Recombinase, chromosomal translocations and lymphoid neoplasia: targeting mistakes and repair failures

Rearrangement of CRLF2 in B-progenitor- and Down syndrome-associated acute lymphoblastic leukemia

Aneuploidy and translocations are hallmarks of B-progenitor acute lymphoblastic leukemia (ALL), but many individuals with this cancer lack recurring chromosomal alterations. Here we report a recurring interstitial deletion of the pseudoautosomal region 1 of chromosomes X and Y in B-progenitor ALL that juxtaposes the first, noncoding exon of P2RY8 with the coding region of CRLF2. We identified the P2RY8-CRLF2 fusion in 7% of individuals with B-progenitor ALL and 53% of individuals with ALL associated with Down syndrome. CRLF2 alteration was associated with activating JAK mutations, and expression of human P2RY8-CRLF2 together with mutated mouse Jak2 resulted in constitutive Jak-Stat activation and cytokine-independent growth of Ba/F3 cells overexpressing interleukin-7 receptor alpha. Our findings indicate that these two genetic lesions together contribute to leukemogenesis in B-progenitor ALL.

RAG: a recombinase diversified

During B cell and T cell development, the lymphoid-specific proteins RAG-1 and RAG-2 act together to initiate the assembly of antigen receptor genes through a series of site-specific somatic DNA rearrangements that are collectively called variable-diversity-joining (V(D)J) recombination. In the past 20 years, a great deal has been learned about the enzymatic activities of the RAG-1-RAG-2 complex. Recent studies have identified several new and exciting regulatory functions of the RAG-1-RAG-2 complex. Here we discuss some of these functions and suggest that the RAG-1-RAG-2 complex nucleates a specialized subnuclear compartment that we call the 'V(D)J recombination factory'.

Too many ends: aberrant transposition

A recent study by Zhang and colleagues published in the March 15, 2009, issue of Genes & Development (pp. 755-765) demonstrates that maize Ac/Ds transposons mediate translocations and other rearrangements through aberrant execution of the normal transposition process. Ac transposase uses one end from each of two neighboring elements in these events, which may happen more commonly than previously thought. In genomes where there can be many transposon ends scattered across all the chromosomes, such mistakes can have important consequences.

HMGB1/2 can target DNA for illegitimate cleavage by the RAG1/2 complex

Background

V(D)J recombination is initiated in antigen receptor loci by the pairwise cleavage of recombination signal sequences (RSSs) by the RAG1 and RAG2 proteins via a nick-hairpin mechanism. The RSS contains highly conserved heptamer (consensus: 5'-CACAGTG) and nonamer (consensus: 5'-ACAAAAACC) motifs separated by either 12- or 23-base pairs of poorly conserved sequence. The high mobility group proteins HMGB1 and HMGB2 (HMGB1/2) are highly abundant architectural DNA binding proteins known to promote RAG-mediated synapsis and cleavage of consensus recombination signals in vitro by facilitating RSS binding and bending by the RAG1/2 complex. HMGB1/2 are known to recognize distorted DNA structures such as four-way junctions, and damaged or modified DNA. Whether HMGB1/2 can promote RAG-mediated DNA cleavage at sites lacking a canonical RSS by targeting or stabilizing structural distortions is unclear, but is important for understanding the etiology of chromosomal translocations involving antigen receptor genes and proto-oncogene sequences that do not contain an obvious RSS-like element.

Results

Here we identify a novel DNA breakpoint site in the plasmid V(D)J recombination substrate pGG49 (bps6197) that is cleaved by the RAG proteins via a nick-hairpin mechanism. The bps6197 sequence lacks a recognizable heptamer at the breakpoint (5'-CCTGACG-3') but contains a nonamer-like element (5'-ACATTAACC-3') 30 base pairs from the cleavage site. We find that RAG-mediated bps6197 cleavage is promoted by HMGB1/2, requiring both HMG-box domains to be intact to facilitate RAG-mediated cleavage, and is stimulated by synapsis with a 12-RSS. A dyad-symmetric inverted repeat sequence lying 5' to the breakpoint is implicated as a target for HMGB1/2 activity.

Conclusion

We have identified a novel DNA sequence, called bps6197, that supports standard V(D)J-type cleavage despite the absence of an apparent heptamer motif. Efficient RAG-mediated bps6197 cleavage requires the presence of HMGB1/2, is stimulated by synapsis with a 12-RSS partner, and is directed in part by an inverted repeat sequence adjacent to the DNA cleavage site. These results have important implications for understanding how the RAG proteins can introduce a DNA double-strand break at DNA sequences that do not contain an obvious heptamer-like motif.

