Somatic G17V RHOA mutations were found in 50–70% of angioimmunoblastic T-cell lymphoma (AITL). The mutant RHOA lacks GTP binding capacity, suggesting defects in the classical RHOA signaling. Here, we discovered the novel function of the G17V RHOA: VAV1 was identified as a G17V RHOA-specific binding partner via high-throughput screening. We found that binding of G17V RHOA to VAV1 augmented its adaptor function through phosphorylation of 174Tyr, resulting in acceleration of T-cell receptor (TCR) signaling. Enrichment of cytokine and chemokine-related pathways was also evident by the expression of G17V RHOA. We further identified VAV1 mutations and a new translocation, VAV1–STAP2, in seven of the 85 RHOA mutation-negative samples (8.2%), whereas none of the 41 RHOA mutation-positive samples exhibited VAV1 mutations. Augmentation of 174Tyr phosphorylation was also demonstrated in VAV1–STAP2. Dasatinib, a multikinase inhibitor, efficiently blocked the accelerated VAV1 phosphorylation and the associating TCR signaling by both G17V RHOA and VAV1–STAP2 expression. Phospho-VAV1 staining was demonstrated in the clinical specimens harboring G17V RHOA and VAV1 mutations at a higher frequency than those without. Our findings indicate that the G17V RHOA–VAV1 axis may provide a new therapeutic target in AITL.

◥ Recurrent hotspot (p.Gly17Val) mutations in RHOA encoding a small GTPase, together with loss-of-function mutations in TET2 encoding an epigenetic regulator, are genetic hallmarks of angioimmunoblastic T-cell lymphoma (AITL). Mice expressing the p.Gly17Val RHOA mutant on a Tet2-null background succumbed to AITL-like T-cell lymphomas due to deregulated T-cell receptor (TCR) signaling. Using these mice to investigate therapeutics for AITL, we found that dasatinib, a multikinase inhibitor prolonged their survival through inhibition of hyperactivated TCR signaling. A phase I clinical trial study of dasatinib monotherapy in 5 patients with relapsed/refractory AITL was performed. Dasatinib was started at a dose of 100 mg/body once a day and continued until days 10-78 (median day 58). All the evaluable patients achieved partial responses. Our findings suggest that AITL is highly dependent on TCR signaling and that dasatinib could be a promising candidate drug for AITL treatment. Significance: Deregulated T-cell receptor signaling is a critical molecular event in angioimmunoblastic T-cell lymphoma and can be targeted with dasatinib.

Activating mutations in the Vav guanine nucleotide exchange factor 1 (VAV1) gene are reported in various subtypes of mature T-cell neoplasms (TCN). However, oncogenic activities associated with VAV1 mutations in TCN remain unclear. To define them, we established transgenic mice expressing VAV1 mutants cloned from human TCN. Although we observed no tumors in these mice for up to a year, tumors did develop in comparably-aged mice on a p53-null background (p53-/- VAV1-Tg), and p53-/- VAV1-Tg mice died with shorter latencies than did p53-null (p53-/-) mice. Notably, various TCN with tendency of maturation developed in p53-/- VAV1-Tg mice, while p53-/- mice exhibited only immature TCN. Mature TCN in p53-/- VAV1-Tg mice mimicked human peripheral T-cell lymphoma (PTCL)-GATA3 and exhibited features of type2 T helper (TH2) cells. Phenotypes seen following transplantation of either p53-/- VAV1 or p53-/- tumor cells into nude mice were comparable, indicating cell-autonomous tumor-initiating capacity. Whole transcriptome analysis (WTA) showed enrichment of multiple Myc-related pathways in TCN from p53-/- VAV1-Tg mice relative to p53-/- or wild-type T cells. Remarkably, amplification of Myc locus were found recurrently in TCN of p53-/- VAV1-Tg mice. Finally, treatment of nude mice transplanted with p53-/- VAV1-Tg tumor cells with JQ1, a bromodomain inhibitor, which targets the Myc pathway, prolonged survival of mice. We conclude that VAV1 mutations function in malignant transformation of T cells in vivo and that VAV1-mutant expressing mice could provide an efficient tool for screening new therapeutic targets in TCN harboring these mutations. We conclude that VAV1 mutations function in malignant transformation of T cells in vivo and that VAV1-mutant expressing mice could provide an efficient tool for screening new therapeutic targets in TCN harboring these mutations.

Angioimmunoblastic T‐cell lymphoma (AITL) is an age‐related malignant lymphoma, characterized by immune system‐dysregulated symptoms. Recent sequencing studies have clarified the recurrent mutations in ras homology family member A (RHOA) and in genes encoding epigenetic regulators, tet methyl cytosine dioxygenase 2 (TET2), DNA methyl transferase 3 alpha (DNMT3A) and isocitrate dehydrogenase 2, mitochondrial (IDH2), as well as those related to the T‐cell receptor signaling pathway in AITL. In this review, we focus on how this genetic information has changed the understanding of the developmental process of AITL and will in future lead to individualized therapies for AITL patients.