Monitoring ctDNA RAS Mutational Status in Metastatic Colorectal Cancer: A Trial Protocol of RAS-trace and RAS-trace-2 Studies.

Background: Spatial and temporal heterogeneities of RAS and other molecular genes should be considered in the treatment of metastatic colorectal cancer (mCRC) treated with anti-epidermal growth factor receptor (EGFR) monoclonal antibodies (mAbs); acquired RAS mutation is sometimes observed at disease progression of treatment with the anti-EGFR mAb. At the same time, discrepancy of RAS status from tissues and circulating tumor DNA (ctDNA) in the same patient is sometimes observed. Based on this, we commenced two observational studies to clarify these heterogeneities of RAS and BRAF in mCRC, using next generation sequencing from liquid biopsy. Methods/Design: RAS-trace study is an observational study to monitor ctDNA RAS/BRAF/PIK3CA status every 4-12 weeks using the Plasma-SeqSensei™ CRC RUO Kit (Sysmex Inostics GmbH) in mCRC with RAS/BRAF wild-type (wt) on tumor tissue. The primary endpoint was the time to the acquired RAS mutations. A total of 42 patients has been accrued. RAS-trace-2 study is also an observational study aimed at comparing the efficacy of the anti-EGFR mAb in ctDNA RAS/BRAF wt with ctDNA RAS or BRAF mutant mCRC patients, whose RAS/BRAF are wt in tumor tissue. The primary endpoint was progression-free survival in patients with ctDNA RAS/BRAF wt and RAS or BRAF mutant. A total of 240 patients will be accrued over 2 years. Discussion: These trials will help us understanding the clinical significance of spatial and temporal heterogeneities of RAS, BRAF and other genes, while optimizing the anti-EGFR mAb treatment strategies in mCRC.

Pre-Analytical Evaluation of Streck Cell-Free DNA Blood Collection Tubes for Liquid Profiling in Oncology.

Excellent pre-analytical stability is an essential precondition for reliable molecular profiling of circulating tumor DNA (ctDNA) in oncological diagnostics. Therefore, in vitro degradation of ctDNA and the additional release of contaminating genomic DNA from lysed blood cells must be prevented. Streck Cell-Free DNA blood collection tubes (cfDNA BCTs) have proposed advantages over standard K2EDTA tubes, but mainly have been tested in healthy individuals. Blood was collected from cancer patients (n = 53) suffering from colorectal (n = 21), pancreatic (n = 11), and non-small-cell lung cancer (n = 21) using cfDNA BCT tubes and K2EDTA tubes that were processed immediately or after 3 days (BCTs) or 6 hours (K2EDTA) at room temperature. The cfDNA isolated from these samples was characterized in terms of yield using LINE-1 qPCR; the level of gDNA contamination; and the mutation status of KRAS, NRAS, and EGFR genes using BEAMing ddPCR. CfDNA yield and gDNA levels were comparable in both tube types and were not affected by prolonged storage of blood samples for at least 3 days in cfDNA BCTs or 6 hours in K2EDTA tubes. In addition, biospecimens collected in K2EDTA tubes and cfDNA BCTs stored for up to 3 days demonstrated highly comparable levels of mutational load across all respective cancer patient cohorts and a wide range of concentrations. Our data support the applicability of clinical oncology specimens collected and stored in cfDNA BCTs for up to 3 days for reliable cfDNA and mutation analyses.

Next-generation sequencing (NGS) profiling of matched tumor and circulating tumor DNA (ctDNA) in head and neck squamous cell carcinoma (HNSCC).

OBJECTIVES: The aim of this pilot study was to evaluate the presence of somatic mutations in matched tumor and circulating DNA (ctDNA) samples from patients with primary head and neck squamous cell carcinoma (HNSCC) and assess the association of changes in ctDNA levels with survival. MATERIALS AND METHODS: Our study included 62 patients with stage I-IVB HNSCC treated with surgery or radical chemoradiotherapy with curative intent. Plasma samples were obtained at baseline, at the end of treatment (EOT), and at disease progression. Tumor DNA was extracted from plasma (ctDNA) and tumor tissue (tDNA). The Safe Sequencing System was used assess the presence of pathogenic variants in four genes (TP53, CDKN2A, HRAS and PI3KCA) in both ctDNA and tDNA. RESULTS: Forty-five patients had available tissue and plasma samples. Concordance of genotyping results between tDNA and ctDNA at baseline was 53.3%. TP53 mutations were most commonly identified at baseline in both ctDNA (32.6%) and tDNA (40%). The presence of mutations in this restricted set of 4 genes in tissue samples at baseline was associated with decreased overall survival (OS) [median 58.3 months for patients with mutations vs. 89 months for patients without mutations, p < 0.013]. Similarly, patients presenting with mutations in ctDNA had shorter OS [median 53.8 vs. 78.6 months, p < 0.037]. CtDNA clearance at EOT did not show any association with PFS or OS. CONCLUSIONS: Liquid biopsy enables real-time molecular characterization of HNSCC and might predict survival. Larger studies are needed to validate the utility of ctDNA as a biomarker in HNSCC.