Alternative Ac/Ds transposition induces major chromosomal rearrangements in maize

Barbara McClintock reported that the Ac/Ds transposable element system can generate major chromosomal rearrangements (MCRs), but the underlying mechanism has not been determined. Here, we identified a series of chromosome rearrangements derived from maize lines containing pairs of closely linked Ac transposable element termini. Molecular and cytogenetic analyses showed that the MCRs in these lines comprised 17 reciprocal translocations and two large inversions. The breakpoints of all 19 MCRs are delineated by Ac termini and characteristic 8-base-pair target site duplications, indicating that the MCRs were generated by precise transposition reactions involving the Ac termini of two closely linked elements. This alternative transposition mechanism may have contributed to chromosome evolution and may also occur during V(D)J recombination resulting in oncogenic translocations.

Initiation of V(D)J recombination by Dbeta-associated recombination signal sequences: a critical control point in TCRbeta gene assembly

T cell receptor (TCR) β gene assembly by V(D)J recombination proceeds via successive Dβ-to-Jβ and Vβ-to-DJβ rearrangements. This two-step process is enforced by a constraint, termed beyond (B)12/23, which prohibits direct Vβ-to-Jβ rearrangements. However the B12/23 restriction does not explain the order of TCRβ assembly for which the regulation remains an unresolved issue. The initiation of V(D)J recombination consists of the introduction of single-strand DNA nicks at recombination signal sequences (RSSs) containing a 12 base-pairs spacer. An RSS containing a 23 base-pairs spacer is then captured to form a 12/23 RSSs synapse leading to coupled DNA cleavage. Herein, we probed RSS nicks at the TCRβ locus and found that nicks were only detectable at Dβ-associated RSSs. This pattern implies that Dβ 12RSS and, unexpectedly, Dβ 23RSS initiate V(D)J recombination and capture their respective Vβ or Jβ RSS partner. Using both in vitro and in vivo assays, we further demonstrate that the Dβ1 23RSS impedes cleavage at the adjacent Dβ1 12RSS and consequently Vβ-to-Dβ1 rearrangement first requires the Dβ1 23RSS excision. Altogether, our results provide the molecular explanation to the B12/23 constraint and also uncover a ‘Dβ1 23RSS-mediated’ restriction operating beyond chromatin accessibility, which directs Dβ1 ordered rearrangements.

Leaky severe combined immunodeficiency and aberrant DNA rearrangements due to a hypomorphic RAG1 mutation

The RAG1/2 endonuclease initiates programmed DNA rearrangements in progenitor lymphocytes by generating double-strand breaks at specific recombination signal sequences. This process, known as V(D)J recombination, assembles the vastly diverse antigen receptor genes from numerous V, D, and J coding segments. In vitro biochemical and cellular transfection studies suggest that RAG1/2 may also play postcleavage roles by forming complexes with the recombining ends to facilitate DNA end processing and ligation. In the current study, we examine the in vivo consequences of a mutant form of RAG1, RAG1-S723C, that is proficient for DNA cleavage, yet exhibits defects in postcleavage complex formation and end joining in vitro. We generated a knockin mouse model harboring the RAG1-S723C hypomorphic mutation and examined the immune system in this fully in vivo setting. RAG1-S723C homozygous mice exhibit impaired lymphocyte development and decreased V(D)J rearrangements. Distinct from RAG nullizygosity, the RAG1-S723C hypomorph results in aberrant DNA double-strand breaks within rearranging loci. RAG1-S723C also predisposes to thymic lymphomas associated with chromosomal translocations in a p53 mutant background, and heterozygosity for the mutant allele accelerates age-associated immune system dysfunction. Thus, our study provides in vivo evidence that implicates aberrant RAG1/2 activity in lymphoid tumor development and premature immunosenescence.

