Lanfang Qin

Tamoxifen Resistance in Breast Tumors Is Driven by Growth Factor Receptor Signaling with Repression of Classic Estrogen Receptor Genomic Function

Not all breast cancers respond to tamoxifen, and many develop resistance despite initial benefit. We used an in vivo model of estrogen receptor (ER)-positive breast cancer (MCF-7 xenografts) to investigate mechanisms of this resistance and develop strategies to circumvent it. Epidermal growth factor receptor (EGFR) and HER2, which were barely detected in control estrogen-treated tumors, increased slightly with tamoxifen and were markedly increased when tumors became resistant. Gefitinib, which inhibits EGFR/HER2, improved the antitumor effect of tamoxifen and delayed acquired resistance, but had no effect on estrogen-stimulated growth. Phosphorylated levels of p42/44 and p38 mitogen-activated protein kinases (both downstream of EGFR/HER2) were increased in the tamoxifen-resistant tumors and were suppressed by gefitinib. There was no apparent increase in phosphorylated AKT (also downstream of EGFR/HER2) in resistant tumors, but it was nonetheless suppressed by gefitinib. Phosphorylated insulin-like growth factor-IR (IGF-IR), which can interact with both EGFR and membrane ER, was elevated in the tamoxifen-resistant tumors compared with the sensitive group. However, ER-regulated gene products, including total IGF-IR itself and progesterone receptor, remained suppressed even at the time of acquired resistance. Tamoxifens antagonism of classic ER genomic function was retained in these resistant tumors and even in tumors that overexpress HER2 (MCF-7 HER2/18) and are de novo tamoxifen-resistant. In conclusion, EGFR/HER2 may mediate tamoxifen resistance in ER-positive breast cancer despite continued suppression of ER genomic function by tamoxifen. IGF-IR expression remains dependent on ER but is activated in the tamoxifen-resistant tumors. This study provides a rationale to combine HER inhibitors with tamoxifen in clinical studies, even in tumors that do not initially overexpress EGFR/HER2.

HER2 Reactivation through Acquisition of the HER2 L755S Mutation as a Mechanism of Acquired Resistance to HER2-targeted Therapy in HER2+ Breast Cancer

Purpose: Resistance to anti-HER2 therapies in HER2+ breast cancer can occur through activation of alternative survival pathways or reactivation of the HER signaling network. Here we employed BT474 parental and treatment-resistant cell line models to investigate a mechanism by which HER2+ breast cancer can reactivate the HER network under potent HER2-targeted therapies. Experimental Design: Resistant derivatives to lapatinib (L), trastuzumab (T), or the combination (LR/TR/LTR) were developed independently from two independent estrogen receptor ER+/HER2+ BT474 cell lines (AZ/ATCC). Two derivatives resistant to the lapatinib-containing regimens (BT474/AZ-LR and BT474/ATCC-LTR lines) that showed HER2 reactivation at the time of resistance were subjected to massive parallel sequencing and compared with parental lines. Ectopic expression and mutant-specific siRNA interference were applied to analyze the mutation functionally. In vitro and in vivo experiments were performed to test alternative therapies for mutant HER2 inhibition. Results: Genomic analyses revealed that the HER2L755S mutation was the only common somatic mutation gained in the BT474/AZ-LR and BT474/ATCC-LTR lines. Ectopic expression of HER2L755S induced acquired lapatinib resistance in the BT474/AZ, SK-BR-3, and AU565 parental cell lines. HER2L755S-specific siRNA knockdown reversed the resistance in BT474/AZ-LR and BT474/ATCC-LTR lines. The HER1/2–irreversible inhibitors afatinib and neratinib substantially inhibited both resistant cell growth and the HER2 and downstream AKT/MAPK signaling driven by HER2L755S in vitro and in vivo. Conclusions: HER2 reactivation through acquisition of the HER2L755S mutation was identified as a mechanism of acquired resistance to lapatinib-containing HER2-targeted therapy in preclinical HER2-amplified breast cancer models, which can be overcome by irreversible HER1/2 inhibitors. Clin Cancer Res; 23(17); 5123–34. ©2017 AACR.