Clearance of ctDNA in triple-negative and HER2-positive breast cancer patients during neoadjuvant treatment is correlated with pathologic complete response.

Background: Although the standard of care is to perform surgery of primary breast cancer (BC) after neoadjuvant chemotherapy (NAC), for certain patients achieving clinical complete response (cCR) and pathologic complete response (pCR), omission of surgical treatment may be an option. Levels of circulating tumor DNA (ctDNA) during and after therapy could identify patients achieving minimal residual disease. In this study, we evaluated whether ctDNA clearance during NAC could be a correlate to effective response in human epidermal growth factor receptor 2 positive (HER2+) and triple-negative (TN) BC patients. Methods: A prospective study was conducted to identify patient-specific PIK3CA and TP53 mutations in tissue using next-generation sequencing, which could then be used to track the presence/absence of mutations prior to, during, and following NAC using Sysmex SafeSEQ technology. All patients underwent a surgical excision after NAC, and pCR was assessed. Results: A total of 29 TN and HER2+ BC patients were examined and 20 that carried mutations in the PIK3CA and/or TP53 genes were recruited. Overall, 19 of these 20 patients harbored at least one tumor-specific mutation in their plasma at baseline. After NAC, 15 patients (75.0%) achieved pCR according to the histopathologic evaluation of the surgical specimen, and 15 patients (75.0%) had a cCR; 18 of 20 patients (90.0%) had concordant pCR and cCR. The status of 'no mutation detected' (NMD) following NAC in cCR patients correctly identified the pCR in 14 of 15 patients (93.33%), as well as correctly ruled out pCR in three patients, with an accuracy of 89.47%. During the 12-month follow-up after surgery, 40 plasma samples collected from 15 patients all showed no detectable ctDNA (NMD), and no patient recurred. Conclusion: These findings prompt further research of the value of ctDNA for non-invasive prediction of clinical/pathological response, raising the possibility of sparing surgery following NAC in selected BC patients.

Broad spectrum of regorafenib activity on mutant KIT and absence of clonal selection in gastrointestinal stromal tumor (GIST): correlative analysis from the GRID trial.

BACKGROUND: In the phase 3 GRID trial, regorafenib improved progression-free survival (PFS) independent of KIT mutations in exons 9 and 11. In this retrospective, exploratory analysis of the GRID trial, we investigated whether a more comprehensive KIT mutation analysis could identify mutations that impact treatment outcome with regorafenib and a regorafenib-induced mutation pattern. METHODS: Archived tumor samples, collected at any time prior to enrollment in GRID, were analyzed by Sanger sequencing (n = 102) and next-generation sequencing (FoundationONE; n = 47). Plasma samples collected at baseline were analyzed by BEAMing (n = 163) and SafeSEQ (n = 96). RESULTS: In archived tumor samples, 67% (68/102) had a KIT mutation; 61% (62/102) had primary KIT mutations (exons 9 and 11) and 12% (12/102) had secondary mutations (exons 13, 14, 17, and 18). At baseline, 81% of samples (78/96) had KIT mutations by SafeSEQ, including the M541L polymorphism (sole event in 6 patients). Coexisting mutations in other oncogenes were rare, as were mutations in PDGFR, KRAS, and BRAF. Regorafenib showed PFS benefit across all primary and secondary KIT mutational subgroups examined. Available patient-matched samples taken at baseline and end of treatment (n = 41; SafeSEQ), revealed heterogeneous KIT mutational changes with no specific mutation pattern emerging upon regorafenib treatment. CONCLUSION: These data support the results of the GRID trial, and suggest that patients may benefit from regorafenib in the presence of KIT mutations and without the selection of particular mutation patterns that confer resistance. The study was not powered to address biomarker-related questions, and the results are exploratory and hypothesis-generating.

