To the Editor
To find cancer-related genome alterations in T-cell large granular lymphocytic leukemia, Koskela et al. [Citation1] analyzed whole exomes of the cancer cells by next-generation exome sequencing. They found that signal transducer and activator of transcription 3 gene (STAT3) was frequently mutated in T-cell large granular lymphocytic leukemia (31 of 77 patients (40%) [Citation1]. Of note, all of the STAT3 mutations were found in exon 21, encoding the Src homology 2 (SH2) domain that mediates dimerization and activation of STAT3 protein. The recurrent STAT3 mutations consisted of p.Y640F, p.D661V, p.D661Y and p.N647I. The mutations increased transcriptional activities of STAT3, suggesting their roles in the tumorigenesis [Citation1].
STAT3 is a transcription factor that is activated in response to a large number of cytokines and growth factors [Citation2]. STAT3 transcriptional targets include bcl-2, mcl-1 and cyclin D1, reflecting its oncogenic roles in promoting cell survival and cell cycle progression [Citation1,Citation3]. Also, activated STAT3 maintains constitutive NF-κB activity in cancers [Citation4]. STAT signaling is frequently activated in a wide spectrum of hematologic malignancies and solid tumors [Citation4]. To see whether STAT3 exon 21 mutation is responsible for activated STAT signaling in common human malignancies, we analyzed the mutation in a large number of human tumors from various origins by polymerase chain reaction (PCR)-based single strand conformation polymorphism (SSCP) assay in this study.
For this, 2677 randomly collected tumor tissues from Korean patients () were used for this study. The hepatocellular carcinomas consisted of two grade I, 14 grade II and 20 grade III cancers by Edmondson's classification. All of the breast carcinomas were invasive ductal carcinomas [estrogen receptor positive (ER+): 66%, progesterone receptor positive (PR+): 58%, ER+/PR+: 51%]. The lung carcinomas consisted of 81 adenocarcinomas and 75 squamous cell carcinomas. The colorectal carcinomas originated from cecum (N = 10), ascending colon (N = 58), transverse colon (N = 18), descending colon (N = 20), sigmoid colon (N = 114) and rectum (N = 191). The GC consisted of 110 diffuse-type, 95 intestinal-type and 35 mixed-type gastric adenocarcinomas by Lauren's classification, and 220 advanced and 20 early gastric carcinomas according to the depth of invasion. The PCA consisted of 17 Gleason's score 6, 125 score 7, 100 score 8 and 33 score 9 cancers. The tumors did not include T-cell large granular lymphocytic leukemia. For the solid tumors, malignant cells and normal cells were selectively procured from hematoxylin and eosin-stained slides by microdissection as described previously [Citation5]. Approval for this study was obtained from the Catholic University of Korea, College of Medicine's institutional review board. Genomic DNA each from tumor cells and normal cells of the same patients were amplified by PCR with a primer pair (5’-CTGTGACAGGTAAGACCCAGAT-3’ and 5’-CGACAATACTTTCCGAATGC-3’; product size: 184 base pairs). Radioisotope ([32P]dCTP) was incorporated into the PCR products for detection by autoradiogram. Other procedures of the PCR-SSCP were described in our previous studies [Citation5]. After SSCP, direct DNA sequencing reactions were performed in the cancers with the mobility shifts in the SSCP.
Table I. STAT3 exon 21 mutations analyzed in 2677 tumors.
PCR and subsequent SSCP analysis identified two tumors with aberrantly migrating bands. None of the corresponding normal samples from the same patients showed evidence of mutations by SSCP, indicating the mutations had risen somatically. Direct DNA sequencing analysis of the tumors with the aberrantly migrating bands led us to identify that these aberrant bands represented STAT3 mutations. One of the STAT3 mutations [c.1967G> A (p.G656D)] was detected in a peripheral T-cell lymphoma and the other [c.1927C> A (p.C643K)] was detected in a rectal adenocarcinoma. None of these were the recurrent mutations identified in T-cell large granular lymphocytic leukemia. We repeated the experiments twice in the two mutated cases, including tissue microdissection, PCR, SSCP and direct DNA sequencing analysis to ensure the specificity of the results, and found that the data were consistent.
One of the main concerns in cancer genetics is to identify whether any mutation found in a cancer is common in other cancer types. In this study, however, we could not indentify STAT3 exon 21 mutations that had been recurrently detected in T-cell large granular lymphocytic leukemia. Also, the incidence of STAT3 exon 21 mutations in our study was very low (2/2,677), suggesting that these mutations may be passenger mutations. Taken together, our results suggest that STAT3 exon 21 mutation is rare in most human cancers and that the recurrent mutation may be specific to T-cell large granular lymphocytic leukemia. As activated STAT3 is considered to play a role in cancer development, the discovery of STAT3 recurrent mutations in T-cell large granular lymphocytic leukemia offered an opportunity for developing therapeutic tools targeting human cancers. Our data, however, suggest that development and application of therapies targeting STAT3 exon 21 mutants should be limited to T-cell large granular lymphocytic leukemias.
Declaration of interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.
This study was supported by a grant from Korea Research Foundation (2012047939).
References
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- Germain D, Frank DA. Targeting the cytoplasmic and nuclear functions of signal transducers and activators of transcription 3 for cancer therapy. Clin Cancer Res 2007;13:5665–9.
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- Lee H, Herrmann A, Deng JH, Kujawski M, Niu G, Li Z, et al. Persistently activated Stat3 maintains constitutive NF-kappaB activity in tumors. Cancer Cell 2009;15:283–93.
- Lee JW, Soung YH, Kim SY, Lee HW, Park WS, Nam SW, et al. PIK3CA gene is frequently mutated in breast carcinomas and hepatocellular carcinomas. Oncogene 2005;24:1477–80.