SUMMARY Genetic screening identifies the atypical tetraspanin TM4SF1 as a strong mediator of metastatic reactivation of breast cancer. Intriguingly, TM4SF1 couples the collagen receptor tyrosine kinase DDR1 to the cortical adaptor syntenin 2 and, hence, to PKCα. The latter kinase phosphorylates and activates JAK2, leading to the activation of STAT3. This non-canonical mechanism of signaling induces the expression of SOX2 and NANOG, sustains the manifestation of cancer stem cell traits, and drives metastatic reactivation in the lung, bone, and brain. Bioinformatic analyses and pathological studies corroborate the clinical relevance of these findings. We conclude that non-canonical DDR1 signaling enables breast cancer cells to exploit the ubiquitous interstitial matrix component collagen I to undergo metastatic reactivation in multiple target organs.

Summary Driver mutations in oncogenes encode proteins with gain-of-function properties that enhance fitness. Heterozygous mutations are thus viewed as sufficient for tumorigenesis. We describe widespread oncogenic mutant allele imbalance in 13,448 prospectively characterized cancers. Imbalance was selected for through modest dosage increases of gain-of-fitness mutations. Negative selection targeted haplo-essential effectors of the spliceosome. Loss of the normal allele comprised a distinct class of imbalance driven by competitive fitness, which correlated with enhanced response to targeted therapies. In many cancers, an antecedent oncogenic mutation drove evolutionarily dependent allele-specific imbalance. In other instances, oncogenic mutations co-opted independent copy number changes via the evolutionary process of exaptation. Oncogenic allele imbalance is a pervasive evolutionary innovation that enhances fitness and modulates sensitivity to targeted therapy.

Altered choline phospholipid metabolism is a hallmark of cancer, leading to malignant choline metabolite profiles consisting of low glycerophosphocholine (GPC) and high phosphocholine (PC) in human breast cancers. Glycerophosphocholine phosphodiesterase (GPC-PDE) catalyzes the degradation of GPC to free choline and glycerol-3-phosphate. The gene(s) encoding for the GPC-PDE(s) responsible for GPC degradation in breast cancers have not yet been identified. Here we have demonstrated for the first time that the GPC-PDE encoded by glycerophosphodiester phosphodiesterase domain containing 5 (GDPD5) is associated with breast cancer malignancy. Two human breast cancer cell lines (n=8 and 10) and primary human breast tumor samples (n=19) were studied with combined magnetic resonance spectroscopy (MRS) and qRT-PCR to investigate several isoforms of GDPD expression with respect to choline phospholipid metabolite levels. Out of five GDPDs tested, GDPD5 was found to be significantly overexpressed in highly malignant estrogen receptor negative (ER−) compared to weakly malignant estrogen receptor positive (ER+) human breast cancer cells (P=0.027) and breast tumors from patients (P=0.015). GDPD5 showed significantly positive correlations with PC (P<0.001), total choline (tCho) (P=0.007) and PC/GPC (P<0.001) levels in human breast tumors. GDPD5 showed a trend towards negative correlation with GPC levels (P=0.130). Human breast cancers with malignant choline metabolite profiles consisting of low GPC and high PC levels highly co-expressed GDPD5, choline kinase alpha (CHKA), and phosphatidylcholine-specific phospholipase D1 (PLD1), while cancers containing high GPC and relatively low PC levels displayed low co-expression of GDPD5, CHKA, and PLD1. GDPD5, CHKA and PLD1 were significantly overexpressed in highly malignant ER− tumors in our patient cohort. Our study identified GDPD5 as a GPC-PDE that likely participates in regulating choline phospholipid metabolism in breast cancer, which possibly occurs in cooperation with CHKA and PLD1.