1. Introduction
Lung cancer is a widespread form of cancer. The chances of surviving NSCLC vary from 19%-63% depending on the stage and available treatment options [
1,
2]. In real-world data analysis, the median overall survival was 351 days for all patients and 571 days for those with pathophysiologically relevant molecular aberrations.[
3] Recent advances in precision medicine have improved outcomes for select patients with NSCLC who have activating mutations and/or activating translocations, particularly those with pulmonary adenocarcinomas.[
4] Moreover, immune therapy with checkpoint inhibitors, particularly PD-1 and PD-L1 inhibitors, has shown promising outcomes with partial or even complete tumor regression in the case of pulmonary adenocarcinomas [
5,
6]. Biomarkers have been developed to identify molecular targets and select the most effective medications for individual patients. To treat lung cancer accurately, it is necessary to perform a precise diagnosis of the underlying tumor type, followed by a molecular panel that includes immunohistochemistry and next-generation sequencing (NGS) with RNA and DNA analyses in the case of NSCLC.[
7]
In our routine investigation, we identify molecular targets relevant to therapy. These include mutations in EGFR, BRAF, KRAS, ERBB2, STK11, and rearrangements such as ALK, RET, ROS1, NTRK, and cMET Exon 14 splice mutation. Tumors such as lung cancer alter their microenvironment to decrease the effectiveness of physiological anti-tumor mechanisms[
8]. Cancer cells upregulate PD-L1 to evade immune-regulated destruction [
9]. PD-1 and PD-L1 checkpoint inhibitors block PD-L1, enabling them to destroy tumor cells again. Therefore, the expression of PD-L1 measured with the tumor proportion score (TPS) is used as a biomarker to predict the probability of immune-therapy response in lung cancer [
10,
11]. TPS is the percentage of stained viable tumor cells with complete or partial membranous staining at any intensity. The higher the TPS, the better the individual’s response to checkpoint inhibitor therapy is assumed[
12]. Immune or target monotherapies can be combined with chemo- and radiotherapy in later therapy lines to improve and prolong therapy success[
13]. Some patients become long-term survivors with a chronification of lung cancer.[
14] Changes in the immune status result in changes in the methylation status in large tumor regions[
15]. We conducted a comprehensive epigenomic methylation analysis to identify such molecular mechanisms in lung cancer with different PD-L1 statuses (i.e., PD-L1 high, TPS>50% and PD.L1 low, TPS<1%) and correlated the results with PD-L1 expression. Database searches such as GeneCards, National Institutes of Health, Alliance of Genome Resources, and The Cancer Genome Atlas (TCGA) outlined the pathobiology functions of genes and promotors significantly differentially methylated.
2. Materials and Methods
2.1. Tissue Collection and Immunohistochemical Analysis
Our study used formalin-fixed and paraffin-embedded (FFPE) archived samples from the University Institute of Pathology, University Hospital Salzburg, and Paracelsus Medical University. Salzburg, Austria. All tumors were primary pulmonary acinar adenocarcinomas with either high PD-L1 expression (TPS > 50%; n=10) or low PD-L1 expression (TPS < 1%; n=10). Standard molecular analyses were performed during routine workup, including DNA and RNA panels (described below). Only cases without molecular events in target genes or gene regions were included in the present study. The PD-L1 status of tumor cells was also determined during routine utilizing the DAKO 22C3pharm DX kit from Agilent, USA. Immunostainings were performed on a DAKO-Omnis System(Agilent, USA), and the tumor proportion score (TPS) was calculated according to the rules determined by the US Food and Drug Administration (FDA) and the National Comprehensive Cancer Network (NCCN).[
11,
16] For our differential methylation analysis, we selected 10 cases with high PD-L1 expression (TPS >50%, mean 82%) and ten patients with low PD-L1 expression (TPS <1%, mean 0,5%), as defined in
Figure 1 and Example Images 1-4.
2.2. Molecular and Genetic Analysis
Tumor specimens with a minimum of 60% tumor content were used to ensure accurate mutation analysis. In some cases, we increased the tumor content by microdissecting the area of interest in tissue slides. We followed the manufacturer’s protocol for DNA extraction using a Maxwell system from Promega in Fitchburg, WI, USA. For mutation analysis of lung adenocarcinomas, we used either the AmpliSeq for Illumina Cancer Hotspot Panel v2 or the AmpliSeq for Illumina Focus Panel (Illumina, San Diego, California, USA). Next-generation sequencing was performed on an Illumina MiniSeq device following the manufacturer’s protocol. We used an Infinium Methylation EPIC Bead Chip and the Illumina protocol to identify methylation patterns for genome-wide screening[
17].
