Asthma reconsidered: a cumulative data perspective on airway smooth muscle

Extract

Asthma is a chronic, heterogeneous respiratory disease affecting >260 million people worldwide [1]. It is characterised by a complex interplay of airway hyperresponsiveness, inflammation and structural remodelling [2, 3]. While current pharmacological interventions can effectively mitigate inflammation and bronchoconstriction in people living with asthma, reversing or preventing airway remodelling remains a significant clinical challenge.

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A meta-analysis identifies NLRP2, Th2 signalling and ERVW-1 as key dysregulated pathways in asthma, highlighting novel targets while underscoring the need for phenotype-specific and single-cell insights https://bit.ly/4sOYUrz

Asthma is a chronic, heterogeneous respiratory disease affecting >260 million people worldwide [1]. It is characterised by a complex interplay of airway hyperresponsiveness, inflammation and structural remodelling [2, 3]. While current pharmacological interventions can effectively mitigate inflammation and bronchoconstriction in people living with asthma, reversing or preventing airway remodelling remains a significant clinical challenge. Airway smooth muscle (ASM) is central to this pathology. Long considered merely the contractile “engine” of airway narrowing, ASM is now recognised as a dynamic immunomodulator that contributes to the disease's “synthetic” phenotype by secreting cytokines and extracellular matrix components [4–7].

Despite the pivotal role of ASM, a comprehensive view of its transcriptional landscape has been elusive. Although numerous transcriptomic studies of the airway epithelium have been reported [8–10], studies focusing specifically on human ASM with disease phenotype are limited in number and often constrained by small sample sizes. In this issue of ERJ Open Research, the Roy et al. [11] present a timely important meta-analysis entitled “Airway smooth muscle in asthma: insights from a transcriptomic meta-analysis” that seeks to reconsider our understanding of ASM biology through a cumulative data perspective in the context of asthma.

The study integrates data from four publicly available RNA sequencing datasets (GSE119578, GSE119579, GSE58434 and GSE94335) comprising primary human ASM cells isolated from 26 donors with asthma and 32 healthy controls. By pooling these resources, the authors overcame the statistical power limitations associated with individual small-cohort studies. This approach allowed for the identification of a core set of 150 differentially expressed genes (DEGs) that are consistently dysregulated across independent study populations. This cumulative perspective is vital because individual studies can be moderated by variability arising from technical differences, culture conditions or patient heterogeneity. By focusing on genes with directional similarity across all datasets, the authors filtered out these inconsistencies to reveal a stable and reproducible transcriptional signature of the human asthmatic ASM.

The meta-analysis demonstrates the progression of the ASM cell towards a pro-inflammatory state (figure 1). The most striking finding is the robust upregulation of NLRP2 (Nod-like receptor pyrin domain containing 2), which emerged as the most strongly upregulated gene in asthmatic ASM sets. While the NLRP3 inflammasome is well-documented in neutrophilic asthma and severe disease [12, 13], NLRP2 has received comparatively little attention. The notable identification of NLRP2 upregulation here suggests that it may represent a distinct, ASM-specific component of the inflammatory response, potentially offering a novel therapeutic target.

FIGURE 1.

Schematic overview of key mechanisms driving asthmatic airway smooth muscle (ASM) dysfunction identified via transcriptomic meta-analysis. In asthma, ASM cells undergo phenotypic switching from a healthy contractile state to a synthetic, pro-inflammatory state. This transition is characterised by upregulation of the NLRP2 (Nod-like receptor pyrin-domain containing 2) inflammasome, the interleukin-13 (IL13)/interleukin-4 (IL4) signalling pathway (interleukin-13 receptor-α₂ (IL13RA2), interleukin-1β (IL1B) and interleukin-1α (IL1A)), and the novel activation of endogenous retroviral elements (ERVW1), along with downregulation of contractile markers such as smooth muscle myosin heavy chain 11 (MYH11) and γ₂-actin (ACTG2). This phenotypic shift leads to loss of contractility, contributes to airflow obstruction and results in impaired bronchodilator responsiveness. The figure was created using licensed version of Biorender.

