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Tropomyosin receptor kinase (TRK) inhibitors have been approved for metastatic solid tumors harboring NTRK fusions, but the detection of NTRK fusions is challenging. International guidelines recommend pan-TRK immunohistochemistry (IHC) screening followed by next generation sequencing (NGS) in tumor types with low prevalence of NTRK fusions, including metastatic colorectal cancer (mCRC). RNA-based NGS is preferred, but is expensive, time-consuming, and extracting good-quality RNA from FFPE tissue is challenging. Alternatives in daily clinical practice are warranted. We assessed the diagnostic performance of RNA-NGS, FFPE-targeted locus capture (FFPE-TLC), fluorescence in situ hybridization (FISH), and the 5′/3′ imbalance quantitative RT-PCR (qRT-PCR) after IHC screening in 268 patients with microsatellite-instability-high mCRC, the subgroup in which NTRK fusions are most prevalent (1–5%). A consensus result was determined after review of all assay results. In 16 IHC positive tumors, 10 NTRK fusions were detected. In 33 IHC negative samples, no additional transcribed NTRK fusions were found, underscoring the high sensitivity of IHC. Sensitivity of RNA-NGS, FFPE-TLC, FISH, and qRT-PCR was 90%, 90%, 78%, and 100%, respectively. Specificity was 100% for all assays. Robustness, defined as the percentage of samples that provided an interpretable result in the first run, was 100% for FFPE-TLC, yet more limited for RNA-NGS (85%), FISH (70%), and qRT-PCR (70%). Overall, we do not recommend FISH for the detection of NTRK fusions in mCRC due to its low sensitivity and limited robustness. We conclude that RNA-NGS, FFPE-TLC, and qRT-PCR are appropriate assays for NTRK fusion detection, after enrichment with pan-TRK IHC, in routine clinical practice.

Minimal residual disease (MRD) diagnostics are implemented in most clinical protocols for patients with acute lymphoblastic leukaemia (ALL) and are mostly performed using rearranged immunoglobulin (IG) and/or T-cell receptor (TR) gene rearrangements as molecular polymerase chain reaction targets. Unfortunately, in 5–10% of patients no or no sensitive IG/TR targets are available, and patients therefore cannot be stratified appropriately. In the present study, we used fusion genes and genomic deletions as alternative MRD targets in these patients, which retrospectively revealed appropriate MDR stratification in 79% of patients with no (sensitive) IG/TR target, and a different risk group stratification in more than half of the cases.

Introduction: Structural variants (SVs) caused by chromosomal rearrangements or LINE retrotranspositions are a highly prevalent type of somatic DNA alterations in colorectal cancer (CRC). Studies about the function of SVs in tumor biology and their putative application as biomarkers are currently hampered by the need for fresh frozen tumor material to detect SVs by long-read or deep whole genome sequencing. This impedes the identification of SVs in retrospective cohorts and implementation in routine diagnostics (where only formalin-fixed paraffin-embedded (FFPE) tissue is available). In this study, we explored the applicability of FFPE-Targeted Locus Capture (FFPE-TLC) as a novel technology for targeted detection of SVs in CRC. Materials and methods: We defined the regions most frequently presenting SVs in CRC and included these in a customized capture panel. FFPE-TLC followed by targeted sequencing was performed on primary tumors of 29 metastatic CRC patients and eight patient-matched normal colon tissues. SV-specific droplet digital PCR (ddPCR) assays were designed for orthogonal validation of SVs in tumor tissue and detection of SVs in cell-free plasma circulating tumor DNA (ctDNA). Results: The regions associated with most SVs in metastatic CRC were four common fragile site (CFS) genes; MACROD2, PARK2, FHIT, WWOX, and three LINE source elements, and were included in a 2.7 Mb targeted panel. After filtering for germline variation, we identified 168 somatic SVs in 26/29 patients (90%), median SVs per patient: 3, range: 1-22. 110 SVs were found in CFS genes in 19/29 patients (66%), 12 of them presenting SVs in MACROD2. 58 SVs were caused by LINE integrations in 19/29 patients (66%), with different LINE behaviors observed: one source LINE generated 19 SVs in 13 patients, whilst another LINE caused 33 SVs in 5 patients. For selected cases, the tumor-specific origin of SVs was verified and its applicability as a plasma ctDNA biomarker demonstrated. Discussion: This study demonstrates, for the first time, the feasibility of targeted detection of SVs in CFS genes and SVs caused by LINE retrotransposition in routinely obtained FFPE tissue without prior knowledge of the breakpoint location. The high prevalence of SVs offers opportunities to detect many somatic alterations by targeting few genes and LINEs, e.g. for translation into ctDNA biomarker assays. We conclude that FFPE-TLC enables the investigation of SV mutations in retrospective cohorts and opens new avenues for mutation analysis in routine diagnostics. Citation Format: Carmen Rubio-Alarcon, Ellen Stelloo, Daan C. Vessies, Iris van ‘t Erve, Nienke Mekkes, Joost Swennenhuis, Soufyan Lakbir, Elise van Bree, Marianne Tijssen, Pien Delis-van Diemen, Mirthe Lanfermeijer, Theodora C. Linders, Daan van den Broek, Cornelis J. Punt, Jaap Heringa, Gerrit A. Meijer, Sanne Abeln, Harma Feitsma, Remond J. Fijneman. High frequency of structural variants in FFPE colorectal cancer tissue detected by targeting selected common fragile site genes and LINE transposable elements [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 222.

