TO THE EDITOR
Prurigo nodularis (PN) is a chronic inflammatory skin disease characterized by intensely pruritic, hyperkeratotic nodules on the trunk and bilateral extremities (Huang et al., 2020). PN’s strong association with several systemic comorbidities suggests that its pathogenesis involves dysregulation of both cutaneous and systemic inflammatory processes (Wongvibulsin et al., 2021a). This is supported by a recent study showing increased γδT cells with Vδ2+ γδT–enrichment in circulating blood from patients with PN and activated T cells uniquely expressing IL-22 (Belzberg et al., 2021). Cutaneous transcriptomics also revealed T helper (Th) 22 and Th17 immune signatures in PN lesions. The Th2 axis also appears to play a role as activated downstream mediators have been observed in the epidermis (Fukushi et al., 2011). Upregulation of these pathways is particularly interesting because it highlights shared cutaneous gene signatures between PN, psoriasis, and atopic dermatitis (AD), although PN had systemic Th22 but not Th17 immune polarization (Wongvibulsin et al., 2021b). Therefore, we compared the cutaneous lesional and nonlesional skin transcriptomes of patients with PN, psoriasis, and AD to uncover novel pathophysiological pathways specific to PN.
Under an Institutional Review Board–approved study, lesional and nonlesional skin biopsies were previously collected from 13 patients with PN without an underlying history of atopy or psoriasis (Belzberg et al., 2021). All patients provided written informed consent. RNA was extracted from biopsies, RNA sequencing libraries were constructed, and sequencing was performed using Illumina NovaSeq 6000 (Illumina, San Diego, CA). Complete methods for patient enrollment and RNA sequencing are available in our previous study (Belzberg et al., 2021). Psoriasis transcriptomes were obtained from the Gene Expression Omnibus (Bethesda, MD) database (GSE117468), which contained lesional and nonlesional gene expression profiles of patients from a phase 3 clinical trial of brodalumab (Tomalin et al., 2020). AD transcriptomes were also obtained from Gene Expression Omnibus (GSE137430), which contained lesional and nonlesional gene expression profiles of patients from a phase 2 clinical trial of secukinumab (Ungar et al., 2021). For both psoriasis and AD, only baseline untreated samples were used. To limit the effects of different sample sizes, only 13 lesional and nonlesional samples of psoriasis and AD were used. Psoriasis and AD samples were selected to resemble the demographic profile of the PN cohort as closely as possible (Table 1). Lesional PN, psoriasis, and AD samples were compared with only their respective, matched nonlesional samples to control batch effects from different experiments. Differentially expressed genes (DEGs) were calculated using the DESeq2 package for R 4.0.5 (Vienna, Austria). Shared DEGs were genes that were differentially expressed in both PN and psoriasis with similar directional change. Gene Ontology (GO) enrichment analysis for 20,506 GO categories was performed using the GOseq package for R. On the basis of top GO categories and existing literature on PN pathogenesis, specific fibroproliferative and neural pathways were explored using gene set variation analysis (GSVA). GSVA enrichment scores were compared using the limma package for R. P-values for GO and GSVA were adjusted with the Benjamini–Hochberg method.
Table 1.
Demographic Characteristics of Patient Cohorts
| Characteristics | Prurigo Nodularis | Psoriasis | Atopic Dermatitis |
|---|---|---|---|
| Age (y), mean ± SD | 54.7 ± 14.2 | 44.7 ± 12.6 | 40.2 ± 15.6 |
| Sex | |||
| Male, % (n) | 15 (2) | 31 (4) | 31 (4) |
| Female, % (n) | 85 (9) | 69 (9) | 69 (9) |
| Race | |||
| Caucasian, % (n) | 23 (3) | 100 (13) | 62 (8) |
| African American, % (n) | 77 (10) | 0 (0) | 38 (5) |
A total of 26 PN (13 lesional and 13 nonlesional), 26 psoriasis (13 lesional and 13 nonlesional), and 26 AD (13 lesional and 13 nonlesional) samples were analyzed. PN had the most unique DEGs followed by psoriasis and AD (Figure 1a). PN and psoriasis shared 3,775 DEGs, PN and AD shared 1,551 DEGs, psoriasis and AD shared 1,565 DEGs, and all three dermatoses shared 1,082 DEGs. PN and psoriasis shared the most Th17/22-related DEGs, including CXCL1, DEFB4A, LCN2, PI3, IL8, S100A7/8/9/12, and SERPINB1/4. PN and AD shared CXCL1/2, PI3, S100A7/8/9, and SERPINB1/4. All three dermatoses shared CXCL1, PI3, S1007/8/9, and SERPINB1/4. The top GO categories for each disease are shown in Figure 1b. Using GSVA, PN but not psoriasis or AD lesions showed significant upregulation of TGFβ-induced epithelial-to-mesenchymal transition (log fold change 0.44, P = 0.001), epidermal acanthosis (log fold change 0.61, P < 0.001), axon regeneration (log fold change 0.46, P = 0.011), and VEGF activity (log fold change 0.23, P = 0.014) (Figure 1c).
