Physiological levels of Pik3ca(H1047R) mutation in the mouse mammary gland results in ductal hyperplasia and formation of ERα-positive tumors

Tumor antigen presentation and the associated signal transduction during carcinogenesis

Numerous developments have been achieved in the study and treatment of cancer throughout the decades that it has been common. After decades of research, about 100 different kinds of cancer have been found, each with unique subgroups within certain organs. This has significantly expanded our understanding of the illness. A mix of genetic, environmental, and behavioral variables contribute to the complicated and diverse process of cancer formation. Mutations, or changes in the DNA sequence, are crucial to the development of cancer. These mutations have the ability to downregulate the expression and function of Major Histocompatibility Complex class I (MHC I) and MHCII receptors, as well as activate oncogenes and inactivate tumor suppressor genes. Cancer cells use this tactic to avoid being recognized by cytotoxic CD8+T lymphocytes, which causes issues with antigen presentation and processing. This review goes into great length into the PI3K pathway, changes to MHC I, and positive impacts of tsMHC-II on disease-free survival and overall survival and the involvement of dendritic cells (DCs) in different tumor microenvironments. The vital functions that the PI3K pathway and its link to the mTOR pathway are highlighted and difficulties in developing effective cancer targeted therapies and feedback systems has also been mentioned, where resistance mechanisms include RAS-mediated oncogenic changes and active PI3K signalling.

CarcSeq detection of lorcaserin-induced clonal expansion of Pik3ca H1047R mutants in rat mammary tissue

Lorcaserin is a 5-hydroxytryptamine 2C (serotonin) receptor agonist and a nongenotoxic rat carcinogen, which induced mammary tumors in male and female rats in a 2-yr bioassay. Female Sprague Dawley rats were treated by gavage daily with 0, 30, or 100 mg/kg lorcaserin, replicating bioassay dosing but for shorter duration, 12 or 24 wk. To characterize exposure and eliminate possible confounding by a potentially genotoxic degradation product, lorcaserin and N-nitroso-lorcaserin were quantified in dosing solutions, terminal plasma, mammary, and liver samples using ultra-high-performance liquid chromatography-electrospray tandem mass spectrometry. N-nitroso-lorcaserin was not detected, supporting lorcaserin classification as nongenotoxic carcinogen. Mammary DNA samples (n = 6/dose/timepoint) were used to synthesize PCR products from gene segments encompassing hotspot cancer driver mutations, namely regions of Apc, Braf, Egfr, Hras, Kras, Nfe2l2, Pik3ca, Setbp1, Stk11, and Tp53. Mutant fractions (MFs) in the amplicons were quantified by CarcSeq, an error-corrected next-generation sequencing approach. Considering all recovered mutants, no significant differences between lorcaserin dose groups were observed. However, significant dose-responsive increases in Pik3ca H1047R mutation were observed at both timepoints (ANOVA, P < 0.05), with greater numbers of mutants and mutants with greater MFs observed at 24 wk as compared with 12 wk. These observations suggest lorcaserin promotes outgrowth of spontaneously occurring Pik3ca H1047R mutant clones leading to mammary carcinogenesis. Importantly, this work reports approaches to analyze clonal expansion and demonstrates CarcSeq detection of the carcinogenic impact (selective Pik3ca H0147R mutant expansion) of a nongenotoxic carcinogen using a treatment duration as short as 3 months.

Navigating the complexity of PI3K/AKT pathway in HER-2 negative breast cancer: biomarkers and beyond

The results of the SOLAR-1 and CAPItello-291, highlight the benefit of the ɑ-selective phosphoinositide 3-Kinase Pathway inhibitor (PI3Ki) alpelisib and the AKT inhibitor (AKTi) capivasertib in patients with hormone receptor-positive (HR+)/Human Epidermal Growth Factor Receptor 2 (HER2)- negative metastatic breast cancer (mBC) that have PIK3CA/AKT1/PTEN tumour alterations. Although effective, these drugs are associated with significant toxicities, which often limit their use, particularly in frail patients. Following the recent incorporation of these agents into clinical practice, and with many others currently in development, significant challenges have emerged, particularly those regarding biomarkers for patient selection. This review will discuss biomarkers of response and their resistance to PI3K/AKT inhibitors (PI3K/AKTis) in HR+/HER- BC in early and advanced settings to ascertain which populations will most benefit from these drugs. Of the biomarkers that were analysed, such as PIK3CA, AKT, PTEN mutations, insulin levels, 18 F-FDG-PET/TC, only the PIK3CA-mutations (PIK3CA-mut) and the AKT pathway alterations seem to have a predictive value for treatments with alpelisib and capivasertib. However, due to the retrospective and exploratory nature of the study, the data did not provide conclusive results. In addition, the different methods used to detect PIK3CA/AKT1/PTEN alterations underline the fact that the optimal diagnostic companion has yet to be established. We have summarised the clinical data on the approved and discontinued agents targeting this pathway and have assessed the drugs development, successes, and failures. Finally, because of tumour heterogeneity, we emphasise the importance of reassessing the mutational status of PI3KCA in both metastatic tissue and blood at the time of disease progression to better tailor treatment for patients.

Advances in Immunocompetent Mouse and Rat Models

Rodent models of breast cancer have played critical roles in our understanding of breast cancer development and progression as well as preclinical testing of cancer prevention and therapeutics. In this article, we first review the values and challenges of conventional genetically engineered mouse (GEM) models and newer iterations of these models, especially those with inducible or conditional regulation of oncogenes and tumor suppressors. Then, we discuss nongermline (somatic) GEM models of breast cancer with temporospatial control, made possible by intraductal injection of viral vectors to deliver oncogenes or to manipulate the genome of mammary epithelial cells. Next, we introduce the latest development in precision editing of endogenous genes using in vivo CRISPR-Cas9 technology. We conclude with the recent development in generating somatic rat models for modeling estrogen receptor-positive breast cancer, something that has been difficult to accomplish in mice.

The mechanisms of class 1A PI3K and Wnt/β-catenin coupled signaling in breast cancer

The class IA PI3K signaling pathway is activated by growth factor stimulation and regulates a signaling cascade that promotes diverse events including cell growth, proliferation, migration and metabolism. PI3K signaling is one of the most commonly hyperactivated pathways in breast cancer, leading to increased tumor growth and progression. PI3K hyperactivation occurs via a number of genetic and epigenetic mechanisms including mutation or amplification of PIK3CA, the gene encoding the p110α subunit of PI3Kα, as well as via dysregulation of the upstream growth factor receptors or downstream signaling effectors. Over the past decade, extensive efforts to develop therapeutics that suppress oncogenic PI3K signaling have been undertaken. Although FDA-approved PI3K inhibitors are now emerging, their clinical success remains limited due to adverse effects and negative feedback mechanisms which contribute to their reduced efficacy. There is an emerging body of evidence demonstrating crosstalk between the PI3K and Wnt/β-catenin pathways in breast cancer. However, PI3K exhibits opposing effects on Wnt/β-catenin signaling in distinct tumor subsets, whereby PI3K promotes Wnt/β-catenin activation in ER+ cancers, but paradoxically suppresses this pathway in ER- breast cancers. This review discusses the molecular mechanisms for PI3K-Wnt crosstalk in breast cancer, and how Wnt-targeted therapies have the potential to contribute to treatment regimens for breast cancers with PI3K dysregulation.

The mutational landscape of a US Midwestern breast cancer cohort reveals subtype-specific cancer drivers and prognostic markers

BACKGROUND

Female breast cancer remains the second leading cause of cancer-related death in the USA. The heterogeneity in the tumor morphology across the cohort and within patients can lead to unpredictable therapy resistance, metastasis, and clinical outcome. Hence, supplementing classic pathological markers with intrinsic tumor molecular markers can help identify novel molecular subtypes and the discovery of actionable biomarkers.

METHODS

We conducted a large multi-institutional genomic analysis of paired normal and tumor samples from breast cancer patients to profile the complex genomic architecture of breast tumors. Long-term patient follow-up, therapeutic regimens, and treatment response for this cohort are documented using the Breast Cancer Collaborative Registry. The majority of the patients in this study were at tumor stage 1 (51.4%) and stage 2 (36.3%) at the time of diagnosis. Whole-exome sequencing data from 554 patients were used for mutational profiling and identifying cancer drivers.

