Angioimmunoblastic T-cell lymphoma contains multiple clonal T-cell populations derived from a common TET2 mutant progenitor cell

Abstract

Angioimmunoblastic T-cell lymphoma (AITL) is a neoplastic proliferation of T follicular helper cells with clinical and histological presentations suggesting a role of antigenic drive in its development. Genetically, it is characterized by a stepwise acquisition of somatic mutations, with early mutations involving epigenetic regulators (TET2, DNMT3A) and occurring in haematopoietic stem cells, with subsequent changes involving signaling molecules (RHOA, VAV1, PLCG1, CD28) critical for T-cell biology. To search for evidence of potential oncogenic cooperation between genetic changes and intrinsic T cell receptor (TCR) signaling, we investigated somatic mutations and T-cell receptor β (TRB) rearrangement in 119 AITL, 11 peripheral T-cell lymphomas with T follicular helper phenotype (PTCL-TFH), and 25 PTCL-NOS using Fluidigm polymerase chain reaction (PCR) and Illumina MiSeq sequencing. We confirmed frequent TET2, DNMT3A, and RHOA mutations in AITL (72%, 34%, 61%) and PTCL-TFH (73%, 36%, 45%) and showed multiple TET2 mutations (2 or 3) in 57% of the involved AITL and PTCL-TFH. Clonal TRB rearrangement was seen in 76 cases with multiple functional rearrangements (2-4) in 18 cases (24%). In selected cases, we confirmed bi-clonal T-cell populations and further demonstrated that these independent T-cell populations harboured identical TET2 mutations by using BaseScope in situ hybridization, suggesting their derivation from a common TET2 mutant progenitor cell population. Furthermore, both T-cell populations expressed CD4. Finally, in comparison with tonsillar TFH cells, both AITL and PTCL-TFH showed a significant overrepresentation of several TRB variable family members, particularly TRBV19*01. Our findings suggest the presence of parallel neoplastic evolutions from a common TET2 mutant haematopoietic progenitor pool in AITL and PTCL-TFH, albeit to be confirmed in a large series of cases. The biased TRBV usage in these lymphomas suggests that antigenic stimulation may play an important role in predilection of T cells to clonal expansion and malignant transformation. © 2019 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

Keywords: AITL; TET2 mutation; clonality; lymphoma genesis; progenitor cells.

Figure 1

Mutation profile in AITL, PTCL‐TFH, and PTCL‐NOS.

Figure 2

TRB functional rearrangements in AITL, PTCL‐TFH, and PTCL‐NOS. (A) Number of functional TRB rearrangements and their clonal sizes. (B) Comparison between the loads of TET2 mutations and the size of clonal TRB rearrangements on select cases.

Figure 3

Confirmation of two clonal T‐cell populations by BaseScope ISH with TRB clone specific probes. (A) BaseScope probe design with the unique VDJ junctional sequence and TET2 mutation highlighted in red. (B) BaseScope in situ hybridization in case AITL030 with tonsil serving as a negative control. The TRB V7‐J1 probe identifies a diffuse cell population, whereas the TRB V27‐J2 probe reveals only scattered cells, confirming that they represent two independent clonal T‐cell populations.

Figure 4

Demonstration of TET2 mutations and CD4 positivity in two clonal T‐cell populations of AITL030. (A) Double BaseScope in situ hybridization shows the presence of both TET2 mutations in the two clonal T‐cell populations of AITL030. Please note that the BaseScope probes were designed to hybridize to mRNA, thus yielding stronger signals for highly expressed (TRB) than lowly expressed (TET2) genes. (B) Double BaseScope ISH and immunohistochemistry demonstrate that both clonal T‐cell populations are CD4 positive, but CD8 negative.

Figure 5

Biased usage of TRB variable genes in AITL and PTCL‐TFH. (A) Comparison of TRB variable gene usage between AITL/PTCL‐TFH and tonsillar TFH. (B) Comparison of CDR3 sequences among cases harbouring TRBV19 functional rearrangements. AITL031 and AITL036 have a nearly identical CDR3 sequence, suggesting possible recognition of a common antigen.

Figure 6

The proposed model of multistage development in AITL. Mutations in DNA methylation regulators—namely TET2, DNMT3A, and IDH2—are early events, with TET2 and DNMT3A mutation occurring in hematopoietic stem cells. These mutations enhance the self‐renewal of haematopoietic stem cells, increase the risk of lymphomagenesis, and may frequently generate bi‐ or oligoclonal T‐cell populations in AITL or PTCL‐TFH following acquisition of additional genetic changes in genes important for T‐cell biology. The TET2 mutant hematopoietic progenitor cells also give rise to non‐neoplastic polyclonal B and CD8 positive T‐cells as reported in several previous studies 7, 13, 14, 16, 21. Finally, there is also a possibility that minor T‐cell clones are not related to TET2 or DNMT3A mutation but are the consequence of immunodeficiency and EBV reactivation in patients with AITL.