Molecular landscape of eyelid sebaceous gland carcinoma: A comprehensive review

Abstract

Eyelid sebaceous gland carcinoma (SGC) is an aggressive skin cancer characterized by a heightened risk of recurrence and metastasis. While surgical excision is the primary treatment, unraveling the molecular intricacies of SGC is imperative for advancing targeted therapeutic interventions and enhancing patient outcomes. This comprehensive review delves into the molecular landscape of eyelid SGC, emphasizing key genetic alterations, signaling pathways, epigenetic modifications, and potential therapeutic targets. Significant findings include aberrations in critical signaling pathways (β-catenin, lymphoid enhancer binding factor, hedgehog, epidermal growth factor receptor, P53, and P21WAF1) associated with SGC progression and poor prognosis. Notably, eyelid SGC manifests a distinctive mutational profile, lacking ultraviolet signature mutations in tumor protein 53 (TP53), indicating alternative mutagenic mechanisms. Next-generation sequencing identifies actionable mutations in genes such as phosphatase and tensin homolog (PTEN) and Erb-B2 receptor tyrosine kinase 2 (ERBB2), facilitating the emergence of personalized medicine approaches. Molecular chaperones, specifically X-linked inhibitor of apoptosis protein (XIAP) and BAG3, emerge as pivotal players in promoting tumor survival and proliferation. The review underscores the role of epithelial–mesenchymal transition, where regulators like E-cadherin, vimentin, and ZEB2 contribute to SGC aggressiveness. Epigenetic modifications, encompassing DNA methylation and microRNA dysregulation, further elucidate the molecular landscape. This review consolidates a comprehensive understanding of the molecular drivers of eyelid SGC, shedding light on potential therapeutic targets and providing a foundation for future investigations in diagnostic, prognostic, and personalized treatment strategies for this formidable malignancy.

Sebaceous gland carcinoma (SGC) is a rare yet potentially aggressive skin cancer. About 40% of these carcinomas develop in the periocular region, constituting around 5% of all eyelid malignancies.[1,2] This aggressive malignancy is characterized by a heightened risk of recurrence and metastasis, necessitating immediate attention for effective management.[3] Presently, surgical excision remains the sole available treatment option, with some cases requiring surgical exenteration to address the aggressive nature of SGC.[4,5] The emerging role of neoadjuvant systemic chemotherapy (NACT) is gaining attention, particularly for advanced tumors infiltrating the orbital structures beyond the eyelid.[6]

Intriguingly, SGC has been found to co-occur with Muir–Torre syndrome (MTS), a subtype of hereditary nonpolyposis colorectal cancer.[7] MTS patients often harbor germline pathogenic variants in DNA mismatch repair (MMR) genes, emphasizing the significance of exploring genetic markers associated with SGC.[8] However, it is important to note that periocular and extraocular tumors exhibit distinct behaviors and genetic characteristics.[9]

Recent advancements in molecular research have shed light on crucial cell signaling mechanisms linked to differentiation and metastasis, which have implications not only in various skin tumors but also in noncutaneous malignancies. Notably, aberrations in β-catenin, lymphoid enhancer binding factor-1, Indian hedgehog (IHH) signaling, sonic hedgehog (SHH), cyclooxygenase-2 (COX-2), epidermal growth factor receptor (EGFR), P53, and P21 pathways have been closely associated with poor prognoses in SGC patients.[10,11,12,13,14,15] Of particular interest is the lack of ultraviolet (UV) signature mutations in the TP53 gene, a common occurrence in basal cell carcinoma (BCC) and squamous cell carcinoma (SCC), pointing toward alternative mechanisms of mutations in eyelid SGC.[16]

Advancements in high-throughput analysis, such as next-generation sequencing (NGS), have further unraveled potentially actionable mutations in genes like phosphatase and tensin homolog deleted on chromosome 10 (PTEN), phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit alpha (PI3K3CA), neurofibromatosis type 1 (NF1), MutS homolog 2 (MSH2), MutL homolog 1 (MLH1), pRB, protocadherin-15 (PCDH15), neurogenic locus notch homolog protein 1 (NOTCH1), and P53 in the context of eyelid SGC, opening new avenues for genotype-matched targeted therapies.[15,17]

In addition to genetic factors, molecular chaperones have emerged as vital players in protecting cells against stress conditions. Overexpression of XIAP in eyelid SGC suggests its involvement in promoting tumor progression and proliferation through prosurvival and antiapoptotic mechanisms.[17] Similarly, elevated levels of BAG3, an anti-apoptotic and pro-survival protein, have been observed in various malignancies, including SGC.[12,13]

Epithelial–mesenchymal transition (EMT) is a pivotal process intricately involved in the aggressive nature of SGC. Key molecular players such as E-cadherin, vimentin, and ZEB2 have been associated to orchestrate this transition in eyelid SGC.[18,19,20]

Epigenetic modifications, such as DNA promoter methylation, have been implicated in the dysregulation of crucial genes involved in cell adhesion and signaling, like E-cadherin and cyclin-dependent kinase inhibitor 2A (CDKN2A).[18,21] Furthermore, the discovery of microRNAs (miRNAs) and their intricate roles in gene expression regulation has provided valuable insights into SGC pathogenesis. miRNAs, such as those belonging to the miRNA-200 family, have been identified as pivotal players in tumor progression and metastasis.[22,23,24]

This comprehensive review aims to provide an up-to-date overview of the molecular genetic framework underlying the drivers of eyelid SGC. By meticulously examining the latest research and literature on SGC-associated genes, signaling pathways, epigenetic factors, and potential therapeutic targets, this review seeks to contribute to a deeper understanding of this life-threatening condition and pave the way for future advancements in its management and treatment. Genes associated with eyelid SGC were meticulously identified using PubMed as a valuable and reliable literature source. All studies published in English up to October 2023 have been included in this review. Key terms such as “eyelid,” “ocular,” “sebaceous gland carcinoma,” “sebaceous cell carcinoma,” “meibomian gland carcinoma,” and “sebaceous carcinoma” were utilized in the literature search to ensure a thorough compilation of relevant studies and findings. However, as for the exclusion criteria, “sebaceous gland carcinoma of the skin,” “stye,” and “conjunctivitis” were excluded.