In routine diagnostic pathology, cancer biopsies are preserved by formalin-fixed, paraffin-embedding (FFPE) procedures for examination of (intra-) cellular morphology. Such procedures inadvertently induce DNA fragmentation, which compromises sequencing-based analyses of chromosomal rearrangements. Yet, rearrangements drive many types of hematolymphoid malignancies and solid tumors, and their manifestation is instructive for diagnosis, prognosis, and treatment. Here, we present FFPE-targeted locus capture (FFPE-TLC) for targeted sequencing of proximity-ligation products formed in FFPE tissue blocks, and PLIER, a computational framework that allows automated identification and characterization of rearrangements involving selected, clinically relevant, loci. FFPE-TLC, blindly applied to 149 lymphoma and control FFPE samples, identifies the known and previously uncharacterized rearrangement partners. It outperforms fluorescence in situ hybridization (FISH) in sensitivity and specificity, and shows clear advantages over standard capture-NGS methods, finding rearrangements involving repetitive sequences which they typically miss. FFPE-TLC is therefore a powerful clinical diagnostics tool for accurate targeted rearrangement detection in FFPE specimens., Preservation of cancer biopsies by FFPE introduces DNA fragmentation, hindering analysis of rearrangements. Here the authors introduce FFPE Targeted Locus Capture for identification of translocations in preserved samples.

