Clonal Hematopoiesis in Patients With ASCVD

Despite major progress in the pharmacological management of traditional cardiovascular risk factors, cardiovascular disease (CVD) remains the leading cause of mortality worldwide. There is increasing evidence that a substantial residual risk of atherosclerotic cardiovascular disease (ASCVD) persists even in patients who have achieved an apparently optimal control of conventional modifiable risk factors.^[1] Moreover, noninvasive imaging studies suggest that more than one-third of asymptomatic middle-aged individuals with a very low cardiovascular risk profile may exhibit significant atherosclerosis,^[2] the underlying cause of the most frequent cardiovascular and cerebrovascular disorders. Therefore, although targeting well-established cardiovascular risk factors is still crucial, it is also evident that identifying and characterizing new nonconventional risk factors for atherosclerosis is essential to improve the prevention of CVD. In this regard, clonal hematopoiesis is emerging as a new and quite common risk factor for ASCVD.^[3] This condition occurs when a substantial proportion of an individual's blood cells is derived from a single hematopoietic stem cell clone. It is most frequently driven by the random acquisition of somatic mutations that provide a selective advantage to the mutant hematopoietic stem cell, allowing for the mutant cells to expand throughout the hematopoietic system and its progeny, including immune cells. Thus, individuals with clonal hematopoiesis carry a substantial proportion of immune cells that harbor the acquired mutation, which evidently has a high potential of affecting the inflammatory responses that are at the center of CVD. When clonal hematopoiesis is driven by single nucleotide variants or small insertions/deletions in known hematological malignancy-related genes, it is often referred to as clonal hematopoiesis of indeterminate potential (CHIP). Genetic analyses in humans and experiments in mice suggest that some CHIP mutations are causally connected with an increased risk of ASCVD, independently of conventional cardiovascular risk factors.^[3–6] This has sparked stimulating discussions on the translation of this knowledge into new preventive strategies against ASCVD. Indeed, several institutions have created specialized clinics to counsel patients with CHIP mutations.^[7] However, CHIP screening is not yet recommended in the context of CVD, because there is a lack of evidence-based interventions to mitigate the heightened CVD risk associated with carrying these mutations. Experimental studies in mice have identified several mechanisms linking CHIP mutations to accelerated atherosclerosis development,^[4–6] which represent potential targets for the development of personalized preventive strategies tailored to CHIP mutation carriers. The most promising target is likely the connection between somatic mutations in the epigenetic regulatory gene TET2 and IL-1β–driven inflammation,^[5,6] which is supported by retrospective analyses in the CANTOS clinical trial.^[8] Nevertheless, the translation of these research findings into clinical practice requires validation through prospective clinical trials. The proper design and execution of such trials depend on resolving several pending issues about the relationship between CHIP and atherosclerosis. In this context, a relevant outstanding question in this field concerns the frequency and effects of CHIP mutations in patients with established ASCVD, because clinical trials in individuals with CHIP would most likely begin in the context of secondary prevention.

In this issue of the Journal of the American College of Cardiology, Gumuser et al^[9] expand our understanding of the clinical implications of CHIP in such patients by reporting the effects of CHIP mutations on clinical outcomes in a large population of individuals with ASCVD. Previous smaller studies had shown that CHIP mutations are associated with adverse clinical progression of patients with ischemic or nonischemic heart failure,^[10–12] ST-segment elevation myocardial infarction,^[13] and cardiogenic shock.^[14,15] In the current study, the authors analyzed whole exome sequencing (WES) data from 13,129 participants in the UK Biobank affected by a broader set of atherosclerotic conditions at enrollment, including coronary artery disease, ischemic stroke, and/or peripheral artery disease. Then, they examined the association between carrying CHIP mutations at enrollment and the occurrence of secondary atherosclerotic events or death over a median follow-up of 10.8 years. Three major findings emerged from this endeavor. First, carrying CHIP mutations with an allelic fraction (VAF) >10% (ie, >20% mutant blood cells, assuming the mutations are monoallelic) was clearly associated with a greater risk of a composite outcome of all-cause death or secondary ASCVD events, with consistent associations observed for all-cause death and ASCVD, separately. Second, in gene-specific analysis, mutations in the epigenetic regulatory gene TET2 or splicing-related genes SF3B1/SRSF2/U2AF1 were most strongly associated with adverse clinical outcomes in ASCVD patients. Considering the frequency of TET2 mutations, their strong impact on outcomes, and available knowledge derived from preclinical models,^[5,6] carriers of TET2 mutations may represent an ideal target population for prospective clinical trials. Third, the association between CHIP mutations and adverse outcomes was generally independent of high-sensitivity C-reactive protein values and neutrophil-to-leukocyte ratios, suggesting that the increased ASCVD risk associated with CHIP is not captured by these frequently used biomarkers of the overall inflammatory status of an individual.

