Defective targeting of PNPLA1 to lipid droplets causes ichthyosis in ABHD5-syndromic epidermal differentiation disorder.

ABHD5-syndromic epidermal differentiation disorder (ABHD5-sEDD; also known as Chanarin-Dorfman syndrome) is a rare autosomal recessive disorder caused by mutations in the α/β-hydrolase domain-containing 5 (ABHD5) gene, leading to systemic accumulation of neutral lipids and ichthyosis due to impaired activation of patatin-like phospholipase domain-containing (PNPLAs) proteins. While ABHD5 is a well-known co-activator of adipose triglyceride lipase (ATGL, also referred to as PNPLA2), its role in epidermal lipid metabolism is incompletely understood. Here, we identify ABHD5 as a key regulator of PNPLA1, an enzyme essential for ω-O-acylceramide (acylCer) synthesis and skin barrier formation. We analyzed seven disease-associated ABHD5 missense mutations and found that they disrupt PNPLA1 localization and function by distinct mechanisms: (i) mutations affecting the PNPLA1 binding region of ABHD5 impair PNPLA1 recruitment to intracellular lipid droplets (LDs), thus reducing acylCer synthesis; (ii) mutations in potential perilipin-binding domains of ABHD5 prevent ABHD5 association with LDs, thereby disrupting PNPLA1-LD localization. Despite these defects, restoring co-localization of ABHD5 mutants with PNPLA1 in proteoliposomes rescued full PNPLA1 enzyme activity, indicating that spatial proximity rather than direct protein binding is sufficient to facilitate acylCer formation. In summary, our findings establish a co-localization-driven model of PNPLA1 regulation, in which ABHD5 ensures proper PNPLA1 targeting to LDs and simultaneously enables its enzymatic activation. This model suggests that pharmacological strategies aimed at restoring PNPLA1 localization to LDs may represent a potential therapeutic approach for ichthyosis in ABHD5-sEDD. By elucidating the molecular mechanisms underlying disease pathogenesis, our study provides important new insights into epidermal lipid metabolism and therapeutic targeting.