Furthermore, the study reinforces the importance of type 2 inflammation. The authors report significant upregulation of the interleukin-4 (IL4) and interleukin-13 (IL13) signalling pathways. Key genes involved in this pathway, including IL13RA2, IL1B and IL1A, were upregulated. This confirms that ASM cells are not passive bystanders but active contributors in the Th2 inflammatory milieu. The upregulation of IL13RA2 is particularly notable, as IL13 is known to promote ASM proliferation and fibrosis [4, 14], directly linking inflammation induced functional changes in ASM.

Parallel to upregulation of inflammatory markers, the study provides clear evidence of a loss of contractile phenotype. The meta-analysis revealed a significant downregulation of MYH11 (smooth muscle myosin heavy chain 11) and ACTG2 (γ₂-actin). MYH11 is a definitive marker of contractile smooth muscle [15] and its reduced expression supports the hypothesis of “phenotypic switching”. This model implies how asthmatic ASM cells go from a contractile state to a proliferative, synthetic state, which can drive structural remodelling. Clinically, this loss of contractility in ASM is highly significant. It helps to explain why some patients fail standard asthma breathing tests (methacholine challenge) or do not respond to common bronchodilators, ultimately leading to false-negative diagnoses.

Perhaps the most intriguing and novel finding of this meta-analysis is the emergence of ERVW1 (endogenous retrovirus group W member 1) as a significantly upregulated gene in asthmatic ASM. Human endogenous retroviruses (HERVs) are remnants of ancient viral infections that have been integrated into the human genome over millions of years [16]. While typically silenced, the reactivation of HERV genes has been linked to several autoimmune and chronic diseases [17], yet their role in asthma has not been documented in this context. The upregulation of ERVW1 opens a fascinating new avenue for understanding environmental triggers in asthma.

While this meta-analysis provides a clearer signal amidst the noise of transcriptomic data, several significant limitations warrant careful consideration. The analysis is constrained by a relatively small sample size, integrating just four datasets with 26 asthma and 32 healthy donors. This limited “N” restricts the statistical power necessary to fully capture the vast heterogeneity of the asthmatic population.

Asthma is a highly pleiotropic disease with diverse phenotypes, ranging from mild to moderate asthma to severe, steroid-resistant or fatal asthma [3, 18]. A critical limitation of this study is the combination of data from these distinct phenotypes into a single “asthma” category. The datasets included samples from both “fatal asthma” (often associated with severe remodelling) and “mild-to-moderate asthma” (driven by specific immune responses) without distinguishing between them in the final analysis. By aggregating these subtypes, the study may obscure phenotype-specific mechanisms. For instance, the NLRP2 upregulation observed might be driven predominantly by one subtype, while the loss of MYH11 might be more pronounced in another. Future work must strive to stratify these distinct biological entities to provide truly personalised insights.

The data utilised in this meta-analysis are derived exclusively from bulk RNA sequencing. Bulk sequencing averages gene expression across millions of cells, potentially masking the heterogeneity of cellular states within the ASM itself. ASM is not a monolith; it may contain subpopulations of cells in varying states of differentiation (contractile versus synthetic) or senescence. Single-cell RNA sequencing would be required to resolve whether the observed “phenotypic switch” is a uniform change across all cells or the expansion of a specific, dysregulated subpopulation.

The authors acknowledge the inability to adjust for critical covariates such as age, sex and medication history due to limited data availability across the source studies. This is not a trivial limitation. Corticosteroids, a mainstay of asthma treatment, have profound effects on gene expression. It remains unclear whether the signatures identified, such as the downregulation of inflammatory “brakes”, are intrinsic features of the disease or secondary effects of chronic steroid exposure.

In summary, “Asthma reconsidered” through the lens of this meta-analysis validates the concept of ASM phenotypic switching, from a contractile engine to a synthetic, inflammatory driver. The identification of NLRP2 alongside established Th2 markers provides a prioritised list of candidates for future mechanistic validation. Furthermore, the unexpected emergence of ERVW1 as a significant DEG opens a novel line of inquiry into the role of endogenous viral elements in sterile airway inflammation. However, the study also highlights the complexity of asthma. As we move forward, the field must embrace the diversity of the disease. Future cumulative analyses will need to incorporate single-cell analysis and rigorous clinical stratification to fully unveil the molecular distinctiveness of mild to moderate versus severe asthma phenotypes. Only then can we develop precise brakes to halt the remodelling machinery specific to each patient's disease.