Expression of glypican 3 in hepatoblastoma: an immunohistochemical study of 65 cases

Summary

Glypican 3 is a heparin sulfate proteoglycan bound to the cell surface that is theorized to participate in cell signaling, resulting in embryonic cell growth and differentiation. The GPC3 gene is mutated in Simpson-Golabi-Behmel syndrome, whose features include numerous developmental abnormalities, tissue overgrowth, and an increased risk for embryonal malignancies, including hepatoblastoma. GPC3 has been detected in hepatic stem cells and was recently identified as one of the most overexpressed genes in hepatoblastoma by microarray analysis. The purpose of this study was to analyze the expression of GPC3 in a large series of hepatoblastoma using immunohistochemistry to assess its use as a diagnostic marker. The GPC3 immunoreactivity was semiquantitatively evaluated in 65 cases of hepatoblastoma. In addition, histologic patterns in each tumor were individually assessed for immunoreactivity. All 65 hepatoblastomas had cytoplasmic immunoreactivity for GPC3 with greater than 90% of cases showing strong, diffuse positivity. There was no reactivity in benign liver tissue. Fetal, embryonal, and small cell undifferentiated patterns were diffusely positive in almost all cases, whereas mesenchymal and teratoid patterns were nearly all negative. The high GPC3 expression consistently demonstrated in this study suggests that GPC3 may play a role in the tumorigenesis of hepatoblastoma.

Introduction

Glypicans belong to a family of heparin sulfate proteoglycans that are localized to the cell membrane and are bound to the exocytoplasmic surface by a glycophosphatidylinositol anchor [1], [2]. There are 6 members identified in the glypican family in mammals, designated glypican 1 through 6. The gene encoding glypican 3 (GPC3), located on Xq26, is mutated in Simpson-Golabi-Behmel (SGB) syndrome [3]. This disorder is characterized by prenatal and postnatal overgrowth and numerous craniofacial, genitourinary, and skeletal developmental abnormalities [3]. In addition, patients have an increased risk for certain malignancies including hepatoblastoma, neuroblastoma, gonadoblastoma, Wilms tumor, and hepatocellular carcinoma [4], [5], [6]. The molecular mechanism by which GPC3 functions in development and tumorigenesis has not been fully elucidated [7], [8]. In organs with no adult expression, GPC3 may act as an oncofetal protein [9], [10], [11], [12], [13], [14], [15], [16], [17]. In contrast to its function as an oncofetal protein, GPC3 has been also suggested to be a tumor suppressor in other tissues that show adult expression [8], [18], [19], [20], [21].

Hepatoblastoma is the most common pediatric primary hepatic neoplasm accounting for more than 50% of malignant pediatric liver tumors. It is thought to arise from liver precursor cells as the tumor recapitulates fetal liver and is capable of epithelial and mesenchymal differentiation. Histologically, hepatoblastoma can be divided into 6 types based on patterns of differentiation including pure fetal epithelial, combined fetal and embryonal epithelial, macrotrabecular, small cell undifferentiated, mixed epithelial and mesenchymal, and mixed epithelial and mesenchymal with teratoid features. The least differentiated is the small cell undifferentiated pattern. Areas of the tumor that closely resemble the developing liver are designated as fetal, whereas the embryonal pattern appears more primitive. Epithelial elements are often mixed with mesenchymal cells and are classified as teratoid when containing elements typically not seen in developing or adult liver, such as squamous, mucinous, neural, or melanocytic cells. However, knowledge of the initial molecular events leading to tumor formation and subsequent differentiation is yet to be defined.

Prior studies have shown that GPC3 is overexpressed in hepatoblastoma by microarray analysis [22]. Furthermore, SGB syndrome, caused by mutations in the GPC3 gene, can be associated with hepatoblastoma [3], [11]. Therefore, GPC3 is an attractive candidate marker for hepatoblastoma. There is limited information on GPC3 immunoreactivity in hepatoblastoma, particularly in specific subtypes, and metastatic tumors as well as treatment effect on expression. The aim of this study was to conduct a detailed immunohistochemical study focusing on the expression of GPC3 in hepatoblastoma and evaluate its potential diagnostic use.