Beyond conventional microscopy: Observing kidney tissues by means of fourier ptychography.

Kidney microscopy is a mainstay in studying the morphological structure, physiology and pathology of kidney tissues, as histology provides important results for a reliable diagnosis. A microscopy modality providing at same time high-resolution images and a wide field of view could be very useful for analyzing the whole architecture and the functioning of the renal tissue. Recently, Fourier Ptychography (FP) has been proofed to yield images of biology samples such as tissues and in vitro cells while providing high resolution and large field of view, thus making it a unique and attractive opportunity for histopathology. Moreover, FP offers tissue imaging with high contrast assuring visualization of small desirable features, although with a stain-free mode that avoids any chemical process in histopathology. Here we report an experimental measuring campaign for creating the first comprehensive and extensive collection of images of kidney tissues captured by this FP microscope. We show that FP microscopy unlocks a new opportunity for the physicians to observe and judge renal tissue slides through the novel FP quantitative phase-contrast microscopy. Phase-contrast images of kidney tissue are analyzed by comparing them with the corresponding renal images taken under a conventional bright-field microscope both for stained and unstained tissue samples of different thicknesses. In depth discussion on the advantages and limitations of this new stain-free microscopy modality is reported, showing its usefulness over the classical light microscopy and opening a potential route for using FP in clinical practice for histopathology of kidney.

Primary cutaneous B-cell lymphoma other than marginal zone: clinicopathologic analysis of 161 cases: Comparison with current classification and definition of prognostic markers.

Categorization of primary cutaneous B-cell lymphomas (PCBCL) other than marginal zone (MZL) represents a diagnostic challenge with relevant prognostic implications. The 2008 WHO lymphoma classification recognizes only primary cutaneous follicular center cell lymphoma (PCFCCL) and primary cutaneous diffuse large B-cell lymphoma, leg type (PCDLBCL-LT), whereas the previous 2005 WHO/EORTC classification also included an intermediate form, namely PCDLBCL, other. We conducted a retrospective, multicentric, consensus-based revision of the clinicopathologic characteristics of 161 cases of PCBCL other than MZL. Upon the histologic features that are listed in the WHO classification, 96 cases were classified as PCFCCL and 25 as PCDLBCL-LT; 40 further cases did not fit in the former subgroups in terms of cytology and/or architecture, thus were classified as PCDLBCL, not otherwise specified (PCDLBCL-NOS). We assigned all the cases a histogenetic profile, based on the immunohistochemical detection of CD10, BCL6, and MUM1, and a "double hit score" upon positivity for BCL2 and MYC. PCDLBCL-NOS had a clinical presentation more similar to PCFCCL, whereas the histology was more consistent with the picture of a diffuse large B-cell lymphoma, as predominantly composed of centroblasts but with intermixed a reactive infiltrate of small lymphocytes. Its behavior was intermediate between the other two forms, particularly when considering only cases with a "non-germinal B-cell" profile, whereas "germinal center" cases resembled PCFCCL. Our data confirmed the aggressive behavior of PCDLBC-LT, which often coexpressed MYC and BCL2. The impact of single factors on 5-year survival was documented, particularly histogenetic profile in PCDLBCL and BCL2 translocation in PCFCCL. Our study confirms that a further group-PCDLBCL-NOS-exists, which can be recognized through a careful combination of histopathologic criteria coupled with adequate clinical information.

Assessment of a HER2 scoring system for colorectal cancer: results from a validation study.

We sought to develop criteria for ERBB2-positivity (HER2) in colorectal cancer to ensure accurate identification of ERBB2-amplified metastatic colorectal cancer patients suitable for enrollment in a phase II trial of ERBB2-targeted therapy (HERACLES trial). A two-step approach was used. In step 1, a consensus panel of pathologists adapted existing protocols for use in colorectal cancer to test ERBB2 expression and amplification. Collegial revision of an archival test cohort of colorectal cancer samples led to specific recommendations for adapting current breast and gastric cancer criteria for scoring ERBB2 in colorectal cancer. In step 2, from September 2012 to January 2015, colorectal-specific ERBB2 testing protocols and ERBB2 scoring criteria were used to centrally screen for ERBB2-positive KRAS wild-type colorectal cancer patients to be enrolled in the HERACLES trial (clinical validation cohort). In both archival test (N=256) and clinical validation (N=830) cohorts, a clinically sizeable 5% fraction of KRAS wild-type colorectal cancer patients was found to be ERBB2-positive according to the colorectal cancer-specific ERBB2 scoring criteria. ERBB2-positive tumors showed ERBB2 immunostaining consisting of intense membranous ERBB2 protein expression, corresponding to homogenous ERBB2 amplification, in >50% of cells. None of the immunohistochemistry 0 or 1+ cases was amplified. Concordance between SISH and FISH was 100%. In conclusion, we propose specific criteria for defining ERBB2-positivity in colorectal cancer (HERACLES Diagnostic Criteria). In a phase II trial of trastuzumab and lapatinib in a cetuximab-resistant population, HERACLES Diagnostic Criteria shaped the selection of patients and defined ERBB2 as a predictive marker for response to ERBB2-targeted therapy in metastatic colorectal cancer.

