Keratinocyte carcinomas are among the most prevalent malignancies worldwide. Basal cell carcinoma (BCC) and cutaneous squamous cell carcinoma (cSCC) are the two cancers recognized as keratinocyte carcinomas. The standard of care for treating these cancers includes surgery and ablative therapies. Several cancer-testis antigens (CTA) and developmental genes (including embryonic stem cell factors and fetal genes) are ectopically expressed in BCC and cSCC. When ectopically expressed in malignant tissues, functions of these genes may be recaptured to promote tumorigenesis. CTAs and developmental genes are emerging as important players in the pathogenesis of BCC and cSCC, positioning themselves as attractive candidate biomarkers and therapeutic targets requiring rigorous testing.
1. Introduction
Cancer cells evolve to express genes that are favorable to their growth, survival and dissemination. Reactivation of genes restricted to early mammalian ontogenesis confers selective advantages to cancer cells [1]. Reproductive germ cell and placental genes are the most common lineage-specific genes that are ectopically expressed in non-germ cell cancers [2]. Gametogenic and placental genes that can be ectopically expressed in non-germ cell cancers are called cancer-testis genes, or cancer-testis antigens (CTA) [1]. CTA expression represents a partial trans-differentiation of cancer cells towards gamete or placental phenotypes [3]. Gametogenic and placental cells possess phenotypic parallels to cancer cells including high proliferation rates, impaired differentiation, migratory phenotypes, genomic instability and immunological privilege [4]. These phenotypes are often attributable to the endogenous functions of CTAs. Over 200 CTAs have been documented in the literature [4]. CTAs can be classified as X or non-X chromosome linked and further subdivided into various subfamilies. Their pathological relevance has been studied in a plethora of malignancies, including skin, lung, breast, gastrointestinal, genitourinary and hematological cancers [5][6][7][8][9]. Beyond their relevance to cancer immunology, ectopic CTA expression influences cancer cell biology, as their natural functions are often recaptured in a novel way to promote carcinogenesis.
Cancer cells can additionally benefit from adopting genes from the developmental transcriptome: embryonic stem cell (ESC) and oncofetal genes
[10]. ESCs constitute the inner cell mass after fertilization and formation of a blastocyst
[11]. These cells exhibit high proliferation rates, replicative immortality and pluripotency
[12]. Some ESC genes orchestrate gastrulation and the subsequent stages of histogenesis and organogenesis. As a collective, ESC genes coordinate developmental dynamics in the early embryo. Perturbing their expression has significant consequences. ESC genes that are re-expressed in cancer include the core transcription factors (Sox2, Oct-4, and Nanog) that are necessary for pluripotency and cell survival as well as factors that regulate epithelial mesenchymal transition (EMT), the first step towards cancer cell migration and invasion
[8][10][12][13]. Other developmental genes may also be expressed during fetal development (after the establishment of functional organ systems) and re-expressed in cancer. These genes are termed oncofetal antigens
[14]. Fetal proteins coordinate diverse functions during development, including cell adhesion, EMT and growth
[15]. Expression of ESC and oncofetal antigens in cancer cells promotes deleterious phenotypes. In particular, developmental genes are associated with the emergence of cancer stem cells (CSCs)
[10][16]. CSCs are subpopulations of malignant cells with the ability to self-renew and spawn differentiated progeny
[17]. These subpopulations are believed to contribute to disease recurrence, clonal evolution, dissemination and heterogeneity
[17]. ESC genes can also modulate other aspects of neoplastic biology beyond the CSC hypothesis, including proliferation, metabolism, dissemination and therapeutic responses. Investigating how misappropriated CTA and developmental genes influence malignant progression may unveil new prognostic, diagnostic or therapeutic avenues for future studies.
