TET2 Mutation Associated with Organ Infiltrations in ATLL

Introduction: Adult T-cell leukemia/lymphoma (ATLL) is a peripheral T-cell lymphoma that is caused by HTLV-1. The prognosis of acute and lymphomatous variants of ATLL is poor, ranging from 2 weeks to >1 year. Compared to other types of malignant lymphomas, the organ infiltration is frequently observed in ATLL (Yamada et al. Leuk Lymphoma 1997). We previously reported the landscape of genetic mutations in ATLL, and showed that various mutations occurred in the TCR-NFκB pathway in more than 90% of ATLL cases (Kataoka et al. Nat Genet 2015). These somatic mutations are thought to develop ATLL in combination with viral genes such as HTLV-1 bZIP factor (HBZ). Among them, mutations in TET2, an epigenetic regulator, was observed in about 10% of ATLL cases. Higher frequencies inTET2 mutation was reported in other types of peripheral T-cell lymphoma (PTCL); it was observed in about 80% of angioimmunoblastic T-cell lymphoma (AITL) and in about 50% of PTCL, not otherwise specified. In PTCL, it has been reported that additional mutations in lymphoid progenitors derived from TET2 mutated hematopoietic stem cells cause increased cell proliferation and anti-apoptosis, leading to the disease progression. In ATLL, the role of TET2 mutation in disease progression is still unknown. In this study, we investigated the role of TET2 mutation in ATLL using mouse model and acute and lymphomatous variant ATLL cohort.

Materials and methods: As an animal model of HTLV-1 infection or ATLL, transgenic mice expressing HBZ under the control of the mouse CD4 promoter (HBZ-Tg) were generated with C57BL/6 background. Heterozygous TET2 knock-down mice (TET2KD) were generated with C57BL/6 background by gene trapping (Tang et al. Transgenic Res 2008; Shide et al. Leukemia 2012). HBZ-Tg/TET2KD compound mice (double mutant) were generated by crossing them. HBZ-Tg, TET2KD, and double mutant mice were investigated by cell counts, organ weight, FACS analysis, pathological analysis, and survival analysis. The relationship between the TET2 mutation status and the clinical feature was investigated using our acute and lymphomatous variant ATLL cohort (n=115).

Result: At 12 months, compared to wild type mouse (WT), sporadic splenomegaly and lymphadenopathy were observed in HBZ-Tg. No significant increase was observed in peripheral blood (PB) leukocyte and mononuclear cell (MNC) of BM and spleen, but an increase was observed in the estimated whole body MNC (Femur x 100/6 + spleen) (WT vs. HBZ-Tg; estimated whole body MNC (x10⁶ cells/body), 416±162 vs. 621±147, p=0.01). In FACS analysis, the frequency of CD4⁺ T-cell was increased in PB, spleen, and BM (WT vs. HBZ-Tg; PB-CD4⁺ T-cell%, 4.9±0.9 vs. 28.2±22.8, p<0.05; spleen CD4⁺ T-cell%, 10.3±3.1 vs. 18.2±3.3, P<0.01; BM-CD4⁺ T-cell%, 1.2±0.6 vs. 2.5±1.1, p<0.05), and the estimated whole body CD4⁺ T-cell count was also increased (WT vs. HBZ-Tg; CD4⁺ T-cells (x10⁶ cells/body) 13.6±7.6 vs. 41.9±24.4, p<0.01). In the survival analysis, compared to WT, the shortened overall survival (OS) was observed in HBZ-Tg (median survival time (MST, month), unreached vs. 11.1, p<0.01). In pathological analysis, HBZ-Tg showed increased leukocyte infiltration to various organs such as lung and liver, and the infiltrated cells were mainly composed of T-cells. In the lung, in addition to the cell infiltration, alveolar edema was observed, which was presumed to be the main cause of death.

Next, to elucidate the role of TET2 mutation in ATLL, the double mutant was analyzed. At six months, compared to HBZ-Tg, no increase was observed in the number of PB leukocyte, spleen-MNC, and BM-MNC, and also in the frequency and the number of CD4⁺ T-cells in PB, spleen and BM. However, in pathological and survival analysis, the double mutant showed severe cell infiltration in lung and liver and demonstrated inferior OS (median OS (month), 11.1 vs. 6.0, p<0.05). Further, the double mutant showed increased frequency of CD103 (integrin alpha E), an adhesion molecule, expressing cells (CD4⁺CD103⁺% in spleen; 6.7±1.0 vs. 10.9±1.9, p<0.05). In the acute and lymphomatous variant ATLL cohort analysis, genetic and clinical investigation revealed that organ infiltration detectable by imaging studies was frequently observed in TET2 mutated patients (WT-Pt (n=100) vs. TET2 mutated-Pt (n=15); extra nodular lesion, 78/100 vs. 14/15).

Conclusion: In both mice model and human cohort, TET2 mutation exacerbated organ infiltration of ATLL cells.