Rare double-hit with two translocations involving IGH both, with BCL2 and BCL3, in a monoclonal B-cell lymphoma/leukemia

Chronic Lymphocytic Leukemia (CLL) is a lymphoproliferative disease characterized by multiple recurring clonal cytogenetic anomalies and is the most common leukemia in adults. Chromosomal abnormalities associated with CLL include trisomy 12 and IGH;BCL3 rearrangement [t(14;19)(q32;q13)] that juxtaposes a proto-oncogenic gene BCL3 and an immunoglobulin heavy chain, a translocation that may be associated with shorter survival. In addition to the IGH;BCL3 rearrangement, other translocations involving 14q32 locus are involved in various lymphoproliferative pathologies pointing toward the significance of IGH locus in oncogenic progression. Significantly, in the majority of B-cell neoplasms that carry an IGH;BCL3 rearrangement, it is a sole translocation involving an IGH locus. We report a patient who, in addition to trisomy 12, carried a rare double-hit translocation characterized by the IGH;BCL3 translocation and an additional clonal IGH;BCL2 translocation involving IGH and another proto-oncogene BCL2, t(14;18)(q32;q21), commonly found in follicular lymphoma. Further single nucleotide polymorphism (SNP) array-based analysis detected a duplication of the 58.8 kb region at 19q13.32 adjacent to the BCL3 translocation junction on chromosome 19q13. Interestingly, the duplicated region contained ERCC2 gene, which encodes a DNA excision repair protein involved in the cancer-prone syndrome, xeroderma pigmentosum. Taken together our findings indicate the existence of double-translocation driven oncogenic events involving both IGH loci and proto-oncogenes BCL2 and BCL3. Importantly, the IGH;BCL3 translocation was characterized by the duplication of the genomic region adjacent to BCL3, containing a major DNA repair factor, ERCC2.


Background
Chronic lymphocytic leukemia (CLL) is a genetically heterogeneous neoplasm characterized by the progressive accumulation of B cells in bone marrow, lymph nodes and blood. The progression of the disease is highly variable, ranging from the indolent state to the highly aggressive leukemia marked by short survival times. Numerous chromosomal abnormalities have been shown to contribute to CLL, including but not limited to trisomy 12, loss of 11q22-q23 containing the ATM gene, loss of 13q14.3 and 6q, loss of 17p13 containing TP53 gene, and others [1].
A specific translocation [t(14;19)(q32;q13)] which juxtaposes the immunoglobulin heavy chain locus (IGH) sequence (HUGO, 14q32.33) and the gene encoding an anti-apoptotic protein BCL3 resulting in overexpression of BCL3 [2], is of a particular interest because it does not occur frequently, and it is usually associated with shorter survival [1]. In addition, this translocation as well as other translocations involving the IGH locus, although found in CLL, have been also described in poorly clinicopathologically described B cell lymphomas, categorized as atypical CLL [3]. Another translocation involving IGH locus and an antiapoptotic protein BCL2, t(14;18)(q32;q21), is considered a hallmark of aggressive lymphomas, such as follicular lymphomas (FL) [4,5] and diffuse large B-cell lymphomas (DLBCL) [6][7][8].
We report a patient who was evaluated for leukocytosis with lymphocytosis, and who was found to have a marked bone marrow involvement by neoplastic lymphocytes showing a B-cell line of differentiation as determined by flow cytometry and immunohistochemistry. Morphologically, the lymphocytes were small to medium in size, and a subset showed knobby cytoplasmic blebbing. Further immunophenotypic studies ruled out involvement by typical CLL or mantle cell lymphoma in this patient, highlighting an atypical characteristic of this malignancy. Surprisingly, in addition to trisomy 12, which is commonly found in CLL, chromosome analysis at the haploid band resolution 300-400 revealed the presence of clones carrying a single t(14;19)(q32;q13) IGH;BCL3 translocation or both, t(14;19)(q32;q13) IGH;BCL3 and t(14;18)(q32;q21) IGH;BCL2 translocations. Whereas single translocation events involving IGH and BCL3 or BCL2 loci can be detected in lymphoid cancers, double-hit translocations involving both IGH;BCL2 and IGH;BCL3 rearrangements in the same patient are exceptionally rare. Additionally, using SNP array analysis we detected a local microduplication event at the BCL3 translocation junction on chromosome 19 involving a DNA repair factor ERCC2. An additional file describing experimental procedures is available (see Additional file 1). However, due to the nature of the SNP array platform we cannot exclude the possibility that ERCC2 duplication occurred on the non-rearranged chromosome 19. Given the absence of classical CLL and mantle cell lymphoma immunophenotype, size and morphology of the neoplastic lymphocytes, and the presence of B-cell lymphoproliferative genetic markers characteristic of aggressive B-cell lymphomas, we favor the diagnosis of an aggressive monoclonal B-cell lymphoma/ leukemia, likely B-prolymphocytic leukemia in this patient driven by the IGH;BCL3 and IGH;BCL2 mediated mechanisms.