Normal and pathological V(D)J recombination: contribution to the understanding of human lymphoid malignancies

The majority of haematological cancers involve the lymphoid system. They include acute lymphoblastic leukemias (ALL), which are arrested at variable stages of development and present with blood and bone marrow involvement and chronic leukemias, lymphomas and myelomas, which present with infiltration of a large variety of hematopoietic and non hematopoietic tissues by mature lymphoid cells which express a surface antigen receptor. The majority involve the B-cell lineage and the vast majority have undergone clonal rearrangement of their Ig and/or TCR rearrangements. Analysis of Ig/TCR genomic V(D)J repertoires by PCR based lymphoid clonality analysis within a diagnostic setting allows distinction of clonal from reactive lymphoproliferative disorders, clonal tracking for evidence of tumor dissemination and follow-up, identification of a lymphoid origin in undiagnosed tumors and evaluation of clonal evolution. Ig/TCR VDJ errors are also at the origin of recombinase mediated deregulated expression of a variety of proto-oncogenes in ALL, whereas in lymphoma it is increasingly clear that IgH containing translocations result from abnormalities other than VDJ errors (somatic hypermutation and/or isotype switching). In addition to this mechanistic contribution to lymphoid oncogenesis, it is possible that failure to successfully complete expression of an appropriate Ig or TCR may lead to maturation arrest in a lymphoid precursor, which may in itself contribute to altered tissue homeostasis, particularly if the arrest occurs at a stage of cellular expansion.

Mus81-dependent double-strand DNA breaks at in vivo-generated cruciform structures in S. cerevisiae

Long DNA palindromes are implicated in chromosomal rearrangement, but their roles in the underlying molecular events remain a matter of conjecture. One notion is that palindromes induce DNA breaks after assuming a cruciform structure, the four-way DNA junction providing a target for cleavage by Holliday junction (HJ)-specific enzymes. Though compelling, few components of the "cruciform resolution" proposal are established. Here we address fundamental properties and genetic dependencies of palindromic DNA metabolism in eukaryotes. Plasmid-borne palindromes introduced into S. cerevisiae are site-specifically broken in vivo, and the breaks exhibit unique hallmarks of an HJ resolvase mechanism. In vivo resolution requires Mus81, for which the bacterial HJ resolvase RusA will substitute. These results provide confirmation of cruciform extrusion and resolution in the context of eukaryotic chromatin. Related observations are that, unchecked by a nuclease function provided by Mre11, episomal palindromes launch a self-perpetuating breakage-fusion-bridge-independent copy number increase termed "escape."

MMEJ repair of double-strand breaks (director's cut): deleted sequences and alternative endings

DNA double-strand breaks are normal consequences of cell division and differentiation and must be repaired faithfully to maintain genome stability. Two mechanistically distinct pathways are known to efficiently repair double-strand breaks: homologous recombination and Ku-dependent non-homologous end joining. Recently, a third, less characterized repair mechanism, named microhomology-mediated end joining (MMEJ), has received increasing attention. MMEJ repairs DNA breaks via the use of substantial microhomology and always results in deletions. Furthermore, it probably contributes to oncogenic chromosome rearrangements and genetic variation in humans. Here, we summarize the genetic attributes of MMEJ from several model systems and discuss the relationship between MMEJ and 'alternative end joining'. We propose a mechanistic model for MMEJ and highlight important questions for future research.

RAG2's non-core domain contributes to the ordered regulation of V(D)J recombination

Variable (diversity) joining [V(D)J] recombination of immune gene loci proceeds in an ordered manner with D to J portions recombining first and then an upstream V joins that recombinant. We present evidence that the non-core domain of recombination activating gene (RAG) protein 2 is involved in the regulation of recombinatorial order. In mice lacking the non-core domain of RAG2 the ordered rearrangement is disturbed and direct V to D rearrangements are 10- to 1000-times increased in tri-partite immune gene loci. Some forms of inter-chromosomal translocations between TCRβ and TCRδ D gene segments are also increased in the core RAG2 animals as compared with their wild-type (WT) counterparts. In addition, the concise use of proper recombination signal sequences (RSSs) appears to be disturbed in the core RAG2 mice as compared with WT RAG2 animals.

Chromosomal translocations in cancer

Genetic alterations in DNA can lead to cancer when it is present in proto-oncogenes, tumor suppressor genes, DNA repair genes etc. Examples of such alterations include deletions, inversions and chromosomal translocations. Among these rearrangements chromosomal translocations are considered as the primary cause for many cancers including lymphoma, leukemia and some solid tumors. Chromosomal translocations in certain cases can result either in the fusion of genes or in bringing genes close to enhancer or promoter elements, hence leading to their altered expression. Moreover, chromosomal translocations are used as diagnostic markers for cancer and its therapeutics. In the first part of this review, we summarize the well-studied chromosomal translocations in cancer. Although the mechanism of formation of most of these translocations is still unclear, in the second part we discuss the recent advances in this area of research.

