Progression and Dormancy in Metastatic Thyroid Cancer: Concepts and Clinical Implications

Abstract

Distant metastasis classically has been defined as a late stage event in cancer progression. However, it has become clear that metastases also may occur early in the “lifetime” of a cancer and that they may remain stable at distant sites. This stability of metastatic cancer deposits has been termed “metastatic dormancy” or, as we term it, “metastatic progression dormancy” as the progression either may reflect growth of already existing metastases or new cancer spread. Biologically, dormancy is the presence of non-growing, static metastatic cells that survive over time. Clinically, dormancy is defined by stability in tumor markers, imaging, and clinical course. Metastatic well-differentiated thyroid cancer offers an excellent tumor-type to understand these processes for several reasons: 1) primary therapy often includes removal of the entire gland with ablation of residual normal tissue thereby removing one source for new metastases; 2) the presence of a sensitive biochemical and radiographic monitoring tests enabling monitoring of metastasis throughout the progression process; and 3) its tendency toward prolonged clinical dormancy that can last for years or decades be followed by progression. This latter factor provides opportunities to define therapeutic targets and/or markers of progression. In this review, we will discuss concepts of metastatic progression dormancy and the factors that drive both long-term stability and loss of dormancy with a focus on thyroid cancer.

Brief Clinical Case

In 1991, a 40 year old woman was diagnosed with a 2.2x1.3x1.4 cm intrathyroidal well-differentiated papillary thyroid cancer (PTC). She was treated with total thyroidectomy and 81 mCi of I-131 following levothyroxine withdrawal. Pre and post-therapy scans demonstrated thyroid bed uptake. Thyroglobulin (Tg) level on levothyroxine with TSH ~0.1 mU/L was < 2 ng/ml (undetectable) after initial therapy. Radioiodine (RAI) scan was negative in 1993 but TSH-stimulated Tg level was 12 ng/ml. Chest CT had a 4 mm left lobe lung nodule. Over the following 10 years, Tg remained undetectable on L-T4. Following rhTSH-stimulation in 1994, and 1998, Tg was 10 and 12 ng/ml, respectively with negative RAI scans. In 2003, with changes in Tg assay, Tg was 0.7 ng/ml with a TSH<0.1 mU/L. Chest CTs in 1994, 1998, 2003, and 2005 were stable. Tg increased to 2.2 with a TSH <0.1 mU/L in 2005. She received 154 mCi I–131 with a TSH-stimulated Tg of 13 ng/ml, had a negative post-therapy scan, and was referred for “thyroglobulin, scan negative” thyroid cancer.

Serial Tg levels and annual chest CTs were stable until 2011 when Tg increased to 8 ng/ml with TSH<0.1 and the lung nodule increase to 9 mm. PET/CT was positive in the lung nodule (SUV 3.3) but not in other locations. CT-guided FNA of the lung nodule confirmed well-differentiated PTC. In 2013, Tg increased to 620 ng/ml with a suppressed TSH. PET/CT had new bone metastases in the left scapula and left iliac crest; the lung nodule increased in size to 14 mm. Molecular analysis of the primary tumor using a 248 gene panel identified a BRAF^V600E mutation and biopsy of the scapula bone metastasis showed PTC with no additional pathogenic mutations. The bone lesions were treated with external beam radiation that achieved pain control and stabilization. In 2017, the lung lesions grew >50% in size and increased in number. She was started on Sorafenib and after initial period of stable disease, the lung nodules increased in size. Since 2018 she has been managed on Lenvatinib with stable disease, albeit with side effects that require occasional “drug holidays.”