2.3. Computational Data Analysis
We adhered to the three customary procedures for computational analysis of DNA methylation data, which included (I) data processing and quality control, (II) data visualization and statistical analysis, and (III) validation and interpretation.
We analyzed methylation data using the Illumina Genome Studio Methylation Module and RnBeads on R statistical software[
18]. Our mapping was based on the human genome reference builds (GRCh38.p14 patch release of the hg38 assembly). DNA methylation beta values were used, indicating the ratio of the intensity of the methylated bead type in variables between 0 and 1, depending on the combined locus intensity (ranging from 0% to 100% methylation). To ensure the reliability of our results, we removed probes enriched with single nucleotide polymorphisms (SNPs) and used Greedycut filtering to eliminate unreliable measurements with p>0.05 in an iterative algorithm. We also removed context-specific probes and those on sex chromosomes. The Dasen method was utilized to normalize beta values.[
19] (please see Figures 2 and 3). Probes were marked with four genomic areas, including tiling regions (which were 5000 nucleotides long), genes (version Ensemble genes 75), promoters (version Ensemble genes 75), and CpG islands (CpG island track of the UCSC Genome Browser)[
20]. We used the Limma method (linear model for microarray data) for on-site level p-value computation, a hierarchical linear model using an empirical Bayes approach.[
21] To identify sites exhibiting differential variability between two sample groups, we used the diffVar method integrated into the missMethyl package and nominal p-values (nominal significances) as performed in other Epigenome-Wide Association Studies (EWAS)[
22,
23].
3. Results
Our analysis focused on identifying significant differences in the methylation status between PD-L1 high adenocarcinomas (TPS > 50%) and PD-L1 low cases (TPS < 1%) without any known relevant molecular aberration, with a particular interest in gene and promoter methylation status. The results showed a significantly different methylation pattern in both groups. (please see Figures 4 and 5)
We analyzed 20 samples with 866,895 methylation sites but had to exclude 17,371 sites due to overlapping with SNPs and 7,532 sites after applying the Greedycut algorithm. Additionally, we removed 18,597 probes located on sex chromosomes and 2,915 context-specific probes. We kept all samples but removed 46,415 probes, thus retaining 820,480 probes for final analyses. (Figure 3)
Our analysis identified 252,729 annotations in the tiling regions (length 5,000), 34,988 annotations in genes, 44,852 annotations in promoters, and 26,540 annotations in CpG islands. We only report the highest-ranking results previously linked to lung cancer in GeneCards and Alliance of Genome Resources. The detailed outcomes of the highest-ranking results can be found in
Table 1.
We classified genes and promoters based on the percentage of hypermethylation and hypomethylation and the most significant differences (delta values) in methylation in cases with high and low PD-L1 expression.
In our study, hypermethylation in the PD-L1 high group ranged from 73% to 90% methylation with p-values from 0.001 to 0.04. (Please refer to TABLE 1 for precise values). Hypermethylation was observed in the following locations in this group: SNORD114-14 (C/D Box 114-14, Small Nucleolar RNAs, snoRNAs), DCAF4L2 (DDB1 Associated Factor 4 Like 2 ), CELF2-AS1 (CELF2 Antisense RNA 1), LINCMD1 (Long Intergenic Non-Protein Coding RNA, Muscle Differentiation, MIR133BHG), LINC00528 (Long Intergenic Non-Protein Coding RNA 528). The most significant difference in methylation (delta value) between PD-L1 high and low expression groups was found in S100A7L2 (S100 Calcium Binding Protein A7 Like 2) (delta value = 17%, p-value = 0.002) and SOD1P3 (Superoxide Dismutase 1 Pseudogene 3) (delta value =14% and p-value = 0,02).
Hypomethylation was observed in genes and promoters of PD.L1 high-expressing adenocarcinomas at rates ranging from 14% to 21%, with p-values between 0.007 and 0.03. This was seen in the following locations: CAPS2 (cyclase-associated protein 2, calcyphosine 2), GLIPR1L2 (GLI Pathogenesis Related 1 Like 2), and IFITM3 (Interferon Induced Transmembrane Protein 3).
Our study found that tumors with low PD-L1 expression had hypermethylation in SNORD114-14 (methylation 90 % and p-value 0,04), while all other top results were pseudogenes or irrelevant to humans.
Locations with the lowest methylation in PD-L1 low expressing cases were found in miR124-3 (MicroRNA124-3), TRIM71 (Tripartite Motif Containing 71, LIN41), CAPS2 (Calcyphosine 2), UBE2QL1 (Ubiquitin Conjugating Enzyme E2 Q Family Like 1), and GLIPR1L2 (GLIPR1 Like 2). Methylation rates ranged from 15% to 30%, with p-values between 0.007 and 0. 01. The most significant methylation delta values between PD-L1 low/high were found for the NUMB gene (NUMB Endocytic Adaptor Protein) and LINC00528, with delta values of 19% / 20% and p-values of 0.001 and 0,0005.
