Germline MPO Variants Predispose to Myeloid Neoplasia: Potential Mechanisms Suggested By In Vivo and in Vitro Studies

In recent decades, there has been a substantial increase in recognition of germline (GL) predisposition to MN. Some congenital blood disorders predispose to subsequent myeloid neoplasia (MN). Severe congenital neutropenia (SCN) and Fanconi anemia serving as prominent examples. There is emerging evidence that heterozygous variants of disease recessive conditions may constitute risk alleles for late developing MN. However, their low penetrance may preclude identification of such alterations. Our previous analysisof allelic burden among selected combined potential GL predisposition genes to MN identified that the myeloperoxidase (MPO) gene (17q22-23) carried the highest risk of pathogenic alterations (Li, S.T, Leukemia, 2020). GL MPO mutations cause MPO deficiency syndrome, one of the most common inherited disorder associated with phagocyte defects and variable clinical penetrance. Despite the high prevalence, inherited MPO deficiency patients are asymptomatic and may remain unrecognized.

We investigated 3280 MN and bone marrow failure syndromes (BMF) pts including 1138 MDS, 260 MDS/MPN, 1661 AML and 221 AA for presence of MPO mutations. In total, 38 different germline MPO mutations were identified in 143 cases. With a stringent bioanalytic pipeline, 28 pathogenic/likely pathogenic variants from 100 MN patients were included in the study. Germline MPO variants were significantly enriched in MN compared to control population (294 vs. 125 per 10 ⁴ individuals; P<.0001) with an odds ratio of 2 (95%CI=1.6-2.5, P<.0001) for AML and 1.8 (95%CI=1.4-2.4, P<.0001) for MDS. The most common pathogenic/likely pathogenic variant (46%; 46/100) was a 3' splice site of intron11 (c.2031-2A>C) followed by R569W (13/100), M519fs* (13/100) and Y173C (6/100) with none being biallelic. While no differences were found in distribution of -7/7q, del5q, or del20q, MPO variants carriers harbored less tri-8 compared to wild type counterparts (2% vs. 12%; P=.008). Only FLT3 and NRAS were significantly associated with MPO mutations.

We then investigated the effects of MPO deficiency on hematopoietic function in murine model using competitive repopulation assays, whereby the difference in CD45.1/CD45.2 isotypes transplanted in to ROSA26 ^{(tdTomato-EGFP)} recipients assayed by flow cytometry allowed distinction of the 2 grafts. Mpo^-/- cells gained over time proliferative advantage over normal murine bone marrow cells. Reverse combinations (graft mix vs recipient) also replicated this result. This effect was not due to increased HSC content in Mpo^-/- marrow as there were no significant differences in LSK, CMP and MEPs cells. We then investigated the clonogenic consequences of Mpo^-/- cells in the setting of H ₂O ₂ induced oxidative stress, after exposing to hydrogen peroxide. Mpo^-/- cells increased clonogenic potential after second serially replating. This prompted us to investigate an MPO inhibitor, AZD5904 in human leukemia cells with different MPO expression (HL-60, high; K562, low). High MPO expressors retained higher cell viability following H ₂O ₂ and MPOi addition compared to those without MPOi (85±10 vs.59±8, P<.0001) while no change was observed in low MPO stage cells with or without MPOi (84±2 vs.89±10, P=0.7).

In conclusion, for the first time we demonstrated that germline MPO variants constitute risk alleles for MN evolution and replicated the potential replicative advantage of MPO deficient cells in murine model and potential mechanisms of the MPO deficiency in vitro including activation of non-homologous DNA repair response and error-prone DNA repair favoring replication. Repeated cycles of stress hematopoiesis e.g., due to increased infection rate may provide conditions in which MPO variants contribute to the risk of MN.