Possible two-way links between blood fat molecules and trigeminal nerve pain

TN is a chronic neurological disorder characterized by sudden, intense, brief episodes of sharp, unilateral facial pain along the trigeminal nerve distribution [1, 2]. Everyday actions like eating, talking, or even brushing teeth can trigger these painful episodes [3]. It affects about 12.6–27.0 people per 100,000 each year, with women being more likely to develop it than men (Female: Male ratio ≈ 3:2) [4]. The intense, unpredictable pain often leads to anxiety, depression, and sleep problems, greatly reducing patients' quality of life [5]. While treatments like pain medications and surgery can help manage symptoms, they do not address the root cause of TN [6]. This gap highlights the need for research into the underlying mechanisms of TN to develop better treatments.

Lipids play pivotal roles in human physiological homeostasis, with accumulating evidence demonstrating their pathophysiological associations with diverse clinical diseases, like atrial fibrillation [7], Alzheimer's disease [8], and diabetes mellitus [9]. The rapid expansion of GWAS in lipidomics [10] and TN [11] has created novel methodological paradigms for investigating potential causal relationships between lipid metabolism dysregulation and TN development.

Mendelian randomization (MR) represents an advanced genetic epidemiological method that leverages genetic variants as instrumental variables (IVs) to infer causal relationships. By leveraging the random assortment of alleles during meiosis, MR minimizes confounding biases and avoids reverse causation—limitations that frequently compromise observational studies. This methodology provides a robust alternative to randomized controlled trials (RCTs) when clinical trials are ethically or practically unfeasible [12, 13]. In the present study, we employ a bidirectional two-sample MR design to systematically evaluate the causal association between plasma lipidome and TN. Our investigation aims to not only elucidate potential pathological mechanisms but also identify novel targets for both prevention and therapeutic intervention.

Our study represents the first comprehensive investigation into the complex causal relationships between plasma lipidome and TN. The results show that diacylglycerol, triacylglycerol, PC, PE, and PI have a certain relationship with the occurrence of TN. Meanwhile, we also found that different lipids have different effects on the risk of TN, and different fatty acid compositions of the same lipids also have different effects on the risk of TN. Through rigorous sensitivity analyses employing MR-Egger regression and weighted median methods, we established the absence of substantial heterogeneity and detected no evidence of horizontal pleiotropy. Therefore, we believe that my research results are relatively reliable.

In our analysis, we primarily relied on IVW as our main analytical approach, given its established statistical power under valid IV assumptions and its widespread adoption as the gold standard in MR Analysis. The results of the sensitivity analysis showed that the lipids used for MR analysis had no heterogeneity (Q-p value > 0.05), nor was there horizontal pleiotropy (Mr-Egger's intercept method p > 0.05). While some supporting methods (MR-Egger regression, weighted median, etc.) showed nonsignificant p values (p > 0.05) in Figures 2, 4, their effect estimates consistently aligned in direction with the IVW results. Importantly, as supported by Chen et al. [20], when the IVW method yields significant results (p < 0.05) in the absence of pleiotropy and heterogeneity as confirmed by our sensitivity analyses—it provides reliable causal estimates even if other methods are nonsignificant [21, 22]. This methodological consistency, combined with the concordant directional effects across approaches, reinforces the validity of our findings.

In this study, our MR analyses revealed distinct risk profiles for TN associated with different levels of PC and PE. Given the paucity of prior evidence regarding different levels of plasma lipids effects on TN, we hypothesized that different levels of the same type of lipids may play different roles in the inflammatory process involved in TN. Notably, four levels of PC (16:0_22:5, 18:0_20:5, 18:0_22:5, and O-16:1_20:4) exhibited inverse correlations with TN risk, potentially through modulating the biosynthesis of pro-inflammatory mediators such as eicosanoids and cytokines [23, 24]. This mechanistic hypothesis aligns with converging evidence implicating neuroinflammation in both TN disease progression and etiopathogenesis [25, 26]. Phosphatidyl ethanolamine is a major phospholipid found in cell membranes, and specific stimulation can induce changes in the composition of phosphatidyl inositol, thereby regulating the phosphoinositol 3-kinase (PI3K) signaling pathway [27]. PI3K is an enzyme involved in the inositol cycle and the production of inositol triphosphate (IP3), an important second-messenger phospholipid that binds to IP3 receptors in the endoplasmic reticulum, releases intracellular calcium stores, and regulates cell proliferation and autophagy [28, 29]. Calcium surges regulated by IP3 can directly or indirectly induce apoptosis [30]. Therefore, different levels of PI may have different effects on TN. These observations warrant further investigation to elucidate the molecular mechanisms at play and explore the clinical implications of these findings.

