A Gain-of-Function CCR4 Mutations in Adult T-Cell Leukemia/Lymphoma (ATLL) Enhance the Chemotactic Abilities and PI3K/AKT Activation

High expression of CC chemokine receptor 4 (CCR4) has been identified as a hallmark gene in ATLL, an aggressive peripheral T-cell neoplasm. CCR4 is a chemokine receptor, which has a critical role in immune cell trafficking including Th2, T-reg and skin-homing memory T-cells. CCR4 ligands, CCL17 and CCL22, were produced in lymph nodes and skin from dendritic cells, macrophages and Langerhans cells. Most ATLL cases express surface CCR4 (90%) and infiltrate to lymph nodes and skin. These observations suggest that CCR4 could have a role in ATLL biology, but it is still unclear whether dysregulation of CCR4 function contributes to ATLL pathogenesis.

We performed RNA-Seq for two primary ATLL cases and discovered recurrent non-sense mutations in CCR4. Though an extended analysis using Sanger sequencing, CCR4 mutations were detected in 14/53 ATLL samples (26%) and consisted exclusively of nonsense or frameshift mutations that truncated the coding region at C329, Q330 or Y331 in the carboxy-terminus. We noticed that the location of the CCR4 mutations in ATLL were reminiscent of mutations affecting the chemokine receptor CXCR4 in the WHIM syndrome, a human immunodeficiency disease. Most CXCR4 mutations in the WHIM syndrome are nonsense or frameshift, resulting in carboxy-terminal truncation of the protein and conferring a gain-of-function phenotype with respect to chemotaxis towards the CXCR4 ligand SDF1. We therefore hypothesized that mutant CCR4 isoforms might enhance chemotaxis of the affected cells to CCR4 ligands.

Chemotaxic assay using 32Db, a mouse myeloid cell line, and ED40515(+), an ATLL cell line, clarified that the ectopic expression of CCR4-Q330* enhanced the chemotactic ability of the transduced cells toward CCL17 and CCL22 rather than CCR4-WT transduced cells with statistical significance. To understand the mechanism of this enhanced chemotactic ability, we studied the change in surface CCR4 levels after CCL22 exposure in CCR4-WT-and CCR4-Q330*-reconstituted ED40515(+) cells. Compared with CCR4-WT, CCR4 internalization in CCR4-Q330*-reconstituted cells was significantly impaired. Thus, the ATLL CCR4 mutants impair desensitization by ligand, which likely contributes to the enhanced chemotaxis of cells bearing these mutants.

We explored the influence of the ATLL CCR4 mutants on PI(3) kinase (PI3K)-dependent activation of AKT since it has been reported that binding of CCL22 to CCR4 activates AKT in CEM leukemic T-cells and in human Th2 cells. CCR4-Q330*-reconstituted ED40515(+) showed strong activation of AKT with CCL22 ligation compared with CCR4-WT-reconstituted cell. The AKT activation was cancelled with pan-PI3K inhibitor, BKM120, indicating that CCR4-mediated AKT activation was PI3K dependent.

Lastly, we tested whether the acquisition of CCR4 mutations by ATLL cells imparts a selective growth advantage relative to cells with wild type CCR4. CCR4-Q330*-reconstituted cells had a selective growth advantage in the presence of CCL22, supporting at least in part the hypothesis that CCR4 mutation are able to provide the affected cells a positive selection pressure through CCL22 ligation and contributes to ATLL pathogenesis.

We discovered for the first time recurrent somatic mutations in CCR4 in ATLL. CCR4 mutations were detected in 14/53 ATLL samples (26%) and consisted exclusively of nonsense or frameshift mutations that truncated the coding region at C329, Q330 or Y331 in the carboxy-terminus. Functionally, the CCR4-Q330* was a gain-of-function since it increased cell migration towards the CCR4 ligands CCL17 and CCL22, in part by impairing receptor internalization. This mutant enhanced PI(3) kinase/AKT activation following receptor engagement by CCL22 in ATLL cells, and conferred a growth advantage in in vitro cultures. Our findings provide a rationale to test whether inhibition of CCR4 signaling might have a therapeutic potential for patients with ATLL.