Introduction

Classical cancer models describe metastatic progression as a late stage event in the “lifetime” of a malignancy, and that when it occurs it leads to rapid growth that results in patient mortality [1]. The development of more sensitive biochemical markers, such as Tg in thyroid cancer; PSA in prostate cancer, circulating tumor cell detection, circulating free DNA assays, and more sensitive imaging tests have challenged this model [1-6]. Data from multiple tumor-types demonstrate that some cancer cells have the ability to circulate, survive, and survive in a metastatic tissue environment while maintaining stability or growing slowly, demonstrating that metastatic progression also can be an early event. Because many cancers display specific patterns of distant metastases it was hypothesized that primary cancers secrete factors such as microvesicles, cytokines, or chemokines that enable metastasis early in the cancer process. This “seed and soil” relationship for cancer metastasis, where the location of metastatic progression has tissue specificity, was suggested more than a century ago by Paget [7-9]. In thyroid cancer, the specificity of location of metastases for the different thyroid cancer subtypes was affirmed in a recent 25 year autopsy study that included more than 650 cases from the Netherlands [10]. Additionally, primary cancers are now recognized to be comprised from a non-uniform heterogeneous group of cancer cells along with stromal and immune cells that can both constrain and promote progression depending on their cell type [11]. Resistant populations further can be selected for surviving treatment and environmental challenges. Thus, different populations of cells may have metastatic and/or invasive potential while others may not have such properties, or they may be located in a sub-environment that does not support this behavior [11].

Recurrent vs Residual Thyroid Cancer

With this evolution in our understanding of the timing of metastatic progression, it is important to recognize that from a clinical perspective, “recurrence”, or the new identification of cancer after a period remission, may reflect instead the growth of preexisting small metastatic lesions to a detectable volume. This concept is particularly evident in thyroid cancer in which often the entire primary organ is removed and residual normal tissue is ablated with I-131. Per this model, if the primary source of metastatic cells has been removed and ablated, the later-identified metastases either must have predated the initial therapy or are secondary metastases from other subclinical sites. The concept of clinically silent early metastasis in thyroid cancer is incorporated into the American Thyroid Association Guidelines in which Tg levels are used in the response criteria and incorporated into the concept of “risk of structural recurrence” [12]. Importantly, the rate of progression after metastasis has occurred often reflects the aggressiveness of the primary tumor. Thus, it is slower typically for well-differentiated thyroid cancers and faster for poorly differentiated or anaplastic thyroid cancers. This high frequency of silent metastases is supported by the aforementioned autopsy series in which 15.6% of the 359 patients diagnosed with thyroid cancer incidentally at autopsy also had distant metastases [10]. Thus, in thyroid cancer, it seems likely that the majority of “recurrence” at distant sites represents growth of pre-existing metastases occurring as early stage events that are biologically dormant (i.e. not growing at the molecular level) or growing at a pace and volume below the threshold of detection (i.e. clinically dormant).

Once growth is detected in a metastatic site either biochemically or radiographically, well-differentiated thyroid cancer metastases tend to progress at a consistent log-linear rate over time [13,14]. This information has led to utilization of “doubling times” to monitor the pace of progression even for radiographically silent disease. The likelihood of radiographic detection increases concordantly with the Tg levels that can result in changes in clinical care [12,15]. Thus, not only the pace of progression, but also the size of lesions and Tg levels are used to determine the frequency of monitoring and the timing of initiation of therapies, even if “doubling times” are constant. For example, the clinical response to a tumor that doubles from 3 to 6 mm may not be the same as one that doubles 3 to 6 cm over the same period of time, despite a stable “doubling time.” Indeed, measurement increases of 20% when tumor masses are >1 cm defines progression in most clinical trials (e.g. RECIST criteria), thereby emphasizing the dual importance of progression and tumor volume. In these cases, TSH suppression to levels <0.1 mU/L is recommended to avoid TSH-induced growth of thyroid cancers that still maintain expression of the TSH receptor [12,15]. TSH suppression is less relevant for anaplastic thyroid cancers that lose TSH receptor expression. Importantly, in some cases, as exemplified in the case report, the doubling time decreases and new and/or more rapidly growing metastases are identified. These changes in growth rate represent changes in biological behavior that serve as “inflection points” in the progression of the cancer. Such alterations imply that secondary biological changes have occured in the metastatic deposits and lead typically to more extensive imaging and often more therapy not always targeting the secondary changes. On imaging, the growth may be limited to one or several lesions or may be consistent for all deposits. The molecular events that lead to changes in the biological behavior in metastases include the development of new driver events or loss of progression restraining events in the cancer cells that enhance their intrinsic growth rate and/or induce changes in the tumor microenvironment that facilitate progression [16]. In addition, the identification of metastasis in a new anatomic site may be due to loss of dormancy in metastatic deposits already present or could represent secondary metastases arising from another metastatic site [17]. As more information becomes available from both molecular analysis of human metastatic tumors and mouse models, it will be critical to fully define the mechanisms behind late stage progression to identify new therapeutic targets and biomarkers. Pathways that are currently targeted and those that may be exploited in the future are highlighted in this review.

