https://orcid.org/0000-0001-7580-2276
Abstract
This editorial refers to ‘Clonal haematopoiesis in chronic ischaemic heart failure: prognostic role of clone size for DNMT3A- and TET2-driver gene mutations’†, by B. Assmus et al., on page .
Clonal haematopoiesis is a prevalent condition whereby a substantial portion of mature blood cells are derived from a single dominant haematopoietic stem cell (HSC) clone.1 Many individuals with clonal haematopoiesis harbour mutations within specific ‘driver’ genes that are recurrently mutated in haematological malignancies, such as DNMT3A, TET2, and ASXL1.2 , 3 These mutations are thought to provide the HSC with a competitive advantage, allowing for its clonal expansion and giving rise to a genetically distinct population of leucocytes over time.1 Notably, the majority of individuals with clonal haematopoiesis do not exhibit overt changes in white blood cell counts, and clonal haematopoiesis cannot be simply determined by a routine blood work. Studies have shown that the frequency of these mutations rises sharply with age, thus this condition is sometimes referred to as age-related clonal haematopoiesis (ARCH). While individuals with clonal haematopoiesis display an increased risk of developing a subsequent blood cancer, epidemiological studies show that most will never develop a malignancy, as these conditions are relatively rare and generally require the acquisition of multiple mutations.4 Based upon this uncertainty in diagnostic criteria, this condition has often been referred to as clonal haematopoiesis of indeterminate potential, or CHIP.4
In 2014, independent epidemiological studies revealed that CHIP was associated with a substantial increase in the risk of all-cause mortality.2 , 3 Unexpectedly, it was revealed that this increase in all-cause mortality could, at least in part, be attributed to a large increase in the frequency and death due to atherosclerotic cardiovascular conditions, such as coronary heart disease and ischaemic stroke.3 , 5 Since this surprising discovery, there has been increased interest in better understanding the relationship between CHIP and cardiovascular disease. Indeed, recent experimental work has played an instrumental role in better understanding the relationship between these two phenomena and has shed light on mechanisms by which CHIP promotes cardiovascular disease. Specifically, it has been found that haematopoietic mutations in common driver genes, Dnmt3a, Tet2, and JAK2V671F, can accelerate experimental atherosclerosis and/or heart failure by generating a pool of myeloid cells with an augmented proinflammatory profile.5–9 Prognostic studies in humans have corroborated the experimental findings and found that ischaemic heart failure patients that carry haematopoietic mutations in DNMT3A and/or TET2 have a worse long-term clinical outcome compared with non-carriers.10 Similarly, data suggest that individuals who harbour these mutations may have greater levels of systemic inflammation compared with controls.11 Overall, recent experimental and clinical evidence suggests that CHIP represents a new casual risk factor for cardiovascular disease that is mechanistically quite distinct from the traditional risk factors that have been appreciated for decades.
From the initial epidemiological studies, CHIP was defined by the presence of an expanded blood cell clone carrying a mutation in a known driver gene of haematological malignancy at a variant allele frequency (VAF) of at least 2% without meeting the standard diagnostic criteria for malignancy (i.e. cytopenia, abnormal blood cell counts, or both).4 This distinction, while convenient, is largely based on the limits of mutation detection by sequencing methodology, rather than being based on epidemiological data or biological principles. Studies reporting an association between CHIP and cardiovascular disease have largely used a biased exome sequencing approach, limited to a VAF detection rate of ∼3.5% or higher.3 , 5 , 11 In some cases, studies were only able to detect a significant association between CHIP and cardiovascular disease in individuals with a VAF ≥10%. However, several important advances in sequencing technology have allowed researchers to examine mutations in an even greater depth. The development of error-corrected sequencing, which allows for detection of clonal events at as low as 0.03% VAF, has suggested that clonal haematopoiesis is more prevalent than previously thought.12 This raises an important question of the biological significance of smaller clones. While epidemiological studies suggest that smaller clones (<2% VAF) do not appear to pose an increased risk of haematological malignancy,3 , 13 it remains unknown what impact they may have on cardiovascular disease. Could smaller clones be pathological in the setting of cardiovascular disease and, if so, what would be the cut-off VAF at which these clones start to have clinical consequences?