Epidemiology

SGC represents a particularly aggressive form of eyelid malignancy, with around 75% of cases occurring in the periocular regions. While this kind of ocular cancer is relatively rare among Caucasians, it is more prevalent among Asian populations. In the USA, the reported occurrence of SGC among all eyelid carcinomas ranges from 0.5% to 5%.[2] SGC is more common in Asians, ranking second to BCC in periocular tumors.[25] In China and India, it constitutes a significant portion, accounting for 28%–32.7% of all eyelid malignancies, with some studies suggesting even higher proportions of 30%–40% in India.[25,26]

Mortality and morbidity

Eyelid SGC is recognized as an aggressive tumor, displaying a propensity for both local recurrence and distant metastasis. The tumor’s spread can encompass direct invasion into the orbital region, lymphatic dissemination to nearby lymph nodes, and hematogenous dissemination to distant organs.[27] Notably, documented local recurrence rates fluctuate between 9% and 36%, with larger-scale studies indicating rates around 30%.[28,29,30] Metastasis begins with lymph nodes and can spread to the liver, lungs, bones, and brain.[28,29] Surprisingly, metastasis can occur up to 5 years postdiagnosis, with mortality estimates ranging from 9% to 40%.[31]

SGC risk factors and prognostic indicators

Eyelid SGC is very rare, slow growing, and commonly found in elderly population with female predisposition. Mean age at diagnosis is mid-60s. Ocular SGC tends to occur in older adults 60–80 years of age; summary of the data of several large series shows that the mean age of onset ranges between 57 and 75 years.[32] There are also reports of ocular sebaceous carcinoma developing in children after radiation therapy for retinoblastoma.[33,34] In addition, individuals with genetic predisposition to MTS or, possibly, familial retinoblastoma have a higher susceptibility to developing this tumor.[33,34,35,36,37]

Several other etiological factors have been implicated in the development of SGC, although the exact causes remain elusive. Notably, previous radiation therapy to the head-and-neck region, administered for the treatment of other cancers, longstanding immunosuppression due to conditions like solid organ transplantation and human immunodeficiency virus infection or acquired immunodeficiency syndrome have been identified as potential risk factors for eyelid SGC.[34,37]

Studies have shown that human papilloma virus (HPV) infection is a significant factor in a subset of eyelid SGC cases. In particular, a relationship has been observed between the absence of mutations in tumor suppressor genes TP53 and retinoblastoma protein (RB1) and the presence of transcriptionally active high-risk HPV infection [Fig. 1]. It is important to note that while HPV is implicated in a subset of eyelid SGC cases, not all cases are associated with HPV.[38,39]

Figure 1.

Critical pathways identified in eyelid SGC (a) Dysregulated Wnt/β-catenin signaling in carcinoma leads to β-catenin accumulation and nuclear translocation. In the nucleus, β-catenin complexes with TCF/LEF transcription factors, activating genes that promote cell proliferation, survival, metastasis, and cancer progression. (b) Key cell cycle regulation and HPV disruption: p53 and RB are critical tumor suppressors. p16INK4a and p14ARF modulate their activity. RB controls G1 cell cycle arrest and entry into S phase. p53 mediates G1 and G2 arrest and apoptosis, preventing genomic instability. Cyclin-CDK complexes regulate RB via phosphorylation. HPV’s E6 and E7 oncoproteins disrupt this control by binding to p53 and Rb, respectively. p16INK4a inhibits CDKs, maintaining RB in an inactive state. Aberrant promoter methylation of p16 is linked to eyelid sebaceous carcinoma. (c) PD-1 and PD-L1 immune response regulation: PD-1 and PD-L1 orchestrate immune responses by regulating T-cell interaction with cancer cells. PD-L1 on cancer cells binds to PD-1 on T cells, suppressing immune activity. Inhibitors like pembrolizumab disrupt this interaction, reinvigorating T-cell–mediated antitumor responses. HPV = human papilloma virus, PD-1 = programmed cell death receptor-1, TCF = T-cell factor

SGC origin, histology, immunohistochemical profile, and differential diagnosis

Sebaceous gland tumors occurring in the ocular region typically have their roots in various glandular structures such as the meibomian glands, glands of Zeis, caruncle, and the skin of the eyebrow.[40] Among these, the meibomian glands found in the upper eyelid emerge as the predominant site for these tumors due to their increased occurrence [Fig. 2].[41] An aggressive attribute of SGC is its potential to manifest as multiple lesions, termed multicentricity, a phenomenon observed in a small subset of patients and often indicative of an elevated risk of localized recurrence.[1,41,42]

Figure 2.