RESULTS

We identified 54 tumors having at least 1000 mutations and 185 tumors with less than 100 mutations. Tumor mutational burden varied across the classified subtypes, and the top ten mutated genes include MUC4, MUC16, PIK3CA, TTN, TP53, NBPF10, NBPF1, CDC27, AHNAK2, and MUC2. Patients were classified based on seven biological and tumor-specific parameters, including grade, stage, hormone receptor status, histological subtype, Ki67 expression, lymph node status, race, and mutational profiles compared across different subtypes. Mutual exclusion of mutations in PIK3CA and TP53 was pronounced across different tumor grades. Cancer drivers specific to each subtype include TP53, PIK3CA, CDC27, CDH1, STK39, CBFB, MAP3K1, and GATA3, and mutations associated with patient survival were identified in our cohort.

CONCLUSIONS

This extensive study has revealed tumor burden, driver genes, co-occurrence, mutual exclusivity, and survival effects of mutations on a US Midwestern breast cancer cohort, paving the way for developing personalized therapeutic strategies.

Familial CCM Genes Might Not Be Main Drivers for Pathogenesis of Sporadic CCMs-Genetic Similarity between Cancers and Vascular Malformations

Cerebral cavernous malformations (CCMs) are abnormally dilated intracranial capillaries that form cerebrovascular lesions with a high risk of hemorrhagic stroke. Recently, several somatic "activating" gain-of-function (GOF) point mutations in PIK3CA (phosphatidylinositol-4, 5-bisphosphate 3-kinase catalytic subunit p110α) were discovered as a dominant mutation in the lesions of sporadic forms of cerebral cavernous malformation (sCCM), raising the possibility that CCMs, like other types of vascular malformations, fall in the PIK3CA-related overgrowth spectrum (PROS). However, this possibility has been challenged with different interpretations. In this review, we will continue our efforts to expound the phenomenon of the coexistence of gain-of-function (GOF) point mutations in the PIK3CA gene and loss-of-function (LOF) mutations in CCM genes in the CCM lesions of sCCM and try to delineate the relationship between mutagenic events with CCM lesions in a temporospatial manner. Since GOF PIK3CA point mutations have been well studied in reproductive cancers, especially breast cancer as a driver oncogene, we will perform a comparative meta-analysis for GOF PIK3CA point mutations in an attempt to demonstrate the genetic similarities shared by both cancers and vascular anomalies.

Contributions of nanotechnology to the intraductal drug delivery for local treatment and prevention of breast cancer

Breast cancer is a major public health problem, affecting millions of people. It is a very heterogeneous disease, with localized and invasive forms, and treatment generally consists of a combination of surgery and radiotherapy followed by administration of estrogen receptor modulators or aromatase inhibitors. Given its heterogeneity, management strategies that take into consideration the type of disease and biological markers and can provide more personalized and local treatment are required. More recently, the intraductal administration (i.e., into the breast ducts) of drugs has attracted significant attention due to its ability of providing drug distribution through the ductal tree in a minimally invasive manner. Although promising, intraductal administration is not trivial, and difficulties in duct identification and cannulation are important challenges to the further development of this route. New drug delivery strategies such as nanostructured systems can help to achieve the full benefits of the route due to the possibility of prolonging tissue retention, improving targeting and selectivity, increasing cytotoxicity and reducing the frequency of administration. This review aims at discussing the potential benefits and challenges of intraductal administration, focusing on the design and use of nanocarriers as innovative and feasible strategies for local breast cancer therapy and prevention.

Functional and Phenotypic Characterisations of Common Syngeneic Tumour Cell Lines as Estrogen Receptor-Positive Breast Cancer Models

Estrogen receptor-positive breast cancers (ER+ BCas) are the most common form of BCa and are increasing in incidence, largely due to changes in reproductive practices in recent decades. Tamoxifen is prescribed as a component of standard-of-care endocrine therapy for the treatment and prevention of ER+ BCa. However, it is poorly tolerated, leading to low uptake of the drug in the preventative setting. Alternative therapies and preventatives for ER+ BCa are needed but development is hampered due to a paucity of syngeneic ER+ preclinical mouse models that allow pre-clinical experimentation in immunocompetent mice. Two ER-positive models, J110 and SSM3, have been reported in addition to other tumour models occasionally shown to express ER (for example 4T1.2, 67NR, EO771, D2.0R and D2A1). Here, we have assessed ER expression and protein levels in seven mouse mammary tumour cell lines and their corresponding tumours, in addition to their cellular composition, tamoxifen sensitivity and molecular phenotype. By immunohistochemical assessment, SSM3 and, to a lesser extent, 67NR cells are ER+. Using flow cytometry and transcript expression we show that SSM3 cells are luminal in nature, whilst D2.0R and J110 cells are stromal/basal. The remainder are also stromal/basal in nature; displaying a stromal or basal Epcam/CD49f FACS phenotype and stromal and basal gene expression signatures are overrepresented in their transcript profile. Consistent with a luminal identity for SSM3 cells, they also show sensitivity to tamoxifen in vitro and in vivo. In conclusion, the data indicate that the SSM3 syngeneic cell line is the only definitively ER+ mouse mammary tumour cell line widely available for pre-clinical research.

Short-term PI3K Inhibition Prevents Breast Cancer in Preclinical Models

Antiestrogen medication is the only chemoprevention currently available for women at a high risk of developing breast cancer; however, antiestrogen therapy requires years to achieve efficacy and has adverse side effects. Therefore, it is important to develop an efficacious chemoprevention strategy that requires only a short course of treatment. PIK3CA is commonly activated in breast atypical hyperplasia, the known precancerous precursor of breast cancer. Targeting PI3K signaling in these precancerous lesions may offer a new strategy for chemoprevention. Here, we first established a mouse model that mimics the progression from precancerous lesions to breast cancer. Next, we demonstrated that a short-course prophylactic treatment with the clinically approved PI3K inhibitor alpelisib slowed early lesion expansion and prevented cancer formation in this model. Furthermore, we showed that alpelisib suppressed ex vivo expansion of patient-derived atypical hyperplasia. Together, these data indicate that the progression of precancerous breast lesions heavily depends on the PI3K signaling, and that prophylactic targeting of PI3K activity can prevent breast cancer.

PREVENTION RELEVANCE

PI3K protein is abnormally high in breast precancerous lesions. This preclinical study demonstrates that the FDA-approved anti-PI3K inhibitor alpelisib can prevent breast cancer and thus warrant future clinical trials in high-risk women.

The FDA-Approved Drug Pyrvinium Selectively Targets ER+ Breast Cancer Cells with High INPP4B Expression

The majority of breast cancers are estrogen receptor-positive (ER⁺), and endocrine therapies that suppress ER signaling are the standard-of-care treatment for this subset. However, up to half of all ER⁺ cancers eventually relapse, highlighting a need for improved clinical therapies. The phosphoinositide phosphatase, INPP4B, is overexpressed in almost half of all ER⁺ breast cancers, and promotes Wnt/β-catenin signaling, cell proliferation and tumor growth. Here, using cell viability assays, we report that INPP4B overexpression does not affect the sensitivity of ER⁺ breast cancer cells to standard-of-care treatments including the anti-estrogen 4-hydroxytamoxifen (4-OHT) or the PI3Kα inhibitor alpelisib. Examination of four small molecule Wnt inhibitors revealed that ER⁺ breast cancer cells with INPP4B overexpression were more sensitive to the FDA-approved drug pyrvinium and a 4-OHT-pyrvinium combination treatment. Using 3D culture models, we demonstrated that pyrvinium selectively reduced the size of INPP4B-overexpressing ER⁺ breast cancer spheroids in the presence and absence of 4-OHT. These findings suggest that repurposing pyrvinium as a Wnt inhibitor may be an effective therapeutic strategy for human ER⁺ breast cancers with high INPP4B levels.

