Absence of Somatically Acquired JAK1 Mutations in Adult T-Cell Leukemia/Lymphoma.

Janus kinase 1 (JAK1) plays a critical role in lymphocyte proliferation and differentiation. Somatic JAK1 mutations are found in 18% of adult precursor T acute lymphoblastic leukemias (T-ALL). Some of the mutations were shown to induce the phosphorylation of JAK1 and STAT5 and lead to cytokine-independent proliferation. These data suggest that dysregulation of JAK1 can be involved in the development or progression of T-ALL (Flex et al. J Exp Med. 2008;205:751-758). Adult T-cell leukemia/lymphoma (ATLL) is a type of T-cell neoplasm, and the activation of JAK/STAT is sometimes observed in the tumor cells. Therefore, we investigated JAK1 mutations in ATLL patients.

Twenty Japanese ATLL patients whose percentage of peripheral abnormal lymphocytes was greater than 30% total cell count were sequentially enrolled into the study from 2000 to 2007. Diagnosis of ATLL was made on the basis of clinical features and laboratory characteristics. All cases tested positive for the serum anti-HTLV-1 antibody. The diagnosis was confirmed by observing monoclonal insertion of the HTLV-1 viral genome into leukemia cells by Southern blot hybridization. Peripheral blood mononuclear cells (PBMCs) were isolated and cryopreserved at -80°C. These PBMCs were thawed and genomic DNA was isolated using standard protocol. The entire coding sequence of the JAK1 gene (exons 2 through 25) was amplified by the polymerase chain reaction (PCR) method. The sequence of PCR primers were kindly provided by Dr. Marco Tartaglia (Istituto Superiore di Sanità, Roma, PhD). The nucleotide sequences were determined by fluorescent dye chemistry sequencing and analyzed by sequencing analysis software. By referencing the assembled sequence in the Ensembl genome database, the presence of homozygous mutations was first checked and then candidates for heterozygous mutations or single nucleotide polypeptides (SNPs) on each allele were screened by comparing the ratio of different bases calculated with the height of the peaks seen from sequencing to the reference genome when the ratio was between 0.15 and 1.0.

The percentage of abnormal lymphocytes ranged from 30-90%, and the mean value was 55.4%. The mean value of WBC and lymphocyte number was 40.5×10⁹/L and 33.4×10⁹/L, respectively. The mean value of LDH, Ca²⁺ or sIL-2R was 609 IU/L, 11.4 mg/dL, or 54748 U/mL, respectively. According to Shimoyama criteria (Shimoyama et al. Br J Haematol. 1991;79:428-437), 19 cases were diagnosed as acute-type ATLL, and one case was diagnosed as chronic-type ATLL. The surface markers of all but one abnormal PBMC were CD3+CD4+CD8-CD25+. In that one exception, loss of CD4 expression was observed. We examined the entire coding sequence of the JAK1 gene in 20 ATLL patients and identified no nonsynonymous or nonsense mutations and five types of silent substitutions in 12 cases. All silent substitutions were synonymous SNPs, as determined from referencing the base sequence in the Ensembl genome database. In the ATLL patients examined, the genotype frequency (%) is c546-AA/AG/GG, 97.5/2.5/0; c1590-CC/CT/TT, 97.5/2.5/0; c2049-CC/CT/TT, 50/50/0; c2097-CC/CG/GG, 95/5/0; c2199-AA/AG/GG, 60/40/0. There is no statistical difference in genotype frequency pattern of these SNPs, between the Japanese ATLL patients examined and the general Asian population on the Ensembl database.

Mutations in the coding region of JAK1 do not associate with either activation of the JAK/STAT pathway or leukemogenesis in ATLL. We only examined the coding region of JAK1, and the regulatory region of JAK1 remains to be investigated. Further investigation including downstream signaling molecules and inhibitory molecules in the JAK/STAT signaling pathway is necessary to clarify the mechanism contributing to the leukemogenesis of ATLL.

No relevant conflicts of interest to declare.