Background: Cancer is caused by somatic DNA alterations, comprising single/small nucleotide variants (SNVs), somatic copy number alterations (SCNAs) and chromosomal rearrangement structural variants (SVs). We previously demonstrated that SVs are recurrently identified in hundreds of genes and are highly prevalent in common fragile site genes, e.g., in MACROD2 in >40% of colorectal cancers (CRCs). However, computational methods that discriminate SV-driver from SV-passenger events are lacking and laboratory methods to detect SVs at nucleotide resolution from routinely obtained formalin-fixed paraffin-embedded (FFPE) tumor tissue material are underdeveloped. Therefore, despite the abundant presence of SVs, knowledge about their biological and clinical impact is limited. Aim: The aim of our studies is to identify genes of which the function is frequently affected by SV, to understand how these genes contribute to CRC pathogenesis, and to translate these SVs into clinically relevant biomarkers. Methods: We made use of publicly available deep whole genome DNA sequencing data and tumor-matched RNA sequencing data from the Hartwig Medical Foundation to develop the algorithm ‘CoBRA’: Computation of Biologically Relevant Alterations. Adenoma-derived organoids were used for CRISPR/Cas9-mediated gene modulation for functional analysis of SV-driver events. Cergentis’ targeted locus capture (FFPE-TLC) technology was used to detect SVs at nucleotide resolution from FFPE material, which were translated into droplet digital PCR (ddPCR) assays for the detection of SVs in cell-free circulating tumor DNA (ctDNA) in liquid biopsies. Results: The CoBRA algorithm associated the presence of SV-events in frequently affected genes to the extent in which genome-wide RNA sequencing data were altered. In this way, CoBRA ranked SV-events in genes according to their putative impact on tumor biology. SVs in MACROD2 ranked among those with the highest impact on tumor biology. Therefore, we generated focal deletions in MACROD2 in adenoma-derived organoids for functional analyses. Moreover, using FFPE tumor tissue material we detected SVs at nucleotide resolution in MACROD2 and three other genes in 21 out of 29 patients. SVs were verified by PCR on tumor tissue and subsequently translated into ddPCR biomarker assays for detection of SVs in ctDNA in blood from the same patients. Conclusions: We developed the computational method CoBRA and succeeded to detect SVs with high impact on tumor biology. These SVs are prioritized for functional analysis in pre-malignant adenoma-derived organoids; for targeted detection in routinely obtained FFPE tumor tissue material; and for translation into liquid biopsy ctDNA assays. Proof of concept was delivered for MACROD2. Our novel computational and laboratory methodologies provide valuable tools to effectively explore the biological and clinical impact of SVs, which will contribute to our understanding of these common recurrent somatic alterations in CRC and their translation into clinically relevant biomarker applications. Citation Format: Elise van Bree, Carmen Rubio Alarcón, Soufyan Lakbir, Ellen Stelloo, Caterina Buranelli, Amber Hondema, Iris van 't Erve, Daan Vessies, Pien Delis-van Diemen, Marianne Tijssen, Anne Bolijn, Mirthe Lanfermeijer, Dorothe Linders, Joost Swennenhuis, Daan van den Broek, Jaap Heringa, Gerrit Meijer, Beatriz Carvalho, Harma Feitsma, Sanne Abeln, Remond J. A. Fijneman. Structural variants in the pathogenesis of colorectal cancer: The elephant in the room [abstract]. In: Proceedings of the AACR Special Conference on Colorectal Cancer; 2022 Oct 1-4; Portland, OR. Philadelphia (PA): AACR; Cancer Res 2022;82(23 Suppl_1):Abstract nr A020.

Background: Human papillomavirus (HPV) infections are the principal cause of cervical cancers, subsets of anogenital and head and neck cancers (HNC). During persistent infection, viral DNA integration into the host genome may occur, which is suggested to affect carcinogenesis. One of the most critical limitations of currently used HPV integration detection techniques (PCR-based or NGS-based) is their application to FFPE material, because of DNA fragmentation. The aim of this study was to assess HPV integration in HPV-positive HNSCC cell lines and FFPE tissue comparing the new Targeted Locus Amplification (TLA) technology with previously used PCR technology (APOT/DIPS). Methods: Seven HPV-positive cell lines and FFPE material of 10 HPV-positive HNSCC were used for HPV integration detection by TLA, a proximity ligation-based next-generation sequencing technique. Crosslinked DNA is digested with restriction enzymes, and re-ligated into chimeric DNA molecules. For cell lines, a PCR based HPV16 target enrichment is performed, and for FFPE material a capture-based target enrichment is performed for HPV16 and HPV18 sequences, both followed by Illumina sequencing. Results: TLA was able to sequence up to 100 kb around the target, detecting exact HPV integration loci, structural variants, and chromosomal rearrangements. In all cell lines, one or more integration sites were identified, in accordance with APOT/DIPS PCR data and the literature. In the FFPE tissue samples, TLA identified integrated HPV in 6 out of 10 tumors, with simple and complex integration patterns. In general, TLA confirmed PCR data and detected additional integration sites. Conclusion: TLA provides the opportunity for reliable and robust detection of HPV integration in HNSCC cell lines and FFPE tissue. This new sequencing technology could be a useful tool for further research on HPV integration in disease and patient outcome and eventually in clinical diagnostics. Citation Format: Imke Demers, Harini Balaji, Harma Feitsma, Irinia Sergeeva, Joost Swennenhuis, Nora Wuerdemann, Steffen Wagner, Bernd Kremer, Christian Huebbers, Jens Peter Klussmann, Ernst-Jan Speel. Targeted locus amplification based sequencing for mapping viral integration sites in human papillomavirus positive head and neck squamous cell carcinomas [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2229.