Abstract P6-04-25: Genomic Deregulation and Therapeutic Role of Nemo-like Kinase in Luminal B Breast Cancer

Luminal B breast tumors are notoriously known as high grade breast cancer prone to Tamoxifen resistance and early recurrence. Chromosomal abnormalities are a crucial class of drug targets in cancer which can be classified into numerical and structural rearrangements. Gene amplification is the result of numerical rearrangement that generates multiple copies of an otherwise normal human gene, such as Her2 amplification observed in breast cancer. Gene fusion is the result of structural rearrangement that fuses together pieces of two different genes, such as BCR-ABL gene fusion identified in leukemia. Due to their critical role in causing human cancers and their presence exclusively in the tumor tissues, gene amplifications and fusions provide highly specific drug targets with only minimal side effects. In order to identify these chromosome aberrations that can serve as new therapeutic targets in breast cancer, we developed a novel bioinformatics analysis called “copy number signature analysis”. This is based on the observation that driving gene fusions and their original (wild-type) genes are often amplified in fusion-positive tumors, or tumors without the fusions respectively. This suggests that gene fusions and amplifications may synergize to activate important oncogenes; if directly druggable, these aberrations will provide substantial opportunities for new therapies. Applying this analysis to the newly emerged genomic data from the Cancer Genome Atlas (TCGA) project nominated Nemo-like kinase ( NLK ) as a lead therapeutic target for invasive breast cancer. NLK encodes a serine-threonine mitogen activated protein kinase that plays a pivotal role in transforming growth factor beta pathway and embryo development. Genomic amplifications and rearrangements at NLK locus happen in 7%–9% of breast tumors, which may lead to deregulated serine-threonine kinase activity. Of note, these aberrations preferentially present in luminal B or Her2 positive breast tumors, suggesting their association with these high grade tumors. Most interestingly, transient inhibition of NLK through small RNA interference in multiple breast cancer cell lines harboring NLK deregulations, we observed potent inhibition of cell growth and capability of forming tumor-like colonies, as well as increased sensitivity to Tamoxifen treatment. These data suggest that NLK might be deregulated by genetic events which may promote growth-factor signaling and Tamoxifen resistant phenotype. Molecular assays detecting NLK chromosomal aberrations may serve as a predictive biomarker for endocrine resistance in breast cancer as well as selection of patients for appropriate treatment. Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P6-04-25.

SRC-1 Promotes Breast Cancer Metastasis by Activating MAPK Pathway.

Background: Steroid receptor coactivator (SRC-1) was defined as a nuclear receptor coactivator. It interacts with nuclear receptor and other transcriptional factors and promotes target genes expression. In breast cancer, SRC-1 expression correlates positively with HER2 expression, disease recurrence and resistance to endocrine therapy. Our previous work demonstrated that knockout of SRC-1 in mice suppressed breast cancer metastasis without affecting primary tumor initiation and growth in the MMTV-PyMT transgenic mice. However, the molecular mechanism responsible for SRC-1 in breast cancer metastasis is still unclear.Material and Methods: To further study the function of SRC-1 in breast cancer metastasis, we generated transgenic mice with overexpression of human SRC-1 in the mouse mammary epithelial cells. The mammary glands of these mice exhibited normal development. After crossing these mice with MMTV-TVA (avian subgroup A receptor gene, TVA) transgenic mice, we obtained MMTV-TVA/MMTV-hSRC-1 (tester) and MMTV-TVA/WT (control) biogenic mice. RCAS (avian leukosis virus) viruses, which contain MMTV-PyMT expression cassette and can bind to TVA, were injected into mammary gland to induce mammary tumor. Tumor initiation, growth and metastasis were carefully examined with these mice. To investigate mechanism, multiple PyMT tumor cell lines were generated form individual primary tumor with or without SRC-1 and cell migration and invasion were compared with those cells and human breast cancer cells.Results and discuss: No difference in tumor initiation and growth was observed in those mice, but metastasis incidence of breast cancer to lung increased significantly in PyMT/TVA/hSRC-1 mice compared with PyMT/TVA/WT control mice. Tumor metastasis in lung also presented higher index in PyMT/TVA/hSRC-1 mice. In vitro study with PyMT/SRC-1 WT and KO cell lines showed SRC-1 deficiency disturbed cells migration and invasion. WT tumor cells adhered faster to extracellular matrix fibronecetin (FN) and presented high levels of phosphorylated forms of FAK, c-Src, Erk, Jnk and activated Rac1. Western blotting and real-time RT-PCR revealed lower integrin alpha5 and Gem, one of GTP binding protein, levels in SRC-1 deficient tumor cells. In human breast cancer MDA-MB-231 cells, SRC-1 knockdown led to decreased cell migration and invasion. Therefore, SRC-1 may regulate integrin expression and MAPK pathway to promote cancer cell migration and metastasis. Our study provides a possibility to inhibit breast cancer metastasis by targeting SRC-1 function. Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 6150.

