AbstractBackgroundHereditary palmoplantar keratodermas (hPPKs) comprise a heterogeneous group of skin disorders characterized by persistent palmoplantar hyperkeratosis. Loss‐of‐function variants in a serine peptidase inhibitor, SERPINA12, have recently been implicated in autosomal recessive diffuse hPPK. The disorder appears to share similarities with another hPPK associated with protease overactivity, namely Nagashima‐type PPK (NPPK) caused by biallelic variants in SERPINB7.ObjectivesThe aim of this study was to enhance the understanding of the clinical and genetic characteristics of serine protease‐related hPPKs caused by variants in SERPINA12 and SERPINB7.MethodsWhole‐exome sequencing (WES) was performed for hPPK patients. Haplotype analysis was completed for the patients with identified recessive SERPINA12 variants and their available family members. In addition, the current literature of SERPINA12‐ and SERPINB7‐related hPPKs was summarized.ResultsThe phenotype of SERPINA12‐related hPPK was confirmed by reporting three new SERPINA12 patients, the first of European origin. A novel SERPINA12 c.1100G>A p.(Gly367Glu) missense variant was identified confirming that the variant spectrum of SERPINA12 include both truncating and missense variants. The previously reported SERPINA12 c.631C>T p.(Arg211*) was indicated enriched in the Finnish population due to a plausible founder effect. In addition, SERPINA12 hPPK patients were shown to share a similar phenotype to patients with recessive variants in SERPINB7. The shared phenotype included diffuse transgradient PPK since birth or early childhood and frequent palmoplantar hyperhidrosis, aquagenic whitening and additional hyperkeratotic lesions in non‐palmoplantar areas. SERPINA12 and SERPINB7 hPPK patients cannot be distinguished without genetic analysis.ConclusionsRecessive variants in SERPINA12 and SERPINB7 leading to protease overactivity and hPPK produce a similar phenotype, indistinguishable without genetic analysis. SERPINA12 variants should be assessed also in non‐Asian patients with diffuse transgradient PPK. Understanding the role of serine protease inhibitors will provide insights into the complex proteolytic network in epidermal homeostasis.