphatase treatment showed that paratarg-8 is hyperphosphorylated (pP-8) in the affected family members compared to healthy controls and that pP-8 carrier state is also inherited in a dominant fashion. This finding prompted us to check previously identified paraprotein targets for hyperphosphorylation, which, in addition to paratarg-7 and paratarg-8 was possible for paratarg 2, 5, 6, 9, 10, 11. In all 8/8 cases, IEF before and after phosphatase treatment revealed that the patients were carriers of a hyperphosphorylated version of their paraprotein target compared to healthy controls.

The paraproteins of members affected by familial MGUS/MM are directed against family-specific antigenic targets, suggesting that the genetic background shared by these patients contains a chronic auto-immune response. Paratarg-7 and paratarg-8 are the first family-typic antigens that have been molecularly defined to date. The fact that all antigenic targets of paraproteins molecularly defined to date are hyperphosphorylated in patients compared to healthy controls implies a significant role of the hyperphosphorylation state in these neoplasms. Analysis of the T-cell response against normo- and hyperphosphorylated paraprotein targets in affected and non-affected family members is underway as are genome-wide association to determine the SNP or mutation which is associated with the hyperphosphorylation of paraprotein targets.

Lynch:State of Nebraska LB595 support: Research Funding; Charles F. and Mary C. Heider Chair at Creighton University.: Research Funding.