GPCRs profiling and identification of GPR110 as a potential new target in HER2+ breast cancer

PurposeG protein-coupled receptors (GPCRs) represent the largest family of druggable targets in human genome. Although several GPCRs can cross-talk with the human epidermal growth factor receptors (HERs), the expression and function of most GPCRs remain unknown in HER2+ breast cancer (BC). In this study, we aimed to evaluate gene expression of GPCRs in tumorigenic or anti-HER2 drug-resistant cells and to understand the potential role of candidate GPCRs in HER2+ BC.MethodsGene expression of 352 GPCRs was profiled in Aldeflur+ tumorigenic versus Aldeflur− population and anti-HER2 therapy-resistant derivatives versus parental cells of HER2+ BT474 cells. The GPCR candidates were confirmed in 7 additional HER2+ BC cell line models and publicly available patient dataset. Anchorage-dependent and anchorage-independent cell growth, mammosphere formation, and migration/invasion were evaluated upon GPR110 knockdown by siRNA in BT474 and SKBR3 parental and lapatinib+ trastuzumab-resistant (LTR) cells.ResultsAdhesion and class A GPCRs were overexpressed in Aldeflur+ and anti-HER2 therapy-resistant population of BT474 cells, respectively. GPR110 was the only GPCR overexpressed in Aldeflur+ and anti-HER2 therapy-resistant population in BT474, SKBR3, HCC1569, MDA-MB-361, AU565, and/or HCC202 cells and in HER2+ BC subtype in patient tumors. Using BT474 and SKBR3 parental and LTR cells, we found that GPR110 knockdown significantly reduced anchorage-dependent/independent cell growth as well as migration/invasion of parental and LTR cells and mammosphere formation in LTR derivatives and not in parental cells.ConclusionOur data suggest a potential role of GPR110 in tumorigenicity and in tumor cell dissemination in HER2+ BC.

Abstract 1223: Evaluation of the dual p38/NLK kinase inhibitor as potential new therapeutic agent for tamoxifen-resistant breast cancer

Background: Endocrine therapy has been considered an effective initial treatment for ER positive breast cancer and tamoxifen is the most commonly used endocrine agent. However, about half of the patients develop resistance or relapse eventually. No effective targeted therapy exists to overcome it. In our previous study, we have identified the role of nemo-like kinase (NLK) in breast cancer endocrine resistance – a serine-threonine kinase that functions in stress response and neurite outgrowth. In addition to NLK, activation of the other stress kinases such as p38 MAPK has been reported to modulate ER signaling and promote endocrine resistance. In this study, we have identified a highly selective dual p38 and NLK kinase inhibitor (PNKI). This study aimed to evaluate the therapeutic effect of the PNKI inhibitor in tamoxifen-resistant breast cancers using in vitro and preclinical mouse models. Experimental design and methods: To determine the effect of PNKI on tamoxifen-resistant breast cancer cells, we treated a primary tamoxifen-resistant breast cancer cell line MDAMB415 and an acquired-resistant line MCF7 TamR, with 0.5 uM PNKI in the presence of different doses of Tamoxifen. To evaluate the therapeutic effect of PNKI in a T47D-derived xenograft tumor model with acquired tamoxifen resistance, we administered PNKI alone or in combination with Fulvestrant, or with the mTOR inhibitor Everolimus. Mice bearing T47D-TamR xenografts were randomized into six treatment groups (Vehicle, PNKI, Fulvestrant, Fulvestrant + PNKI, Everolimus, Everolimus + PNKI). Tumor growth was tracked closely. The tumors harvested 2 weeks following treatments were profiled with Reverse Phase Protein Array (RPPA) to assess the early signaling changes after treatments. Furthermore, the therapeutic effect of PNKI were also evaluated in a patient-derived xenograft (PDX) model of de novo endocrine resistant breast cancer. Mice bearing the PDX tumors were randomized to one of four treatment groups (Vehicle, PNKI, Everolimus, Everolimus + PNKI) and tumor growth curve was measured. Summary of the Results: Breast cancer cell lines with either de novo or acquired Tamoxifen resistance became more sensitive to tamoxifen when treated with 0.5 uM PNKI. The concomitant treatment of PNKI and Everolimus results in significant decrease of tumor growth in the T47D-TamR xenograft tumor compared to Fulvestrant, Fulvestrant + PNKI, PNKI, or Everolimus treatments. RPPA data revealed that a majority of key survival signaling in breast cancer are repressed only when PNKI are combined with Everolimus. The de novo endocrine-resistant PDX tumors showed diverse response to PNKI mono-treatment, whereas the combination of PNKI and Everolimus resulted in significantly decreased tumor growth. Conclusion: The PNKI exhibited potential therapeutic value as adjuvant agent to the mTOR inhibitor everolimus for acquired or de novo tamoxifen-resistant breast cancer. Citation Format: XIAN WANG, Xixi Cao, Jamunarani Veeraraghavan, Lanfang Qin, Jin-Ah Kim, Ying Tan, Susan G. Hilsenbeck, Rachel Schiff, Xiaosong Wang. Evaluation of the dual p38/NLK kinase inhibitor as potential new therapeutic agent for tamoxifen-resistant breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1223. doi:10.1158/1538-7445.AM2017-1223