Spatiotemporal control of ERK pulse frequency coordinates fate decisions during mammary acinar morphogenesis

The signaling events controlling proliferation, survival, and apoptosis during mammary epithelial acinar morphogenesis remain poorly characterized. By imaging single-cell ERK activity dynamics in MCF10A acini, we find that these fates depend on the average frequency of non-periodic ERK pulses. High pulse frequency is observed during initial acinus growth, correlating with rapid cell motility and proliferation. Subsequent decrease in motility correlates with lower ERK pulse frequency and quiescence. Later, during lumen formation, coordinated multicellular ERK waves emerge, correlating with high and low ERK pulse frequencies in outer surviving and inner dying cells, respectively. Optogenetic entrainment of ERK pulses causally connects high ERK pulse frequency with inner cell survival. Acini harboring the PIK3CA H1047R mutation display increased ERK pulse frequency and inner cell survival. Thus, fate decisions during acinar morphogenesis are coordinated by different spatiotemporal modalities of ERK pulse frequency.

Generation and functional characterization of murine mammary organoids

3D cultures of mammary epithelial cells purified from murine models provide a unique resource to study genetically defined breast cancer and response to targeted therapies. Here, we describe step-by-step experimental procedures for the successful establishment of murine mammary organoid lines isolated from mammary glands or mammary tumors driven by mutations in components of the PI3K pathway. These detailed protocols also include procedures to perform assays that can be adopted to screen response to drug treatments and to inform better therapies. For details on potential applications and use of this protocol, please refer to Yip et al. (2020).

INPP4B promotes PI3Kα-dependent late endosome formation and Wnt/β-catenin signaling in breast cancer

INPP4B suppresses PI3K/AKT signaling by converting PI(3,4)P₂ to PI(3)P and INPP4B inactivation is common in triple-negative breast cancer. Paradoxically, INPP4B is also a reported oncogene in other cancers. How these opposing INPP4B roles relate to PI3K regulation is unclear. We report PIK3CA-mutant ER⁺ breast cancers exhibit increased INPP4B mRNA and protein expression and INPP4B increased the proliferation and tumor growth of PIK3CA-mutant ER⁺ breast cancer cells, despite suppression of AKT signaling. We used integrated proteomics, transcriptomics and imaging to demonstrate INPP4B localized to late endosomes via interaction with Rab7, which increased endosomal PI3Kα-dependent PI(3,4)P₂ to PI(3)P conversion, late endosome/lysosome number and cargo trafficking, resulting in enhanced GSK3β lysosomal degradation and activation of Wnt/β-catenin signaling. Mechanistically, Wnt inhibition or depletion of the PI(3)P-effector, Hrs, reduced INPP4B-mediated cell proliferation and tumor growth. Therefore, INPP4B facilitates PI3Kα crosstalk with Wnt signaling in ER⁺ breast cancer via PI(3,4)P₂ to PI(3)P conversion on late endosomes, suggesting these tumors may be targeted with combined PI3K and Wnt/β-catenin therapies.

Intraductal Injection of Lentivirus Vectors for Stably Introducing Genes into Rat Mammary Epithelial Cells in Vivo

Various retroviral and lentiviral vectors have been used for up-the-teat intraductal injection to deliver markers, oncogenes, and other genes into mammary epithelial cells in mice. These methods along with the large number of genetically engineered mouse lines have greatly helped us learn normal breast development and tumorigenesis. Rats are also valuable models for studying human breast development and cancer. However, genetically engineered rats are still uncommon, and previous reports of intraductal injection of retroviral vectors into rats appear to be inefficient in generating mammary tumors. Here, we report, and describe the method for, stably introducing marker genes and oncogenes into mammary glands in rats using intraductal injection of commonly used lentiviral vectors. This method can infect mammary epithelial cells efficiently, and the infected cells can initiate tumorigenesis, including estrogen receptor-positive and hormone-dependent tumors, which are the most common subtype of human breast cancer but are yet still difficult to model in mice. This technique provides another tool for studying formation, prevention, and treatment of breast cancer, especially estrogen receptor-positive breast cancer.

Control of Glucocorticoid Receptor Levels by PTEN Establishes a Failsafe Mechanism for Tumor Suppression

The PTEN tumor suppressor controls cell death and survival by regulating functions of various molecular targets. While the role of PTEN lipid-phosphatase activity on PtdIns(3,4,5)P3 and inhibition of PI3K pathway is well characterized, the biological relevance of PTEN protein-phosphatase activity remains undefined. Here, using knockin (KI) mice harboring cancer-associated and functionally relevant missense mutations, we show that although loss of PTEN lipid-phosphatase function cooperates with oncogenic PI3K to promote rapid mammary tumorigenesis, the additional loss of PTEN protein-phosphatase activity triggered an extensive cell death response evident in early and advanced mammary tumors. Omics and drug-targeting studies revealed that PI3Ks act to reduce glucocorticoid receptor (GR) levels, which are rescued by loss of PTEN protein-phosphatase activity to restrain cell survival. Thus, we find that the dual regulation of GR by PI3K and PTEN functions as a rheostat that can be exploited for the treatment of PTEN loss-driven cancers.

Parity reduces mammary repopulating activity but does not affect mammary stem cells defined as CD24 + CD29/CD49fhi in mice

BACKGROUND

Breast cancer (BCa) mortality is decreasing with early detection and improvement in therapies. The incidence of BCa, however, continues to increase, particularly estrogen-receptor-positive (ER +) subtypes. One of the greatest modifiers of ER + BCa risk is childbearing (parity), with BCa risk halved in young multiparous mothers. Despite convincing epidemiological data, the biology that underpins this protection remains unclear. Parity-induced protection has been postulated to be due to a decrease in mammary stem cells (MaSCs); however, reports to date have provided conflicting data.

METHODS

We have completed rigorous functional testing of repopulating activity in parous mice using unfractionated and MaSC (CD24^midCD49f^hi)-enriched populations. We also developed a novel serial transplant method to enable us to assess self-renewal of MaSC following pregnancy. Lastly, as each pregnancy confers additional BCa protection, we subjected mice to multiple rounds of pregnancy to assess whether additional pregnancies impact MaSC activity.

RESULTS

Here, we report that while repopulating activity in the mammary gland is reduced by parity in the unfractionated gland, it is not due to a loss in the classically defined MaSC (CD24⁺CD49f^hi) numbers or function. Self-renewal was unaffected by parity and additional rounds of pregnancy also did not lead to a decrease in MaSC activity.

CONCLUSIONS

Our data show instead that parity impacts on the stem-like activity of cells outside the MaSC population.

PIK3CA and p53 Mutations Promote 4NQO-Initated Head and Neck Tumor Progression and Metastasis in Mice

The PI3K signaling pathway is frequently mutated in head and neck squamous cell carcinoma (HNSCC), often via gain-of-function (GOF) mutations in the PIK3CA gene. Here, we present novel genetically engineered mouse models (GEMM) carrying a GOF allele Loxp-STOP-Loxp(LSL)-PIK3CAH1047R (E20) alone or in combination with heterozygous LSL-p53+/R172H (p53) mutation with tissue-specific expression to interrogate the role of oncogenic PIK3CA in transformation of upper aerodigestive track epithelium. We demonstrated that the GOF PIK3CA mutation promoted progression of 4-nitroquinoline 1-oxide-induced oral squamous cell carcinoma (OSCC) in both E20 single mutant and E20/p53 double mutant mice, with frequent distal metastasis detected only in E20/p53 GEMM. Similar to in human OSCC, loss of p16 was associated with progression of OSCC in these mice. RNA-seq analyses revealed that among the common genes differentially expressed in primary OSCC cell lines derived from E20, p53, and E20/p53 GEMMs compared with those from the wild-type mice, genes associated with proliferation and cell cycle were predominantly represented, which is consistent with the progressive loss of p16 detected in these GEMMs. Importantly, all of these OSCC primary cell lines exhibited enhanced sensitivity to BYL719 and cisplatin combination treatment in comparison with cisplatin alone in vitro and in vivo, regardless of p53 and/or p16 status. Given the prevalence of mutations in p53 and the PI3K pathways in HNSCC in conjunction with loss of p16 genetically or epigenetically, this universal increased sensitivity to cisplatin and BYL719 combination therapy in cancer cells with PIK3CA mutation represents an opportunity to a subset of patients with HNSCC. IMPLICATIONS: Our results suggest that combination therapy of cisplatin and PI3K inhibitor may be worthy of consideration in patients with HNSCC with PIK3CA mutation.

