BackgroundThe somatic mutation in the FOXL2 gene c.402C>G (p.Cys134Trp) has recently been identified in the vast majority of adult ovarian granulosa cell tumors (OGCTs) studied. In addition, this mutation seems to be specific to adult OGCTs and is likely to be a driver of malignant transformation. However, its pathogenic mechanisms remain elusive.Methodology/Principal FindingsWe have sequenced the FOXL2 open reading frame in a panel of tumor cell lines (NCI-60, colorectal carcinoma cell lines, JEG-3, and KGN cells). We found the FOXL2 c.402C>G mutation in the adult OGCT-derived KGN cell line. All other cell lines analyzed were negative for the mutation. In order to gain insights into the pathogenic mechanism of the p.Cys134Trp mutation, the subcellular localization and mobility of the mutant protein were studied and found to be no different from those of the wild type (WT). Furthermore, its transactivation ability was in most cases similar to that of the WT protein, including in conditions of oxidative stress. A notable exception was an artificial promoter known to be coregulated by FOXL2 and Smad3, suggesting a potential modification of their interaction. We generated a 3D structural model of the p.Cys134Trp variant and our analysis suggests that homodimer formation might also be disturbed by the mutation.Conclusions/SignificanceHere, we confirm the specificity of the FOXL2 c.402C>G mutation in adult OGCTs and begin the exploration of its molecular significance. This is the first study demonstrating that the p.Cys134Trp mutant does not have a strong impact on FOXL2 localization, solubility, and transactivation abilities on a panel of proven target promoters, behaving neither as a dominant-negative nor as a loss-of-function mutation. Further studies are required to understand the specific molecular effects of this outstanding FOXL2 mutation.

Polyalanine (polyAla) tract expansions have been associated with an increasing number of human diseases. Here, we have undertaken a functional study of the effects of polyAla expansions in the context of the transcription factor FOXL2, involved in cranio-facial and ovarian development. Using two cellular models, we show that FOXL2 polyAla expansions lead to protein mislocalization and aggregation in a length-dependent manner. The fraction of cells containing cytoplasmic staining displays a sigmoidal relationship with respect to the length of the polyAla tract, suggesting the existence of a threshold length above which protein mislocalization occurs. The existence of such a threshold might be rationalized if we consider that the longer the polyAla tract is, the higher its tendency to misfolding or to inducing spurious interactions with cytoplasmic components. To study the intranuclear dynamics of polyAla-expanded FOXL2, we performed fluorescence recovery after photobleaching experiments. The most unexpected result concerned the pathogenic protein containing 19 Ala residues in the run, which was virtually immobile, although this variant does not present a classical aggregation pattern. Luciferase assays and real time RT-PCR of many potential target genes showed that polyAla expansions induce different losses of activity according to the target promoters tested. We provide molecular explanations for these findings. Although our main focus is the mechanisms of pathogenesis of polyAla-expanded proteins, we discuss the potential relevance of polyAla length variation in micro- and macroevolution because polyAla-containing proteins tend to be transcription factors.