Case presentation
Our patient was an 82-year-old African-American female who presented to her oncologist for leukocytosis. Complete blood count (CBC) data showed a white blood cell (WBC) count of 24.5 K/ MicroL with relative and absolute lymphocytosis of 70 % and 17.2K/ MicroL respectively. A bone marrow biopsy and aspirate was performed and the specimen was sent to the hematopathologist for evaluation. Flow cytometric analysis showed 40 % monoclonal B-cells with lambda light chain restriction of moderate intensity with dim CD5 co-expression and the following immunophenotype: CD10-, CD19+, CD20+, CD200-, CD23 +/-(dim), FMC7 +/-(dim), CD38-, CD25-, CD103-and CD11c-.
By histology the bone marrow showed marked involvement by lymphocytes with interstitial pattern of distribution estimated at 80-90 % of the total marrow cellularity. H&E staining, CD20 and Pax5 immunohistochemistry confirmed the nature of the lymphocytes consistent with B-cell line of differentiation ( Fig. 1a and b and data not shown). No lymphoid aggregate formation was seen. Morphologically the lymphocytes were small to medium in size and showed clumped nuclear chromatin with small but conspicuous nucleoli and no morphologic features suggestive of involvement by DLBCL, Burkitt lymphoma or other high-grade aggressive B-cell lymphomas.
By immunohistochemistry the B-cells were negative for Cyclin-D1 and, additionally, they were negative for Sox-11, arguing against the diagnosis of mantle cell lymphoma or Cyclin-D1 negative mantle cell lymphoma, respectively. Evaluation of the aspirate smear showed the lymphocytes with occasional small knobby cytoplasmic blebbing but no discernible villous hairy projections.
Overall, based on the immunophenotypic finding by flow cytometric analysis, it is unlikely that this lymphoma represents typical CLL, since it not only expresses monoclonal light chain with moderate intensity, it also shows weak CD23 expression and is negative for CD200. The possibility of mantle cell lymphoma and Cyclin D-1 negative mantle cell lymphoma was considered and further evaluated but ruled out by negative staining with Cyclin D1 and Sox-11 immunohistochemistry respectively. Given the morphologic observation of knobby cytoplasmic blebbing, the diagnosis of B-PLL (B-prolymphocytic leukemia) was considered a strong possibility.
Cytogenetic studies of bone marrow preparations from this patient revealed two related abnormal clones in eight of twenty-two metaphase spreads examined. The first clone (stemline [sl]), six cells, contained a translocation between the long (q) arm of chromosomes 14 and 19 (Fig. 2a, arrows), and gain of one copy of chromosome 12 (underlined). The second clone, a composite of two cells, was a doubling of the stemline clone (the first clone) with two copies of an additional translocation between 14q and 18q (Fig. 3a, red arrows, six copies of chromosome 12 are underlined). The remaining fourteen cells contained a normal karyotype. Additionally, FISH studies using BCL3 and IGH break-apart probes confirmed BCL3 and IGH translocations in the first clone ( Fig. 2b and c, arrows). With respect to the second Fig. 1 a H&E staining of the bone marrow showing marked interstitial involvement by medium sized cells with small but conspicuous nucleoli. b CD20 immunohistochemistry highlights marked involvement of the bone marrow by abnormal B-cells Fig. 2 a Karyotype of the first clone. The first clone (stemline [sl]), six cells, contained a translocation between the long (q) arms of chromosomes 14 and 19 (arrows), and gain of one copy of chromosome 12 (underlined). b BCL3 break-apart FISH probe (green/red) indicates rearrangement of a BCL3 locus in the first clone (arrows). Normal FISH signal is shown on the right side of the panel for comparison. c IGH break-apart FISH probe (green/red) indicates rearrangement of an IGH locus in the first clone (arrows). Normal FISH signal is shown on the right side of the panel for comparison clone, IGH;BCL2 dual-fusion probe picked up eight signals for IGH (Fig. 3b, black and white IGH image) which, in combination with karyotype data, was indicative of the presence of eight IGH derivative translocation products. This suggests that all IGH loci are rearranged in the second clone in addition to the near duplication of the chromosome content. BCL2 probe picked up six BCL2 signals (Fig. 3b, black and white BCL2 image) indicating on the presence of four translocated BCL2 loci and two intact BCL2 loci from the non-translocated chromosome 18. Importantly, IGH and BCL2 signals formed four fusions (Fig. 3b, "merge" panel, arrows) confirming two derivative IGH;BCL2 loci on chromosomes 14 and chromosome 18. In addition, IGH break-apart probe confirmed the presence of four translocated IGH loci (Fig. 3b, right panel, four red and four green signals). In summary, first clone was characterized by the IGH;BCL3 translocation event resulting in derivative chromosomes, 14 and 19 (Fig. 4a) and second clone was characterized by the presence of IGH;BCL2 translocation in addition to IGH;BCL3 translocation and near tetraploid chromosome content resulting in four derivative chromosomes 14 and two derivative chromosomes 18 and 19 (Fig. 4b).