Embryonic Barcoding of Equipotent Mammary Progenitors Functionally Identifies Breast Cancer Drivers

Identification of clinically relevant drivers of breast cancers in intact mammary epithelium is critical for understanding tumorigenesis yet has proven challenging. Here, we show that intra-amniotic lentiviral injection can efficiently transduce progenitor cells of the adult mammary gland and use that as a platform to functionally screen over 500 genetic lesions for functional roles in tumor formation. Targeted progenitors establish long-term clones of both luminal and myoepithelial lineages in adult animals, and via lineage tracing with stable barcodes, we found that each mouse mammary gland is generated from a defined number of ∼120 early progenitor cells that expand uniformly with equal growth potential. We then designed an in vivo screen to test genetic interactions in breast cancer and identified candidates that drove not only tumor formation but also molecular subtypes. Thus, this methodology enables rapid and high-throughput cancer driver discovery in mammary epithelium.

S100a4 upregulation in Pik3caH1047R;Trp53R270H;MMTV-Cre-driven mammary tumors promotes metastasis

Background

PIK3CA mutations are frequent in human breast cancer. Pik3caH1047R mutant expression in mouse mammary gland promotes tumorigenesis. TP53 mutations co-occur with PIK3CA mutations in human breast cancers. We previously generated a conditionally activatable Pik3caH1047R;MMTV-Cre mouse model and found a few malignant sarcomatoid (spindle cell) carcinomas that had acquired spontaneous dominant-negative Trp53 mutations.

Methods

A Pik3caH1047R;Trp53R270H;MMTV-Cre double mutant mouse breast cancer model was generated. Tumors were characterized by histology, marker analysis, transcriptional profiling, single-cell RNA-seq, and bioinformatics. Cell lines were developed from mutant tumors and used to identify and confirm genes involved in metastasis.

Results

We found Pik3caH1047R and Trp53R270H cooperate in driving oncogenesis in mammary glands leading to a shorter latency than either alone. Double mutant mice develop multiple histologically distinct mammary tumors, including adenocarcinoma and sarcomatoid (spindle cell) carcinoma. We found some tumors to be invasive and a few metastasized to the lung and/or the lymph node. Single-cell RNA-seq analysis of the tumors identified epithelial, stromal, myeloid, and T cell groups. Expression analysis of the metastatic tumors identified S100a4 as a top candidate gene associated with metastasis. Metastatic tumors contained a much higher percentage of epithelial–mesenchymal transition (EMT)-signature positive and S100a4-expressing cells. CRISPR/CAS9-mediated knockout of S100a4 in a metastatic tumor-derived cell line disrupted its metastatic potential indicating a role for S100a4 in metastasis.

Conclusions

Pik3caH1047R;Trp53R270H;MMTV-Cre mouse provides a preclinical model to mimic a subtype of human breast cancers that carry both PIK3CA and TP53 mutations. It also allows for understanding the cooperation between the two mutant genes in tumorigenesis. Our model also provides a system to study metastasis and develop therapeutic strategies for PIK3CA/TP53 double-positive cancers. S100a4 found involved in metastasis in this model can be a potential diagnostic and therapeutic target.

Genetic alterations of epithelial polarity genes are associated with loss of polarity in invasive breast cancer

Breast cancer remains a leading cause of cancer-related death for women. The stepwise development of breast cancer through preinvasive to invasive disease is associated with progressive disruption of cellular and tissue organization. Apical-basal polarity is thought to be a barrier to breast cancer development, but the extent and potential mechanisms that contribute to disrupted polarity are incompletely understood. To investigate the cell polarity status of invasive breast cancers, we performed multiplex imaging of polarity markers on tissue cores from 432 patients from a spectrum of grades, stages and molecular subtypes. Apical-basal cell polarity was lost in 100% of cells in all cases studied, indicating that loss of epithelial polarity may be a universal feature of invasive breast cancer. We then analyzed genomic events from the TCGA dataset for an 18-gene set of core polarity genes. Coamplification of polarity genes with established breast oncogenes was found, which is consistent with functional cooperation within signaling amplicons. Gene-expression levels of several polarity genes were significantly associated with survival, and protein localization of Par6 correlated with higher grade, nodal metastasis and molecular subtype. Finally, multiple hotspot mutations in protein-protein interaction domains critical for cell polarity were identified. Our data indicate that genomic events likely contribute to pervasive disruption of epithelial polarity observed in invasive breast cancer.

Association of Somatic Driver Alterations With Prognosis in Postmenopausal, Hormone Receptor-Positive, HER2-Negative Early Breast Cancer: A Secondary Analysis of the BIG 1-98 Randomized Clinical Trial

Importance

A range of somatic driver alterations has been described in estrogen receptor-positive, HER2-negative (ER+/HER2-) early breast cancer (BC); however, the clinical relevance is unknown.

Objective

To investigate associations of driver alterations with prognosis and the role of PIK3CA mutations in prediction of benefit associated with endocrine therapy in postmenopausal patients with ER+/HER2- early BC treated with tamoxifen or letrozole.

Design, Setting, and Participants

The Breast International Group (BIG) 1-98 trial randomized 8010 postmenopausal patients with hormone receptor-positive, operable, invasive BC to monotherapy with letrozole, tamoxifen, or a sequential strategy for 5 years. Driver alterations were characterized using next-generation sequencing in primary tumors from a subset of 764 patients from 7329 eligible patients with ER+/HER2- BC, with 841 distant recurrences after a median of 8.1 years of follow-up. To correct for the oversampling of distant recurrences, weighted analysis methods were used. This analysis was conducted from April 4, 2016, to November 30, 2016.

Main Outcomes and Measures

The prevalence of driver alterations, associations with clinicopathologic factors, distant recurrence-free interval, and treatment interactions were analyzed. Multivariable analyses were performed to adjust for clinicopathologic factors.

Results

Of 764 samples, 538 (70.4%), including 140 distant recurrence events, were successfully sequenced. Nineteen driver alterations were observed with 5% or greater frequency, with a mean of 4 alterations (range, 0-15) per tumor. PIK3CA mutations were the most common (49%) and were significantly associated with reduction in the risk for distant recurrence (hazard ratio [HR], 0.57; 95% CI, 0.38-0.85; P = .006). TP53 mutations (HR, 1.92; 95% CI, 1.21-3.04; P = .006), amplifications on 11q13 (HR, 2.14; 95% CI, 1.36-3.37; P = .001) and 8p11 (HR, 3.02; 95% CI, 1.88-4.84; P < .001), and increasing number of driver alterations (HR per additional alteration, 1.18; 95% CI, 1.11-1.25; P < .001) were associated with significantly greater risk. Amplifications on 11q13 and 8p11 remained significant predictors in multivariable analysis, but not PIK3CA and TP53 mutations. Patients with tumors harboring kinase or helical domain PIK3CA mutations derived significantly greater benefit from letrozole over tamoxifen than patients whose tumors did not (P interaction = .002).

Conclusions and Relevance

In ER+/HER2- postmenopausal, early-stage BC, amplifications on 11q13 and 8p11 were significantly associated with increased risk for distant recurrence and PIK3CA mutations were predictive of greater magnitude of benefit from letrozole. With these findings, DNA-based classification may aid adjuvant treatment decision making in this setting.

Trial Registration

ClinicalTrials.gov Identifier: NCT00004205.

Oncogenic Roles of the PI3K/AKT/mTOR Axis

The PI3K/AKT/mTOR pathway is frequently activated in various human cancers and has been considered a promising therapeutic target. Many of the positive regulators of the PI3K/AKT/mTOR axis, including the catalytic (p110α) and regulatory (p85α), of class IA PI3K, AKT, RHEB, mTOR, and eIF4E, possess oncogenic potentials, as demonstrated by transformation assays in vitro and by genetically engineered mouse models in vivo. Genetic evidences also indicate their roles in malignancies induced by activation of the upstream oncoproteins including receptor tyrosine kinases and RAS and those induced by the loss of the negative regulators of the PI3K/AKT/mTOR pathway such as PTEN, TSC1/2, LKB1, and PIPP. Possible mechanisms by which the PI3K/AKT/mTOR axis contributes to oncogenic transformation include stimulation of proliferation, survival, metabolic reprogramming, and invasion/metastasis, as well as suppression of autophagy and senescence. These phenotypic changes are mediated by eIF4E-induced translation of a subset of mRNAs and by other downstream effectors of mTORC1 including S6K, HIF-1α, PGC-1α, SREBP, and ULK1 complex.