Prospective study evaluating dynamic changes of cell-free HPV DNA in locoregional viral-associated oropharyngeal cancer treated with induction chemotherapy and response-adaptive treatment.

BACKGROUND: Human papillomavirus (HPV)-associated oropharyngeal cancer (OPC) has a favorable prognosis which has led to efforts to de-intensify treatment. Response-adaptive de-escalated treatment is promising, however improved biomarkers are needed. Quantitative cell-free HPV-DNA (cfHPV-DNA) in plasma represents an attractive non-invasive biomarker for grading treatment response and post-treatment surveillance. This prospective study evaluates dynamic changes in cfHPV-DNA during induction therapy, definitive (chemo)radiotherapy, and post-treatment surveillance in the context of risk and response-adaptive treatment for HPV + OPC. METHODS: Patients with locoregional HPV + OPC are stratified into two cohorts: High risk (HR) (T4, N3, [Formula: see text] 20 pack-year smoking history (PYH), or non-HPV16 subtype); Low risk (LR) (all other patients). All patients receive induction chemotherapy with three cycles of carboplatin and paclitaxel. LR with ≥ 50% response receive treatment on the single-modality arm (minimally-invasive surgery or radiation alone to 50 Gy). HR with ≥ 50% response or LR with ≥ 30% and < 50% response receive treatment on the intermediate de-escalation arm (chemoradiation to 50 Gy with cisplatin). All other patients receive treatment on the regular dose arm with chemoradiation to 70 Gy with concurrent cisplatin. Plasma cfHPV-DNA is assessed during induction, (chemo)radiation, and post-treatment surveillance. The primary endpoint is correlation of quantitative cfHPV-DNA with radiographic response. DISCUSSION: A de-escalation treatment paradigm that reduces toxicity without compromising survival outcomes is urgently needed for HPV + OPC. Response to induction chemotherapy is predictive and prognostic and can select candidates for de-escalated definitive therapy. Assessment of quantitative cfHPV-DNA in the context of response-adaptive treatment of represents a promising reliable and convenient biomarker-driven strategy to guide personalized treatment in HPV + OPC. TRIAL REGISTRATION: This trial is registered with ClinicalTrials.gov on October 1st, 2020 with Identifier: NCT04572100 .

Detection of TP53 and PIK3CA Mutations in Circulating Tumor DNA Using Next-Generation Sequencing in the Screening Process for Early Breast Cancer Diagnosis.

Circulating tumor DNA (ctDNA) has emerged as a non-invasive "liquid biopsy" for early breast cancer diagnosis. We evaluated the suitability of ctDNA analysis in the diagnosis of early breast cancer after mammography findings, comparing PIK3CA and TP53 mutations between tumor biopsies and pre-biopsy circulating DNA. Matched plasma and frozen fresh tissue biopsies from patients with Breast Imaging-Reporting and Data System (BIRADS) 4c/5 mammography findings and subsequent diagnosis of primary breast cancer were analyzed using NGS TruSeq Custom Amplicon Low Input Panel (Illumina) and plasma SafeSEQ (Sysmex Inostics). The same plasma and tumor mutations were observed in eight of 29 patients (27.6%) with four in TP53 and five in PIK3CA mutations. Sequencing analysis also revealed four additional ctDNA mutations (three in TP53 and one in PIK3CA) previously not identified in three patients tissue biopsy. One of these patients had mutations in both genes. Age, tumor grade and size, immunohistochemical (IHC) subtype, BIRADS category, and lymph node positivity were significantly associated with the detectability of these blood tumor-derived mutations. In conclusion, ctDNA analysis could be used in early breast cancer diagnosis, providing critical clinical information to improve patient diagnosis.

The deubiquitinating enzyme USP5 promotes pancreatic cancer via modulating cell cycle regulators.

Pancreatic ductal adenocarcinoma (PDAC) is one of the most lethal solid tumors. With an overall five-year survival rate remaining below 6%, there is an explicit need to search for new molecular targets for therapeutic interventions. We undertook a barcode labelled short-hairpin (shRNA) library screen in pancreatic cancer cells in order to identify novel genes promoting cancer survival and progression. Among the candidate genes identified in this screen was the deubiquitinase USP5, which subsequent gene expression analyses demonstrated to be significantly upregulated in primary human pancreatic cancer tissues. Using different knockdown approaches, we show that expression of USP5 is essential for the proliferation and survival of pancreatic cancer cells, tested under different 2D and 3D cell culture conditions as well as in in vivo experiments. These growth inhibition effects upon knockdown of USP5 are mediated primarily by the attenuation of G1/S phase transition in the cells, which is accompanied by accumulation of DNA damage, upregulation of p27, and increased apoptosis rates. Since USP5 is overexpressed in cancer tissues, it can thus potentially serve as a new target for therapeutic interventions, especially given the fact that deubiquitinases are currently emerging as new class of attractive drug targets in cancer.

