HMGA2 mRNA Expression in Patients with Myelodysplastic/Myeloproliferative Neoplasms (MDS/MPN)

MDS/MPN comprise a group of uncommon myeloid neoplasms with clinical and hematological features overlapping MDS and MPN. Pathogenesis remain elusive. HMGA2 on chromosome 12q plays a crucial role in self-renewal and proliferation of fetal hematopoietic stem cells (HSCs), but not adult HSCs because of repression by binding of MIRLET-7 with the 3'-untranslated region (3'UTR) of HMGA2 (Copley et al, Nat Cell Biol, 2013). Trimethylation of histone H3 at lysine 27, which is catalyzed by EZH2 - a member of polycomb recessive complex 2 (PRC2) - also suppresses HMGA2 (Sashida et al, JEM, 2016). We have demonstrated that expression of Hmga2 cDNA lacking 3'UTR caused proliferative hematopoiesis mimicking MPN in young mice and led to anemia with ineffective erythropoiesis and leukocytosis like MDS/MPN in old mice (Ikeda et al, Blood, 2011; Ueda et al, Blood Adv, 2017), suggesting that HMGA2 may contribute to the pathogenesis of MDS/MPN. In fact, we found high HMGA2 mRNA levels in granulocytes of MPN including >90% of patients with myelofibrosis (MF) and about 30% with essential thrombocythemia (ET), and either reduced MIRLET-7c levels or PRC2-related mutations were correlated with high HMGA2 mRNA in MF, while reduced MIRLET-7c with no particular mutation was correlated with high HMGA2 mRNA in ET (Harada-Shirado et al, BJH, 2015; Blood Adv, 2017). However, in MDS/MPN, only one case series has reported high HMGA2 mRNA due to the 12q rearrangement that removed the 3'UTR of HMGA2 in a few patients (Odero et al, Leukemia, 2005). So far, expression of HMGA2 has not been studied in MDS/MPN without the 12q rearrangement.

To clarify the expression of HMGA2 in MDS/MPN in relation to clinical and other genetic findings, we investigated HMGA2 mRNA and MIRLET-7c levels by real-time RT-PCR and sought PRC2-related mutations by targeted deep sequencing in peripheral leukocytes of 11 patients with MDS/MPN [5 chronic myelomonocytic leukemia (CMML), 2 atypical chronic myeloid leukemia (aCML), and 4 unclassifiable MDS/MPN (MDS/MPN-u)]. In addition, to identify the features specific for MDS/MPN, we also evaluated 10 patients with low-risk MDS [MDS with multilineage dysplasia (MDS-MLD)]. No MDS/MPN or MDS-MLD patients had 12q rearrangement.

In MDS/MPN, high HMGA2 mRNA [>1.0 relative to HPRT1 mRNA above mean + 2SD of healthy volunteers, per our previous study (Blood Adv, 2017)] was found in each subtype: 2 of 5 (40%) in CMML, 1 of 2 (50%) in aCML, and 3 of 4 (75%) in MDS/MPN-u. Therefore, more than half of patients with MDS/MPN (6 of 11, 54.5%) showed high HMGA2 mRNA levels. In contrast, high HMGA2 mRNA was found in only 1 of 10 (10%) patients with MDS-MLD (P = 0.06). Reduction in MIRLET-7c was seen in 5 of 11 (45.5%) MDS/MPN and 3 of 10 (30.0%) MDS-MLD (P = 0.16). Among these 5 MDS/MPN patients with low MIRLET-7c, only 2 patients had high HMGA2 mRNA. Thus, MIRLET-7c reduction did not correlate with deregulation of HMGA2 mRNA in MDS/MPN as frequently as in MPN. In contrast, HMGA2 mRNA levels were high in all 3 patients with MDS/MPN carrying EZH2 mutations, suggesting that loss of EZH2 might augment HMGA2 in MDS/MPN. Laboratory findings of MDS/MPN patients with high HMGA2 mRNA were not different from those without high HMGA2 mRNA. However, MDS/MPN patients with both high HMGA2 mRNA and an EZH2 mutation showed more severe anemia compared with those with high HMGA2 mRNA without an EZH2 mutation (hemoglobin level: 9.6±2.0 vs. 11.0±2.3 g/dL, P = 0.05).

In conclusion, we demonstrated that MDS/MPN cases highly express HMGA2 mRNA even without 12q rearrangement, more often than MDS-MLD. Unlike MPN, HMGA2 mRNA levels did not correlate with reduced MIRLET-7c, but probably with mutations in EZH2, which may also contribute to anemia in MDS/MPN.