Mapping the genetic landscape of DNA double-strand break repair.

Cells repair DNA double-strand breaks (DSBs) through a complex set of pathways critical for maintaining genomic integrity. To systematically map these pathways, we developed a high-throughput screening approach called Repair-seq that measures the effects of thousands of genetic perturbations on mutations introduced at targeted DNA lesions. Using Repair-seq, we profiled DSB repair products induced by two programmable nucleases (Cas9 and Cas12a) in the presence or absence of oligonucleotides for homology-directed repair (HDR) after knockdown of 476 genes involved in DSB repair or associated processes. The resulting data enabled principled, data-driven inference of DSB end joining and HDR pathways. Systematic interrogation of this data uncovered unexpected relationships among DSB repair genes and demonstrated that repair outcomes with superficially similar sequence architectures can have markedly different genetic dependencies. This work provides a foundation for mapping DNA repair pathways and for optimizing genome editing across diverse modalities.

The BRAF pseudogene functions as a competitive endogenous RNA and induces lymphoma in vivo.

Research over the past decade has suggested important roles for pseudogenes in physiology and disease. In vitro experiments demonstrated that pseudogenes contribute to cell transformation through several mechanisms. However, in vivo evidence for a causal role of pseudogenes in cancer development is lacking. Here, we report that mice engineered to overexpress either the full-length murine B-Raf pseudogene Braf-rs1 or its pseudo "CDS" or "3' UTR" develop an aggressive malignancy resembling human diffuse large B cell lymphoma. We show that Braf-rs1 and its human ortholog, BRAFP1, elicit their oncogenic activity, at least in part, as competitive endogenous RNAs (ceRNAs) that elevate BRAF expression and MAPK activation in vitro and in vivo. Notably, we find that transcriptional or genomic aberrations of BRAFP1 occur frequently in multiple human cancers, including B cell lymphomas. Our engineered mouse models demonstrate the oncogenic potential of pseudogenes and indicate that ceRNA-mediated microRNA sequestration may contribute to the development of cancer.

Translocation-capture sequencing reveals the extent and nature of chromosomal rearrangements in B lymphocytes.

Chromosomal rearrangements, including translocations, require formation and joining of DNA double strand breaks (DSBs). These events disrupt the integrity of the genome and are frequently involved in producing leukemias, lymphomas and sarcomas. Despite the importance of these events, current understanding of their genesis is limited. To examine the origins of chromosomal rearrangements we developed Translocation Capture Sequencing (TC-Seq), a method to document chromosomal rearrangements genome-wide, in primary cells. We examined over 180,000 rearrangements obtained from 400 million B lymphocytes, revealing that proximity between DSBs, transcriptional activity and chromosome territories are key determinants of genome rearrangement. Specifically, rearrangements tend to occur in cis and to transcribed genes. Finally, we find that activation-induced cytidine deaminase (AID) induces the rearrangement of many genes found as translocation partners in mature B cell lymphoma.

53BP1 inhibits homologous recombination in Brca1-deficient cells by blocking resection of DNA breaks.

Defective DNA repair by homologous recombination (HR) is thought to be a major contributor to tumorigenesis in individuals carrying Brca1 mutations. Here, we show that DNA breaks in Brca1-deficient cells are aberrantly joined into complex chromosome rearrangements by a process dependent on the nonhomologous end-joining (NHEJ) factors 53BP1 and DNA ligase 4. Loss of 53BP1 alleviates hypersensitivity of Brca1 mutant cells to PARP inhibition and restores error-free repair by HR. Mechanistically, 53BP1 deletion promotes ATM-dependent processing of broken DNA ends to produce recombinogenic single-stranded DNA competent for HR. In contrast, Lig4 deficiency does not rescue the HR defect in Brca1 mutant cells but prevents the joining of chromatid breaks into chromosome rearrangements. Our results illustrate that HR and NHEJ compete to process DNA breaks that arise during DNA replication and that shifting the balance between these pathways can be exploited to selectively protect or kill cells harboring Brca1 mutations.

Stress from Nucleotide Depletion Activates the Transcriptional Regulator HEXIM1 to Suppress Melanoma.

Studying cancer metabolism gives insight into tumorigenic survival mechanisms and susceptibilities. In melanoma, we identify HEXIM1, a transcription elongation regulator, as a melanoma tumor suppressor that responds to nucleotide stress. HEXIM1 expression is low in melanoma. Its overexpression in a zebrafish melanoma model suppresses cancer formation, while its inactivation accelerates tumor onset in vivo. Knockdown of HEXIM1 rescues zebrafish neural crest defects and human melanoma proliferation defects that arise from nucleotide depletion. Under nucleotide stress, HEXIM1 is induced to form an inhibitory complex with P-TEFb, the kinase that initiates transcription elongation, to inhibit elongation at tumorigenic genes. The resulting alteration in gene expression also causes anti-tumorigenic RNAs to bind to and be stabilized by HEXIM1. HEXIM1 plays an important role in inhibiting cancer cell-specific gene transcription while also facilitating anti-cancer gene expression. Our study reveals an important role for HEXIM1 in coupling nucleotide metabolism with transcriptional regulation in melanoma.

