NK-Cell Lymphoma Shares Strikingly Similar Molecular Features with a Distinct Set of γδ T-Cell Lymphoma and Identification of Aurora Kinase A Inhibitor as a Novel Therapeutic Agent.

Natural Killer (NK) cell lymphomas (NKCL) are rare with aggressive clinical behavior. The majority of these cases belong to extra-nodal NK/T-cell lymphoma of nasal type (ENKTL) of the current World Health Organization (WHO) classification scheme. ENKTL also includes peripheral T-cell lymphomas (PTCL) that are similar in many respects to the NK cell counterpart. Due to rarity of the disease and difficulty in obtaining adequate biopsy specimens, the molecular mechanisms underlining ENKTL are largely unknown. We profiled a series of NK-cell lymphoma cases and many well- characterized cell lines of NK- and T-cell lineages to define molecular classifiers that can distinguish NKCL from PTCL, including lymphomas of cytotoxic T-cells. We also evaluated oncogenic pathways in these tumors and the therapeutic potential of a novel inhibitor of a cell cycle regulator (aurora kinase A).

The gene expression profiling (GEP) of ENKTL (n=21) and PTCL-U (n=50) cases were performed using HG U133 plus 2 arrays (Affymetrix Inc, CA). GEP of other PTCL subtypes (n=90), normal NK and T cells (resting and activated), NK and T cell lines (n=14) and indolent NK- cell/large granular lymphocytic proliferation (NK-LGLP) (n=5) were used for comparative analysis. Immunohistochemistry (IHC) was used to validate the GEP findings. A novel aurora-kinase-A inhibitor (MK-8745) was obtained from Merck & Co (Merck & Co., Inc. NJ, USA) and incubated with the cell lines for 2 -24 hours at 0.1-1 μM concentrations.

The ENKTL showed a male predominance (2:1) with a median age of 55 years at diagnosis and aggressive clinical behavior [5-year OS (<10%)]. Unsupervised hierarchical clustering revealed that ENKTL cases formed a distinct cluster from PTCL entities, with a few PTCL-U cases interspersed in the cluster. The molecular classifier derived for NKCL was composed of 84 transcripts. The majority of the upregulated genes demonstrated the distinct phenotypic and functional characteristics associated with NK-cells, including expression of many cytotoxic molecules and NK cell associated chemokines. The down-regulated genes were largely associated with T-cell biology including T-cell activation and maturation. Most of the classifier genes were contributed by the neoplastic cells, not by stromal cells, as these genes showed similar pattern in normal NK cells and NK cell lines. Surprisingly, three γδ T-cell lines and five PTCL-U cases were re-classified as NKCL. The gene expression profile of these cases was very similar to NKCL but further analysis demonstrated that they expressed genes associated with TCR complex including TCR -γ,-δ, CD3 -γ and -δ mRNA, thus exhibiting γδ T-cell differentiation. Further pathological review indicated that all these tumors showed extranodal involvement and the expression of γδ TCR. These γδ PTCLs could be distinguished readily from a group of (αβ) cytotoxic PTCL and hepatosplenic T-cell lymphoma (HSTCL) by GEP and in the usage of the TCR V region. Pathway analysis revealed enrichment of gene signatures related to TGFβ and VEGF pathway and high proliferation in NKCL, when compared with IL-2 activated normal NK-cells or NK-LGLP. Due to high-proliferation associated with NKCL a mitotic inhibitor was tested. All NK-cell lines were highly sensitive to aurora kinase A inhibitor with a significant increase in apoptosis and cell cycle arrest at G2/M phase. There was concomitant decrease in phosphorylated aurora kinase-A and Survivin levels with induction of TP53.

The NKCL molecular classifier identified lymphomas of NK-cell origin as well as a unique subset of γδ T cell lymphoma. These tumors are derived from cytotoxic cells of the innate immune system associated with epithelial surfaces. They have a different GEP from the T-cell lymphomas derived from αβ cytotoxic T-cell and even other γδ T cell lymphoma such as HSTCL which may be derived from an ontogenically and functionally distinct subset of γδ T cells. The role of enriched pathways in NKCL pathogenesis and biology requires further investigation. The effectiveness of the Aurora-kinase A inhibitor in inducing apoptosis of NK-cell lines suggests that it may be a candidate for use in clinical trials in NKCL.

No relevant conflicts of interest to declare.