Performance of Streck cfDNA Blood Collection Tubes for Liquid Biopsy Testing.

OBJECTIVES: Making liquid biopsy testing widely available requires a concept to ship whole blood at ambient temperatures while retaining the integrity of the cell-free DNA (cfDNA) population and stability of blood cells to prevent dilution of circulating tumor DNA (ctDNA) with wild-type genomic DNA. The cell- and DNA-stabilizing properties of Streck Cell-Free DNA BCT blood collection tubes (cfDNA BCTs) were evaluated to determine if they can be utilized in combination with highly sensitive mutation detection technologies. METHODS: Venous blood from healthy donors or patients with advanced colorectal cancer (CRC) was collected in cfDNA BCTs and standard K2EDTA tubes. Tubes were stored at different temperatures for various times before plasma preparation and DNA extraction. The isolated cfDNA was analyzed for overall DNA yield of short and long DNA fragments using qPCR as well as for mutational changes using BEAMing and Plasma Safe-Sequencing (Safe-SeqS). RESULTS: Collection of whole blood from healthy individuals in cfDNA BCTs and storage for up to 5 days at room temperature did not affect the DNA yield and mutation background levels (n = 60). Low-frequency mutant DNA spiked into normal blood samples as well as mutant circulating tumor DNA in blood samples from CRC patients collected in cfDNA BCTs were reliably detected after 3 days of storage at room temperature. However, blood samples stored at ≤ 10°C and at 40°C for an extended period of time showed elevated normal genomic DNA levels and an abnormally large cellular plasma interface as well as lower plasma volumes. CONCLUSION: Whole blood shipped in cfDNA BCTs over several days can be used for downstream liquid biopsy testing using BEAMing and Safe-SeqS. Since the shipping temperature is a critical factor, special care has to be taken to maintain a defined room temperature range to obtain reliable mutation testing results.

Detection and Quantification of KIT Mutations in ctDNA by Plasma Safe-SeqS.

We have designed a highly sensitive assay based on the Safe-SeqS technology to detect de novo mutations in the KIT gene and tested its performance. This assay was applied to plasma samples of GIST patients before and after treatment with regorafenib (GRID III trial) and mutations at known and novel sites of potential secondary resistance were identified.

Methylisoindigo preferentially kills cancer stem cells by interfering cell metabolism via inhibition of LKB1 and activation of AMPK in PDACs.

Pancreatic ductal adenocarcinoma (PDAC) clinically has a very poor prognosis. No small molecule is available to reliably achieve cures. Meisoindigo is chemically related to the natural product indirubin and showed substantial efficiency in clinical chemotherapy for CML in China. However, its effect on PDAC is still unknown. Our results showed strong anti-proliferation effect of meisoindigo on gemcitabine-resistant PDACs. Using a recently established primary PDAC cell line, called Jopaca-1 with a larger CSCs population as model, we observed a reduction of CD133+ and ESA+/CD44+/CD24+ populations upon treatment and concomitantly a decreased expression of CSC-associated genes, and reduced cellular mobility and sphere formation. Investigating basic cellular metabolic responses, we detected lower oxygen consumption and glucose uptake, while intracellular ROS levels increased. This was effectively neutralized by the addition of antioxidants, indicating an essential role of the cellular redox balance. Further analysis on energy metabolism related signaling revealed that meisoindigo inhibited LKB1, but activated AMPK. Both of them were involved in cellular apoptosis. Additional in situ hybridization in tissue sections of PDAC patients reproducibly demonstrated co-expression and -localization of LKB1 and CD133 in malignant areas. Finally, we detected that CD133+/CD44+ were more vulnerable to meisoindigo, which could be mimicked by LKB1 siRNAs. Our results provide the first evidence, to our knowledge, that LKB1 sustains the CSC population in PDACs and demonstrate a clear benefit of meisoindigo in treatment of gemcitabine-resistant cells. This novel mechanism may provide a promising new treatment option for PDAC.

High throughput synthetic lethality screen reveals a tumorigenic role of adenylate cyclase in fumarate hydratase-deficient cancer cells.