The ontogeny and fate of NK cells marked by permanent DNA rearrangements

A subset of NK cells bears incomplete V(D)J rearrangements, but neither the consequence to cell activities nor the precise developmental stages in which recombination occurs is known. These are important issues, as recombination errors cause cancers of the B and T lineages. Using transgenic recombination reporter mice to examine NK cell dynamics in vivo, we show that recombination(+) NK cells have distinct developmental patterns in the BM, including reduced homeostatic proliferation and diminished Stat5 phosphorylation. In the periphery, both recombination(+) and recombination(-) NK cells mediate robust functional responses including IFN-gamma production, cytolysis, and tumor homing, suggesting that NK cells with distinct developmental histories can be found together in the periphery. We also show that V(D)J rearrangement marks both human cytolytic (CD56(dim)) and immunoregulatory (CD56(bright)) populations, demonstrating the distribution of permanent DNA rearrangements across major NK cell subsets in man. Finally, direct quantification of rag transcripts throughout NK cell differentiation in both mouse and man establishes the specific developmental stages that are susceptible to V(D)J rearrangement. Together, these data demonstrate that multipotent progenitors rather than lineage-specified NK progenitors are targets of V(D)J recombination and that NK cells bearing the relics of earlier V(D)J rearrangements have different developmental dynamics but robust biological capabilities in vivo.

V(D)J recombinase binding and cleavage of cryptic recombination signal sequences identified from lymphoid malignancies

V(D)J recombination is a process integral to lymphocyte development. However, this process is not always benign, since certain lymphoid malignancies exhibit recurrent chromosomal abnormalities, such as translocations and deletions, that harbor molecular signatures suggesting an origin from aberrant V(D)J recombination. Translocations involving LMO2, TAL1, Ttg-1, and Hox11, as well as a recurrent interstitial deletion at 1p32 involving SIL/SCL, are cited examples of illegitimate V(D)J recombination. Previous studies using extrachromosomal substrates reveal that cryptic recombination signal sequences (cRSSs) identified near the translocation breakpoint in these examples support V(D)J recombination with efficiencies ranging from about 30- to 20,000-fold less than bona fide V(D)J recombination signals. To understand the molecular basis for these large differences, we investigated the binding and cleavage of these cRSSs by the RAG1/2 proteins that initiate V(D)J recombination. We find that the RAG proteins comparably bind all cRSSs tested, albeit more poorly than a consensus RSS. We show that four cRSSs that support levels of V(D)J recombination above background levels in cell culture (LMO2, TAL1, Ttg-1, and SIL) are also cleaved by the RAG proteins in vitro with efficiencies ranging from 18 to 70% of a consensus RSS. Cleavage of LMO2 and Ttg-1 by the RAG proteins can also be detected in cell culture using ligation-mediated PCR. In contrast, Hox11 and SCL are nicked but not cleaved efficiently in vitro, and cleavage at other adventitious sites in plasmid substrates may also limit the ability to detect recombination activity at these cRSSs in cell culture.

Transposable elements and the dynamic somatic genome

Although alterations in the genomes of somatic cells cannot be passed on to future generations, they can have beneficial or detrimental effects on the host organism, depending on the context in which they occur. This review outlines the ways in which transposable elements have important consequences for somatic cell genomes.

Pathophysiological aspects of memory B-cell development

B cells follow two functionally distinct pathways of development: a classical germinal center (GC) T-dependent pathway in which diversification and maturation generate a slow, but virtually unlimited high-affinity response to cognate antigens; and a marginal zone (MZ) T-independent pathway providing a first line of 'innate-like' defense against specific pathogens. Cells populating these two distinct locations are the normal counterparts of two clinically important pathological entities, follicular lymphoma (FL) and MZ lymphoma (MZL). FL and MZ represent paradigms of two rising concepts of lymphomagenesis, protracted preclinical and antigen-driven lymphoproliferation, respectively. Integrating the mechanisms and functions of MZ and GC B cells and the distinctive features of their pathological counterparts should provide essential clues to the understanding of their malignant development, and should offer new insights into the design of effective treatments for B-cell lymphomas.

