Gene expression profiling of skin and blood in hidradenitis suppurativa

Dear Editor, Hidradenitis suppurativa (HS) is characterized by recurrent abscesses, nodules and sinus tract formation and is localized mainly in the body folds. The pathogenesis is poorly understood. Mutations in genes encoding essential compounds of the transmembrane protease γ‐secretase, including NCSTN, PSEN1 and PSENEN, have been identified in familial HS.1 2 These mutations may result in impaired Notch signalling, promoting cyst formation and continuing inflammatory activity.3 Additionally, certain single‐nucleotide polymorphisms at the promoter region of the tumour necrosis factor (TNF)‐α gene were found to be associated with HS.4 Furthermore, elevated levels of interleukin (IL)‐1, TNF‐α and IL‐10 were demonstrated in both lesional and perilesional skin of patients with HS,5 and increased expression of the IL‐23/T helper 17 pathway was found in lesional skin.6 A recent study demonstrated that the production of proinflammatory cytokines in HS is compartmentalized, as the levels were elevated in pus collected from lesional skin but suppressed in circulating blood mononuclear cells.7 To acquire a better understanding of the molecular pathogenesis of HS, we performed mRNA microarray studies to compare gene expression in lesional skin vs. healthy skin of patients with HS. Also, the gene expression profiles in whole blood of patients and unaffected individuals were determined. The study was approved by the university medical ethics committee. Seventeen patients with HS were included. Whole blood was collected from all patients and 10 healthy controls. Skin biopsies of 3 mm were obtained from inflammatory lesions in all 17 subjects. Additionally, an extra biopsy from clinically healthy skin of the upper arm or leg was obtained from 13 of these 17 individuals. The skin samples were immediately frozen in liquid nitrogen and subsequently stored at −80 °C. mRNA was extracted from skin samples using the Qiagen RNeasy Fibrous Tissue Mini Kit (Qiagen Inc., Valencia, CA, U.S.A.) and from whole blood with the Qiagen PAXgene Blood RNA Kit according to the manufacturer's instructions. RNA was hybridized to the GeneChip HT HG‐U133+PM Array (Affymetrix, Santa Clara, CA, U.S.A.). The raw data have been deposited in the National Center for Biotechnology Information's Gene Expression Omnibus under accession number GSE72702. Pathway analyses were conducted with Qiagen's Ingenuity Pathway Analysis. Array Studio software version 8.0 (OmicSoft Corp., St Morrisville, NC, U.S.A.) was used to analyse microarray data. Dysregulated genes were identified using a general linear model with multiple testing correction. Expression modulation with a fold change > 2 or < −2 and a false‐discovery‐rate‐adjusted P‐value < 0·05 were considered significant. A significant difference was observed in mRNA expression between lesional and clinically healthy skin of patients with HS, with 1145 probe sets representing over 800 genes having at least a twofold change (P < 0·05) (Table S1; see Supporting Information). Patients with the most dysregulated gene profiles were more likely to have long‐standing disease (> 15 years) (Fig. 1a). Pathway analyses of the modulated genes showed that they were related mostly to inflammation, including cell adhesion, diapedesis and extravasation, as well as immune cell signalling and communication pathways (Fig. 1b; Table S2; see Supporting Information). An interesting finding is involvement of the atherosclerosis signalling pathway in lesional HS skin, as it supports the current thought that the inflammatory response in HS is associated with metabolic syndrome.8 Gene expression profile in hidradenitis suppurativa (HS) skin. (a) The heat map of differentially expressed genes in HS skin based on mRNA microarray analysis. A significant difference was observed in mRNA expression between lesional (grey boxes) and nonlesional patient skin (red boxes). Patients with the most dysregulated gene profiles were more likely to have long‐standing disease. (b) The top 10 canonical pathways involved in the disease profile of patient skin. Expression of NCSTN, PSEN1 and PSENEN was not modulated in lesional vs. clinically healthy skin of patients. No significant differences were identified in whole‐blood mRNA expression between patients (n = 17) and healthy controls (n = 10) (Fig. S1; see Supporting Information). Nonetheless, the top 50 probe set that were differentially expressed based on raw P‐value are provided in Table S3 (see Supporting Information). The lack of differences between patients with HS and healthy controls in whole blood supports the hypothesis of Kanni et al.,7 proposing that decreased cytokine production in the blood circulation of patients with HS is regulated at a post‐transcriptional level. The authors suggest that elevated TNF‐α levels in serum of patients with HS, as previously demonstrated by Matusiak et al.,9 may have a role in this decreased production, as the number of mononuclear cells was shown to be equal to or even greater than the number in healthy controls. In summary, we have demonstrated significantly altered gene expression in lesional skin compared with clinically healthy skin of patients with HS. This, in addition to the finding that whole‐blood RNA expression did not differ between patients with HS and healthy subjects, indicates that activated cells in HS reside in affected tissue. This may be due to activation and migration of leucocytes from the circulation into skin tissue in response to released inflammatory chemokines. However, our results must be interpreted with caution, as the sample size was relatively small. Additionally, reverse‐transcriptase polymerase chain reaction techniques or immunohistochemical analyses were not performed to confirm our findings. As perilesional skin of patients with HS already shows histological abnormalities,10 one may hypothesize that the skin type of patients with HS in general is genetically different from that of normal human skin, making patients prone to the development of HS lesions under certain mechanical and lifestyle triggers. Therefore, investigating differences in gene expression between clinically healthy skin from predilection sites of patients with HS and skin of unaffected individuals from the same sites would be an interesting additional study. Funding sources: this study was supported by Janssen Cilag Inc. (the Netherlands). Conflicts of interest: C.B. and K.L. are both employees at Janssen Research and Development. Fig S1. Principle component analysis showing (a) separation between patients with hidradenitis suppurativa (HS) with lesional vs. nonlesional skin; and (b) lack of expression modulation between patients with HS and healthy controls in whole blood. Table S1. The P‐values of the implicated genes in hidradenitis suppurativa lesional and nonlesional skin. Table S2. The top 10 canonical pathways in the hidradenitis suppurativa skin disease profile. The 1145 probe sets were significantly (false discovery rate P‐value < 0·05 and fold change > 2) dysregulated between lesional (n = 17) and nonlesional skin (n = 13). Table S3. The top 50 probe sets differentially expressed in whole‐blood mRNA based on the raw P‐values.