The development of SHPT which is a common consequence in CKD patients is influenced by a variety of factors. The elevated concentration of extracellular phosphate and decreased of ionized calcium, a result of low calcitriol levels and decreased kidney function, leads to SHPT which is characterized by elevated PTH secretion and parathyroid gland hyperplasia. Meanwhile, in response to systemic hypocalcemia, PTH production increases mediated by the CaR. It has been found that when the expression of CaR and VDR was assessed, the presence of both receptors was significantly reduced in parathyroid glands from patients with SHPT compared to normal individuals[15]. The down-regulation of CaR and VDR is associated with parathyroid cell proliferation, which may limit the ability of calcium and calcitriol to regulate PTH secretion as SHPT progresses[16–18]. In addition, the disturbance of fibroblast growth factor 23 (FGF23)-Klotho axis also affects PTH synthesis[19]. A correlation between high levels of FGF-23 and increased mortality in dialysis patients and refractory SHPT patients has also been reported[20].
At present, the mechanism of cell proliferation in SHPT is not completely clear. We have used the RNA-Seq technique to analyze the differentially expressed genes in parathyroid gland tissue of the patients who underwent parathyroidectomy. In our study, we constructed a 4-mRNA signature considering U2AF1L5, LTBP2, RGN and RAP1GAP2. The signature could divide secondary hyperparathyroidism patients into high-risk and low-risk groups with different prognoses and to the best of our knowledge, no studies evaluating above genes as biomarkers for SHPT have been reported.
LTBP2, an extracellular matrix (ECM) protein with a molecular weight of 195–240 kDa, is part of the Latent TGFβ1-Binding Protein family which have been shown to participate in the regulation of TGFβ signaling. But unlike LTBP1, LTBP3, and LTBP4, the function of LTBP2 has not been fully clarified. Several studies suggested that LTBP2 which had an indirect role in the storage and activation of TGF-β may be associated with fibrosis or tissue remodeling in several organs, including the skin, heart, and kidneys[21–23]. The results from Park S, et al. supported the possibility of LTBP2 being used as a marker for myocardial fibrosis[24]. In addition, Toshihide et al.[25] reported that LTBP2 can be used to evaluate acute-phase and ongoing fibrogenesis and they also found the upregulation of LTBP2 induced by TGF-β1 at the mRNA and protein levels in human lung myofibroblasts was consistent with a previous study which suggested that Ras or another related small GTP-binding protein in the signaling pathway might be involved[26].
Regucalcin (RGN) otherwise known as the senescence marker protein 30 (SMP30) is a calcium-binding protein which preferentially expressed in cytosol of hepatocytes and renal tubular epithelia and plays a crucial role in protective effects against apoptosis and oxidative stress. In some previous studies, the anti-RGN antibody level was considered a biomarker for diagnosing hepatocellular carcinoma (HCC) especially for the patients with AFP negative while the expression of RGN decreases in HCC-infected tissues but increases in adjacent cancerous tissues[27, 28]. RGN, functioning as a tumor suppressor, could protect cells from various injuries by enhancement of membrane calcium-pump activity thus rescue effect from cell death caused by calcium influx and effectively inhibit cell proliferation by suppressing DNA and RNA synthesis in liver proliferative cells[29–31].
Rap1 is a guanine nucleotide binding protein that regulates integrin function and plays a key role in adhesion of many cell types[32–34]. Rap1GAP2, the name of which means RAP1 GTPase activating protein 2, is a modular protein encoding a GTPase-activating protein that activates the small guanine-nucleotide-binding protein Rap1 in platelets and regulates secretion of dense granules from platelets at sites of endothelial damage. Smolenski et al. demonstrated that Rap1GAP2, or rather at least part of this C-terminal region, is involved in protein-protein interactions. Then found that Rap1GAP2 interacts with Slp1 and the binding of them increases the release of dense granule[35]. The 14-3-3 proteins also interact with phosphorylated serine 9 at the N terminus of Rap1GAP2, the progress of which is enhanced by ADP and thrombin and disrupted by endothelium-derived factors nitric oxide and prostacyclin[36].
We then compared the serum concentrations of the four differentially identified genes between two groups of patients with secondary hyperparathyroidism and controls. The serum U2AF1L5 concentrations in patients with SHPT were significantly higher than those in healthy controls. Few studies related to the gene of U2AF1L5 have been reported. Gao et al. reported that U2AF1L5 was one of the 4-mRNA signature which could predict the prognosis of nasopharyngeal carcinoma patients and furthermore, the signature was closely associated with cell proliferation and the immune response[37]. U2AF1L5 was also found one of the most up-regulated 10 genes which were regarded as hearing loss related genes and enriched in pathways related to apoptosis such as tumor necrosis factor[38]. In our study, we found that the U2AF1L5 was downregulated in the parathyroid tissue samples and upregulated in the serum compared to the control group.
Our study has several limitations. Firstly, we had a relatively small number of samples available for clinical validation due to the limited availability of positive specimens. Secondly, we did not further investigate the mechanism of the genes and pathways involved. These can be future research topics.