There is a persistent need to identify novel biomarkers and new therapeutic targets to facilitate diagnosis, infer clinical disease prognosis and treat keratinocyte carcinomas (KC). KCs are malignancies that arise from epidermal keratinocytes. Basal cell carcinoma (BCC) and cutaneous squamous cell carcinoma (cSCC) are the two types of KC, representing the first and fifth most prevalent cancers, respectively, worldwide
[18][19][20][21][22]. BCC arises from keratinocytes in the basal layer of the interfollicular epidermis, mechanosensory niches and from hair follicle infundibulum stem cells
[23]. Mutations in the Sonic Hedgehog (Shh)-Patched 1 (PTCH1) signaling axis are integral to BCC tumorigenesis
[24]. Seventy to ninety percent of sporadic BCCs exhibit PTCH1 loss-of-function mutations and 10–30% exhibit smoothened gain-of-function mutations
[25]. Risk factors for developing BCC include fair skin, intermittent ultraviolet radiation (UVR) exposure, prior ionized radiation, immunosuppression and certain genodermatoses, amongst other factors
[26]. cSCC is characterized by malignant transformation of hair follicle bulge stem cells or interfollicular epidermal stem cells. Mutations in
TP53,
CDKN2A,
HRAS and
NOTCH1 are among the most frequent drivers of cSCC tumorigenesis
[27][28][29][30]. cSCCs may also be driven by human papilloma virus infection
[26]. Risk factors for developing cSCC include fair skin, chronic UV exposure, immunosuppression (i.e., in solid organ transplant recipients receiving immunosuppressants) and select genodermatoses, among others
[26][31]. cSCCs may develop in continuity with or progress from actinic keratoses (AK)
[32]. In most cases, the prognoses of BCC and cSCC are relatively favorable. BCCs, in particular, are seldom aggressive or metastatic. Nonetheless, cases of locally invasive disease do occur and can be devastating/mutilating
[33]. Risk of metastasis becomes significant for tumors that are ≥4 cm in diameter on the skin
[34]. Discoveries of novel biomarkers to optimize diagnosis and risk stratification/prognostication remains critical in the ability to manage a large number of keratinocyte carcinomas. Ectopically expressed CTAs and developmental genes are emerging contributors to KC pathogenesis. Investigating the expression patterns and function of these genes in KCs may foster clinically relevant advances.
2. Cancer-Testis Antigens Are Widely Expressed in Keratinocyte Carcinomas
The associations between CTA expression and the hallmarks of cancer are well-documented, and are continually illustrated by emerging research. The contributions of CTAs to KC pathogenesis are, unfortunately, largely unexplored. Walter et al. investigated the expression of 23 CTAs in human KC biopsies, using immunohistochemistry (IHC) and reverse-transcription quantitative polymerase chain reaction (RT-qPCR). Their panel included X-linked and non-X CTAs. At least one CTA was expressed in 81% of BCCs and 40% of cSCC tumors profiled
[35]. At least 40% of all KC tumors analyzed co-expressed two or more CTAs
[35]. Relevant associations between CTA expression, disease histological subtypes and invasiveness were not established
[35]. On average, BCCs co-expressed more CTAs per biopsy compared to cSCCs
[35]. This finding was attributed to decreased cytotoxic T cell immunosurveillance in BCC tumors compared to cSCC tumors and to decreased tumor antigen presentation by BCC cells compared to cSCC cells
[35].
In support of an alternative hypothesis, CTA expression was quantitatively and qualitatively compared between cSCCs from immunocompromised solid organ transplant recipients and immunocompetent patients
[35]. Since immune status did not significantly impact CTA expression in cSCCs, it was concluded that immunosurveillance differences between BCC and cSCC cannot fully explain the differential expression of CTAs between BCC vs. SCC. Importantly, studies on the expression and function of specific CTAs in BCC and cSCC have been conducted. The CTAs known to be expressed in KC thus far may be grouped based on their functions. These groups include phenotypic regulation, invasion and stress response, and CTAs with unknown functions.
2.1. CTAs That Regulate Cell Phenotype May Promote Neoplasia and Impair Differentiation in KC
Precise spatiotemporal control of gene and protein expression are required throughout the process of gametogenesis. Unique transcriptional, post-transcriptional and post-translational regulatory mechanisms are engaged throughout gametogenesis and embryogenesis to orchestrate proper developmental dynamics and ensure timely transition in cell phenotypes
[36]. These gene and protein expression regulatory factors can be recaptured in a novel way by cancer cells.