Predictive value of computerized morphometric analysis of steatosis in donor livers.

OBJECTIVE: To describe, by computerized morphometry, the degree and the type of steatosis in liver transplants that developed primary nonfunction and to compare the results with the quantification by pathologist. STUDY DESIGN: Twelve patients who developed primary nonfunction after liver transplantation were matched with 23 transplanted patients with a regular postoperative clinical course. Morphology of the liver biopsy included many stereological parameters; all cases were evaluated by an operator blinded to the diagnosis and to the clinical history. The assessment of steatosis by morphometry was compared with the pathologist's evaluation. Moreover, to assess the reproducibility of the morphometric model, another operator applied the morphometric model in a blinded fashion to a randomly selected sample of cases. RESULTS: The percentage of hepatocytes with microsteatosis and the ratio of macro/microsteatosis were higher in primary nonfunction. The pathologist's evaluation of steatosis showed a marked overestimation when compared to morphometry. Lastly, the comparison between the results of 2 blinded operators of morphometric analysis showed a high reproducibility with a low interobserver variability. CONCLUSION: Our quantitative estimation of the degree and the quality of steatosis avoids interobserver interpretations. Moreover, our analysis shows that the quantification of steatosis in liver transplantation by the current assessment must be reviewed in order to reevaluate the real impact of steatosis.

MicroRNA as potential biomarker in HCV-associated diffuse large B-cell lymphoma.

AIMS: To identify molecular characteristics to hepatitis C virus (HCV)-associated diffuse large B-cell lymphoma (DLBCL) through a comprehensive miRNAs expression profiling. METHODS: In this study, miRNA profiles were obtained from 37 patients with DLBCLs and 60 patients with reactive lymph nodes, equally distributed according to HCV presence. Germinal centres, from reactive lymph nodes were used as controls. Clinical features at presentation were available for all patients. RESULTS: A set of 52 miRNAs define a signature for HCV-associated DLBCL. Importantly, decreased expression of miR-138-5p and increased expression of miR-147a, miR-147b and miR-511-5p in HCV DLBCL was found to be a poor prognostic factor for HCV-positive DLBCL patients. CONCLUSIONS: These data reveal molecular differences in diffuse DLBCL patients according to HCV presence, potentially useful as novel prognostic or therapeutic biomarkers.

The TPM3-NTRK1 rearrangement is a recurring event in colorectal carcinoma and is associated with tumor sensitivity to TRKA kinase inhibition.

The NTRK1 gene encodes Tropomyosin-related kinase A (TRKA), the high-affinity Nerve Growth Factor Receptor. NTRK1 was originally isolated from a colorectal carcinoma (CRC) sample as component of a somatic rearrangement (TPM3-NTRK1) resulting in expression of the oncogenic chimeric protein TPM3-TRKA, but there has been no subsequent report regarding the relevance of this oncogene in CRC. The KM12 human CRC cell line expresses the chimeric TPM3-TRKA protein and is hypersensitive to TRKA kinase inhibition. We report the detailed characterization of the TPM3-NTRK1 genomic rearrangement in KM12 cells and through a cellular screening approach, the identification of NMS-P626, a novel highly potent and selective TRKA inhibitor. NMS-P626 suppressed TPM3-TRKA phosphorylation and downstream signaling in KM12 cells and showed remarkable antitumor activity in mice bearing KM12 tumors. Finally, using quantitative reverse transcriptase PCR and immunohistochemistry (IHC) we identified the TPM3-NTRK1 rearrangement in a CRC clinical sample, therefore suggesting that this chromosomal translocation is indeed a low frequency recurring event in CRC and that such patients might benefit from therapy with TRKA kinase inhibitors.

