Identification of Genetic Suppressors for a Berardinelli-Seip Congenital Generalized Lipodystrophy Type 2 (BSCL2) Pathogenic Variant in C. elegans

Abstract

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.