Molecular Drivers in Chordoma




Abstract


Chordomas are tumors that arise from the osseous spine and skull base. They are often difficult to manage given their locally destructive behavior and predilection to grow near vascular and critical neural structures. Chordomas have an indolent clinical course, which results in late clinical presentation of patients. Unfortunately, these factors make the complete surgical resection of these tumors difficult to perform without causing significant morbidity. Although modern surgical and high-dose radiation techniques have improved outcomes, adjuvant therapies are the key to further progress, and enhanced understanding of the molecular drivers of chordomas is essential to their development. Recent advances in genomics, the establishment of human chordoma cell lines, and the development of animal models have created an exciting landscape for chordoma research. Here we survey the most recent discoveries in this field and identify important molecular pathways that could be targeted for future drug development.




Keywords

Brachyury, Chordoma, Chromosomal anomalies, Epigenetic deregulations, Molecular biology, Notochord, Pathogenesis, Tyrosine kinase pathways, Zebrafish model

 






  • Outline



  • Pathogenesis and Molecular Biology of Chordoma 23




    • Histological Grading 24



    • Cytogenetic Abnormalities 24



    • Epigenetic Deregulations 24



    • Receptor Tyrosine Kinase Signaling Pathways 26




  • Hypoxia and Angiogenesis in Chordoma 26



  • Apoptosis in Chordoma 26



  • Cancer Stem Cells 26



  • Animal Models of Chordoma 27



  • Molecular Drivers in Chordoma Invasion and Metastasis 28



  • Summary/Conclusions 28



  • References 28


© 2018 Elsevier Inc. All rights reserved. Please note that the copyright for the original figures submitted by the contributors is owned by Contributors.






  • Outline



  • Pathogenesis and Molecular Biology of Chordoma 23




    • Histological Grading 24



    • Cytogenetic Abnormalities 24



    • Epigenetic Deregulations 24



    • Receptor Tyrosine Kinase Signaling Pathways 26




  • Hypoxia and Angiogenesis in Chordoma 26



  • Apoptosis in Chordoma 26



  • Cancer Stem Cells 26



  • Animal Models of Chordoma 27



  • Molecular Drivers in Chordoma Invasion and Metastasis 28



  • Summary/Conclusions 28



  • References 28


© 2018 Elsevier Inc. All rights reserved. Please note that the copyright for the original figures submitted by the contributors is owned by Contributors.




Pathogenesis and Molecular Biology of Chordoma


Chordomas are rare primary bone tumors with an annual incidence of less than 0.1 per 100,000 individuals. They develop from the remnants of the primitive notochord and have a predilection for the axial skeleton, most commonly the sphenooccipital synchondrosis and sacrum. They grow slowly, rarely metastasize, and occur at any age, with a peak incidence in the fifth decade.


Virchow in 1857 initially described the unique intracellular “bubble-like” vacuoles of chordoma cells, which he termed physaliferous. Chordomas were initially thought to be derived from cartilage, but this hypothesis has since been abandoned as more contemporary evidence has confirmed Ribbert’s hypothesis from the 1890s that these tumors are derived from undifferentiated notochordal remnants within the vertebral bodies and throughout the axial skeleton. Studies of human embryos and fetuses and cell tracking experiments in mice have revealed that notochordal cells topographically distribute to sites where chordomas occur. Although direct evidence is still lacking that these cells transform to chordoma cells, the similarities of their distributions and their histological, molecular, and immunophenotypes suggest that these primitive cells are the substrate for transformation into chordoma tumor cells.


Perhaps the most compelling evidence in support of this notochordal hypothesis is the duplication of the T (brachyury) gene in familial chordoma. Brachyury is an important transcription factor in notochord development and is found to be expressed in rests of normal undifferentiated embryonic notochord in the axial skeleton. High-resolution array comparative genomic hybridization (CGH) of the chordoma genome has revealed unique duplications in the 6q27 region of familial chordomas. The duplicated region contains only the T (brachyury) gene, which is uniquely overexpressed in almost all sporadic chordomas compared with other bone or cartilaginous lesions. Brachyury regulates several stem cell genes and has recently been implicated in promoting epithelial–mesenchymal transition (EMT) in other human carcinomas. Although the specific pathogenetic role of brachyury of chordomas is undefined, T gene duplication and its overexpression suggest that brachyury is a critical molecular driver of tumor initiation and propagation.


