Juxtaposition of the BCL11B Gene to a Novel Region at 17q by a t(14;17)(q32;Q21) in Childhood T-Cell Lymphoblastic Lymphoma.

T-cell acute lymphoblastic lymphoma/leukemia is frequently associated with recurrent genetic aberrations that result in the deregulation of transcription factors. In this respect, BCL11B plays a key role in the differentiation and survival during T-cell development. The 3′-located regulatory elements of BCL11B are juxtaposed to TLX3 by a cryptic t(5;14)(q35;q32) in approximately 20% of childhood T-ALL, which leads to inappropriate expression of TLX3. BCL11B can also fuse to TRDC through an inv(14)(q11.2q32.31) resulting in the expression of a BCL11B-TRDC fusion transcript in the absence of wild-type BCL11B. Moreover, a t(6;14) involving BCL11B and the 6q26 region has been described. We have identified a novel BCL11B rearrangement in a case of childhood T-cell lymphoblastic lymphoma. Cytogenetics detected a t(14;17)(q32;q21) and subsequent FISH analysis using BCL11B-spanning and BCL11B 3′-breakpoint-cluster-region flanking BAC clones revealed that BCL11B itself was not disrupted. However, a translocation breakpoint downstream of the BCL11B was observed suggesting the activation of a juxtaposed gene usually residing at 17q by the transcriptional regulatory elements of BCL11B. To narrow down the breakpoint at 17q a FISH-based chromosome-walking strategy using a set of chromosome 17q-specific BACs was employed. A BAC clone encompassing - from centromere to telomere - the genes RAB5C (a member of the RAS oncogene family), KCNH4 (potassium voltage-gated channel, subfamily H (eag-related), member 4), HCRT (hypocretin (orexin) neuropeptide precursor), GHDC (GH3 domain containing; LGP1), STAT5B (signal transducer and activator of transcription 5B), and the 5′-end of STAT5A showed a split signal indicating that one of these genes was juxataposed to the BCL11B enhancer. RAB5C, KCNH4, GHDC, and STAT5B are transcribed in a telomere-centromere orientation, whereas STAT5A shows the opposite transcriptional direction. Together with the FISH pattern observed these data suggested that STAT5A was the most likely candidate gene that might be inappropriately expressed via the regulatory elements of BCL11B. However, semi-quantitative expression analysis showed that neither STAT5A nor STAT5B were significantly upregulated in the affected lymph node as compared to normal bone marrow, peripheral blood, and thymus. In fact, compared to the expression levels in the other tissues STAT5A seemed to be expressed at lower levels. Thus, also the expression levels of RAB5C, KCNH4, and GHDC were analyzed. KCNH4 expression was almost undetectable in bone marrow, peripheral blood, and thymus and for all three genes no elevated expression was observed in the T-cell lymphoma. Owing to the unchanged expression of these genes also the transcription level of STAT3, which is localized further distal to the breakpoint determined by FISH was analyzed, and similar to STAT5A showed lower expression. However, depletion of STATs usually results in reduced cell viability and apoptosis. Together, our data suggest several scenarios:

• rearrangements of the region containing the remote enhancer of BCL11B are not necessarily accompanied by high expression of a gene juxtaposed into the close vicinity,

• expression levels of the juxtaposed gene may be just modulated rather than strongly enhanced,

• the presence of a more complex translocation undetectable by cytogenetics that results in the overexpression of a gene not obviously affected by the translocation or the generation of a fusion gene.