ALBERT

Description: Background TET2 is known as an enzyme which converts 5-methylcytosine (5mC) to 5-hydroxymethylcytosine (5hmC). Loss-of-function mutations in TET2 are frequent in angioimmunoblastic T-cell lymphoma (AITL) and peripheral T-cell lymphoma, not otherwise specified (PTCL-NOS). Normal counterpart of AITL is thought to be follicular helper T cells (Tfh). PTCL-NOS is likely to consist of heterogeneous groups. Some of the PTCL-NOS cases also have features of Tfh. In the last annual meeting, we reported that aged homozygous Tet2 gene-trap mice (Tet2gt/gt), which showed 80% reduction in the Tet2 mRNA level in various hematopoietic cells, developed T-cell lymphoma. To further investigate the mechanism of T-lymphomagenesis, we analyzed methylome, hydroxymethylome, and transcriptome in the lymphoma cells. Material and Method Tet2 gt mice have a trapping vector inserted into the second intron of the Tet2 locus. In all the analyses, we used CD4+ T cells prepared from lymphoma cells developed in these mice, as well as CD4+ T cells prepared from spleen of wild-type mice as a control. To investigate genome-wide methylation and hydroxymethylation statuses, we performed MeDIP and hMeDIP sequencing, and bisulfite sequencing. To examine comprehensive gene expression, we performed microarray-based analysis, followed by Gene Set Enrichment Analysis (GSEA). Result After observation over a year (median, 67 weeks), 5 out of 7 Tet2gt/gt mice developed T-cell lymphomas with Tfh-like immunostaining pattern. No differences were found in the average levels of 5mC between lymphoma and control CD4+ cells throughout the regions around transcription start sites (TSS) +/- 5 kb. In contrast, when the same regions were analyzed for 5hmC levels, those in the lymphoma cells were significantly lower at the regions around TSS +/- 1 kb. When focused on regions having high 5mC contents (MACS score〉5.0), lymphoma cells demonstrated a significant enrichment at regions around TSS +/- 1 kb, intragenic regions, and CpG islands (p=0.013, 0.006, 0.022 respectively). On the other hand, 5hmC was significantly decreased at regions around TSS +/- 1 kb in lymphoma cells than control cells (p=0.018). In a set of genes whose expression was higher in lymphoma cells than control cells, 5hmC levels were significantly lower in lymphoma cells. GSEA analysis revealed upregulation of Tfh-associated genes such as Bcl6 and cMaf (FDR q value=0.0004), key transcription factors for Tfh differentiation, in lymphoma cells compared with control cells. The expression of upregulated Tfh-associated genes was validated by real-time PCR. We focused on the epigenetic change of Bcl6 because it is among the most important transcription factors for Tfh development. It was reported that hypermethylation at intron 1 of Bcl6 upregulated its transcriptional activity in B cell lymphomas. Bisulfite sequencing revealed that the CpG sites in the intron 1 of Bcl6 were massively methylated/hydroxymathylated in lymphoma cells, whereas those in control cells were mostly at an unmodified status. MeDIP sequencing indicated that intron 1 of Bcl6 had more methylated status in lymphoma cells than control cells. We also found that CpG sites in the same region were densely methylated/hydroxymathylated in EL4, mouse T-cell lymphoma cell line, and that the decitabine treatment converted them into unmodified CpG along with the decrease in the Bcl6 expression levels. Discussion and Conclusion Tet2 gt/gt mice developed T-cell lymphoma with both Tfh-like immunohistological character and gene expression pattern. We found distinct changes in methylome, hydroxymethylome, and transcriptome. We also found a tight linkage between the increased methylation of intron 1 of Bcl6 and increased expression of its mRNA level in lymphoma cells developed in Tet2 knockdown mice. The same scenario is indicated in a T-cell lymphoma cell line. These observations imply that, in normal CD4+ T cells, reduced Tet2 function might increase the methylation status of the CpG sites in intron 1 of Bcl6, which may result in upregulation of Bcl6 expression and deviated Tfh generation. These processes might be an initiating event for the development of T-cell lymphoma with the Tfh features. Because of the long latency before lymphoma development in Tet2gt/gtmice, it is likely that additional hits are necessary. Disclosures: No relevant conflicts of interest to declare.