Preferentially expressed antigen of melanoma (PRAME) is a leucine-rich repeat protein ectopically expressed in a broad spectrum of cancers, including KC
[35][37]. PRAME is the founding member of a multi-gene family of CTAs. In gametogenic cells, the PRAME protein maintains pluripotency and regulates proliferation
[38][39][40]. In the presence of retinoic acid, PRAME binds retinoic acid receptors (RAR) at retinoic acid response elements (RARE)
[38]. PRAME recruits EZH2 to the RARE, where EZH2 deposits trimethylation of histone 3 lysine 27 (h3k27me3), repressing transcription of RAR-target genes
[38][41]. In addition to regulating transcription, PRAME regulates protein expression via Cullin-2, which targets cell-specific signaling factors for proteasomal degradation
[39][42].
PRAME’s function, its correlation with clinicopathological parameters and therapeutic relevance may be important to KC treatment. PRAME’s activity as a retinoid signaling repressor makes it an attractive focus for future research on KC prevention and treatment. Via RAR signaling, retinoids normalize keratinization and promote epidermal turnover
[43][44][45]. Systemic retinoids (e.g., acitretin) are effective chemoprophylactic therapies for AKs and KCs in immunosuppressed solid organ transplant recipients (SOTR)
[44][46]. Retinoids are also used clinically to treat acute promyelocytic leukemia and pediatric neuroblastoma, and demonstrate potential as therapeutic agents for other cancers (e.g., squamous cell carcinomas)
[47]. Considering the use of retinoids for KC chemoprophylaxis, there is a need to evaluate the contributions of ectopic PRAME expression/function to AK/KC pathogenesis and retinoid response in lesional skin. An adhesive patch pigmented lesion assay that probes PRAME and LINC00518 expression indicated a positive result in a small sample of actinic keratoses, and PRAME expression in premalignant tumors was previously described
[48][49]. As a transcriptional repressor of retinoid signaling, PRAME expression may modulate sensitivity to retinoids in epidermal tumors.
Whilst PRAME exerts transcriptional control of gene expression, insulin-like growth factor mRNA binding protein 3 (IMP3—also called IGF2BP3) exerts post-transcriptional gene regulation in gametes and ESCs
[50]. The three members of the IMP family of mRNA binding proteins form ribonucleoprotein complexes that surround nascent mRNA transcripts
[50]. IMPs protect against mRNA decay, prevent unnecessary translation, form transcript storage units and control intracellular trafficking of mRNA transcripts
[50]. Ectopic expression of IMP3 in cancer correlates with adverse outcomes (e.g., higher tumor grade in bladder carcinoma, metastasis and poor survival in esophageal adenocarcinoma)
[51].
IMP3 supports proliferation and disrupts differentiation in cSCCs. Using RT-qPCR, Kanzaki et al. demonstrated that IMP3 was highly expressed in the human cSCC cell lines HSC-1 and HSC-5, and in HaCaT immortalized lesional keratinocytes but not expressed in normal skin
[52]. IMP3 siRNA knockdown in HaCaT and SCC cells resulted in decreased proliferation or migration
[52].
CTAs can leverage the ubiquitin proteasome system to regulate protein expression in gametes
[36]. As mentioned earlier, PRAME serves as a substrate-recognition subunit for the Cullin-2 E3 ubiquitin ligase
[39]. PRAME orchestrates the degradation of p14/ARF, which supports proliferation
[42]. Additionally, PRAME can direct degradation of Lin28 in testicular germ cell tumors—regulating a key pluripotency pathway
[53]. Members of the Melanoma Antigen-A (MAGE-A) family of CTAs are widely expressed in KC, guiding proteolysis to promote cancer progression. MAGEs coordinate spatiotemporal localization of E3 ubiquitin ligases, substrate targeting and enzymatic activity
[54]. There are over 40 members of the MAGE family, approximately two thirds of which are CTAs
[54]. Type 1 MAGEs (MAGE-A, -B, and -C) are CTAs with restricted expression patterns, while Type 2 MAGEs are expressed in various somatic tissues
[54]. There are 12 members of the MAGE-A subfamily
[54]. MAGE-As are expressed primarily in spermatogonial stem cells, where they function to protect the genome from genotoxic and starvation stresses
[55]. These proteins possess MAGE homology domains that bind to RING E3 ubiquitin ligases
[54]. Targets of MAGE-RING complexes include substrates implicated in cell signaling and oncogenesis, such as AMP-activated Protein Kinase (AMPK) and p53
[56]. Ectopic expression of MAGE-As promote malignant phenotypes including hyperproliferation, invasiveness and resistance to glycolysis inhibitors
[54][55]. Previous reports indicate that MAGE-A3, -A4, -A9, -A10 and -A12 are expressed in KC tumors and contribute to disease progression
[35][57][58].
