Pathogenesis and therapeutic implications of EBV-associated epithelial cancers

Abstract

Epstein-Barr virus (EBV), one of the most common human viruses, has been associated with both lymphoid and epithelial cancers. Undifferentiated nasopharyngeal carcinoma (NPC), EBV associated gastric cancer (EBVaGC) and lymphoepithelioma-like carcinoma (LELC) are amongst the few common epithelial cancers that EBV has been associated with. The pathogenesis of EBV-associated NPC has been well described, however, the same cannot be said for primary pulmonary LELC (PPLELC) owing to the rarity of the cancer. In this review, we outline the pathogenesis of EBV-associated NPC and EBVaGCs and their recent advances. By drawing on similarities between NPC and PPLELC, we then also postulated the pathogenesis of PPLELC. A deeper understanding about the pathogenesis of EBV enables us to postulate the pathogenesis of other EBV associated cancers such as PPLELC.

Keywords: Epstein-Barr virus; lymphoepithelioma-like carcinoma; nasopharyngeal cancer; pathogenesis; primary pulmonary lymphoepithelioma-like carcinoma.

Figure 1

Figure 1 describes the proposed multistep pathogenesis of PPLELC. We propose that PPLELC follows a multistep pathogenesis process similar to that in NPC. Early events Like NPC, distinct genetic profiles (unidentified HLA haplotype, and TLR subtypes variations) and early EBV infection may result in susceptibility to EBV infection. Early infection results in a type II latency program, suggested by a molecular profile that is highly resembling that of NPC. Additional risk factors such as being of a Chinese race, female, and nonsmokers are also associated with increased incidences of PPLELC. However, our knowledge of the role of epigenetic and environmental risk factors in PPLELC is still in its infancy. Type II latency program In these non-malignant respiratory ciliated columnar cells, EBV can establish latency and subsequently express latent EBV oncoproteins (LMP1, LMP2A, BART miRBA). Mutations in TP53, JAK/STAT, and cell cycle genes such as CDKN2A and CCND1 further drive non-malignant respiratory epithelium to PPLELC. The molecular and mutational landscape in PPLELC bears a striking resemblance to the molecular profile of NPC, suggesting that EBV enters a type II latency program in PPLELC. While it is unclear how these type II latency oncoproteins confer oncogenic traits in PPLELC, extrapolations can be drawn from our understanding of their established roles in NPC pathogenesis. It is likely that these oncoproteins and their downstream signaling pathways help enhance cell survival and facilitate tumorigenesis, resulting in the transformation from non-malignant respiratory epithelium to PPLELC.

Figure 2

Figure 2 illustrates the multistep model of the pathogenesis of EBV positive NPC. A complex interplay of known genetic variations with environmental risk factors increases susceptibility to NPC. Subsequent chromosomal loss (3p, 9p, 11a), loss of function mutations (CDKN2A, cCDN1, RASSF1A, PTEN), and widespread hypermethylation lead to high-grade preinvasive lesions. Mutations in the NF-kB, MAPK/PI3K/STAT and TP53 pathways further drive tumorigenesis. Concurrently, EBV oncogenes upregulate the expression of various genes, perpetuating tumorigenesis. Early events in NPC pathogenesis and epigenetic changes Numerous environmental factors have stood out in its relation to NPC. Dietary habits, such as preserved fish and foods, as well as occupational exposures to wood dust, formaldehyde and cigarette smoke emerged as significant contributors which heightened risk of NPC. In early stages of NPC, in addition to various TLR and HLA polymorphisms, epigenetic alterations such as CpG Island hypermethylation, and chromosomal loss (3p, 9p21) play a pivotal role in silencing tumor suppressor genes, contributing to disease initiation and progression. Abnormalities in signaling pathways Dysregulation of the NF-KB, MAPK PI3K and STAT pathways are well known in NPC. LMP1 activates NF-KB by engaging both the canonical and non-canonical pathway, facilitating apoptotic evasion and immune escape. The MAPK, PI3K and STAT pathways are also activated via the TNF receptor, upregulating anti-apoptotic genes and pro-survival signals. LMP1 and tumorigenic properties LMP1 is one of the key players of tumor progression, orchestrating various mechanisms that drive malignancy. Through activation of the FGFR1, mTOR and, the NF-KB pathway, upregulation of MMP, fibronectin and integrin-a5, and enhanced VEGF expression, LMP1 stimulates new vessel formation, a critical factor facilitating tumor proliferation and metastasis. Simultaneously, LMP1 disrupts immune surveillance by hampering antigen presentation and amplifying anti-apoptotic signals, allowing it to evade immune detection and circumvent cell cycle checkpoints. EBV fosters a tumor suppressive microenvironment and this is enabled by LMP1 upregulation of IL-10 and upregulation of T helper cells. Uncontrolled cell proliferation is attained through LMP1 hyperphosphorylation of DK2 and Rb thus promoting G1/S progression, and LMP1 regulation of telomerases resulting in cell immortality. Other prominent EBV products include LMP2 and EBNA1, which are further detailed under Figure 3. An overview of the development of NPC In summary, Figure 2 provides a brief overview of the multi step model of NPC pathogenesis. This model posits that the development of NPC is a highly complex, multistage process of cumulative environmental exposures and genetic changes, spanning from initial infection and environmental encounters leading to epithelial dysplasia, and subsequent genetic and epigenetic alterations over time activating oncogenesis. Products of type II latency program then confer tumorigenic properties to NPC cells, enabling growth and aggressive invasion.