Description: Abstract 1299 Background: Loss-of-function mutations in TET2 are frequent in human myeloid and lymphoid malignancies. Especially, TET2 mutations were found in 30–47% of angioimmunoblastic T-cell lymphoma (AITL) and 10–38% of peripheral T-cell lymphoma, not otherwise specified (PTCL, NOS). The tumor origin of AITL is thought to be follicular helper T cells (Tfh). PTCL, NOS is a group of heterogenous T-cell-derived lymphomas, whereas some cases of PTCL, NOS also appear to be derived from Tfh. Several recent papers reported that Tet2 knockout mice demonstrated premalignant status of myeloid lineages, implicating that TET2 mutations have a driver's role in myeloid malignancies; however, there are no clues to how impaired Tet2 function provokes T-cell lymphomas. To address this issue, we analyzed Tet2 gene trap (Tet2gt) mice. Materials and methods: Tet2gt mice (Transgenic Res 17:599, 2008) have a trapping vector inserted into the second intron of Tet2 locus. Lineage-, Sca1+, and c-kit+ (LSK) cells were sorted from Tet2gt E15.5 fetal liver (FL) cells and transplanted into lethally irradiated Ly5.1 wild-type (WT) mice. Bone marrow, spleen, and tumors when developed were analyzed in the recipient mice as well as adult Tet2gtmice by flow cytometory and immunohistochemical staining. Result: Adult Tet2gt mice were obtained under the following ratio; Tet2+/+:Tet2+/gt:Tet2gt/gt=48:52:25. TET2 mRNA expression level in Tet2gt/gt Lin- FL cells was reduced to 20% of that in WT Lin- FL cells, while the 5-hydroxymethyl cytosine level was reduced to around a half of that in WT cells. At 40 weeks of age, the proportion of CD4+/PD1+/Cxcr5+ cells, immunophenotypically similar to Tfh cells, significantly increased in Tet2gt/gt mice compared to WT mice (P=0.022). Two out of 8 Tet2gt/gt mice developed marked splenomegaly (more than 300 mg) at 40 weeks, and 4 out of 6 developed marked splenomegaly with swollen lymph nodes and multi-nodular tumors in liver and lungs at 60 weeks or older (median, 67 weeks). Histological examination of the enlarged spleen and swollen lymph nodes in these 4 mice demonstrated completely destroyed follicular structures. The spleen, lymph nodes, and nodular tumors showed infiltration of morphologically abnormal lymphocytes, which were proven to be CD4+/PD1+ T cells. These cells were also weakly positive for Cxcr5. CD4+ cells purified from the tumors revealed restricted rearrangement patterns of T-cell receptor genes, implying T-cell lymphoma. There were no increased vascular structures representing AITL. CD4+/PD1+ cells purified from the tumors expressed Bcl6 and cMaf, two key transcription factors of Tfh, at significantly higher levels compared to CD4+/PD1+ cells purified from spleen of WT mice. Meanwhile, one of 6 mice transplanted with Tet2gt/gt LSK cells also developed lymphoma with similar pattern of CD4+/PD1+/CXCR5 expression at 40 weeks after transplantation. Discussion: Our observations indicate that reduced expression of TET2 may cause skewed differentiation into Tfh, which eventually develop tumors that pathologically recapitulate PTCL, NOS in human, while the tumor-comprising cells show characteristics of Tfh. The long latency period required for tumor development indicates secondary genetic hits in addition to the downregulation of TET2. These secondary events might determine the specificity of the hematologic malignancies, particularly AITL and PCTL, NOS with propensity for AITL. Disclosures: No relevant conflicts of interest to declare.

