Expression of Activated BRAF Induces Cyst Formation and Accelerates Disease Progression in ADPKD mice

Autosomal dominant polycystic kidney disease (ADPKD) is characterized by the formation and progressive enlargement of fluid‐filled cysts, leading to massively enlarged kidneys, interstitial fibrosis and a decline in renal function. Intracellular cAMP promotes cyst growth by stimulating the MEK/ERK pathway and proliferation of the cystic cells. By contrast, cAMP is anti‐mitogenic in normal kidney cells. This aberrant proliferative response to cAMP appears to involve BRAF, a kinase upstream of the MEK/ERK pathway. In normal cells, BRAF is repressed by a Ca2+‐dependent mechanism, thereby preventing cAMP activation of ERK and cell proliferation. In ADPKD, mutations in the PKD genes disrupt intracellular Ca2+ homeostasis, relieving BRAF inhibition and allowing cAMP stimulation of the BRAF/MEK/ERK pathway. Currently, it is unclear if aberrant BRAF activity induces cyst formation and accelerates cyst growth and progression of ADPKD.To determine if BRAF activation induces cyst formation, we generated a novel transgenic mouse that conditionally expresses constitutively active BRAFV600E. BRAFV600E mice were bred to Pkhd1‐Cre mice to selectively overexpress active BRAF in collecting duct (CD) cells. BRAFV600E mice were also bred to Pkd1RC/RC mice, an orthologous model of ADPKD, to determine if BRAF activation accelerates disease progression. At 10 weeks, mice were sacrificed for measurement of kidney weight as percent body weight (KW%BW), cystic area, interstitial fibrosis, cell proliferation and blood urea nitrogen (BUN), a marker of renal function.Expression of active BRAF in CD cells caused a small but significant increase in KW%BW (1.8% compared to 1.2% in wild‐type (WT) mice). There was an 8‐fold increase in the percentage of cells that stained positive for Ki‐67, a marker for cell proliferation, and formation of numerous cysts within the BRAFV600E kidneys. These mice also displayed immune cell infiltration and interstitial fibrosis. Expression of active BRAF in Pkd1RC/RC mice caused a 2.3‐fold increase in KW%BW, 2.3‐fold increase in the number of cysts and a 3.6‐fold increase in cystic area. These changes were accompanied by an increase in the number of proliferating cells in the cysts and interstitium. Pkd1RC/RC: BRAFV600E mice also had a significant increase in interstitial fibrosis (86% compared to 33% in Pkd1RC/RC mice) and a higher BUN, indicating a more rapid decline in renal function. We conclude that aberrant BRAF activation is sufficient to induce cyst formation and accelerates disease progression in ADPKD mice.Support or Funding InformationThis work was supported by a grant from the National Institutes of Diabetes and Digestive and Kidney Disease (R01‐DK081579 to D.P. Wallace). This work was also supported, in part, by funds provided by the Allen Foundation Trust, Milwaukee, Wisconsin (D.P. Wallace). The opinions and conclusions herein are those of the authors and do not necessarily represent those of the Allen Foundation Trust or any of its programs.