RAC1 mutation is not a predictive biomarker for PI3'-kinase- selective pathway-targeted therapy.

Mutational activation of RAC1 is detected in similar to 7% of cutaneous melanoma, with the most frequent mutation (RAC1(C85T)) encoding for RAC1(P29S). RAC1(P29S) is a fast-cycling GTPase that leads to accumulation of RAC1(P29S)-GTP, which has potentially pleiotropic regulatory functions in melanoma cell signaling and biology. However, the precise mechanism by which mutationally activated RAC1(P29S) propagates its pro-tumorigenic effects remains unclear. RAC1-GTP is reported to activate the beta isoform of PI3'-kinase (PIK3CB/PI3K beta) leading to downstream activation of PI3'-lipid signaling. Hence, we employed both genetic and isoform-selective pharmacological inhibitors to test if RAC1(P29S) propagates its oncogenic signaling in melanoma through PI3K beta. We observed that RAC1(P29S)-expressing melanoma cells were largely insensitive to inhibitors of PI3K beta. Furthermore, RAC1(P29S) melanoma cell lines showed variable sensitivity to pan-class 1 (alpha/beta/gamma/delta) PI3'-kinase inhibitors, suggesting that RAC1-mutated melanoma cells may not rely on PI3'-lipid signaling for their proliferation. Lastly, we observed that RAC1(P29S)-expressing cell lines also showed variable sensitivity to pharmacological inhibition of the RAC1 -> PAK1 signaling pathway, questioning the relevance of inhibitors of this pathway for the treatment of patients with RAC1-mutated melanoma.