{"type": "root", "children": [{"type": "p", "children": [{"type": "t", "text": "\n CD99, a 32‐kDa transmembrane glycoprotein encoded by the MIC2 gene, plays multiple roles in cell biology that appear to be context‐ and cell type–dependent. In mesenchymal and bone‐derived tumors, such as osteosarcoma, CD99 has been shown to act as a tumor suppressor by regulating cell–cell adhesion and migration. Forced re‐expression of full‐length CD99 (CD99wt) promotes the recruitment of adhesion molecules such as N‐cadherin and β‐catenin to adherens junctions, while inhibiting key regulators of actin cytoskeletal remodelling (including ACTR2, ARPC1A, ROCK2, and ezrin). This reactivation of “stop‐migration” signals appears to favor a more differentiated, less metastatic phenotype and even primes osteosarcoma cells to undergo terminal osteoblastic differentiation via ERK pathway modulation."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "1"}]}, {"type": "t", "text": ""}]}, {"type": "t", "text": "\n \n "}, {"type": "p", "children": [{"type": "t", "text": "\n In T cells, in contrast, CD99 plays a nuanced role in regulating programmed cell death. Engagement of distinct CD99 epitopes elicits different responses: ligation via one epitope (recognised by the DN16 monoclonal antibody) induces apoptosis, whereas targeting another epitope (recognized by YG32) does not directly induce T‐cell death but instead enhances Fas–mediated apoptosis by promoting Fas receptor oligomerization."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "3"}]}, {"type": "t", "text": "\n "}]}, {"type": "t", "text": "\n \n "}, {"type": "p", "children": [{"type": "t", "text": "\n In classical Hodgkin lymphoma, low levels of CD99 are linked to a block in terminal B‐cell differentiation. Upregulation of CD99 in Hodgkin/Reed–Sternberg cells triggers downstream changes—including cytoskeletal re‐organisation mediated by differential expression of proteins such as SEPTIN2 and STATHMIN—that promote features of terminal B‐cell differentiation and alter diagnostic markers."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "4"}]}, {"type": "t", "text": "\n "}]}, {"type": "t", "text": "\n \n "}, {"type": "p", "children": [{"type": "t", "text": "\n In the context of hematologic malignancies, a distinct CD34+ leukemia‐associated immunophenotype co‐expressing CD99 together with CD123 and CD25 has been identified. This profile correlates with FLT3‐ITD mutations in acute myeloid leukemia, thereby underscoring the diagnostic and potentially prognostic utility of CD99 expression."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "6"}]}, {"type": "t", "text": "\n "}]}, {"type": "t", "text": "\n \n "}, {"type": "p", "children": [{"type": "t", "text": "\n Outside hematologic cancers, CD99 expression has also been implicated in chemoresistance. In ovarian cancer cells, higher CD99 expression is associated with cisplatin resistance; mechanistic studies indicate that the transcription factor Nrf2 upregulates CD99, and that suppression of CD99 can reverse chemoresistance."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "7"}]}, {"type": "t", "text": "\n "}]}, {"type": "t", "text": "\n \n "}, {"type": "p", "children": [{"type": "t", "text": "\n CD99 also functions via its interactions with specific ligands expressed on monocytes, natural killer cells, and dendritic cells. Recombinant CD99 proteins have been shown to upregulate cytokine production (notably IL‐6 and TNF‐α) in activated T cells and other leukocytes without affecting early activation markers, suggesting that CD99–ligand interactions help fine‐tune immune responses."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "8"}]}, {"type": "t", "text": "\n "}]}, {"type": "t", "text": "\n \n "}, {"type": "p", "children": [{"type": "t", "text": "\n The regulation of CD99 gene expression is linked to differentiation signals as well. For example, in osteoblast‐lineage cells, the transcription factor Cbfa1 (also known as RUNX2) directly influences CD99/MIC2 expression, further connecting CD99 to differentiation programs in mesenchymal cells."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "9"}]}, {"type": "t", "text": "\n "}]}, {"type": "t", "text": "\n \n "}, {"type": "p", "children": [{"type": "t", "text": "\n From a diagnostic perspective, the immunophenotypic detection of CD99 is valuable in the classification of small round cell tumors – including Ewing sarcoma, Ewing‐like sarcomas, synovial sarcoma, and even Merkel cell carcinoma – wherein the pattern of CD99 expression (ranging from diffuse membranous to paranuclear dot‐like staining) can aid in differential diagnosis."}, {"type": "fg", "children": [{"type": "fg_fs", "start_ref": "10", "end_ref": "14"}]}, {"type": "t", "text": "\n "}]}, {"type": "t", "text": "\n \n "}, {"type": "p", "children": [{"type": "t", "text": "\n Finally, structural studies have provided insights into the molecular basis of CD99 action. Nuclear magnetic resonance and circular dichroism analyses of the CD99 Type I cytoplasmic domain revealed a hairpin‐shaped structure with flexible loops and a modest intrinsic helical propensity. Such structural features may underpin its capacity to engage in diverse intracellular protein–protein interactions, thereby influencing signal transduction pathways."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "15"}]}, {"type": "t", "text": "\n "}]}, {"type": "t", "text": "\n \n "}, {"type": "p", "children": [{"type": "t", "text": "\n In summary, CD99 is a multifunctional protein that influences cell adhesion, migration, apoptosis, differentiation, immune regulation, and even chemotherapeutic responses. Its diverse roles, demonstrated across a variety of cell types and tumor contexts, underscore its potential as both a diagnostic marker and a therapeutic target.\n "}]}, {"type": "rg", "children": [{"type": "r", "ref": 1, "children": [{"type": "t", "text": "C Zucchini, M C Manara, R S Pinca, et al. "}, {"type": "b", "children": [{"type": "t", "text": "CD99 suppresses osteosarcoma cell migration through inhibition of ROCK2 activity."}]}, {"type": "t", "text": " "}, {"type": "i", "children": [{"type": "t", "text": "Oncogene (2014)"}]}, {"type": "t", "text": " DOI: "}, {"type": "a", "children": [{"type": "t", "text": "10.1038/onc.2013.152"}], "href": "https://doi.org/10.1038/onc.2013.152"}, {"type": "t", "text": " PMID: "}, {"type": "a", "children": [{"type": "t", "text": "23644663"}], "href": "https://pubmed.ncbi.nlm.nih.gov/23644663"}]}, {"type": "r", "ref": 2, "children": [{"type": "t", "text": "Marika Sciandra, Maria Teresa Marino, Maria Cristina Manara, et al. 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