The spontaneous mouse mutant low set ears (Lse) is caused by tandem duplication of Fgf3 and Fgf4.

The external ear develops from an organized convergence of ventrally migrating neural crest cells into the first and second branchial arches. Defects in external ear position are often symptomatic of complex syndromes such as Apert, Treacher-Collins, and Crouzon Syndrome. The low set ears (Lse) spontaneous mouse mutant is characterized by the dominant inheritance of a ventrally shifted external ear position and an abnormal external auditory meatus (EAM). We identified the causative mutation as a 148 Kb tandem duplication on Chromosome 7, which includes the entire coding sequences of Fgf3 and Fgf4. Duplications of FGF3 and FGF4 occur in 11q duplication syndrome in humans and are associated with craniofacial anomalies, among other features. Intercrosses of Lse-affected mice revealed perinatal lethality in homozygotes, and Lse/Lse embryos display additional phenotypes including polydactyly, abnormal eye morphology, and cleft secondary palate. The duplication results in increased Fgf3 and Fgf4 expression in the branchial arches and additional discrete domains in the developing embryo. This ectopic overexpression resulted in functional FGF signaling, demonstrated by increased Spry2 and Etv5 expression in overlapping domains of the developing arches. Finally, a genetic interaction between Fgf3/4 overexpression and Twist1, a regulator of skull suture development, resulted in perinatal lethality, cleft palate, and polydactyly in compound heterozygotes. These data indicate a role for Fgf3 and Fgf4 in external ear and palate development and provide a novel mouse model for further interrogation of the biological consequences of human FGF3/4 duplication.

Fibroblast Growth Factor 3 Is Associated with Tongue Squamous Cell Carcinoma: A Controlled Study.

Objective: To explore the effects of fibroblast growth factor 3 (FGF3) on the proliferation, cell cycle, and apoptosis of the tongue squamous cell carcinoma SCC-9 cell line (SCC-9). Methods: We measured the proliferation of SCC-9 cells in a control group, an FGF3 intervention group, and a fibroblast growth factor (FGFR) inhibitor intervention group in cholecystokinin octapeptide (CCK-8) experiments. We studied effects of FGF3 on the cell cycle and apoptosis of tongue cancer cells using flow cytometry. We further explored the IRS1/PI3K/AKT signaling pathway by measuring BCL-2 and Bcl-2 Associated X-protein (BAX) mRNA and protein levels with RT-PCR and western blot, respectively. Results: Results from the CCK-8 experiment showed that survival rates of cells in the control group, FGF3 intervention group, and FGFR inhibitor intervention group were 100.000% ± 4.026%, 136.330% ± 9.779%, and 83.199% ± 4.954%, respectively; survival rates of SCC-9 cells in all three groups were statistically significant (P < 0.05). Compared with that in the control group, the ratio of cells in G0/G1 phase in the FGFR inhibitor intervention group was higher (P < 0.05) and that in G2/M phase was lower, while the FGF3 intervention group showed opposite results (P < 0.05). The apoptosis rate of tongue cancer cells differed significantly between the FGFR inhibitor intervention and the control groups (P < 0.05). The mRNA and protein expression levels of IRS1, PI3K, and BCL-2 were all increased in the FGF3 intervention group (P < 0.05), while BAX mRNA and protein expression levels were decreased (P < 0.05). The mRNA expression levels of protein kinase B (AKT) showed no differences between groups. The p-AKT protein was overexpressed, while the total amount of AKT protein remained stable (P < 0.05). Conclusion: FGF3 contributes to the proliferation of SCC-9 cells by increasing the proportion of cells in G2/M phase. Therefore, appropriately timed inhibition of FGF3 can potentially promote tumor apoptosis through the IRS1/PI3K/AKT signaling pathway. Our results demonstrate the role of FGF3 in the tumor microenvironment in tongue squamous cell carcinoma SCC-9 cells and suggest new therapeutic targets.

Inactivation of Fgf3 and Fgf4 within the Fgf3/Fgf4/Fgf15 gene cluster reveals their redundant requirement for mouse inner ear induction and embryonic survival.