MAGE-A4 is expressed in 25% of KC tumors
[35]. Muelheisen et al. performed a more exhaustive assessment of MAGE-A4 expression in epidermal tumors. Patient immune status was accounted for, allowing comparisons between tumors excised from immunocompromised SOTRs and immunocompetent patients
[57]. In addition to cSCCs, MAGE-A4 expression was detected in AKs, Bowenoid AKs and SCC in-situ (Bowens disease) samples
[57]. Expression was most common in Bowenoid AKs (71% of tumors) from immunocompetent patients and lowest in Bowenoid AKs from SOTR (25% of tumor profiles)
[57]. Immunoreactivity was comparable between tumor types. Expression levels and immunostaining patterns varied depending on immune status, though the study lacked the power to statistically validate the former observation
[57].
The aforementioned gene and protein regulatory CTAs are expressed in KC tumors. Gametes use these factors to support multiple phenotypes, including high proliferation rates and impaired differentiation (
Figure 1). Likewise, in cancer, these CTAs can regulate proliferation and differentiation. PRAME, MAGE-As and IMP3 regulate expression of cell cycle regulatory factors (e.g., p14, p53, cyclins, etc.) In KC cells, IMP3 and MAGE-A regulate proliferation. Regarding differentiation, PRAME is more often expressed in poorly differentiated compared to well-differentiated cSCCs. Likewise, MAGE-A expression correlates with poorly differentiated cSCCs. Facilitating migratory phenotypes in cSCC cells may implicate IMP3 in EMT—a manifestation of cellular plasticity and a documented consequence of IMP3 expression
[59][60].
Figure 1. Ectopic CTA gene expression regulation at the transcriptional, post-transcriptional and post-translational levels. PRAME mediates transcriptional repression of retinoid signaling. IMP3 can regulate post-transcriptional regulation of gene expression, controlling mRNA trafficking and storage, translation and degradation. Both PRAME and MAGE regulate proteasomal degradation of certain substrates. Figure created using BioRender.com.
2.2. CTAs May Influence Invasion and Stress Response Phenotypes in KC
Cancer cells and gametogenic cells are attuned to navigate and thrive in their microenvironments. These cells express genes that regulate their motility and their capacity to withstand and adapt to threats to their viability, such as reactive oxygen species (ROS), genotoxic and metabolic stresses. Genes that control motility and stress response in gametogenic cells can be misappropriated by cancer cells to support their survival, migration and increase their fitness.
CTAs can alter invasive phenotypes in cancer cells by modulating their interactions with the extracellular matrix (ECM). Testis Expressed 101 (TEX101) is a membrane-anchored glycoprotein that is required for sperm capacitation (the process whereby spermatozoa become motile and competent to fertilize an oocyte)
[61]. Studies probing TEX101 expression and function in cancer are lacking. Yin et al. demonstrated that TEX101 suppresses invasion and metastasis by de-activating multiple ECM degrading enzymes, including urokinase plasminogen activator and matrix metalloproteases (MMP) 2 and 9, which mediate ECM degradation, promoting invasion and metastasis
[62]. Consequently, ectopic TEX101 expression may be a negative regulator of invasion. This hypothesis is supported by additional studies demonstrating negative correlations between TEX101 expression and metastasis
[63].