Overview of Metastatic Spread, Dormancy, and Progression

Metastatic spread

The ability of cancer cells to metastasize often is linked to tumor growth, but because it may occur even in small tumors, growth is not necessary for metastasis. By distinction, progression of metastatic cancer more broadly includes both metastasis and subsequent growth of metastatic lesions over time. The ability of cancer cells to metastasize requires invasion and intravasation into the vascular and/or lymphatic systems, the ability to survive in circulation, extravasation and invasion in a new microenvironment, and subsequent survival (Figure 1). Intravasation is essential to the metastatic process [18]. Access to lymphatic vessels allows cancer cell spread to local and regional nodes while vascular invasion allows for distant metastases. Intravasation can occur as individual cells, which express enzymes to degrade local stromal elements such as metalloproteinases and collagenases, or can occur through collective migration where a subset of outer cells form a leading edge with those properties [19]. The process also can involve angiogenesis, or the formation of new blood vessels around these clusters of cells. Circulating individuals or clusters of cells can be identified and isolated from blood samples by genetic signatures and cell isolation methods and serve predictors for metastatic progression [4-6]. It has been demonstrated that the number of circulating cancer cells is much higher than previously thought, suggesting that the metastatic process is relatively inefficient [1]. Metastasis is regulated further by elements derived from the cancer cells and factors derived from the stroma [20]. The cancer-derived factors can contribute to metastasis both by affecting the invasive capacity of the cancer cells through paracrine interactions and by influencing how the cancer cells and the local and target organ micro-environments interact [20]. They can subdivided into two groups; 1) factors that regulate extracellular environments such as those that induce angiogenesis, facilitate a prometastatic local immune environment and/or that regulate the pre-metastatic niche in target organs and 2) factors that regulate cancer cell biology including those that result in epithelial – to – mesenchymal transition (EMT) to enable local invasion and those that enable survival of cancer stem cell (CSC) populations [20]. EMT is one of the hallmarks of invasive cancers and is the transition of cancer cells from an organ tissue-type morphology (e.g. polarized epithelial cuboidal thyroid follicular cells) to a fibroblastoid morphology that enables invasive capacity [21,22]. Cancer stem cells (CSC’s) are a slowly dividing, relatively treatment-resistant population of cells that exhibit plasticity in response to therapeutic or environmental challenges and express genes and surface markers [21,23-25]. Cells that have undergone EMT and CSCs are critical in all components of the metastatic cascade and demonstrate the considerable “plasticity” of cancer cells.

Figure 1. Model of Thyroid Cancer Metastatic Progression.

Primary tumors release factors (blue circles) that include chemokines and cytokines, nucleic acids, and exosomes and other microvesicles to create a receptive environment for cancer cells. The cancer cells (red pentagons) along with stromal cells invade locally and also into blood vessels and follow the chemokines to metastatic sites with relative specificity for different subtypes. Once in the metastatic sites, the cancer deposits can remain dormant influenced by MPSs (such as KiSS-1, NM23, and RCAN 1.4 in thyroid cancer) and a tumor growth inhibiting microenvironment characterized by absent angiogenesis, the inability to degrade tissue matrix, and immune surveillance. Progression in the metastatic sites is enhanced by loss of MPSs, the development of new growth promoting changes, and the development of a tumor growth promoting microenvironment with enhanced angiogenesis, the development of invasive properties (e.g EMT), and a tumor promoting immune environment.