BACKGROUND: Synthetic lethality is an appealing technique for selectively targeting cancer cells which have acquired molecular changes that distinguish them from normal cells. High-throughput RNAi-based screens have been successfully used to identify synthetic lethal pathways with well-characterized tumor suppressors and oncogenes. The recent identification of metabolic tumor suppressors suggests that the concept of synthetic lethality can be applied to selectively target cancer metabolism as well. RESULTS: Here, we perform a high-throughput RNAi screen to identify synthetic lethal genes with fumarate hydratase (FH), a metabolic tumor suppressor whose loss-of-function has been associated with hereditary leiomyomatosis and renal cell carcinoma (HLRCC). Our unbiased screen identified synthetic lethality between FH and several genes in heme metabolism, in accordance with recent findings. Furthermore, we identified an enrichment of synthetic lethality with adenylate cyclases. The effects were validated in an embryonic kidney cell line (HEK293T) and in HLRCC-patient derived cells (UOK262) via both genetic and pharmacological inhibition. The reliance on adenylate cyclases in FH-deficient cells is consistent with increased cyclic-AMP levels, which may act to regulate cellular energy metabolism. CONCLUSIONS: The identified synthetic lethality of FH with adenylate cyclases suggests a new potential target for treating HLRCC patients.

A mammosphere formation RNAi screen reveals that ATG4A promotes a breast cancer stem-like phenotype.

INTRODUCTION: Breast cancer stem cells are suspected to be responsible for tumour recurrence, metastasis formation as well as chemoresistance. Consequently, great efforts have been made to understand the molecular mechanisms underlying cancer stem cell maintenance. In order to study these rare cells in-vitro, they are typically enriched via mammosphere culture. Here we developed a mammosphere-based negative selection shRNAi screening system suitable to analyse the involvement of thousands of genes in the survival of cells with cancer stem cell properties. METHODS: We describe a sub-population expressing the stem-like marker CD44(+)/CD24(-/low) in SUM149 that were enriched in mammospheres. To identify genes functionally involved in the maintenance of the sub-population with cancer stem cell properties, we targeted over 5000 genes by RNAi and tested their ability to grow as mammospheres. The identified candidate ATG4A was validated in mammosphere and soft agar colony formation assays. Further, we evaluated the influence of ATG4A expression on the sub-population expressing the stem-like marker CD44(+)/CD24(low). Next, the tumorigenic potential of SUM149 after up- or down-regulation of ATG4A was examined by xenograft experiments. RESULTS: Using this method, Jak-STAT as well as cytokine signalling were identified to be involved in mammosphere formation. Furthermore, the autophagy regulator ATG4A was found to be essential for the maintenance of a sub-population with cancer stem cell properties and to regulate breast cancer cell tumourigenicity in vivo. CONCLUSION: In summary, we present a high-throughput screening system to identify genes involved in cancer stem cell maintenance and demonstrate its utility by means of ATG4A.

Depletion of RAD17 sensitizes pancreatic cancer cells to gemcitabine.

Chemotherapy of advanced pancreatic cancer has mainly been gemcitabine-based for the past 15 years, with only limited effect. Recently, combination therapy that also targets checkpoint kinase 1 (CHK1) has become an attractive option. The central role of CHK1 in many DNA-damage response pathways, however, may result in undesired cytotoxicity in normal cells, causing side effects. We were searching for other target molecules of similar function that may be more specific and thus better suited for combination therapy. To this end a negative selection RNAi screen was performed in cell lines with small hairpin RNA molecules targeting over 10,000 genes. Genes that were found to be synthetically lethal with gemcitabine and whose proteins act upstream of CHK1 were characterised in more detail. In particular, the inhibition of RAD17 potentiated gemcitabine cytotoxicity in the pancreatic cancer cell lines BxPC-3 and MiaPaca-2 and in the primary cell line JoPaca-1 that closely resembles primary tumour tissue. Further analysis showed that the synergistic effect of RAD17 knockdown and gemcitabine leads to forced mitotic entry of cells arrested in S phase by gemcitabine treatment, resulting in asymmetric DNA distribution during anaphase followed by DNA fragmentation and finally cell death by mitotic catastrophe. Our data suggest RAD17 as a novel target protein for gemcitabine combination therapy supplementing or complementing inhibition of CHK1. In contrast to CHK1, RAD17 knockdown by itself does not lead to abnormal DNA segregation, suggesting a more specific action.