Sperm Associated Antigen 9 (SPAG9--also known as JIP4 or JLP) is a member of the c-jun N-terminal Kinase interacting protein (JIP) family that is implicated in cancer cell invasion
[64]. Widely expressed in KC tumors, this cell-surface scaffolding protein is endogenously expressed on the acrosomal compartment of spermatozoa, serving as a signaling mediator of spermatozoa-oocyte interactions via the p38 MAP kinase and the c-jun N-terminal Kinase (JNK) pathways
[64].
Akin to SPAG9, Testis Specific Gene A10 (TSGA10) is another CTA expressed in KC that may influence stress responses. TGSA10 is a sperm tail sheath protein that is necessary for sperm motility and proper mitochondrial organization in the midpiece of mature spermatozoa
[65]. This protein binds and prevents nuclear localization of the transcription factor hypoxia inducible factor 1α (HIF-1α) in spermatids
[66]. Ectopic TSGA10 is detected in a multitude of cancers and has been proposed as a diagnostic biomarker for select malignancies
[67][68]. In cancer, TSGA10 has been shown to inhibit angiogenesis by sequestering HIF1-α
[69][70][71]. TSGA10 expression is also associated with decreased cell migration, repression of EMT and decreased metastatic potential
[71][72].
2.3. CTAs of Unknown Significance Are Expressed in KC and in Normal Epidermis
Other CTAs have been detected in KC cells, but their contributions to cancer biology are poorly understood (summarized in Table 1). Either their functions in gametes, their functions in cancer, or both have not been elucidated. As some of these genes exhibit considerable expression in KC tumors, evaluating their function is a compelling prospect.
Table 1. Expression and functions of CTAs that are known to be expressed in KCs. * Description for all MAGE-A proteins are the same. +: positive expression. N.P: Not Profiled. AK: Actinic Keratosis.
Gene |
Description |
Expression in BCC |
Expression in cSCC |
Expression in Other Skin Tumors |
Functions in Keratinocyte Carcinoma |
References |
A. GENE AND PROTEIN REGULATION |
PRAME |
Repressor of retinoic acid signaling in gametes and embryonic stem cells Substrate-recognition subunit for Cullin-E3 ubiquitin ligase |
+ |
+ |
N.P. |
Associated with poorly differentiated tumors. Enriched in acantholytic SCC cells. |
[35][73] |
MAGE-A3 |
* Spatiotemporal localization and regulation of RING E3 ubiquitin ligases and substrates Protects the germline from environmental stressors |
+ |
+ |
N.P. |
Associated with PNI and poor differentiation in cSCC. Regulation of cyclins. |
[35][74][75] |
MAGE-A4 |
* |
+ |
+ |
AK + Bowenoid AK+ |
PNI and poor differentiation in cSCC. |
[35][57][74][75] |
MAGE-A9 |
* |
+ |
+ |
N.P. |
Functions not investigated in KC. |
[35] |
MAGE-A10 |
* |
+ |
+ |
N.P. |
Functions not investigated in KC. |
[35][58] |
MAGE-A12 |
* |
N.P. |
+ |
N.P. |
Proliferation and invasion in cSCC. |
[76] |
IMP-3 |
mRNA binding protein regulating mRNA localization, stability and degradation |
+ |
+ |
N.P. |
Proliferation and invasion in SCC. |
[51][52][77] |
B. INVASION AND STRESS RESPONSE |
SPAG9 |
c-Jun N terminal kinase interacting protein expressed on spermatocyte acrosome |
+ |
+ |
N.P. |
Functions not investigated in KC. |
[78] |
TSGA10 |
Mitochondrial biogenesis and organization |
+ |
+ |
N.P. |
Functions not investigated in KC. |
[67] |
TEX101 |
GPI anchored acrosomal protein for fertilization |
+ |
+ |
N.P. |
Functions not investigated in KC. |
[79] |
C. UNKNOWN SIGNIFICANCE |
TSPY1 |
Unknown functions |
N.P. |
+ |
N.P. |
Functions not investigated in KC. |
[74] |
NY-ESO-1 |
Unknown functions |
+ |
+ |
N.P. |
Functions not investigated in KC. |
[35] |
SPATA19 |
Mitochondrial biogenesis and organization |
+ |
N.P. |
N.P. |
Functions not investigated in KC. |
[79] |
ODF |
Sperm tail structural protein |
+ |
N.P. |
N.P. |
Functions not investigated in KC. |
[79] |
3. Embryonic Stem Cell Genes Are Ectopically Expressed in Keratinocyte Carcinomas
3.1. Core ESC Transcription Factors Support Multiple Hallmarks of Cancer in KC
The core ESC transcription factors that are ectopically expressed in KC regulate tumor initiation, stemness and several other outcomes. The three core transcription factors are sex determining region Y-box protein 2 (Sox2), octamer binding transcription factor 4 (Oct-4) and Nanog
[11]. Effectively, these genes promote a stable pluripotent state by activating transcription of pluripotency genes and repressing lineage specific genes in ESCs
[11]. Their expression is restricted to the ESCs in the inner cell mass; they are undetectable in adult tissues
[11]. Ectopic activation of these factors in cancer correlates with aggressive disease and the emergence of CSC subpopulations
[10].
