Background: The pathogenesis of hidradenitis suppurativa (HS), with its complex inflammatory network, is still elusive. Imbalances in DNA methylation can lead to genome destabilization and have been assumed to play a role in inflammatory diseases. Global DNA methylation and hydroxymethylation have not been studied in HS yet. Objective: We conducted this study to investigate the global DNA methylation and hydroxymethylation status in lesional and perilesional HS skin compared to healthy controls. Methods: Immunohistochemical analysis was performed for 5-methylcytosine (5-mC) and 5-hydroxymethylcytosine (5-hmC) in 30 lesional and 30 corresponding healthy-appearing perilesional HS tissue samples. We included 30 healthy subjects as an interindividual control group. Results: 5-hmC levels were significantly lower in healthy-appearing perilesional (p < 0.0001) and lesional HS skin (p < 0.0001) when compared to healthy controls. There was no significant difference between lesional HS skin and perilesional HS skin regarding 5-hmC levels (p = 0.6654). In contrast to 5-hmC, 5-mC staining showed no significant changes between the 3 groups. Univariate analysis revealed no significant association between patients' characteristics, disease severity, and the levels of 5-mC and 5-hmC. Conclusion: Our findings indicate that imbalances in DNA hydroxymethylation may play a role in the pathogenesis of HS rather than DNA methylation. Further studies are warranted to investigate the significance of DNA hydroxymethylation and the regulating enzymes in HS in order to advance our knowledge of the inflammatory network in this disease.

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