Cancer-associated fibroblasts (CAFs) are a major constituent of the tumor microenvironment, although their origin and roles in shaping disease initiation, progression and treatment response remain unclear due to significant heterogeneity. Here, following a negative selection strategy combined with single-cell RNA sequencing of 768 transcriptomes of mesenchymal cells from a genetically engineered mouse model of breast cancer, we define three distinct subpopulations of CAFs. Validation at the transcriptional and protein level in several experimental models of cancer and human tumors reveal spatial separation of the CAF subclasses attributable to different origins, including the peri-vascular niche, the mammary fat pad and the transformed epithelium. Gene profiles for each CAF subtype correlate to distinctive functional programs and hold independent prognostic capability in clinical cohorts by association to metastatic disease. In conclusion, the improved resolution of the widely defined CAF population opens the possibility for biomarker-driven development of drugs for precision targeting of CAFs.

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BACKGROUND Multiple synchronous, ipsilateral, invasive foci of breast carcinomas are frequent and are associated with a poorer prognosis. Few studies have investigated the prognostic and therapeutic implications of heterogeneity of such foci. METHODS The authors reviewed the tumor type, grade, and size of all invasive foci in a series of 110 multifocal breast carcinomas documented on large‐format slides. Molecular phenotype was determined by immunohistochemistry in tissue microarray blocks using 3 classification systems. The survival of patients who had tumors with microscopic (tumor type and/or grade) heterogeneity and of those who had tumors with phenotypic heterogeneity was compared with the survival of patients who had multifocal homogeneous tumors using Kaplan‐Meier curves. The hazard ratio of dying from breast cancer was also calculated. RESULTS Intertumoral heterogeneity in tumor type and grade was detected in 16 of 110 tumors (14.6%) and in 6 of 110 tumors (5.5%), respectively. The molecular phenotype of invasive tumor foci within the same breast differed in 10% to 12.7% of patients (11‐14 of 110 tumors), depending on the classification system used. Patients who had phenotypically heterogeneous, multifocal cancers had a greater risk of dying from disease (HR=2.879; 95%CI=1.084−7.649; P = .034) and had significantly shorter survival (P = .016). Phenotypic differences were most common in patients who had tumors that were homogeneous in terms of tumor type (11 of 18 tumors) and histology grade (14 of 18 tumors). Phenotyping additional tumor foci had the potential to influence the therapeutic decisions in up to 8 patients. CONCLUSIONS Phenotyping more than 1 invasive focus of multifocal breast carcinomas only if the individual foci deviate microscopically appears to be insufficient, because phenotypic intertumoral heterogeneity may be observed in microscopically identical foci and has potential prognostic and therapeutic consequences. Cancer 2014;120:26–34. © 2013 American Cancer Society.

IMPORTANCE Trastuzumab emtansine (T-DM1) is presently approved for treatment of advanced breast cancer and after incomplete response to neoadjuvant therapy, but the potential of T-DM1 as monotherapy is so far unknown.OBJECTIVE To assess pathologic complete response (pCR) to standard neoadjuvant therapy of combination docetaxel, trastuzumab, and pertuzumab (DTP) vs T-DM1 monotherapy in patients with ERBB2 (formerly HER2)-positive breast cancer. DESIGN, SETTING, AND PARTICIPANTSThis randomized phase 2 trial, conducted at 9 sites in Sweden, enrolled 202 patients between December 1, 2014, and October 31, 2018. Participants were 18 years or older, with ERBB2-positive tumors larger than 20 mm and/or verified lymph node metastases. Analysis was performed on an intention-to-treat basis.INTERVENTIONS Patients were randomized to receive 6 cycles of DTP (standard group) or T-DM1 (investigational group). Crossover was recommended at lack of response or occurrence of intolerable toxic effects. Assessment with fluorine 18-labeled fluorodeoxyglucose ( 18 F-FDG) positron emission tomography combined with computed tomography (PET-CT) was performed at baseline and after 2 and 6 treatment cycles.MAIN OUTCOME AND MEASURES Pathologic complete response, defined as ypT0 or Tis ypN0. Secondary end points were clinical and radiologic objective response; event-free survival, invasive disease-free survival, distant disease-free survival, and overall survival; safety; health-related quality of life (HRQoL); functional and biological tumor characteristics; and frequency of breast-conserving surgery. RESULTSOverall, 202 patients were randomized; 197 (99 women in the standard group [median age, 51 years (range, 26-73 years)] and 98 women in the investigational group [median age, 53 years (range, 28-74 years)]) were evaluable for the primary end point. Pathologic complete response was achieved in 45 patients in the standard group (45.5%; 95% CI 35.4%-55.8%) and 43 patients in the investigational group (43.9%; 95% CI 33.9%-54.3%). The difference was not statistically significant (P = .82). In a subgroup analysis, the pCR rate was higher in hormone receptor-negative tumors than in hormone receptor-positive tumors in both treatment groups (45 of 72 [62.5%] vs 45 of 125 [36.0%]). Three patients in the T-DM1 group experienced progression during therapy. In an exploratory analysis, tumor-infiltrating lymphocytes at 10% or more (median) estimated pCR significantly (odds ratio, 2.76; 95% CI, 1.42-5.36; P = .003). Response evaluation with 18 F-FDG PET-CT revealed a relative decrease of maximum standardized uptake value by equal to or greater than 68.7% (median) was associated with pCR (odds ratio, 6.74, 95% CI, 2.75-16.51; P < .001). CONCLUSIONS AND RELEVANCEIn this study, treatment with standard neoadjuvant combination DTP was equal to T-DM1.