Chromosomal translocations with immunoglobin (IG) loci are the classic drivers in a large subset of B-cell lymphomas. Detection of these translocations is important for confirmation of diagnosis and for prognosis and therapy decisions. Currently, molecular diagnosis of translocations in lymphomas is not addressed well by next-generation sequencing (NGS). The standard method for detection of translocations is fluorescence in situ hybridization (FISH), which is labor intensive and can be difficult to interpret. There is a need for a robust technology that can be standardized. Targeted Locus Capture (TLC) selectively enriches and sequences entire genes based on the crosslinking of physically proximal sequences, and thereby enables complete sequencing of genes of interest, including detection of large structural variants. Because the technology is based on the crosslinking and fragmenting of DNA, it has particular advantages in the analysis of formalin-fixed, paraffin-embedded (FFPE) samples in which DNA is inherently crosslinked and fragmented. In order to validate the FFPE-TLC technology as a novel approach for translocation detection in lymphoma samples, we have developed a panel assay containing genes with frequent translocations (MYC, BCL2, BCL6, IG loci). With this assay we have analyzed >140 lymphoma and control FFPE samples of variable input amounts and qualities that had previously been analyzed with FISH, and a subset also with standard targeted NGS. Good concordance with FISH results was observed for both translocation-positive and -negative samples. In 10 cases for which FFPE-TLC analysis resulted in a different finding than FISH, discordance could be explained by higher sensitivity of FFPE-TLC or by inconclusive FISH results. In a specific case, FFPE-TLC detected a small-distance rearrangement on chromosome 3 that caused a BCL6 fusion but led to insufficient and therefore undetectable break-apart with FISH. Secondly, the FFPE-TLC approach was tested on a set of 19 B-cell lymphoma FFPE samples that had previously been analyzed using standard targeted NGS and FISH and was enriched for discordant results between these methods. FFPE-TLC-based NGS enables more robust translocation calling as the detection relies on broad sequencing coverage across the translocation partner rather than on breakpoint sequences only. In 3 cases, FFPE-TLC could proof false negative calls in standard targeted NGS due to breakpoints located in regions difficult to capture or to sequence. In 1 case, standard targeted NGS had made a false positive call on a breakpoint sequence that was shown to be caused by a small insertion rather than a genuine translocation. This study shows that FFPE-TLC promises to be a robust alternative for FISH analysis and standard targeted NGS procedures in lymphoma diagnostics and in other cancers with frequent structural variants. The FFPE-TLC approach enables a single, DNA-based NGS test detecting both small mutations and translocations. Citation Format: Amin Allahyar, Mark Pieterse, Joost Swennenhuis, Tjitske Los-de Vries, Mehmet Yilmaz, Roos Leguit, Ruud Meijers, Nathalie Hijmering, Daphne de Jong, Bauke Ylstra, Robert van der Geize, Joost Vermaat, Arjen Cleven, Tom van Wezel, Arjan Diepstra, Leon van Kempen, Karima Hajo, Harma Feitsma, Marieke Simonis, Max van Min, Erik Splinter, Wouter de Laat. Robust detection of translocations in lymphoma FFPE samples using Targeted Locus Capture-based sequencing [abstract]. In: Proceedings of the AACR Virtual Meeting: Advances in Malignant Lymphoma; 2020 Aug 17-19. Philadelphia (PA): AACR; Blood Cancer Discov 2020;1(3_Suppl):Abstract nr PO-45.