Metastatic Dormancy

In both primary and metastatic environments, cancer dormancy is governed by a complex interplay between cancer cells and host factors that regulate angiogenesis, the immunologic response to the cancer, and cancer cell proliferation and apoptosis [26]. Angiogenic dormancy occurs when there is sufficient blood supply to support cell survival, but insufficient blood supply to support proliferation. Angiogenic responses are regulated largely by tissue hypoxia leading to secretion of growth factors from cancer and stromal cells with angiogenic and anti-angiogenic functions [27]. Dormancy is also maintained by immune-surveillance whereby immune cells, such as tumor infiltrating lymphocytes (TILs) are recruited that suppresses progression (e.g. cytotoxic CD8+ T cells) [28]. By contrast, subsets of TILs can suppress immune surveillance, and non-lymphocyte immune cell population such as tumor-associated macrophages (TAMs) can either inhibit or promote cancer progression depending on the subtype [29,30]. Tumor promoting TAMs have been shown to be highly prevalent in poorly differentiated and anaplastic thyroid cancer and play a functional role in progression in mouse models [31,32]. In addition, programmed death-ligand 1 (PD-L1) expression on cancer cells allows for avoidance of host immune system T cells and its overexpression predicts therapeutic response to checkpoint inhibitors that target either PD-L1 or its receptor [33,34].

Intrinsic cellular dormancy, which can be maintained by MPS genes, allows for cell survival in nutrient and oxygen-limiting environments whereby cells are pushed into the G0 phase of cell cycle, where they remain in a quiescent state until enough oxygen or other nutrients are provided to allow for growth [35,36]. Additional microenvironment factors also constrain or activate progression, including the tissue modulus (hardness), collagens and other protein complexes in the stroma, and other stromal cell types, such as cancer associated fibroblasts (CAFs) [37-39]. These processes are described in more detail later in this manuscript.

Metastatic Progression

In addition to loss of MPS gene expression, thyroid cancer growth rate in metastatic tissues often can be predicted by the histology and genotype of the primary tumor. For example, in papillary thyroid cancer the combination of a BRAF^V600E mutation with a mutation in the promoter of hTERT is associated with a more aggressive clinical course [40-42]. Similarly, the loss of P53, or co-occurrence of a BRAFV600E MAPK pathway activating mutation with PI3K pathway activation together lead to dedifferentiation, rapid growth, and poor prognosis [43-47]. FTCs, that more frequently have activation of the PI3K pathway, tend to metastasize hematogenously while PTCs tend to metastasize to regional nodes [10,12,48-50]. In general, metastatic lesions tend to retain clonal expression of driver mutations present in the primary cancer. However, in some cases, additional mutations such as those in the SWI/SNF and DNA Damage Repair pathways are identified uniquely in metastatic lesions [51,52]. Thus, in some cases the progression is predicted by the mutation profile of the primary tumor, but in other cases, new genetic mutations can be identified in the metastatic lesions that may be important in late-stage progression. Because most of the available data have focused on analysis of DNA (i.e. identification of mutations), changes in gene expression and the roles of DNA hypermethylation and microRNAs are incompletely defined. Finally, similar to primary tumors, metastatic lesions are also comprised of subpopulations of thyroid cancer cells, immune cells, and stromal elements, all of which contribute to the growth or stability of metastatic lesions [53].

Regulation of Metastasis

Both stimulatory and inhibitory pathways regulate cancer progression. We will focus on inhibitory and activated pathways in cancer cells that modulate metastatic potential and changes in the microenvironment that restrain or facilitate metastatic progression. Although we are considering them separately, these features are closely linked and dynamically regulate one another. In addition, rates of growth of metastatic lesions may be influenced by specific features of metastatic sites, such as the blood-brain barrier, bone marrow vs bone cortex, and lung inflammatory responses. General mechanistic concepts relevant to thyroid cancer are reviewed below.