(a) Histological depiction of a well-differentiated sebaceous carcinoma of the eyelid, revealing a prominent population of multivacuolated cells (arrow) that exhibit nuclear indentation, pleomorphism, and a notable presence of mitotic activity (hematoxylin–eosin staining, 200×). (b) Clinical presentation of eyelid sebaceous carcinoma with upper eyelid involvement. (c) SGC patient with large tumor and lymph node involvement (arrow). (d) CT scan from a patient with lower lid SGC. CT = computed tomography, SGC = sebaceous gland carcinoma

Clinically, SGC may present as a solitary nodule or a diffuse process on the eyelids. The more common presentation is a firm, painless subcutaneous nodule fixed to the tarsus or arising from the gland of Zeis. It may mimic chalazion, but can be distinguished by its characteristic loss of eyelashes.[43] In some cases, SGC can cause diffuse thickening of the eyelid and may involve the forniceal and bulbar conjunctiva, leading to initial misdiagnosis as persistent unilateral blepharitis or conjunctivitis.[43]

Histopathologically, SGC can be classified based on the degree of cellular differentiation. Well-differentiated tumors show neoplastic cells with sebaceous differentiation, featuring vacuolated foamy–frothy cytoplasm [Fig. 2].[2]

SGC often exhibits intraepithelial spread into the eyelid epidermis and conjunctival epithelium, known as “pagetoid spread.” It may also invade directly into adjacent structures, such as the orbit, paranasal sinus, and the intracranial cavity.[32,44] The poorly differentiated form may show perineural infiltration and lymphatic invasion, leading to metastasis in regional lymph nodes and distant organs [Fig. 2].

Timely diagnosis and proper management significantly enhance the prognosis of SGC. However, its clinical presentation varies, often mimicking other benign or malignant conditions such as SCC or BCC, which can lead to delays in diagnosis. Notably, SGC may resemble benign entities like chalazion and blepharoconjunctivitis.

In assessing eyelid lesions, especially in populations with higher incidence rates, considering SGC in the differential diagnosis is crucial. Microscopic examination remains the cornerstone for diagnosing and confirming SGC. Typically, SGC presents as moderately differentiated tumors characterized by sheet or lobule formation, occasionally exhibiting central comedo necrosis. The tumor cells display distinct cell membranes, clear to vacuolated cytoplasm, vesicular nuclei with prominent nucleoli, numerous mitoses, and apoptotic cells.

While staining techniques like Oil Red O or Sudan IV on frozen or formalin-fixed tissues were once used to detect intracytoplasmic lipid vacuoles, immunohistochemical staining has largely supplanted them. Recent studies have shown that over 75% of SGC cases stain positively for anti-cytokeratin 5 (anti-CK5) and anti-cytokeratin 15 (anti-CK15) while being negative for other cytokeratins.[45,46] Antibodies targeting phosphinothricin-N-acetyl transferase or perilipin family proteins, particularly adipophilin (ADP), have also been employed to identify lipid droplets, although ADP may lack specificity.

Immunoreactivity for epithelial membrane antigen (EMA), BRST-1, and Cam 5.2 antibodies further aids in SGC diagnosis. Notably, androgen receptor (AR) staining has emerged as a reliable marker for sebaceous differentiation. Combining antibodies such as carcinoembryonic antigen (CEA), EMA, AR, Ber-EP4, ADP, CA15-3, and CA19-9 in a panel has been suggested to differentiate SGC from SCC and BCC.[47,48,49,50]

To aid in the diagnosis of SGC, various immunohistochemical markers are employed. Human milk fat globulin-1, anti-CK5, anti-CK15, and anti-CK1 are some of the markers used to differentiate SGC from other eyelid malignancies like SCC and BCC.[45,46] A proposed panel of antibodies, including CEA, EMA, AR, Ber-EP4, ADP, CA15-3, and CA19-9, has been put forth as a potential method for distinguishing SGC from SCC and BCC.[47] ADP and Transcription factors that bind to G-A-T-A Nucleotide sequence (GATA) 6 immunostaining has emerged as a sensitive marker for lipid droplets in SGC, replacing traditional stains like Oil Red O or Sudan IV.[48,49] In addition, immunostaining for AR has been extensively used as a reliable marker of sebaceous differentiation.[50]

MTS and sebaceous carcinoma

Sebaceous carcinomas are rare malignancies arising from sebaceous glands, not only on the eyelids but also in the skin, head, or neck.[51] They can occur sporadically or be linked to MTS, a genetic condition involving skin tumors and internal cancers. MTS is part of hereditary non-polyposis colon cancer syndrome, or Lynch syndrome, marked by sebaceous growths.[52] These growths encompass benign sebaceous adenomas, sebaceomas, and sebaceous carcinomas. MTS patients exhibit microsatellite instability (MSI) and inherit autosomal dominant mutations in DNA MMR genes like MLH1, MSH2, and sometimes MSH6, PMS1, or PMS2. Consequently, assessing MMR gene status is crucial in diagnosing sebaceous adenomas, epitheliomas, and non-eyelid sebaceous carcinomas.[8] Notably, extraocular sebaceous carcinomas more often exhibit loss of MMR protein expression and/or MSI than eyelid SGC.[2]

By using immunohistochemical nuclear expression of MutL homolog 1 (hMLH1) and hMSH2, it is possible to identify tumors with MMR deficiencies, which serves as a reliable screening method for the diagnosis of DNA MMR-deficient MTS in sebaceous neoplasms. While some studies suggest a strong link between periocular sebaceous carcinoma and MTS, with reported rates as high as 30%, most research findings generally indicate that this association is relatively rare.[53]

The extent of the correlation between periocular sebaceous carcinoma and MTS is a subject of ongoing debate within the medical community.[54]

Wingless-related integration site/β-catenin signaling and SHH in sebaceous carcinoma