BACKGROUND: Fibroblast growth factors (Fgfs) are required for survival and organ formation during embryogenesis. Fgfs often execute their functions redundantly. Previous analysis of Fgf3 mutants revealed effects on inner ear formation and embryonic survival with incomplete penetrance. RESULTS: Here, we show that presence of a neomycin resistance gene (neo) replacing the Fgf3 coding region leads to reduced survival during embryogenesis and an increased penetrance of inner ear defects. Fgf3neo/neo mutants showed reduced expression of Fgf4, which is positioned in close proximity to the Fgf3 locus in the mouse genome. Conditional inactivation of Fgf4 during inner ear development on a Fgf3 null background using Fgf3/4 cis mice revealed a redundant requirement between these Fgfs during otic placode induction. In contrast, inactivation of Fgf3 and Fgf4 in the pharyngeal region where both Fgfs are also co-expressed using a Foxg1-Cre driver did not affect development of the pharyngeal arches. However, these mutants showed reduced perinatal survival. CONCLUSIONS: These results highlight the importance of Fgf signaling during development. In particular, different members of the Fgf family act redundantly to guarantee inner ear formation and embryonic survival.

Single gene variants causing deafness in Asian Indians.

Congenital deafness is one of the common disorders, with some common genes accounting for most of the cases. One in 1000 children are born with sensorineural hearing loss, and of that 50% are hereditary. In the Mediterranean Europeans, 80% of the nonsyndromic recessive deafness is due to homozygous mutation in GJB2, the 35del G allele. InWestern population, the GJB2 variation have been found in up to 30-40% cases. In Indians, the GJB2 variants have been found in up to 20% cases, mostly from central and southern India. In the present study, DNA was extracted from blood using standard methods. This was used to perform targeted gene capture using a custom capture kit. Multiple genes causing deafness were sequenced by next-generation sequencing to mean >80-100x coverage on Illumina sequencing platform. We found variants in GJB2, WFS1, FGF3, EYA4, MYO7A. and CHD7 genes. Most of these variants were pathogenic and novel, and possibly causative. Deafness is most commonly due to the autosomal dominant genes but in severe cases of early onset deafness, autosomal recessive genes may contribute in our population. In selected families of severe prelingual deafness, prenatal diagnosis can be done.

Acquired FGFR and FGF Alterations Confer Resistance to Estrogen Receptor (ER) Targeted Therapy in ER+ Metastatic Breast Cancer.

PURPOSE: To identify clinically relevant mechanisms of resistance to ER-directed therapies in ER+ breast cancer. EXPERIMENTAL DESIGN: We conducted a genome-scale functional screen spanning 10,135 genes to investigate genes whose overexpression confer resistance to selective estrogen receptor degraders. In parallel, we performed whole-exome sequencing in paired pretreatment and postresistance biopsies from 60 patients with ER+ metastatic breast cancer who had developed resistance to ER-targeted therapy. Furthermore, we performed experiments to validate resistance genes/pathways and to identify drug combinations to overcome resistance. RESULTS: Pathway analysis of candidate resistance genes demonstrated that the FGFR, ERBB, insulin receptor, and MAPK pathways represented key modalities of resistance. The FGFR pathway was altered via FGFR1, FGFR2, or FGF3 amplifications or FGFR2 mutations in 24 (40%) of the postresistance biopsies. In 12 of the 24 postresistance tumors exhibiting FGFR/FGF alterations, these alterations were acquired or enriched under the selective pressure of ER-directed therapy. In vitro experiments in ER+ breast cancer cells confirmed that FGFR/FGF alterations led to fulvestrant resistance as well as cross-resistance to the CDK4/6 inhibitor palbociclib. RNA sequencing of resistant cell lines demonstrated that FGFR/FGF induced resistance through ER reprogramming and activation of the MAPK pathway. The resistance phenotypes were reversed by FGFR inhibitors, a MEK inhibitor, and/or a SHP2 inhibitor. CONCLUSIONS: Our results suggest that FGFR pathway is a distinct mechanism of acquired resistance to ER-directed therapy that can be overcome by FGFR and/or MAPK pathway inhibitors.

Slc26a9P2ACre : a new CRE driver to regulate gene expression in the otic placode lineage and other FGFR2b-dependent epithelia.

Pan-otic CRE drivers enable gene regulation throughout the otic placode lineage, comprising the inner ear epithelium and neurons. However, intersection of extra-otic gene-of-interest expression with the CRE lineage can compromise viability and impede auditory analyses. Furthermore, extant pan-otic CREs recombine in auditory and vestibular brain nuclei, making it difficult to ascribe resulting phenotypes solely to the inner ear. We have previously identified Slc26a9 as an otic placode-specific target of the FGFR2b ligands FGF3 and FGF10. We show here that Slc26a9 is otic specific through E10.5, but is not required for hearing. We targeted P2ACre to the Slc26a9 stop codon, generating Slc26a9P2ACre mice, and observed CRE activity throughout the otic epithelium and neurons, with little activity evident in the brain. Notably, recombination was detected in many FGFR2b ligand-dependent epithelia. We generated Fgf10 and Fgf8 conditional mutants, and activated an FGFR2b ligand trap from E17.5 to P3. In contrast to analogous mice generated with other pan-otic CREs, these were viable. Auditory thresholds were elevated in mutants, and correlated with cochlear epithelial cell losses. Thus, Slc26a9P2ACre provides a useful complement to existing pan-otic CRE drivers, particularly for postnatal analyses.