Overall, this carefully executed work adds to the growing body of evidence supporting the relevance of CHIP as a cardiovascular risk factor, as well as its prognostic value in patients with established CVD. A major strength of this study is its large sample size, which provided sufficient statistical power to examine some gene-specific relationships between CHIP and secondary adverse clinical outcomes. A weakness to consider is the limited sensitivity to detect CHIP based on WES data, which have modest sequencing depth. Although WES provides good capability to detect mutant clones with >5% VAF (ie, over 10% mutant blood cells), it may miss many smaller clones, which have been shown to have clinical consequences in previous studies.^[10–13] Additionally, small differences in variant interpretation and filtering can significantly affect the results when using WES data to study CHIP, as illustrated by recent studies conducted by independent groups that arrived at somewhat conflicting conclusions about the magnitude of the association between CHIP and ASCVD and its modulation by inherited genetic variation, despite utilizing the same WES data set.^[16,17] In this regard, the current study used a particularly stringent approach to calling CHIP, which may reduce the rate of false positives, but also decrease further the ability to detect relatively small mutant clones, as rightfully acknowledged by the authors. This possibility is supported by the fact that only one-third of the CHIP mutations detected in this study had a VAF lower than 10%, in contrast to previous high-sensitivity analyses showing that such relatively small mutant clones account for most CHIP mutations in both CVD patients and healthy individuals.^[10–13,18] Furthermore, the proportion of participants with CHIP in this study (5.1%) was much lower than those reported in previous high-sensitivity analyses of CVD patients, which ranged from 12% to 30% approximately when using a 2% VAF threshold.^[11–15] Thus, additional high-sensitivity analyses will be required to accurately determine the frequency of CHIP in ASCVD patients and the threshold of mutant clone size that defines clinically-relevant CHIP.

A notable finding in this study is the negligible impact on clinical outcomes of mutations in the epigenetic regulatory gene DNMT3A, which is the most frequently mutated gene in CHIP. This result is in line with previous WES-based studies reporting a modest effect of DNMT3A mutations on incident ASCVD,^[4,6] but contrasts with recent high-sensitivity studies that revealed a significant association between DNMT3A-mutant CHIP and a higher risk of adverse clinical outcomes in patients with ST-segment elevation myocardial infarction^[13] or ischemic heart failure.^[10,12] Moreover, experimental studies in mice suggest an important role for DNMT3A mutations in the resolution of inflammatory responses and the remodeling of atherosclerotic plaques.^[19] Given that DNMT3A mutations are by far the most prevalent CHIP mutations, clarifying their role in ASCVD and other cardiovascular conditions will be essential to grasp the clinical significance of CHIP and implement precision medicine approaches accordingly.

In summary, the novel results reported by Gumuser et al^[9] provide strong evidence of the significant clinical impact of certain CHIP mutations in patients with established ASCVD. The findings of this work bring us one step closer to designing and executing prospective clinical trials that test personalized preventive care strategies tailored to carriers of specific CHIP mutations.