In vivo reinsertion of excised episomes by the V(D)J recombinase: a potential threat to genomic stability

It has long been thought that signal joints, the byproducts of V(D)J recombination, are not involved in the dynamics of the rearrangement process. Evidence has now started to accumulate that this is not the case, and that signal joints play unsuspected roles in events that might compromise genomic integrity. Here we show both ex vivo and in vivo that the episomal circles excised during the normal process of receptor gene rearrangement may be reintegrated into the genome through trans-V(D)J recombination occurring between the episomal signal joint and an immunoglobulin/T-cell receptor target. We further demonstrate that cryptic recombination sites involved in T-cell acute lymphoblastic leukemia–associated chromosomal translocations constitute hotspots of insertion. Eventually, the identification of two in vivo cases associating episomal reintegration and chromosomal translocation suggests that reintegration events are linked to genomic instability. Altogether, our data suggest that V(D)J-mediated reintegration of episomal circles, an event likely eluding classical cytogenetic screenings, might represent an additional potent source of genomic instability and lymphoid cancer.

Lymphoid cells recognize billions of pathogens as a result of gene rearrangements that generate pathogen-specific B- and T-cell receptors. This genetic reshuffling, called V(D)J recombination, occasionally misfires and damages genomic integrity. When such aberrations dysregulate proto-oncogenes, cancer ensues. It has become increasingly clear that multiple oncogenes acting in different cellular pathways can cooperate to cause cancer. Nevertheless, in the case of T-cell acute lymphoblastic leukemia, about a third of cases display oncogene activation in the absence of identified aberration, suggesting the presence of additional mechanisms of chromosomal alteration. In the hunt for such mechanisms, episomal circles (DNA segments that are excised during V(D)J recombination) have recently drawn attention. Moreover, signal joints, short sequences formed after gene rearrangements, once considered harmless, now appear to take part in events that might compromise genomic integrity. Using ex vivo recombination assays and genetically modified mice, we demonstrate that episomal circles may be reintegrated into the genome through recombination occurring between the episomal signal joints and a T-cell receptor target. Furthermore, we show that cryptic recombination sites located in the vicinity of oncogenes constitute hotspots of episomal insertion. Altogether, our results suggest that reintegration of excised episomal circles constitute a potential source of genomic instability and cancer in leukemia and lymphoma.

Episomal DNA circles are the by-products of immunoreceptor gene rearrangements in lymphoid cells. Episomal circles can be reintegrated into the genome by trans-V(D)J recombination and cause oncogene deregulation.

Formation of NHEJ-derived reciprocal chromosomal translocations does not require Ku70

Chromosomal translocations in lymphoid tumours can involve antigen-receptor loci undergoing V(D)J recombination. Here, we show that translocations are recovered from the joining of RAG-generated double-strand breaks (DSBs) on one chromosome to an endonuclease-generated DSB on a second chromosome, providing evidence for the participation of non-RAG DSBs in some lymphoid translocations. Surprisingly, translocations are increased in cells deficient for the nonhomologous end-joining (NHEJ) protein Ku70, implicating non-canonical joining pathways in their etiology.

Chromosomal reinsertion of broken RSS ends during T cell development

The V(D)J recombinase catalyzes DNA transposition and translocation both in vitro and in vivo. Because lymphoid malignancies contain chromosomal translocations involving antigen receptor and protooncogene loci, it is critical to understand the types of “mistakes” made by the recombinase. Using a newly devised assay, we characterized 48 unique TCRβ recombination signal sequence (RSS) end insertions in murine thymocyte and splenocyte genomic DNA samples. Nearly half of these events targeted “cryptic” RSS-like elements. In no instance did we detect target-site duplications, which is a hallmark of recombinase-mediated transposition in vitro. Rather, these insertions were most likely caused by either V(D)J recombination between a bona fide RSS and a cryptic RSS or the insertion of signal circles into chromosomal loci via a V(D)J recombination-like mechanism. Although wild-type, p53, p53 x scid, H2Ax, and ATM mutant thymocytes all showed similar levels of RSS end insertions, core-RAG2 mutant thymocytes showed a sevenfold greater frequency of such events. Thus, the noncore domain of RAG2 serves to limit the extent to which the integrity of the genome is threatened by mistargeting of V(D)J recombination.

Excised V(D)J recombination byproducts threaten genomic integrity

Signal joints were long considered to be inert byproducts of V(D)J recombination that protect the genome from illegitimate rearrangements. However, increasing evidence suggests that signal joints are not inert and could pose a threat to genomic stability. A recent study from Nadel and colleagues shows that episomal signal joints readily undergo trans recombination, resulting in their insertion into chromosomal DNA.