AID is required for the chromosomal breaks in c-myc that lead to c-myc/IgH translocations.

Chromosomal translocation requires formation of paired double-strand DNA breaks (DSBs) on heterologous chromosomes. One of the most well characterized oncogenic translocations juxtaposes c-myc and the immunoglobulin heavy-chain locus (IgH) and is found in Burkitt's lymphomas in humans and plasmacytomas in mice. DNA breaks in IgH leading to c-myc/IgH translocations are created by activation-induced cytidine deaminase (AID) during antibody class switch recombination or somatic hypermutation. However, the source of DNA breaks at c-myc is not known. Here, we provide evidence for the c-myc promoter region being required in targeting AID-mediated DNA damage to produce DSBs in c-myc that lead to c-myc/IgH translocations in primary B lymphocytes. Thus, in addition to producing somatic mutations and DNA breaks in antibody genes, AID is also responsible for the DNA lesions in oncogenes that are required for their translocation.

Regulation of DNA end joining, resection, and immunoglobulin class switch recombination by 53BP1.

53BP1 is a DNA damage protein that forms phosphorylated H2AX (γ-H2AX) dependent foci in a 1 Mb region surrounding DNA double-strand breaks (DSBs). In addition, 53BP1 promotes genomic stability by regulating the metabolism of DNA ends. We have compared the joining rates of paired DSBs separated by 1.2 kb to 27 Mb on chromosome 12 in the presence or absence of 53BP1. 53BP1 facilitates joining of intrachromosomal DSBs but only at distances corresponding to γ-H2AX spreading. In contrast, DNA end protection by 53BP1 is distance independent. Furthermore, analysis of 53BP1 mutants shows that chromatin association, oligomerization, and N-terminal ATM phosphorylation are all required for DNA end protection and joining as measured by immunoglobulin class switch recombination. These data elucidate the molecular events that are required for 53BP1 to maintain genomic stability and point to a model wherein 53BP1 and H2AX cooperate to repress resection of DSBs.

UDiTaS™, a genome editing detection method for indels and genome rearrangements.

BACKGROUND: Understanding the diversity of repair outcomes after introducing a genomic cut is essential for realizing the therapeutic potential of genomic editing technologies. Targeted PCR amplification combined with Next Generation Sequencing (NGS) or enzymatic digestion, while broadly used in the genome editing field, has critical limitations for detecting and quantifying structural variants such as large deletions (greater than approximately 100 base pairs), inversions, and translocations. RESULTS: To overcome these limitations, we have developed a Uni-Directional Targeted Sequencing methodology, UDiTaS, that is quantitative, removes biases associated with variable-length PCR amplification, and can measure structural changes in addition to small insertion and deletion events (indels), all in a single reaction. We have applied UDiTaS to a variety of samples, including those treated with a clinically relevant pair of S. aureus Cas9 single guide RNAs (sgRNAs) targeting CEP290, and a pair of S. pyogenes Cas9 sgRNAs at T-cell relevant loci. In both cases, we have simultaneously measured small and large edits, including inversions and translocations, exemplifying UDiTaS as a valuable tool for the analysis of genome editing outcomes. CONCLUSIONS: UDiTaS is a robust and streamlined sequencing method useful for measuring small indels as well as structural rearrangements, like translocations, in a single reaction. UDiTaS is especially useful for pre-clinical and clinical application of gene editing to measure on- and off-target editing, large and small.

DNA damage defines sites of recurrent chromosomal translocations in B lymphocytes.

Recurrent chromosomal translocations underlie both haematopoietic and solid tumours. Their origin has been ascribed to selection of random rearrangements, targeted DNA damage, or frequent nuclear interactions between translocation partners; however, the relative contribution of each of these elements has not been measured directly or on a large scale. Here we examine the role of nuclear architecture and frequency of DNA damage in the genesis of chromosomal translocations by measuring these parameters simultaneously in cultured mouse B lymphocytes. In the absence of recurrent DNA damage, translocations between Igh or Myc and all other genes are directly related to their contact frequency. Conversely, translocations associated with recurrent site-directed DNA damage are proportional to the rate of DNA break formation, as measured by replication protein A accumulation at the site of damage. Thus, non-targeted rearrangements reflect nuclear organization whereas DNA break formation governs the location and frequency of recurrent translocations, including those driving B-cell malignancies.