FGF3 from the Hypothalamus Regulates the Guidance of Thalamocortical Axons.

Thalamus is an important sensory relay station: afferent sensory information, except olfactory signals, is transmitted by thalamocortical axons (TCAs) to the cerebral cortex. The pathway choice of TCAs depends on diverse diffusible or substrate-bound guidance cues in the environment. Not only classical guidance cues (ephrins, slits, semaphorins, and netrins), morphogens, which exerts patterning effects during early embryonic development, can also help axons navigate to their targets at later development stages. Here, expression analyses reveal that morphogen Fibroblast growth factor (FGF)-3 is expressed in the chick ventral diencephalon, hypothalamus, during the pathfinding of TCAs. Then, using in vitro analyses in chick explants, we identify a concentration-dependent effect of FGF3 on thalamic axons: attractant 100 ng/mL FGF3 transforms to a repellent at high concentration 500 ng/mL. Moreover, inhibition of FGF3 guidance functions indicates that FGF3 signaling is necessary for the correct navigation of thalamic axons. Together, these studies demonstrate a direct effect for the member of FGF7 subfamily, FGF3, in the axonal pathfinding of TCAs.

Polymorphisms in FGF3, FGF10, and FGF13 May Contribute to the Presence of Temporomandibular Disorders in Patients Who Required Orthognathic Surgery.

BACKGROUND: To evaluate whether genetic polymorphisms in FGF3, FGF10, and FGF13 are associated with temporomandibular disorders (TMD) in patients that presented dentofacial deformities requiring orthognathic surgery. MATERIAL AND METHODS: The sample comprised a total of 113 patients of both sexes. The diagnosis of TMD was performed before orthognathic surgery between Research Diagnostic Criteria for Temporomandibular Disorders (RDC-TMD). According to the TMD assessment, the patients were divided into 3 major groups: myofascial pain, articular disc displacements and other TMD conditions (arthralgia, arthritis, and arthrosis). Genomic DNA was collected from saliva samples and genetic polymorphisms in FGF3 (rs1893047 and rs7932320), FGF10 (rs900379) and FGF13 (rs5931572 and rs5974804) were analyzed by real-time polymerase chain reactions. The association between the TMD conditions and the genetic polymorphisms assessed were analyzed by Poisson Regression. The model was calculated on bivariate and adjusted by sex. The established alpha was 5%. Data were analyzed by using SPSS software (IBM, Armonk, NY). RESULTS: The genetic polymorphisms rs7932320 in FGF3 (P < 0.001) and rs900379 in FGF10 (P < 0.05) were associated with the presence of muscle disorder. The genetic polymorphisms rs1893047 in FGF3, rs900379 in FGF10, and rs5974804 and rs5931572 in FGF13, were associated with the presence of disk displacement (P < 0.05). The genetic polymorphisms rs1893047 and rs7932320 in FGF3, rs900379 in FGF10, and rs900379 in FGF10 were associated with other TMD conditions (P < 0.05). CONCLUSION: Genetic polymorphisms in FGF3, FGF10, and FGF13 genes were associated with temporomandibular disorders in a population with dentofacial deformities.

Three New Mutations and Mild, Asymmetrical Phenotype in the Highly Distinctive LAMM Syndrome: A Report of Eight Further Cases.

Labyrinthine aplasia, microtia, and microdontia (LAMM) is an autosomal recessive condition causing profound congenital deafness, complete absence of inner ear structures (usually Michel's aplasia), microtia (usually type 1) and microdontia. To date, several families have been described with this condition and a number of mutations has been reported. We report on eight further cases of LAMM syndrome including three novel mutations, c. 173T>C p.L58P; c. 284G>A p.(Arg95Gln) and c.325_327delinsA p.(Glu109Thrfs*18). Congenital deafness was the primary presenting feature in all affected individuals and consanguinity in all but two families. We compare the features in our patients to those previously reported in LAMM, and describe a milder, asymmetrical phenotype associated with FGF3 mutations.

