CHIP: is clonal hematopoiesis a surrogate for aging and other disease?

Somatic mutations are an unavoidable consequence of aging tissues. Even though most mutations are functionally silent, some may affect genes critical to proper tissue self-renewal and differentiation, resulting in the outgrowth of affected cells, also known as clonal expansion. In hematopoietic tissue such clonal dominance is known as clonal hematopoiesis (CH). Sporadic CH is frequent in aging and affects over 10% of individuals beyond the fifth decade of life. It has been associated with an increased risk of hematologic malignancies and cardiovascular disease. In addition to aging, CH has been observed in other hematologic conditions and confers an adaptation of hematopoietic stem cells (HSCs) to various environmental stressors and cell-intrinsic defects. In the presence of extrinsic stressors such as genotoxic therapies, T-cell-mediated immune attack, or inflammation, somatic mutations may result in augmentation of HSC fitness. Such attuned HSCs can evade the environmental insults and outcompete their unadapted counterparts. Similarly, in inherited bone marrow failures, somatic mutations in HSCs frequently lead to the reversion of inherited defects. This may occur via the direct correction of germline mutations or indirect compensatory mechanisms. Occasionally, such adaptation may involve oncogenes or tumor suppressors, resulting in malignant transformation. In this brief article, we focus on the mechanisms of clonal dominance in various clinical and biological contexts.