Here we present diverse lines of evidence that long-term molecular effects of doxorubicin action in ALL survivors include changes of miRNA abundance in circulation that may contribute to the development of cardiomyopathy, a major life-threatening long-term side effect of anthracycline treatment21,22. We sequenced miRNA to find those differentially expressed between ALL survivors and healthy subjects, characterized miRNA distribution between total plasma and extracellular vesicles, and used several bioinformatic tools to suggest processes active in these subjects, Subsequently, we compared our findings with those previously described in cardiomyopathy patients. Finally, we searched for correlations between identified miRNA and discrete echocardiographic parameters, that may be suggestive of incipient cardiac dysfunction.
First, we confirmed that circulating miRNAs that are differentially expressed in ALL survivors in comparison to healthy people may indicate transcriptional alterations related to cardiac disease development.
KEGG enrichment analyses revealed that differentially expressed miRNA in EVs as well as miRNA that are differentially distributed between plasma and EVs are related to ‘dilated cardiomyopathy’ or ‘arrhythmogenic cardiomyopathy’. Both morphological and functional changes in doxorubicin-induced cardiomyopathy have been reported as similar to those of dilated cardiomyopathy. It involves presence of fibrotic areas, myofilaments loss with visible Z-discs disorganization. In advanced pathology, chambers dilation is present with concomitant reduction of ejection fraction and diastolic dysfunction23. We also found other KEGG terms significantly related to DCM, like ‘ERBB signaling’ which pathway plays a key role in maintaining cardiac structure 24,25 as well as in restoring cardiac function after injury26. Its postnatal disruption leads to dilated cardiomyopathy27 and sensitizes heart to drug-induced toxicity28. In addition, molecular pathways related to cardiac rhythm and contraction are altered in ALL survivors, which is indicated by KEGG terms arrhythmogenic right ventricular cardiomyopathy’, ‘axon guidance’ or ‘neurotrophin signaling’, which is essential for normal cardiac rhythm through the regulation of cardiac Ca2+ cycling29–31. This remains in line with reports showing that doxorubicin affects cardiac electrophysiological properties and may cause various type of arrhythmias23. Analysis of gene ontologies further supports this finding as among significant ontologies we identified such as related to heart, muscle, cardiocyte, cardiac structures (ventricle, valve), endocardial cushion and cardiac contraction and relaxation.
Our analyses of molecular pathways that are disturbed in ALL survivors revealed that differentially expressed miRNA are involved in the regulation of pathways related to DNA damage, which belongs to canonical effects of doxorubicin action involved in cardiac complications8 and to pathological cardiac remodeling like NFKβ and TNFα signaling32. Among processes regulated specifically by miRNA differentially expressed in EVs we identified epithelial-to-mesenchymal- transition (EMT), process linked to therapy-triggered fibrosis33,34 and senescence, which was described as a consequence of genotoxic treatment35,36 and was suggested to reinforce long-term cardiac complications of anticancer treatment37.
Additionally, both this and GO analysis show that particularly miRNAs encapsulated in EVs are involved in TGFβ signaling. TGFβ is a master regulator of EMT 38,39, which expression can be increased in the heart tissue many weeks after doxorubicin treatment40. It was shown that maintaining balance within this pathway is critical for cardiac contractile function, sarcomere kinetics, ion-channel gene expression, and cardiomyocyte survival41,42.
Multiple of differentially expressed miRNAs in our study correlate with cardiac function parameters in ALL survivors, which supports the role of particular miRNAs in cardiac system functioning. However, due to redundancy of miRNA in transcriptomic network and interrelatedness between echocardiographic variables, it is difficult to identify miRNAs predictive of cardiac system function. Therefore, we used another approach, based on selecting the set of miRNAs that expression is most variable between groups in each of compartments. Strikingly, among plasma most variable miRNAs, let-7g-5p, correlated with the highest number of echocardiographic parameters, has been reported by Fu et al as involved in cardiac cells response to doxorubicin43.
We also show that similarities in miRNA expression between ALL survivors and patients with advanced, clinically manifested cardiomyopathies exist, despite that subjects in our studied group have not developed any significant cardiac phenotype yet, most probably due to young age and short time span between doxorubicin exposure and sample collection.
Of the most variable miRNA set, miR-144, miR-10b and miR-101 are common for plasma and EVs in ALL survivors and ICM/DCM patients. miR-144-3p, having the highest statistical significance in EVs, is crucial for cardiac function as its loss worsened heart failure phenotype resulting with impaired late remodeling and decreased LVEF44. Moreover, mir-144 was identified as an important regulatory node in DCM45 and its expression was down-regulated both in samples from DCM patients and in a doxorubicin-induced rodent model of cardiomyopathy46. Another study supporting important role of mir-144 shows that its loss resulted in ventricular dilation and impaired contractility, whereas intravenous delivery of this miRNA reduced infarcted area and improved cardiac function including LV fractional shortening, end-systolic volume, end-diastolic volume and ejection fraction47. Our study shows that miR-144-3p is positively correlated with the highest number of cardiac parameters including ejection fraction, a parameter that is used to define and to monitor the progress of anthracycline-induced cardiac disease48.
Interestingly, only vesicular expression of this miRNA is informative in terms of cardiac functioning. This phenomenon might be related to the specificity of both the EVs packaging and release as well as EVs uptake – being precise vesicular mir-144 might have either slightly different cellular origin or target than that circulating outside EVs. Abundance of RNA in extracellular space depends among others on the cellular system of RNA binding proteins (RBP) that are part of the cell cargo packing and exporting system, shown to be affected by doxorubicin49. E.g. doxorubicin changes expression of RBPs in rodent cardiomyocytes and in human induced pluripotent stem cell-derived cardiomyocytes. Additionally, our analyses reveal that especially miRNA that are differentially distributed between plasma and vesicles (i.e. miRNA that are more or less abundant in EVs than in plasma when compared to healthy people) in ALL survivors indicate processes related to cardiomyopathy. Of note, the term ‘protein secretion’ was present among enriched hallmarks. This supports the notion that changes in miRNA presence that we observe are closely linked to the alterations in RNA secretory mechanisms, possibly related to RNA binding proteins.