Tumor Cell Factors

Metastasis Progression Suppressors and Pathways

The term “metastasis suppressor” describes genes whose loss is sufficient to enable metastatic spread of cancer cells but not sufficient for cellular transformation [2,54]. They are distinct from tumor suppressor genes whose loss is sufficient to induce cellular transformation but may or may not regulate metastasis. Because these genes often inhibit many facets of cancer cell behavior, including proliferation, invasion, and EMT, we propose the term “metastasis progression suppressors” (MPS). More than 20 MPS genes have been described [55]. MPSs are typically expressed (or overexpressed) in primary cancers and function as intrinsic “brakes” to slow cancer cell growth. Reduced expression in progressive metastatic lesions and and/or in subcomponents of primary tumors are hallmarks of these genes, thus loss of expression is a potential biomarker for aggressive disease [56]. Moreover, it is possible that their re-expression, or inhibition of the consequently activated downstream pathways, represent potential therapeutic approaches [57]. Because metastatic dormancy is typical for thyroid cancer, it serves as an excellent model to study pathways that regulate dormancy in cancer cells.

Several of the known MPSs, and their functions, have been studied in thyroid cancer, including KiSS1, RCAN1.4, and NM23 [2,57-60]. The KiSS1 gene products, termed “kisspeptins”, are the endogenous ligands for G-protein coupled receptor (GPR) 54. In the hypothalamus, mutations in GPR54 or KiSS-1 cause infertility due to reduced development and function of the gonadotropin-releasing cells [61,62]. In addition to this function, KiSS-1-activated GPR54, and KiSS-1 activation of other pathways, together reduce the proliferative, invasive, and metastatic capacities of melanoma, breast and thyroid cancer cells [58,59,63-65]. In addition, in solid tumors, KiSS1 expression follows the typical expression pattern of a MPS [66,60,67,68]. The inducibility of GPR54 by kisspeptins created a system to identify downstream metastasis suppression pathways. Subsequent studies demonstrated that GPR54 activity resulted in an increase expression and function of the regulator of calcineurin isoform 4 (RCAN1.4) leading to reduced calcineurin signaling in thyroid cancer, colon cancer, and melanoma cells [56,58]. RCAN 1.4 overexpression and activity were shown to be necessary for GPR54 effects in vitro, RCAN 1.4 overexpression was sufficient independently to inhibit cell motility and invasion, and its loss increased cell invasion and proliferation but was not for transformation, consistent with MPS activity [56,58,60]. The mechanisms by which RCAN 1.4 levels are reduced in include promoter hypermethylation and increased protein degradation following phosphorylation by MAPK and TGF beta-activated-kinase −1 (TAK1) [59,69]. In vivo studies confirmed that RCAN 1.4 is a bonafide MPS in thyroid cancer. This MPS function required the transcription factor NFE2L3 (Nrf3) which is overexpressed in metastatic thyroid cancer lesions [60]. Similar data ascribing cancer progression activity to NFE2L3 was demonstrated in gastric and hepatic cancers [70].

Interestingly, RCAN 1.4 is known to be important in the biology of Down’s syndrome, or trisomy 21, which is associated with a markedly reduced frequency of solid tumors [71]. RCAN 1.4 is the primary inducible transcribed mRNA of the RCAN1 gene located on chromosome 21 that was originally identified as the Down’s Syndrome Critical Region 1 (DSCR1) gene [67]. The low frequency of cancer is recapitulated in mice that are engineered to have three copies of most chromosome 16 (the mouse homolog of human chromosome 21) and this feature is dependent on expression of three copies of RCAN 1 [71]. Promoter expression studies demonstrate that this effect is driven by RCAN 1.4 via NFAT-mediated regulation vascular endothelial growth factor (VEGF) release and angiogenesis [59,67,72]. Thus, it appears that RCAN 1.4 regulates cancer progression both by impacting cancer cells and, in the context of Down’s syndrome, on host cell biology.