Oncogenic PIK3CA promotes cellular stemness in an allele dose-dependent manner

The PIK3CA gene, which encodes the p110α catalytic subunit of PI3 kinase (PI3K), is mutationally activated in cancer and in overgrowth disorders known as PIK3CA-related overgrowth spectrum (PROS). To determine the consequences of genetic PIK3CA activation in a developmental context of relevance to both PROS and cancer, we engineered isogenic human induced pluripotent stem cells (iPSCs) with heterozygous or homozygous knockin of PIK3CAH1047R While heterozygous iPSCs remained largely similar to wild-type cells, homozygosity for PIK3CAH1047R caused widespread, cancer-like transcriptional remodeling, partial loss of epithelial morphology, up-regulation of stemness markers, and impaired differentiation to all three germ layers in vitro and in vivo. Genetic analysis of PIK3CA-associated cancers revealed that 64% had multiple oncogenic PIK3CA copies (39%) or additional PI3K signaling pathway-activating "hits" (25%). This contrasts with the prevailing view that PIK3CA mutations occur heterozygously in cancer. Our findings suggest that a PI3K activity threshold determines pathological consequences of oncogenic PIK3CA activation and provide insight into the specific role of this pathway in human pluripotent stem cells.

Mouse Models for Exploring the Biological Consequences and Clinical Significance of PIK3CA Mutations

The phosphatidylinositol 3-kinase (PI3K) pathway is involved in a myriad of cellular signalling pathways that regulate cell growth, metabolism, proliferation and survival. As a result, alterations in the PI3K pathway are frequently associated with human cancers. Indeed, PIK3CA-the gene encoding the p110α catalytic subunit of PI3K-is one of the most commonly mutated human oncogenes. PIK3CA mutations have also been implicated in non-malignant conditions including congenital overgrowth syndromes and vascular malformations. In order to study the role of PIK3CA mutations in driving tumorigenesis and tissue overgrowth and to test potential therapeutic interventions for these conditions, model systems are essential. In this review we discuss the various mouse models currently available for preclinical studies into the biological consequences and clinical significance of PIK3CA mutations.

The PTEN⁻PI3K Axis in Cancer

The PI3K-AKT-mTOR signal transduction pathway regulates a variety of biological processes including cell growth, cell cycle progression and proliferation, cellular metabolism, and cytoskeleton reorganization. Fine-tuning of the phosphatidylinositol 3-kinase (PI3K) pathway signaling output is essential for the maintenance of tissue homeostasis and uncontrolled activation of this cascade leads to a number of human pathologies including cancer. Inactivation of the tumor suppressor phosphatase and tensin homologue deleted on Chromosome 10 (PTEN) and/or activating mutations in the proto-typical lipid kinase PI3K have emerged as some of the most frequent events associated with human cancer and as a result the PI3K pathway has become a highly sought-after target for cancer therapies. In this review we summarize the essential role of the PTEN-PI3K axis in controlling cellular behaviors by modulating activation of key proto-oncogenic molecular nodes and functional targets. Further, we highlight important functional redundancies and peculiarities of these two critical enzymes that over the last few decades have become a central part of the cancer research field and have instructed hundreds of pre-clinical and clinical trials to better cancer treatments.

Challenges for the Clinical Development of PI3K Inhibitors: Strategies to Improve Their Impact in Solid Tumors

The PI3K pathway is mutated and aberrantly activated in many cancers and plays a central role in tumor cell proliferation and survival, making it a rational therapeutic target. Until recently, however, results from clinical trials with PI3K inhibitors in solid tumors have been largely disappointing. Here, we describe several factors that have limited the success of these agents, including the weak driver oncogenic activity of mutant PI3K, suboptimal patient selection in trials, drug-related toxicities, feedback upregulation of compensatory mechanisms when PI3K is blocked, increased insulin production upon PI3Kα inhibition, lack of mutant-specific inhibitors, and a relative scarcity of studies using combinations with PI3K antagonists. We also suggest strategies to improve the impact of these agents in solid tumors. Despite these challenges, we are optimistic that isoform-specific PI3K inhibitors, particularly in combination with other agents, may be valuable in treating appropriately selected patients with PI3K-dependent tumors. SIGNIFICANCE: Despite the modest clinical activity of PI3K inhibitors in solid tumors, there is an increasing understanding of the factors that may have limited their success. Strategies to ameliorate drug-related toxicities, use of rational combinations with PI3K antagonists, development of mutant-selective PI3K inhibitors, and better patient selection should improve the success of these targeted agents against solid tumors.

Mouse ER+/PIK3CAH1047R breast cancers caused by exogenous estrogen are heterogeneously dependent on estrogen and undergo BIM-dependent apoptosis with BH3 and PI3K agents

Estrogen dependence is major driver of ER + breast cancer, which is associated with PI3K mutation. PI3K inhibition (PI3Ki) can restore dependence on ER signaling for some hormone therapy-resistant ER + breast cancers, but is ineffective in others. Here we show that short-term supplementation with estrogen strongly enhanced Pik3caH1047R-induced mammary tumorigenesis in mice that resulted exclusively in ER + tumors, demonstrating the cooperation of the hormone and the oncogene in tumor development. Similar to human ER + breast cancers that are endocrine-dependent or endocrine-independent at diagnosis, tumor lines from this model retained ER expression but were sensitive or resistant to hormonal therapies. PI3Ki did not induce cell death but did cause upregulation of the pro-apoptotic gene BIM. BH3 mimetics or PI3Ki were unable to restore hormone sensitivity in several resistant mouse and human tumor lines. Importantly however, combination of PI3Ki and BH3 mimetics had a profound, BIM-dependent cytotoxic effect in PIK3CA-mutant cancer cells while sparing normal cells. We propose that addition of BH3 mimetics offers a therapeutic strategy to markedly improve the cytotoxic activity of PI3Ki in hormonal therapy-resistant and ER-independent PIK3CA-mutant breast cancer.

Oncogenic activation of PI3K induces progenitor cell differentiation to suppress epidermal growth

Oncogenic lesions are surprisingly common in morphologically and functionally normal human skin, however, the cellular and molecular mechanisms that suppress their cancer-driving potential to maintain tissue homeostasis are unknown. By employing assays for direct and quantitative assessment of cell fate choices in vivo, we show that oncogenic activation of PI3K/AKT, the most commonly activated oncogenic pathway in cancer, promotes differentiation and cell-cycle exit of epidermal progenitors. As a result, oncogenic PI3K/AKT activated epidermis exhibits growth disadvantage even though its cells are more proliferative. To uncover the underlying mechanism behind oncogene-induced differentiation, we conduct a series of genetic screens in vivo, and identify an AKT substrate SH3RF1 as a specific promoter of epidermal differentiation that has no effect on proliferation. Our study provides evidence for a direct, cell autonomous mechanism that can suppresses progenitor cell renewal and block clonal expansion of epidermal cells bearing a common and activating mutation in Pik3ca.

Cancer-Associated PIK3CA Mutations in Overgrowth Disorders

PIK3CA is one of the most commonly mutated genes in solid cancers. PIK3CA mutations are also found in benign overgrowth syndromes, collectively known as PIK3CA-related overgrowth spectrum (PROS). As in cancer, PIK3CA mutations in PROS arise postzygotically, but unlike in cancer, these mutations arise during embryonic development, with their timing and location critically influencing the resulting disease phenotype. Recent evidence indicates that phosphoinositide 3-kinase (PI3K) pathway inhibitors undergoing trials in cancer can provide a therapy for PROS. Conversely, PROS highlights gaps in our understanding of PI3K's role during embryogenesis and in cancer development. Here, we summarize current knowledge of PROS, evaluate challenges and strategies for disease modeling, and consider the implications of PROS as a paradigm for understanding activating PIK3CA mutations in human development and cancer.

