1. Introduction
Intracranial Epidermoid Cysts (IECs) are congenital inclusions arising from the neuroectodermal epithelial rests that result from the defective closure of the neural tube between the third and fifth week of embryonic life[
1], [
2]. IECs are extremely rare, representing only 0.3-1.8% of all intracranial tumors[
2], [
3].They typically occur in both genders, between the ages of 20–60 years, with a peak incidence in the fourth decade[
4]. IECs are often located in the cerebellopontine angle (37.3%) and parasellar region (30%), spreading in the subarachnoid space of the basal cisterns[
5]. They may also be found in the middle cranial fossa (18%), diploe (16%), spinal canal (5%)[
5], [
6], and rarely in the brain stem or medulla[
7], [
8], [
9]. Magnetic resonance imaging (MRI) is the diagnostic approach to identify IECs. Diffusion-weighted imaging allows greater accuracy in the preoperative differential diagnosis[
10].
Despite their benign intercourse, tumor growth can be driven by the division of the stratified squamous epithelium lining its cavity. Cyst contents are largely composed of acellular keratin debris and cholesterol inclusions. Liquefaction of the cyst contents is associated with infection or loss of vascularity[
11].
IECs have a strong propensity to adhere to critical neurovascular structures, resulting in significant morbidity and neurologic impairment[
12], [
13]. In addition, recurrent malignant transformation of IECs has been reported [
14], [
15], [
16], including the development of secondary lesions such as malignant melanoma[
17] and Squamous Cell Carcinoma (SCC)[
18]
The primary treatment for IECs is surgery. Gross total resection is critical to minimize progression[
19]; however, cyst adherence results in difficult removal. A subtotal resection yields recurrence rates of up to 93%[
20]. Radiation does not seem to have a role in the management of IECs but it has been used as adjuvant therapy for cysts that exhibit malignant transformation[
21]. Due to the rarity of IECs, limited research has been conducted focusing on cyst prognosis, diagnosis, and treatment. There is a lack of timely access to molecular testing to determine eligibility for treatment with targeted therapies, difficulty enrolling sufficient numbers of patients in clinical trials, and limited incentives for drug development. Due to the insufficient understanding of the pathophysiology and the molecular characteristics of IECs, no effective systemic or targeted therapy for these patients is available.
Genomic instability is a characteristic of human cancers[
22]. With the emergence of exome sequencing, the genetic landscape of many diseases has been unveiled, enabling the identification of actionable targets for new drug development[
23] and for which FDA-approved options already exist, highlighting the potential repositioning of these drugs to a new indication[
24], and reshaping cancer treatment.
The genetics of IECs remain underexplored. There have been suggestions of a familial inheritance pattern observed for epidermoid cysts of the spleen[
25]. Mice lacking the IL-1 receptor (IL-1R) (
IL1r–/–) or deficient in IL1-β developed immunosuppression and multiple epidermal cysts after chronic UVB[
26], suggesting that induced somatic events and an
altered innate immune response may be involved in the initiation of epidermoid cysts. However, the role of somatic genetic variants in driving trapped fibroblasts to form skull base epidermoid cysts is completely unknown.
In this study, we applied Whole Exome Sequencing (WES) to establish the somatic signature of IECs. A particular focus of our efforts was to gain insights into the biology of IECs. Improved knowledge about the mechanisms of cyst development and progression could enable the identification of potentially actionable variants of clinical impact, and ultimately, bring treatment advances to these patients who still depend on a very limited standard of care.
2. Materials and Methods
Specimens, Patients, and Clinical Data
Studies were conducted following the U.S. Common Rule ethical guidelines. Tumor tissue was resected from participants with the diagnosis of IEC who underwent surgery between 1995 and 2021 at the University of Washington hospitals (Seattle, WA, USA). The respective clinical data was extracted from the University of Washington, School of Medicine clinical database. Data and specimen collection were reviewed and approved by the University of Washington Institutional Review (STUDY R1813). Written informed consent was obtained from all subjects. Samples were collected and stored in tumor biobank (STUDY 00002162) until further processing. Specimens were reviewed by three neuropathologists and neurosurgeons. Clinical data was gathered regarding date, history, demographics, imaging, neuropathology reports, operative information, pre-and post-operative symptoms, adjuvant treatment, and outcomes.
