In this study, we found that reproductive factors, especially AFS and AFB, were dominant risk factors for psoriasis; there were mediating effects by educational attainment and BMI. Genetically predicted early AFS led to an increased risk of overall psoriasis; 36.13% of this effect was mediated through BMI and 47.79% through educational attainment. The direct negative casual association between AFB-PsA was dominant, with 49.61% proportion of the mediation due to BMI. The mediating effect was also found for BMI on the AFS-PV relationship, which accounted for 26.27% of the proportion. Consistent null associations were identified via sensitivity and multivariable MR analyses, demonstrating the robustness of our findings.
This is the first application of an MR mediation analysis to study the mediators of the relationship between reproductive factors and psoriasis risk. In this study, we corroborate the genetic correlation between reproductive factors shown in previous studies, while showing additional correlations that were not previously investigated. For example, one study showed that women with psoriasis were younger at the time of first delivery and had greater mean durations between the first and last birth and a greater mean interpregnancy interval[18]. Another study investigated reproductive factors related to sexual history among heterosexual women, finding no significant differences in the age of the first sexual encounter between those with and without psoriasis (weighted difference -0.54 years, 95% CI -1.27 to 0.19)[47]. Several factors could underlie these discrepancies.
Firstly, reproductive factors are complex and heterogeneous traits influenced by both genetic and environmental factors. Genetics alone cannot fully capture the phenotypic variance of these traits. For instance, AFS is considered a human behavioral trait influenced by genetics, psychosocial, cultural, and financial factors. Therefore, traditional observational designs' results are susceptible to the influence of complex confounding variables that are challenging to capture and model accurately. To address this issue, we utilized MVMR to control for adiposity, education, smoking, and alcohol intake effects. The negative results obtained corroborated our findings that AMA, AMP, ALB, NEB, and NSP were not significantly associated with psoriasis. It is also likely that the true causal effects of these reproductive factors on psoriasis were modest; as such, our study may have been underpowered to identify them. Notably, the association between AFS and PV became nonsignificant in the merged MVMR model. This may stem from an interaction between all mediators in the merged MVMR model; the effects of any single mediator could not eliminate the causal association between AFS and PV in the separate MVMR models.
The biological underlying association between hormonal factors and psoriasis development is not yet fully understood. Sex hormones have been shown to have an impact on the immune system and their interaction with environmental and genetic factors may partly explain the higher prevalence of psoriasis in women. Estrogen, in particular, is a complex modulator of the immune system with both stimulatory and inhibitory effects. For instance, estrogen levels during periovulatory to pregnancy periods may stimulate B cells and Th2 response, while also supporting the survival of auto-reactive T and B cell clones. Moreover, estrogens can suppress cell-mediated responses like Th17 cell differentiation[48-50]. Despite these findings, further research is needed to determine the exact relationship between modifiable reproductive factors and the incidence of psoriasis, as prospective studies on this topic remain limited and their findings are controversial.
Regarding the relationship of early AFS and AFB with increased risk of psoriasis, and educational attainment and BMI played a mediating role in the association, the following are some possible underlying mechanistic analyses: Firstly, mothers who give birth before the age of 20 tend to have a lower socio-economic status. Early motherhood often results in a heavier burden of childbearing, premature childbearing age, and health-related behaviors. Early childbearing may have an impact on a woman's lifestyle and health behaviors[51,52]. For example, women who have children early may have less time and resources for higher education, which can lead to poor health literacy and reduced ability to prevent and manage chronic diseases such as psoriasis[53,54]. Secondly, lower educational attainment is often associated with poorer health cognition and behavior. People with less education may lack adequate health knowledge to effectively manage and prevent chronic diseases[55,56]. This may affect psoriasis risk in several ways: on the one hand, people with higher levels of education are generally better equipped to understand health and adopt healthy behaviors, such as eating a reasonable diet, quitting smoking, limiting alcohol, and maintaining a healthy weight[57]. For another, those with higher levels of education are generally better able to use medical resources, have regular medical check-ups, and detect and treat diseases early[58,59].
Our results revealed that BMI may also played an important role in the association between early reproductive age and psoriasis risk. Women who have children early may be more likely to be overweight or obese for a variety of reasons, such as unbalanced nutrition, lack of exercise, psychological stress, and so on. Women who become pregnant at an early age may experience greater difficulty returning to their pre-pregnancy weight[60]. Obesity can lead to chronic low-grade inflammation in the body, and psoriasis is an inflammatory disease. Obesity may induce or aggravate psoriasis by increasing the level of systemic inflammation[61,62]. High BMI is associated with metabolic syndromes (such as high blood pressure, diabetes, and dyslipidemia), and as such, a tendency for these metabolic disorders may progress to increase the risk of psoriasis[63].
In addition, those with younger AFB and AFS were also more likely to have had unintended pregnancies and may face more psychological and socioeconomic stresses that may increase the risk of psoriasis through a variety of mechanisms. Long-term psychological stress can affect immune system function through neuroendocrine pathways, inducing or aggravating psoriasis[64,65]. Besides, early childbearing can lead to increased financial burdens and weak social support systems, which can also increase psychological stress and affect health[66,67] .
The association between early reproductive age and increased risk of psoriasis is the result of a combination of factors. Educational attainment and BMI play mediating roles, and these factors might work together through influencing health behaviors, medical resource utilization, inflammation levels, and psychological stress. For the potential avenues for further investigation, progressive research can test these hypotheses through longitudinal data and multivariate analysis, and gain insight into their specific mechanisms to develop more effective prevention and intervention strategies.
To the best of our knowledge, no MR has been conducted to investigate the association between reproductive factors and psoriasis prior to this study. Our research possesses several notable strengths. We incorporated seven different but complementary reproductive traits. In the context of mediation, MR provided further robustness to non-differential measurement error in the mediator. However, we recognize some limitations. First, the available GWAS databases provide summary statistics that were limited to individuals of European descent. Second, MVMR is a powerful tool for investigating the causal relationships between multiple factors and an outcome of interest and has the potential to generate important insights into the underlying mechanisms of complex diseases, however, it has some unresolved limitations. As with any observational study, the results of a multivariable MR should be interpreted with caution, as residual confounding or unmeasured variables may still exist. For instance, we detected an abnormal result of the MVMR for AFS and PV. One potential explanation for such results is that the different mediators are related to each other in a way that is not fully captured by the MVMR model and this residual correlation is causing confounding or masking the causal effect of an individual mediator. Another possibility is that the effects of the different mediators interact with each other in complex ways, such that the net effect of multiple mediators is different from the effects of each mediator alone. In addition, there is a potential weak instrumental bias in the causal estimates generated by the MVMR, and such analyses can potentially be affected by some indeterminate pleiotropy via pathways that are not captured by the included variables. Larger and higher-quality datasets are needed to mitigate such potential weak instrumental bias and verify our findings from the MVMR. Besides that, despite using multiple methods to verify and mitigate these biases, genetic confounding could still be a potential problem. Given the complex etiology of psoriasis and the limitations of currently available datasets, it is not feasible to account for all potential mediators to eliminate the bias entirely. Therefore, the interpretation of these results should be approached with caution. Finally, the statistical power was suboptimal (< 80%), due to the relatively low overall incidence of psoriasis in the FinnGen population, and thus we urge caution in interpreting the negative results of our analysis.