Background Structural variants in DNA such as translocations are increasingly found as key oncogenic events in tumors, and detection is clinically relevant for therapy choice in many conditions. Despite developments in targeted and whole genome gene sequencing, the robust detection of structural variants remains a challenge, especially in FFPE specimens. Methods Targeted proximity ligation assays selectively amplify and sequence entire genes based on the crosslinking of physically proximal sequences, and thereby enable complete sequencing of genes of interest, including single nucleotide and structural variants. Because these assays are based on the crosslinking and fragmenting of DNA, they have particular advantages in the analysis of FFPE samples, in which DNA is inherently crosslinked and fragmented. Results We will show that we have successfully developed targeted proximity ligation assays on representative clinical FFPE samples with considerably fragmented DNA. By making use of the DNA crosslinks in these samples, we can generate very broad sequence information over 100’s of kb’s, where other targeted NGS technologies are limited by the fragment size. We will show that we can detect NTRK fusions at DNA level, including determination of the fusion partner and the exact translocation breakpoint sequence. The latter can be used as a very sensitive and cancer-specific marker for minimal residual disease (MRD) testing and ctDNA monitoring with PCR. Conclusions Our newly developed targeted proximity ligation assays enable robust detection of structural variation in FFPE samples and the development of personalized MRD/ctDNA tests. Legal entity responsible for the study Cergentis. Funding Cergentis via Horizon2020 SME funding. Disclosure H. Feitsma: Full / Part-time employment: Cergentis. M. Yilmaz: Full / Part-time employment: Cergentis. J. Swennenhuis: Full / Part-time employment: Cergentis. A. Rakszewska: Full / Part-time employment: Cergentis. K. Hajo: Full / Part-time employment: Cergentis. E. Splinter: Full / Part-time employment: Cergentis. M. Simonis: Full / Part-time employment: Cergentis. M. Van Min: Shareholder / Stockholder / Stock options, Full / Part-time employment: Cergentis. All other authors have declared no conflicts of interest.

Among the cellular properties that are essential for the organization of tissues during animal development, the importance of cell polarity in the plane of epithelial sheets has become increasingly clear in the past decades. Planar cell polarity (PCP) signaling in vertebrates has indispensable roles in many aspects of their development, in particular, controlling alignment of various types of epithelial cells. Disrupted PCP has been linked to developmental defects in animals and to human pathology. Neural tube closure defects (NTD) and disorganization of the mechanosensory cells of the organ of Corti are commonly known consequences of disturbed PCP signaling in mammals. We report here a typical PCP phenotype in a mouse mutant for the Sec24b gene, including the severe NTD craniorachischisis, abnormal arrangement of outflow tract vessels and disturbed development of the cochlea. In addition, we observed genetic interaction between Sec24b and the known PCP gene, scribble. Sec24b is a component of the COPII coat protein complex that is part of the endoplasmic reticulum (ER)-derived transport vesicles. Sec24 isoforms are thought to be directly involved in cargo selection, and we present evidence that Sec24b deficiency specifically affects transport of the PCP core protein Vangl2, based on experiments in embryos and in cultured primary cells.

Mlh1 is a member of DNA mismatch repair (MMR) machinery and is also essential for the stabilization of crossovers during the first meiotic division. Recently, we have shown that zebrafish mlh1 mutant males are completely infertile because of a block in metaphase I, whereas females are fertile but have aneuploid progeny. When studying fertility in males in a two-fold more inbred background, we have however observed low numbers of fertilized eggs (approximately 0.4%). Histological examination of the testis has revealed that all spermatogenic stages prior to spermatids (spermatogonia, primary spermatocytes, and secondary spermatocytes) are significantly increased in the mutant, whereas the total weight of spermatids and spermatozoa is highly decreased (1.8 mg in wild-type vs. 0.1 mg in mutants), a result clearly different from our previous study in which outbred males lack secondary spermatocytes or postmeiotic cells. Thus, a delay of both meiotic divisions occurs rather than complete arrest during meiosis I in these males. Eggs fertilized with mutant sperm develop as malformed embryos and are aneuploid making this male phenotype much more similar to that previously described in the mutant females. Therefore, crossovers are still essential for proper meiosis, but meiotic cell divisions can progress without it, suggesting that this mutant is a suitable model for studying the cellular mechanisms of completing meiosis without crossover stabilization.