Whole-Exome Sequencing
Genomic DNA (gDNA) was isolated from six fresh-frozen tissue and matching blood (when available) using the QIAmp DNA Mini Kit (Qiagen, Hilden, Germany) and QIAmp DNA Blood Mini Kit (Qiagen, Hilden, Germany), respectively, following the manufacturer’s recommended protocol. DNA was quantified using the Qubit fluorometer (Invitrogen, Carlsbad, CA). Sequencing of gDNA was carried out at the Northwest Genomics Center, Seattle, WA. All sequencing and library preparation were performed at the University of Washington Northwest Genomics Center. Automated library construction and exome capture were carried out in 96-well plate format, using Perkin-Elmer Janus II (PerkinElmer, Waltham, MA) equipment. Five hundred nanograms of genomic DNA were subjected to a series of shotgun library construction steps, including fragmentation through acoustic sonication (Covaris, Woburn, MA), end-polishing, and A-tailing ligation of sequencing adaptors, followed by PCR amplification with dual 10bp barcodes for multiplexing. Libraries underwent exome capture using the Twist+RefSeq exome (36.5MB target) (Twist Biosciences, San Francisco, CA). Briefly, 187.5ng of shotgun library was hybridized to biotinylated capture probes for 16-18 hours. Enriched fragments were recovered via streptavidin beads and PCR amplified. Since each library was uniquely barcoded, samples were captured in multiplex. Prior to sequencing, the pooled library concentration was determined by fluorometric assay, and molecular weight distributions were verified on the Agilent Bioanalyzer (Agilent, Santa Clara, CA) (consistently 180 ± 15bp). Parallel sequencing-by-synthesis with fluorescently labeled, reversibly terminating nucleotides was carried out on the NovaSeq6000 instrument (RTA 3.1.5) (Illumina, San Diego, CA).
Read Processing, Quality Control, and Somatic Variant Calling
Base calls generated in real-time on the NovaSeq6000 were demultiplexed. Raw reads were assessed for Phred score quality using the FastQC tool kit v0.11.9 (
https://www.bioinformatics.babraham.ac.uk/projects/fastqc/). Trimmomatic[
27] was used to detect and remove sequencing adapters, primers, and low-quality nucleotides. Trimmed reads were aligned against the human reference genome GRCh38 using the Burrows-Wheeler Aligner (BWA) (v0.7.15)[
28]. All aligned read data were sorted and subject to duplicate removal using Picard (v2.6.0) (
https://broadinstitute.github.io/picard/). Base qualities were recalibrated with GATK BaseRecalibrator (v4.2.6.1) (
https://gatk.broadinstitute.org/). Somatic variant calling in unmatched samples was performed against a panel of normal (PoN), using three variant callers: Mutect2 (v4.2.6.1) (GATK), Varscan (v2.4.2)[
29], and Vardict[
30]. Tumor/normal variant calling was conducted with Mutect2(v4.2.6.1) (GATK), Varscan (v2.4.2)[
29], MuSe (v1.0) (MD Anderson Cancer Center), and Strelka[
31]. To identify high-confidence mutations, a joint analysis was applied by combining at least three softwares (three-caller pipeline approach)[
32]. Mutation calls were selected through a stringent filtering process and functionally annotated with ANNOVAR[
33].
Somatic Copy-Number Inference
Somatic Copy Number Variations (CNVs) were identified using CNVkit (v0.9.9)[
34]. Whole-exome sequencing read alignments in BAM format, the capture bait locations, and a pre-built human reference were the inputs to the program. All additional data files used in the workflow, such as GC content and the location of sequence repeats, were extracted from the genome sequence in FASTA format using scripts included with the CNVkit distribution. Log
2 copy ratios across the genome for each sample were calculated based on on-target reads and the nonspecifically captured off-target reads. The baseline of normalized sequencing depth on targeted regions was first constructed based on normal samples. For each tumor sample, log2 copy number ratio of normalized sequencing depth on each targeted region between the tumor sample and the baseline was then calculated. A built-in segmentation algorithm was applied to the log
2 ratio values to infer discrete copy number segments[
34]. The log
2 ratios and segments were used as inputs for the genomic Identification of Significant Targets in Cancer (GISTIC) v2.0.23[
35], allowing the identification of copy number regions that were significantly amplified or deleted across the set of samples.
