Malleable immunoglobulin genes and hematopathology - the good, the bad, and the ugly: a paper from the 2007 William Beaumont hospital symposium on molecular pathology

Abstract

Immunoglobulin gene rearrangement analysis is one of the more commonly performed assays available on the hematopathology menu of clinical molecular pathology laboratories. The analysis of these rearrangements provides useful information on a number of different levels in the evaluation of lymphoproliferations. An appreciation of the various mechanisms involved in the numerous physiological pathways affecting the immunoglobulin genes, and hence antibody molecules, is central to an understanding of B-cell development vis-à-vis the generation of immunological diversity. Knowledge about the intricate complexities of these mechanisms is also germane to an evaluation of testing methodologies. With this information, it is easier to develop an understanding of how contemporary molecular testing of immunoglobulin gene rearrangements has evolved, from historically quite heterogeneous, fairly flawed, and rather ugly approaches to current more-standardized protocols. In addition, recognition of how such genetic changes with good intentions can turn bad has fostered increasing insights into the pathogenesis of B-cell lymphomas and leukemias. Despite the significant improvements in the design of immunoglobulin gene rearrangement assays, numerous pitfalls and caveats remain. Accordingly, it is crucial to consider such testing a tool, and although most useful, it is one of many tools that may be required to build cogent diagnoses.

Figure 1

Major stages of development of the IG repertoire. Three major phases during which IG gene diversity and effector function is generated are noted at the top: VDJ recombination, SHM (somatic hypermutation), and CSR (class switch recombination), and the enzymes mediating these events (RAG1/2, recombinase-activating genes 1 and 2; AID, activation-induced cytidine deaminase). The generation of combinatorial diversity is not noted (see text for details). The two major sites at which these events occur are in the middle: bone marrow and germinal center of lymph nodes. The nongerminal center (T-cell independent) pathway for acquisition of memory (ie, a B cell can acquire memory and become a marginal zone cell, independent of the germinal center) is not shown. The respective stages of B-cell maturation are noted at the bottom.

Figure 2

IGH@ VDJ recombination and detection of PCR products. A: D-J rearrangement precedes full V-DJ rearrangement (see text for details). The intervening DNA sequences are deleted. In between the rearranged V and D, and D and J, are N (nucleotide) sequences, inserted by TdT (terminal deoxynucleotidyl transferase). Only a rearranged VDJ gene can be PCR amplified with V and J primers, since in the germline (non-rearranged) configuration the V and J gene segments are too far apart to be successfully amplified. The primers are directed at the FRs (framework regions), with most of the length and composition heterogeneity generated by CDR3 (complementarity determining region 3). B: PCR products analyzed by gel electrophoresis. Lane 1, size markers; lane 2, polyclonal smear; lane 3, two monoclonal bands, consistent with bi-allelic rearrangement; lane 4, specimen from same patient as analyzed in lane 3, now 2 years later, with the apparently identical rearrangements most suggestive of relapse rather than a second primary; lane 5, monoclonal; lane 6, polyclonal smear; lane 7, neither discrete band nor smear, suggestive of either an absence of B cells or failure of these primers (seen as band at the bottom) to amplify a monoclonal rearrangement. C: PCR products analyzed by capillary electrophoresis. A Gaussian distribution is seen with polyclonal, reactive B cells, using FR1, FR2, and FR3 primers. The x axis reflects the lengths of the products in base pairs, whereas the y axis reflects the height of the peaks in arbitrary fluorescent units. Original images kindly provided by Jason Merker, MD, Stanford University. D: PCR products analyzed by capillary electrophoresis. A discrete peak in the absence of a Gaussian distribution is seen with monoclonal B cells, using FR1, FR2, and FR3 primers. The axes are as noted above in C. Original images kindly provided by Jason Merker, MD, Stanford University.