Clonal hematopoiesis in acquired aplastic anemia

Abstract

Clonal hematopoiesis (CH) in aplastic anemia (AA) has been closely linked to the evolution of late clonal disorders, including paroxysmal nocturnal hemoglobinuria and myelodysplastic syndromes (MDS)/acute myeloid leukemia (AML), which are common complications after successful immunosuppressive therapy (IST). With the advent of high-throughput sequencing of recent years, the molecular aspect of CH in AA has been clarified by comprehensive detection of somatic mutations that drive clonal evolution. Genetic abnormalities are found in ∼50% of patients with AA and, except for PIGA mutations and copy-neutral loss-of-heterozygosity, or uniparental disomy (UPD) in 6p (6pUPD), are most frequently represented by mutations involving genes commonly mutated in myeloid malignancies, including DNMT3A, ASXL1, and BCOR/BCORL1 Mutations exhibit distinct chronological profiles and clinical impacts. BCOR/BCORL1 and PIGA mutations tend to disappear or show stable clone size and predict a better response to IST and a significantly better clinical outcome compared with mutations in DNMT3A, ASXL1, and other genes, which are likely to increase their clone size, are associated with a faster progression to MDS/AML, and predict an unfavorable survival. High frequency of 6pUPD and overrepresentation of PIGA and BCOR/BCORL1 mutations are unique to AA, suggesting the role of autoimmunity in clonal selection. By contrast, DNMT3A and ASXL1 mutations, also commonly seen in CH in the general population, indicate a close link to CH in the aged bone marrow, in terms of the mechanism for selection. Detection and close monitoring of somatic mutations/evolution may help with prediction and diagnosis of clonal evolution of MDS/AML and better management of patients with AA.

Figure 1

Immune-mediated mechanism of AA and clonal evolution. (A) AA is thought to be initiated by recognition and destruction of HSCs by CTLs, which recognize some unknown antigen present on HSCs via their HLA class I molecule. Supporting this hypothesis is a limited usage of T-cell receptor Vβ subsets at diagnosis, suggestive of the presence of oligoclonal expansion of CD8⁺CD28⁻ T cells, which diminish or disappear with successful IST.^- Overproduction of inflammatory cytokines, including interferon γ (IFNγ) and tumor necrosis factor α (TNFα), from aberrantly activated immune cells and stromal microenvironments is also suggested to make a significant contribution to BM failure, in which the role of FAS-mediated apoptosis has been implicated. (B) During and/or after immune-mediated BM destruction, a rapid expansion of residual cells (which escaped destruction) occurs, whereby cells carrying mutations achieve clonal dominance and may progress to malignant proliferation. CTL, cytotoxic T cell; HSC, hematopoietic stem cell; IST, immunosuppressive therapy.

Figure 2

Overlaps between AA, PNH, and MDS. Although conceptually representing discrete disease entities, AA, PNH, and MDS frequently coexist or show mutual transitions within a same patient, apart from the ambiguity in actual diagnosis due to the lack of conclusive evidence or misdiagnosis because of resembling clinical presentations. Immune-mediated BM destruction can occur at the same time of emergence of PNH or evolution of malignant clones, causing a diagnostic overlap between these different disease concepts. Increased ring sideroblasts (RS) are rarely seen in AA. ICUS, idiopathic cytopenia of undetermined significance.

Figure 3

Genetic alterations in AA. (A) Somatic mutations and other genetic lesions in AA. Mutations and CNAs indicated on the left are shown for individual patients (shown horizontally). Frequency of each genetic lesion is presented on the right. (B) Frequency of mutations in AA, MDS, and ARCH, according to the reports from Yoshizato et al, Haferlach et al, and Jaiswal et al, respectively, for which the denominators are the total numbers of mutations in each report. ARCH, age-related CH; CHIP, CH with indeterminate potential; NIH, National Institutes of Health.

Figure 4

A possible model for clonal evolution in AA. Upon immune-mediated destruction of BM, some clones are thought to be resistant to the inciting autoimmune insult and/or show faster cycling/less apoptosis than others upon BM recovery to achieve a clonal dominance. In some cases, the dominant clones, especially those having DNMT3A, ASXL1, and other unfavorable mutations, increase their clone size, giving rise to more selectively dominant clones therein. In other cases, typically those carrying PIGA mutations (and possibly BCOR/BCOR mutations), initially dominant clones may regress or remain stable over years.

Figure 5

Putative mechanism of immune escape of 6pUPD-positive clones. (A) A schematic diagram of 6pUPD, which is thought to result from a recombination between 2 homologous chromosomes, invariably involving 6pter distally. (B) Breakpoint mapping of 6pUPD in different patients, showing a prominent breakpoint cluster within the HLA class I region. Breakpoints in critical cases are shown by arrows. (C) 6pUPD-positive (+) HSCs permanently lose the relevant HLA class I molecule required for the presentation of the putative antigen and are thereby thought to escape destruction by CTLs. The 6pUPD-mediated mechanism for immune escape in AA is also supported by the presence of multiple independent 6pUPD clones affecting the same parental HLA allele in some patients.

Figure 6

Clonality and MDS diagnosis in AA and other refractory cytopenia. (A) Clonality (orange) is common (>50%) in AA when assessed by gene mutations and other genetic lesions, such as cytogenetic abnormalities, using advanced genomics. However, the clonal evolution does not necessarily mean the development of diseases, especially malignant ones, such as MDS. According to the current World Health Organization (WHO) criteria, patients with AA are diagnosed as having MDS when cytopenia is present and clonality is evidenced by MDS-specific cytogenetic abnormalities, such as monosomy 7, even in the absence of dysplasia or increased blast counts (pink circle). Their clone size is not relevant as long as they are found at ≥2 metaphases. On the other hand, not all clonality is associated with MDS diagnosis (and vice versa). For example, isolated abnormalities of trisomy 8, del(20q), and –Y, are not considered to be sufficient evidence of MDS diagnosis without morphological evidence or increased blast counts, because these isolated lesions may be found in normal individuals and do not likely to correlate with typical MDS pictures. Similarly, without other evidence of MDS, somatic mutations, including those affecting common targets of myeloid malignancies, are not thought to be evidence of malignant clonal evolution by themselves. In fact, in many AA cases, somatic mutations or other clonality can be compatible with normal or almost normal blood counts. By contrast, patients with unfavorable mutations (blue circle) are more likely to show poor prognosis and to satisfy the WHO criteria for MDS/AML during their clinical course than patients with other mutations. (B) A parallel relationship is also found in unexplained cytopenia in the general population. According to the WHO criteria, the clonality (cytogenetic) criteria of MDS are reserved for MDS-specific lesions, which suffice for the diagnosis of MDS, even without evidence of dysplasia or increased blast counts. Those patients with unexplained refractory cytopenia having no evidence of MDS (between red and blue circles) are designated as ICUS. Some ICUS patients show evidence of clonal evolution (CCUS), and recent reports have suggested some similarity between CCUS and MDS (see main text), for which further confirmation is needed. CCUS, clonal ICUS.