Establishment and characterization of a highly tumourigenic and cancer stem cell enriched pancreatic cancer cell line as a well defined model system.

Standard cancer cell lines do not model the intratumoural heterogeneity situation sufficiently. Clonal selection leads to a homogeneous population of cells by genetic drift. Heterogeneity of tumour cells, however, is particularly critical for therapeutically relevant studies, since it is a prerequisite for acquiring drug resistance and reoccurrence of tumours. Here, we report the isolation of a highly tumourigenic primary pancreatic cancer cell line, called JoPaca-1 and its detailed characterization at multiple levels. Implantation of as few as 100 JoPaca-1 cells into immunodeficient mice gave rise to tumours that were histologically very similar to the primary tumour. The high heterogeneity of JoPaca-1 was reflected by diverse cell morphology and a substantial number of chromosomal aberrations. Comparative whole-genome sequencing of JoPaca-1 and BxPC-3 revealed mutations in genes frequently altered in pancreatic cancer. Exceptionally high expression of cancer stem cell markers and a high clonogenic potential in vitro and in vivo was observed. All of these attributes make this cell line an extremely valuable model to study the biology of and pharmaceutical effects on pancreatic cancer.

Bioinformatic, structural, and functional analyses support release factor-like MTRF1 as a protein able to decode nonstandard stop codons beginning with adenine in vertebrate mitochondria.

Vertebrate mitochondria use stop codons UAA and UAG decoded by the release factor (RF) MTRF1L and two reassigned arginine codons, AGA and AGG. A second highly conserved RF-like factor, MTRF1, which evolved from a gene duplication of an ancestral mitochondrial RF1 and not a RF2, is a good candidate for recognizing the nonstandard codons. MTRF1 differs from other RFs by having insertions in the two external loops important for stop codon recognition (tip of helix alpha5 and recognition loop) and by having key substitutions that are involved in stop codon interactions in eubacterial RF/ribosome structures. These changes may allow recognition of the larger purine base in the first position of AGA/G and, uniquely for RFs, only of G at position 2. In contrast, residues that support A and G recognition in the third position in RF1 are conserved as would be required for recognition of AGA and AGG. Since an assay with vertebrate mitochondrial ribosomes has not been established, we modified Escherichia coli RF1 at the helix alpha5 and recognition loop regions to mimic MTRF1. There was loss of peptidyl-tRNA hydrolysis activity with standard stop codons beginning with U (e.g., UAG), but a gain of activity with codons beginning with A (AAG in particular). A lower level of activity with AGA could be enhanced by solvent modification. These observations imply that MTRF1 has the characteristics to recognize A as the first base of a stop codon as would be required to decode the nonstandard codons AGA and AGG.

Decoding pooled RNAi screens by means of barcode tiling arrays.

BACKGROUND: RNAi screens via pooled short hairpin RNAs (shRNAs) have recently become a powerful tool for the identification of essential genes in mammalian cells. In the past years, several pooled large-scale shRNA screens have identified a variety of genes involved in cancer cell proliferation. All of those studies employed microarray analysis, utilizing either the shRNA's half hairpin sequence or an additional shRNA-associated 60 nt barcode sequence as a molecular tag. Here we describe a novel method to decode pooled RNAi screens, namely barcode tiling array analysis, and demonstrate how this approach can be used to precisely quantify the abundance of individual shRNAs from a pool. RESULTS: We synthesized DNA microarrays with six overlapping 25 nt long tiling probes complementary to each unique 60 nt molecular barcode sequence associated with every shRNA expression construct. By analyzing dilution series of expression constructs we show how our approach allows quantification of shRNA abundance from a pool and how it clearly outperforms the commonly used analysis via the shRNA's half hairpin sequences. We further demonstrate how barcode tiling arrays can be used to predict anti-proliferative effects of individual shRNAs from pooled negative selection screens. Out of a pool of 305 shRNAs, we identified 28 candidate shRNAs to fully or partially impair the viability of the breast carcinoma cell line MDA-MB-231. Individual validation of a subset of eleven shRNA expression constructs with potential inhibitory, as well as non-inhibitory, effects on the cell line proliferation provides further evidence for the accuracy of the barcode tiling approach. CONCLUSIONS: In summary, we present an improved method for the rapid, quantitative and statistically robust analysis of pooled RNAi screens. Our experimental approach, coupled with commercially available lentiviral vector shRNA libraries, has the potential to greatly facilitate the discovery of putative targets for cancer therapy as well as sensitizers of drug toxicity.