P-Rex1 is dispensable for Erk activation and mitogenesis in breast cancer

Phosphatidylinositol-3,4,5-Trisphosphate Dependent Rac Exchange Factor 1 (P-Rex1) is a key mediator of growth factor-induced activation of Rac1, a small GTP-binding protein widely implicated in actin cytoskeleton reorganization. This Guanine nucleotide Exchange Factor (GEF) is overexpressed in human luminal breast cancer, and its expression associates with disease progression, metastatic dissemination and poor outcome. Despite the established contribution of P-Rex1 to Rac activation and cell locomotion, whether this Rac-GEF has any relevant role in mitogenesis has been a subject of controversy. To tackle the discrepancies among various reports, we carried out an exhaustive analysis of the potential involvement of P-Rex1 on the activation of the mitogenic Erk pathway. Using a range of luminal breast cancer cellular models, we unequivocally showed that silencing P-Rex1 (transiently, stably, using multiple siRNA sequences) had no effect on the phospho-Erk response upon stimulation with growth factors (EGF, heregulin, IGF-I) or a GPCR ligand (SDF-1). The lack of involvement of P-Rex1 in Erk activation was confirmed at the single cell level using a fluorescent biosensor of Erk kinase activity. Depletion of P-Rex1 from breast cancer cells failed to affect cell cycle progression, cyclin D1 induction, Akt activation and apoptotic responses. In addition, mammary-specific P-Rex1 transgenic mice (MMTV-P-Rex1) did not show any obvious hyperproliferative phenotype. Therefore, despite its crucial role in Rac1 activation and cell motility, P-Rex1 is dispensable for mitogenic or survival responses in breast cancer cells.

Tumor PIK3CA Genotype and Prognosis in Early-Stage Breast Cancer: A Pooled Analysis of Individual Patient Data

Purpose Phosphatidylinositol-4, 5-bisphosphate 3-kinase catalytic subunit alpha ( PIK3CA) mutations are frequently observed in primary breast cancer. We evaluated their prognostic relevance by performing a pooled analysis of individual patient data. Patients and Methods Associations between PIK3CA status and clinicopathologic characteristics were tested by applying Cox regression models adjusted for age, tumor size, nodes, grade, estrogen receptor (ER) status, human epidermal growth factor receptor 2 (HER2) status, treatment, and study. Invasive disease-free survival (IDFS) was the primary end point; distant disease-free survival (DDFS) and overall survival (OS) were also assessed, overall and by breast cancer subtypes. Results Data from 10,319 patients from 19 studies were included (median OS follow-up, 6.9 years); 1,787 patients (17%) received chemotherapy, 4,036 (39%) received endocrine monotherapy, 3,583 (35%) received both, and 913 (9%) received none or their treatment was unknown. PIK3CA mutations occurred in 32% of patients, with significant associations with ER positivity, increasing age, lower grade, and smaller size (all P < .001). Prevalence of PIK3CA mutations was 18%, 22%, and 37% in the ER-negative/HER2-negative, HER2-positive, and ER-positive/HER2-negative subtypes, respectively. In univariable analysis, PIK3CA mutations were associated with better IDFS (HR, 0.77; 95% CI, 0.71 to 0.84; P < .001), with evidence for a stronger effect in the first years of follow-up (0 to 5 years: HR, 0.73; 95% CI, 0.66 to 0.81; P < .001; 5 to 10 years: HR, 0.82; 95% CI, 0.68 to 0.99; P = .037); > 10 years: (HR, 1.15; 95% CI, 0.84 to 1.58; P = .38; P heterogeneity = .02). In multivariable analysis, PIK3CA genotype remained significant for improved IDFS ( P = .043), but not for the DDFS and OS end points. Conclusion In this large pooled analysis, PIK3CA mutations were significantly associated with a better IDFS, DDFS, and OS, but had a lesser prognostic effect after adjustment for other prognostic factors.

Oncogenic PIK3CA induces centrosome amplification and tolerance to genome doubling

Mutations in PIK3CA are very frequent in cancer and lead to sustained PI3K pathway activation. The impact of acute expression of mutant PIK3CA during early stages of malignancy is unknown. Using a mouse model to activate the Pik3ca H1047R hotspot mutation in the heterozygous state from its endogenous locus, we here report that mutant Pik3ca induces centrosome amplification in cultured cells (through a pathway involving AKT, ROCK and CDK2/Cyclin E-nucleophosmin) and in mouse tissues, and increased in vitro cellular tolerance to spontaneous genome doubling. We also present evidence that the majority of PIK3CA H1047R mutations in the TCGA breast cancer cohort precede genome doubling. These previously unappreciated roles of PIK3CA mutation show that PI3K signalling can contribute to the generation of irreversible genomic changes in cancer. While this can limit the impact of PI3K-targeted therapies, these findings also open the opportunity for therapeutic approaches aimed at limiting tumour heterogeneity and evolution.

PI3K/AKT/mTOR: role in breast cancer progression, drug resistance, and treatment

Anti-cancer cancer-targeted therapies are designed to exploit a particular vulnerability in the tumor, which in most cases results from its dependence on an oncogene and/or loss of a tumor suppressor. Mutations in the phosphoinositide 3-kinase (PI3K)/AKT/mTOR pathway are freqcuently found in breast cancers and associated with cellular transformation, tumorigenesis, cancer progression, and drug resistance. Several drugs targeting PI3K/ATK/mTOR are currently in clinical trials, mainly in combination with endocrine therapy and anti-HER2 therapy. These drugs are the focus of this review.

Inhibitors of STAT3, β-catenin, and IGF-1R sensitize mouse PIK3CA-mutant breast cancer to PI3K inhibitors

Although mutations in the phosphoinositide 3‐kinase catalytic subunit (PIK3CA) are common in breast cancer, PI3K inhibitors alone have shown modest efficacy. We sought to identify additional pathways altered in PIK3CA‐mutant tumors that might be targeted in combination with PI3K inhibitors. We generated two transgenic mouse models expressing the human PIK3CA‐H1047R‐ and the ‐E545K hotspot‐mutant genes in the mammary gland and evaluated their effects on development and tumor formation. Molecular analysis identified pathways altered in these mutant tumors, which were also targeted in multiple cell lines derived from the PIK3CA tumors. Finally, public databases were analyzed to determine whether novel pathways identified in the mouse tumors were altered in human tumors harboring mutant PIK3CA. Mutant mice showed increased branching and delayed involution of the mammary gland compared to parental FVB/N mice. Mammary tumors arose in 30% of the MMTV‐PIK3CA‐H1047R and in 13% of ‐E545K mice. Compared to MMTV‐Her‐2 transgenic mouse mammary tumors, H1047R tumors showed increased upregulation of Wnt/β‐catenin/Axin2, hepatocyte growth factor (Hgf)/Stat3, insulin‐like growth factor 2 (Igf‐2), and Igf‐1R pathways. Inhibitors of STAT3, β‐catenin, and IGF‐1R sensitized H1047R‐derived mouse tumor cells and PIK3CA‐H1047R overexpressing human HS578T breast cancer cells to the cytotoxic effects of PI3K inhibitors. Analysis of The Cancer Genome Atlas database showed that, unlike primary PIK3CA‐wild‐type and HER‐2⁺ breast carcinomas, PIK3CA‐mutant tumors display increased expression of AXIN2, HGF, STAT3, IGF‐1, and IGF‐2 mRNA and activation of AKT, IGF1‐MTOR, and WNT canonical signaling pathways. Drugs targeting additional pathways that are altered in PIK3CA‐mutant tumors may improve treatment regimens using PI3K inhibitors alone.

Constitutively activated PI3K accelerates tumor initiation and modifies histopathology of breast cancer

The gene encoding phosphatidylinositol 3-kinase catalytic subunit α-isoform (PIK3CA, p110α) is frequently activated by mutation in human cancers. Based on detection in some breast cancer precursors, PIK3CA mutations have been proposed to have a role in tumor initiation. To investigate this hypothesis, we generated a novel mouse model with a Cre-recombinase regulated allele of p110α (myristoylated-p110α, myr-p110α) along with p53^fl/fl deletion and Kras^G12D also regulated by Cre-recombinase. After instillation of adenovirus-expressing Cre-recombinase into mammary ducts, we found that myr-p110α accelerated breast tumor initiation in a copy number-dependent manner. Breast tumors induced by p53^fl/fl;Kras^G12D with no or one copy of myr-p110α had predominantly sarcomatoid features, whereas two copies of myr-p110α resulted in tumors with a carcinoma phenotype. This novel model provides experimental support for importance of active p110α in breast tumor initiation, and shows that the amount of PI3K activity can affect the rate of tumor initiation and modify the histological phenotype of breast cancer.

