Abstract LB-043: Driver mutation and collaborating signaling in phenotype patterning in human myeloproliferative neoplasms

Abstract

Abstract Mutually exclusive mutations in JAK2 (JAK2V617F and exon 12), MPL, and calreticulin (CALR) constitute the driving force of Philadelphia-negative myeloproliferative neoplasms (MPN), as they have been shown to promote disease propagation through dysregulated JAK-STAT signaling pathways. However, none of these mutations have been proved to be specific to disease subtype, and they cannot be used in the molecular sub-classification of MPNs. It remains a major mystery why the same driver mutation could lead to discrepant phenotypes. High mobility group AT-hook 2 (HMGA2) is an architectural transcriptional factor that is negatively regulated by Let-7 microRNA through binding to its 3’-untranslated region and is known to promote cell proliferation and survival. Transgenic mice expressing HMGA2 with a truncation of its 3’-UTR has been leading to increased megakaryopoiesis as well as MPN phenotypes in animal models. To decipher the Let-7-HMGA2 axis in myeloproliferative neoplasms (MPN), we employed an in vitro model supplemented with clinical correlation. Ba/F3 cells with inducible JAK2V617F expression (Ton.JAK2.V617F cells) showed up-regulation of HMGA2 with concurrent let-7a repression. Ton.JAK2.V617F cells treated with a let-7a inhibitor exhibited further escalation of HMGA2 expression, while a let-7a mimic diminished the HMGA2 transcript level. HMGA2 overexpression conferred JAK2-mutated cells a survival advantage through inhibited apoptosis. Pan-JAK inhibitor INC424 increased the expression of let-7a, down-regulated the level of HMGA2, and led to increased apoptosis in Ton.JAK2.V617F cells in a dose-dependent manner. Furthermore, up-regulation of HMGA2 was significantly associated with MPN patients carrying the JAK2V617F mutation. In vitro studies showed that Ba/F3 cells carried JAK2V617F (Ba/F3-JAK2V617F) had decreased let-7a and up-regulated HMGA2. Silencing of HMGA2 in Ba/F3-JAK2V617F cells resulted in growth inhibition coupled with a significant increase in apoptosis. In our model cells, HMGA2 up-regulation is seen in both JAK2-mutated and CALR-mutated cells, indicating its profound participation in the disease patterning and a phenotype modifier in MPN. Hence, we studied a cohort of 151 MPN patients. Overexpressed HMGA2 was detected in about one-fifth of the cases, and it was more commonly seen in ET (26.9%, vs. 12.7% in PV, p=0.044). Compared to their counterparts, HMGA2-overexpressing patients had higher platelet counts, increased thromboembolic risk, and inferior thrombosis-free survival. Fluorescence in situ hybridization analysis showed that chromosomal translocation was not a major cause of HMGA2 overexpression in MPN patients, yet there was an inverse correlation between the expression levels of let-7a and HMGA2. Our findings suggest that, in a subset of MPN patients, Let-7-HMGA2 axis plays a prominent role in the pathogenesis of the disease that leads to unique clinical phenotypes in JAK2 and CALR mutations. Citation Format: Chia-Chen Hsu, Jie-Yu You, Cih-En Huang, Yi-Yang Chen, Hsing-Ying Ho, Chian-Pei Li, Chang-Hsien Lu, Kuan-Der Lee, Jyh-Pyng Gau, Yu-Wei Leu, Chih-Cheng Chen. Driver mutation and collaborating signaling in phenotype patterning in human myeloproliferative neoplasms [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr LB-043. doi:10.1158/1538-7445.AM2017-LB-043