Functional skin barrier requires sphingolipid homeostasis. 3-ketodihydrosphingosine reductase or KDSR is a key enzyme of sphingolipid anabolism catalyzing the reduction of 3-ketodihydrosphingosine to sphinganine. Biallelic mutations in the KDSR gene may cause erythrokeratoderma variabilis et progressive-4, later specified as PERIOPTER syndrome, emphasizing a characteristic periorifical and ptychotropic erythrokeratoderma. We report another patient with compound heterozygous mutations in KDSR, born with generalized harlequin ichthyosis, which progressed into palmoplantar keratoderma. To determine whether patient-associated KDSR mutations lead to KDSR substrate accumulation and/or unrecognized sphingolipid downstream products in stratum corneum we analyzed lipids of this and previously published patients with non-identical biallelic mutations in KDSR. In stratum corneum of both patients we identified hitherto unobserved skin ceramides with an unusual keto-type sphingoid base in lesional and non-lesional areas, which accounted for up to 10% of the measured ceramide species. Furthermore, an overall shorter mean chain length of free and bound sphingoid bases was observed—shorter mean chain length of free sphingoid bases was also observed in lesional psoriasis vulgaris SC, but not generally in lesional atopic dermatitis SC. Formation of keto-type ceramides is probably due to a bottle neck in metabolic flux through KDSR and a bypass by ceramide synthases, which highlights the importance of tight intermediate regulation during sphingolipid anabolism and reveals substrate deprivation as potential therapy.

Ceramides belong to sphingolipids, an important group of cellular and extracellular lipids. Their physiological functions range from cell signaling to participation in the formation of barriers against water evaporation. In the skin, they are essential for the permeability barrier, together with free fatty acids and cholesterol. We examined the periodical structure and permeability of lipid films composed of ceramides (Cer; namely, N-lignoceroyl 6-hydroxysphingosine, CerNH24, and N-lignoceroyl sphingosine, CerNS24), lignoceric acid (LIG; 24:0), and cholesterol (Chol). X-ray diffraction experiments showed that the CerNH24-based samples form either a short lamellar phase (SLP, d ∼ 5.4 nm) or a medium lamellar phase (MLP, d = 10.63−10.78 nm) depending on the annealing conditions. The proposed molecular arrangement of the MLP based on extended Cer molecules also agreed with the relative neutron scattering length density profiles obtained from the neutron diffraction data. The presence of MLP increased the lipid film permeability to the lipophilic model permeant (indomethacin) relative to the CerNS24based control samples and the samples that had the same lipid composition but formed an SLP. Thus, the arrangement of lipids in various nanostructures is responsive to external conditions during sample preparation. This polymorphic behavior directly affects the barrier properties, which could also be (patho)physiologically relevant.

Functional skin barrier requires sphingolipid homeostasis. 3‐ketodihydrosphingosine reductase or KDSR is a key enzyme of sphingolipid anabolism and catalyzes the reduction of 3‐ketodihydrosphingosine to sphinganine. Biallelic mutations in the KDSR gene may cause skin pathologies of the erythrokeratoderma spectrum. The underlying molecular changes in sphingolipid metabolism remain largely unexplored. We report a new patient with compound heterozygous mutations in KDSR. The patient was born with generalized harlequin ichthyosis, which progressed into palmoplantar keratoderma. In stratum corneum of this and another patient with KDSR mutations we identified skin ceramides with an unusual sphingoid base. These unobserved keto‐type ceramides were found in lesional and non‐lesional skin areas and accounted for up to 10% of the measured ceramide species. We observed on average a shorter chain length of sphingoid bases and associated fatty acids in both patients. Formation of keto‐type ceramides is probably due to the utilization of accumulating 3‐ketodihydrosphingosine as an alternative substrate by ceramide synthases, which highlights the importance of tight intermediate regulation during sphingolipid anabolism. It is conceivable that the observed alterations affect lamellar lipid organization leading to defective skin barrier and keratinocyte differentiation.