Beyond the learning curve of the Retzius-sparing approach for robot-assisted laparoscopic radical prostatectomy: oncologic and functional results of the first 200 patients with ≥ 1 year of follow-up.

BACKGROUND: Robot-assisted laparoscopic radical prostatectomy (RARP) has become the main surgical option for localized prostate cancer. We recently developed a new approach for RARP, passing through the pouch of Douglas and avoiding all the Retzius structures involved in continence and potency preservation. OBJECTIVE: To report the functional and oncologic results of our first 200 patients operated on using this new approach. DESIGN, SETTING, AND PARTICIPANTS: This was a prospective, noncontrolled case series including the first 200 consecutive patients undergoing this kind of surgery (January the 1st, 2010 to December the 31st, 2011). SURGICAL PROCEDURE: Retzius-sparing RARP. OUTCOME MEASUREMENTS AND STATISTICAL ANALYSIS: All perioperative, oncologic, and functional data were prospectively recorded. Potency was defined as an International Index of Erectile Function-5 questionnaire score >17; continence was defined as use of no pad or of one safety liner. Oncologic results were reported as positive surgical margins (PSM) and 1-yr biochemical disease-free survival (1y-bDFS). Recurrence was defined as a repeated prostate-specific antigen >0.2 ng/ml. Complications were graded according to the Clavien-Dindo system. The first 100 patients (group 1) were compared with the second 100 (group 2) to evaluate the learning curve effects. RESULTS AND LIMITATIONS: The median patient age was 65 yr. Comparing the two groups, transfusions were needed in 8% versus 4% of cases in groups 1 and 2, respectively (p=0.02). There was one Clavien-Dindo grade 3b in group 1 versus one grade 3a complication in group 2. In patients with pT2 disease, PSMs were recorded in 22.4% of those in group 1 versus 10.1% in group 2 (p=0.045). 1y-bDFS was 89% in group 1 versus 92% in group 2. For groups 1 and 2, respectively, immediate continence was reached in 92% versus 90% of patients, and the 1-yr continence rate was 96% versus 96%. Considering the 77 potent patients aged <65 yr who underwent bilateral intrafascial nerve-sparing surgery, 40.4% of those in group 1 versus 40% of those in group 2 reached their first intercourse within 1 mo; at 1 yr of follow-up, these figures had increased to 81% versus 71%, respectively (p=0.162). The main limitation of this study is its noncontrolled nature. CONCLUSIONS: We demonstrated Retzius-sparing RARP to be oncologically safe and to result in high early continence and potency rates. Long-term, prospective, comparative, and possibly randomized studies are needed.

The reticulin algorithm for adrenocortical tumor diagnosis: a multicentric validation study on 245 unpublished cases.

The pathologic diagnosis of adrenocortical carcinoma (ACC) still needs to be improved, because the renowned Weiss Score (WS) system has a poor reproducibility of some parameters and is difficult to apply in borderline cases and in ACC variants. The "reticulin algorithm" (RA) defines malignancy through an altered reticulin framework associated with 1 of the 3 following parameter: necrosis, high mitotic rate, and vascular invasion. This study aimed at validating the interobserver reproducibility of reticulin stain evaluation in an unpublished series of 245 adrenocortical tumors (61 adenomas and 184 carcinomas) from 5 Italian centers, classified according to the WS. Eight pathologists reviewed all reticulin-stained slides. After training, a second round of evaluation on discordant cases was performed 10 weeks later. The RA reclassified 67 cases (27%) as adenomas, including 44 with no reticulin alterations and 23 with an altered reticulin framework but lacking the subsequent parameters of the triad. The other 178 cases (73%) were carcinomas according to the above-mentioned criteria. A complete (8/8 pathologists) interobserver agreement was reached in 75% of cases (κ=0.702), irrespective of case derivation, pathologists' experience, and histologic variants, and was further improved when only those cases with high WS and clinically malignant behavior were considered. After the training, the overall agreement increased to 86%. We conclude that reticulin staining is a reliable technique and an easy-to-interpret system in adrenocortical tumors; moreover, it has a high interobserver reproducibility, which supports the notion of using such a method in the proposed 2-step RA approach for ACC diagnosis.

Amplification of the MET receptor drives resistance to anti-EGFR therapies in colorectal cancer.

