Background: Syndecan-1 (CD138) is a heparan sulfate bearing proteoglycan found on various epithelial cells as well as on B lineage cells depending on its stage of development. Syndecan-1 (CD138) is abundantly expressed by plasma cells, especially myeloma cells. The extra cellular domain along with the heparan sulfate side chains can be cleaved off the cell surface and can be detected in the serum as soluble syndecan. Syndecan possibly plays a multifunctional role in the biology of myeloma. It has been shown to be an independent prognostic factor in patients with multiple myeloma. It has also been shown to promote myeloma cell growth through different mechanisms. Its expression has also been suggested to correlate with bone disease in MM.

Methods: In this study we studied serum levels of soluble syndecan in newly diagnosed MM patients enrolled in the Eastern Cooperative Oncology Group (ECOG) E9486 and its associated correlative laboratory clinical trial E9487. We evaluated the prognostic value of syndecan in MM and its relationship to other known prognostic factors for this disease. In addition, syndecan levels were correlated with clinical and laboratory markers of bone disease.

Results: A total of 501 patients were studied and the median serum syndecan-1 was 158 ng/mL. Syndecan levels correlated positively with other prognostic factors and markers of tumor burden such as β2-microglobulin (correlation coefficient 0.3; P <0.00001), labeling index (0.25; <0.0001), creatinine (0.23; <0.0001), soluble IL6 receptor (0.3; <0.0001), BM plasma cell percentage (0.16; <0.0006), and disease stage (P=0.0007). Significant differences in the overall and progression free survival was found between two groups of patient separated using the median value as cut-off. The High syndecan group had a median overall survival of 36.3 months compared to 49.3 months for the low syndecan group (P < 0.0001). Similarly, the high syndecan group had progression free survival of 25.4 months compared to 33.5 months for the low syndecan group (P < 0.0001). In a proportional hazards model including syndecan-1 as well as labeling index, β2M, Platelet count, IL-6R, syndecan-1 retained its prognostic value for overall survival (HR 1.3, P = 0.021). Syndecan levels were correlated with various bone markers including C-terminal telopeptide of type I collagen (ICTP), osteocalcin (OC), C-terminal type I procollagen (PICP), bone-specific alkaline phosphatase (BAP), and tartrate resistant alkaline phosphatase (TRAP) and were found to correlate only with ICTP (0.25, P < 0.0001). No correlation was found between clinical markers of bone disease including presence of lytic lesions, osteoporosis and pathologic fractures on X-rays or bone pain.

Conclusion: In this large study, we once again confirm the prognostic value of serum syndecan-1 levels in large group of patients with newly diagnosed myeloma. Syndecan-1 level correlates with other disease markers. Syndecan levels also correlated with ICTP, a marker of bone turnover, though no strong correlation was found between syndecan levels and clinical markers of myeloma bone disease. The biological basis of these finding needs further evaluation.

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