The wingless-related integration site (Wnt)/β-catenin signaling pathway is a complex and tightly regulated mechanism that controls various cellular processes, including the formation of sebaceous glands and the differentiation of stem cells in the epidermis.[55] This pathway regulates the balance between hair follicle and sebaceous gland differentiation. Overexpression of β-catenin in transgenic mouse epidermis leads to the development of hair follicle tumors. However, overexpression of N-terminally truncated LEF-1, which blocks β-catenin signaling, results in spontaneous sebaceous tumors.[2,3] Dysregulation of the Wnt/β-catenin pathway resulting in increased expression of cytoplasmic β-catenin has been observed eyelid SGC. This leads to the transcription of Wnt target genes, including cyclin D1 and c-Myc, suggesting a potential dysregulation of the Wnt/β-catenin pathway in these tumors [Fig. 1].[10] Mutations in the N-terminus of Lef1 that prevent β-catenin binding are found in human benign sebaceous tumors and eyelid SGC.[13,56] In addition, SHH has been observed to promote the proliferation of progenitors of hair lineages, while another related molecule, IHH, stimulates the proliferation of sebocyte precursors.[55,57]

In a study involving 37 cases of eyelid SGC, it was observed that both SHH and Wnt signaling pathways were activated in the cancerous tissue.[57] Notably, patients with metastasis displayed higher expression levels of SHH, ATP binding cassette subfamily G member 2 (ABCG2), and Wnt compared to those without metastasis.

NGS and potentially actionable genes in SGC

Recent studies have shed light on the genetic landscape of SGC using NGS techniques, offering valuable insights into the underlying pathogenesis of this cancer. In a recent study conducted by Tetzlaff et al.,[38] whole-exome NGS of 27 SGCs highlighted the prevalence of mutations in key cancer-associated genes, including TP53, RB1, PIK3CA, PTEN, ERBB2, and NF1. This comprehensive analysis also identified potentially clinically actionable mutations in a subset of patients, offering new avenues for targeted therapies. Among the identified actionable genes are Bruton tyrosine kinase (BTK), FGFR2, PDGFRB, Harvey rat sarcoma virus (HRAS), and NF1 mutations, which hold promise as potential therapeutic targets in the treatment of SGC [Table 1].

Table 1:

Potentially actionable genes in sebaceous gland carcinoma identified through NGS

Gene	Molecular role	Pathways	Inhibitor	Classification	References
PIK3CA	Phosphatidylinositol-4,5- bisphosphate 3-kinase catalytic subunit alpha	PI3K–Akt signaling pathway	PI3K inhibitors (e.g., alpelisib, idelalisib)	Oncogene	Tetzlaff et al., Na et al.
ATM	ATM serine/threonine kinase	DNA damage response, cell cycle checkpoints	PARP inhibitors (e.g., olaparib, rucaparib)	Tumor suppressor	Na et al.
KMT2A	Lysine methyltransferase 2A	Transcription regulation	Not specified	-	Peterson et al.
ERBB2	Receptor tyrosine-protein kinase ErbB-2	MAPK signaling pathway, PI3K–Akt signaling pathway	Pan-HER kinase inhibitors (e.g., lapatinib, neratinib)	Oncogene	Peterson et al., Tetzlaff et al.
EGFR	Epidermal growth factor receptor	MAPK signaling pathway, PI3K–Akt signaling pathway	EGFR inhibitors (e.g., erlotinib, gefitinib)	Oncogene	Erovic et al.
MMR genes	DNA MMR genes	DNA MMR pathway, MMR-deficient tumors	Immunotherapy (with MSI validation)	Tumor suppressor	Shields et al., Gaskin et al., Shalin et al.
RUNX1	Runt-related transcription factor 1	Transcription regulation	Not specified	-	Na et al.
FGFR2	Fibroblast growth factor receptor 2	FGFR signaling pathway	FGFR Inhibitors (e.g., erdafitinib, infigratinib)	Oncogene	Tetzlaff et al.
PDGFRB	Platelet-derived growth factor receptor beta	PDGF signaling pathway	PDGFR inhibitors (e.g., imatinib, nilotinib)	Oncogene	Tetzlaff et al., Erovic et al.
HRAS	HRas proto-oncogene, GTPase	RAS/MAPK signaling pathway	RAS inhibitors (e.g., tipifarnib)	Oncogene	Tetzlaff et al.
NF1	Neurofibromin 1	RAS/MAPK signaling pathway	RAS inhibitors (e.g., tramatinib)		Tetzlaff et al.

MAPK=Mitogen-activated protein kinase, MMR=Mismatch repair, MSI=Microsatellite instability, NGS=Next-generation sequencing, PARP=Poly (ADP-ribose) polymerase, PDGF=Platelet-derived growthfactor, FEFR= Fibroblasts growth factor receptor, RAS= Rat sarcoma

In a study by Peterson et al.,[15] NGS was employed to analyze 13 SGC samples, identifying recurrent mutations in tumor suppressor genes, including TP53 and RB1, in 76.9% (10/13) and 53.8% (7/13) of cases, respectively. The study also demonstrated the amplification of the Myelocytomatosis oncogene (MYC) locus in select cases, further supporting its contribution to SGC oncogenesis. Interestingly, the presence of other clinically actionable gene mutations, such as NF1, PMS2, ROS1, KMT2C, MNX1, NOTCH1, piccolo presynaptic cytomatrix protein (PCLO), and PTPRT, highlighted potential targets for therapeutic interventions in SGC.

