HCC is one of the most fatal cancers in the world which occurrence and development are a multi-gene, multi-step and multi-stage process[3, 4]. With the development of molecular biology, more and more biomarkers have been found. However, the prognosis of HCC is still poor due to the lack of effective biomarkers to predict early-stage HCC[5, 6]. Therefore, it is of great significance to find a specific and sensitive tumor marker to assist in the diagnosis and treatment of early-stage HCC. This study is the first time to propose that CYSTM1 could be used as a biomarker for the prognosis of HCC, and the overexpression of CYSTM1 is significantly related to the clinicopathological characteristics and prognosis of HCC patients. In addition, multivariate Cox proportional hazard regression model showed that CYSTM1 was an independent risk factor for survival of HCC patients.
Nearly 10 years ago, CYSTM1 was first proposed and characterized, when it was proved to be a cysteine-rich transmembrane module. It is noteworthy that CYSTM1 is also expressed and located on the cell membrane of human tissues. In the past researches, CYSTM1 has only a few reports in some eukaryotes, and it has been proved that CYSTM family proteins play an important role in resistance to drug, resistance to metal ions, and resistance to viruses[8, 9]. These functions may be related to cysteines, the acid residues and the cytoplasmic polar disordered head on CYSTM1, and these structures are highly conserved in different species. The C-terminal transmembrane helix of CYSTM1 contains 5–6 cysteines, among which 3–4 continuous cysteines constitute the cysteine patch. This may change the redox potential or radical quenching of the membrane, thus playing an antioxidant role. CDT1, a member of CYSTM1 subfamily which rich in cysteine polypeptide, is heterologously expressed in yeast to prevent cadmium from entering cells. Vallee and Margoshe reported for the first time that metallothioneins (MTs) are cadmium binding proteins in horse kidney cortex, and it contained a high proportion of cysteines. These results suggest that CYSTM1 may act as a metallothionein-like protein in cell membrane.
Through KEGG enrichment analysis, we found that CYSTM1 co-expression genes are mainly concentrated in pathway of neurodegeneration - multiple diseases, especially huntington's disease, which is consistent with the results of transcriptome analysis by Mastrokolias using next-generation sequencing to predict biomarkers of huntington's disease. Copper could increase the aggregation of poly-glutamine (polyQ) in vivo and in vitro, but MTs could protect huntington model cells from the toxic effects of polyQ. Then through GO enrichment analysis, it is concluded that CYSTM1 co-expressed genes are mainly enriched in unfolded protein binding and NADH dehydrogenase activity. The former function is related to the results of KEGG enrichment analysis, because neuronal cells are highly sensitive to unfolded protein. Long-term accumulation of unfolded protein will cause endoplasmic reticulum stress, which may lead to cell apoptosis and necrosis if stress exists permanently. The function of the latter may be related to the resistance to cellular oxidative stress. The tumor microenvironment also contains a large amount of reactive oxygen species (ROS). These ROS could be produced by tumor-related fibroblasts, inflammatory cells, vascular endothelial cells, hypoxic internal environment through a variety of ways. At the same time, the effect of ROS on tumor is bidirectional. On the one hand, it could promote tumor growth and progression by stimulating tumor cells migration and invasion. On the other hand, high levels of ROS could cause cell apoptosis or necrosis, which is detrimental to the progress of tumor. However, MTs could protect DNA from damage by exchanging various toxic metal ions and oxygen free radicals. Therefore, the up-regulation of CYSTM1 expression may be to protect tumor cells from apoptosis by resisting high ROS levels. Of course, these results need a large number of scientific experiments to verify, and it is also one of our follow-up research topics.