Abstract P2-05-23: TWIST1 silences FOXA1 transcription to promote breast cancer progression

TWIST1 promotes epithelial-mesenchymal transition (EMT), invasion and metastasis of breast cancer cells, but the underlying mechanism is still not well understood. We generated mammary gland tumor specific Twist1 knock out mouse model and found that TWIST1 does not affect PyMT-induced mammary tumor initiation and growth but promotes tumor lung metastasis. We identified FOXA1 as a novel direct target of TWIST1 in both mouse and human breast cancer. We further found that TWIST1 inhibits FOXA1 expression through direct binding to its proximal promoter region and recruiting Mi2/nucleosome remodeling and deacetylase (Mi2/NuRD) transcriptional repressor complex. Moreover, TWIST1 also diminished transcriptional activator AP1 binding to FOXA1 promoter. TWIST1 mediated FOXA1 down-regulation is essential for promoting breast cancer migration, invasion and metastasis. FOXA1 significantly inhibits TWIST1 dependent cell migration and invasion capability of MCF7 cells through inhibiting integrin α5, β1 and MMP9 expression. Importantly, TWIST1high FOXA1low correlates with the poorest prognosis in breast cancer patients. Citation Format: Xu Y, Feng Z, Xu Y, Mo Q, Qin L, Sun T, Wu H, Li Y, Liao L, Xu J. TWIST1 silences FOXA1 transcription to promote breast cancer progression. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr P2-05-23.

Abstract P2-04-01: The role of Jmjd1a in mammary gland development and breast tumor growth

Histone modification alters chromatin architecture and thereby influences gene transcription. Histone methylation status is reversible and counter-regulated by methyltransferases and demethylases. Jmjd1a (also known as KDM3A, TSGA, JMJD1, JHDM2A and JHMD2A) is a histone demethylase. It belongs to JmjC domain-containing protein family and could specifically remove di- and mono- methyl residues from di or mono-methylated histone H3K9 (H3K9me2/me1). Recent studies showed that Jmjd1a plays an important role in embryonic stem cell self-renewal, spermatogenesis, regulation of metabolic gene expression and body weight, sex determination, tumor angiogenesis, and macrophage infiltration. However, its role in mammary gland (MG) development, breast carcinogenesis and breast cancer progression hasn9t been systemically investigated. In this study, we found that Jmjd1a is expressed in mouse luminal epithelial cells. Genetic disruption of the Jmjd1a gene significantly slowed down MG development as indicated by retarded MG elongation and decreased ductal density in virgin mice observed at the ages of 4, 6 and 8 weeks. In agreement with the retarded MG development, the expression of Ki67 and cyclinD1 in epithelial cells of MGs from Jmjd1a knockout (KO) mice dramatically reduced compared with that from wild type (WT) mice. H3K9me1 and H3K9me2 levels in the epithelial cells of KO MGs are much higher than that in WT MGs. To assess the role of Jmjd1a in breast cancer progression, we crossbred Tg(Jmjd1a-/-) mice with MMTV-TVA(RCAS-PyMT) mice and obtained Tg(Jmjd1a-/-)×MMTV-TVA(RCAS-PyMT) mice. Infection of the TVA-expressing MG epithelial cells with the RCAS-PyMT virus induced mammary tumors in these mice and MMTV-TVA(RCAS-PyMT) control mice. We found that KO of Jmjd1a slightly accelerated mammary tumor initiation but significantly decreased tumor growth. Ki67 and cyclinD1 expression statistically reduced in KO tumors versus WT tumors. At the molecular level, Jmjd1a expression positively correlated with cyclin D1 expression in mammary epithelial cells and mammary tumors. Knockdown of Jmjd1a in MCF-7 cells significantly reduced cyclin D1 expression, while ectopic expression of Jmjd1a in MCF-7 cells increased cyclin D1 expression. ChIP assay revealed that Jmjd1a is associated with a promoter region of cyclin D1. Co-expression of c-Myc and Jmjd1a boosted the activity of the cyclin D1 reporter. In conclusion, our study indicated that Jmjd1a plays an important role in promoting mammary gland development and breast tumor growth by up-regulating cyclin D1 expression. Targeting Jmjd1a may inhibit breast cancer progression. Citation Format: Qin L, Xu Y, Wu Y, Yu X, Toneff MJ, Liao L, Li Y, Xu J. The role of Jmjd1a in mammary gland development and breast tumor growth. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr P2-04-01.