Mesenchymal Wnt/ß-catenin signaling induces Wnt and BMP antagonists in dental epithelium.

Previous studies indicated that the elevated mesenchymal Wnt/ß-catenin signaling deprived dental mesenchyme of odontogenic fate. By utilizing ex vivo or pharmacological approaches, Wnt/ß-catenin signaling in the developing dental mesenchyme was suggested to suppress the odontogenic fate by disrupting the balance between Axin2 and Runx2. In our study, the Osr2-creKI; Ctnnb1ex3f mouse was used to explore how mesenchymal Wnt/ß-catenin signaling suppressed the odontogenic fate in vivo. We found that all of the incisor and half of the molar germs of Osr2-creKI; Ctnnb1ex3fmice started to regress at E14.5 and almost disappeared at birth. The expression of Fgf3 and Msx1 was dramatically down-regulated in the E14.5 Osr2-creKI; Ctnnb1ex3f incisor and molar mesenchyme, while Runx2transcription was only diminished in incisor mesenchyme. Intriguingly, in the E14.5 Osr2-creKI; Ctnnb1ex3f incisor epithelium, the expression of Noggin was activated, while Shh was abrogated. Similarly, the Wnt and BMP antagonists, Ectodin and Noggin were also ectopically activated in the E14.5 Osr2-creKI; Ctnnb1ex3f molar epithelium. Recombination of E13.5 Osr2-creKI; Ctnnb1ex3f molar mesenchyme with E10.5 and E13.5 WT dental epithelia failed to develop tooth. Taken together, the mesenchymal Wnt/ß-catenin signaling resulted in the loss of odontogenic fate in vivo not only by directly suppressing odontogenic genes expression but also by inducing Wnt and BMP antagonists in dental epithelium.

Engraftment of Human Stem Cell-Derived Otic Progenitors in the Damaged Cochlea.

Most cases of sensorineural deafness are caused by degeneration of hair cells. Although stem/progenitor cell therapy is becoming a promising treatment strategy in a variety of organ systems, cell engraftment in the adult mammalian cochlea has not yet been demonstrated. In this study, we generated human otic progenitor cells (hOPCs) from induced pluripotent stem cells (iPSCs) in vitro and identified these cells by the expression of known otic markers. We showed successful cell transplantation of iPSC-derived-hOPCs in an in vivo adult guinea pig model of ototoxicity. The delivered hOPCs migrated throughout the cochlea, engrafted in non-sensory regions, and survived up to 4 weeks post-transplantation. Some of the engrafted hOPCs responded to environmental cues within the cochlear sensory epithelium and displayed molecular features of early sensory differentiation. We confirmed these results with hair cell progenitors derived from Atoh1-GFP mice as donor cells. These mouse otic progenitors transplanted using the same in vivo delivery system migrated into damaged cochlear sensory epithelium and adopted a partial sensory cell fate. This is the first report of the survival and differentiation of hOPCs in ototoxic-injured mature cochlear epithelium, and it should stimulate further research into cell-based therapies for treatment of deafness.

Gain of FGF4 is a frequent event in KIT/PDGFRA/SDH/RAS-P WT GIST.

Gastrointestinal stromal tumors (GIST) lacking mutations in KIT/PDGFRA or RAS pathways and retaining an intact SDH complex are usually referred to as KIT/PDGFRA/SDH/RAS-P WT GIST or more simply quadruple WT GIST (~5% of all GIST). Despite efforts made, no recurrent genetic event in quadruple WT GIST has been identified so far. To further investigate this disease, we performed high throughput copy number analysis on quadruple WT GIST specimens identifying a recurrent focal gain in band 11q13.3 (involving FGF3/FGF4) in 6/8 cases. This event was not found in the other molecular GIST subgroups. FGF3/FGF4 duplication was associated with high expression of FGF4, both at mRNA and protein level, a growth factor normally not expressed in adult tissues or in KIT/PDGFRA-mutated GIST. FGFR1 was found to be the predominant FGF receptor expressed and phosphorylation of AKT was detected, suggesting that a FGF4-FGFR1 autocrine loop could stimulate downstream signaling in quadruple WT GIST. Together with the recent reports of quadruple WT cases carrying FGFR1 activating alterations, these findings strengthen the hypothesis of a potential involvement of FGFR pathway deregulation in quadruple WT GIST, which may represent a rationale for novel therapeutic approaches.

Spatial and temporal inhibition of FGFR2b ligands reveals continuous requirements and novel targets in mouse inner ear morphogenesis.

