The process of cell fusion is a slowly-growing area of research attention, despite fusion’s frequent occurrence throughout the human body. Cell-cell fusion is a complex and energy-consuming process that requires cytoplasmic and/or nuclear contents to mix, yet is fundamental to the development and survival of many human tissues, both healthy (Brukman et al., 2019, Kloc et al., 2021, Emans et al., 1993, Wang et al., 2020, Hernández and Podbilewicz, 2017, Ahmadzadeh et al., 2022, Kloc et al., 2022, Elson et al., 2022, Soe et al., 2011, Kloc et al., 2022, Petrany and Millay, 2019, Brukman et al., 2022, Lu and Ikawa, 2022, Deneke and Pauli, 2021) and diseased (Kloc et al., 2021, Rajah et al., 2022, Leroy et al., 2020, Lazebnik, 2021, Hernández and Podbilewicz, 2017, Ahmadzadeh et al., 2022, Parris, 2013, Boveri, 2008, Hass et al., 2021a) (Figure 1). Oftentimes, the process of cell fusion imitates that of viral entry into cells, which is a direct result of one of its evolutionary origins (Brukman et al., 2019, Kloc et al., 2021, Leroy et al., 2020, Soe et al., 2011, Hernández and Podbilewicz, 2017, Brukman et al., 2022, Sapir et al., 2008, Harrison, 2008, Yang and Margam, 2021, Modis, 2013). In fact, in some cases, cell-cell fusion and the resulting syncytia formation occur secondary to viral infection (Leroy et al., 2020, Lazebnik, 2021, Rajah et al., 2022). While not a traditional “cell” fusion, a closely related example is the fusion of vesicular membranes with cell membranes, such as during synapsis (Emans et al., 1993, Pérez-Vargas et al., 2014) and autophagy (Wang, 2016). Regardless of the exact mechanism at play, cell-cell fusion is defined by key characteristics that have been coined the “hallmarks of cell fusion” (Hernández and Podbilewicz, 2017).
The primary examples of cell-cell fusion in the human body include the formation of multinucleated giant cells (Bühler et al., 2022, Ahmadzadeh et al., 2022, Leroy et al., 2020, Kloc et al., 2022, Skokos et al., 2011, Zhang et al., 2022), osteoclasts (Elson et al., 2022, Kloc et al., 2022, Soe et al., 2011), skeletal muscle cells (Schejter, 2016, Gibbs and Pyle, 2023, Lehka and Rȩdowicz, 2020, Petrany and Millay, 2019), and placentation (Aguilar et al., 2013, Hernández and Podbilewicz, 2017, Kloc et al., 2021, Liang et al., 2010), which all produce syncytia (multiple-nucleated cells secondary to cell fusion). Some processes of cell-cell fusion involve nuclear fusion as well; in particular, this is characteristic of gamete-gamete fusion in human fertilization and embryogenesis (Brukman et al., 2022, Deneke and Pauli, 2021, Lu and Ikawa, 2022).
Because fusion can occur in different tissues throughout the body at baseline, it should come as no surprise that fusion can be a characteristic of cancer as well (Parris, 2013). It is well established that all of the classic cancer hallmarks are derived from variations on normal cellular functionality, which in turn promote cancer growth by increasing cellular fitness, and thus yield improved survival of the cancer cells relative to their healthy somatic counterparts (Hanahan and Weinberg, 2011). Despite being first hypothesized as a potential mechanism of carcinogenesis over 120 years ago by Theodore Boveri (Boveri, 2008), it was not until recently that cell-cell fusion in cancer became a more prominent point of discussion in the literature. The past few decades in particular have yielded an increasing number of research articles looking into the mechanism and potential functions of fusion in cancer (Bjerregaard et al., 2006, Demin et al., 2022, Dittmar, 2022, Dittmar et al., 2021, Gast et al., 2018, Hass et al., 2021b,a). There is a steadily growing evidence base that suggests that cell-cell fusion may provide the answers to several key questions in cancer research, including the basis for the organotropic patterns of metastasis (Arena et al., 2023, Sieler et al., 2021, Tretyakova et al., 2022, Zhang et al., 2022), immune evasion (Bates et al., 2023, Tretyakova et al., 2022, Bateman et al., 2000, Shin et al., 2021, Hass et al., 2021a), cancer stemness (Uygur et al., 2019, Zhang et al., 2014, Dittmar and Hass, 2023, Hass et al., 2021a,Hass et al., 2021b, Melzer et al., 2018, Warrier et al., 2023), genomic instability and the resulting tumor heterogeneity (Wang et al., 2020, Ogle et al., 2005, Archetti, 2022, Gast et al., 2018, Hass et al., 2021a, Mirzayans and Murray, 2023), and patterns of both chemo- and radio-therapeutic resistance (Casotti et al., 2023, Druzhkova et al., 2023, Guo et al., 2023, Hass et al., 2021a, Mirzayans and Murray, 2023, Pienta et al., 2022, Uygur et al., 2019, Sieler et al., 2021, Yan et al., 2016).