In a subsequent research endeavor, Xu et al.[17] employed whole-exome sequencing on 31 instances of ocular adnexal sebaceous carcinomas. Their study also revealed a majority of tumors with TP53 mutations (71% of the 22 cases), often accompanied by mutations in RB1 (in 10 cases), ZNF750 (in 10 cases), and NOTCH1 (in seven cases) within a subgroup of instances. Interestingly, mutations in the PCDH15 gene were detected in five tumors, with three of these cases also having TP53 mutations. It is worth mentioning that four out of these five tumors with PCDH15 mutations subsequently progressed to a metastatic stage.

A study by Na et al.[58] demonstrated that SGC patients exhibited recurrent mutations in TP53 (13/30, 65%) and PIK3CA (4/20, 20%). Importantly, the identification of mutations in runt-related transcription factor 1 (RUNX1; 2/20, 10%) and ATM (3/20, 15%) was associated with the development of distant metastases, providing valuable insights into the potential mechanisms underlying SGC metastasis.[58]

Telomerase reverse transcriptase (TERT) mutations have emerged as significant genetic alterations in various cancer types, contributing to the maintenance of telomere length and enabling limitless replicative potential in cancer cells. Muñoz-Jiménez et al.[59] conducted an analysis of TERT mutations and revealed the presence of mutations in 26.7% (eight out of 29) of the examined cases. Table 1 summarizes the potentially actionable genes identified through NGS in eyelid SGC and their associated molecular roles, pathways, potential targeted therapies, and classification as oncogenes or tumor suppressors.

Epigenetic aspects of SGC

Epigenetics, a pivotal field unraveling heritable changes in gene expression independent of DNA sequence alterations, has emerged as a cornerstone for delving into the intricate molecular landscape of diverse cancers.[60] Recent research has highlighted the impact of promoter methylation on key genes in SGC. Notably, the E-cadherin gene, vital for cell adhesion and signaling, shows promoter methylation in 72% (25/36) of SGC cases. This epigenetic modification is correlated with the loss of membranous E-cadherin and membranous β-catenin expression.[18] Clinically, the presence of E-cadherin gene promoter methylation was marginally associated with reduced disease-free survival in patients with eyelid SGC.

Another study by Liau et al.[21] explored the epigenetic landscape of eyelid SGC and found CDKN2A promoter hypermethylation in nearly 45% (11/24) of cases. The CDKN2A gene, also known as “p16INK4a,” (p16) encodes for tumor suppressor proteins involved in cell cycle regulation. Interestingly, CDKN2A promoter hypermethylation was associated with younger patient age in eyelid SGC cases. This suggests that epigenetic alterations may play a role in the early development of SGC in younger individuals.

EMT-associated genes in SGC

EMT is a critical phenomenon involved in the progression of various epithelial malignancies, including SGC.[20,61] Some of the well-studied EMT markers in SGC include E-cadherin, vimentin, and ZEB2. The loss of cell adhesion molecule E-cadherin and the upregulation of mesenchymal markers, such as vimentin, are considered hallmark events of EMT in SGC [Fig. 3].[61]

Figure 3.

Outline of the key epithelial–mesenchymal transition events affected in SGC discussed in the present review article. The figure depicts the direct regulatory influence of the miR-141, miR-200b, and miR-205 miRNA families on factors that suppress the transcription of E-cadherin. In epithelial cells, these specific miRNA families (miR-141, miR-200b, and miR-205) actively impede the expression of transcription factors known as E-box interactors, which include ZEB1 and ZEB2. As a result of this inhibition, the transcription of E-cadherin is facilitated. However, when undergoing an EMT, there is a reduction in the expression of these miRNAs. This reduction subsequently allows ZEB1 and ZEB2 to switch roles and actively suppress the transcription of E-cadherin. Moreover, ZEB2 also directly promotes the increase in vimentin expression. SGC = sebaceous gland carcinoma

ZEB2, a transcription factor, has been identified as a key regulator of E-cadherin expression during EMT. Studies by Bhardwaj et al.[19] revealed that ZEB2 overexpression is present at the invasive front of eyelid SGC, suggesting its involvement in EMT of this malignancy. Furthermore, E-cadherin gene silencing through promoter hypermethylation was observed in a significant portion of ocular SGC cases, and mutations in the E-cadherin gene may also contribute to E-cadherin loss.[18] The association between ZEB2 overexpression and E-cadherin loss in SGC underscores their invasion-promoting roles in this aggressive cancer. In addition, their dysregulation correlated with poor clinical outcomes, further emphasizing the importance of EMT in driving aggressive behavior in SGC.

Zhao et al.[23] identified abnormal expression of miRNA-651-5p and ZEB2 in SGC tissues and SZ95 cell lines, compared to control tissues and cells. Through a series of in vitro and in vivo experiments, they demonstrated that miR-651-5p overexpression and ZEB2 knockdown suppressed malignant behaviors of SGC cells. Previous studies have highlighted the significance of aberrant expression of EMT markers, including ZEB2, E-cadherin, and miRNA-200 family members (miRNA-200c/141), in predicting the prognosis of eyelid SGC patients [Fig. 3].[22]

Vimentin, an intermediate filament protein in normal mesenchymal cells, maintains cellular integrity and stress resistance. In cancer, vimentin overexpression is linked to tumor advancement, metastasis, and poor prognosis. Eyelid SGC cases show significant cytoplasmic vimentin overexpression, absent in nearby normal epithelium. High vimentin levels signal adverse outcomes and predict lymph node metastasis, indicating EMT in aggressive SGC.[20]

Role of miRNA in SGC

miRNAs are small, single-stranded, noncoding RNA molecules, about 17–25 nucleotides long, that regulate the gene expression posttranscription. They play crucial roles in cellular processes like differentiation, proliferation, angiogenesis, and apoptosis. Depending on their target genes, miRNAs can function as oncogenes or tumor suppressors, influencing cancer development and progression.[62]

