Testing strategies for the molecular abnormalities of BCL is evolving as the number of genes important for diagnosis and prognosis of BCL increases almost daily. Thus, the molecular testing performed to confirm a primary diagnosis, predict BCL prognosis, or identify appropriate targeted therapies for subtypes of BCL often will not be the same as testing performed for detection of MRD after therapy. At initial diagnosis or relapse of BCL, the laboratory often needs to rapidly evaluate multiple genetic targets, but with only a low level of assay sensitivity. This is in distinct contrast to testing for residual disease after therapy, for which the IGH gene is used as the sole molec ular target in most BCL subtypes and for which a very sensitive assay is needed. The specific chromosomal abnormalities and optimal molecular approaches for identification of clinically important diagnostic or prognostic factors for each BCL were presented in the previous section under the different BCL headings. The reader is referred to these sections and to the summary in Table 32-2, as these abnormalities only are discussed here in a general sense.
There are several major clinical indications for molecular testing in BCL: (1) to distinguish between a reactive and neoplastic proliferation of B cells, (2) to identify specific chromosomal abnormalities to aid in accurate subclassification of a BCL, (3) to predict the prognosis within a BCL subtype when alternative therapies related to specific molecular abnormalities are defined, and (4) to evaluate response to therapy by molecular analysis for detection of MRD. When there is a need to distinguish between reactive and neoplastic B-cell proliferations, assays for detection of B-cell clonality are most frequently used because they are applicable to virtually all BCLs. The IGH gene is the most common target evaluated for confirmation of B-cell clonality at diagnosis; the IGH gene is also the most common target for detection of persistent BCLs following therapy. The specific features of different molecular methods used for detection of B-cell clonality are discussed later (see "Assay Methods for Molecular Analysis of BCL"), as is the interpretation of the testing (see "Interpretation of Test Results").
Because substantial morphologic and immunologic overlap exists between different BCL subtypes, subclassification is sometimes difficult with routine histologic and phenotypic evaluation alone, and accurate subclassification may require evaluation for characteristic chromosomal abnormalities. Assays for specific BCL-associated chromosomal alterations have limited clinical utility in that they are applicable to only a subset of BCL patients, but they may provide essential information for accurate subclassification and prediction of prognosis. Assessment of BCL for molecular features related to adverse prognosis is being performed on a more frequent basis than in the past, due to not only the identification of increased numbers of significant molecular factors but also to the development of many new alternative therapies. Some prognostic factors for poor outcome appear to be relevant only to a particular subtype of BCL,but other factors, such as TP53 dysfunction, seem to be associated with poor prognosis or progressive disease in many BCL subtypes. Patients with BCL manifesting molecular markers of poor prognosis often are treated more aggressively at the time of initial diagnosis.
Maintaining multiple PCR-based molecular assays for detection of the many BCL-subtype-specific chromosomal abnormalities is impractical for clinical molecular laboratories, since assays other than for IGH gene rearrangements are applicable to only a small subset of BCL patients. The development of molecular probes against many of the relevant genes involved in BCL for use in FISH assays has obviated this problem and provided a valuable alternative method for the rapid evaluation of multiple genes. The use of FISH assays for molecular evaluation of BCL at diagnosis and relapse has expanded remarkably over the past several years and will likely continue to expand as new molecular probes are identified. However, it must be stressed that the sensitivity of even the best FISH assay is not sufficient for detection of MRD in treated BCL patients. There is an increasing need for the development and implementation of standardized, sensitive, reproducible, and quantitative PCR testing for detection of MRD in BCL.
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