DNA-Reparaturdefekte und Krebs

Die Zellen des lymphatischen Systems sind durch das hohe Ausmaß somatischer Rekombination und die damit verbundene Hypermutabilität gekennzeichnet. Dabei werden DNA-Doppelstrang-Brüche (DSB) induziert und „repariert“, wobei die beteiligten Gene auch in die allgemeine Reparatur von DSB einbezogen sind. Keimbahnmutationen in diesen Genen gehen mit einem besonders hohen Risiko für Lymphome einher. Diese genetisch bedingten Krankheiten sowie die charakteristischen somatischen Mutationen in Lymphomen haben wesentlich zum Verständnis von DNA-Reparaturdefekten bei der Kanzerogenese beigetragen.

The C-MYB locus is involved in chromosomal translocation and genomic duplications in human T-cell acute leukemia (T-ALL), the translocation defining a new T-ALL subtype in very young children

The C-Myb transcription factor is essential for hematopoiesis, including in the T-cell lineage. The C-Myb locus is a common site of retroviral insertional mutagenesis, however no recurrent genomic involvement has been reported in human malignancies. Here, we identified 2 types of genomic alterations involving the C-MYB locus at 6q23 in human T-cell acute leukemia (T-ALL). First, we found a reciprocal translocation, t(6;7)(q23;q34), that juxtaposed the TCRB and C-MYB loci (n = 6 cases). Second, a genome-wide copy-number analysis by array-based comparative genomic hybridization (array-CGH) identified short somatic duplications that include C-MYB (MYB(dup), n = 13 cases of 84 T-ALL, 15%). Expression analysis, including allele-specific approaches, showed stronger C-MYB expression in the MYB-rearranged cases compared with other T-ALLs, and a dramatically skewed C-MYB allele expression in the TCRB-MYB cases, which suggests that a translocation-driven deregulated expression may overcome a cellular attempt to down-regulate C-MYB. Strikingly, profiling of the T-ALLs by clinical, genomic, and large-scale gene expression analyses shows that the TCRB-MYB translocation defines a new T-ALL subtype associated with a very young age for T-cell leukemia (median, 2.2 years) and with a proliferation/mitosis expression signature. By contrast, the MYB(dup) alteration was associated with the previously defined T-ALL subtypes.

Different chromosomal breakpoints impact the level of LMO2 expression in T-ALL

The t(11;14)(p13;q11) is presumed to arise from an erroneous T-cell receptor delta TCRD V(D)J recombination and to result in LMO2 activation. However, the mechanisms underlying this translocation and the resulting LMO2 activation are poorly defined. We performed combined in vivo, ex vivo, and in silico analyses on 9 new t(11;14)(p13;q11)-positive T-cell acute lymphoblastic leukemia (T-ALL) as well as normal thymocytes. Our data support the involvement of 2 distinct t(11;14)(p13;q11) V(D)J-related translocation mechanisms. We provide compelling evidence that removal of a negative regulatory element from the LMO2 locus, rather than juxtaposition to the TCRD enhancer, is the main determinant for LMO2 activation in the majority of t(11;14)(p13;q11) translocations. Furthermore, the position of the LMO2 breakpoints in T-ALL in the light of the occurrence of TCRD-LMO2 translocations in normal thymocytes points to a critical role for the exact breakpoint location in determining LMO2 activation levels and the consequent pressure for T-ALL development.

Roles of nonhomologous DNA end joining, V(D)J recombination, and class switch recombination in chromosomal translocations

When a single double-strand break arises in the genome, nonhomologous DNA end joining (NHEJ) is a major pathway for its repair. When double-strand breaks arise at two nonhomologous sites in the genome, NHEJ also appears to be a major pathway by which the translocated ends are joined. The mechanism of NHEJ is briefly summarized, and alternative enzymes are also discussed. V(D)J recombination and class switch recombination are specialized processes designed to create double-strand DNA breaks at specific locations in the genomes of lymphoid cells. Sporadic Burkitt's lymphoma and myelomas can arise due to translocation of the c-myc gene into the Ig heavy chain locus during class switch recombination. In other lymphoid neoplasms, the RAG complex can create double-strand breaks that result in a translocation. Such RAG-generated breaks occur at very specific nucleotides that are directly adjacent to sequences that resemble canonical heptamer/nonamer sequences characteristic of normal V(D)J recombination. This occurs in some T cell leukemias and lymphomas. The RAG complex also appears capable of recognizing regions for their altered DNA structure rather than their primary sequence, and this may account for the action by RAGs at some chromosomal translocation sites, such as at the bcl-2 major breakpoint region in the follicular lymphomas that arise in B lymphocytes.