Chronic hemolysis, anemia and vaso-occlusive crises represent a characteristic triad in patients with SCA. While transfusion may improve disease complications of SCA patients, iron overload is a dreaded and inevitable consequence of ongoing transfusion therapy. The role of iron in the pathogenesis of cardiac damage is undisputed, but the underlying molecular mechanisms remain incompletely understood. 3
MMP-9 is one among a large family of zinc-dependent endopeptidases. It plays a crucial role in ECM degradation in many physiological and pathophysiological processes that involve tissue remodeling. A bunch of studies offer strong evidence for the role of MMP-9 in LV remodeling. 4, 5
To the best of our knowledge, this is the first study to evaluate the relationship between MMP-9 and cardiac T2* in sickle cell disease patients. However there is a large body of literature confirming the role of MMP-9 in cardiovascular diseases including in atherosclerosis, hypertension, heart failure and acute myocardial infarction. 6,7
Our results showed that patients with sickle cell disease had significantly higher plasma levels of MMP-9 than controls (150.9 versus 32.0 ng/ml respectively, p <0.001).
These results were consistent with the results of Carvalho et al. 8 who evaluated hematological, biochemical and immunological parameters in 101 stable SCA patients, 23 SCA patients in vaso-occlusive crisis (crisis state) and 146 healthy control individuals. MMP9 levels were significantly increased in patients with stable SCA (p value <0.0001) and patients with SCA in VOC (p value 0.0091) group than healthy controls.
In their previous study, Carvalho et al. 9 found that steady-state SCD patients had the highest values of LTB4, PGE2, TIMP1, MMP9, IL-8, and IL-12 concentration compared with SCA patients in crisis and healthy control groups (p < 0.0001).
Our results were also in agreement with a previous study conducted by Franco-Penteado et al. 10 where MMP-9 levels were significantly increased in the plasma of SCD patients (20.99 ± 1.52, n=32) compared to healthy controls (13.96 ± 1.64, n=16, p=0.02).
Gümüs et al. 11 found significantly higher MMP-9 levels in patients with thalassemia major compared to healthy controls (p = 0.042, respectively).
In this study, patients with serum ferritin >1000 ng/ml had significantly higher levels of MMP-9 than those with serum ferritin <1000 ng/ml (197.5 versus 93.9 respectively). Also there was highly significant correlation between serum ferritin and MMP-9 (r = 0.23, p < 0.05).
The effects of iron on MMPs expression have previously been demonstrated. In normal fibroblasts, iron has been shown to increase MMP-1, 2, 3 and 9 expressions in culture via JNK2 activity. Iron could also stimulate MMP-2 activity in rat hepatic stellate cells leading to an increase in matrix degradation. Furthermore, increased amount of iron released in serum of patients with chronic venous disease could lead to an over-expression of MMP-9 and increased tissue destruction. In agreement with these previous studies, Kaomongkolgit et al. 12 provided new evidence that iron could also upregulate MMP-9 expression in head and neck squamous carcinoma cell lines.
Higher MMP-9 levels in patients with high serum ferritin level has been explained by Zamboni et al. 13 who reported that the iron-driven pathway is one of the recognized mechanisms of MMP hyperactivation and suggested that, the iron-driven pathway as a further mechanism for MMP hyperexpression leading to tissue lesion.
Our results showed that patients receiving iron chelation had significantly higher levels of MMP-9 than those who did not receive iron chelation therapy <1000 ng/ml (181.5 versus 76 respectively). This can be attributed to iron overload that necessitated chelation.
Vaso-occlusion is the most common outcome and the hallmark of SCD. It is responsible for many clinical complications of SCD, including osteomyelitis, osteonecrosis, stroke, splenic infarct, renal insufficiency, and acute chest syndrome. Endothelial damage plays a crucial role in VOC occurrence in SCD. 1
Our study showed that, patients with vaso-occlusive crises (VOC) > 5/year had significantly higher levels of MMP-9 than those with VOC ≤ 5 /year.
Carvalho et al. 9 assessed 129 SCD steady-state patients (SP), 23 SCD in crisis patients (CP), and 67 healthy individuals (HC) age- and sex-matched with patients groups. They found that Steady-state SCD patients had the highest values of MMP9 concentration compared with CP and HC groups (p < 0.0001). However, crisis-state SCD patients had the lowest levels of MMP9 (p < 0.0001).
In a more recent study, Carvalho et al. 8 found both Steady-state SCD patients and SCD patients in crises had higher levels of MMP-9 than healthy controls. However there was no statistical significant difference between Steady-state SCD patients and SCD patients in crises.
One of the limitations of our study was that we did not measure MMP-9 in SCA patients during crises. We only evaluated the relationship between MMP-9 levels and the frequency of VOC/ year.
The finding of difference in inflammatory markers level in steady state and crisis SCD patients suggest that these markers can be used to monitoring patients and to predict crisis events.
While ECG, echocardiography, and serum ferritin levels are very easy and cheap methods for detection of cardiac iron overload, they are either not specific for cardiac overload or not enough sensitive for the detection of iron in the heart. Since iron is able to reduce the magnetic resonance signal, the parameter called T2 measures the amount of iron in heart as the loss of signal in iron-loaded tissue. cMRI-T2, which is based on the evaluation of T2, is the most efficient diagnostic technique for the evaluation of cardiac iron overload. T2* values lower than 20 ms indicate presence of iron overload and, as T2* changes according to the concentration of iron, values lower than 10 ms indicate the presence of severe iron overload with high risk of developing cardiac dysfunction within 1 year. 14
Indeed, prolonged increase in myocardial fibrotic activity results in stiffening of the heart and is an indicator of adverse outcomes related to systolic and diastolic dysfunction, as well as arrhythmogenesis. Regarding fibrosis, matrix metalloproteinases (MMPs), that are important for pathophysiological tissue repair, appear to be involved in cardiac remodeling. The finding that an iron-binding protein like Lipocalin-2 is induced in a porcine model of heart failure and associates with MMP-9 suggest that iron may also directly influence heart fibrotic response. 15
Though we have found a significant relationship and correlation between MMP-9 and serum ferritin. Yet, we did not find a significant correlation between MMP-9 and cardiac T2* (r = 0.09, p >0.05).
This can be attributed to normal cardiac T2* in all patients with a mean of 34.2 ms and a range of 23.3-50.9 ms and also can be owed to small sample size in our study.