Histological Grading


Microscopically, chordomas display a lobular architecture in which fibrous bands encapsulate groups of vacuolated (physaliferous) atypical epithelioid neoplastic cells embedded in a myxoid stromal cellular matrix and classically display reactivity for cytokeratin, and a variable expression for S-100 and epithelial membrane antigen (EMA) on immunohistochemistry (IH) panel ( Fig. 3.1 ). Chordomas are classified into four different subtypes based on histological and IH findings: conventional (i.e., classical), representing the majority of cases; chondroid, containing a matrix with cartilaginous differentiation and carrying a better prognosis; dedifferentiated, with highly undifferentiated spindle cells, no immunoreactivity for cytokeratin, and worse prognosis; and sarcomatoid, similar to the dedifferentiated subtype but with cytokeratin reactivity.




Figure 3.1


Chordoma immunohistochemistry and cytogenetics.

(A) Chordomas display a lobular architecture in which fibrous bands encapsulate groups of vacuolated (physaliferous) atypical epithelioid neoplastic cells embedded in a myxoid stromal cellular matrix, The first panel demonstrates an MRI of a sacral chordoma, and the second and third panels are hematoxylin-eosin stains of a chordoma exhibiting this classical cytoarchitecture. (B) Chordoma display reactivity for cytokeratin and have variable expression for S-100 and epithelial membrane antigen, and almost all express the transcription factor brachyury. (C) Karyotype analysis of G-banded chromosomes in metaphase demonstrating numerical, structural, and clonal abnormalities in a human chordoma cell.

Data previously published: Hsu W, Mohyeldin A, Shah SR, et al. Generation of chordoma cell line JHC7 and the identification of Brachyury as a novel molecular target. J Neurosurg 2011; 115 (4):760–69.


Classically, chordomas were pathologically identified by their physaliferous cells and immunoreactivity for S-100 and epithelial markers such as EMA and cytokeratins. Distinguishing between chondroid chordomas and chondrosarcomas was challenging because they share S-100 immunoreactivity and interpretation of cytokeratin expression on small biopsies is difficult until the discovery that the notochord developmental transcription factor, brachyury, is a discriminating novel biomarker for chordomas. A tissue microarray analysis of 103 skull base/head and neck chondroid tumors found 98% sensitivity and 100% specificity for brachyury and cytokeratin staining in detecting chordoma ( Fig. 3.1 ).


Cytogenetic Abnormalities


Chordomas may display different karyotypic and chromosomal anomalies depending on their stage (i.e., primary vs. recurrent), histopathology (i.e., classical vs. dedifferentiated), and anatomical site (skull base vs. spine).


Many chordomas have nonrandom multiple copy number alterations in which gains are less frequent than losses. Chromosome 7 polysomy and alterations were seen in 73% of chordomas, in 62% of primary tumors, and in all recurrent chordomas. Gains were most commonly seen for 7q and 20q. Gains of chromosome 7 occurred at loci 7q22, 7q33, 7q34, 7q36, 7p15, and 7p21–p22. These 7q gains correlated significantly with the expression of proto-oncogene tyrosine kinase, cMET. These findings suggest that alteration of chromosome 7 occurs early in chordoma development. Gene duplication of brachyury is the most common single gene anomaly in familial chordoma, and mutations have been found in four different oncogenes (ALK, CTNNB1, NRAS, PIK3CA). 22