The miRNA-200 family acts as a tumor suppressor by regulating EMT in tumor progression and metastasis. Low expression of miRNA-200c and miRNA-141 in eyelid SGC is associated with high-risk features like large tumor size, poor differentiation, advanced stages, and pagetoid spread. The study also revealed an inverse correlation between ZEB2 and miRNA-141, suggesting an miRNA-mediated feedforward loop that stabilizes EMT and promotes tumor progression and invasion.[23] Recent studies have shown that miR-651-5p overexpression promotes apoptosis and inhibits invasion, migration, and EMT in UV-treated SGC cells by targeting the EMT regulator ZEB2.[24]

Zhang et al.[63] investigated miR-3907 and its role in eyelid SGC. Experiments revealed that miR-3907 directly targets and negatively regulates the tumor-suppressor gene THBS1, which plays crucial roles in platelet aggregation, angiogenesis, and tumor development. The upregulated expression of miR-3907 was observed along with downregulated expression of THBS1 in SGC tissues.

Studies conducted by Bladen et al.[64] identified differentially expressed (DE) miRNAs in two subtypes of SGC: nodular and pagetoid. There were 39 DE miRNAs common to both subtypes, with hsa-miR-34a significantly overexpressed in both. Hsa-miR-34a plays a role in the TP53 suppressor network and has a different impact on target genes in each subtype.

For instance, MYC, a target of hsa-miR-34a, was significantly overexpressed in pagetoid SGC, but remained unchanged in nodular SGC. In addition, B-cell lymphoma 2 (BCL2) and MYC, both targeted by hsa-miR-34a, showed significant overexpression only in the pagetoid subtype, suggesting a potential synergistic oncogenic role. In nodular SGC, 75 DE miRNAs were unique to this subtype, with hsa-miR-150 and hsa-miR-143 being the most overexpressed. Hsa-miR-150 is known to act as a tumor suppressor by targeting ZEB1, a transcriptional repressor associated with cancer invasion and metastasis. Hsa-miR-143, which targets BCL2, may also contribute to preventing cancer progression in this subtype. However, BCL2 was significantly overexpressed in the pagetoid subtype, possibly promoting its aggressive nature. Pagetoid SGC specifically expressed 53 DE genes, including hsa-miR-205, which was significantly upregulated. Hsa-miR-205 has been associated with invasive properties in other cancers and targets EZH2, which is significantly overexpressed in the pagetoid subtype and is linked to cancer development and therefore may contribute to its aggressiveness. Hsa-miR-200a, hsa-miR-141, and hsa-miR-603 were also upregulated in pagetoid SGC, with potential roles in enhancing tumor growth and regulating EMT-related pathways.[65]

The miRNA–target gene networks revealed the involvement of various pathways. In nodular SGC, miRNAs appeared to act as tumor suppressors, potentially inhibiting the mitogen-activated protein kinase (MAPK)/ Extracellular signal-regulated kinase (ERK) pathway. In pagetoid SGC, miRNAs may promote tumor growth and influence pathways such as PTEN and MAPK. Hsa-miR-199, specifically downregulated in pagetoid SGC, targets CD44, which is significantly overexpressed in this subtype and may contribute to its aggressive behavior.

Hirano et al.[66] investigated DE miRNAs and their interactions with cell cycle regulatory mRNAs in eyelid SGC. Dysregulation of cell cycle-related miRNAs, such as miR-146a-5p, miR-195-5p, and miR-4671-3p, led to the upregulation of cell cycle-related genes, including CCNE1, CCNE2, and CDKN3. Table 2 summarizes the miRNAs found to play a role in the pathogenicity of SGC and their target genes.

Table 2:

Differential expression of miRNAs with their target genes and biological roles in SGC

miRNA	Expression	Target gene	Biological role	miRNA type	Study group
miRNA-200c	Downregulated	ZEB2	EMT	Tumor suppressor	Bhardwaj et al.
miRNA-141	Downregulated	ZEB2	EMT	Tumor suppressor	Bhardwaj et al.
miR-651-5p	Downregulated	ZEB2	EMT	Tumor suppressor	Zhao et al.
miR-3907	Upregulated	THBS1	Tumorigenesis, cell proliferation, migration, and metastasis	Oncogenic	Zhang et al.
miR-205	Upregulated	EZR, LMNA, ZEB1, PTEN, PHLPP2, EZH2	Promotes invasion and metastasis, regulates epithelial phenotype	Oncogenic	Bladen et al.
miR-200a	Upregulated	MAPK14	Enhances oxidative stress tumor growth response	Oncogenic	Mateescu et al.
miR-141	Upregulated	MAPK14	Enhances oxidative stress tumor growth response	Oncogenic	Mateescu et al.
miR-199	Downregulated	CD44	Suppression of CD44, potential stem cell biomarker	Tumor suppressor	Zhang et al.
miR-150	Upregulated	ZEB1	Tumor suppressor, reduced invasion and metastasis	Tumor suppressor/oncogenic	Zhang et al.
miR-143	Upregulated	BCL2	Tumor suppressor, prevents cancer progression	Tumor suppressor	Zhang et al.
miR-603	Downregulated	E2F1	Regulation of E2F1 and cell behavior	Tumor suppressor	Zhang et al.
miR-34a	Upregulated	MYC, BCL2	TP53 suppressor network, MYC-mediated cell survival, regulation of EMT	Oncogenic	Zhang et al.
miR-146a-5p	Downregulated	CCNE1	Cell cycle	Tumor suppressor	Hirano et al.
miR-195-5p	Downregulated	CCNE2	Cell cycle	Tumor suppressor	Hirano et al.
miR-4671-3p	Downregulated	CDKN3	Cell cycle	Tumor suppressor	Hirano et al.