The zebrafish has developed into an important model organism for biomedical research over the last decades. Although the main focus of zebrafish research has traditionally been on developmental biology, keeping and observing zebrafish in the lab led to the identification of diseases similar to humans, such as cancer, which subsequently became a subject for study. As a result, about 50 articles have been published since 2000 in which zebrafish were used as a cancer model. Strategies used include carcinogenic treatments, transplantation of mammalian cancer cells, forward genetic screens for proliferation or genomic instability, reverse genetic target-selected mutagenesis to inactivate known tumor suppressor genes, and the generation of transgenics to express human oncogenes. Zebrafish have been found to develop almost any tumor type known from human, with similar morphology and, according to gene expression array studies, comparable signaling pathways. However, tumor incidences are relatively low, albeit highly comparable between different mutants, and tumors develop late in life. In addition, tumor spectra are sometimes different when compared with mice and humans. Nevertheless, the zebrafish model has created its own niche in cancer research, complementing existing models with its specific experimental advantages and characteristics. Examples of these are imaging of tumor progression in living fish by fluorescence, treatment with chemical compounds, and screening possibilities not only for chemical modifiers but also for genetic enhancers and suppressors. This review aims to provide a comprehensive overview of the state of the art of zebrafish as a model in cancer research. (Mol Cancer Res 2008;6(5):685–94)

Defective mismatch repair (MMR) in humans causes hereditary nonpolyposis colorectal cancer. This genetic predisposition to colon cancer is linked to heterozygous familial mutations, and loss-of-heterozygosity is necessary for tumor development. In contrast, the rare cases with biallelic MMR mutations are juvenile patients with brain tumors, skin neurofibromas, and café-au-lait spots, resembling the neurofibromatosis syndrome. Many of them also display lymphomas and leukemias, which phenotypically resembles the frequent lymphoma development in mouse MMR knockouts. Here, we describe the identification and characterization of novel knockout mutants of the three major MMR genes, mlh1, msh2, and msh6, in zebrafish and show that they develop tumors at low frequencies. Predominantly, neurofibromas/malignant peripheral nerve sheath tumors were observed; however, a range of other tumor types was also observed. Our findings indicate that zebrafish mimic distinct features of the human disease and are complementary to mouse models. [Cancer Res 2008;68(13):5059–66]

Background— Identifying molecular pathways regulating the development of pacemaking and coordinated heartbeat is crucial for a comprehensive mechanistic understanding of arrhythmia-related diseases. Elucidation of these pathways has been complicated mainly by an insufficient definition of the developmental structures involved in these processes and the unavailability of animal models specifically targeting the relevant tissues. Here, we report on a highly restricted expression pattern of the homeodomain transcription factor Shox2 in the sinus venosus myocardium, including the sinoatrial nodal region and the venous valves. Methods and Results— To investigate its function in vivo, we have generated mouse lines carrying a targeted mutation of the Shox2 gene. Although heterozygous animals did not exhibit obvious defects, homozygosity of the targeted allele led to embryonic lethality at 11.5 to 13.5 dpc. Shox2 −/− embryos exhibited severe hypoplasia of the sinus venosus myocardium in the posterior heart field, including the sinoatrial nodal region and venous valves. We furthermore demonstrate aberrant expression of connexin 40 and connexin 43 and the transcription factor Nkx2.5 in vivo specifically within the sinoatrial nodal region and show that Shox2 deficiency interferes with pacemaking function in zebrafish embryos. Conclusions— From these results, we postulate a critical function of Shox2 in the recruitment of sinus venosus myocardium comprising the sinoatrial nodal region.

