https://orcid.org/0000-0001-5983-688X
In cardiomyocytes, BRCA1 can induce Circ-INSR up-regulation (upwards arrow) which in turn binds SSBP1, eventually leading to reduced mitochondrial fragmentation (‘no symbol’) and to a healthy heart. Up-regulation of Circ-INSR can thus significantly safeguard the heart against mitochondrial damage and the consequent cardiotoxicity induced by the anticancer drug doxorubicin.
This editorial refers to ‘A circular RNA derived from the insulin receptor locus protects against doxorubicin-induced cardiotoxicity’, by D. Lu et al., https://doi.org/10.1093/eurheartj/ehac337.
In cardiomyocytes, BRCA1 can induce Circ-INSR up-regulation (upwards arrow) which in turn binds SSBP1, eventually leading to reduced mitochondrial fragmentation (‘no symbol’) and to a healthy heart. Up-regulation of Circ-INSR can thus significantly safeguard the heart against mitochondrial damage and the consequent cardiotoxicity induced by the anticancer drug doxorubicin.
In recent years, non-coding RNAs (ncRNAs), especially microRNAs and long ncRNAs, have emerged as powerful targets for prevention and treatment of cardiovascular diseases (CVDs) in pre-clinical models and beyond.1 Indeed, the first human clinical heart failure (HF) trial that took advantage of ncRNA modulators showed promising results, thus highlighting the translational potential for a newer generation of RNA-based HF treatments.2,3 Beyond these ncRNA molecules, circular RNAs (circRNAs) are starting to receive more focus as innovative therapeutic targets in the setting of CVDs.
Anthracyclines, such as doxorubicin, are among the most effective and widely used anticancer drugs, but unfortunately their broad-spectrum cytotoxicity can affect not just the tumour, but also other metabolically active cells such as cardiomyocytes, thus leading to cardiomyopathy by many different mechanisms.4 Beside a baseline cardiovascular assessment, and strict follow-up for prevention and early recognition of doxorubicin cardiotoxicity, once anthracycline-induced HF is detected, treatment is based on legacy standard HF drugs, but specific therapeutic options to reduce or reverse cardiotoxicity are not yet available.5,6 Although protective strategies have been proposed to interfere with the nitroso-redox balance,7 with autophagy within the myocardium,8 or with pre-conditioning,9 new treatments are still needed for doxorubicin-induced cardiomyopathy.
In this issue of the Euopean Heart Journal, Lu and colleagues10 show in an elegant series of early pre-clinical studies that a circRNA derived from the insulin receptor locus may protect against doxorubicin-induced cardiac dysfunction. Specifically, the authors found that loss of this evolutionarily conserved circRNA, named Circ-INSR, is involved in the development of HF and particularly in the form resulting from doxorubicin cardiotoxicity (Graphical Abstract). In agreement with this, doxorubicin induced a down-regulation of Circ-INSR, and the loss of Circ-INSR correlated with mitochondrial damage, cardiomyocyte death, and eventually heart dysfunction in both rodents and humans. Conversely, Circ-INSR overexpression as well as administration of its in vitro transcribed form prevented doxorubicin-induced cardiac toxicity not only in vitro, in rodent and human cardiomyocytes, but also in vivo, in a mouse model of anthracycline-induced cardiomyopathy. Although the mechanism underlying Circ-INSR function is still only partially understood, Lu et al. show that this circRNA physically interacts with the single-stranded DNA-binding protein 1 (SSBP1; Graphical Abstract), a molecule involved in the maintenance of healthy and functional mitochondria, the main target subcellular organelles of doxorubicin-induced toxicity.11 By means of detailed transcriptomic and proteomic analyses, Circ-INSR was then found to reduce the mitochondrial fragmentation typical of anthracycline-induced toxicity and to preserve key metabolic pathways in cardiac myocytes, eventually protecting them from apoptosis. How doxorubicin affects Circ-INSR transcription is still largely unclear. However, the authors embarked on a series of screening assays to define what could induce the expression of this protective Circ-INSR, and identified Breast cancer type 1 susceptibility protein (Brca1) as a key upstream activator of formation of this circRNA. Whether and how Brca1 is involved in Circ-INSR circularization or other processes regulating circRNA production is presently unknown, and future studies are expected to better unveil mechanistic details of this functional interdependence. Notably, besides being a well-established tumour suppressor gene, Brca1 is as a gatekeeper of cardiac function and survival.12 More importantly, mutations in Brca1 (as well as in its homologue Brca2) not only constitute key drivers of the tumour types treated with doxorubicin, such as breast cancer, but are also associated with increased individual susceptibility to doxorubicin-induced cardiotoxicity. Animal studies show that Brca2 deficiency exaggerates doxorubicin-induced cardiomyocyte apoptosis and cardiac failure,13 but whether this is also true in patients with BRCA1/2 mutations remains controversial.14 Hence, future studies are awaited to define to what extent BRCA1/2 alterations can account for predisposition to both cancer development and anthracycline-induced cardiomyopathy.
While the up-to-date technologies used by Lu et al. show clear beneficial effects of Circ-INSR up-regulation, several caveats should still be considered. For example, most of the studies in cardiomyocytes were conducted either in induced pluripotent stem cell (iPSC)-derived cardiomyocytes or in neonatal rat cardiomyocytes that generally display a foetal-like metabolism, essentially relying on anaerobic glycolysis rather than mitochondrial respiration which is, however, one of the main cellular processes affected by doxorubicin.11 This issue was mitigated by in vivo studies in adult mice, but such experiments with doxorubicin dosing schemes adapted to rodents might represent only a partial proxy of what is observed in patients. In the same vein, authors have not assessed the efficacy and safety of the Circ-INSR-based therapy in tumour-bearing animals which is, from a therapeutic perspective, pivotal to exclude any impact on the anticancer action of chemotherapy or on the growth/relapse of the tumour itself. This is particularly relevant for a Circ-INSR up-regulation approach considering the prominent role of the host gene INSR in cancer development and progression.15 Another potential issue to consider for translational studies is the finding that the induction of Circ-INSR up-regulation in vivo seems to require high efficiency of viral transduction. This suggests that a gene therapy approach might suffer from lack of efficacy due to insufficient overexpression. On the other hand, in vitro transcribed Circ-INSR was able to provide pharmacological activity at least in in vitro assays, eventually supporting the future possibility of formulating a therapeutic RNA or RNA derivative to be delivered without a viral vector.
Overall, the body of pre-clinical data provided by the study by Lu et al.10 altogether suggests a strong translational potential of Circ-INSR. This novel approach may retain its potential not only in doxorubicin-induced cardiomyopathy but also in response to other cardiotoxic common anticancer agents, such as, for example, taxanes. Further detailed investigations are warranted in this context, but the group of Thomas Thum has unveiled a novel avenue towards treatment of cardiotoxicity which deserves to be pursued in a further drug discovery process as well as clinical testing.
Data availability
No new data were generated or analysed in support of this research.
References
Author notes
The opinions expressed in this article are not necessarily those of the Editors of the European Heart Journal or of the European Society of Cardiology.
Conflict of interest: C.G.T. has received funding from Amgen and personal fees from Vivalyfe, outside of the submitted work, and is listed as an inventor on two heart failure patents. E.H. and A.G. are founders, shareholders, and advisors of Kither Biotech Srl, a pharmaceutical product company developing PI3K inhibitors for the treatment of respiratory diseases not in conflict with statements made in this article.