EMT=Epithelial–mesenchymal transition, SGC=sebaceous gland carcinoma

Cancer stem cell markers in eyelid SGC

Cancer stem cells (CSCs) are central in cancer development and resistance. Notably, CSCs exhibit increased motility, invasiveness, and resistance to DNA damage-induced apoptosis, all of which are critical traits contributing to metastatic potential.[67]

Kim et al.[68] investigated the clinical significance of CSCs in eyelid SGC and focused on seven proteins previously identified as potential CSC markers based on relevant literature, namely ALDH1, CD44, CD133, ABCG2, Sox 4, Sox 9, and Slug. Examinations revealed that these six markers were notably and selectively highly expressed in eyelid SGC cells compared to normal tissue, affirming their stem cell characteristics. Further study indicated patients with positive CD133 showed significantly shorter metastasis-free survival compared to those in the negative groups for these markers.[69]

Receptor tyrosine kinases in eyelid SGC

Receptor tyrosine kinases (RTKs) are a class of cell surface receptors that are integral components of cell communication and are involved in essential functions such as cell growth, proliferation, differentiation, survival, and migration.[70,71] In recent investigations, a comprehensive analysis of 49 eyelid SGC patient samples revealed intriguing insights into the molecular landscape of this malignancy.[72] Among the examined RTKs, HER2, EGFR, C-MET, and FGFR1 protein expression were observed in distinct proportions of patient samples.

In a study conducted by Erovic et al.,[73] a comprehensive investigation was undertaken utilizing a tissue microarray comprising 115 core biopsies extracted from 20 patients afflicted with eyelid SGC. Utilizing immunohistochemistry, the study assessed protein expressions associated with pivotal processes, including carcinogenesis, angiogenesis, inflammation, apoptosis, and cell-to-cell interactions. These investigations unveiled numerous proteins with elevated expression levels in eyelid SGC. Noteworthy among these were vascular endothelial growth factor receptor 2 (VEGFR-2), platelet-derived growth factor receptor alpha and beta (PDGFR-a/-b), EGFR, Cox-1/-2, myeloid cell leukemia sequence 1 (Mcl-1), matrix metalloproteinase 1 (MMP-1), CD9, Bmi-1, 14-3-3r, glutathione S-transferase pi (GSTP), as well as various constituents of the SHH, AKT, and WNT pathways. Notably, the study’s most significant observations pertained to the prominent expression of VEGFR2, EGFRR2, PDGFR-a, and PDGFR-b. Drawing parallels with the successful implementation of tyrosine kinase receptor inhibitors in the treatment of other malignancies, these findings raise the prospect of employing agents like sunitinib, cetuximab, bevacizumab, and pazopanib for the treatment of eyelid SGC.

Cell cycle dysregulation in eyelid SGC

The tightly regulated cell cycle governs cell growth, replication, and division, with dysregulation being a hallmark of cancer.[74] In eyelid SGC, abnormal expression of cell cycle regulatory proteins like p21, p27, cyclin E, and CDKN2A (p16) has been noted.[75] To delve into SGC’s molecular mechanisms, a global microarray analysis identified highly expressed cell cycle-related genes, including CDKN2A (p16), CDK1, and CCNE1, forming a gene network.[75]

Immunohistochemical analyses have provided fresh perspectives on the disruption of essential cell cycle pathways in eyelid SGC, with specific emphasis on the p53–p21–p27–cyclin E–CDK2 and p16–cyclin D–CDK4/6–Rb–E2F pathways. Kim et al.[76] investigated the dysregulation of the p53–p21–p27–cyclin E–CDK2 pathway in 43 cases of eyelid SGC. Among these, p53, a notable tumor suppressor, exhibited positive immunoreactivity in 52.4% of tumor cases. In addition, p21, a vital cyclin-dependent kinase inhibitor responsible for enforcing cell cycle arrest, displayed positivity in 79.5% of tumors. Significantly, p27, a critical CDK inhibitor, exhibited widespread positivity in 95.4% of cases, accentuating its potential role in disease progression. Cyclin E, a pivotal driver of the G1–S transition, was found to be overexpressed in 68.4% of tumors, collectively indicating aberrant activation of the p53–p21–p27–cyclin E–CDK2 pathway. The study also unveiled perturbations within the p16–cyclin D–CDK4/6–Rb–E2F pathway, an additional crucial axis governing cell cycle progression. The tumor suppressor p16, a key regulator of G1–S transition, demonstrated positive immunostaining in 79.1% of cases. Cyclin D1, a critical orchestrator of G1 phase progression, was overexpressed in 30.8% of tumors. Importantly, loss of pRb expression was observed in 39.0% of cases, suggesting compromised control of the G1 checkpoint.

