HLA Class I Allele-Lacking Hematopoietic Stem/Progenitor Cells Support Long-Term Clonal Hematopoiesis without Oncogenic Driver Mutations in Acquired Aplastic Anemia
Authors: Imi Tatsuya , Katagiri Takamasa , Hosomichi Kazuyoshi , Nakagawa Noriharu , Zaimoku Yoshitaka , Ishiyama Ken , Tajima Atsushi , Nakao Shinji 
Published: November 2019

Abstract
[Background] Clonal hematopoiesis is currently known to be common in patients with acquired aplastic anemia (AA). One of the most common abnormalities underlying clonal hematopoiesis in AA patients is copy-number neutral loss of heterozygosity (LOH) in the short of 6 chromosome (6pLOH) caused by acquired uniparental disomy. Hematopoietic stem/progenitor cells (HSPCs) having undergone 6pLOH are thought to evade attack by cytotoxic T lymphocytes (CTLs) specific to auto-antigens by lacking particular HLA-A alleles. These HSPCs then produce HLA class I allele-lacking [HLA(-)] leukocytes to support hematopoiesis in patients with AA patients in remission. Our recent study showed that HLA(-) granulocytes are detected in about 24% of newly-diagnosed AA patients, and the aberrant granulocytes often account for more than 95% of the total granulocytes and persist for many years. The sustainability of 6pLOH(+) HSPC clones suggests that these HSPCs may suffer from secondary somatic mutations that confer a proliferative advantage on them over normal HSPCs. Alternatively, 6pLOH(+) HSPCs may persist and continue to support hematopoiesis according to their inherent sustainability, just like the PIGA mutant HSPCs we previously described (Katagiri et al. Stem Cells, 2013). To test these hypotheses, we determined the sequences of genes associated with the clonal expansion of HSPCs in HLA(-) granulocytes. [Patients and Methods] Eleven AA patients whose percentages of HLA(-) granulocytes ranged 6.4%-99.8% (median 94.2%) of the total granulocyte population were chosen for this study. The patients (male/female, 5/6 and age 27-79 [median 53] years) had been diagnosed with severe (n=5) or non-severe (n=6) AA 2-25 [median 12.5] years earlier, and 7 and 4 patients achieved complete response and partial response, respectively after treatments with cyclosporine (CsA) alone (n=4), CsA+antithymocyte globulin (ATG, n=3), CsA+anabolic steroids (AS, n=2), AS+romiplostim (n=1), and AS alone (n=1). The lineage combinations of HLA(-) cells were granulocyte, monocytes, B cells and T cells (GMBT) in 6, GMB in 4 and GM in 1. HLA(-) and normal [HLA(+)] granulocytes were sorted from the blood leukocytes of the 11 patients and the DNA of each cell population as well as that of buccal mucosa cells was subjected to target sequencing of 61 myelodysplastic syndrome (MDS)-related genes with MiSeq. DNA samples from 5 patients including 4 patients whose HLA(-) cell percentages were greater than 95% were further analyzed by whole-exome sequencing (WES) using HiSeq. The percentage of 6pLOH(+) cells in the total granulocytes or sorted HLA(-) granulocytes were estimated using digital droplet PCR or deep sequencing of HLA alleles. [Results] Target sequencing of 8 of the 11 patients revealed somatic mutations in the HLA(-) granulocytes of 3 patients. HLA(-) granulocytes-specific mutations were found in DNMT3A, PRR5L, SMC3A, and LRCH1 (Table). The variant allele frequencies (VAF) of these mutations were far lower (5.1%-20%) than those of HLA(-) granulocytes that accounted for 95% of sorted cells. WES revealed 22 non-synonymous and 9 synonymous mutations in the HLA(-) granulocytes from 4 of the 5 patientsthat included 3 new patients and 2 patients whose samples were negative for mutations revealed by the target sequencing. The VAF of these mutations ranged from 20.7-52.5% (median 44.1%, Table). Very-high VAFs of several mutant genes suggested that these mutations occurred simultaneously with or soon after the occurrence of 6pLOH. A patient who achieved remission after romiplostim therapy without ATG showed various gene mutations that were thought to have occurred after 6pLOH. Despite of their highly biased hematopoiesis supported by single or few clones, recurrent or MDS-related oncogenic mutations were not detected in any of the 11 patients. Of note, the percentages of 6pLOH(+) cells in the sorted HLA(-) granulocytes were ≤75% (36.7%, 46%, 74%, and 75%) in 4 patients, indicating the presence of granulocytes lacking HLA-A alleles through mechanisms other than 6pLOH. [Conclusions] HLA(-) HSPCs caused by 6pLOH or other unknown mechanisms support long-term hematopoiesis without the development of oncogenic driver mutations that are associated with clonal hematopoiesis of MDS; as such, clonal hematopoiesis by 6pLOH(+) HSPCs may not portend a poor prognosis.

Disclosures
Nakao: Alexion Pharmaceuticals: Honoraria, Research Funding.