Morphogenesis of the inner ear epithelium requires coordinated deployment of several signaling pathways, and disruptions cause abnormalities of hearing and/or balance. The FGFR2b ligands FGF3 and FGF10 are expressed throughout otic development and are required individually for normal morphogenesis, but their prior and redundant roles in otic placode induction complicates investigation of subsequent combinatorial functions in morphogenesis. To interrogate these roles and identify new effectors of FGF3 and FGF10 signaling at the earliest stages of otic morphogenesis, we used conditional gene ablation after otic placode induction, and temporal inhibition of signaling with a secreted, dominant-negative FGFR2b ectodomain. We show that both ligands are required continuously after otocyst formation for maintenance of otic neuroblasts and for patterning and proliferation of the epithelium, leading to normal morphogenesis of both the cochlear and vestibular domains. Furthermore, the first genome-wide identification of proximal targets of FGFR2b signaling in the early otocyst reveals novel candidate genes for inner ear development and function.

Sporadic and Hereditary Breast Cancer Genetics.

Genetic research provides the basis for sporadic and hereditary breast cancer diagnoses. Several mutated genes in sporadic breast cancer (eg, ERBB2 and myc) and hereditary breast cancer (eg, BRCA1 and BRCA2) influence how health care professionals assess breast cancer screenings and risks. The knowledge of these genetic mutations in the context of risk management, genetic counseling, genetic testing, and ethics might improve breast cancer treatment, prevention, and awareness.

Maldevelopment of the submandibular gland in a mouse model of apert syndrome.

BACKGROUND: Apert syndrome is characterized by craniosynostosis and bony syndactyly of the hands and feet. The cause of Apert syndrome is a single nucleotide substitution mutation (S252W or P253R) in fibroblast growth factor receptor 2 (FGFR2). Clinical experience suggests increased production of saliva by Apert syndrome patients, but this has not been formally investigated. FGFR2 signaling is known to regulate branching morphogenesis of the submandibular glands (SMGs). With the Apert syndrome mouse model (Ap mouse), we investigated the role of FGFR2 in SMGs and analyzed the SMG pathology of Apert syndrome. RESULTS: Ap mice demonstrated significantly greater SMG and sublingual gland (SMG/SLG complex) mass/body weight and percentage of parenchyma per unit area of the SMG compared with control mice. Furthermore, gene expression of Fgf1, Fgf2, Fgf3, Pdgfra, Pdgfrb, Mmp2, Bmp4, Lama5, Etv5, and Dusp6 was significantly higher in the SMG/SLG complex of Ap mice. FGF3 and BMP4 exhibited altered detection patterns. The numbers of macrophages were significantly greater in SMGs of Ap mice than in controls. Regarding functional evaluations of the salivary glands, no significant differences were observed. CONCLUSIONS: These results suggest that the gain-of-function mutation in FGFR2 in the SMGs of Ap mice enhances branching morphogenesis. Developmental Dynamics 247:1175-1185, 2018. © 2018 Wiley Periodicals, Inc.

Inhibitory effects of int-2 gene on the invasion and metastasis of oral cancer cells.

OBJECTIVE: The purpose of this study was to investigate the effects of int-2 transfection on the invasiveness and metastasis of oral cancer BcaCD885 cells, and to determine the relevant mechanisms of action. MATERIALS AND METHODS: High-purity int-2 eukaryotic expression plasmids were prepared and transfected using a modified cationic liposome-mediated transfection protocol. Nucleoside diphosphate kinase A (NDPKA) expression before and after transfection was examined, as well as changes in cell invasiveness and metastasis capabilities. RESULTS: Int-2 was confirmed to be stably expressed post-transfection into oral cancer cells. Expression of int-2 in BcaCD885 cells was significantly different before and after transfection. The proportion of invasive cells were 70.3%±8.2% and 46.5%±5.7%, and the proportion of chemotaxis cells were 78.5%±7.9% and 49.6%±7.5%, in the in the control and experimental groups respectively. The adhesion capability of cells in the experimental group was also significantly reduced. CONCLUSIONS: Upregulation of int-2 expression can significantly inhibit the invasion and metastasis of BcaCD885 cells.

Hypomethylated Fgf3 is a potential biomarker for early detection of oral cancer in mice treated with the tobacco carcinogen dibenzo[def,p]chrysene.

