Abstract 3542: Abnormal intracytoplasmic accumulation of autophagy-related protein p62/SQSTM1 characterizes giant cells of giant cell tumor of bone
Authors: Motoi Toru , Yoshida Akihiko , Motoi Noriko , Kato Ikuma , Okuma Tomotake , Tonooka Akiko , Horiguchi Shinichiro , Goto Takahiro , Hishima Tsunekazu 
Published: July 2016

Abstract
Background
Giant cell tumors of bone (GCTs) are locally aggressive neoplasms composed of osteoclast-like giant cells and mononuclear cells. Mononuclear cells are accepted as true neoplastic elements, while osteoclast-like giant cells are non-neoplastic. However, close interaction between two cellular populations is indispensable for tumor formation. Large giant cells with hypermultinucleation of GCT are distinct from normal osteoclasts; however, their abnormality has not yet been sufficiently investigated. Recently, a germline mutation of the gene encoding autophagy-related protein p62/SQSTM1 was reported in Paget disease of bone, which implicates a role of autophagy in the differentiation and function of osteoclasts. In order to understand the relationship between autophagy and osteoclastogenesis in giant cell lesions, we studied clinical samples of GCTs and related tumors for immunoexpression of p62 (an autophagy flux marker), LC3 (an autophagosome marker), and Cathepsin K (a known osteoclastic marker).
Design
Formalin-fixed paraffin-embedded specimens of 46 GCTs, 17 chondroblastomas (CBLs), 3 giant cell reparative granulomata (GRGs), 3 chondromyxoid fibromas (CMFs) and one each of aneurysmal bone cyst (ABC) and brown tumor (BRT) were retrieved. Immunohistochemistry was performed by using antibodies against p62, LC3, and Cathepsin K. Results were scored as 3+ (strongly positive in ≥50% of cells), 1+ (weakly positive in &lt;10% of cells), 2+ (intermediate between 1+ and 2+), or 0 (negative). Giant cells and mononuclear cells were separately evaluated for nuclear and cytoplasmic staining.
Results
For giant cells, p62 was positive in a dot-like manner in all tumor types. Strong (3+) staining was found in nuclei and/or cytoplasms in GCTs (27/46, 59%), CBLs (14/17, 82%), GRGs (2/3, 67%), and other lesions (5/5, 100%). In addition, abnormal intracytoplasmic coarse aggregates were specifically noted in GCTs (21/46, 46%) and CBLs (3/17, 18%). In contrast, mononuclear cells showed a lower prevalence of 3+ p62 positivity in GCTs (2/46, 4%), CBLs (5/17, 29%), and an ABC (1/1), where positive cells were only presumable osteoclastic precursor cells around giant cells. LC3 showed a limited cytoplasmic 3+ positivity in giant cells of GCTs (6/46, 13%), CBLs (7/17, 41%), CMFs (1/3, 33%), and BRT (1/1). Cytoplasmic staining of Cathepsin K was constantly observed in giant cells.
Conclusion
Abnormal intracytoplasmic accumulation of p62 in a form of coarse staining aggregate characterizes giant cells of GCTs and CBLs. This finding may be a potential diagnostic marker for distinguishing GCTs and CBLs from other giant cell lesions. The presence of these p62 aggregates and low LC3 expression may indicate disturbance of autophagy-driven protein clearance. As p62 immunostain recognizes a wider osteoclastic population than Cathepsin K, p62 may be involved in abnormal osteoclastogenesis.
Citation Format: Toru Motoi, Akihiko Yoshida, Noriko Motoi, Ikuma Kato, Tomotake Okuma, Akiko Tonooka, Shinichiro Horiguchi, Takahiro Goto, Tsunekazu Hishima. Abnormal intracytoplasmic accumulation of autophagy-related protein p62/SQSTM1 characterizes giant cells of giant cell tumor of bone. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3542.