Novel Biological Effects and Distinct Patterns of Rhoa Mutations in Adult T-Cell Leukemia/Lymphoma and Angioimmunoblastic T Cell Lymphoma

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.