Description: Adult T-cell leukemia/lymphoma (ATL) is a distinct form of peripheral T-cell lymphoma (PTCL). ATL is initiated by immortalization of T-cells by human T-lymphotropic virus type I (HTLV-1) infection during early infancy, followed by an accumulation of multiple genetic hits to develop leukemia. However, little has been known about these genetic hits. Recently, frequent somatic mutations in RHOA, DNMT3A, IDH2 and TET2 have been reported in angioimmunoblastic T cell lymphoma (AITL) and PTCL not otherwise specified (PTCL-NOS), which are characterized by follicular helper T-cell phenotypes. Most of the RHOA mutations in AITL invariably cause an identical amino acid change (Gly17Val), which was shown to inhibit wild-type RHOA function in a dominant-negative manner. In addition, RHOA mutations have been also identified in gastric cancer and Burkitt lymphoma, with mutational hotspots including Tyr42 and Arg5. Interestingly, some of them were reported as working in a gain-of function manner. In the current study, we investigated the mutational status of RHOA, as well as TET2, IDH2, and DNMT3A in a total of 205 patients with ATL in order to reveal their role in ATL pathogenesis. In targeted deep sequencing, RHOA was mutated in 32 (15.6%) cases. In contrast to the exclusive G17V mutation found in AITL and PTCL-NOS, mutations in ATL was widely distributed within the regions responsible for GTP-binding with Cys16Arg being most frequently mutated (N = 11; 5.4%), although Gly17Val mutations were also found in 4 cases (1.9%). Being commonly located in the same amino acid with the GTP binding site, Ala161Pro and Ala161Glu mutations were exclusively found in ATL and AITL/PTCL-NOS, respectively. We found no significant differences in the RHOA mutational status among different ATL subtypes, and no significant prognostic impact of RHOA mutations on survival. TET2 mutations were very common and tightly associated with RHOA mutation in AITL, but much less common (N = 21; 10.2%) and may not associated with RHOA mutations in ATL. Mutations of IDH2 (N = 2; 1%) and DNMT3A (N = 2; 1%) were rare in ATL. RHOA encodes a ras-related GTP-binding protein that functions as a molecular switch in a variety of biological processes through cycling between an active (GTP-bound) state and an inactive (GDP-bound) state. Known biological functions of RHOA are related to actin organization, cell migration and transcriptional activation. Modeling of three-dimensional structures suggested that most of mutations identified in ATL located near nucleotide binding pocket, which affected the GTP-binding ability of RHOA protein. Indeed, we performed luciferase assay to measure transcriptional activities of wild-type and mutant RHOA. As previously reported, dominant-negative Gly17Val RHOA mutant suppressed transcriptional activity. In contrast, ATL specific RHOA mutant such as Cys16Arg or Ala161Pro enhanced transcriptional activity, similar to or even more than wild-type RHOA. In addition, when we assessed actin stress fiber formation in cells transduced with wild-type or mutant RHOA, Gly17Val or Ala161Glu RHOA attenuated actin stress fiber formations, whereas Cys16Arg or Ala161Pro RHOA induced actin stress fiber formation, similar to known gain-of-function Gly14Val RHOA mutant. These functional assays implicated Cys16Arg and Ala161Pro RHOA mutant showed different biological behaviors from Gly17Val and Ala161Glu RHOA mutant, indicating gain-of-function mechanisms of Cys16Arg or Ala161Pro mutants. In summary, RHOA mutations were also common in ATL as in AITL and PTCL-NOS. Nevertheless, mutational patterns and functional consequences of RHOA mutations identified in ATL were distinct from those in other PTCLs, suggesting differential driver roles of RHOA mutations in ATL compared to other PTCLs. Disclosures No relevant conflicts of interest to declare.

Description: Backgrounds Angioimmunoblastic T-cell lymphoma (AITL) is a distinct subtype of T-cell lymphoma, characterized by generalized lymphadenopathy and frequent autoimmune-like manifestations. The diagnosis of AITL is sometimes challenging for hematopathologists, because the tumor cell content is generally low and relatively large reactive lymphocytes are confused as tumor cells. Possibly because of the low tumor cell frequency, clonal rearrangement of T-cell receptor gene is undetectable in 30% of the cases. We identified recurrent mutations in RHOA at c.G50T, predicting to generate p.G17V in 70% of AITL and PTCL-NOS harboring AITL features, which implies diagnostic properties for AITL (M S-Y and SC, unpublished). Purpose To establish a novel cost-effective method to diagnose AITL, we performed allele-specific realtime PCR to detect RHOA G17V mutation. Methods Genomic DNA was extracted from 