IDENTIFICATION OF TBC1D7 AS A METABOLIC PROGNOSTIC BIOMARKER IN BREAST CANCER

Cancer cells undergo metabolic alterations that serve the anabolic and energy demands of the growing tumor and promote its immune evasion [1]. These metabolic alterations might change during tumor evolution to support adaptation of cancer cells to environmental stresses and nutrient deprivation [2]. However, the molecular determinants that support this metabolic plasticity are still largely uncharacterized. Mechanistic insights might come from the study of membrane trafficking since metabolism depends on the uptake/scavenging of nutrients and on lysosomal degradation, all membrane-based processes that requires biogenesis and trafficking of vesicles. We, therefore, hypothesized that genetic deregulation of genes encoding membrane trafficking proteins might promote metabolic alterations increasing tumor aggressiveness. To test this hypothesis, we focused on a family of forty-five genes that encode for membrane trafficking regulators: the TBC-domain containing proteins (TBC1Ds) studying their involvement in breast cancer (BC) since these tumors are characterized by a high molecular heterogeneity that mirrors specific metabolic features and impact disease progression and patient outcome [1]. The involvement of TBC1Ds in BC metabolism and aggressiveness was evaluated with two parallel approaches: i) we analyzed alterations of these genes in BC patients and the correlation with patients’ survival using the Metabric dataset [3]. ii) We performed metabolic screenings by individually silencing the expression of each TBC1D molecule in BC cells measuring the concentration of intracellular lactate as a read-out of glycolysis, one of the major metabolic alterations in breast cancer [1]. By crossing these data, we identified three top hits (TBC1D31, TBC1D22B and TBC1D7). Among them, we focused on TBC1D7 since it is highly expressed in Triple negative BC (TNBC) behaving as an independent prognostic biomarker in this subtype. Moreover, it strongly affects lactate production in BC cells. TBC1D7 is a small protein mostly consisting of a non-functional GAP domain that participates in the TSC-complex [4]. Here we identified a novel 4 function of TBC1D7 in increasing glucose and lipid metabolism in BC cells independently from its well-known participation in the TSC complex and the consequent downregulation of the mTORC1 pathway. Of note, TBC1D7 was also found to facilitate overgrowth of spheroids. To gain insights into the molecular mechanism of these phenotypes, we identify an N-terminal point mutant of TBC1D7 defective both in lipid storage and 3D spheroids growth. We employed this mutant in mass-spectrometry analyses aimed at identifying partners of TBC1D7 in its lipid pathway, currently under validation. Together, our results highlight how TBC1D7 connect BC aggressiveness and metabolic plasticity, representing a promising prognostic tool in TNBC.