In ESCs, Sox2 forms a complex with Oct-4 that is necessary for establishing pluripotency
[11]. Li et al. performed Sox2 knockdown and overexpression in primary human BCC cells. Transient Sox2 knockdown attenuated proliferation and migration
[80]. Knockdown also increased expression of E-cadherin protein whilst decreasing fibronectin and vimentin
[80]. Opposite effects were observed when Sox2 was overexpressed
[80]. These findings implicate Sox2 expression in BCC cells with heightened proliferative and invasive phenotypes.
In cSCC, ectopically-expressed Sox2 is implicated in tumor initiation and malignant progression. Boumahdi et al. reported that Sox2 protein is expressed in 29 of 40 AKs and 25 of 39 invasive cSCC tumors
[81]. Sox2 expression was also observed in murine papillomas (analogous to human AKs) and cSCCs tumors that were induced using the chemical carcinogen 7,12-Dimethylbenz[a]anthracene/12-O-tetradecanoylphorbol 13-acetate (DMBA/TPA)
[81]. Isolated Sox2-expressing murine tumor cells had a greater tumor-initiating potential compared to Sox2-negative cells
[81]. Roles for Sox2 in early-stage malignant progression were investigated. Deletion of Sox2 in established papillomas and cSCCs resulted in tumor regression
[81]. Transcriptional profiling revealed that Sox2 expressing cells are enriched in genes involved in proliferation, adhesion and stemness
[81].
3.2. Ectopic Expression of EMT Factors in KC
EMT is a physiological process during embryogenesis that is necessary for the establishment of the three embryonic germ layers
[82]. The induction of EMT in transformed tissues is an early event in invasion and metastasis and is the most recognized manifestation of phenotypic plasticity
[83]. EMT is coordinated by a complement of embryo-specific transcription factors including Snail1, Slug, Twist1 and Zeb1/2 in both the embryo and transformed tissues
[84]. While Snail1 and Slug are shown to be expressed at low to moderate levels in normal epidermis, the other factors are absent from normal adult skin and are re-expressed in malignant tissues, promoting invasive phenotypes
[85].
The transcription factor Twist-related protein 1 (Twist1) is essential for mesoderm formation during embryonic development
[86]. A role for Twist1 in KC is controversial due to paradoxical findings regarding its expression in healthy human skin
[87]. Vand-Rajabpour et al. performed RT-qPCR on primary BCC biopsies and reported that Twist-1 mRNA is decreased in BCCs compared to normal skin
[87]. It was hypothesized that this reduced Twist1 mRNA expression in BCC explains the low propensity for these cancers to become invasive
[87]. Contrary to these findings, Beck et al. demonstrated that Twist1 protein is absent from normal epidermis, but is ectopically expressed in murine and human hyperplastic skin, pre-cancerous lesions and cSCCs
[88]. A low level of Twist-1 protein was detectable in the dermis. Conditional deletion of Twist1 expression in murine epidermis attenuated DMBA/TPA-induced SCC formation
[88]. cSCC cells with Twist1 knockdown demonstrate decreased tumor propagating potential
[88]. Deletion of Twist1 in papillomas caused tumor regression without impacting markers of EMT, suggesting that Twist1 is necessary for tumor maintenance independent of EMT
[88]. It was determined that Twist1 regulates p53 stabilization, which in turn enables and sustains skin tumors
[88]. Hence, beyond EMT and invasion, Twist1 plays a crucial role in tumorigenesis, stemness and tumor maintenance. These results were corroborated by a subsequent study
[89]. Eguiarte-Solomon et al. further found that conditional Twist1 deletion in murine epidermis attenuated UV-induced hyperproliferation and tumorigenesis
[90]. Twist1 overexpression in keratinocytes also impaired keratinocyte differentiation in vitro
[90]. These combined results indicate that Twist1 may be implicated in tumorigenesis and shifting the balance between proliferation and differentiation.
