Congenital generalized lipodystrophy type 2 (CGL2) is caused by mutations in the BSCL2 gene, which encodes the protein seipin. However, how seipin loss causes adipose tissue failure remains unclear. Using human adipocyte progenitor cells capable of robust differentiation in vitro and in vivo, we reveal two unexpected findings that redefine CGL2 pathogenesis. First, seipin is dispensable for lipid droplet biogenesis but essential for recruiting the major adipocyte scaffold protein Perilipin 1 (PLIN1) to the lipid droplet surface. Second, we discover that the integrity of the lipid droplet serves as an organelle to nucleus quality control checkpoint enforcing adipocyte identity. Without seipin-dependent PLIN1 recruitment, adipocytes exhibit enhanced lipolysis and ceramide accumulation, triggering an unexpected cellular response of de-differentiation into a progenitor-like state. From this de-differentiated state, cells can undergo additional cycles of differentiation and de-differentiation upon repeated adipogenic stimuli. However, some cells escape de-differentiation, instead forming a single large droplet and displaying severe cellular structural abnormalities. Consistent with this model, we find functional adipose tissue can form in vivo from seipin-deficient cells, yet ultimately fails. These findings resolve conflicting models of CGL2 pathogenesis by reframing seipin as a regulator of PLIN1 recruitment, rather than droplet formation per se, and reveal the fundamental role of lipid droplet integrity in the development of functional human adipocytes.

We describe our 8-year clinical experience with metreleptin in a Brazilian adult female patient with congenital generalized lipodystrophy type 2 (due to a mutation in the BSCL2 gene) and severe insulin resistance. The patient was initially treated with antidiabetic medications due to the unavailability of metreleptin. Metreleptin was initiated at age 20 years. Reductions from baseline for glycated hemoglobin (HbA1c) and triglycerides on metreleptin were sustained over a 5-year treatment period. The greatest reductions in HbA1c (from 10.8% [95 mmol/mol] to 6.0% [42 mmol/mol], -4.8%) and triglycerides (from 398 mg/dL [4.5 mmol/mL] to 104 mg/dL [1.2 mmol/L], -74%) occurred after 39 months, accompanied by a -95% decrease in total daily insulin usage (from 1600 to 88 IU/day). No significant adverse events occurred throughout metreleptin therapy. Metreleptin therapy was interrupted for 36 months due to limited access to the medication, during which time metabolic parameters deteriorated, returning to near-baseline levels. Thereafter, metreleptin was restarted. At the most recent clinic evaluation (3 months after resuming metreleptin), HbA1c, triglycerides, and liver enzyme levels reduced relative to the last measurements taken during treatment interruption. These findings provide support for the long-term and continuous use of metreleptin in patients with generalized lipodystrophy.

Congenital generalized lipodystrophy type 2 (CGL2) is a rare autosomal recessive disorder characterized by the near-total absence of adipose tissue, leading to various metabolic complications. We present the case of a one-year-old male who exhibited progressive abdominal distension from six months of age. Physical examination revealed distinctive features including triangular facies, hypertelorism, an emaciated appearance with absent buccal fat, and hepatosplenomegaly. Laboratory investigations showed elevated transaminases and a deranged lipid profile, while imaging confirmed hepatosplenomegaly without systemic anomalies. A liver biopsy indicated macrovesicular steatosis and impending cirrhosis. Genetic testing revealed a homozygous pathogenic variant in the BSCL2 gene (c.604C>T), confirming CGL2. The child is under regular follow-up, with genetic counseling provided to the parents. This case underscores the importance of early recognition, genetic diagnosis, and regular monitoring in managing this rare condition.

Seipin (BSCL2), a conserved endoplasmic reticulum protein, plays a critical role in lipid droplet (LD) biogenesis and in regulating LD morphology, pathogenic variants of which are associated with Berardinelli-Seip congenital generalized lipodystrophy type 2 (BSCL2). To model BSCL2 disease, we generated an orthologous BSCL2 variant, seip-1(A185P), in Caenorhabditis elegans. In this study, we conducted an unbiased chemical mutagenesis screen to identify genetic suppressors that restore embryonic viability in the seip-1(A185P) mutant background. A total of five suppressor lines were isolated and recovered from the screen. The defective phenotypes of seip-1(A185P), including embryonic lethality and impaired eggshell formation, were significantly suppressed in each suppressor line. Two of the five suppressor lines also alleviated the enlarged LDs in the oocytes. We then mapped a suppressor candidate gene, lmbr-1, which is an ortholog of human limb development membrane protein 1 (LMBR1). The CRISPR/Cas9 edited lmbr-1 suppressor alleles, lmbr-1(S647F) and lmbr-1(P314L), both significantly suppressed embryonic lethality and defective eggshell formation in the seip-1(A185P) background. The newly identified suppressor lines offer valuable insights into potential genetic interactors and pathways that may regulate seipin in the lipodystrophy model.

Maintaining the metabolic homeostasis of fatty acids is crucial for human health. Excess fatty acids are stored in lipid droplets (LDs), the primary energy reservoir that helps regulate fat and lipid homeostasis in nearly all cell types. Seipin (BSCL2), a conserved endoplasmic reticulum protein, plays a critical role in LD biogenesis and regulating LD morphology. Pathogenic variants of seipin are associated with multiple human genetic diseases, including Berardinelli-Seip Congenital Generalized Lipodystrophy Type 2 (BSCL2). However, the cellular and molecular mechanisms by which dysfunctional seipin leads to these diseases remain unclear. To model BSCL2 disease, we generated an orthologous BSCL2 pathogenic variant seip-1(A185P) using CRISPR/Cas9 genome editing in Caenorhabditis elegans . This variant led to severe developmental and cellular defects, including embryonic lethality, impaired eggshell formation, and abnormally enlarged LDs. We set out to identify genetic determinants that could suppress these defective phenotypes in the seip-1(A185P) mutant background. To this end, we conducted an unbiased chemical mutagenesis screen to identify genetic suppressors that restore embryonic viability in the seip-1(A185P) mutant background. A total of five suppressor lines were isolated and recovered from the screen. The defective phenotypes of seip-1(A185P) , including embryonic lethality and impaired eggshell formation, were significantly suppressed in each suppressor line. Two of the five suppressor lines also alleviated the enlarged LDs in the oocytes. We then mapped a suppressor candidate gene, R05D3.2 (renamed as lmbr-1 ), which is an ortholog of human LMBR1 (limb development membrane protein 1). The CRISPR/Cas9 edited lmbr-1 suppressor alleles, lmbr-1(Ser647Phe) and lmbr-1(Pro314Leu) , both significantly suppressed embryonic lethality and defective eggshell formation in the seip-1(A185P) background. The newly identified suppressor lines offer valuable insights into potential genetic interactors and pathways that may regulate seipin in the lipodystrophy model.