STAT3 Gene Mutations and the Association with Pure Red Cell Aplasia in Large Granular Lymphocyte Leukemia.

Abstract 2668

Large granular lymphocyte leukemia (LGL-L) is a proliferative disorder of cytotoxic T cells or NK cells frequently complicated with cytopenia and autoimmune phenomena. In the current WHO classification, T-cell large granular lymphocyte leukemia (T LGL-L), and chronic lymphoproliferative disorders of NK cells (CLPD-NK) are included in this category. Aggressive NK cell leukemia (ANKL) and Epstein-Barr virus (EBV)-positive T-cell lymphoproliferative disease of childhood (EBV-T LPD) are also categorized as entities in the classification based on the clinical characteristics and EBV-positivity, and their morphologies closely resemble LGL. There has been controversy concerning the nature of these diseases, whether they are reactive processes or neoplasms. Recently, recurrent somatic mutations in the src homology (SH) 2 domain of the signal transducer and activator of transcription 3 (STAT3) gene have been found in T LGL-L and CLPD-NK, leading to constitutive activation of STAT3 and dysregulation of genes downstream of STAT3. Since LGL-L consists of various disorders and is suggested to differ based on racial backgrounds, these findings prompted us to investigate mutations in STAT3 in a Japanese cohort of LGL-L.

The study included a total of 36 patients (pts) with LGL. Genes of exons 19 to 24 in STAT3 were amplified by PCR and sequenced directly using genomic DNA isolated from peripheral blood mononuclear cells. Since Y604F and D661Y mutations in STAT3 gene were representative and frequently recognized, allele specific PCR (AS-PCR) assays for these mutations were also performed.

Pts consisted of 18 with T LGL-L (14 with αβ T cell receptor (TCR) type and 4 with γΔ TCR type), 11 with CLPD-NK, 5 with ANKL, and 2 with EBV-T LPD; one pt with αβ TCR type and 1 with γΔ TCR type, as well as 1 with reactive NK lymphocytosis used as control.

By direct sequencing, two mutations, Y640F and D661Y, were identified. Y640F was recognized in 1 pt with αβ T LGL-L and D661Y in 3 pts with CLPD-NK. By AS-PCR, 10 additional pts were found to be positive for mutations of either Y604F or D661Y. A pt with T LGL-L was positive for both mutations by AS-PCR, although no mutations were detected by direct sequencing. A pt with CLPD-NK was positive for Y640F by AS-PCR in addition to D661Y recognized by direct sequencing. All 4 pts positive for the mutations by direct sequencing were confirmed to be positive by AS-PCR. In total, 7 pts with αβ TCR type T LGL-L and three T LGL-L pts with γΔ TCR type were positive for mutations. All these mutations were heterozygous. Mutations in SH2 domains of the STAT3 gene were not found in ANKL or EBV T-LPD pts by either direct sequencing or AS-PCR. Samples from fifty healthy controls were examined by AS-PCR and all were negative for Y604F or D661Y mutations. Reactive NKL lymphocytosis and three cell lines, Jurkat, NKL and NK92, were also negative for the mutations. The frequencies of STAT3 mutations in T LGL-L and CLPD-NK were 55.6% and 27.3%, respectively (P = 0.25). Among the pts with T LGL-L and CLPD-NK, the frequency of pure red cell aplasia (PRCA) was significantly higher in pts with the mutations (8/13) than in those without the mutations (3/16) (P = 0.03). In three T LGL-L pts positive for the mutations examined on subsequent occasions, the mutation became undetectable after cyclosporine A treatment in one pt, and was persistently found at stable amounts in two other pts by quantitative PCR.

Our results indicate that mutations in the SH2 domain of the STAT3 gene frequently occur in T LGL-L and CLPD-NK in a Japanese cohort and these mutations are closely associated with PRCA and treatment requirements. STAT3 mutation thus likely contributes to the pathogenesis of T LGL-L and CLPD-NK, while EBV-associated LGL diseases, such as ANKL or EBV T-LPD, might be driven by mechanisms other than STAT3-associated pathways.