EGF receptor (EGFR)-targeted monoclonal antibodies are effective in a subset of metastatic colorectal cancers. Inevitably, all patients develop resistance, which occurs through emergence of KRAS mutations in approximately 50% of the cases. We show that amplification of the MET proto-oncogene is associated with acquired resistance in tumors that do not develop KRAS mutations during anti-EGFR therapy. Amplification of the MET locus was present in circulating tumor DNA before relapse was clinically evident. Functional studies show that MET activation confers resistance to anti-EGFR therapy both in vitro and in vivo. Notably, in patient-derived colorectal cancer xenografts, MET amplification correlated with resistance to EGFR blockade, which could be overcome by MET kinase inhibitors. These results highlight the role of MET in mediating primary and secondary resistance to anti-EGFR therapies in colorectal cancer and encourage the use of MET inhibitors in patients displaying resistance as a result of MET amplification.

Postsurgical intrapericardial adhesions: mechanisms of formation and prevention.

Postsurgical intrapericardial adhesions are still considered an unavoidable consequence of cardiothoracic operations. They increase the technical difficulty and the risk of reoperations. The pathogenesis of postsurgical adhesions is a multistep process, and the main key players are (1) loss of mesothelial cells, (2) accumulation of fibrin in areas devoid of mesothelial cells, (3) loss of normal pericardial fibrinolysis, and (4) local inflammation. Today, very promising methods to reduce adhesions are available for clinical use. This report reviews the process of formation of adhesions and the methods to prevent them, classified according to the mechanism of action.

Results of the First Italian External Quality Assurance Scheme for somatic EGFR mutation testing in non-small-cell lung cancer.

INTRODUCTION: The Italian Association of Medical Oncology (AIOM) and the Italian Society of Pathology and Cytology organized an external quality assessment (EQA) scheme for EGFR mutation testing in non-small-cell lung cancer. METHODS: Ten specimens, including three small biopsies with known epidermal growth factor receptor (EGFR) mutation status, were validated in three referral laboratories and provided to 47 participating centers. The participants were requested to perform mutational analysis, using their usual method, and to submit results within a 4-week time frame. According to a predefined scoring system, two points were assigned to correct genotype and zero points to false-negative or false-positive results. The threshold to pass the EQA was set at higher than 18 of 20 points. Two rounds were preplanned. RESULTS: All participating centers submitted the results within the time frame. Polymerase chain reaction (PCR)/sequencing was the main methodology used (n = 37 laboratories), although a few centers did use pyrosequencing (n = 8) or real-time PCR (n = 2). A significant number of analytical errors were observed (n = 20), with a high frequency of false-positive results (n = 16). The lower scores were obtained for the small biopsies. Fourteen of 47 centers (30%) that did not pass the first round, having a score less than or equal to 18 points, used PCR/sequencing, whereas 10 of 10 laboratories, using pyrosequencing or real-time PCR, passed the first round. Eight laboratories passed the second round. Overall, 41of 47 centers (87%) passed the EQA. CONCLUSION: The results of the EQA for EGFR testing in non-small-cell lung cancer suggest that good quality EGFR mutational analysis is performed in Italian laboratories, although differences between testing methods were observed, especially for small biopsies.

Histological findings following use of CoSeal in a patient with a left ventricular assist device.

Adhesions are a formidable challenge in patients undergoing reoperative cardiac surgery, particularly in those supported by an intracorporeal left ventricular assist device (LVAD) and undergoing heart transplantation. This report describes the pathological findings following the clinical use of a surgical sealant (CoSeal, Baxter Healthcare, Fremont, CA), in a patient who underwent LVAD implantation. On the treated surfaces, a minimal amount of adhesions were observed, whereas in untreated surfaces adhesions were present.

KRAS gene amplification in colorectal cancer and impact on response to EGFR-targeted therapy.

KRAS mutations are the most common oncogenic event in colorectal cancer (CRC) progression and their occurrence is associated with lack of response to anti epidermal growth factor receptor (EGFR) targeted therapies. Using preclinical models and patients' samples we recently reported that the emergence of KRAS mutations but also KRAS amplification is associated with acquired resistance to the EGFR inhibitors cetuximab or panitumumab. We reasoned that KRAS amplification may also be responsible for primary resistance to these agents. Furthermore, while the prevalence of KRAS mutations has been well established in CRC, little is known about the frequency of KRAS amplification in large CRC series. We performed a screening of 1,039 CRC samples to assess the prevalence of KRAS amplification in this tumor type and further evaluated the role of this genetic alteration on the sensitivity to anti EGFR therapies. We detected KRAS amplification in 7/1,039 (0.67%) and 1/102 evaluable CRC specimens and cell lines, respectively. KRAS amplification was mutually exclusive with KRAS mutations. Tumors or cell lines harboring this genetic lesion are not responsive to anti-EGFR inhibitors. Although KRAS amplification is an infrequent event in CRC, it might be responsible for precluding response to anti-EGFR treatment in a small proportion of patients.