Abstract P3-05-13: Overexpression of insulin receptor substrate 4 can mediate acquired resistance to lapatinib-containing regimens in HER2+ breast cancer cells

Background: The HER2 pathway can be inhibited by potent targeting agents such as lapatinib (L), trastuzumab (T), or their combination (LT), but acquired and de novo resistance still occur. Resistance to these drugs remains a major hurdle in the management of HER2+ breast cancer. Consequently, elucidation of mechanisms of acquired therapeutic resistance to HER2-directed therapies is of critical importance. Methods: To obtain clues to the mechanisms for resistance we developed a panel of HER2+ breast cancer cell lines resistant to L, T, or LT. Parental cells and resistant derivatives of the HER2+ BT474 cell line were characterized by RNA-seq. Genes that were overexpressed in resistant compared to parental cells were confirmed by RT-PCR, Western blotting, and immunohistochemistry (IHC). Cell growth and cell signaling were assessed in parental and resistant cell lines after down-regulation (by siRNA) or overexpression (via an inducible cDNA) of IRS4 in the presence or absence of treatment. The effect of IRS4 overexpression on L resistance was assessed in a BT474 xenograft model. The proteins that interact with IRS4 were identified by co-immunoprecipitation with IRS4 followed by separation of the associated proteins by SDS-PAGE and microsequencing by mass spectrometry. Results: RNA-seq analysis revealed that IRS4 was the most up-regulated gene in BT474 L or LT resistant derivatives in which HER2 signaling is effectively inhibited, but not T alone, where HER2 signaling is reactivated. Western blotting and IHC validated this result and identified membrane localization of IRS4. Knockdown of IRS4 in L- or LT-resistant cells reversed resistance and restored growth inhibition. IRS4 knockdown also inhibited downstream signaling, with a reduction in pAKT but not in pMAPK. Induction of the cell cycle regulator p27 and down-regulation of survivin were observed after IRS4 knockdown. Overexpression of IRS4 cDNA in parental BT474 and SKBR3 cells led to resistance to L/LT, increased pAkt, and decreased the apoptotic marker cleaved PARP in the presence of L or the LT combination. The BT474 xenograft model showed that IRS4 overexpression in the absence of treatment had no effect on tumor growth but it significantly reduced the inhibitory effect of lapatinib (p=0.002). A group of proteins that interact with IRS4 in BT474 L-resistant cells were identified by mass spectrometry. The roles of these proteins in IRS4-mediated resistance to lapatinb-containing regimens are under investigation. Conclusion: IRS4 overexpression is a critical factor in causing resistance to lapatinib-containing regimens in BT474 cells. Investigation of IRS4 and its signaling partners in HER2+ human tumors resistant to lapatinib will be important to determine if this mechanism is also operative in patients. Citation Format: Lanfang Qin, Maria B Hahn, Xiaoyong Fu, Martin J Shea, Mario Giuliano, Sarmistha Nanda, Xiaowei Xu, Huizhong Hu, Sung Yun Jung, Laura M Heiser, Nicholas Wang, Joe W Gray, Susan G Hilsenbeck, Chad Creighton, Chad A Shaw, Gary C Chamness, Dean P Edwards, Sabrina Herrera, Carolina Gutierrez, C Kent Osborne, Rachel Schiff. Overexpression of insulin receptor substrate 4 can mediate acquired resistance to lapatinib-containing regimens in HER2+ breast cancer cells [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr P3-05-13.