Conclusions
In this report we describe a case of a monoclonal B-cell lymphoma/leukemia with extensive bone marrow involvement and CLL-like immunophenotype showing CD5 expression by flow cytometric analysis and furthermore displaying a double translocation event between IGH loci and both BCL3 and BCL2 gene loci. Additionally, we detected a microduplication event in the genomic locus adjacent to BCL3 gene involving a nucleotide excision repair protein ERCC2. Our karyotype analysis identified two abnormal clones in this patient. The first clone contained, in addition to trisomy 12, a single cytogenetically defined translocation t(14;19)(q32;q13) involving IGH;BCL3 loci. The second clone represented a near duplication of the chromosome content of the first clone and the presence of two copies of an additional translocation involving IGH locus and BCL2 gene t(14;18)(q32;q21). Therefore, all IGH loci located on the chromosome 14 were rearranged in the second clone which was confirmed by our FISH studies.
These results are consistent with a neoplastic process in this patient's bone marrow specimen. Importantly, the translocation t(14;19)(q32;q13) involving IGH and BCL3 loci, which results in altered BCL3 expression, has been described in aggressive forms of lymphomas and atypical CLLs [3,10]. The translocation t(14;18)(q32;q21) is a recurring abnormality in B-cell lymphomas. This translocation places the oncogene BCL2 on 18q21 within the immunoglobulin heavy chain locus on 14q32 and therefore deregulates the BCL2 function. Juxtaposition of both Fig. 4 Schematic representation of the rearrangements described in clone 1 and clone 2. Chromosomes involved in rearrangements are shown on the right. a Rearrangements in clone 1 involving IGH (14q32) and BCL3 (19q13.1) which results in two derivative chromosomes (der(19) and der (14). Rearranged chromosomes 14 and 19 are indicated by an arrow on the right. b Rearrangements in clone 2 which includes chromosome duplication event and IGH (14q32) and BCL2 (18q21) translocation in addition to IGH;BCL3 rearrangements. Therefore clone 2 contains 8 derivative chromosomes (a pair of each der(19), der(18), der(14)t(14;19), and der(14)t(14;18)). Rearranged chromosomes 18 and 19 are indicated by the arrows on the right BCL3 and BCL2 genes next to the active IGH locus leads to their altered expression, decreased apoptosis, and leukemia progression [10,11]. Concurrent rearrangements of BCL2(18q21) and BCL3(19q13) have been reported in atypical chronic lymphocytic leukemia at Richter's transformation [12]. Both BCL3 and BCL2 are protooncogenes, however mechanistically, BCL2 functions as a key regulator of apoptosis through the mitochondrial pathways [11], whereas BCL3 is a predominantly nuclear protein recruited to the NFκB-responsive promoters where it regulates apoptotic program [13], suggesting that rearrangements of BCL2 and BCL3 may differentially modulate leukemic progression through distinct molecular pathways. The appearance of the IGH;BCL3 clone followed by the genome near duplication event and the appearance of the second clone containing both IGH;BCL3 and IGH;BCL2 rearrangements in our patient might, therefore, have an additive effect on the development of this neoplasm.
ERCC2 duplication in this patient is an intriguing finding, because of the role of ERCC2 in DNA nucleotide excision repair process. Specific ERCC2 polymorphisms are implicated in susceptibility to melanoma [14] as well as triple negative breast cancer [15]. However, according to the Database of Genomic Variants (http://dgv.tcag.ca/dgv/ app/home), duplications of ERCC2 genomic region are also found in healthy individuals. Therefore, the significance of ERCC2 duplication in context of IGH;BCL3 and