Cancer-associated systemic inflammation is strongly linked with poor disease outcome in cancer patients 1,2. For most human epithelial tumour types, high systemic neutrophil-tolymphocyte ratios are associated with poor overall survival 3 , and experimental studies have demonstrated a causal relationship between neutrophils and metastasis 4,5. However, the cancer cell-intrinsic mechanisms dictating the substantial heterogeneity in systemic neutrophilic inflammation between tumour-bearing hosts are largely unresolved. Using a panel of 16 distinct genetically engineered mouse models (GEMMs) for breast cancer, we have uncovered a novel role for cancer cell-intrinsic p53 as a key regulator of pro-metastatic neutrophils. Mechanistically, p53 loss in cancer cells induced secretion of Wnt ligands that stimulate IL-1β production by tumour-associated macrophages, which drives systemic inflammation. Pharmacological and genetic blockade of Wnt secretion in p53-null cancer cells reverses IL-1β expression by macrophages and subsequent neutrophilic inflammation, resulting in reduced metastasis formation. Collectively, we demonstrate a novel mechanistic link between loss of p53 in cancer cells, Wnt ligand secretion and systemic neutrophilia that potentiates metastatic progression. These insights illustrate the importance of the genetic makeup of breast tumours in dictating pro-metastatic systemic inflammation, and set the stage for personalized immune intervention strategies for cancer patients. 4 Main text To determine how pro-metastatic systemic inflammation is influenced by genetic aberrations in tumours, we studied 16 GEMMs for breast cancer carrying different tissue-specific mutations. These GEMMs represent most subtypes of human breast cancer, including ductal and lobular carcinoma, oestrogen receptor-positive (luminal A), HER2 + , triple-negative and basal-like breast cancer. Because we and others have demonstrated that neutrophils expand systemically and promote metastasis 5-10 , we evaluated circulating neutrophil levels as a marker for systemic inflammation in mammary tumour-bearing mice with end-stage disease. As expected, most tumour-bearing mice displayed an increase in circulating neutrophils as compared to non-tumour-bearing animals (wild-type [WT]) (Fig. 1a). Like the inter-patient heterogeneity in systemic inflammation in human breast cancer 11 , we observed a striking variability in the extent of neutrophilia between the different tumour-bearing GEMMs (Fig. 1a, Extended Data Fig. 1a). We found that the models exhibiting high neutrophil expansion displayed a subset of neutrophils expressing the stem cell marker cKIT (Fig. 1b), indicative of an immature neutrophil phenotype 5. We subsequently searched for commonalities and differences among the 16 GEMMs with regards to high versus low systemic neutrophil levels. Strikingly, mice bearing tumours with a p53 deletion exhibited the most pronounced circulating neutrophil levels (Fig. 1a). The difference in magnitude of systemic inflammation between p53proficient and p...

Large-scale sequencing studies are rapidly identifying putative oncogenic mutations in human tumors. However, discrimination between passenger and driver events in tumorigenesis remains challenging and requires in vivo validation studies in reliable animal models of human cancer. In this study, we describe a novel strategy for in vivo validation of candidate tumor suppressors implicated in invasive lobular breast carcinoma (ILC), which is hallmarked by loss of the cell-cell adhesion molecule E-cadherin. We describe an approach to model ILC by intraductal injection of lentiviral vectors encoding Cre recombinase, the CRISPR/Cas9 system, or both in female mice carrying conditional alleles of the Cdh1 gene, encoding for E-cadherin. Using this approach, we were able to target ILC-initiating cells and induce specific gene disruption of Pten by CRISPR/Cas9-mediated somatic gene editing. Whereas intraductal injection of Cas9-encoding lentiviruses induced Cas9-specific immune responses and development of tumors that did not resemble ILC, lentiviral delivery of a Pten targeting single-guide RNA (sgRNA) in mice with mammary gland-specific loss of E-cadherin and expression of Cas9 efficiently induced ILC development. This versatile platform can be used for rapid in vivo testing of putative tumor suppressor genes implicated in ILC, providing new opportunities for modeling invasive lobular breast carcinoma in mice.

Invasive lobular carcinoma (ILC) is the second most common breast cancer subtype and accounts for 8-14% of all cases. Although the majority of human ILCs are characterized by the functional loss of E-cadherin (encoded by CDH1), inactivation of Cdh1 does not predispose mice to develop mammary tumors, implying that mutations in additional genes are required for ILC formation in mice. To identify these genes, we performed an insertional mutagenesis screen using the Sleeping Beauty transposon system in mice with mammary-specific inactivation of Cdh1. These mice developed multiple independent mammary tumors of which the majority resembled human ILC in terms of morphology and gene expression. Recurrent and mutually exclusive transposon insertions were identified in Myh9, Ppp1r12a, Ppp1r12b and Trp53bp2, whose products have been implicated in the regulation of the actin cytoskeleton. Notably, MYH9, PPP1R12B and TP53BP2 were also frequently aberrated in human ILC, highlighting these genes as drivers of a novel oncogenic pathway underlying ILC development.