Apoptosis

Apoptosis is a tightly regulated process crucial for maintaining tissue homeostasis and eliminating damaged or unwanted cells. Among the key regulators of apoptosis is the X-linked inhibitor of apoptosis protein (XIAP), a potent caspase inhibitor that can hinder the activation of apoptosis-promoting caspases (caspase 3, 7, and 9), blocking both the extrinsic and intrinsic apoptotic pathways.[77] Recent studies have highlighted the role of XIAP in the pathogenesis of several cancers, including melanoma, prostate cancer, and lung cancer. In a study investigating the expression of XIAP in eyelid SGC, it was found that increased cytoplasmic XIAP expression was significantly associated with advanced age, large tumor size, and reduced disease-free survival.[78]

Management of SGC

Various methods are employed to manage patients with SGC, with the choice of approach contingent upon the tumor’s stage at presentation. Strategies encompass excision, orbital exenteration, radiotherapy, and chemotherapy. The primary approach for managing sebaceous carcinoma, both periocular and extraocular, is in accordance with recent clinical practice guidelines authored by a panel of experts in the field.[4] These guidelines endorse Mohs micrographic surgery or wide-local excision as the preferred initial treatment.[79]

In cases of periocular tumors involving underlying orbital or periorbital structures extensively, orbital exenteration might be necessary.[5,80] To assess disease extent in stage IIC or higher tumors, particularly those around the eyes, a sentinel lymph node biopsy is an option, given the 15% positivity rate for metastasis (Sa HS).[81,82]

The role of radiotherapy in eyelid SGC has been extensively explored, yielding varied outcomes.[83,84] Postoperative radiotherapy following orbital exenteration is superior in disease control compared to sole exenteration.[28,85] Cryotherapy can serve as a supportive intervention.[86] The application of mitomycin C, a non-cell cycle–specific alkylating agent, is controversial, with its utility debated.[87] It has been employed to mitigate the need for further invasive procedures in cases of SGC with pagetoid spread.[88]

Neoadjuvant chemotherapy (NACT) has emerged as a transformative approach in the treatment landscape of SGC. Recent developments, documented in case series and isolated case reports, have shed light on the role of NACT in SGC treatment, yielding promising results that warrant further exploration.[89,90,91] Drawing inspiration from established chemotherapy protocols utilized in managing head-and-neck SCC, researchers and clinicians have adapted these regimens to SGC treatment. Platinum-based agents, such as cisplatin and carboplatin, have been combined with 5-fluorouracil and taxanes, forming the basis of these NACT strategies. The rationale behind these choices lies in the success of these agents in targeting cancer cells in related malignancies, which paves the way for potential effectiveness in SGC.[6]

In cases where advanced or unresectable tumors exhibit MSI or a high tumor mutation burden, the Food and Drug Administration (FDA) has granted approval for the use of pembrolizumab, an immune checkpoint inhibitor (ICI).[91,92] However, it is important to note that there is a lack of FDA-approved treatments specifically tailored to sebaceous carcinoma.[5]

Immune checkpoint inhibitors

Immune checkpoint molecules are vital regulators that prevent excessive immune responses and maintain self-tolerance. A well-studied interaction in carcinoma involves programmed cell death receptor-1 (PD-1) and Programmed cell death ligand 1 (PD-L1). PD-1 on T cells, B cells, and natural killer cells interacts with PD-L1 on tumor cells, inhibiting T-cell activation and cytokine production [Fig. 1].[93] While surgical excision remains the primary treatment in SGC, the pursuit of adjuvant therapies for aggressive cases has led to exploration in the field of cancer immunotherapy, specifically ICIs[94,95] Among these, PD-1 blockade has emerged as a promising adjuvant therapy.

Studies have revealed significant PD-1 expression on tumor infiltrating lymphocytes (TILs) in SGC cases, indicating the presence of an active immune response within the tumor microenvironment.[96] In addition, PD-L1 expression on tumor cells suggests potential immune evasion through PD-1/PD-L1 interactions. Jayaraj and Sen[97] observed immunohistochemical co-expression of PD-1 on TILs and PD-L1 on tumor cells in 46% of SGC cases, which further supports the viability of immune checkpoint blockade in SGC.

FDA-approved PD-1 inhibitors such as nivolumab and pembrolizumab have demonstrated favorable outcomes in various advanced malignancies and may offer potential benefits for SGC patients.[93,94] Recent case reports have illustrated the off-label use of pembrolizumab in advanced SGC, leading to near-complete responses and remissions in metastatic and recurrent cases.[94] Nevertheless, large-scale prospective studies are imperative to evaluate the full potential of anti-PD-L1 immunotherapy as an adjuvant treatment for SGC.

Conclusion

In conclusion, the present review delves into the intricate landscape of eyelid SGC, a rare yet aggressive skin cancer that poses significant challenges in its management. The aggressive nature of SGC is underscored by its high recurrence and metastasis rates, necessitating urgent attention for effective therapeutic strategies.

The interplay between SGC and MTS adds a layer of complexity to understanding the genetic basis of SGC. With insights from recent molecular research, crucial signaling pathways related to differentiation and metastasis have come to the forefront. Distinct genetic aberrations in pathways such as β-catenin, hedgehog signaling, COX-2, EGFR, and others have been linked to unfavorable prognosis in SGC patients. Importantly, the absence of UV signature mutations in TP53 hints at unique mutational mechanisms in SGC.

Advancements in high-throughput analysis, like NGS, have illuminated actionable mutations in genes such as PTEN, ERBB2, and PI3K3CA, offering potential avenues for genotype-specific targeted therapies. EMT has been identified as a pivotal process in SGC’s aggressiveness. Epigenetic modifications and miRNA dysregulation have also been unraveled as critical players in SGC pathogenesis. These alterations influence cell adhesion, signaling, and metastasis-related genes, highlighting their potential as prognostic markers and therapeutic targets.

Surgical excision remains the primary treatment for SGC. Concurrently, NACT is emerging as a noteworthy consideration in SGC treatment. However, aggressive cases prompt exploration of cancer immunotherapy, specifically ICIs targeting the PD-1 pathway. Comprehensive large-scale studies are imperative to assess the adjuvant potential of anti-PD-L1 immunotherapy for SGC. Ongoing research, rigorous trials, and collaborative efforts are essential to unveil ICIs as a transformative approach in managing eyelid SGC.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.