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A common HLA nonsense mutation in aplastic anemia
gesting the involvement of these alleles in the develop- ment of AA.
Loss of HLA-B expression from Exon1mut-positive leukocytes
Although Exon1mut in leukocytes is expected to result in lack of the corresponding HLA allele, the phenotype of these leukocytes is difficult to examine since the VAF of Exon1mut is very low. We previously established six induced pluripotent stem (iPS) cell clones from peripheral blood monocytes of an AA patient (UPN 333) whose monocytes included approximately 60% HLA-A24+Bw6– cells (Figure 6A).17 Deep sequencing revealed the presence of Exon1mut in sorted HLA-A24+Bw6– cells and also in one (clone C1) of the six iPS cell clones. When a wild-type iPS clone (clone E1) and clone C1 were induced to differentiate into CD34+ cells, all clone E1-derived CD34+ cells expressed HLA-Bw6 (B5401), while all clone C1-derived CD34+ cells lacked HLA-Bw6 (Figure 6B). The ddPCR assay using DNA from wild-type and Exon1mut-positive iPS cell-derived CD34+ cells revealed that the VAF of Exon1mut were 0.041% and 49%, respectively, as expected (Figure 6C).
A
Clinical characteristics of aplastic anemia patients with Exon1mut
Of the 291 patients whose peripheral blood samples were examined for Exon1mut and GPI– cells before treat- ment, 151 were evaluable for response to immunosup- pressive therapy (CsA alone, n=68; CsA+rATG, n=83). The other 140 patients were excluded from the analysis of the relationship between the response to immunosuppres- sive therapy and the presence of Exon1mut or GPI– cells because no data on the response to immunosuppressive therapy were available in 84, and the remaining 56 received no treatment (n=25) or treatments other than immunosuppressive therapy (n=31), such as anabolic steroids and thrombopoietin receptor agonists, and allo- geneic stem cell transplantation. An increase in GPI– cells was noted in 76% (34/45) of patients with Exon1mut and in 76% (81/106) without Exon1mut (P=1.0). In terms of response to immunosuppressive therapy, 82% (37/45) of patients with Exon1mut responded to CsA (n=14) or CsA+rATG (n=23), while 75% (79/106) of those without Exon1mut responded to CsA (n=35) or CsA+rATG (n=44) (P=0.40). The response rate to immunosuppressive thera-
B
Figure 5. HLA alleles that acquired Exon1mut. (A) The number of cases that acquired Exon1mut in the corresponding HLA alleles are shown. HLA numbers shown below the bar graph denote the HLA class I supertypes to which each allele belongs. (B) Proportions that the 12 HLA alleles account for in the different populations. AA: aplastic anemia; 6pLOH: copy number neutral loss of heterozygosity of the short arm of chromosome 6.
haematologica | 2021; 106(6)
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