Data Analysis and Visualization
Search Tool for the Retrieval of Interacting Genes (STRING,
http://www.string-db.org/) was used to construct the Protein-Protein Interaction (PPI) network. Genomic data analysis, visualization, and annotation were conducted in R through the usage of Bioconductor packages. Functional enrichment analysis was performed by enrichR[
36] with the inclusion of the General Ontology (GO) Molecular Function, Biological Process, and Molecular Component databases[
37]. Pathway enrichment analysis was carried out using the Kyoto Encyclopedia of Genes and Genomes (KEGG) 2021 Human database[
38]. We utilized the sigminer[
39], clusterProfiler[
40] , and Maftools[
41] packages to extract, analyze, and visualize copy-number aberrations and mutational signatures. For all analyses, p < 0.05 was considered as statistically significant, unless stated otherwise. Biorender software (Toronto, CA) was used for scientific illustration.
Data will be deposited in a publicly available database prior to publication.
4. Discussion
There is an urgent need to understand the mechanisms as well as identify oncogenic drivers underlying the progression of IECs to improve treatment options. Due to the rarity of these tumors, limited research has been done; consequently, these patients continue to rely on a very limited standard of care, resulting in neurological impairment and poor outcomes.
Next-generation sequencing has dramatically reshaped oncological treatment through the identification of genetic variants that provide prognostic information, and aid in therapeutic selection, resulting in hypothesis-driven clinical trials[
120]. Since the genetics of IECs was completely unexplored, we sought to apply WES on resected IECs to improve our knowledge of the mechanisms of IEC's oncogenic transformation and identify possible actionable drivers of potential clinical value.
Tumor cells stimulate significant molecular, cellular, and physical changes in their surrounding tumor microenvironment (TME) to keep it continuously evolving as the tumor develops, acting as an active promoter of cancer progression[
121]. The GO analysis of the altered gene set identified in IECs revealed a strong association with the MHC complex. The pathway enrichment indicated a tight involvement in antigen processing and presentation, ECM-receptor, and cellular adhesion. Since the TME is composed of blood vessels, extracellular matrix, stromal and immune cells[
121], [
122], our results demonstrate that genetic alterations in these immune, ECM-receptor, and adhesion genes may have a role in shaping an epidermoid cyst-permissive TME.
IECs were characterized by an altered immune repertoire. The immune cells of the TME are the ones involved in the acquisition of immune escape mechanisms for cancer progression and development[
121], [
122]. Tumor cells with stronger immunogenicity can be recognized and eliminated by the immune system, while some others can escape from it by several mechanisms and then develop into cancers[
123]. Somatic mutations in immune genes affected 100% of the cohort. Top altered immune-related genes included HLA and KIR genes. HLA molecules are fundamental for triggering anti-tumor immunity[
124]. KIR is a superfamily of immunoglobulins located on the surface of natural killer (NK) cells, consisting of Ig-like domains that bind to HLA class-I molecules, which help them distinguish between what is “the self” and what is “the non-self”[
125], [
126]. The innate immune response is the first mechanism activated for tumor immune response, where NK cells play a significant function. The interaction between HLA class-I molecules on normal tissue cells, and inhibitory KIRs on the surface of NK cells shapes the autoimmune tolerance[
127]. Taken together, a mechanism of immune evasion in IECs may be based on the HLA-KIR interaction, where the tumor can insert mutations on its LA molecules to decrease antigen presentation[
123], [
128]. Interestingly, preliminary data demonstrated that mice lacking the IL-1 receptor (IL-1R) (
IL1r–/–) or deficient in IL1-β developed immunosuppression and multiple epidermal cysts after chronic UVB[
26], suggesting that induced somatic events and an
altered innate immune response may be involved in the initiation of epidermoid cysts, strongly corroborating our findings. The blockade of, for example, the inhibitory KIRs might be a promising immunotherapy strategy to optimize anti-tumor response.
