Abstract LB-043: Driver mutation and collaborating signaling in phenotype patterning in human myeloproliferative neoplasms
Authors: Hsu Chia-Chen , You Jie-Yu , Huang Cih-En , Chen Yi-Yang , Ho Hsing-Ying , Li Chian-Pei , Lu Chang-Hsien , Lee Kuan-Der , Gau Jyh-Pyng , Leu Yu-Wei , Chen Chih-Cheng 
Published: July 2017

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