Reactivation of multipotency by oncogenic PIK3CA induces breast tumour heterogeneity

Breast cancer is the most frequent cancer in women and consists of heterogeneous types of tumours that are classified into different histological and molecular subtypes. PIK3CA and P53 (also known as TP53) are the two most frequently mutated genes and are associated with different types of human breast cancers. The cellular origin and the mechanisms leading to PIK3CA-induced tumour heterogeneity remain unknown. Here we used a genetic approach in mice to define the cellular origin of Pik3ca-derived tumours and the impact of mutations in this gene on tumour heterogeneity. Surprisingly, oncogenic Pik3ca(H1047R) mutant expression at physiological levels in basal cells using keratin (K)5-CreER(T2) mice induced the formation of luminal oestrogen receptor (ER)-positive/progesterone receptor (PR)-positive tumours, while its expression in luminal cells using K8-CReER(T2) mice gave rise to luminal ER(+)PR(+) tumours or basal-like ER(-)PR(-) tumours. Concomitant deletion of p53 and expression of Pik3ca(H1047R) accelerated tumour development and induced more aggressive mammary tumours. Interestingly, expression of Pik3ca(H1047R) in unipotent basal cells gave rise to luminal-like cells, while its expression in unipotent luminal cells gave rise to basal-like cells before progressing into invasive tumours. Transcriptional profiling of cells that underwent cell fate transition upon Pik3ca(H1047R) expression in unipotent progenitors demonstrated a profound oncogene-induced reprogramming of these newly formed cells and identified gene signatures characteristic of the different cell fate switches that occur upon Pik3ca(H1047R) expression in basal and luminal cells, which correlated with the cell of origin, tumour type and different clinical outcomes. Altogether our study identifies the cellular origin of Pik3ca-induced tumours and reveals that oncogenic Pik3ca(H1047R) activates a multipotent genetic program in normally lineage-restricted populations at the early stage of tumour initiation, setting the stage for future intratumoural heterogeneity. These results have important implications for our understanding of the mechanisms controlling tumour heterogeneity and the development of new strategies to block PIK3CA breast cancer initiation.

The Inositol Polyphosphate 5-Phosphatase PIPP Regulates AKT1-Dependent Breast Cancer Growth and Metastasis

Metastasis is the major cause of breast cancer mortality. Phosphoinositide 3-kinase (PI3K) generated PtdIns(3,4,5)P3 activates AKT, which promotes breast cancer cell proliferation and regulates migration. To date, none of the inositol polyphosphate 5-phosphatases that inhibit PI3K/AKT signaling have been reported as tumor suppressors in breast cancer. Here, we show depletion of the inositol polyphosphate 5-phosphatase PIPP (INPP5J) increases breast cancer cell transformation, but reduces cell migration and invasion. Pipp ablation accelerates oncogene-driven breast cancer tumor growth in vivo, but paradoxically reduces metastasis by regulating AKT1-dependent tumor cell migration. PIPP mRNA expression is reduced in human ER-negative breast cancers associated with reduced long-term outcome. Collectively, our findings identify PIPP as a suppressor of oncogenic PI3K/AKT signaling in breast cancer.

Functional and molecular characterisation of EO771.LMB tumours, a new C57BL/6-mouse-derived model of spontaneously metastatic mammary cancer

The translation of basic research into improved therapies for breast cancer patients requires relevant preclinical models that incorporate spontaneous metastasis. We have completed a functional and molecular characterisation of a new isogenic C57BL/6 mouse model of breast cancer metastasis, comparing and contrasting it with the established BALB/c 4T1 model. Metastatic EO771.LMB tumours were derived from poorly metastatic parental EO771 mammary tumours. Functional differences were evaluated using both in vitro assays and spontaneous metastasis assays in mice. Results were compared to non-metastatic 67NR and metastatic 4T1.2 tumours of the 4T1 model. Protein and transcript levels of markers of human breast cancer molecular subtypes were measured in the four tumour lines, as well as p53 (Tp53) tumour-suppressor gene status and responses to tamoxifen in vivo and in vitro. Array-based expression profiling of whole tumours identified genes and pathways that were deregulated in metastatic tumours. EO771.LMB cells metastasised spontaneously to lung in C57BL/6 mice and displayed increased invasive capacity compared with parental EO771. By immunohistochemical assessment, EO771 and EO771.LMB were basal-like, as was the 4T1.2 tumour, whereas 67NR had a luminal phenotype. Primary tumours from all lines were negative for progesterone receptor, Erb-b2/Neu and cytokeratin 5/6, but positive for epidermal growth factor receptor (EGFR). Only 67NR displayed nuclear estrogen receptor alpha (ERα) positivity. EO771 and EO771.LMB expressed mutant p53, whereas 67NR and 4T1.2 were p53-null. Integrated molecular analysis of both the EO771/EO771.LMB and 67NR/4T1.2 pairs indicated that upregulation of matrix metalloproteinase-3 (MMP-3), parathyroid hormone-like hormone (Pthlh) and S100 calcium binding protein A8 (S100a8) and downregulation of the thrombospondin receptor (Cd36) might be causally involved in metastatic dissemination of breast cancer.

Mouse models of breast cancer

Breast cancer is the most common cause of cancer death in women worldwide. This malignancy is a complex disease, which is defined by an intrinsic heterogeneity on the histopathological and molecular level as well as response to therapy and outcome. In addition to classical histopathological features, breast cancer can be categorized into at least five major subtypes based on comprehensive gene expression profiling: luminal A, luminal B, basal-like, ERBB2-positive, and normal-like breast cancer. Genetically engineered mouse models can serve as tools to study the molecular underpinnings for this disease. Given the genetic complexity that drives the initiation and progression of individual breast cancer subtypes, it is evident that certain models can reflect only particular aspects of this malignancy. In this book chapter, we will primarily focus on advances in modeling breast cancer at defined stages of carcinogenesis using genetically engineered mice. We will discuss the ability as well as shortcomings of these models to faithfully recapitulate the spectrum of human breast cancer subtypes.

Ubiquitous expression of the Pik3caH1047R mutation promotes hypoglycemia, hypoinsulinemia, and organomegaly

Mutations in PIK3CA, the gene encoding the p110α catalytic subunit of PI3K, are among the most common mutations found in human cancer and have also recently been implicated in a range of overgrowth syndromes in humans. We have used a novel inducible "exon-switch" approach to knock in the constitutively active Pik3ca(H1047R) mutation into the endogenous Pik3ca gene of the mouse. Ubiquitous expression of the Pik3ca(H1047R) mutation throughout the body resulted in a dramatic increase in body weight within 3 weeks of induction (mutant 150 ± 5%; wild-type 117 ± 3%, mean ± sem), which was associated with increased organ size rather than adiposity. Severe metabolic effects, including a reduction in blood glucose levels to 59 ± 4% of baseline (11 days postinduction) and undetectable insulin levels, were also observed. Pik3ca(H1047R) mutant mice died earlier (median survival 46.5 d post-mutation induction) than wild-type control mice (100% survival > 250 days). Although deletion of Akt2 increased median survival by 44%, neither organ overgrowth, nor hypoglycemia were rescued, indicating that both the growth and metabolic functions of constitutive PI3K activity can be Akt2 independent. This mouse model demonstrates the critical role of PI3K in the regulation of both organ size and glucose metabolism at the whole animal level.

The PTEN/PI3K/AKT Pathway in vivo, Cancer Mouse Models

When PI3K (phosphatidylinositol-3 kinase) is activated by receptor tyrosine kinases, it phosphorylates PIP2 to generate PIP3 and activates the signaling pathway. Phosphatase and tensin homolog deleted on chromosome 10 dephosphorylates PIP3 to PIP2, and thus, negatively regulates the pathway. AKT (v-akt murine thymoma viral oncogene homolog; protein kinase B) is activated downstream of PIP3 and mediates physiological processes. Furthermore, substantial crosstalk exists with other signaling networks at all levels of the PI3K pathway. Because of its diverse array, gene mutations, and amplifications and also as a consequence of its central role in several signal transduction pathways, the PI3K-dependent axis is frequently activated in many tumors and is an attractive therapeutic target. The preclinical testing and analysis of these novel therapies requires appropriate and well-tailored systems. Mouse models in which this pathway has been genetically modified have been essential in understanding the role that this pathway plays in the tumorigenesis process. Here, we review cancer mouse models in which the PI3K/AKT pathway has been genetically modified.