Oncogene Signaling and Metastatic Progression

Early stage well-differentiated thyroid cancers typically are genomically stable compared with other tumors [73]. Early thyroid oncogenic drivers include constitutively active forms of BRAF and translocations that involve RET that both display MAPK-dependent transformation for PTC; and RAS and PI3K activation for FTC. Hürthle Cell cancers appear to be driven by mTOR signaling and are characterized by chromosomal loss [74-77]. When these genomically stable tumors metastasize, they typically progress slowly while more genomically complex poorly differentiated tumors often progress more rapidly [52,78]. Targeting oncogenic drivers such as BRAFV600E, RET/PTC, ALK, NTRK, and others can result in robust initial anti-tumor responses in metastatic sites, although long-term disease control is rare [79].

As noted above, thyroid cancers display tumor specific patterns of distant metastases [10]. The reasons for these tissue distributions are uncertain but likely are driven by the oncogenic drives or the primary tumors through the release of factors that create a “premetastatic niche” in particular locations (Figure 1). For example, it has been shown that tumor-derived secreted factors, such as extracellular vesicles (EVs) (exosomes and microvesicles) carry DNA, RNA, proteins, cell surface receptors and other “cargo” that contribute to the development of the premetastatic niche [80-82]. In thyroid and breast cancer cells, the components of secreted exosomes are regulated by MAPK and PI3K pathways [83,84]. In addition to microvesicles, cancer cell secrete a number of chemical factors (TGFβ, VEGF, MMP9, LOX, CCL2, etc.) and RNA species that can regulate the premetastatic niche [81]. The factors together influence the host environment to regulate metastatic progression (Figure 1) [85,86].

To better target the more rapidly growing metastatic lesions with specificity, it is important to identify changes in those tissues that result in escape from biological and/or clinical dormancy [87]. Several groups have examined the genomics of growing metastatic lesions and in some cases, compared them with the primary tumors [51,52,88-91] and themes are beginning to emerge from these data: 1) there is continued expression of the initial driver oncogenes, 2) Mutations in genes involved in DNA Damage Repair are common and 3) Mutational loss of members of the SWI/SNF chromatin-remodeling complex are common. These data suggest the abnormal DNA repair and regulation of transcription may be crucial to late stage thyroid cancer progression [51,52,88,92]. Ongoing studies using RNA sequencing, methylation profiling, and single cell analyses are still required to more fully characterize the metastatic lesions.

Host Factors

Angiogenesis

Angiogenesis, or the process by which new blood vessels form, is a vital function, required for growth, development, wound healing, and in the tissue response to nutrient deprivation and hypoxia. Hypoxia is a potent “angiogenic switch” that signals quiescent endothelium to proliferate and migration leading to tube formation and the subsequent development of new blood vessels [93,94]. Limitations in the formation of new blood vessels is known to be one of the key features used to promote dormancy in cancer [1]. In addition to being needed for growth and metastasis at the primary tumor sites, angiogenesis also is required for the survival and growth of metastatic deposits [95]. The vascular endothelial growth factor (VEGF) is one of the critical hypoxia-stimulated angiogenic signals that has been targeted to treat thyroid cancer [96]. Higher levels of VEGF expression have been associated with tumor development, size, progression, and invasiveness [97-101]. The FDA-approved drugs for progressive metastatic thyroid cancer, Sorafenib and Lenvatinib, share VEGF receptors as targets and both reduce tumor vascularity, thus anti-angiogenesis is felt to be critical to the efficacy of these agents [102,103].

Extracellular matrix and Cancer-Associated Fibroblasts

The extracellular matrix (ECM) is the non-cellular tissue network that maintains tissue architecture by providing both structural and chemical support. It serves as a natural barrier to cell movement. Cancer cells, particularly those that have undergone EMT, secrete proteins that enzymatically degrade ECM to enable invasion by either individual cells or cluster of cells (i.e. individual or collective invasion) [104-106]. The biochemical and biophysical properties of the ECM also provides signals that promote cellular proliferation, migration and invasion [107,108]. Collagen is a principle component of the ECM. Changes in the balance between collagen deposition or degradation can result in the loss of homeostasis and enable intravasation [109,110]. For example, degradation of the ECM by matrix metalloproteinases (MMPs) is critical and perhaps necessary for cancer cell invasion [111]. MMP over-expression is associated with increased cancer aggressiveness [112] and have been therapeutically targeted [112,113]. Matrix stiffness affects intracellular and intercellular mechanical interactions through signals transmitted by integrins and other receptor/signaling pathways [114]. In papillary thyroid cancer, activation of integrin signaling is associated with local invasion and EMT, and inhibition of key downstream effectors, including PAK, AKT, SRC kinase, and FAK reduce cancer invasion and metastasis in vitro and in vivo [25,78,115-117].

