Apolipoprotein A-I and serum amyloid A plasma levels are biomarkers of acute painful episodes in patients with sickle cell disease

Abstract

Background: Acute painful episodes are the clinical hallmark of sickle cell disease and have been linked to morbidity and mortality in the sickle cell population.

Design and methods: We undertook exploratory proteomic studies on paired plasma samples collected from a cohort of 26 adult sickle cell patients during steady state and on the first day of an acute painful episode. We screened for changes in abundance of specific protein peaks via surface-enhanced laser desorption/ionization time of flight mass spectrometry (SELDI-TOF MS), and confirmed the identify of candidate protein peaks by specific immunoassays.

Results: The levels of hemoglobin, hematocrit, total protein, and albumin were lower and the levels of lactate dehydrogenase and absolute reticulocytes higher during acute painful episodes than during the steady state. Surface-enhanced laser desorption/ionization time of flight mass spectrometry spectral analysis consistently showed a mass-to-charge peak at 11.7 kDa with elevated intensities during acute painful episodes, which correlated significantly with the serum amyloid A immunoassay. Serum amyloid A levels were significantly elevated during acute painful episodes, especially in four patients with marked end-organ complications of such episodes. A second, recurring peak, less abundant during acute painful episodes, was present at 28.1 kDa; this peak was correlated significantly with immunoassay measurements of apolipoprotein A1.

Conclusions: On the average, plasma serum amyloid A rises and apolipoprotein AI falls during acute painful episodes. The serum amyloid A/apolipoprotein AI ratio increased in 81% of the patients during acute painful episodes, potentially making it a useful objective marker of such episodes. We propose that these protein alterations, known to contribute to endothelial dysfunction in other settings, might do likewise acutely in acute painful episodes and present a new target for therapeutic intervention in sickle cell disease. (ClincalTrials.gov Identifier: NCT00081523).

Figure 1.

Serum amyloid A (SAA) levels in SCD in steady state versus acute painful episode. (A) Correlation of peak intensity of 11.7 kDa charge-to-mass ratio peak determined by SELDI TOF MS to SAA levels quantified by immunoassay in sickle cell patients (log-log scale). Data are representative of both steady state and acute painful episodes. The two values correlate significantly (r=0.89, P=0.002, Spearmans correlation). (B) Parallel immunoassay revealed significantly elevated SAA levels in patients during an acute painful episode compared to in the same patients at steady state (P=0.002, n = 26 paired observations)(box and whisker plot, logarithmic scale).

Figure 2.

Apolipoprotein A-I (apoA-I) levels in sickle cell disease at steady state versus acute painful episode. (A) Correlation of intensity of a 28.3 kDa charge-to-mass ratio peak determined by SELDI-TOF MS with apoA-I levels quantified by immunoassay in sickle cell patients. Data are representative of both steady state and acute painful episodes. Peak intensity analysis showed a difference in levels of the 28.3 kDa peak between steady state and acute painful episode. (B) Immunoassay demonstrated a significant decrease in apoA-I levels in patients during acute painful episode when compared to during steady state (P<0.008). Data are representative of 26 matched pairs.

Figure 3.

The SAA/Apo A-I ratio is significantly higher during acute painful episodes than during the steady state. Ratios were derived for each patient from the immunoassay data in the previous figures. Presented on a logarithmic scale, the ratio is significantly higher in SCD patients during acute painful episodes (p<0.004, box and whisker plot). Data are representative of 26 matched pairs.