Genetically engineered knock-in and conditional knock-in mouse models of cancer

Classical transgenic models are useful for quickly gauging the impact of transgene overexpression, but they are limited by the absence of the innate, subtle regulatory elements encoded in introns and other untranslated regions. Moreover, the widespread, high-level expression of oncogenes often leads to tumors that lack the histopathological and acquired genetic features of human cancers. Targeted mutation of endogenous loci, or knock-in (KI) alleles, facilitates more accurate modeling of human tumors by allowing for the expression of mutant alleles under normal physiological regulation. Advanced strategies enable the stochastic activation of such alleles in somatic cells, such that genotypically wild-type cells surround individual mutant cells. More recent technologies, such as site-specific engineered nucleases, have also accelerated the design and implementation of KI strategies. Together, these tools aid in the development of advanced mouse models that better recapitulate the features of human disease.

Paradoxical zinc toxicity and oxidative stress in the mammary gland during marginal dietary zinc deficiency

Zinc (Zn) regulates numerous cellular functions. Zn deficiency is common in females; ∼80% of women and 40% of adolescent girls consume inadequate Zn. Zn deficiency enhances oxidative stress, inflammation and DNA damage. Oxidative stress and inflammation is associated with breast disease. We hypothesized that Zn deficiency increases oxidative stress in the mammary gland, altering the microenvironment and architecture. Zn accumulated in the mammary glands of Zn deficient mice and this was associated with macrophage infiltration, enhanced oxidative stress and over-expression of estrogen receptor α. Ductal and stromal hypercellularity was associated with aberrant collagen deposition and disorganized e-cadherin. Importantly, these microenvironmental alterations were associated with substantial impairments in ductal expansion and mammary gland development. This is the first study to show that marginal Zn deficiency creates a toxic microenvironment in the mammary gland impairing breast development. These changes are consistent with hallmarks of potential increased risk for breast disease and cancer.

Selective CREB-dependent cyclin expression mediated by the PI3K and MAPK pathways supports glioma cell proliferation

The cyclic-AMP response element binding (CREB) protein has been shown to have a pivotal role in cell survival and cell proliferation. Transgenic rodent models have revealed a role for CREB in higher-order brain functions, such as memory and drug addiction behaviors. CREB overexpression in transgenic animals imparts oncogenic properties on cells in various tissues, and aberrant CREB expression is associated with tumours. It is the central position of CREB, downstream from key developmental and growth signalling pathways, which gives CREB this ability to influence a spectrum of cellular activities, such as cell survival, growth and differentiation, in both normal and cancer cells. We show that CREB is highly expressed and constitutively activated in patient glioma tissue and that this activation closely correlates with tumour grade. The mechanism by which CREB regulates glioblastoma (GBM) tumour cell proliferation involves activities downstream from both the mitogen-activated protein kinase and phosphoinositide 3-kinase (PI3K) pathways that then modulate the expression of three key cell cycle factors, cyclin B, D and proliferating cell nuclear antigen (PCNA). Cyclin D1 is highly CREB-dependent, whereas cyclin B1 and PCNA are co-regulated by both CREB-dependent and -independent mechanisms. The precise regulatory network involved appears to differ depending on the tumour-suppressor phosphatase and tensin homolog status of the GBM cells, which in turn allows CREB to regulate the activity of the PI3K itself. Given that CREB sits at the hub of key cancer cell signalling pathways, understanding the role of glioma-specific CREB function may lead to improved novel combinatorial anti-tumour therapies, which can complement existing PI3K-specific drugs undergoing early phase clinical trials.

Frequent phosphatidylinositol-3-kinase mutations in proliferative breast lesions

The phosphatidylinositol-3-kinase pathway is one of the most commonly altered molecular pathways in invasive breast carcinoma, with phosphatidylinositol-3-kinase catalytic subunit (PIK3CA) mutations in 25% of invasive carcinomas. Ductal carcinoma in situ (DCIS), benign papillomas, and small numbers of columnar cell lesions harbor an analogous spectrum of PIK3CA and AKT1 mutations, yet there is little data on usual ductal hyperplasia and atypical ductal and lobular neoplasias. We screened 192 formalin-fixed paraffin-embedded breast lesions from 75 patients for point mutations using a multiplexed panel encompassing 643 point mutations across 53 genes, including 58 PIK3CA substitutions. PIK3CA point mutations were identified in 31/62 (50%) proliferative lesions (usual ductal hyperplasia and columnar cell change), 10/14 (71%) atypical hyperplasias (atypical ductal hyperplasia and flat epithelial atypia), 7/16 (44%) lobular neoplasias (atypical lobular hyperplasia and lobular carcinoma in situ), 10/21 (48%) DCIS, and 13/37 (35%) invasive carcinomas. In genotyping multiple lesions of different stage from the same patient/specimen, we found considerable heterogeneity; most notably, in 12 specimens the proliferative lesion was PIK3CA mutant but the concurrent carcinoma was wild type. In 11 additional specimens, proliferative epithelium and cancer contained different point mutations. The frequently discordant genotypes of usual ductal hyperplasia/columnar cell change and concurrent carcinoma support a role for PIK3CA-activating point mutations in breast epithelial proliferation, perhaps more so than transformation. Further, these data suggest that proliferative breast lesions are heterogeneous and may represent non-obligate precursors of invasive carcinoma.

Assessing the subcellular distribution of oncogenic phosphoinositide 3-kinase using microinjection into live cells

Oncogenic mutations in PIK3CA lead to an increase in intrinsic phosphoinositide kinase activity, but it is thought that increased access of PI3Kα (phosphoinositide 3-kinase α) to its PM (plasma membrane) localized substrate is also required for increased levels of downstream PIP₃/Akt [phosphoinositide-3,4,5-trisphosphate/also called PKB (protein kinase B)] signalling. We have studied the subcellular localization of wild-type and the two most common oncogenic mutants of PI3Kα in cells maintained in growth media, and starved or stimulated cells using a novel method in which PI3Kα is pre-formed as a 1:1 p110α:p85α complex in vitro then introduced into live cells by microinjection. Oncogenic E545K and H1047R mutants did not constitutively interact with membrane lipids in vitro or in cells maintained in 10% (v/v) FBS. Following stimulation of RTKs (receptor tyrosine kinases), microinjected PI3Kα was recruited to the PM, but oncogenic forms of PI3Kα were not recruited to the PM to a greater extent and did not reside at the PM longer than the wild-type PI3Kα. Instead, the E545K mutant specifically bound activated Cdc42 in vitro and microinjection of E545K was associated with the formation of cellular protrusions, providing some preliminary evidence that changes in protein–protein interactions may play a role in the oncogenicity of the E545K mutant in addition to the well-known changes in lipid kinase activity.

Oncogenic forms of PI3Kα (phosphoinositide 3-kinase α) microinjected into live cells are not recruited to the PM (plasma membrane) to a greater extent, and do not reside at the PM longer, than wild-type PI3Kα.

PI3K and cancer: lessons, challenges and opportunities

The central role of phosphoinositide 3-kinase (PI3K) activation in tumour cell biology has prompted a sizeable effort to target PI3K and/or downstream kinases such as AKT and mammalian target of rapamycin (mTOR) in cancer. However, emerging clinical data show limited single-agent activity of inhibitors targeting PI3K, AKT or mTOR at tolerated doses. One exception is the response to PI3Kδ inhibitors in chronic lymphocytic leukaemia, where a combination of cell-intrinsic and -extrinsic activities drive efficacy. Here, we review key challenges and opportunities for the clinical development of inhibitors targeting the PI3K-AKT-mTOR pathway. Through a greater focus on patient selection, increased understanding of immune modulation and strategic application of rational combinations, it should be possible to realize the potential of this promising class of targeted anticancer agents.