Colon cancer is a fatal malignancy of the digestive system and the prognosis remains very poor19. Currently, conventional treatment options also have limitations in improving patient survival outcomes, including surgery and chemotherapy 20–22. At the same time, immune checkpoint inhibitor therapy offers patients better monitoring opportunities, unique treatment options, and greater hope for prolonged survival 3. Therefore, the discovery of new biomarkers associated with immunomodulation in colon cancer is crucial for its diagnosis, treatment and prognosis.
The DLD gene encodes a member of the class I pyridine nucleotide-disulfide oxidoreductase family that plays multiple roles in the decarboxylation of pyruvate and the oxidation of dihydrolipoamide and is involved in constituting α-keto acid dehydrogenase complexes (e.g. α-ketoglutarate dehydrogenase, α-ketoadipate dehydrogenase and glycine decarboxylase) 6, 23–25.DLD is a mitochondrial respiration-dependent copper ion carrier induced novel regulated positive regulators of cell death that regulate copper-dependent cell death by affecting the tumor TCA cycle and the underlying biosynthetic processes that support tumor growth, which may provide new ideas for the therapeutic action of certain cancer-targeted drugs 5,26. Previous studies have found that DLD is involved in constituting pyruvate dehydrogenase complex and α-ketoglutarate dehydrogenase complex, which are associated with several fatal metabolic diseases and have been identified as important metabolic targets in cancer27–30. Evidence from studies suggests that DLD downregulation may affect mitochondrial metabolism, thereby decreasing the levels of downstream metabolites in the TCA cycle, inducing melanoma cell death, and inhibiting tumor progression in humans by promoting ROS production and altering energy metabolism 31. However, more bioactivities of DLD as a promising therapeutic target need to be found in different diseases.
Expression profiling based on the TCGA-COAD database and the GSE:39582 dataset showed that DLD expression was significantly lower in CC than in normal tissues (Fig. 1C). We obtained DLD protein expression levels in human normal colon tissues and colon cancer tissues in the HPA database, and the results were consistent with DLD mRNA expression levels (Fig. 3A). In our study, low DLD expression was associated with malignant clinicopathological features such as staging and grading. We then evaluated the prognostic value of DLD in CC using two cohorts and found that high DLD expression was significantly associated with OS prognosis of CC patients (Fig. 2B).
To reveal the hidden mechanism behind their reduced invasive growth pattern, we constructed a DLD gene co-expression network in the TCGA-COAD dataset and performed GO and KEGG enrichment analysis. In this study, the expression of CYCS, NDUFA5, SDHD, and SNX4 in CC showed the highest correlation with DLD. Copper ion-induced ROS leads to cytochrome C (CYCS) release from mitochondria into the cytoplasm, resulting in caspases activation and DNA breakage in PC12 pheochromocytoma cells 32. deubiquitination of CYCS inhibits hepatocellular carcinoma 33. nDUFA5 (Ubiquinone Oxidoreductase Subunit A5) is involved in encoding the B13 subunit of the mitochondrial respiratory chain complex I, a protective prognostic gene in ovarian cancer 34,35. sdhd benefits patients with COAD and is a novel biomarker 36. A retrospective analysis based on GSE66229 data showed that SNX4 was associated with good survival outcomes in gastric cancer37. Analysis of GO and KEGG enrichment by co-expression correlated with many classical signaling pathways such as metabolic pathways, oxidative phosphorylation and peroxisome. GSEA analysis showed that differential genes based on DLD high expression grouping were mainly enriched in some important tumor associated KEGG pathways. This study is the first to reveal that DLD is associated with pathways in CC in cancer, hedgehog signaling pathway, focal adhesion, ECM, WNT signaling, cell adhesion molecules, leukocyte transendothelial migration, regulation of actin cytoskeleton, calcium signaling pathway, and chemokine signaling pathway.
