Response to Letter Regarding Article, “β2-Microglobulin as a Biomarker in Peripheral Arterial Disease: Proteomic Profiling and Clinical Studies”
Schroecksnadel and collaborators seem a bit skeptical about our recent publication on β2 microglobulin (β2M)1 and about the laudatory editorial2 that accompanied it. Our article described the discovery that blood levels of β2M correlate with the severity of peripheral arterial disease (PAD) as assessed by the ankle-brachial index or treadmill testing. This novel finding arose from an agnostic high-throughput proteomic profiling effort using surface-enhanced laser desorption and ionization time-of-flight mass spectroscopy. This is a candidate-generating approach, in contrast to the more common candidate-based approach to proteomic profiling. The major advantage of the candidate-generating approach is that it provides for the discovery of new biomarkers for disease and potentially new insights into pathobiology. We hypothesized that repeated bouts of ischemia–reperfusion in the lower extremities could cause the expression and release of proteins that would be characteristic of the ischemic musculoskeletal circulation and that these could be detected in the systemic circulation. This agnostic approach led to an unexpected finding, the correlation of β2M with the severity of PAD. Notwithstanding the unreferenced comment of Schroeksnadel and colleagues, there are no previous reports suggesting β2M as a biomarker for PAD.
On the other hand, we recognize that β2M has been used previously as a biomarker of renal insufficiency, a point raised by our Austrian colleagues. Accordingly, we performed a multivariate analysis that included the estimated glomerular filtration rate as a covariable. Even after consideration of renal function, β2M remained a predictor for the ankle-brachial index. The Innsbruck group reminds us that β2M is elevated in inflammatory states, an association that is familiar to us. Inflammation is an integral component of the pathobiology of PAD, and the level of C-reactive protein (an accepted biomarker of inflammation) has been correlated with severity of PAD.3 In our study, the odds ratio for the diagnosis of PAD for an elevated C-reactive protein level was 1.3, as opposed to an odds ratio of 7.2 for an elevated β2M.
Our findings are now supported by an elegant study from Shinkai and coworkers.4 In their prospective longitudinal study of elderly Japanese subjects, β2M was an independent predictor of death after consideration of renal insufficiency and inflammation. Indeed, β2M seems to be a better prognosticator than cystatin C (thought to be a superior marker for renal dysfunction)5 or C-reactive protein. Shinkai and colleagues4 did not measure the ankle-brachial index; however, they made the intriguing observation that β2M was negatively associated with usual walking speed. It is well known that patients with PAD have reduced walking speeds.6 This casual observation of Shinkai and coworkers4 may have reflected the prevalence of PAD among their subjects with elevated β2M levels.
We do not disagree with our Austrian colleagues that β2M levels will be elevated by renal insufficiency and inflammation. The elevation of β2M levels in our patients with PAD is likely multifactorial, reflecting these and other adverse influences on the vessel wall. Taken together with the study of Shinkai and coworkers,4 however, our study suggests that β2M provides independent predictive value for the diagnosis of PAD. Interesting areas for future investigation include elucidation of the mechanisms that increase β2M in vascular disease; determination of its role in vascular pathogenesis; characterization of the effect of medical therapy on β2M levels; and assessment of therapeutic interventions targeting β2M so as to reduce the morbidity and mortality rates associated with PAD.
Stanford University owns intellectual property related to the use of β2 microglobulin as a marker for peripheral arterial disease. Drs Cooke, Kimura, and Wilson are inventors and may receive royalties from this intellectual property. Drs Fung, Zhang, and Meng are employees of Vermillion Inc, which is developing the β2 microglobulin for PAD. The other authors report no conflicts.
Wilson AM, Kimura E, Harada RK, Nair N, Narasimhan B, Meng X-Y, Zhang F, Beck KR, Olin JW, Fung ET, Cooke JP. β2-microglobulin as a biomarker in peripheral arterial disease: proteomic profiling and clinical studies. Circulation. 2007; 116: 1396–1403.
Annex BH. Is a simple biomarker for peripheral arterial disease on the horizon? Circulation. 2007; 116: 1346–1348.
Ridker PM, Cushman M, Stampfer MJ, Tracy RP, Hennekens CH. Plasma concentration of C-reactive protein and risk of developing peripheral vascular disease. Circulation. 1998; 97: 425–428.