AID produces DNA double-strand breaks in non-Ig genes and mature B cell lymphomas with reciprocal chromosome translocations.

Cancer-initiating translocations such as those associated with lymphomas require the formation of paired DNA double-strand breaks (DSBs). Activation-induced cytidine deaminase (AID) produces widespread somatic mutation in mature B cells; however, the extent of "off-target" DSB formation and its role in translocation-associated malignancy is unknown. Here, we show that deregulated expression of AID causes widespread genome instability, which alone is insufficient to induce B cell lymphoma; transformation requires concomitant loss of the tumor suppressor p53. Mature B cell lymphomas arising as a result of deregulated AID expression are phenotypically diverse and harbor clonal reciprocal translocations involving a group of Immunoglobulin (Ig) and non-Ig genes that are direct targets of AID. This group includes miR-142, a previously unknown micro-RNA target that is translocated in human B cell malignancy. We conclude that AID produces DSBs throughout the genome, which can lead to lymphoma-associated chromosome translocations in mature B cells.

Role of 53BP1 oligomerization in regulating double-strand break repair.

Tumor suppressor p53-binding protein 1 (53BP1) regulates the repair of dysfunctional telomeres lacking the shelterin protein TRF2 by promoting their mobility, their nonhomologous end-joining (NHEJ), and, as we show here, by blocking 5' resection by CtIP. We report that these functions of 53BP1 required its N-terminal ATM/ATR target sites and its association with H4K20diMe, but not the BRCT domain, the GAR domain, or the binding of 53BP1 to dynein. A mutant lacking the oligomerization domain (53BP1(oligo)) was only modestly impaired in promoting NHEJ of dysfunctional telomeres and showed no defect with regard to the repression of CtIP. This 53BP1(oligo) allele was previously found to be unable to support class switch recombination or to promote radial chromosome formation in PARP1 inhibitor-treated Brca1-deficient cells. The data therefore support two conclusions. First, the requirements for 53BP1 in mediating NHEJ at dysfunctional telomeres and in class switch recombination are not identical. Second, 53BP1-dependent repression of CtIP at double-strand breaks (DSBs) is unlikely to be sufficient for the generation of radial chromosomes in PARP1 inhibitor-treated Brca1-deficient cells.

Detection and Modulation of DNA Translocations During Multi-Gene Genome Editing in T Cells.

Multiplexed genome editing with DNA endonucleases has broad application, including for cellular therapies, but chromosomal translocations, natural byproducts of inducing simultaneous genomic breaks, have not been explored in detail. Here we apply various CRISPR-Cas nucleases to edit the T cell receptor alpha and beta 2 microglobulin genes in human primary T cells and comprehensively evaluate the frequency and stability of the resulting translocations. A thorough translocation frequency analysis using three orthogonal methods (droplet digital PCR, unidirectional sequencing, and metaphase fluorescence in situ hybridization) yielded comparable results and an overall translocation rate of ∼7% between two simultaneous CRISPR-Cas9 induced edits. In addition, we show that chromosomal translocations can be reduced when using different nuclease combinations, or by the presence of a homologous single stranded oligo donor for multiplexed genome editing. Importantly, the two different approaches for translocation reduction are compatible with cell therapy applications.

Germline NPM1 mutations lead to altered rRNA 2'-O-methylation and cause dyskeratosis congenita.

RNA modifications are emerging as key determinants of gene expression. However, compelling genetic demonstrations of their relevance to human disease are lacking. Here, we link ribosomal RNA 2'-O-methylation (2'-O-Me) to the etiology of dyskeratosis congenita. We identify nucleophosmin (NPM1) as an essential regulator of 2'-O-Me on rRNA by directly binding C/D box small nucleolar RNAs, thereby modulating translation. We demonstrate the importance of 2'-O-Me-regulated translation for cellular growth, differentiation and hematopoietic stem cell maintenance, and show that Npm1 inactivation in adult hematopoietic stem cells results in bone marrow failure. We identify NPM1 germline mutations in patients with dyskeratosis congenita presenting with bone marrow failure and demonstrate that they are deficient in small nucleolar RNA binding. Mice harboring a dyskeratosis congenita germline Npm1 mutation recapitulate both hematological and nonhematological features of dyskeratosis congenita. Thus, our findings indicate that impaired 2'-O-Me can be etiological to human disease.

Characterization of the interplay between DNA repair and CRISPR/Cas9-induced DNA lesions at an endogenous locus.

