In this study, PPGL patients were divided into SDHB high expression group and SDHB low expression group for analysis.The SDHB high expression group obtained higher stromal cell score and microenvironment score, lower tumor purity score, while there was no significant difference in immune cell score. This indicates that the tumor composition is more complex and heterogeneous, which may be associated with its poor prognosis, which needs to be further verified.
The results of PDCD1LG2, BTLA, and HAVCR2 suggest that SDHB score is positively correlated with ICGs expression and may be valuable in predicting the response to immunotherapy in PPGL. Among them, PDCD1LG2 is a programmed death ligand 2, which can be used as an immune checkpoint for Hodgkin's lymphoma, primary testicular lymphoma and gastric cancer in the previous literature[45–47].B and T lymphocyte attenuator BTLA can induce immune suppression by inhibiting the activation and proliferation of B and T cells to mediate the related immune effects[48].In recent years, a large number of studies have found that BTLA is involved in a variety of physiological and pathological processes, such as tumors, inflammatory diseases, autoimmune diseases, infectious diseases, and transplant rejection[48–50].HAVCR2 is the hepatitis A virus receptor 2 and is commonly known as T cell immunoglobulin and mucin domain-containing protein 3 TIM3 in terms of immune checkpoints. It was originally identified as a receptor expressed on CD4 + and CD8 + T cells that produce interferon-γ[51].However, recent studies have shown that TIM3 inhibition enhances the anti-tumor effect of PD1 blockade[52].
Differential alternative splicing enzymes can regulate the expression of SDHB and provide potential therapeutic targets for treatment.Among them, researchers have found that PRMT5 can structurally and functionally interact with Menin in hereditary multiple endocrine neoplasia type 1 (MEN1) syndrome, acting as a scaffold protein to regulate the cellular homeostasis of endocrine organs[53].The other alternative splicing enzymes (CBP1, HSPA8, EIF3A, DHX9, DDX46, CHERP, ZFR, SRSF9, RNF40, RAVER1) have not been reported and need further verification.
The differentially expressed genes were involved in vascular smooth muscle proliferation, transcriptional activity and regulation of oxygen content in PPGL. At the same time, KEGG results showed that the differentially expressed genes were closely related to cell apoptosis and a variety of immune-related pathways.These results suggest that SDHB may affect the biological behavior of PPGL by altering the immune response, transcriptional activity and apoptosis.In addition, correlation analysis of tumor burden showed that there was no significant difference in tumor burden between the SDHB high expression group and the SDHB low expression group.
For the potential mechanism of SDHB in PPGL, functional enrichment suggested that the differential expression of SDHB could affect the biological behavior of PPGL through immune response, transcriptional activity and apoptosis.Immune evasion, genomic instability, and tumor promoted inflammation are also hallmarks of cancer.Similarly, the correlation between SDHB genes and lncrnas has been reported in a variety of tumors such as colon adenocarcinoma and osteosarcoma.Similar to these studies, our results provide clues to the potential functional mechanism of the SDHB gene in PPGL. Functional enrichment analysis suggested that altered immune response might be one of the potential mechanisms of SDHB gene in pheochromocytoma/paraganglioma.However, how the differential expression of SDHB gene affects the immune response of PPGL remains unclear.Our immunological correlation analysis showed that samples with high SDHB expression had higher immune scores, promoted macrophage infiltration, widely increased immune function scores, and generally upregulated ICG expression.Interestingly, in addition to tumor-promoting GAM and Treg cells, most other immune functions were also activated in the high SDHB expression group.Perhaps their function is masked by GAM and Treg activation.Alternatively, perhaps these common tumor suppressor immune components are reprogrammed in PPGL to play different biological roles.But all these hypotheses need to be further verified by follow-up experiments.Tumor heterogeneity, including genomic heterogeneity, largely affects the SDHB expression of PPGL.
TMB, the total mutation frequency per tumor sample, is a fundamental type of genomic heterogeneity.Theoretically, higher TMB is associated with a better immunotherapy response and prognosis in patients, in large part due to the potential for immunogenic neoantigens to be generated by tumor mutations[54, 55].However, our results showed no significant difference in TMB between the high expression group and the low expression group, which seems to contradict previous studies.The main reason for this may be the heterogeneity among different tumor types.The above correlation between TMB and immunotherapy response was mainly observed in lung cancer.However, a pan-cancer study published in Annals of Oncology showed that high TMB was associated with greater immunotherapy sensitivity only in cancers in which CD8 T-cell abundance was positively correlated with neoantigen load, such as melanoma, lung cancer, and bladder cancer.For cancers in which there was no correlation between CD8 T cell levels and neoantigen load, such as breast cancer, prostate cancer, and glioma, higher TMB was instead associated with immune response and tolerance to immunotherapy.
Since TMB did not predict the immune response to PPGL, to explore new treatment options and screen potential synergistic drugs for PPGL, we further attempted to predict the preferential sensitivity of various drugs in different risk groups to PPGL samples.Based on our results, SDHB low expression group tumors may be more sensitive to NPK-76-Ⅱ-72-1, cyclopamine and sunitinib, while SDHB high expression group tumors may be more sensitive to crizotinib, YM155 and JW7-24-1.In addition, pazopanib, a commonly used anti-PPGL drug, also showed preferential sensitivity to SDHB-low expression samples[56],which is in line with our expectation that low SDHB expression is associated with a worse prognosis[22, 23, 57–59].NPK-76-Ⅱ-72-1 is a minichromosome maintenance protein 2, which is believed to regulate the expression of PLK3CA, thereby regulating cell proliferation.Cyclopamine can specifically inhibit the Hedgehog pathway and also play an important role as an antagonist of SMO[60].However, the prospects of cyclopamine as a therapeutic drug are limited, not only because of the acid instability of this small molecule, but also because of the strict concentration requirements of this small molecule[61].Sunitinib is a selective inhibitor of multiple receptor tyrosine kinases associated with tumor growth and angiogenesis[62].Sunitinib has been shown to play an important role in the treatment of patients with metastatic PPGL, especially in patients with SDHB mutations[63].Crizotinib inhibits ROS1, MET and ALK and is considered a first-line treatment for lung cancer[64].There are no studies on this drug and PPGL.YM155(Sepatronium Bromide) is an inhibitor of Survivin, an anti-apoptotic protein that is overexpressed in cancer cells. It can inhibit apoptosis, promote proliferation and enhance invasion, and is a logical target for potential cancer therapy[65, 66].By overexpressing survivin, cancer cells can avoid apoptosis and often become resistant to treatment[67].There are no studies on this drug and PPGL.It is possible that YM 155 is also a DNA damage inducer, introducing another anticancer mechanism, and that the concentration of DNA damage induced is much lower than that required to inhibit survivin[65].JW7-24-1 can regulate lymphocyte-specific protein tyrosine kinase LCK, which plays a key role in the activation and differentiation of T lymphocytes, and under certain conditions, Lck is also involved in the induction of apoptosis [68].Pazopanib is a tyrosine kinase inhibitor that exerts anticancer effects by inhibiting angiogenesis and oncogenic signaling pathways [69].Chemotherapy with temozolomide or cyclophosphamide + vincristine + dacarbazine is the first-line treatment option for metastatic PPGL, and other treatments (sunitinib, cabozantinib, everolimus, and PD-1/PDL-1 inhibitors) have shown moderate efficacy[70].In our study, temozolomide showed no significant difference between SDHB high and low expression groups.PPGL needs individualized treatment, and further research and differentiated treatment methods are needed.Taken together, our findings reveal the possibility of using these drugs in synergy with SDHB to inhibit PPGL growth.