4. Discussion
The expression of genes can differ due to various epigenetic mechanisms cells use to regulate DNA functions. One of these mechanisms is DNA methylation, which alters gene expression without changing its sequence. This modification can affect gene expression during cell differentiation. Methylation of a promoter region can regulate nearby gene expression, and excessive methylation can cause silencing mainly of DNA repair genes[
24]. This is believed to be an early step towards cancer progression, as hypermethylation of DNA upstream blocks access to transcription factors and enzymes, ultimately inhibiting downstream gene activity[
25]. Conversely, many tumors exhibit hypomethylated carcinogenic genes when compared to normal tissue[
26,
27]. Activating typically silenced genes can contribute to developing malignant neoplasias as individuals age. Both hypermethylation and hypomethylation are leading causes of oncogenesis, the former being more frequent and occurring at the CpG islands in the promoter region of the genes. In contrast, the latter occurs globally in various genomic sequences[
28].
Several genes exhibit significant methylation status, directly influencing angiogenesis, active oxygen, calcium, and vessels closely related to the tumor microenvironment. [
29] Additionally, the methylation status of several genes and promoters is associated with modifying the status of immune cells such as macrophages, lymphocytes, or neutrophils.[
30] These genes and promoters help create an optimized microenvironment for tumor growth and development.[
24,
31]
Our research revealed significant differences in the methylation levels of genes and promoters between pulmonary adenocarcinomas with high and low expression of PD-L1. A few genes and promoters had similar methylation status simultaneously in both groups.
5. Conclusions
PD-L1 high-expressing tumors show hypermethylated genes and promotors with strong oncogenic effects (SNORD114-14, DCAF4L2, S100A7L2, and SOD1P3) and hypermethylated genes and promotors with tumor suppressor effects (CELF2-AS1, LINCMD1). Especially LINC00528 helps to create a tumor microenvironment that supports cancer development, growth, and progression. Hypomethylated genes and promotors in this group lose their tumor-suppressing effects (IFITM3 and GLIPR1L2) and also have a reduction of oncogene activity (CAPS2).
PD-L1 low-expressing tumors show a single potent hypermethylated oncogene (SNORD114-4) and hypomethylated genes and promotors which lose their suppressor function (MIR124-3, TRIM71, GLIPR1L2, and NUMB) and additionally reduce the functionality of CAPS2, an oncogene. Besides, UBE2QL1 and LINC0528 form a microenvironment that supports cancer growth.
PD-L1 independent genes and promotors with similar results in both tumor groups with low and high levels of PD-L1 show hypermethylation of SNORD114-14 and hypomethylation of CAPS2 and GLIPR1L2.
Lung carcinomas exhibiting high and low PD-L1 expression demonstrate distinct methylation patterns, indicating different paths through various mechanisms by which PD-L1 high-expressing and PD-L1 low-expressing lung cancers develop. It appears that the development of oncogenic effects primarily drives tumors with high expression of PD-L1, and carcinomas with low expression of PD-L1 tend to develop tumors mainly by reducing suppressor mechanisms. If genes and promoters are hypermethylated, leading to simultaneous upregulation of suppressor and oncogenic effects, suppressors seem less effective than dominant oncogenic mechanisms. We concluded that activation of oncogenes is equivalent to the more aggressive behavior of tumors in the PD-L1 high group. Remarkably, the tumor microenvironment supports tumor growth in both groups.
Author Contributions
Conceptualization, G.H., T.F.J.K.; KS. methodology, G.H., T.F.J.K., B.A.S., software, G.H., T.F.J.K.; validation, G.H., T.F.J.K.,B.A.S., KS.; formal analysis, G.H., T.F.J.K. investigation, G.H., T.F.J.K.; resources, K.S.; data curation, G.H., T.F.J.K.; writing—original draft preparation, G.H.; writing—review and editing, G.H., K.S.; visualization, G.H., T.F.J.K.; supervision, G.H., and K.S.; project administration, K.S.; All authors have read and agreed to the published version of the manuscript.
Funding
This research received no external funding.
Institutional Review Board Statement
All procedures performed in this study were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki Declaration and its later amendments or comparable ethical standards and approved by the Institutional Review Board and Ethics Committee of the district of Salzburg (415-E/2509/2- 2019, 24 April 2019), all samples were anonymized before study inclusion (non-identifiable samples).
Informed Consent Statement
Not applicable.
Data Availability Statement
Data is available from the corresponding author on request.
Conflicts of Interest
The authors declare no conflict of interest.
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