https://orcid.org/0000-0002-4648-5926
In this issue of Blood, Pershad et al1 report longitudinal analysis of clonal hematopoiesis (CH) at variant allele fraction (VAF) as low as 0.5% in 6976 participants enrolled on the Women’s Health Initiative Long Life Study (LLS). The work highlights distinct determinants that contributed to expansion of CH clones over a period of 16 years: the initial clone size, mutated gene, germ line polymorphisms, telomere length (TL), and inflammatory environment. Increased clonal expansion was associated with increased risk of cytopenia, leukemia, and all-cause mortality (see figure).
Factors impacting rate of CH clonal expansion. Participants in the Women’s Health Initiative were screened for CH at baseline and after 15 years using sequencing with minimal VAF of 0.5%. Rate of expansion correlated with increased risk of cytopenia, leukemia, and all-cause mortality. Important determinants of expansion included initial clone size, gene mutated, germ line polymorphisms, TL, and IL-6 signaling.
Factors impacting rate of CH clonal expansion. Participants in the Women’s Health Initiative were screened for CH at baseline and after 15 years using sequencing with minimal VAF of 0.5%. Rate of expansion correlated with increased risk of cytopenia, leukemia, and all-cause mortality. Important determinants of expansion included initial clone size, gene mutated, germ line polymorphisms, TL, and IL-6 signaling.
CH of indeterminate potential (CHIP), defined as somatic mutations in specific genes associated with malignancies detected in people without cytopenia or hematologic diseases at a VAF of ≥2%, is associated with increased risk of malignancy, cardiovascular disease, and all-cause mortality. The expansion of CHIP in an individual’s blood over time is etiologic in the development of hematologic cancers, however, longitudinal studies of CH to identify predictors of clonal expansion are challenging and limited by a lack of available serial samples in large cohorts for analysis. As a result, only a few studies have investigated clonal expansion over time, mostly using whole-genome sequencing approaches, to infer gene-specific fitness effects from single time points.2,3 Longitudinal dynamics of CH derived from serial sampling have identified mutations with the highest growth rates and risk of malignancy,4 but correlation with other determinants of clonal expansion remains poorly explored.
Here, the authors sequenced a large cohort of individuals at 2 time points for CH using a targeted panel covering 15 genes. Their data corroborated the important gene-specific differences in clonal expansion that occur over time and influence risk of leukemia and survival; DNMT3A and ZNF381 exhibited the slowest growth rates, whereas JAK2 and SF3B1 more rapidly expanded. Interestingly, clonal growth was dependent on clone size at baseline. Dynamics of CH detected at initial VAFs >0.5% but <2% (termed micro-CH) were variable: approximately half of mutations expanded to VAFs >2% at longitudinal examination (LLS), whereas the other half either disappeared or were stable, even those in genes of poor prognosis (eg, TP53). In contrast, CH detected at VAFs >2% at baseline mostly persisted or expanded.
The more novel aspects of this work include further characterization of germ line determinants influencing CH expansion, which may in turn impact cell function. The TERT rs2853677, TCLA1 rs2887399, and MSI2 rs11868362 polymorphisms, all previously associated with CHIP prevalence and/or expansion by genome-wide association studies, were associated with a lower rate of expansion of gene-specific mutated clones; this likely explains the variable patterns of clonal expansion involving mutations in the same gene. Also, TL at baseline, measured in ∼20% of individuals by Southern blot, was associated with presence of PPM1D and DNMT3A mutations. These findings are supported by previous studies showing that patients with telomere biology disorders due to germ line variants in telomere-related genes, and those very long telomeres and with cancer predisposition syndromes due to POT1 germ line mutations, have a clonal signature involving PPM1D and DNMT3A, respectively.5,6 TL is also a marker of a cell’s proliferative history and has been previously associated with clonal expansion.7,8 Pershad et al found that only TET2 clonal expansion was associated with increased TL shortening.
Numerous studies have highlighted the association of inflammatory cytokines and CH, in particular interleukin-1b (IL-1b), tumor necrosis factor-α, and IL-6, though directionality remains unclear.9 In this study, the IL6R p.Asp358Ala polymorphism, which has previously been shown to attenuate classical IL-6 signaling by increasing soluble IL6 receptor (sIL6R) levels, and to predict treatment response to tocilizumab in giant cell arteritis,10 was associated with reduced clonal growth of TET2 clones. In addition, higher sIL6R and IL-6 levels were associated with reduced and greater TET2 clonal expansion, respectively, highlighting the complex interplay of germ line genetics on the environment and on clonal selection.
From a practical standpoint, these data highlight the importance of monitoring clonal dynamics over time in patients with CH and the value of developing dynamic scoring systems. Rate of clonal expansion was a predictor of malignant progression and mortality, contributing a significant effect to the leukemia prediction using the CH risk score. From a therapeutic standpoint, modulation of cytokines to decrease clonal progression is attractive with clinical trials currently underway, such as the IMPACT study, which is assessing the efficacy IL-1B in high-risk individuals with cytopenia of uncertain significance (ClinicalTrials.gov identifier: NCT05641831). Tocilizumab, an IL-6 inhibitor has been shown to slow expansion of TET2 mutations in a nonhuman primate model. Given the high prevalence of CH, particularly DNMT3A and TET2, in hematologic and inflammatory disorders, and the direct impact of clonal growth in predicting cytopenia and malignant evolution, further studies examining the effect of immune modulation on CH dynamics may help to refine risk stratification algorithms.
Conflict-of-interest: The authors declare no competing financial interests.