Tumor mutation burden (TMB) reflects the number of mutations in the tumor and generates immunogenic neoantigens, thus increasing the likelihood of T cell recognition and correlating with the response to immune checkpoint inhibitors (ICIs) 38. Consistent with PD-L1 expression, TMB may provide a reference for the selection of ICIs for the treatment of tumor patients 39,40. In this study, a somatic mutation analysis was performed in the TCGA database based on DLD expression levels. We listed the top 30 genes with the highest mutation rates in the high and low DLD groups (Fig. 6A, B).TTN, APC, TP53, KRAS, MUC16, and SYNE1 were the most frequently mutated genes in both groups, while PIK3CA appeared in the top 5 in the low DLD group.PIK3CA mutations identified colorectal cancer as a poor prognostic factor associated with solid organ transplantation and mismatch repair deficiency, and PIK3CA mutations predict reduced response to anti-EGFR therapy in patients with metastatic colorectal cancer 41,42. In conclusion, low expression DLD has a higher tumor mutation rate compared to high expression DLD, which would provide some direction for the selection of immunotherapy regimens for patients with low expression DLD.
This study investigated the potential mechanisms of DLD high expression prognosis based on COAD single cell function and immune infiltration levels.The CancerSEA database reveals the functional states of different cancer cells at the single cell level, involving 25 functional states of 41,900 cancer single cells from 14 cancer types 43. In this study, DLD expression was positively correlated with tumor cell "invasion" and " DNA repair" as analyzed by the CancerSEA database. In contrast, DLD expression was negatively correlated with "angiogenesis". These results suggest that high expression of DLD plays a crucial role in the suppression of colon tumors. Recent evidence suggests that DLD is highly correlated with lipid acylated protein abundance in a variety of human tumor cells and that cell lines with high levels of lipid acylated proteins are sensitive to copper-induced cell death 5. This may be another important factor in the inhibition of tumor invasion by DLD.
Immune infiltrating cells are an important component of the tumor microenvironment and play an important role in influencing tumor growth, progression, treatment outcome, and patient prognosis 44–46. Higher immune infiltration in colorectal cancer is associated with improved time to recurrence, OS, and DFS 47,48. TIMER was used to explore the correlation between DLD expression and the level of immune cell infiltration in tumors and showed that samples with high DLD expression tended to infiltrate more immunocytes. By analyzing the infiltration levels of 28 immune cells in the high and low DLD expression groups using the ssGSEA algorithm, we also observed that the infiltration levels of CD4 T cells, type 1 T helper cells, type 2 T helper cells, regulatory T cells, memory B cells, and NK cells in the high DLD group were elevated, while CD8 T cells, Tfh cells, gamma delta T cells, type 17 T helper cells, activated B cells, immature B cells, CD56dim natural killer cells, myeloid derived suppressor cell, activated dendritic cell, plasmacytoid dendritic cell, immature dendritic cell, macrophage, eosinophil, mast cell, monocyte ,neutrophil infiltration levels were reduced. A higher percentage of NK cells were activated in the high DLD group, a cell particularly known in oncology for its innate ability to recognize and spontaneously kill tumor cells 49. Thus, a greater proportion of NK cells being activated may reduce tumor immune escape in the tumor immune microenvironment, which may be a potential reason for the prognostic benefit of high DLD expression in CC patients. Interestingly, the high DLD group also possessed a higher proportion of Tregs. it is known that Tregs contribute to suppress excessive immune activation and act as immunosuppressive cells to coordinate tumor immune escape and are considered targets of systemic immunotherapy 50. These analyses suggest that DLD is involved in regulating the immune mechanisms of the CC tumor microenvironment, particularly the regulation of NK cells. we hypothesize that DLD plays an important role in recruiting infiltrating immune cells and regulating immunity in CC, thereby affecting prognosis. However, more studies are needed to confirm this hypothesis, especially the effect of DLD on NK cells in the CC microenvironment.