Somatic G17V Rhoa Mutation Specifies Angioimmunoblastic T-Cell Lymphoma
Authors: Sakata-Yanagimoto Mamiko , Enami Terukazu , Yoshida Kenichi , Shiraishi Yuichi , Ishii Ryohei , Miyake Yasuyuki , Muto Hideharu , Tsuyama Naoko , Okuno Yusuke , Sakata Seiji , Kamada Yuhei , Nakamoto-Matsubara Rie , Nguyen Tran , Izutsu Koji , Ohta Yasunori , Furuta Junichi , Shimizu Seiichi , Komeno Takuya , Sato Yuji , Ito Takayoshi , Noguchi Masayuki , Noguchi Emiko , Sanada Masashi , Chiba Kenichi , Tanaka Hiroko , Suzukawa Kazumi , Nanmoku Toru , Hasegawa Yuichi , Nureki Osamu , Miyano Satoru , Nakamura Naoya , Takeuchi Kengo , Ogawa Seishi , Chiba Shigeru 
Published: October 2019

Abstract

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.