In this issue of the European Heart Journal, Assmus et al. aimed to address the aforementioned questions, by defining the threshold of VAF which could impact on the prognosis of chronic ischaemic heart failure.14 The authors focused their investigation on mutations in DNMT3A and TET2, the two most commonly mutated driver genes in both aged individuals and patients with cardiovascular disease. Using error-corrected deep-amplicon sequencing, which is able to reliably detect mutations at a VAF of <0.5%, the authors analysed peripheral blood or bone marrow mononuclear cell samples from 419 chronic ischaemic heart failure patients. In accordance with previous observations, it was noted that the prevalence of clonal haematopoiesis associated with mutations in DNMT3A and/or TET2 increased with age. A receiver operator curve (ROC) analysis revealed that the cut-off VAF to predict the 5-year survival of heart failure patients was 1.15% and 0.73% for mutations in DNMT3A and TET2, respectively. Whilst the initial studies examining the clinical relationship between clonal haematopoiesis and cardiovascular disease primarily focused on risk of cardiovascular disease rather than prognosis, these VAF cut-offs are considerably lower than some previous estimates whereby a statistically significant effect was only observed for a VAF ≥10%(Take home figure).3 , 5 , 11 An earlier preliminary report by the same group found a dose-dependent relationship between clone size and clinical outcome, including clone sizes between 1% and 2% VAF; however this study was not optimized to determine the threshold of VAF at which clones start to become pathological.10 It should also be noted that this study only focused on mutations in DNMT3A and TET2, whereas previous studies examined mutations in any known candidate driver gene.3 , 5 It is therefore conceivable that smaller clones of DNMT3A and/or TET2 may be more pathological with respect to cardiovascular disease than other candidate driver mutations, although this concept warrants further investigation.
Clones with high variant allele fraction (VAF) are the tip of the iceberg.
Mechanistically, both experimental studies suggest that haematopoietic mutations in DNMT3A and TET2 result in leucocyte progeny with an altered inflammatory cytokine profile.5–8 While it may be difficult to comprehend that cytokines derived from a small clone could drive cardiovascular disease progression, the potential underlying mechanisms may not be too dissimilar to what has been observed in experimental studies. For instance, it has been noted that TET2 loss of function in haematopoietic cells results in a pool of macrophages with augmented interleukin (IL)-1β production.6–8 In a mouse model of atherosclerosis, it was found that the elevated IL-1β production by Tet2-deficient cells in the plaque wall promoted local endothelial cell activation and an up-regulation of P-selectin.6 This in turn led to the recruitment of more monocytes, regardless of genotype, into the plaque. It is known that IL-1β can stimulate its own expression by an autoregulatory feedback loop in multiple cell types15 and thus could conceivably augment IL-1β production by other cells within damaged cardiovascular tissues. Therefore, it is possible that small clones could act in a catalytic manner within damaged cardiovascular tissue, aggravating and perpetuating inflammation, resulting in further injury, although this concept warrants further exploration within a clinical setting.
The study of Assmus et al. provides an important step forward, as it reveals that small clones are also associated with a worse prognosis in chronic heart failure patients. Moreover, it challenges the quantitative definition of CHIP and its relationship to cardiovascular disease. Nevertheless, several important questions remain to be addressed by future studies. First, it will be essential to know whether these new threshold VAFs are only applicable to chronic ischaemic heart failure or whether they extend to other cardiovascular conditions, particularly other forms of heart failure. Secondly, it will be of interest to determine whether the presence of small clones with other driver mutations, such as ASXL1 and JAK2, may also lead to a poorer prognosis of chronic ischaemic heart failure. Ultimately, answering these questions may help to determine the risk of a poor prognosis following an ischaemic cardiac event and may help dictate an individual treatment plan.
Conflict of interest: M.A.E. has an American Heart Association Postdoctoral fellowship. S.S. and K.W. have no relevant conflicts to declare.
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.
Footnotes
† doi:10.1093/eurheartj/ehaa845.
References
