Mechanism of the t(14;18) chromosomal translocation: structural analysis of both derivative 14 and 18 reciprocal partners.

Abstract

To elucidate the mechanism of the t(14;18)(q32;q21) chromosomal translocation found in follicular lymphoma, we examined the structure of both derivative (der) chromosomal breakpoints as well as their germ-line predecessors. We noted that chromosome segment 18q21 was juxtaposed with immunoglobulin heavy (H) chain gene diversity (DH) regions on all five der(18) chromosomes we examined, and we confirmed the juncture with immunoglobulin H-chain genejoining (JH) regions on the der(14) chromosome. However, the t(14;18) was not fully reciprocal in that chromosome 14 DNA between the DH andJH regions was deleted. Furthermore, extra nucleotides, reminiscent of "N" segments, were present at the der(14) and possibly der(18) junctions. This indicates that despite the mature B-cell phenotype of follicular lymphoma, the t(14;18) occurs during attempted DH-JH joining, the earliest event in immunoglobulin rearrangement in a pre-Bcell. Our detailed analysis of the germ-line 18q21 region indicated that most breakpoints clustered within a 150-basepair major breakpoint region. However, we found no evidence for evolutionarily conserved immunoglobulin-like recombinational signals at 18q21, arguing against a role for immunoglobulin recombinase in chromosome 18 breakage. Instead, a direct repeat duplication of chromosome 18 sequences was discovered at both chromosomal junctures, typical of the repair of a naturally occurring staggered double-stranded DNA break. These results prompt a translocation model with illegitimate pairing of a staggered double-stranded DNA break at 18q21 and an immunoglobulin endonuclease-mediated break at 14q32 and with N-segment addition, repair, and ligation to generate der(14) and der(18) chromosomes.Specific chromosomal translocations are highly associated with distinct neoplasms, implicating them in the origin or maintenance of malignancy (1, 2). These translocation breakpoints have been noted near previously identified oncogenes or new putative transforming genes, suggesting that they deregulate expression of these genes and play a direct role in tumor development (3). More than 80% of follicular B-cell lymphomas possess a t(14;18)(q32;q21) chromosomal translocation (4). Some of us and others (5-7) have cloned and characterized the site of chromosomal juncture on the derivative (der) chromosome 14 and identified a new transcriptional unit (BCL2) at 18q21. This interchromosomal recombination juxtaposed the immunoglobulin heavy (H) chain joining region (JH) with the potential transforming gene on chromosome 18.Despite the molecular cloning of the der(14) chromosomal breakpoint, the precise mechanism by which this interchromosomal recombination occurs remained unresolved.We approached this question in t(14;18) lymphomas by comparing both germ-line substrates at 14q32 and 18q21 with both derivative products of this recombination. The observation that the site of juncture on the der(14) chromosome occurred at the 5' ends of JH regions suggested that immunoglobulin recombinase may be mediating the breaka...

DNA structures at chromosomal translocation sites

“…This is the most-common chromosomal translocation in human cancer, accounting for almost all follicular lymphomas. (3,(6)(7)(8)(9)(10)(11) The t(14,18)(q32;q21) translocation occurs at the bcl-2 gene of chromosome 18, activates the protooncogene bcl-2, leads to bcl-2 overexpression and aborts programmed cell death, which in turn leads to the lymphoma. (3,5,12) Although the bcl-2 gene is more than 200 kb long, T-cells or B-cells about 75% of translocations occur within a relatively small 150 bp sequence located in the untranslated portion of the third exon, designated as the major breakpoint region (Mbr) ( Fig.…”

“…(1,14,15) The recombination signal sequences are recognized by the recombinase, the RAG-1 and RAG-2 proteins, along with HMG1. (6,7) RAG-1 and RAG-2 proteins act in concert to assemble an appropriate pair of RSSs into a synaptic complex and introduce double-strand breaks (DSBs). (16)(17)(18)(19)(20)(21)(22)(23) The induced DSBs are then repaired by nonhomologous DNA end joining with addition or deletion of nucleotides.…”

“…(6,7,9,29) Though such misrecognition of unintended sites applies to the T-cell ALL sites described above, it does not apply to the bcl-2 Mbr. A study by Marculescu et al and our recent study rule out such a mechanism.…”

t(14;18), a journey to eternity

“…In the reciprocal chromosome 18q − , a D H gene segment is coupled to the remaining bcl-2 sequences. [17][18][19] Random nucleotides, denoted 'N-nucleotides', are present at both translocational junctions, and resemble random nucleotides which are present in the V H -D H and D H -J H junctions and which are randomly inserted by terminal-deoxy transferase during the IgH assembly process. It is now generally believed that the t(14;18) most likely results from a mistake during the IgH rearrangement process in the pre-B cell stage.…”

“…It is now generally believed that the t(14;18) most likely results from a mistake during the IgH rearrangement process in the pre-B cell stage. 5,18 In agreement with this, occasional t(14;18)s have been described which are the result of erroneous IgH rearrangements during either the D H to D H Refs molecular USA 4/9 -----130 USA 29/48 -----131, 132 USA 14/20 -----133 World 37/64 -----134 USA -9/18 2/18 12/18 a --5 6 USA -21/36 11/36 32/36 --135 USA -11/17 ----136 USA 10/13 21/31 ----137 USA -11/37 ---- …”