A cellular model provides insights into the pathogenicity of the oncogenic FOXL2 somatic variant p.Cys134Trp

FOXL2 is a transcription factor expressed in ovarian granulosa cells. A somatic variant of FOXL2 (c.402 C > G, p.Cys134Trp) is the hallmark of adult-type granulosa cell tumours.

We generated KGN cell clones either heterozygous for this variant (MUT) or homozygous for the wild-type (WT) allele by CRISPR/Cas9 editing. They underwent RNA-Seq and bioinformatics analyses to uncover pathways impacted by deregulated genes. Cell morphology and migration were studied.

The differentially expressed genes (DEGs) between WT/MUT and WT/WT KGN cells (DEGs-_WT/MUT), pointed to several dysregulated pathways, like TGF-beta pathway, cell adhesion and migration. Consistently, WT/MUT cells were rounder than WT/WT cells and displayed a different distribution of stress fibres and paxillin staining. A comparison of the DEGs-_WT/MUT with those found when FOXL2 was knocked down (KD) in WT/WT KGN cells showed that most DEGs-_WT/MUT cells were not so in the KD experiment, supporting a gain-of-function (GOF) scenario. MUT-FOXL2 also displayed a stronger interaction with SMAD3.

Our work, aiming at better understanding the GOF scenario, shows that the dysregulated genes and pathways are consistent with this idea. Besides, we propose that GOF might result from an enhanced interaction with SMAD3 that could underlie an ectopic capacity of mutated FOXL2 to bind SMAD4.