Figure 1. Comparison of the transcriptomic profiles of PN, Pso, and AD.
(a) Venn diagram of shared and unique DEGs for each disease. (b) Top enriched Gene Ontology categories for each disease. The dotted line represents P = 0.0025 (P = 0.05 corrected for multiplicity: 0.05/20). (c) Gene set variation analysis enrichment scores for several pathways in PN and Pso. *P < 0.05, **P < 0.01, ***P < 0.001. AD, atopic dermatitis; DEG, differentially expressed gene; down, downregulated; EMT, epithelial-to-mesenchymal transition; FDR, false discovery rate; L, lesional; NL, nonlesional; PN, prurigo nodularis; Pso, psoriasis; up, upregulated.
Our results highlight several distinct features between PN, psoriasis, and AD. GO analysis also showed different patterns of top enriched categories. Although psoriasis and AD showed more predominant immunological mechanisms such as cytokine/Fc signaling and response to IFN-γ, PN showed dysregulation of pathways also related to epithelium development, nervous system development, cornification, vasculature development, and mesenchymal pathways. These patterns were reinforced by GSVA, which showed upregulation of fibroproliferative pathways in PN but not in psoriasis or AD lesional skin.
The identified pathways are concordant with the phenotypic features of PN. TGFβ-induced epithelial-to-mesenchymal transition increases keratinocyte growth and is connected to fibrosis, consistent with the hyperkeratotic and fibrotic nodules observed in PN (Huang et al., 2020; Liarte et al., 2020). Epidermal acanthosis and dermal neural hyperplasia are also features of the characteristic raised nodules of PN, and a reduction in intraepidermal nerve fiber density has been described previously in lesional and nonlesional skin (Schuhknecht et al., 2011). Increased VEGF activity has also been described in PN and may correlate with itch intensity (Krause et al., 2013). These GSVA pathways were not dysregulated in psoriasis or AD, highlighting the unique pathogenesis of PN. Because PN, psoriasis, and AD feature systemic inflammation, future studies should compare circulating blood biomarkers in these dermatoses and their correlation with disease comorbidities.
Limitations of this study include small sample sizes restricting the generalizability of our findings. Lack of information about biopsy site location for psoriasis and AD samples is a limitation of this study. Despite these limitations, we present several findings about the PN pathogenesis, including fibroproliferative, neuropathic, and angiopathic features. Further research with larger cohorts is needed to validate our findings.
ACKNOWLEDGMENTS
SGK is supported by the National Institute of Arthritis and Musculoskeletal and Skin Diseases of the National Institutes of Health under award number K23AR077073. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health. This study is approved by the Johns Hopkins Institutional Review Board (IRB00119007).
Abbreviations:
- AD
atopic dermatitis
- DEG
differentially expressed gene
- GO
Gene Ontology
- GSVA
gene set variation analysis
- PN
prurigo nodularis
- Th
T helper
Footnotes
CONFLICT OF INTEREST
SGK is an advisory board member/consultant for AbbVie, Celldex Therapeutics, Galderma, Incyte Corporation, Pfizer, Regeneron Pharmaceuticals, and Kiniksa Pharmaceuticals and has served as an investigator for Galderma, Kiniksa Pharmaceuticals, Pfizer, and Sanofi. NKA has received financial support from Pfizer. The remaining authors state no conflict of interest.
Data availability statement
Psoriasis and atopic dermatitis datasets related to this article can be found at https://www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSE117468 and https://www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSM4079244, hosted at the Gene Expression Omnibus database. The prurigo nodularis data are not publicly available owing to patient wishes but are available upon reasonable request to the corresponding author.
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Data Availability Statement
Psoriasis and atopic dermatitis datasets related to this article can be found at https://www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSE117468 and https://www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSM4079244, hosted at the Gene Expression Omnibus database. The prurigo nodularis data are not publicly available owing to patient wishes but are available upon reasonable request to the corresponding author.