A dominant CIDEC variant segregates with familial obesity by increasing lipid droplet size in adipocytes

Adipose tissue dysfunction in obesity is a major global public health risk, contributing to insulin resistance and chronic diseases such as diabetes and cardiovascular disorders. Here, we identify a dominant c.37A>G p.(Arg13Gly) variant in the long isoform of CIDEC (CIDEC-L), a key regulator of lipid droplet (LD) size, as the underlying cause of familial obesity. Affected individuals display marked subcutaneous fat accumulation in white adipose tissue (WAT), elevated fat content in brown adipose tissue (BAT) and insulin resistance. Accordingly, patient-derived iPSCs differentiated into white adipocytes exhibit accelerated LD growth, a phenotype mirrored by CIDEC-L ^R13G overexpression. Mechanistically, we find that the p.Arg13Gly variant disrupts the N-terminal structural order of CIDEC-L, shifting its phase separation properties to enable, rather than restrict, lipid exchange through condensation plates between LDs. Notably, knock-in mice with the analogous Cidec-L p.(Arg10Gly) mutation recapitulate the human BAT hypertrophy and exhibit impaired thermogenesis. These findings establish the CIDEC-L ^R13G variant as the first example of a dominantly inherited monogenic obesity driven by a dysfunctional adipocyte LD protein, revealing a critical role for CIDEC-L in restraining fat accumulation and maintaining metabolic health.