Recent studies highlight cell-cell fusion in human cancers, which can range in effect from enabling endothelial-mesothelial transition (EMT), the first step in local invasion and metastasis, via cancer-endothelium fusion (Hass et al., 2020, Parris, 2013, Hass et al., 2021a, Dittmar et al., 2021, Melzer et al., 2019) to the formation of cancer stem cells (CSCs) and immune evasion following cancer-macrophage fusion (Li et al., 2023, Dittmar, 2022, Hass et al., 2021a, Zhang et al., 2022, Dittmar et al., 2021). The examples of cell-cell fusion in healthy tissue and cancer have been summarized in Figure 1.
Thus, there is an increasing evidence base that cell-cell fusion is extremely important to consider in cancer. While one of the first papers to do so was written almost 20 years ago (Ogle et al., 2005), the past 5 years have had a surge in the number of review papers that connect cell-cell fusion to the expanded hallmarks of cancer described by Hanahan and Weinberg in 2011 (Hanahan and Weinberg, 2011). Several recent reviews highlight how cell-cell fusion may promote tumorigenesis, propagate treatment resistance, and worsen patient prognosis (Hass et al., 2021a, Dittmar, 2022, Dittmar et al., 2023,Dittmar et al., 2021, Hass et al., 2021b, Wang et al., 2021, Dietz et al., 2021, Gast et al., 2018, Sieler et al., 2021).
While evidence of fusion has been repeatedly observed in a variety of tissue types in vitro and in vivo (Melzer et al., 2019, Yan et al., 2016, Dietz et al., 2021, Hass et al., 2021a, Ruano et al., 2022), the frequency at which fusion occurs in vivo and the impact of these fusion events are unclear. This has been complicated by difficulties in observing fusion events between cells of shared cell lineages. In clinical studies, cell-cell fusion events between distinct cellular lineages, have been identified using several experimental techniques. For example, using genotyping, Pawelek et al. demonstrated fusion between cells from genetically distinct bone marrow donors (Pawelek, 2014), while other studies have utilized highly multiplexed immuno-histochemical analyses to reveal co-expression of proteins associated with distinct epithelial and lymphocytic or monocytic lineages (Melzer et al., 2019, Ruano et al., 2022). It is only in experimental in vitro and in vivo settings, where the use of fluorescent proteins or barcoding has allowed researchers to detect similar fusion events occurring between cancerous epithelial cells of more closely related lineages (Miroshnychenko et al., 2021, Su et al., 2015, Bjerregaard et al., 2006, Wang et al., 2021). Unlike the studies of cell fusion between independent cell lineages, these epithelial tumor populations did not have unique surface markers to distinguish the fused cells from their parental lineages. Due to these challenges, the research of cell-cell fusion, particularly within tumor cell populations, is still in its earliest stages.
Additionally, the molecular biology of the cell-cell fusion pathway is generally well-studied only in certain biological contexts (e.g. vesicular fusion in synapsis (Emans et al., 1993, Pérez-Vargas et al., 2014), gamete-gamete fusion (Brukman et al., 2022, Deneke and Pauli, 2021, Lu and Ikawa, 2022), and viral-host fusion (Leroy et al., 2020, Brukman et al., 2022, Sapir et al., 2008, Harrison, 2008, Rajah et al., 2022, Lazebnik, 2021)). Further study in other contexts, particularly in settings of pathology and increased biological variation (e.g. across cancer), is necessary. A deeper mechanistic understanding of cell fusion in cancer may unlock new avenues of cancer treatment that limit the fundamental ability of cancer (i.e. pre-malignant lesions) to evolve.
In this review, we utilize the framework of evolutionary oncology (Niculescu, 2023, Gatenby and Brown, 2020, Pepper et al., 2009) to demonstrate our current understanding of how fusion happens, what fitness costs are associated with fusion, how rare fusion events can impact tumor evolution and patient outcomes, and highlight key unanswered questions in this burgeoning field of cancer research. We further exploit this evolutionary framework to emphasize how interclonal cell-cell fusion events in cancer may contribute to oncogenesis, metastasis, and the evolution of therapeutic resistance.