Circulating Adamts-13 Is Reduced in Patients with Waldenstrom's Macroglobulinemia and Is Associated with Increased IgM Levels and Features of the Disease but Not with the Increased Levels of Von Willebrand Factor

ADAMTS-13 is a protease, member of the ADAMTS family (A Disintegrin and Metalloproteinase with ThromboSpondin type 1 repeats-13), which cleaves the cell bound large ultrapolymeric von Willebrand factor (vWF) strings. Circulating ADAMTS-13 is primarily synthesized and released from hepatic stellate and endothelial cells. Acquired functional deficiency of ADAMTS-13, usually due to inhibitory IgG autoantibodies, results in excessive platelet aggregation and disseminated vWF/platelet-rich thrombus formation. A possible association of low levels of ADAMTS-13 Ag with arterial thrombosis and endothelial dysfunction has also been reported. Hivert and colleagues (Blood 2012;120:3214-21) and our group have shown that increased levels of vWF, the only known substrate of ADAMTS-13, are associated with poorer prognosis in patients with symptomatic Waldenstrom’s Macroglobulinemia (WM). Thus, our aim was to investigate the possible association of ADAMTS-13 antigen (Ag) levels with features of WM and possible biologic implications of the ADAMTS-13 / vWF interaction in WM patients’ prognosis.

Our study included 42 patients with symptomatic WM who were treated and followed in the Department of Clinical Therapeutics of the University of Athens (Greece), from 1999 to 2012, 22 patients with asymptomatic WM and 19 healthy controls of matched gender and age. For the purpose of this study we used stored serum, which had been collected before initiation of any therapy. ADAMTS-13 antigen levels were measured using a commercially available kit (R&D Systems, Minneapolis, MN, USA), which has a detection limit for ADAMTS-13 13 (ng/mL) with intra- and inter-Assay Precisions of <3.7% and <5.7%, respectively.

The median age of patients with symptomatic WM was 65 years (range: 37-83 years) and 54% were males. Anemia (Hb <11.5 g/dL) was present in 78% of patients, low platelet counts (<100x10⁹/L) in 17%, beta2-microglobulin >3 mg/dl in 56%, while 7.5% had serum LDH ≥250 U/L and 58% had serum albumin <3.5 g/dL. Median serum IgM was 3340 mg/L (range 246-9563 mg/dL). According to IPSSWM, 22% had low, 43% intermediate and 35% high risk WM, respectively. Main reasons to initiate therapy included cytopenias in 42% of patients, B-symptoms in 15%, hyperviscosity in 12%, neuropathy in 10% and other reasons in 21%. Primary therapy based on rituximab was given in 93% of the patients and 54% achieved at least 50% reduction of IgM.

The median levels of ADAMTS-13 in patients with symptomatic WM were 848 ng/ml (range 471-1622 ng/ml), while those of patients with asymptomatic WM were 875 ng/ml (range 280.5-1466 ng/nl) and of healthy controls were 1170 ng/ml (range 770-1598 ng/ml). Thus, there was a statistically significant difference in the levels of ADAMTS-13 between patients with either symptomatic or asymptomatic WM and normal individuals (p<0.001 and p=0.002 respectively), while there was no difference between those with symptomatic and asymptomatic WM (p=0.934). We then evaluated the association of ADAMTS-13 antigen levels with features of symptomatic WM. There was a negative correlation between low levels of ADAMTS-13 with high levels of IgM (r=-0.544, p<0.001), beta2-microglobulin (r=-0.376, p=0.01) and with the infiltration of the BM by lymphoplasmatic cells (r=-0.338, p=0.03), while there was a positive correlation with serum albumin levels (r=0.373, p=0.016). Accordingly, in patients who required therapy because of hyperviscosity syndrome, serum ADAMTS-13 was lower (p=0.04). There was no association of the levels of ADAMTS-13 and vWF Ag either in patients with symptomatic or asymptomatic WM (p>0.5 for both), and thus, a direct link between increased vWF Ag levels, which are found in patients with WM, and compensatory ADAMTS-13 Ag levels cannot be supported by our data. The levels of ADAMTS-13 Ag had no prognostic association with overall or progression free survival in patients with symptomatic WM.

In conclusion, this is the first evaluation of ADAMTS-13 Ag levels in the serum of patients with WM, in which we found that the levels of ADAMTS-13 Ag are lower compared to healthy controls and these reduced levels inversely correlate with IgM levels, beta2-microglobulin and the degree of bone marrow infiltration. Levels of ADAMTS-13 do not increase in compensation to increased levels of vWF Ag, a marker of endothelial activation and this fact may have biological implications yet to be investigated.