Sustained clonal hematopoiesis by HLA-lacking hematopoietic stem cells without driver mutations in aplastic anemia.

Clonal hematopoiesis by hematopoietic stem progenitor cells (HSPCs) that lack an HLA class I allele (HLA HSPCs) is common in patients with acquired aplastic anemia (AA); however, it remains unknown whether the cytotoxic T lymphocyte (CTL) attack that allows for survival of HLA HSPCs is directed at nonmutated HSPCs or HSPCs with somatic mutations or how escaped HLA HSPC clones support sustained hematopoiesis. We investigated the presence of somatic mutations in HLA granulocytes obtained from 15 AA patients in long-term remission (median, 13 years; range, 2-30 years). Targeted sequencing of HLA granulocytes revealed somatic mutations (, n = 2; , , and , n = 1) in 3 elderly patients between 79 and 92 years of age, but not in 12 other patients aged 27 to 74 years (median, 51.5 years). The chronological and clonogenic analyses of the 3 cases revealed that mutation in 1 case, which occurred in an HLA HSPC with a mutation, was the only mutation associated with expansion of the HSPC clone. Whole-exome sequencing of the sorted HLA granulocytes confirmed the absence of any driver mutations in 5 patients who had a particularly large loss of heterozygosity in chromosome 6p (6pLOH) clone size. Flow-fluorescence in situ hybridization analyses of sorted HLA and HLA granulocytes showed no telomere attrition in HLA granulocytes. The findings suggest that HLA HSPC clones that escape CTL attack are essentially free from somatic mutations related to myeloid malignancies and are able to support long-term clonal hematopoiesis without developing driver mutations in AA patients unless HLA loss occurs in HSPCs with somatic mutations.