Abstract P5-05-03: Clonal evolution of the HER2 L755S mutation leads to acquired HER-targeted therapy resistance that can be reversed by the irreversible HER1/2 inhibitor afatinib

Background: Targeting HER2 with lapatinib (L), trastuzumab (T), or the LT combination, is effective in HER2+ breast cancer (BC), but acquired resistance commonly occurs. In our 12-week neoadjuvant trial (TBCRC006) of LT without chemotherapy in HER2+ BC, the overall pathologic complete response rate (pCR) was 27%. To investigate resistance mechanisms our lab developed 10 HER2+ BC cell lines resistant (R) to these drugs (LR/TR/LTR). To discover potential predictive markers/therapeutic targets to circumvent resistance, we completed genomic profiling of the cell line panel and a subset of pre-treatment baseline specimens from TBCRC006. Methods: Parental (P) lines and LR/TR/LTR derivatives of 9 HER2+ BC cell line models were profiled with whole exome and RNA sequencing. Mutations detected in R lines but not in same-model P lines were identified. cDNAs were assessed by targeted Sanger sequencing. Single cells of the BT474AZ-LR line were cloned and their cDNAs were sequenced. Mutant-specific Q-PCR was designed to sensitively quantify mutations. Whole exome sequencing (minimum depth 100X) of 17 baseline tumor/normal pairs from TBCRC006 were performed on Illumina HiSeq. Results: We found and validated the HER2 L755S mutation in the BT474ATCC-LTR line and the BT474AZ-LR line (∼30% of DNA/RNA/cDNA in BT474AZ-LR), in which the HER pathway was reactivated to cause resistance. Overexpression of this mutation was previously shown to induce L resistance in HER2-negative BC cell lines, suggesting a role as an acquired L/LT resistance driver in HER2+ BC. Sanger sequencing of BT474AZ-LR single cell clones found the HER2 L755S mutation in every clone but only in ∼30% of the HER2 copies. Using sensitive mutant-specific Q-PCR, we found statistically higher levels of HER2 L755S expression in BT474ATCC-P and BT474AZ-P compared to parentals of other HER2+ BC cell lines (UACC812/AU565/SKBR3/SUM190). These data suggest that this mutation exists subclonally within BT474 parental lines and was selected to become the more dominant population in the two resistant lines. The HER1/2 irreversible tyrosine kinase inhibitor (TKI) afatinib (Afa) robustly inhibited growth of both BT474ATCC-LTR/AZ-LR cells (IC50: Afa 0.02µM vs. L 3 µM). Western blots confirmed inhibition of the HER and downstream Akt and MAPK signaling in the LR cells by Afa. Sequencing of TBCRC006 baseline samples found the HER2 L755S mutation in 1/17 subjects. This patient did not achieve pCR after neoadjuvant LT treatment. The variant was present in 2% of the reads, indicating it as a subclonal event in this patient’s baseline tumor. Conclusion: Acquired resistance in two of our BT474 LR/LTR lines is due to selection of HER2 L755S subclones present in the parental cell population. The higher HER2 L755S levels detected in BT474 parentals compared with other HER2+ BC parental lines, and detection of its subclonal presence in a pre-treatment HER2+ BC patient, suggest that sensitive mutation detection methods will be needed to identify patients with potentially actionable HER family mutations in primary tumor. Treating this patient group with an irreversible TKI like Afa may prevent resistance and improve clinical outcome of this subset of HER2+ BC. Citation Format: Xiaowei Xu, Agostina Nardone, Huizhong Hu, Lanfang Qin, Sarmistha Nanda, Laura M Heiser, Nicholas Wang, Kyle R Covington, Edward S Chen, Alexander Renwick, Tao Wang, Carmine De Angelis, Alejandro Contreras, Carolina Gutierrez, Suzanne AW Fuqua, Gary C Chamness, Chad Shaw, David A Wheeler, Joe W Gray, Susan G Hilsenbeck, Mothaffar F Rimawi, C Kent Osborne, Rachel Schiff. Clonal evolution of the HER2 L755S mutation leads to acquired HER-targeted therapy resistance that can be reversed by the irreversible HER1/2 inhibitor afatinib [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr P5-05-03.