It is worth noting that our research found that PC (18:0–18:2) had a significant correlation in both forward and reverse MR analyses. The dual fatty acid composition of PC (18:0_18:2) may underlie its pathological relevance: the saturated stearic acid (18:0) confers membrane rigidity, while the polyunsaturated linoleic acid (18:2) serves as a precursor for pro-inflammatory eicosanoids (e.g., prostaglandin E2) capable of sensitizing trigeminal nociceptors [31]. The inverse causality direction (TN on lipid alteration) may reflect chronic stress-induced activation of the hypothalamic-pituitary-adrenal (HPA) axis, which upregulates stearoyl-CoA desaturase-1 (SCD1) and subsequently modifies circulating lipid profiles [32]. These findings collectively advance our mechanistic understanding of lipid-mediated TN pathogenesis and highlight potential therapeutic targets.

Intriguingly, the randomization results of MR also pose some problems and challenges for us. In our traditional understanding, monoglycerides, diglycerides, and triglycerides increase the risk of various diseases [33]. However, our findings suggest that triglycerides may be a protective factor for TN. Like most complex diseases, the common variations found in GWAS can only explain a small part of the total heritability of the disease, while rare variations throughout the genome may also play an important role in the development of the disease. Therefore, it is necessary to combine basic research and clinical research to further clarify the causal relationship [34]. It is worth noting that diglyceride and triglyceride exhibit different structural changes. Previous studies have mainly studied diglyceride and triglyceride as a whole, without specific segmentation based on their structural forms. Therefore, further research is needed to determine which specific diglycerol and triglyceride structural forms may play a preventive role. These findings highlight the complexity of lipid levels in TN etiology and the need for further research to uncover the exact mechanisms involved.

Despite the novel insights provided, this study has several limitations. First, since the traditional p < 5 ∗ 10⁻⁸ cannot screen out enough SNPS, in order to obtain enough SNPS when screening liposomal SNPS, we adopted a more lenient threshold of 5 ∗ 10⁻⁵. The use of this threshold may have increased the likelihood of potential bias, but a loose threshold significantly improves statistical power and allows us to detect associations with small effect sizes, which is particularly important in exploratory studies, so we adopted this threshold. Secondly, the majority of participants in this study were of European descent, which limits the generalization of our findings to other populations. Additionally, this study aims to explore the causal relationship between plasma lipids and TN outcomes, with its core analysis based on preselected IVs and exposure-outcome pairs. Although multiple comparisons are involved, MR analysis places greater emphasis on evaluating the robustness of results through sensitivity methods (such as MR-Egger and weighted median) and IV assumption tests (such as heterogeneity Q values and pleiotropy intercepts), rather than relying solely on p value correction. Furthermore, we hope to identify as many potential therapeutic targets as possible, and in line with the research by Yuan et al. [35], we chose not to perform multiple testing correction.

This pioneering MR study establishes a causal nexus between plasma lipids and TN pathogenesis, revealing differential risk modulation contingent upon different levels and class of lipid. PC (16:0_22:5; 18:0_22:5; O-16:1_20:4) and PE (O-16:1_20:4) increased the risk of TN. PC (16:0_18:2; 18:0_18:2; 18:1_18; 1; 18:2_18:2), PE (O-16:1:22:5), PI (18:0_20:3), diacylglycerol (16:0_18:2, 16:1_18:1), and triacylglycerol (50:5; 51:2; 51:3; 53:2; 54:3) reduced the risk of TN. PC (18:0_18:2) showing a particularly notable bidirectional causal relationship with TN. The findings implicate lipid metabolism in TN pathogenesis and highlight potential therapeutic targets.