Clonal hematopoiesis of indeterminate potential and its distinction from myelodysplastic syndromes

Abstract

Recent genetic analyses of large populations have revealed that somatic mutations in hematopoietic cells leading to clonal expansion are commonly acquired during human aging. Clonally restricted hematopoiesis is associated with an increased risk of subsequent diagnosis of myeloid or lymphoid neoplasia and increased all-cause mortality. Although myelodysplastic syndromes (MDS) are defined by cytopenias, dysplastic morphology of blood and marrow cells, and clonal hematopoiesis, most individuals who acquire clonal hematopoiesis during aging will never develop MDS. Therefore, acquisition of somatic mutations that drive clonal expansion in the absence of cytopenias and dysplastic hematopoiesis can be considered clonal hematopoiesis of indeterminate potential (CHIP), analogous to monoclonal gammopathy of undetermined significance and monoclonal B-cell lymphocytosis, which are precursor states for hematologic neoplasms but are usually benign and do not progress. Because mutations are frequently observed in healthy older persons, detection of an MDS-associated somatic mutation in a cytopenic patient without other evidence of MDS may cause diagnostic uncertainty. Here we discuss the nature and prevalence of CHIP, distinction of this state from MDS, and current areas of uncertainty regarding diagnostic criteria for myeloid malignancies.

Figure 1

CHIP as a precursor state for hematological neoplasms. (A) A model for evolution from normal hematopoiesis to CHIP and then, in some cases, to MDS or AML. (B) Comparison of evolution patterns of MGUS, MBL, and CHIP. Hematopoietic progenitor or stem cells commonly acquire mutations throughout the human lifespan; most of these are passenger mutations that have no consequence for hematopoiesis. Certain mutations, however, confer a survival advantage to the mutated cell and its progeny and allow clonal expansion. Serial acquisition of mutations in an expanded clone can lead to a disease phenotype and ultimately morbidity and mortality. Although this article primarily discusses CHIP in the context of its distinction from MDS, CHIP can also directly progress to AML without an intervening MDS stage, and CHIP can progress to other conditions such as myeloproliferative neoplasms or lymphoid neoplasms. Just as with MGUS and MBL, the majority of patients with CHIP will never develop an overt neoplasm, and patients will eventually die of unrelated causes.

Figure 2

Definition of CHIP and its distinction from MDS and non-clonal cytopenic states. (A) A proposed definition of CHIP. A mutation that is commonly associated with clonal expansion of hematopoietic cells in older persons should be present, whereas criteria for other diagnoses should not be met. Evidence of mildly disordered erythropoiesis such as an elevated red cell distribution width or mean corpuscular volume can be compatible with CHIP rather than MDS, and occasional dysplastic cells might be seen, as is common in the general population with careful scrutiny of blood and marrow. CHIP is associated with an increased risk of all-cause mortality and subsequent diagnosis of hematological malignancy. The 19 genes most commonly mutated in healthy older adults in sequencing studies to date are listed. The roster of CHIP-associated mutations will likely change in the future, with some genes being removed and others being added. As a working definition, we propose a variant allele frequency of 2% in order to be considered CHIP (since extremely deep sequencing will detect mutations in almost every person), but this may need to be revised with further population analyses. PNH, paroxysmal nocturnal hemoglobinuria. (B) The spectrum of clonal hematopoiesis, ICUS, and MDS. ICUS is a broad category that includes a heterogeneous group of individuals, some of whom have benign (nonclonal) hematopoiesis. Other patients with ICUS may have CHIP, differing only from lower risk MDS by their lack of dysplasia and, currently, an undetermined disease risk. CHIP can also include patients with clonal hematopoiesis and nonmalignant causes of cytopenias (eg, immune cytopenias, liver disease, or nutritional deficiencies) that would not be considered to have ICUS because of the presence of a clone, but may have a distinct natural history. Obs, observation; BM, bone marrow; CCUS, clonal cytopenias of undetermined significance; HMA, hypomethylating agent (eg, azacitidine); GF, hematopoietic growth factor (eg, epoetin); IMiD, immunomodulatory drug (eg, lenalidomide); IST, immunosuppressive therapy; BSC, best supportive care; HSCT, hematopoietic stem cell transplant.