To identify potential oncogenic drivers of IECs, we considered genes that were found altered in at least 50% of the cohort (>=3 patients), resulting in a set of thirty-three driver candidates. The functional interaction network also strengthened immune evasion as the leading oncogenic mechanism of IECs. Our review of the literature revealed that, except for the PRB1, all the other driver candidates have been previously associated with cancer immune infiltration, TME, and/or endoplasmic reticulum (ER) stress, supporting their role as oncogenic drivers in IECs. Interestingly, many members of the list of IEC driver candidates have been reported in SCC and melanoma, probably due to the epidermoid inclusions consisting of ectodermal epithelial/stratified squamous epithelial tissue applied to connective tissue with accumulation of keratin over the epithelium[
12]. The transformation of IECs into malignant SCC[
18] and melanoma[
17] has been reported. Targeting these overlapping genes may be an interesting approach to potentially block the malignant transformation of IECs.
Recurrent mutations on NOTCH2 and USP8 were identified in IECs, affecting 50% of the cohort. The most frequent alterations included USP8 p.R657W and NOTCH2 p.Q677P, which were observed in 50% and 30% of the cohort, respectively. Both USP8 and NOTCH2 are frequently overexpressed in human cancers[
102], [
103], [
108], [
109]. Consistently, mutations on these genes confer upregulated protein functionality [
110], [
111]. These observations indicate that the USP8 and NOTCH2 variants identified in IECs may display also enhanced protein function; therefore, they may be potential candidates for targeted therapy.
The missense p.Q677P in NOTCH2 is positioned in the EGF-like calcium-binding domain of the protein. USP8 p.R657W sits adjacent to the 14-3-3 binding motif (RSYSS) of the protein. Somatic activating mutations in USP8 located between the amino acids 713 and 720 have been identified in (ACTH)-secreting neuroendocrine tumors. In ACTH pituitary adenomas, USP8 mutations are also positioned within or adjacent to the 14-3-3 binding motif (RSYSS). The characterization of USP8 mutations in neuroendocrine tumors revealed that they promote oncogenic transformation via activation of EGF receptor signaling[
129]. Together, these observations indicate that USP8 and NOTCH2 alterations may result in downstream dysregulation of EGF receptor signaling in IECs. As in ACTH-pituitary adenomas, USP8 mutations identified in the present work may represent a potential diagnostic marker for IECs. Finally, the inhibition of USP8, NOTCH2, or EGFR should be further explored to tailor treatment strategies for patients with the diagnosis of IECs.
Many challenges are involved in drug development for IECs and other ultra-rare cancers, including the insufficient understanding of the cancer pathophysiology, molecular characteristics, natural history, and lack of timely access to molecular testing to determine eligibility for treatment with targeted therapies, difficulty enrolling sufficient numbers of patients to clinical trials. The use of FDA-approved drugs for new indications may promote the development of safe and effective therapeutic approaches to treat patients with ultra-rare cancers[
24]. We performed drug-gene interaction analysis to generate hypotheses about how the potential driver candidates for IECs might be therapeutically targeted. NOTCH2 was highlighted as a clinically actionable target for IECs. The Nirogacestat (Ogsiveo®, SpringWorks Therapeutics) is an FDA-approved NOTCH2 inhibitor indicated for the treatment of adult patients with progressing desmoid tumors. The patient’s treatment with Nirogacestat has provided progression-free survival benefit[
130]. It is worth noting that dermoid cysts and epidermoid cysts are similar in structure and origin; therefore, it may represent a very interesting approach for drug repurposing towards IECs. Finally, considering the high genomic instability in deubiquitinases and the indication of a tumor immune evasion mechanism (both also observed at copy-number level), proteasome inhibitors and immunotherapy may also represent treatment options that deserve further investigation in intracranial epidermoid cysts.
Author Contributions
SK: methodology, data analysis, data mining and curation, investigation, writing – original draft preparation. OP: methodology, data curation and backup, data analysis. CP: methodology, data analysis, data mining and curation, investigation, visualization, supervision, writing – original draft preparation, writing – review and editing. MFJ: Conceptualization, investigation, project administration, case selection, sample collection, funding acquisition. All authors have read and agreed to the published version of the manuscript.