Molecular genetics of osteosarcoma

Abstract

Osteosarcoma is the predominant form of bone cancer, affecting mostly adolescents. Recent progress made in molecular genetic studies of osteosarcoma has changed our view on the cause of the disease and ongoing therapeutic approaches for patients. As we draw closer to gaining more complete catalogs of candidate cancer driver genes in common forms of cancer, the landscape of somatic mutations in osteosarcoma is emerging from its first phase. In this review, we summarize recent whole genome and/or whole exome genomic studies, and then put these findings in the context of genetic hallmarks of somatic mutations and mutational processes in human osteosarcoma. One of the lessons learned here is that the extent of somatic mutations and complexity of the osteosarcoma genome are similar to that of common forms of adult cancer. Thus, a much higher number of samples than those currently obtained are needed to complete the catalog of driver mutations in human osteosarcoma. In parallel, genetic studies in other species have revealed candidate driver genes and their roles in the genesis of osteosarcoma. This review also summarizes newly identified drivers in genetically engineered mouse models (GEMMs) and discusses our understanding of the impact of nature and number of drivers on tumor latency, subtypes, and metastatic potentials of osteosarcoma. It is becoming apparent that a synergistic team composed of three drivers (one 'first driver' and two 'synergistic drivers') may be required to generate an animal model that recapitulates aggressive osteosarcoma with a short latency. Finally, new cancer therapies are urgently needed to improve survival rate and quality of life for osteosarcoma patients. Several vulnerabilities in osteosarcoma are illustrated in this review to exemplify the opportunities for next generation molecularly targeted therapies. However, much work remains in order to complete our understanding of the somatic mutation basis of osteosarcoma, to develop reliable animal models of human disease, and to apply this information to guide new therapeutic approaches for reducing morbidity and mortality of this rare disease.

Keywords: Animal modeling; Bone cancer; Driver mutations; Genomic analysis; Osteosarcoma; Targeted therapy.

Figure 1. Models for development of osteosarcoma subtypes

(a) The “single driver” hypothesis states that each subtype is induced by a subtype-specific first driver (A, B, or C) in proliferative cells, which then determines three osteosarcoma (OS) subtypes: osteoblastic OS, fibroblastic OS, and chondroblastic OS. (b) The “multiple drivers” hypothesis states that each subtype is induced by two or more drivers, illustrated here by three first drivers (D, F, H) in addition to synergistic drivers (E, G, I), which then determines the different OS subtypes. The red arrow indicates a proliferating mesenchymal stem cell-derived osteoblast.

Figure 1. Models for development of osteosarcoma subtypes

(a) The “single driver” hypothesis states that each subtype is induced by a subtype-specific first driver (A, B, or C) in proliferative cells, which then determines three osteosarcoma (OS) subtypes: osteoblastic OS, fibroblastic OS, and chondroblastic OS. (b) The “multiple drivers” hypothesis states that each subtype is induced by two or more drivers, illustrated here by three first drivers (D, F, H) in addition to synergistic drivers (E, G, I), which then determines the different OS subtypes. The red arrow indicates a proliferating mesenchymal stem cell-derived osteoblast.

Figure 2. PI3K-mTORC1 signaling components altered in osteosarcoma

Simplified summary of mutated and/or dysregulated genes in human and mouse osteosarcoma (OS) that participate in the mTROC1 signaling pathway. In the presence of growth factors (e.g. Insulin) or other stimuli, receptor tyrosine kinase (RTK) intracellular domains are trans-phosphorylated, leading to the recruitment of the regulatory subunit of class IA PI3K, p85 (encoded by gene PIK3R1) and release of the catalytic subunit p100 (encoded by gene PIK3CA). One of the signaling components activating PI3K is Ras. Binding of a ligand to RTKs promotes dimerization of the receptor and subsequent autophosphorylation of tyrosine residues. This allows the RTK to interact with SH2 domain–containing proteins, such as Growth Factor Receptor Bound Protein 2 (GRB2), that can bind and activate Son of Sevenless (SOS), which in turn, activates RAS. Neurofibromin 1 (NF1) negatively regulates this process. Thus, p110 is activated by RAS independently of p85. Upon activation, PI3K catalyzes the formation of the second messenger phosphatidylinositol-3,4,5-trisphosphate (PIP3) at the plasma membrane, which can be down-regulated by the tumor suppressor, phosphatase and tensin homolog (PTEN). Increased PIP3 levels lead to the recruitment of PIP3-binding proteins, such as Protein Kinase B Alpha (Akt) and PDK1. At the membrane, PDK1 (encoded by PDPK1) phosphorylates Akt at Thr308 leading to partial activation of Akt. Akt then phosphorylates varied substrates, including tuberin (encoded by TSC2), an inhibitor of mTORC1. This results in the activation of Ras Homolog Enriched in Brain (Rheb). Activated Rheb binds to and activates mTORC1. Active mTORC1 promotes protein synthesis, lipogenesis, and energy metabolism, but inhibits autophagy and lysosome biogenesis. Other regulators of mTORC1 in OS, including Neurofibromin 2 (NF2) and Glycogen Synthase Kinase 3 Beta (GSK3b), which mediates Wnt signaling to phosphorylate and promote TSC2 activity and p53 in response to DNA damage. Rapamycin selectively inhibits mTORC1. Round shape: oncogene. Rectangular shape: tumor suppressor gene (TSG). Red text: mutations identified in human OS samples [28-32]. Asterisks (*) denote mutations identified by Sleeping Beauty forward genetic screen [19]. Green background: change of expression identified by transcriptome analysis of p53 and Notch OS [16]. Gray text: targeted agents being tested in clinical trials. Orange bar-headed lines or arrows: drug mechanism (inhibition or activation, respectively).