Bcl-6

The Bcl-6 protein (B-cell CLL/lymphoma 6, zinc finger protein 51) is a nuclear zinc finger transcription factor with an N-terminal POZ domain. Bcl-6 functions as a transcriptional repressor and is necessary for germinal centre formation. The gene is located at chromosome 3q27. Two alternatively spliced transcript variants that encode the identical protein have been reported for this gene. The Bcl-6 protein acts as a sequence-specific repressor of transcription, and has been shown to modulate the transcription of START-dependent IL-4 responses of B-cells. It can interact with a variety of POZ-containing proteins that function as transcription co-repressors. While it was generally known that this protein is a repressor of transcription, only recently it was highlighted that Bcl-6 suppresses the expression of the p53 gene and modulates DNA damage-induced apoptotic responses in germinal centre B-cells. Bcl-6 represses p53 transcription by binding 2 specific DNA sites within the p53 promoter region and accordingly, p53 expression is absent in germinal centre B-cells where Bcl-6 is highly expressed. Most notably, constitutive expression of Bcl-6 protects B-cell lines from apoptosis induced by DNA damage. These results suggest that an important function of Bcl-6 is to allow germinal centre B-cells to tolerate the physiologic DNA breaks required for immunoglobulin class switch recombination and somatic hypermutation without inducing a p53-dependent apoptotic response. These findings also imply that deregulated Bcl-6 expression contributes to lymphomagenesis in part by functional inactivation of p53. Bcl-6 appears to inhibit differentiation of germinal centre B-cells toward plasma cells by binding to signal transducers and activators of transcription 3, thereby preventing the expression of Blimp-1 (a major regulator of plasma cell development). Mutations of the Bcl-6 gene have been found in normal germinal centre B-cells, and even though it is not clear what role they play in lymphomagenesis, it appears that they may disrupt its negative autoregulation and in this manner promote lymphoma growth. In non-neoplastic lymphoid tissues, Bcl-6 protein is expressed in germinal centre B-cells in secondary follicles in both the dark (centroblast-rich) and light (centrocyte-rich) zones. It is also detectable in a few CD4-positive T cells, both in the germinal centres and in interfollicular areas, and in perifollicular CD30+ lymphoid cells. In addition, Bcl-6 protein is expressed in thymocytes, mainly in the cortex. It was recently also demonstrated in monocytes, but in levels lower than that usually observed in germinal centre B-cells. Bcl-6 is also observed in the squamous epithelium of the epidermis, transitional epithelium and olfactory sensory neurons. In all positive cells, Bcl-6 protein localizes to the nucleus in a diffuse or microgranular pattern.

In malignant lymphomas, Bcl-6 protein is most commonly detected in germinal centre neoplasms including follicular lymphoma, Burkitt lymphoma, diffuse large B-cell lymphoma (40%), lymphocyte predominant Hodgkin lymphoma, and even in peripheral T-cell lymphoma with follicular localization (“follicular T cell lymphoma”). It is also found in CD30+ anaplastic large cell lymphoma. Bcl-6 expression is virtually absent in acute lymphatic leukaemia and mantle cell lymphoma. Translocations of the regulatory part of the Bcl-6 gene juxtapose the gene to promoters from a variety of partner chromosomes (most commonly in Ig chain loci).

Bcl-6 is mainly used in the classification of B-cell lymphomas (see above).

Normal tonsil is recommended as positive and negative tissue control. Virtually all the germinal centre B-cells must show a moderate to strong nuclear staining reaction, while an at least weak to moderate nuclear staining reaction must be seen in the majority of squamous epithelial cells. In the mantle zones and interfollicular areas only dispersed cells should display a positive nuclear staining reaction.

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