We used plasma pQTL data to explore potential therapeutic targets for LUAD using a two-sample MR method, identifying nine proteins (ALAD, FLT1, ICAM5, MDGA2, NTM, PMM2, RNASET2, TFPI and VWC2). Bayesian co-localization indicated that three of them (RNASET2, TFPI, and VWC2) exhibited genetic colocalization evidence with LUAD. Our findings revealed that smoking, maternal smoking around birth, parental history of lung cancer, alcohol consumption, leisure activities, and COPD were causally associated with the risk of LUAD, signifying their significant role in LUAD pathogenesis and aligning with classical epidemiological studies[3]. Furthermore, it is likely that the associations of TFPI and VWC2 with LUAD are mediated by one or more of these risk factors. Finally, a list of two protein-targeted drugs in development and three druggable proteins supported the integration of genomics and proteomics into drug development initiatives. Taken together, these findings illustrate the effectiveness of proteomics analysis in identifying potential therapeutic targets with causal implications for LUAD.
TFPI is a serine protease that inhibits prothrombin clearence via the Tissue Factor (TF) pathway[29]. TF can promote tumor angiogenesis and metastasis, playing a pivotal role in cancer progression[30]. The role of TFPI in cancer remains controversial. Initially, TFPI has been shown as a tumor suppressor. Specifically, TFPI silencing in breast cancer cells resulted in invasive tumor growth, while its overexpression induced apoptosis[31]. An in vivo study in mice revealed an increased metastatic potential in the absence of TFPI[32]. Consistent with our findings, some evidence suggests that TFPI might contribute to tumor progression. Increased TFPI mRNA and protein expression were observed in many invasive tumors[33]. Arnason et al. found that TFPI facilitated the development of multiple drug resistance (MDR) in cancer cells[34]. Phenoscanner revealed associations between TFPI SNP(rs116350534) and LUAD[14]. We also found that higher levels of TFPI were associated with a smoking history. Research has shown that TFPI could differentiate LUAD form high-risk smokers[35]. It suggestes that the pathway through which TFPI influences the risk of LUAD may be related to smoking. In addition, TFPI is currently undergoing clinical trials for hematological malignancies such as hemophilia.
RNASET2 belongs to the T2 family of extracellular ribonucleases and has been linked to anti-tumor activities. Its overexpression inhibits the clonogenicity of ovarian cancer cells in vitro and suppresses tumorigenesis and metastasis in vivo[36]. However, our results indicated that RNASET2 played a role in promoting LUAD carcinogenesis. McKay et al. conducted a GWAS meta-analysis of 29,266 patients and 56,450 controls and found that increased RNASET2 expression was correlated with an increased lung cancer risk[14]. Decreased expression of RNASET2 was associated with the protective allele of rs444210, indicating that RNASET2 promotes lung cancer carcinogenesis[37]. The association between circulating RNASET2 levels and the risk of LUAD remains unclear due to controversial results in previous studies. In our research, we validated RNASET2 using a co-localization method, indicating a higher likelihood of RANSET2 being a causal protein for LUAD. Future investigations are required to examine the causality of these relationships.
VWC2 (Brorin) is a glycoprotein belonging to the Chordin family of secreted BMP regulators. Its expression is reduced in colorectal cancer and shows a negative correlation with tumor stage and the expression of tumor biomarkers. VWC2 inhibits tumor cell growth both in vitro and in vivo[38]. Currently, there have been no studies on the association between VWC2 and LUAD. In our research, we established a link between VWC2 and a reduced risk of LUAD, confirming a causal effect through co-localization analysis. Additionally, we observed that higher levels of VWC2 were linked to leisure activities, suggesting that the protective effect of VWC2 on LUAD can be strengthen through leisure activities.
Tumor angiogenesis is a significant hallmark of cancer, and vascular endothelial growth factors (VEGFs) and their receptors play key roles in pathological angiogenesis in various tumors[39]. FLT1 is expressed on both endothelial and epithelial cells and binds with its ligands, namely VEGF-A, VEGF-B, and placental growth factor (PGF)[40]. The regulatory mechanism of FLT1 in tumor cells is unclear. FLT1 is upregulated in colorectal cancer tissue, and its interaction with ligands promotes epithelial-mesenchymal transition (EMT), consequently enhancing invasiveness[41]. FLT1 is crucial for the proliferation, invasion, and metastasis of LUAD cells, with its expression negatively correlated with survival time and recurrence-free survival rate in LUAD[42]. However, Dylan et al. discovered that the variant allele (C) of the FLT1 SNP rs9582036 leads to reduced FLT1 expression, thereby accelerating NSCLC recurrence through enhanced angiogenesis. One of the mechanisms is that soluble VEGFR-1 dominantly inhibits VEGFA by forming heterodimers with VEGFR-2, the primary receptor responsible for most of the proangiogenic effects of VEGFA[43]. Our results align with this, as the FLT1 protein variant offers protection against LUAD. Furthermore, our PPI analysis revealed that FLT1 interacts with EGFR, KRAS, ERBB2, and CD274 (PD-L1), all of which are targeted by medications involving tyrosine kinase inhibitors and monoclonal antibodies. Consequently, FLT1 may represent a promising novel target for these anti-tumor drugs. Moreover, therapeutic agent targeting FLT1 have been well developed and evaluated in phase II clinical trials.
In addition to the proteins mentioned earlier, our findings suggest that NTM have potential for both pharmacological and clinical applications. NTM is a glycoprotein belonging to the immunoglobulin superfamily and the IgLON family. Lower expression of the NTM gene was observed in LUAD compared to controls[44], a finding confirmed by public datasets, including Oncomine and TCGA. However, our results indicate that NTM is associated with an increased risk of LUAD. PPI analysis has shown that NTM interacts with NTRK2, which are the targets of Larotrectinib and Entrectinib. This suggests that NTM may exert its effects on NTRK to influence LUAD.
This study has several limitations. Firstly, LUAD GWAS data lack stratification based on specific subtypes, preventing stratified analyses in the current study. This presents a potential avenue for future research, to be explored once specific datasets become available. Secondly, when interpreting the posterior probability of hypothesis 4 (PPH4) in colocalization, caution is needed. A low PPH4 may not necessarily indicate the absence of evidence supporting colocalization when PPH3 is also low due to limited statistical power[45]. Thirdly, despite conducting several sensitivity analyses in our MR study, the complete elimination of horizontal pleiotropy and confounding bias remains challenging. Fourth, since both the pQTLs and GWAS data used in this research primarily originated from individuals of European ancestry, it is important to exercise caution when generalizing the applicability of our findings to other populations. Fifth, our study revealed limited genetic prediction of TFPI and VWC2 mediated by smoking and leisure activities. Therefore, additional research is required to quantify the influence of other potential mediators.
In conclusion, our integrated analysis suggests a causal link between genetically determined plasma protein levels and LUAD risk. These identified proteins, especially RNASET2, TFPI, VWC2, FLT1, and NTM, hold promise as potential therapeutic targets for LUAD. Further research is necessary to elucidate the mechanisms of these candidate proteins in LUAD.