LAUSR

To the Editor: Solid pseudopapillary neoplasm (SPN) of the pancreas is an uncommon tumor of low malignant potential. SPN is classified as an epithelial tumor and shows the characteristic cytomorphological features of loosely cohesive cells lining several layers and monotonous round nucleoli. Kosmahl et al. and Geers et al. hypothesized that SPN might immigrate from gonadal embryonic tissues to developing fetal pancreatic tissues on the basis of the following: first, the strikingly high preponderance of SPN in female; second, the close proximity of the genital ridges to the pancreatic angle during embryogenesis; third, the progesterone receptor being consistently present in SPN, in conjunction with the recent report of the expression of the luteinizing hormone receptor in SPN. However, SPNs arising in the ovary are quite rare compared with those in the pancreas and Yan et al. reported the case of SPN colliding with a welldifferentiated pancreatic endocrine neoplasm and suggested that SPN may originate from a multipotential primordial cell. In this study, we performed immunohistochemical examinations of inhibin-a to investigate the hypothesis, furthermore, we performed immunohistochemical examination of Tcf-3 (T cell factor 3) to investigate the tumor genesis in seven cases of SPNs resected at Toho University Ohashi Hospital and Jikei University Hospital. This study was approved by the Toho University Ohashi Hospital Ethical Committee (No.15–51) and Jikei University Hospital Ethical Committee (No.27–169). Formalin-fixed, paraffin-embedded 4mm thick tumor specimens were used for immunohistochemical analysis. The antibodies used in this study were anti-inhibin-a antibody (anti-human inhibin-a polyclonal antibody, clone R1, Neomarkers for Lab Vision Corporation, United States) and anti-Tcf-3 antibody (anti-mouse tcf-3 monoclonal antibody, clone sc8635 Santa Cruz Biochemistry, Shanghai, China, kindly gifted by Professor Shinsuke Ohba, Division of Clinical Biotechnology Center for Disease Biology and Integrative Medicine The University of Tokyo Graduate School of Medicine). The immunoreactive scores (IRS) for inhibin-a were calculated multiplying intensity score (0, negative; 1, weak; 2, moderate; and 3, strong) by percentage of positively-stained cells (0, no staining; 1, 10% staining; 2, 11–50%; 3, 51%–80%; and 4, 81% staining). Positive inhibin-a expression was defined as 1 or more IRS. As for Tcf-3, the immunohistochemical assay was scored as histological score (H-score) multiplying intensity of staining (0, no staining; 1, weak; 2, moderate; or 3, strong) by the distribution of specific staining (0.0–1.0). Positive Tcf3 expression was defined as 0.1 or more H-score. Among seven cases of SPNs, six cases (85.7%) were female, and the mean age at surgery was 27.9 (10–49) years. One of the seven cases showed invasion of atypical cells into vessel, but the other cases did not. None of the seven patients had recurrence after surgical resection. All of the seven patients had typical histopathological features compatible with SPN showing loosely cohesive cells lining several layers and the nucleoli are uniformly round (Fig. 1a). The results of immunohistochemical analysis using anti-b-catenin antibody for all seven cases definitely confirmed the diagnosis of SPNs showing positivity of b-catenin in the nucleoli (Fig. 1b). The specimens of SPN were positively stained by inhibina focally in all of seven cases (Fig. 1c). 10%–90% of cells showed positivity in the cytosol from weak to strong and IRS were calculated as 2–12 (8.43 2.82). As for Tcf-3, the nucleoli were focally positive in tumor cells of five cases (Fig. 1d). Around 10%–20% of tumor cells showed positivity from weak to moderate and H-score were calculated as 0.1–0.4 (0.14 0.05). Inhibin-a is a member of the transforming growth factor-b (TGF-b) family and a regulatory factor in the ovary and testis, with a wide range of functions. Although inhibin-a did not express in the normal tissue of the pancreas, the expression level of the inhibin-a subunit in both ovary and testis; follicle granulosa cells, Sertoli cells, and Leydig cells in late-gestation human fetuses significantly increases during gestational age. Some pancreatic neuroendocrine tumors (pNETs) are positive for inhibin-a, but histopathological features of the SPN are different from pNETs. Tcf-3 is an important transcriptional factor counters pluripotency and is stimulated by b-catenin signaling pathways in embryonic stem cells. In this study, Tcf-3 was weakly positive in the nucleoli of five SPN cases. The pathophysiological mechanisms underlying the development of SPN has not been fully clarified, the ectopic expression of b-catenin in the nucleus is consistently associated with SPN. This aberrant nuclear localization of b-catenin is the result of mutations in exon 3 of the b-catenin gene which would cause b-catenin to bind to Tcf3 and form complex localizing in the nuclei. b-catenin signaling stimulates Tcf-3 associated with differentiation programs, neutralize the destabilizing activity of Tcf-3 in the pluripotency network, and subsequently activate