Hyperphosphorylated Paratarg-7 Is a Frequent Antigenic Target of IgM Paraproteins, Is Dominantly Inherited and Represents a Highly Significant Risk Factor for Monoclonal Gammopathy of Undetermined Significance of the IgM Type (IgM-MGUS) and Waldenstrom's Macroglobulinemia (WM), Allowing for the Identification of Family Members at Risk in Cases of Familial IgM-MGUS and WM.

Antigenic targets of paraproteins in MGUS, multiple Myeloma (MM) and WM might play a role in the pathogenesis of these neoplasms by chronic antigenic stimulation, but very few have been identified, of which most were specific for one individual paraprotein only. In contrast, we recently described paratarg-7, a protein of unknown function which is expressed in all human tissues as the target of 15% of IgA and IgG paraproteins in MGUS and MM (Grass et al.; Lancet Oncology 2009 in press).

To determine if and how frequently paratarg-7 functions as the antigenic target of the IgM paraproteins in MGUS/WM, sera from patients with IgM-MGUS/WM were tested for reactivity with recombinant paratarg-7 by ELISA. The specificity of the paraprotein-mediated reaction was demonstrated by absorption studies with recombinant paratarg-7 and by cloning the B-cell receptor from bone marrow cells of patients with a paratarg-7 specific paraprotein. Lysates of peripheral blood from patients and controls were tested by gel electrophoresis and isoelectric focusing before and after phosphatase treatment. Moreover, paratarg-7 cDNA was sequenced to exclude SNPs and mutations.

The paraproteins of 18 (9 WM and 9 IgM-MGUS) of 161 (11%) sera from patients in Germany, USA and Greece reacted specifically with paratarg-7, proving paratarg-7 as the first antigen identified as a paraprotein target in a significant proportion of patients with IgM-MGUS/WM. Mutations or polymorphisms of paratarg-7 were not found. However, 2D-gelelectrophoresis, isoelectric focusing and phosphatase treatment revealed that paratarg-7 was hyperphosphorylated in all patients with an anti-paratarg-7 specific IgM-paraprotein tested. In contrast, only 4 of 200 (2%) healthy blood donors were carriers of hyperphosphorylated paratarg-7. Thus, carriers of hyperphosphorylated paratarg-7 have a significantly increased risk (odds ratio= 6.5; 95%-CI: 2.1-19.6; p=0.001) for developing IgM-MGUS/MW. Moreover, family analyses of relatives of IgM-MGUS/WM patients with an anti-paratarg-7 specific paraprotein revealed that the hyperphosphorylated state of this protein is inherited as a dominant trait. The results obtained in IgM-MGUS/WM are similar to recent observations made in 252 patients with IgG- or IgA-MGUS/MM where hyperphosphorylated paratarg-7 was also associated with a significantly increased risk of developing IgG- and IgA-MGUS (odds ratio: 7.9; 95% CI, 2.8-22.6; p=0.0001).

Hyperphosphorylated paratarg-7 is a highly significant risk factor for MGUS, WM and MM, with the highest odds ratio of any risk factor reported to date for these diseases. Hyperphosphorylated paratarg-7 is the first molecularly defined and dominantly inherited risk factor identified for any hematological neoplasm reported to date. The carrier state of hyperphosphorylated paratarg-7 explains cases with familial MGUS, MM and WM and enables the identification of family members of patients at increased risk for MGUS/WM/MM. That only MGUS/WM/MM patients who are carriers of hyperphosphorylated paratarg-7 had a paratarg-7 specific paraprotein suggests that the hyperphosphorylation of paratarg-7 induces auto-immunity and is involved in the pathogenesis of these diseases, e.g. by chronic antigenic stimulation. The identification of paratarg-7 as a frequent antigenic target enables the more detailed analysis of tumor-host interactions in these patients and its role in the pathogenesis of these diseases. Moreover, its dominant inheritance and the identification of familial cases with MGUS/MM/WM and hyperphosphorylated paratarg-7 carrier state facilitate genome-wide screens for the identification of the SNP responsible for hyperphosphorylation of this molecule.

No relevant conflicts of interest to declare.