Genetic alterations in Adult T cell leukemia

Abstract

Human T-cell leukemia virus type 1 (HTLV-1), which infects more than 20 million people worldwide, is known to cause adult T-cell leukemia (ATL). Even those patients treated with intense chemotherapy have a poor prognosis [1]. Although the detailed mechanisms on how HTLV-1 transforms T cells are unclear, it is believed that the viral oncoprotein Tax and the accumulation of somatic alterations lead to ATL [2]. In 2015, Seishi Ogawa and colleague used whole-exome sequencing and whole-genome sequencing to comprehensively analyze ATL genetic alterations [2, 3]. They found that fifty genes are significantly mutated, with 13 genes (PLCG1, PRKCB, CCR4, CARD11, STAT3, TP53, VAV1, TBL1XR1, NOTCH1, GATA3, IRF4, FAS, CCR7) affecting more than 10% of ATL patients [2]. In our previous study, we found Notch1 mutations in 30% of ATL patients leading to reduced Fbw7-mediated degradation and stabilization of the intracellular cleaved form of Notch1 (ICN1). In addition, Notch inhibitors reduced ATL tumor formation in a xenograft model [4]. Since FBXW7 has been reported to be mutated in 6% of human tumors, we hypothesized that the deregulation of FBXW7 can accelerate ATL proliferation and transformation. In my first study, we found that FBXW7 is down-regulated and mutated in ATL patients. In contrast to the tumor suppressor role of FBXW7 wild-type, FBXW7 D510E increased cell proliferation and transformation both in vitro and in an ATL xenograft model [5]. Genome-wide H3K27 me3 accumulation has been observed in ATL patients, which can be explained by Polycomb repressive complex 2 hyperactivation [6]. In addition, EZH2 suppressed Fbxw7 expression via H3K27me3, resulting in Notch activation [7]. We hypothesized that the mutations of epigenetic regulators can reduce the FBXW7 expression in ATL. In my second study, we applied whole-genome next-generation sequencing (NGS) of uncultured freshly isolated ATL samples and identified the presence of mutations in SUZ12, DNMT1, DNMT3A, DNMT3B, TET1, TET2, IDH1, IDH2, MLL, MLL2, MLL3 and MLL4. TET2 was the most frequently mutated gene, occurring in 32 % (10/31) of ATL samples analyzed. Consistent with the previous report, Seishi Ogawa demonstrated hypermethylation in promoter-associated CpG islands in ATL [2]. Since the FBXW7 promoter hypermethylation has been reported [8] and a DNA methyltransferase inhibitor can restore the expression of FBXW7, the correlation of TET2 mutations and FBXW7 down-regulation needs to be further examined. FBXW7α R465C/+ knockin mice increased T-ALL formation when in cooperation with a Notch1 mutation [9]. Mechanically, FBXW7α R465C/+ stabilized c-Myc protein half-life, therefore increasing leukemia-initiating cells (LICs) in FBXW7α R465C/+ knockin mice [9]. In my third study, we confirmed the existence of side populations having both self-renewal and leukemia-renewal capacity and representing cancer stem cells (CSC)/ leukemia-initiating cells (LIC) in ATL cell lines and patient samples. We further show that PI3K and the NOTCH1 signaling pathway have opposite functions on the ATL side population. Constitutive activation of NOTCH1 signaling depletes the pool of side population cells in ATL-derived cell lines. Since Notch1 signaling is deregulated and essential for ATL progression, our results indicate another mechanism to explain how Notch1 signaling is constitutively active in ATL patients, implying a unique therapeutic opportunity to target FBXW7 in the future.