119 AITL and PTCL-NOS samples, which include 47 periodate-lysine-paraformaldehyde (PLP)-fixed, 12 formalin-fixed-paraffin- embedded (FFPE) and 60 frozen tissues. Forty-one out of 60 genomic DNA samples, purified from frozen tissue, were amplified by RepliG kit (QIAGEN). Allele-specific primers for RHOA G17V mutant and wild-type sequences were designed by Wangkumhang's algorithm. The [mut] and [WT] values were individually measured by realtime PCR using each primer set, and the [mut]/([mut]+[WT]) values were calculated. Mutant allele frequencies were determined by amplicon-based deep sequencing using MiSeq. Results The [mut] values were distributed from 1.5×10-7 to 7.6×10-2, and the [WT] values were from 7.9×10-5 to 1.3×10-2. The [mut]/[mut]+[WT] values were distributed from 1.9×10-4 to 8.5×10-1. We set a cut-off value to determine existence of mutation as 2% for MiSeq, and 1.3×10-2 for the [mut]/([mut]+[WT]) value. Then we compared these two methods to detect RHOA G17V mutation. Forty-three cases were positive for the RHOA mutation in this study cohort by MiSeq, including 32 AITL and 11 PTCL-NOS cases. The [mut]/([mut]+[WT]) values of DNA from FFPE samples tend to be lower than those from other samples, and 4 out of 12 FFPE samples determined as mutation-positive by MiSeq were not detected by our allele-specific realtime PCR. We therefore excluded the FFPE samples and analyzed the data of 107 DNA samples, purified from PLP-fixed and frozen tissues. Rank correlation coefficient was 0.753. Sensitivity was 97.4%, and specificity was 97.1%. Positive concordance rate was 94.9%, and negative concordance rate was 98.6%. Conclusions We established a method to detect RHOA G17V hotspot mutation for AITL. It is expected to be highly accurate and cost effective. Disclosures: No relevant conflicts of interest to declare.

Description: Key Points RHOA mutations are common in ATLL and show a unique distribution compared with other T-cell lymphomas. Depending on patients, functionally distinct RHOA mutations are clonally selected and involved in the pathogenesis of ATLL.

Description: Background Angioimmunoblastic T-cell lymphoma (AITL) is a distinct subtype of peripheral T-cell lymphoma (PTCL) characterized by generalized lymphadenopathy, hyperglobulinemia, and autoimmune-like manifestations. Frequent mutations in TET2, IDH2, and DNMT3A have been described in AITL, which are commonly found in myeloid malignancies. However, the molecular pathogenesis specific to AITL is still unknown. Methods To clarify the molecular pathogenesis of AITL, we performed comprehensive gene-mutation analysis. Somatic mutations in 3 AITL and 3 PTCL-NOS specimens were explored using whole-exome sequencing (WES). Targeted resequencing for genes identified by WES was also performed in a cohort of 157 patients with AITL/PTCL-NOS. Results We identified a novel recurrent mutation in RHOA (c.G50T/p.G17V) in 3 AITL and one PTCL-NOS samples by WES. Validation in an extended cohort revealed an extremely high frequency of the identical G17V RHOA mutation in AITL (50/72 [69.4%]), together with mutations in TET2 (39/47 [83.0%]), IDH2 (14/47 [29.8%]), and DNMT3A(12/47 [25.5%]). The G17V RHOA mutation was also found in PTCL-NOS samples at a lower frequency (14/85 [16.5%]), especially in those harboring AITL features (PTCL-NOS with AITL features vs PTCL-NOS w/o AITL features: 13/21 [61.9%] vs 0/38 [0%]). Remarkably, mutations in RHOA, TET2, and IDH2 showed striking correlations. All RHOA-mutated samples were accompanied by TET2 mutations. IDH2 mutations were confined to the samples having simultaneous mutations of RHOA and TET2. Mutations in DNMT3A largely overlapped to TET2 mutations, but its correlation with RHOA or IDH2 mutations was much less clear. TET2 mutations showed a consistently higher allelic burden than RHOA mutations. Gene-mutation analysis of tumor cells and infiltrated cells demonstrated that the G17V RHOA mutation specifically existed in tumor cells, but not in non-tumor cells, while TET2 mutations were identified both in tumor and non-tumor cells. RHOA encodes a small GTPase, which operates as a molecular switch that regulates a wide variety of biological processes through cycling between an active (GTP-bound) and an inactive (GDP-bound) state. We demonstrated that the G17V RHOA mutant did not bind GTP and also inhibited GTP-binding of the wild-type RHOA protein. Accordingly, unlike wild-type RHOA, the G17V mutant was not able to activate transcription from the serum response factor-responsive element (SRF-RE). Conclusions Our data suggests that combination of preceding mutations in TET2 and subsequent tumor-specific G17V RHOA mutation determines distinct disease properties of AITL. Disclosures: No relevant conflicts of interest to declare.