The role of the aristaless-related homeobox gene Alx4 in antero-posterior (AP-) patterning of the developing vertebrate limb has remained somewhat elusive. Polydactyly of Alx4 mutant mice is known to be accompanied by ectopic anterior expression of genes like Shh, Fgf4 and 5′Hoxd. We reported previously that polydactyly in Alx4 mutant mice requires SHH signaling, but we now show that in early Alx4−/− limb buds the anterior ectopic expression of Fgf4 and Hoxd13, and therefore disruption of AP-patterning, occurs independently of SHH signaling. To better understand how Alx4 functions in the pathways that regulate AP-patterning, we also studied genomic regulatory sequences that are capable of directing expression of a reporter gene in a pattern corresponding to endogenous Alx4 expression in anterior limb bud mesenchyme. We observed, as expected for authentic Alx4 expression, expansion of reporter construct expression in a Shh−/− background. Total lack of reporter expression in a Gli3−/− background confirms the existence of Gli3-dependent and -independent Alx4 expression in the limb bud. Apparently, these two modules of Alx4 expression are linked to dissimilar functions.

BACKGROUND: Mutagenesis screens in the mouse have been proven useful for the identification of novel gene functions and generation of interesting mutant alleles. Here we describe a phenotype-based screen for recessive mutations affecting embryonic development. METHODOLOGY/PRINCIPAL FINDINGS: Mice were mutagenized with N-ethyl-N-nitrosourea (ENU) and following incrossing the offspring, embryos were analyzed at embryonic day 10.5. Mutant phenotypes that arose in our screen include cardiac and nuchal edema, neural tube defects, situs inversus of the heart, posterior truncation and the absence of limbs and lungs. We isolated amongst others novel mutant alleles for Dll1, Ptprb, Plexin-B2, Fgf10, Wnt3a, Ncx1, Scrib(Scrib, Scribbled homolog [Drosophila]) and Sec24b. We found both nonsense alleles leading to severe protein truncations and mutants with single-amino acid substitutions that are informative at a molecular level. Novel findings include an ectopic neural tube in our Dll1 mutant and lung defects in the planar cell polarity mutants for Sec24b and Scrib. CONCLUSIONS/SIGNIFICANCE: Using a forward genetics approach, we have generated a number of novel mutant alleles that are linked to disturbed morphogenesis during development.

Microarray-based enrichment of selected genomic loci is a powerful method for genome complexity reduction for next-generation sequencing. Since the vast majority of exons in vertebrate genomes are smaller than 150 nt, we explored the use of short fragment libraries (85-110 bp) to achieve higher enrichment specificity by reducing carryover and adverse effects of flanking intronic sequences. High enrichment specificity (60-75%) was obtained with a relative even base coverage. Up to 98% of the target-sequence was covered more than 20x at an average coverage depth of about 200x. To verify the accuracy of SNP/mutation detection, we evaluated 384 known non-reference SNPs in the targeted regions. At approximately 200x average sequence coverage, we were able to survey 96.4% of 1.69 Mb of genomic sequence with only 4.2% false negative calls, mostly due to low coverage. Using the same settings, a total of 1197 novel candidate variants were detected. Verification experiments revealed only eight false positive calls, indicating an overall false positive rate of less than 1 per approximately 200,000 bp. Taken together, short fragment libraries provide highly efficient and flexible enrichment of exonic targets and yield relatively even base coverage, which facilitates accurate SNP and mutation detection. Raw sequencing data, alignment files and called SNPs have been submitted into GEO database http://www.ncbi.nlm.nih.gov/geo/ with accession number GSE18542.

The biological role and structure-function relationship of the Na(+)Ca(2+) exchanger NCX1 have been the subject of much investigation. Subtle mutagenesis to study the function of a protein seems only feasible in in vitro systems, but genetic forward screens have the potential to provide in vivo models to study single amino acid substitutions. In a genetic screen in mouse, we have isolated a mutant line carrying a novel mutant allele of the mouse Ncx1 gene. In this allele, a point mutation causes the substitution of a highly conserved asparagine residue (N874) with lysine. Accepted models for NCX1 structure propose that the affected amino acid is located in one of the reentrant membrane loops and experiments in vitro have identified N874 as critical for the ion transport function of NCX1. We found severe circulation defects and defective placentation in homozygous Ncx1(N87K4) mutant embryos, making the phenotype essentially indistinguishable from those of previously described null mutants. By ex vivo analysis, we demonstrated intrinsic functional abnormalities of cardiomyocytes. Western blot analysis and immunohistochemistry demonstrated normal levels and subcellular localization of the altered protein, ruling out the possibility that the abnormalities are a mere consequence of a major disturbance of protein structure. This study confirms and extends studies in vitro indicating the significance of amino acid N874 for the function of the NCX1 protein. It provides an in vivo model for this mutation and demonstrates the potential of forward genetic screens in a mammalian system.