3.3. Emerging Contributions of Oncofetal Genes to Early Skin Tumorigenesis
In addition to ESC genes, oncofetal genes were also shown to participate in KC tumorigenesis. High mobility group AT-hook 2 (HMGA2) is an oncofetal protein that binds DNA at select promoters/enhancers, remodeling chromatin architecture and serving as a scaffold for additional factors
[91]. Ha et al. investigated HMGA2 expression in UV-induced skin carcinogenesis. RT-qPCR and immunofluorescence were used to demonstrate that HMGA2 is expressed in neonatal foreskin keratinocytes but not in adult epidermis
[92]. HMGA2 is re-expressed in cSCC tumors, and in immortalized lesional HaCaT keratinocytes
[92]. UVR upregulates HMGA2 expression in keratinocytes and in SCC cells in vivo
[92]. Since HMGA2 is expressed in non-lesional and UV-treated skin, it was postulated that HMGA2 is a marker of proliferating keratinocytes rather than transformation
[92]. Li et al. found that HMGA2 nuclear translocation occurs as papillomas progress to cSCC
[93]. All evidence considered, a role for HMGA2 in early tumorigenesis is well-supported.
Cripto-1 is another oncofetal antigen expressed in KC tumors that is implicated in skin tumorigenesis. In embryonic and fetal tissues, Cripto-1 acts as a co-receptor for activin receptor heterodimers
[94]. In coordination with Cripto-1, activin heterodimers transduce signals from transforming growth factor β (TGF-β) family signaling ligands such as Nodal, participating in the establishment of proper body axes and organogenesis
[94]. Welss et al. detected Cripto expression in eight of ten primary human BCC tumors by RT-qPCR
[95]. cSCC tumors did not express Cripto-1
[95].
4. Conclusions
Currently available chemotherapeutics, immunotherapies and targeted therapies for advanced/metastatic BCC and cSCC are wanting in efficacy and tolerability. Given the high prevalence of KCs, optimizing KC diagnosis and chemoprophylaxis are research priorities to minimize the prevalence and burden of disease. Hence, there is a persistent need to investigate the molecular pathogenesis of this cancer, and to develop new and improved approaches for disease management. Ectopically expressed CTAs and developmental genes are appealing to study as putative biomarkers and therapeutic targets, given their limited expression in healthy somatic cells.
Key questions remain. First, some genes that were classified as gametogenic or development-restricted are, in fact, detected in normal skin samples. Whether the expression of these genes in normal skin is a natural occurrence or a manifestation of environmental insults has implications for skin cancer prevention and early biomarker discovery. Analyzing differential expression of ectopically expressed genes between sun exposed versus non-sun exposed skin, and between young versus aged skin, can be an informative strategy to determine sensitive/specific biomarkers of early disease. In the same vein, characterizing CT and developmental gene expression in pre-malignant disease and carcinomas in situ can also yield novel insights and reveal therapeutic targets. In addition, despite recognized contributions to cancer biology, the majority of ectopically expressed CT, ESC and oncofetal genes have not been profiled in epidermal tumors. This remains a challenge that must be addressed. Even though expression for several genes has been detected in KC tumors, functional studies to assess possible contributions to disease phenotypes are lacking. Finally, there is the ever-present question of clinical translation. The pipeline from bench to bedside research is only emerging. Efforts to target and exploit CTAs and developmental gene expression for clinical uses are underway, and have occasionally manifested in concrete results such as clinical trials and biomarkers.