HLA-DPA1 gene is a potential predictor with prognostic values in multiple myeloma

Multiple myeloma (MM) is a hematological malignancy which is characterized by aberrant plasma cells infiltration in the bone marrow and complex heterogeneous cytogenetic abnormalities [1]. Accumulation of abnormal plasma cells replaces normal hematopoietic cells and leads to “CRAB” - hypercalcemia, renal failure, anemia, and bone lesions, even fetal outcome eventually [2]. With the deepening of basic and clinicalresearches, novel drugs mainly including proteasome inhibitors and immunomodulatory drugs have improved patients’ outcome to some extend [3, 4]. Besides, high-dose chemotherapy and tandem autologous stem cell transplant (ASCT), together with supportive care have significantly prolonged patients’ progression-free survival (PFS) and overall survival (OS) [5]. However, MM remains an uncurable disease as underlying molecular mechanisms of pathogenesis and progression are still largely unclear. Quite a few patients cannot get diagnosis and proper treatment in time. Therefore, identifying key mechanisms regulating MM is critically important for early diagnosis and targeted therapy.

With the advances of high-throughput platforms and microarray, more and more molecular heterogeneity on MM has been recognized [6, 7]. Hypoxia plays an important role in occurrence and development of MM [8, 9] and more related pathogenesis is still urgent needs to be explore for better diagnosis and treatment. In order to find potential biomarker of MM related to hypoxia, we analyzed the differentially expressed genes (DEGs) functions and pathways between normoxic and hypoxia-resistant (HR) MM cell lines contained in GSE110113 dataset. Major histocompatibility complex, class II, DP alpha 1 (HLA-DPA1) was finally screened out as a hub gene associated with poor outcome of MM related to hypoxia. In addition, survival analyses and gene expression level were visualized with online clinical data, and the results validated higher HLA-DPA1expression level of MM patients was associated with poor clinical outcome. The findings in this study provide new insights on HLA-DPA1 as a potential biomarker for MM and more research needs to be performed.

In this study, we analyzed 1285 DEGs between normoxic and hypoxic cultured MM cells based on GSE110113 dataset. Enrichment analysis indicated that adaptive immune response was the most significant GO term and herpes simplex virus 1 infection pathway was the most significant KEEG pathway. It is well-known that human immune system can eradicate cancer cells. Cancers’ occurrence and development is critically associated with immune response adaptation and immune escape which have been demonstrated with mice model [22, 23]. Herpes simplex virus (HSV) 1 has antitumor effect which mainly depends on its cytotoxic effect and replication ability with tumor in order to produce more virus for tumor lysis [24]. Previous study indicated HSV was associated with occurrence of MM and Bortezomib could inhibit HSV infection by halting viral capsid transport to the nucleus [25].

Establishment of the PPI network and further analysis with Cytoscape plug-ins cytoHubba identified 3 candidate hub genes, HLA-DPA1, HLA-DQA1 and HLA-DQB1. The major histocompatibility complex (MHC) class II proteins include HLA-DR, HLA-DQ and HLA-DP classical proteins and they only expressed on professional antigen-presenting cells (B lymphocytes, dendritic cells and macrophages) to activate CD4+ T cells [26]. They could participate in cancer development as it has been proved that dysregulation of immune function which involved antigen presentation was associated with cancer [27]. Subsequently, survival analysis based on GSE2658 dataset with PrognoScan revealed HLA-DPA1 as the hub gene associated with DSS of MM patients. Since GSE2658 dataset did not provided detail clinical data of patients’ general condition, multivariate Cox’s proportional hazard regression models could not be constructed to further clarify the relationship between HLA-DPA1 and survival. According to ScanGEO analysis results, gene expression of HLA-DPA1 was significantly lower compared to HDs and MGUS.

Hypoxia is common and essential in various cancers which can bring different gene expression change during metabolic adaptations [28]. As a result, cancer cells can survival and keep high rate proliferation. Previous studies have shown hypoxic bone marrow microenvironment plays a critical role in MM occurrence and progression through different aspects. For instance, endothelial cells (ECs) in MM patients having a hypoxic phenotype could keep up with enhanced angiogenesis in cancer growth and metastasis [8]. Hypoxia induced MM cells dedifferentiation, stem-cell like state acquisition without apoptosis and enhanced drug resistance to proteasome inhibitors [9].

In the GO enrichment analysis, cell adhesion molecule binding was the most significant term. Evidences suggested cell adhesion molecule binding is an important pathway in MM related to hypoxia. Hypoxia reduces the adhesion of tumor cells and accelerates tumor development process [7, 29], manifested as extramedullary (EMD). Central nervous system (CNS) involvement phenotype is an rare, EMD form of MM which indicates unfavorable cytogenetics, shorter survival time even with intensive treatment [30]. Capicua transcriptional repressor (CIC) is a transducer of receptor tyrosine kinase (RTK) signaling that functions through default repression [31]. Marra MA et al. found that CIC deficiency was associated with down-regulated expression of genes involving in cell-cell adhesion which led to tumor progression and over-expression mitogen-activated protein kinase (MAPK) signaling cascade [32]. Another research proved CIC mutation affected the BRAF-RAS pathway and resulted in drug resistance in MM patients [33]. Other several mutations including Ki-ras2 Kirsten rat sarcoma viral oncogene homolog (KRAS), Neuroblastoma Ras viral oncogene homolog (NRAS) also participate in drug resistance of MM [34, 35]. In our study, HLA-DPA1 is also down-regulated under hypoxic condition and we hypothesize that it may play an oncogenic role in MM through hypoxic activated signaling pathway.

HLA-DPA1, also known as HLA-DP1A, HLASB or DPA1, belongs to the HLA class II alpha chain paralogues [36]. As a result, HLA-DPA1 function as an MHC class II receptor to participate in immune response and antigenic peptides presentation. Clinical study on adrenocortical tumors (ACT) indicated low expression of HLA-DPA1 was associated with poor prognosis [37]. Acute myeloid leukemia (AML) relapse after transplantation was analyzed by Christopher MJ et al. It was proved to be associated with dysregulation of pathways which had an influence on immune function. HLA-DPA1 and several other MHC class II genes’ down-regulation were involved as they function in antigen presentation [38]. Other several researches showed MHC class II genes had crucial relationship with cancer immunology, and down-regulation of related genes indicated a poor prognosis [26, 39, 40].

HLA-DPA1 was a hub gene related to hypoxia in MM. Down-regulated expression of HLA-DPA1 was associated with shorter survival time of MM patients. Notably, 3 candidate hub genes were all related to immune response. Based on the findings in our study, further researches investigating immune process of MM pathogenesis may help us to better understand MM. This study provided a novel insight into HLA-DPA1 as a critical potential biomarker for MM.