Clonal Hematopoiesis with Somatic Mutations Is a Common, Age-Related Condition Associated with Adverse Outcomes

Hematological malignancies are associated with recurrent somatic mutations in specific genes, and may be preceded by a pre-malignant state in which only the initial driver mutation(s) are present. For example, monoclonal gammopathy of unknown significance often precedes multiple myeloma and monoclonal B-lymphocytosis can precede chronic lymphocytic leukemia. Recent sequencing studies have identified genes that are recurrently mutated in acute myeloid leukemia, myelodysplastic syndromes, myeloproliferative neoplasms, acute lymphoblastic leukemia, and other hematological neoplasms. We hypothesized that a pre-malignant state comprised of a clonal expansion of cells harboring some of these recurrent mutations would be detectable in the blood of elderly individuals not known to have hematological disorders.

To address this question, we analyzed whole exome sequencing data from peripheral blood cell DNA of 17,182 individuals. Most of these were sequenced for type 2 diabetes genetic association studies and were therefore unselected for hematological phenotypes. We looked for candidate somatic variants by identifying previously characterized single nucleotide variants (SNVs) and small insertions/deletions (indels) in 160 genes recurrently mutated in hematological malignancies. The presence of these variants was analyzed for association to hematological phenotypes, survival, and cardiovascular events.

We identified a total of 805 candidate somatic variants (hereafter referred to as mutations) from 746 individuals in 73 genes. Somatic mutations were rarely detected in individuals younger than 40, but rose appreciably with age (Figure 1). At ages 70-79, 80-89, and 90-108 these clonal mutations were observed in 9.6% (220 out of 2299), 11.7% (37 out of 317), and 18.4% (19 out of 103) of individuals, respectively. The majority of the variants occurred in 3 genes: DNMT3A (403 variants), TET2 (72 variants), and ASXL1 (62 variants). The median variant allele fraction for the detected somatic mutations was 0.09, from which we infer that the pathologic clone represents on average 18% of circulating white blood cells.

Clinical outcome data was available on a subset of subjects, with a median follow-up period of 8 years. Carrying a somatic mutation was associated with increased risk of developing a hematological malignancy (hazard ratio [HR] 11, 95% confidence interval [95% CI] 3.9-33 by competing risks regression). Harboring a mutation was also associated with an increase in all-cause mortality that could not be explained by death due to hematological malignancies alone (HR 1.4, 95% CI 1.1-1.8 by Cox proportional hazards model). We further found that these mutations were associated with type 2 diabetes (odds ratio 1.3, 95% CI 1.1-1.5) and increased risk of incident coronary heart disease (HR 2.0, 95% CI 1.2-3.4) and ischemic stroke (HR 2.6, 95% CI 1.4-4.8) in multivariable regression models.

We conclude that clonal hematopoiesis associated with a somatic mutation in a known cancer-causing gene is a common pre-malignant condition in the elderly. This entity is associated with increased risk of transformation to hematological malignancy, as well as increased all-cause mortality, possibly due to increased cardio-metabolic disease. While the link between somatic mutations and cancer is well established, the relationship between clonal hematopoiesis and cardio-metabolic disease requires further study.

Figure 1 

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Prevalence of somatic mutation by age. Colored bands represent 50, 75, and 95 percent confidence intervals.

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Prevalence of somatic mutation by age. Colored bands represent 50, 75, and 95 percent confidence intervals.

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