Circulating and Synovial Fluid Levels of YKL-40 Protein in Patients with Hemophilia and Arthropathy: Preliminary Observations

Background: In patients with hemophilia (PwH) there is a predilection for bleeding into the joints, which can lead to synovial hypertrophy, cartilage and subchondral bone modifications and finally destruction of the joint, in a process resulting to hemophilic arthropathy (HAP). YKL-40 is a glycoprotein found increased in the synovial fluid in several types of arthritis, with a probable role as a biomarker for monitoring and even for the therapeutic management of joint diseases. In a previous study, we correlated increased circulating levels of YKL-40 of children and adolescents with hemophilia to the degree of HAP. The aim of the present study was to compare the serum levels of YKL-40 protein between pediatric and adult PwH, as well as to compare the serum and the synovial fluid levels of YKL-40, in PwH with HAP.

Patients/Methods: Pediatric and adult PwH were recruited from two hemophilia treatment centers in Athens, Greece. Fifty-two children and adolescents (median age: 14.0 years, range: 10-18 years) with severe (n=43), moderate (n=4) and mild (n=5) hemophilia A (n=47) or B (n=5), on prophylaxis or on demand treatment (n=3), were included in the study. At the time of YKL-40 measurement, 27 patients had no signs of HAP and 25 patients had HAP. HAP was defined as mild in 11 patients and moderate in 14 of the pediatric PwH. Thirty five adult PwH (34 male/1 female) were also included. The median age of the adult PwH was 48.5 years (range: 19-83) and all of them had severe hemophilia A (n=32) or B (n=3). All of the adult patients were on prophylaxis and had severe HAP. Laboratory evaluation of serum levels of YKL-40 were measured with immunoenzymatic method. In addition, levels of YKL-40 were measured in the synovial fluid of 5 adult PwH and severe HAP.

Results: i) In both pediatric and adult PwH and HAP a strong correlation between age and serum levels of YKL-40: r=0.748, p<0.001, was detected. It is worth mentioning that there was a strong exponential increase of circulating levels of YKL-40 along with age, ii) Adult patients with HAP had higher circulating levels of YKL-40 compared to apparently healthy controls of the same age: 80.9±67.1 ng/mL (range: 14.5-274.0 ng/ml) vs. 31.1±14.7 ng/mL (range: 12.2-69.1 ng/ml) respectively, (p < 0.01), iii) Adult PwH with HAP had higher circulating levels of YKL-40 compared to pediatric PwH and HAP: 80.9±67.1 ng/ml (range:14.5-274.0 ng/ml) vs 24.8±12.8 ng/mL (range:12.2-69.1 ng/ml), respectively (p<0.01), iv) Comparing the children who were more affected regarding HAP vs adults with severe HAP, the latter continued to have significantly higher circulating levels of YKL-40: 23.3±8.7 ng/mL (range: 13.5-48.5 ng/ml) vs 80.9±67.1 ng/mL (range: 14.5-274.0 ng/ml) (p<0.01), v) No significant difference was found in serum levels of YKL-40 in pediatric PwH with HAP, regardless of the severity of arthropathy (mild or moderate) (p>0.05), vi) Finally, extremely high concentrations of YKL-40 in the synovial fluid (more than 10 times higher compared to serum levels), were found in adult patients with severe HAP (p<0.001). Synovial fluid report values ranged from 750.0 to 1,100.0 ng/mL.

Conclusions: In this study, significant differences in serum circulating levels of YKL-40 between pediatric and adult PwH with HAP were detected. The higher synovial fluid concentration of YKL-40 reflect local secretion of the protein and potentially influences its circulating levels, suggesting a crucial involvement of YKL-40 in the pathophysiology of the HAP. Further studies are needed to standardize normal values of YKL-40 according to age, as well as to clarify its role as a biomarker with prognostic value in PwH suffering from HAP.