IRTA1 is selectively expressed in nodal and extranodal marginal zone lymphomas.

AIMS: The aim of this study was to search for a molecule selectively expressed by marginal zone (MZ) lymphomas (MZLs), whose diagnosis is currently based on morphological criteria and negativity for markers detectable in other B-cell lymphomas. METHODS AND RESULTS: Two thousand one hundred and four peripheral lymphomas of various types were immunostained with a monoclonal antibody against immunoglobulin superfamily receptor translocation-associated 1 (IRTA1), which recognizes the equivalents of MZ in human lymphoid tissues other than spleen. IRTA1 expression was restricted to extranodal (93%) and nodal MZLs (73%) and to lymphomas with MZ differentiation. Extranodal MZL cells with the strongest IRTA1 expression were usually located adjacent to epithelia, mimicking the IRTA1 expression pattern of normal and acquired mucosa-associated lymphoid tissue (MALT). The cytological features, growth pattern and IRTA1 positivity in nodal MZLs suggest they may derive from IRTA1(+) perifollicular B cells or monocytoid B cells detectable in reactive lymph nodes. Double immunostaining for IRTA1/bcl-6 tracked the colonization of B-cell follicles by MZL cells, and showed modulation of their phenotype (e.g. acquisition of bcl-6) during recirculation through germinal centres. MZL cells differentiating into plasma cells usually lost IRTA1. CONCLUSIONS: These results further expand our knowledge of the biology of MZLs, and highlight IRTA1 as the first positive marker for MZLs, enabling more accurate diagnosis of these neoplasms.

Emergence of KRAS mutations and acquired resistance to anti-EGFR therapy in colorectal cancer.

A main limitation of therapies that selectively target kinase signalling pathways is the emergence of secondary drug resistance. Cetuximab, a monoclonal antibody that binds the extracellular domain of epidermal growth factor receptor (EGFR), is effective in a subset of KRAS wild-type metastatic colorectal cancers. After an initial response, secondary resistance invariably ensues, thereby limiting the clinical benefit of this drug. The molecular bases of secondary resistance to cetuximab in colorectal cancer are poorly understood. Here we show that molecular alterations (in most instances point mutations) of KRAS are causally associated with the onset of acquired resistance to anti-EGFR treatment in colorectal cancers. Expression of mutant KRAS under the control of its endogenous gene promoter was sufficient to confer cetuximab resistance, but resistant cells remained sensitive to combinatorial inhibition of EGFR and mitogen-activated protein-kinase kinase (MEK). Analysis of metastases from patients who developed resistance to cetuximab or panitumumab showed the emergence of KRAS amplification in one sample and acquisition of secondary KRAS mutations in 60% (6 out of 10) of the cases. KRAS mutant alleles were detectable in the blood of cetuximab-treated patients as early as 10 months before radiographic documentation of disease progression. In summary, the results identify KRAS mutations as frequent drivers of acquired resistance to cetuximab in colorectal cancers, indicate that the emergence of KRAS mutant clones can be detected non-invasively months before radiographic progression and suggest early initiation of a MEK inhibitor as a rational strategy for delaying or reversing drug resistance.

Morphometric analysis of hepatocellular nodular lesions in HCV cirrhosis.

We generated a computerized morphometric model to evaluate and quantify the morphological features in large regenerative nodules (LRN), high-grade dysplastic nodules (HGDN) and hepatocellular carcinoma (HCC). Sixteen LRN, 10 HGDN and 16 HCC in HCV-cirrhotic livers were stained with H&E, smooth muscle actin, CD34, CD31 and reticulin to evaluate volume and surface fractions. On H&E stains, the most discriminatory features between LRN, HGDN and HCC were volume fraction and the number of hepatocyte nuclei in unit volume and hepatocyte nuclear/cytoplasmic ratio. On immunohistochemistry, volume fractions of capillarised sinusoids, capillary units and isolated arteries were significantly different among all groups and highest in HCC; surface fraction of reticulin was markedly decreased in HCC. Our morphometric model is an objective method for quantification of the morphological changes of the nodular lesions, and it could be applied to studies involving histological evaluation of the spectrum of nodular lesions arising in the cirrhotic liver.