ENU (N-ethyl-N-nitrosourea) mutagenesis is a widely accepted and proven method to introduce random point mutations in the genome. Because there are no targeted knockout strategies available for zebrafish so far, random mutagenesis is currently the preferred method in both forward and reverse genetic approaches. To obtain high-density mutagenized zebrafish, six consecutive ENU treatments are applied at weekly intervals to adult male zebrafish by bathing them in ENU solution. With this procedure an average germ line mutation load of one mutation every 1.0 x 10(5)-1.5 x 10(5) basepairs is reached routinely in our lab.

ENU (N-ethyl-N-nitrosourea) mutagenesis is a widely accepted and proven method to introduce random point mutations in the genome. Because there are no targeted knockout strategies available for zebrafish so far, random mutagenesis is currently the preferred method in both forward and reverse genetic approaches. To obtain high-density mutagenized zebrafish, six consecutive ENU treatments are applied at weekly intervals to adult male zebrafish by bathing them in ENU solution. With this procedure an average germ line mutation load of one mutation every 1.0 x 10(5)-1.5 x 10(5) basepairs is reached routinely in our lab.

S(N)1-type alkylating agents, like N-methyl-N-nitrosourea (MNU) and N-ethyl-N-nitrosourea (ENU), are potent mutagens. Exposure to alkylating agents gives rise to O(6)-alkylguanine, a modified base that is recognized by DNA mismatch repair (MMR) proteins but is not repairable, resulting in replication fork stalling and cell death. We used a somatic mutation detection assay to study the in vivo effects of alkylation damage on lethality and mutation frequency in developing zebrafish embryos. Consistent with the damage-sensing role of the MMR system, mutant embryos lacking the MMR enzyme MSH6 displayed lower lethality than wild-type embryos after exposure to ENU and MNU. In line with this, alkylation-induced somatic mutation frequencies were found to be higher in wild-type embryos than in the msh6 loss-of-function mutants. These mutations were found to be chromosomal aberrations that may be caused by chromosomal breaks that arise from stalled replication forks. As these chromosomal breaks arise at replication, they are not expected to be repaired by non-homologous end joining. Indeed, Ku70 loss-of-function mutants were found to be equally sensitive to ENU as wild-type embryos. Taken together, our results suggest that in vivo alkylation damage results in chromosomal instability and cell death due to aberrantly processed MMR-induced stalled replication forks.

S(N)1-type alkylating agents such as N-ethyl-N-nitrosourea (ENU) are very potent mutagens. They act by transferring their alkyl group to DNA bases, which, upon mispairing during replication, can cause single base pair mutations in the next replication cycle. As DNA mismatch repair (MMR) proteins are involved in the recognition of alkylation damage, we hypothesized that ENU-induced mutation rates could be increased in a MMR-deficient background, which would be beneficial for mutagenesis approaches. We applied a standard ENU mutagenesis protocol to adult zebrafish deficient in the MMR gene msh6 and heterozygous controls to study the effect of MMR on ENU-induced DNA damage. Dose-dependent lethality was found to be similar for homozygous and heterozygous mutants, indicating that there is no difference in ENU resistance. Mutation discovery by high-throughput dideoxy resequencing of genomic targets in outcrossed progeny of the mutagenized fish did also not reveal any differences in germ line mutation frequency. These results may indicate that the maximum mutation load for zebrafish has been reached with the currently used, highly optimized ENU mutagenesis protocol. Alternatively, the MMR system in the zebrafish germ line may be saturated very rapidly, thereby having a limited effect on high-dose ENU mutagenesis.