Abstract

Introduction

Vaso-occlusive crises (VOCs), the most common acutely painful complication of sickle cell disease (SCD), presumably cause bone infarction. In a preclinical study that exposed sickle cell mice to repeated hypoxia-reperfusion stress-to recapitulate VOCs-the mice over-expressed osteoclast-driven bone resorption markers and suppressed osteoblast-mediated bone formation markers. We hypothesize that bone infarction stems from ongoing hemolysis and inflammation incurred during VOCs, and that the extent of bone degradation can be approximated by increases in concentrations of bone resorption markers in the plasma or urine of adults with SCD. We measured urinary levels of cross-linked carboxyterminal telopeptide (CTX-1, a bone resorption marker) in 52 adults with SCD, both during and after resolution of VOCs. We now report on the association between urinary CTX-1 concentrations and serum markers of hemolysis and inflammation.

Methods

In the Sickle Cell Pain Markers (SCPM) study, adults with SCD were consecutively recruited from Tulane University Sickle Cell Center of Southern Louisiana (2008-2013). Blood and urine specimens were collected for laboratory analyses and symptoms assessed during vaso-occlusive pain crises (visit 1) and at recovery (visit 2). Self-reported symptoms were graded on a 0-10 scale (0=no symptoms, 10=worst symptoms). We restricted our analyses to subjects with complete data obtained at both visits. Three study participants were re-enrolled in SCPM, but only one subject completed data collection and was included in this analysis. We used paired t-tests and non-parametric Wilcoxon sign rank test to assess for between-visit differences in continuous variables and Fisher's exact test to monitor changes in categorical variables. Urinary CTX-1 concentrations, corrected for urine creatinine levels, were measured using enzyme-linked immunosorbent assays performed by the Research Laboratory Services, a core facility of Maine Medical Center Research Institute, Scarborough, ME. We then compared between-visit changes in urinary CTX-1 levels with changes in serum/plasma markers of hemolysis (hemoglobin, total bilirubin, lactate dehydrogenase and reticulocyte counts) and inflammation (white blood cell count, platelets, fibrinogen, C-reactive protein and erythrocyte sedimentation rates). All data were analyzed using R 3.5.1 software, and results presented in standard box plots, with slopes and 95% confidence intervals (CI).

Results

Of the 52 adults enrolled in the SCPM study (62% female, 65% hemoglobin SS/ Sb-thalassemia, 35% hemoglobin SC), we analyzed paired data from 31 subjects (60%). The average duration between visits 1 and 2 was 22 days. Mean concentrations of urinary CTX-1-corrected for urine creatinine-significantly decreased from 3.43±1.95 μg/mmol during vaso-occlusive crises to 2.62±1.34 μg/mmol at recovery (Figure). We found a non-significant positive correlation between changes in urinary CTX-1 levels and days between visits (slope 0.048, 95% CI -0.025, 0.121, p-value 0.184). The correlations between changes in urinary CTX-1 concentration and serum/plasma markers of hemolysis and inflammation were not statistically significant. Secondary analyses of all laboratory markers showed significant increases in serum levels of hemoglobin and platelets between visits 1 and 2 (p<0.05). Pain scores significantly decreased, as expected, between visits; and, shortness of breath, fatigue, nausea and sleep significantly improved with resolution of pain crises.

Conclusions

VOCs accelerate bone degradation in adults with SCD. CTX-1 has low intra-individual variability and can reliably approximate the extent of bone resorption that occur during pain crises. The small sample size (n=31 pairs) likely limited our ability to detect a significant association between changes in urinary CTX-1 concentrations and changes in serum/plasma markers of hemolysis and inflammation. Additional studies are required to better understand how CTX-1 concentrations-relative to bone formation markers e.g., bone-specific alkaline phosphatase-can be used to monitor immediate and long-term deleterious effects of recurrent VOCs on the bones of adults with SCD.

Disclosures

No relevant conflicts of interest to declare.

Author notes

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Asterisk with author names denotes non-ASH members.