Figure 3

Figure 3 presents a comprehensive and intricate depiction of the pivotal roles played by type II latency Epstein-Barr virus (EBV) oncoproteins, specifically Latent Membrane Proteins 1 and 2A (LMP1 and LMP2A), in conjunction with the crucial gene product EBNA1. It sheds light on how their orchestrated actions contribute to the initiation and progression of tumorigenesis by silencing of key tumor suppressor genes and driving cell proliferation and survival. LMP1 and LMP2A: masters of oncogenic signaling At the forefront of this dynamic interplay are the oncoproteins LMP1 and LMP2A. LMP1 is a multifunctional transmembrane protein. By emulating the constitutive activation of CD40, a pivotal B-cell receptor, LMP1 engages the TNF receptor-associated factors (TRAFs) and orchestrates the activation of the canonical NF-kB pathway. This activation culminates in the creation of a proinflammatory milieu that is conducive to cell survival and unbridled proliferation. Furthermore, LMP1’s capacity to activate the non-canonical NF-kB pathway further amplifies these signals, resulting in a sustained NF-kB activity that effectively shields cells from apoptotic cues. LMP1’s influence isn’t confined to NF-kB signaling alone. It interfaces with a multitude of molecules, including Janus kinases (JAKs), STATs, and PI3K, thereby activating pathways that foster cell growth and survival. Moreover, LMP1’s impact on various microRNAs and transcription factors significantly influences gene expression and cellular behavior. Beyond direct oncogenic signaling, LMP1 also modulates the tumor microenvironment, augment cellular invasiveness, and promotes angiogenesis. These help create a supportive niche for tumor progression and metastasis. Complementing LMP1, LMP2A emerges as a critical player in EBV-associated oncogenesis. This transmembrane protein mimics the B-cell receptor (BCR), a critical component in B-cell signaling. By recruiting protein tyrosine kinases, including Lyn and Syk, LMP2A effectively dampens BCR signaling. Activation of the PI3-kinase/Akt axis stimulates GSK3 which culminates in the accumulation of betacatenin, thereby promoting WNT signaling. This strategic inhibition aids in evading the regulatory mechanisms that would otherwise prompt apoptosis of autoreactive B cells. This evasion, in turn, promotes the survival of EBV-infected cells, and similar to LMP1, fosters a cellular milieu that supports enhanced proliferation and survival. EBNA1: guardian of viral persistence and cellular transformation Central to this network of interactions is the gene product EBNA1. This multifaceted protein stands as a linchpin in ensuring the persistence of EBV within the host cell. In addition to its role in maintaining viral genome replication, EBNA1 actively engages cellular machinery to promote cell proliferation and survival. Importantly, EBNA1 orchestrates the silencing of key tumor suppressor genes, notably CDKN2A and TP53. This results in an imbalance that shifts the cellular equilibrium toward unchecked cell growth and survival, fueling the malignant transformation of host cells. A holistic view of tumorigenesis Collectively, Figure 3 encapsulates the multifaceted orchestration of type II latency EBV oncoproteins, LMP1, LMP2A, and EBNA1, in driving the molecular intricacies of tumorigenesis. Their intricate molecular interplay, ranging from direct oncogenic signaling to the modulation of the tumor microenvironment, lays the foundation for cell transformation and the subsequent development of EBV-associated malignancies.