Genetic and epigenetic alterations observed at end stage OSCC formation could be considered as a consequence of cancer development and thus changes in normal or premalignant tissues which had been exposed to oral carcinogens such as Dibenzo[def,p]chrysene (DBP) may better serve as predictive biomarkers of disease development. Many types of DNA damage can induce epigenetic changes which can occur early and in the absence of evident morphological abnormalities. Therefore we used ERRBS to generate genome-scale, single-base resolution DNA methylomes from histologically normal oral tissues of mice treated with DBP under experimental conditions known to induce maximum DNA damage which is essential for the development of OSCC induced by DBP in mice. After genome-wide correction, 30 and 48 differentially methylated sites (DMS) were identified between vehicle control and DBP treated mice using 25% and 10% differences in methylation, respectively. RT-PCR was further performed to examine the expressions of nine selected genes. Among them, Fgf3, a gene frequently amplified in head and neck cancer, showed most prominent and significant gene expression change (2.4× increases), despite the hypomethylation of Fgf3 was identified at >10kb upstream of transcription start site. No difference was observed in protein expression between normal oral tissues treated with DBP or vehicle as examined by immunohistochemistry. Collectively, our results indicate that Fgf3 hypomethylation and gene overexpression, but not protein expression, occurred in the early stage of oral carcinogenesis induced by DBP. Thus, Fgf3 hypomethylation may serve as a potential biomarker for early detection of OSCC.

Efficacy assessment of self-assembled PLGA-PEG-PLGA nanoparticles: Correlation of nano-bio interface interactions, biodistribution, internalization and gene expression studies.

The aim of our study was to develop and compare the biological performance of two types of biodegradable SN-38 loaded nanoparticles (NPs) with various surface properties, composed of low and high Mw triblock PLGA-PEG-PLGA copolymers, applying rational quality and safety by design approach. Therefore, along with the optimization of crucial physico-chemical properties and in order to evaluate the therapeutical potential and biocompatibility of prepared polymeric nanoparticles, analysis of nano-bio interactions, cell internalization, gene expression and biodistribution studies were performed. The optimized formulations, one of low Mw and one composed of high Mw PLGA-PEG-PLGA copolymer, exhibited different characteristics in terms of surface properties, particle size, zeta potential, drug loading, protein adsorption and biodistribution, which may be attributed to the variations in nano-bio interface interactions due to different NP building blocks length and Mw. On the contrary to protein adsorption and biodistribution studies, both types of NPs exhibited similar results during cell internalization and gene expression studies performed in cell culture medium containing serum proteins. This pool of useful data for internalization and efficacy as well as the notable advance in the circulation time of low Mw NPs may be further employed for shaping the potential of the designed nanocarriers.

HER2-Overexpressing Breast Cancers Amplify FGFR Signaling upon Acquisition of Resistance to Dual Therapeutic Blockade of HER2.

Purpose: Dual blockade of HER2 with trastuzumab and lapatinib or pertuzumab has been shown to be superior to single-agent trastuzumab. However, a significant fraction of HER2-overexpressing (HER2+) breast cancers escape from these drug combinations. In this study, we sought to discover the mechanisms of acquired resistance to the combination of lapatinib + trastuzumab.Experimental Design: HER2+ BT474 xenografts were treated with lapatinib + trastuzumab long-term until resistance developed. Potential mechanisms of acquired resistance were evaluated in lapatinib + trastuzumab-resistant (LTR) tumors by targeted capture next-generation sequencing. In vitro experiments were performed to corroborate these findings, and a novel drug combination was tested against LTR xenografts. Gene expression and copy-number analyses were performed to corroborate our findings in clinical samples.Results: LTR tumors exhibited an increase in FGF3/4/19 copy number, together with an increase in FGFR phosphorylation, marked stromal changes in the tumor microenvironment, and reduced tumor uptake of lapatinib. Stimulation of BT474 cells with FGF4 promoted resistance to lapatinib + trastuzumab in vitro Treatment with FGFR tyrosine kinase inhibitors reversed these changes and overcame resistance to lapatinib + trastuzumab. High expression of FGFR1 correlated with a statistically shorter progression-free survival in patients with HER2+ early breast cancer treated with adjuvant trastuzumab. Finally, FGFR1 and/or FGF3 gene amplification correlated with a lower pathologic complete response in patients with HER2+ early breast cancer treated with neoadjuvant anti-HER2 therapy.Conclusions: Amplification of FGFR signaling promotes resistance to HER2 inhibition, which can be diminished by the combination of HER2 and FGFR inhibitors. Clin Cancer Res; 23(15); 4323-34. ©2017 AACR.