A quantitative-PCR protocol rapidly detects αGAL deletions/duplications in patients with Anderson-Fabry disease.

The Anderson-Fabry disease (AFD) is an X-linked glycosphingolipidosis leading to a progressive systemic disease. A particular variant of the disease of AFD presents only with left ventricular hypertrophy (LVH). Molecular diagnosis with bidirectional sequencing fails to detect genomic re-arrangements in female patients because of the presence of the wt allele. We here propose a quantitative PCR-based method alternative/complementary to the MLPA.

Standardisation of EGFR FISH in colorectal cancer: results of an international interlaboratory reproducibility ring study.

AIMS: Epidermal growth factor receptor (EGFR) gene copy number evaluated by fluorescence in situ hybridisation (FISH) can discriminate among KRAS wild-type patients those with better outcome to EGFR-targeted therapy in metastatic colorectal cancer, further enhancing selection of patients. Nevertheless, enumeration of gene copies is challenging and the lack of analytical standardisation has limited incorporation of the test into the clinical practice. We therefore assessed EGFR FISH interlaboratory consensus among five molecular diagnostic reference centres. METHODS: A set of 12 colorectal cancer samples circulated among laboratories, and samples were scored according to commonly agreed guidelines. Reproducibility was quantified using the standard error of measurement (SEM). RESULTS: A SEM of 0.865 and a within-subject coefficient of variation (WSCV) of 26.8% for mean EGFR gene/nuclei and a SEM of 0.235 and a WSCV of 19.4% for the mean EGFR gene/CEP7 ratio were observed. Measurement of the fraction of cells displaying chromosome 7 polysomy showed WSCV of 46.6%, 34.0% and 51.0% for percentage of cells displaying ≤2, ≥3 and ≥4 EGFR signals, respectively. Among different slides of the same specimen, the WSCV was 6.1% for mean EGFR gene/nuclei and 3.9% for mean of EGFR gene/CEP7 ratios. CONCLUSIONS: Molecular diagnosis of EGFR gene copy number by FISH varied largely among pathology centres, with fluctuations covering the whole range of proposed cut-offs of predictive usefulness from literature. Definition of a detailed scoring system and implementation of comprehensive training programmes for laboratories are therefore necessary before including the test into clinical practice.

A molecularly annotated platform of patient-derived xenografts ("xenopatients") identifies HER2 as an effective therapeutic target in cetuximab-resistant colorectal cancer.

UNLABELLED: Only a fraction of patients with metastatic colorectal cancer receive clinical benefit from therapy with anti-epidermal growth factor receptor (EGFR) antibodies, which calls for the identification of novel biomarkers for better personalized medicine. We produced large xenograft cohorts from 85 patient-derived, genetically characterized metastatic colorectal cancer samples ("xenopatients") to discover novel determinants of therapeutic response and new oncoprotein targets. Serially passaged tumors retained the morphologic and genomic features of their original counterparts. A validation trial confirmed the robustness of this approach: xenopatients responded to the anti-EGFR antibody cetuximab with rates and extents analogous to those observed in the clinic and could be prospectively stratified as responders or nonresponders on the basis of several predictive biomarkers. Genotype-response correlations indicated HER2 amplification specifically in a subset of cetuximab-resistant, KRAS/NRAS/BRAF/PIK3CA wild-type cases. Importantly, HER2 amplification was also enriched in clinically nonresponsive KRAS wild-type patients. A proof-of-concept, multiarm study in HER2-amplified xenopatients revealed that the combined inhibition of HER2 and EGFR induced overt, long-lasting tumor regression. Our results suggest promising therapeutic opportunities in cetuximab-resistant patients with metastatic colorectal cancer, whose medical treatment in the chemorefractory setting remains an unmet clinical need. SIGNIFICANCE: Direct transfer xenografts of tumor surgical specimens conserve the interindividual diversity and the genetic heterogeneity typical of the tumors of origin, combining the flexibility of preclinical analysis with the informative value of population-based studies. Our suite of patient-derived xenografts from metastatic colorectal carcinomas reliably mimicked disease response in humans, prospectively recapitulated biomarker-based case stratification, and identified HER2 as a predictor of resistance to anti-epidermal growth factor receptor antibodies and of response to combination therapies against HER2 and epidermal growth factor receptor in this tumor setting.