The BCL2 FISH probe set is designed to detect rearrangements involving regions of the human BCL2 gene located on chromosome band 18q21. The kit contains two differentially labeled Locus Specific Probes (LSP), one covering the 5’ (start) portion of the BCL2 gene and some upstream untranslated genomic sequence, the other covering sequences downstream of the 3’ (end) part of the gene. The two probes are designed to recognize sequences on both sides of an area inside the BCL2 gene where breakpoints are frequently found. In addition to revealing breaks, which lead to translocation of parts of the gene or its fusion to other genes, the probe set can also be used to identify other BCL2 aberrations such as deletions, amplifications or chromosome 18 hyperdiploidy.

BCL2 Gene

Fluorescence in situ hybridization (FISH) is a powerful technique developed to detect presence or absence, location, integrity and amount of genomic sequences in tissue samples or cells.^1-3

Members of the BCL2 family of proteins play important roles in the control of programmed cell death (apoptosis). They comprise proteins that facilitate apoptosis, as well as apoptosis inhibitors.⁴ At least some family members are believed to elicit release of cytochrome C from mitochondria and subsequent cell death. In addition, some members are involved in the activation of members of the caspase family of apoptosis enzymes.

The prototype family member, B-Cell Lymphoma 2 (BCL2), is a powerful inhibitor of cell death. It is expressed in most embryonic and many adult tissues, but particularly in B and T cells. It is a mitochondrial membrane protein of 25,000 molecular mass that can block programmed cell death and, when dysregulated, can extend the survival of many cell lineages, especially in hematopoietic tissues.⁵

BCL2 orthologs have been found in many mammalian species for which deciphered genomes are available. The human BCL2 gene was the first anti-death gene discovered. It consists of three exons spanning an area of about 195kb and is believed to play a crucial role in many cancer types, including skin, breast, prostate, colorectal and lung cancer, and leukemias, and in psychiatric and autoimmune diseases.^6,7

BCL2 and Lymphoma

Lymphomas are malignant conditions of the lymphatic part of the circulatory system. They are the most common blood cancers in the developed world. They also are among the small number of cancer types that has seen significant decreases in mortality in recent years, in large part owing to earlier diagnosis resulting from molecular and cytogenetic characterization.⁸ One of the most frequent subtypes of lymphoma in adults is follicular lymphoma, a cancer originating in certain B cells that is often symptomless in its early stages.⁹ Nearly half of lymphomas diagnosed in adults are of the follicular subtype.

One of the widely recognized molecular characteristics of this subtype of the condition is that BCL2 is regularly dysregulated in follicular lymphomas, most often by gene translocation.¹⁰ In fact, BCL2 protein has been detected in virtually all transformed cells in samples from follicular lymphoma patients, whereas it was not seen in non-neoplastic cells or in normal lymph nodes.¹¹  One particularly common rearrangement is a reciprocal translocation, in which the BCL2 gene, normally on chromosome 18, is moved to the immunoglobulin heavy chain (IGH) gene region on chromosome 14 and is overexpressed under the influence of an enhancer element on that locus.¹² However, fusion of BCL2 with other genes, including AFF3 and the IGHL@ and IGK@ gene loci, has also been observed.

Today, detection of BCL2 rearrangements is routinely used for the diagnostic differentiation of follicular lymphoma from among the many other non-Hodgkin forms of the disease.^13,14 If resulting in overexpression of the gene product, increased BCL2 protein can be detected in tissue samples by immunohistochemistry. Gene translocations can also be determined by PCR. However, there is not a single common breakpoint, although many of the known breakpoints within the BCL2 gene cluster in several small areas near the end of the coding region of the gene. Some breakpoints may be missed by PCR methods. In addition, translocation to genes other than IGH has also been reported, making FISH detection using breakpoint-flanking probe pairs a more definitive and straightforward method of detecting BCL2rearrangements.^15,16

References

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