Cancer associated fibroblasts (CAFs) are stromal cells that modulate the ECM and also can facilitate cancer progression. Studies in genetically engineered mice have demonstrated that loss of Pten only in fibroblasts, can increase invasiveness of breast cancer [118-120] and that CAF-derived platelet derived growth factor beta (PDGFβ) increases brain metastases in breast cancer [121]. In thyroid cancer, the role of CAFs in metastasis has been less well defined. It has been demonstrated; however, that BRAF V600E in mouse thyroid facilitates collagen expression, deposition, and CAF recruitment suggesting an important role for collagen in PTC development and progression [122].

Immune surveillance

Over the past decade, the concepts of immune surveillance to limit cancer progression, immune exhaustion whereby cancer cells evade the immune response, and the development of a prometastatic immune environment have greatly expanded [123]. Clinical studies have long implicated improved outcomes for patients co-existent Hashimoto’s thyroiditis in PTC papillary thyroid cancer [124] and worse outcomes for patients with both thyroid cancer Graves’ disease [125]. While a full review of this topic is beyond the scope of this manuscript, in thyroid cancer, there is evidence that as cancer progress, the immune environment progresses from an inhibitory environment to an exhausted or facilitating environment. Natural killer (NK) cells, cytotoxic T-lymphocytes, and mature dendritic cells all are essential in maintaining immune surveillance of cancers [126]. Several metastasis progression inhibitors have been shown to induce or maintain this immune response including KiSS-1 and RCAN1.4 [60,127]. By contrast, cancer progression is associated with a higher percentage of regulatory T-cells (Tregs), enrichment in lymphocyte expression of programmed death-1 (PD-1) and the PD-1 ligand 1 (PDL1), and enhanced infiltration of tumor associated macrophages (TAM) and immature dendritic cell infiltration that together facilitate cancer progression [128]. One interesting related phenomenon is the “abscopal effect” in which radiation therapy in one location induces an anti-tumor immune response that leads to metastasis regression outside of the radiation field [129]. In thyroid cancer, there is strong evidence that the development of an immune exhausted signature is associated with progressive disease and that TAMs are critical for progression [31,32,130-132]. The role of individual thyroid oncogenes in inducing an immune exhausted environment has been less clear. RET/PTC expression has been associated with lymphocytic infiltration in PTC. BRAFV600E expression has not been associated with an increase in PD1 and PDL1 expression in melanoma or PTC, suggesting that secondary changes and later stage events may be required for PD1/PDL1 overexpression [133]. Interestingly, correlations have been reported between overexpression of NFE2L3 with both a tumor promoting immune environment and distant metastases in thyroid cancer [60,126]. Thus, dormant distant metastases likely are maintained in part by a balance in the cancer:immune interface, and changes in that balance functionally regulates cancer progression.

Summary

Thyroid cancer provides an outstanding model to study regulation of dormancy, as prolonged dormancy is common in the tumor type, but late progression also occurs. Based on biomarker and autopsy studies, it is likely that clinical “recurrence” of metastatic thyroid cancer often represents progression of pre-existing radiographically undetectable lesions. The growth rate of the metastases tends to mirror growth rates in the primary tumors and they are genomically similar. Factors that drive late stage progression include loss of expression of metastasis progression suppressors, gain of new mutations in genes that regulate DNA damage repair and chromatin remodeling, and potentially the expansion of resistant cells with increased proliferation, angiogenesis, invasion, and escape from immune surveillance. Further study and a deeper mechanistic understanding of the biology in progressive distant metastases is needed to improve the outcome for patients with progressive metastatic thyroid cancer.