The CRISPR-Cas9 system provides a versatile toolkit for genome engineering that can introduce various DNA lesions at specific genomic locations. However, a better understanding of the nature of these lesions and the repair pathways engaged is critical to realizing the full potential of this technology. Here we characterize the different lesions arising from each Cas9 variant and the resulting repair pathway engagement. We demonstrate that the presence and polarity of the overhang structure is a critical determinant of double-strand break repair pathway choice. Similarly, single nicks deriving from different Cas9 variants differentially activate repair: D10A but not N863A-induced nicks are repaired by homologous recombination. Finally, we demonstrate that homologous recombination is required for repairing lesions using double-stranded, but not single-stranded DNA as a template. This detailed characterization of repair pathway choice in response to CRISPR-Cas9 enables a more deterministic approach for designing research and therapeutic genome engineering strategies.

RPA accumulation during class switch recombination represents 5'-3' DNA-end resection during the S-G2/M phase of the cell cycle.

Activation-induced cytidine deaminase (AID) promotes chromosomal translocations by inducing DNA double-strand breaks (DSBs) at immunoglobulin (Ig) genes and oncogenes in the G1 phase. RPA is a single-stranded DNA (ssDNA)-binding protein that associates with resected DSBs in the S phase and facilitates the assembly of factors involved in homologous repair (HR), such as Rad51. Notably, RPA deposition also marks sites of AID-mediated damage, but its role in Ig gene recombination remains unclear. Here, we demonstrate that RPA associates asymmetrically with resected ssDNA in response to lesions created by AID, recombination-activating genes (RAG), or other nucleases. Small amounts of RPA are deposited at AID targets in G1 in an ATM-dependent manner. In contrast, recruitment in the S-G2/M phase is extensive, ATM independent, and associated with Rad51 accumulation. In the S-G2/M phase, RPA increases in nonhomologous-end-joining-deficient lymphocytes, where there is more extensive DNA-end resection. Thus, most RPA recruitment during class switch recombination represents salvage of unrepaired breaks by homology-based pathways during the S-G2/M phase of the cell cycle.

Mechanism of DNA resection during intrachromosomal recombination and immunoglobulin class switching.

DNA double-strand breaks (DSBs) are byproducts of normal cellular metabolism and obligate intermediates in antigen receptor diversification reactions. These lesions are potentially dangerous because they can lead to deletion of genetic material or chromosome translocation. The chromatin-binding protein 53BP1 and the histone variant H2AX are required for efficient class switch (CSR) and V(D)J recombination in part because they protect DNA ends from resection and thereby favor nonhomologous end joining (NHEJ). Here, we examine the mechanism of DNA end resection in primary B cells. We find that resection depends on both CtBP-interacting protein (CtIP, Rbbp8) and exonuclease 1 (Exo1). Inhibition of CtIP partially rescues the CSR defect in 53BP1- and H2AX-deficient lymphocytes, as does interference with the RecQ helicases Bloom (Blm) and Werner (Wrn). We conclude that CtIP, Exo1, and RecQ helicases contribute to the metabolism of DNA ends during DSB repair in B lymphocytes and that minimizing resection favors efficient CSR.

Activation-induced cytidine deaminase in antibody diversification and chromosome translocation.

DNA damage, rearrangement, and mutation of the human genome are the basis of carcinogenesis and thought to be avoided at all costs. An exception is the adaptive immune system where lymphocytes utilize programmed DNA damage to effect antigen receptor diversification. Both B and T lymphocytes diversify their antigen receptors through RAG1/2 mediated recombination, but B cells undergo two additional processes--somatic hypermutation (SHM) and class-switch recombination (CSR), both initiated by activation-induced cytidine deaminase (AID). AID deaminates cytidines in DNA resulting in U:G mismatches that are processed into point mutations in SHM or double-strand breaks in CSR. Although AID activity is focused at Immunoglobulin (Ig) gene loci, it also targets a wide array of non-Ig genes including oncogenes associated with lymphomas. Here, we review the molecular basis of AID regulation, targeting, and initiation of CSR and SHM, as well as AID's role in generating chromosome translocations that contribute to lymphomagenesis.

53BP1 regulates DNA resection and the choice between classical and alternative end joining during class switch recombination.

Class switch recombination (CSR) diversifies antibodies by joining highly repetitive DNA elements, which are separated by 60-200 kbp. CSR is initiated by activation-induced cytidine deaminase, an enzyme that produces multiple DNA double-strand breaks (DSBs) in switch regions. Switch regions are joined by a mechanism that requires an intact DNA damage response and classical or alternative nonhomologous end joining (A-NHEJ). Among the DNA damage response factors, 53BP1 has the most profound effect on CSR. We explore the role of 53BP1 in intrachromosomal DNA repair using I-SceI to introduce paired DSBs in the IgH locus. We find that the absence of 53BP1 results in an ataxia telangiectasia mutated-dependent increase in DNA end resection and that resected DNA is preferentially repaired by microhomology-mediated A-NHEJ. We propose that 53BP1 favors long-range CSR in part by protecting DNA ends against resection, which prevents A-NHEJ-dependent short-range rejoining of intra-switch region DSBs.