Chromosomal losses in chordomas occur on 1p, 3p, 9p, 10q, and 13q ( Fig. 3.1C demonstrates the karyotype of a sacral chordoma; published data ). The loss of heterozygosity of tumor suppressor genes and overexpression and/or mutation of oncogenes may contribute to the genesis of chordomas. Mutations occur in tumor suppressor genes located in areas of frequent chromosomal loss and in oncogenes encoding molecular inhibitors that might be of therapeutic interest. Analysis of 865 hotspots for mutation in 111 oncogenes and selected tumor suppressor genes in chordomas identified mutations in ALK (A877S), CTNNB1 (T41A), NRAS (Q61R), PIK3CA (E545K), PTEN (R130), CDKN2A (R58∗), and SMARCB1 (R40∗). Of note, PTEN and CDKN2A lie in chromosomal regions found to have losses in CGH analysis (9p21 and 10q23, respectively), and 40% of mutations occurred in SMARCB1 and its companion chromatin regulatory genes.


SMARCB1 is part of the SWI/SNF chromatin remodeling complex, which controls chromatin compaction and gene expression. The absence of SMARCB1 expression, attributable to SMARCB1/INI1 gene deletions and a feature of rhabdoid tumors, is particularly characteristic of pediatric chordomas, which are poorly differentiated, highly aggressive malignancies with a poor prognosis.


Epigenetic Deregulations


The three main epigenetic mechanisms implicated in the origin of cancer, namely, histone modifications, aberrant DNA methylation, and altered miRNA expression, have been found in chordoma. Histones are a key part of the nucleosome, the basic subunit of chromatin. Histone modifications, such as acetylation and methylation of lysine residues, that alter transcription have been observed in chordomas. Aberrant DNA methylation at cytosine adjacent to a guanine nucleotide (CpG), which promotes silencing of tumor suppressor genes, has been found in 20 different genes in chordomas. Among them, KL gene, a tumor suppressor gene, binds to fibroblast growth factor receptor (FGFR) and inhibits ligand-dependent activation of insulin-like growth factor-1 and insulin pathways. RASSF1 gene, another tumor suppressor gene, is inactive in the presence of CpG hypermethylation in chordomas, and hypermethylation of the MGMT gene predicts chordoma recurrence.


Posttranscriptional gene regulation by miRNA degrades transcription and inhibits translation in ways that alter or disrupt the regulation of cell differentiation and apoptosis. In one study, 33 miRNAs and 2791 mRNAs were deregulated in chordoma compared with embryonic notochord tissue. Downregulation of five miRNAs (miR-149-3p, miR-663a, miR-1908, miR-2861, and miR-3185) was associated with overexpression of the mitogen-activated protein kinase (MAPK) signaling pathway and its related genes, which govern cell proliferation, differentiation, and survival. In chordoma, aberrant miRNA expression of miR-1 and miR-31 may downregulate c-MET, an oncogene overexpressed in many chordomas, and thereby inhibit cell growth and proliferation. Furthermore, miR-608 also downregulates c-MET and miR-34a in the oncogenic pathway containing epidermal growth factor receptor (EGFR)/Bcl-xL.


Receptor Tyrosine Kinase Signaling Pathways


Although nearly all familial and sporadic chordomas express brachyury, the T (brachyury) gene is duplicated only in familial chordoma; the cause of its overexpression in sporadic cases is unknown. Activation of MAPK/ERK (extracellular signal-regulated kinases) kinase (MEK) in the fibroblast growth factor (FGF) signaling cascade directs notochord development and early mesoderm induction by regulating the expression of brachyury. Excess activity in this pathway occurs in chordoma cells. Chordoma cells produce FGF2, and neutralization of FGF2 inhibits MEK/ERK phosphorylation, decreases brachyury expression, and promotes apoptosis, thus inhibiting cell proliferation. Silencing of brachyury expression reduces FGF2 secretion and blocks its tumor-promoting effects; this suggests the important presence of an autocrine feedback loop between FGFR signaling and brachyury expression in chordoma.


The overexpression of vascular endothelial growth factor (VEGF) receptor and EGFR in chordoma and the antiproliferative effect of some tyrosine kinase inhibitors (lapatinib and imatinib) on chordoma cell lines prompted clinical trials that have demonstrated modest antitumor activity.

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Feb 21, 2019 | Posted by in NEUROSURGERY | Comments Off on Molecular Drivers in Chordoma

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