Ichthyosis associated with sphingosine 1-phosphate lyase insufficiency syndrome is due to aberrant calcium and sphingolipid regulation

Sphingosine 1-phosphate lyase (SGPL1) insufficiency (SPLIS) is a syndrome which presents with adrenal insufficiency, steroid resistant nephrotic syndrome, hypothyroidism, neurological disease, and ichthyosis. Where a skin phenotype is reported, 94% presented with abnormalities such as ichthyosis, acanthosis, and hyperpigmentation. To elucidate the disease mechanism and the role SGPL1 plays in the skin barrier we established CRISPR-Cas9 SGPL1 knock-out and a lentiviral induced SGPL1 overexpression in nTERT immortalised keratinocytes and thereafter organotypic skin equivalents. Loss of SGPL1 caused an accumulation of S1P, sphingosine and ceramide while its overexpression caused a reduction of these species. RNAseq analysis showed perturbations in sphingolipid pathway genes, particularly in SGPL1_KO and GSEA revealed polar opposite differential gene expression between SGPL1_KO and _OE in keratinocyte differentiation and Ca2+ signalling genesets. SGPL1_KO upregulated differentiation markers while SGPL1_OE upregulated basal and proliferative markers. The advanced differentiation of SGPL1_KO was confirmed by 3D organotypic models that also presented with a thickened and retained stratum corneum, sphingolipid accumulation in the spinous layer and a breakdown of E-cadherin junctions. We hypothesise that SPLIS associated ichthyosis is a multi-faceted disease caused by sphingolipid imbalance and excessive S1P signalling. Overall design: We generated SGPL1 CRISPR KO nTERT cell lines as well as SGPL1 overexpressing cell lines and compared their transcriptomes using RNAseq. We used 3 biological replicates per group.