Abstract 806: TGFBR1*6A enhances breast cancer susceptibility and promote cell cycle transition
Authors: Agyemang Kojo , Pennison Michael , Wang Minghui , Johansen Allan , Jimenez Hugo , Blackman Carl , Singh Ravi , D'Agostino Ralph , Grippo Paul , Cristofano Antonio Di , Pasche Boris 
Published: July 2021

Abstract
Background: TGFBR1*6A is a polymorphic variant of the Transforming Growth Factor Beta receptor 1 (TGFBR1) that has three GCG synonymous deletions in its signal peptide. It is a low penetrance breast cancer susceptibility allele (OR, 1.15; 95% CI, 1.01 to 1.31) postulated to enhance tumor formation. In this study, a novel breast cancer mouse model carrying humanized TGFBR1*6A (hTGFBR1*6A) knock-in was used to investigate TGFBR1*6A influence on breast cancer formation, and dissect its molecular pathogenicity.
Methods: Humanized mouse models of the two most common TGFBR1*6A genotypes - homozygous TGFBR1*6A/6A and heterozygous TGFBR1*9A/6A, and wild-type TGFBR1 were created by replacing murine Tgfbr1 exon 1 with human TGFBR1*6A or TGFBR1 exon 1. The three genotypes were bred with MMTV.Neu mice to study breast cancer development. Between 29-35 nulliparous females from each genotype were monitored for tumor formation (≥1 tumor), multiple tumor formation (≥2 tumors) and survival. Tumor cells were isolated from the hTGFBR1 and hTGFBR1*6A mice and validated for similar Neu protein expression. They were assessed for TGF-β/FoxO1 signaling as it relates to cell cycle progression and proliferation.
Results: After 548 days, the mice harboring hTGFBR1*6A developed significantly more tumors (p=0.003 for one/more tumors, and p=0.0009 for two/more tumors) than wild-type mice. About 70% hTGFBR1*6A/6A and 28.6% hTGFBR1*9A/6A variants compared to 18.5% hTGFBR1 wild-type mice developed two (2) or more tumors. The hTGFBR1*6A mice also had shorter survival when compared with the wild-type mice (Log rank X2=7.77, p=0.02). The hTGFBR1*6A/6A and hTGFBR1*9A/6A mice survived for 178-210 and 208-282 days respectively compared to 198-342 days among the wild-type. The isolated hTGFBR1*6A tumor cells formed three (3) times more colonies, and exhibited higher growth rate and G1/S cell cycle transition than wild-type tumor cells. Interestingly, the hTGFBR1*6A cells were less sensitive to TGF-β anti-proliferative effects despite showing marked increase in TGF-β/SMAD signaling. Analyses of the TGF-β/FoxO1 growth inhibition pathway showed decreased FoxO1, p21 and p27 gene and protein expression, which was not increased significantly by TGF-β treatment. This suggests that TGFBR1*6A constitutively inhibits the TGF-β/FoxO1 cell cycle tumor suppressor arm.
Conclusions: Altogether, this study provides new evidence in support of TGFBR1*6A as a breast cancer susceptibility allele. It also identifies deregulation of TGF-β/FoxO1 cell cycle inhibition as a potential mechanism underlying its increased cellular proliferation.
Citation Format: Kojo Agyemang, Michael Pennison, Minghui Wang, Allan Johansen, Hugo Jimenez, Carl Blackman, Ravi Singh, Ralph D'Agostino, Paul Grippo, Antonio Di Cristofano, Boris Pasche. TGFBR1*6A enhances breast cancer susceptibility and promote cell cycle transition [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 806.