| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |



From the Department of Pathology,
*
University of Utah Health Sciences Center, and the ARUP Institute of Clinical and Experimental Pathology,
Salt Lake City, Utah
| Abstract |
|---|
|
|
|---|
| Introduction |
|---|
|
|
|---|
PCR-based assays have additional advantages over SBH analysis. PCR requires smaller quantities of DNA, and high molecular weight DNA is not necessary. Therefore, PCR has been applied to small biopsies and fixed paraffin-embedded tissues, which are generally unsuitable for SBH analysis.6
PCR is extremely sensitive and can detect 1 positive cell in a background of 105 negative cells. Immunoglobulin heavy chain (IgH) PCR is capable of detecting 1 monoclonal B cell in a background of 102 to 103 polyclonal B cells.7 The extreme sensitivity of PCR also constitutes a potential source for pitfalls in the interpretation of PCR-based antigen receptor gene rearrangement studies. Clearly, contamination is a frequent concern. Additionally, we have observed discrete bands in samples obtained from small biopsy specimens in which histological and immunophenotypic evaluation revealed a reactive process. We believe that the discrete bands generated in this situation are related to the paucity of B cells in the biopsy specimen and do not represent a true oligoclonal or monoclonal B cell population. For the purpose of this study, we refer to those bands as pseudomonoclonal bands.
We undertook this study to investigate the relationship between the relative numbers of B cells in polyclonal processes and the generation of pseudomonoclonal bands by IgH PCR. We have performed serial dilutions of DNA samples obtained from a variety of well characterized specimens, including reactive tonsils and lymph nodes and malignant B cell lymphomas, and assessed them for clonality using IgH PCR. In addition, we assessed cases of benign chronic active gastritis by IgH PCR. We believe that the results of this study have implications for the interpretation of the PCR-based studies for the assessment of clonal antigen receptor gene rearrangements (IgH or T cell receptor). We also provide some recommendations to aid in the identification of pseudoclonal bands to prevent their misinterpretation as bona fide monoclonal bands.
| Materials and Methods |
|---|
|
|
|---|
Laser Capture Microdissection
Laser capture microdissection of reactive germinal centers was
performed on 5-µm-thick hematoxylin/eosin-stained sections from
formalin-fixed, paraffin-embedded tissue as previously described, using
a PixCell laser capture microscope (Arcturus Engineering, Santa Clara,
CA).9
DNA Extraction
DNA was extracted from whole sections of formalin-fixed,
paraffin-embedded tissue according to standard
procedures.10
The approximate size of the tissue samples
from which DNA was extracted was 2 cm x 1 cm x 5 µm
(three sections each) for the tonsils and lymph node biopsies, 3
mm x 3 mm x 5 µm (three sections each) for the gastric
biopsies, and 80 µm x 60 µm x 5 µm (150 cells) for
each microdissected germinal center. DNA extraction from the
microdissected reactive germinal centers was performed as previously
described.9
Serial Dilution of DNA Templates
DNA extracted from the whole sections of paraffin-embedded tissue
were serially diluted in placental DNA from 1:2 to 1:1500, and then
subjected to IgH PCR.
IgH PCR Analysis
IgH PCR analysis was performed on all samples using a previously
described protocol and primers.11
Briefly, amplification
reactions were performed using a 1605 rapid air thermocycler (Idaho
Technology, Idaho Falls, ID). A heminested approach was used using the
VHA-FRIII (5' ACA CGG C(C/T)G T(G/A)T ATT ACT GT 3') and JHa (5' ACC
TGA GGA GAC GGT GAC C 3') primers for the first round of PCR, which
consisted of 45 cycles of denaturation (94°C for 0 seconds),
annealing (55°C for 0 seconds) and extension (72°C for 1 second).
Heminesting was performed by using 1 µl of the initial amplification
product as the template in a second reaction consisting of 30 cycles of
denaturation (94°C for 0 seconds), annealing (50°C for 0 seconds)
and extension (72°C for 10 seconds) using an internal JH primer (5'
GTG ACC AGG GT(G/T/A/C) CCT TGG CCC CAG 3') and the VHA-FRIII primer.
This rapid cycle PCR assay is performed in glass capillary tubes that
accommodate only 10 µl of reaction solution, providing a high
surface-to-volume ratio and thus a high reaction efficiency. An initial
template concentration of 50 ng per 10-µl reaction maintains a
similar DNA template-to-reaction volume ratio of 5 to 10 ng/µl, as
obtains in conventional PCR assays that use 500 ng to 1 µg in
reaction volumes of up to 100 µl. In our system, utilization of
higher starting DNA template-to-reaction volume ratios are inhibitory
to rapid cycle amplification and result in PCR failure. Thus, 50 ng of
DNA isolated from whole sections or 2 µl of the crude extract from
the microdissected samples were used as the initial quantity of
template DNA per reaction. PCR products were subjected to
electrophoresis at 80V for 2 hours using ethidium bromide-stained 3.5%
metaphor agarose gels (FMC BioProducts, Rockland, ME) and products
detected by ultraviolet transillumination. A non-nested IgH-PCR
approach was also used for assessing the serial dilutions from the
reactive lymph nodes and tonsils. VHa-FRIII and JHa primers were used
in a reaction with reagent concentrations as described for the first
round of the heminested assay described above. The reaction consisted
of 35 cycles of denaturation (94°C for 0 seconds), annealing (55°C
for 0 seconds) and extension (72°C for 1 second). Each amplification
was performed in duplicate and 10 µl of the PCR products were
subjected to electrophoresis at 160V for 2 hours on 10% polyacrylamide
gels (acrylamide:bisacrylamide = 29:1).
DNA Sequencing
The IgH PCR products from two microdissected germinal centers were
subjected to bidirectional double-stranded DNA sequencing using IgH PCR
primers and ABI PRISM 377 sequencer (PE Applied Biosystems, Foster
City, CA). Comparison of sequences and identification of immunoglobulin
heavy chain gene usage was performed using Lasergene software (DNASTAR,
Inc., Madison, WI).
| Results |
|---|
|
|
|---|
|
|
|
|
| Discussion |
|---|
|
|
|---|
PCR has provided enormous benefit because of its sensitivity and its amenability to use with smaller samples. Indeed, several studies have demonstrated the utility of IgH PCR analysis in the assessment of clonality in small tissue biopsy13, 14 and fine needle aspiration specimens,2, 15 and in small groups of cells microdissected from tissue sections15, 16 or scraped from glass slides of cytologic material.17 Although most of these studies have focused on the sensitivity of PCR-based methods for assessment of clonality, only a few reports have addressed issues relating to its specificity in a systematic fashion. Nevertheless, some studies have demonstrated instances of false positivity, particularly when small or microdissected specimens are used,18 and in reactive conditions involving extranodal sites such as the stomach and salivary gland.19, 20
The results of our study clearly show a relationship between a paucity of B cell targets and a tendency for generating pseudomonoclonal or pseudo-oligoclonal bands by IgH PCR. Using DNA obtained from reactive lymph nodes and tonsils, we found that polyclonal results were reliably obtained when the starting quantity of template DNA was 50 ng or more. Serial dilutions in placental DNA revealed pseudomonoclonal bands at dilutions beginning at 1:50 (1 ng) of template. Our results are consistent with those of a previous study which used dilutions of normal peripheral blood lymphocyte DNA.2 These results collectively suggest that small amounts of polyclonal B cell DNA may generate pseudomonoclonal bands based on preferential amplification of the markedly diminished (restricted) IgH targets that are present in the highly dilute sample. Thus, a sufficient number of lymphocytes must be present in a sample to allow the production of heterogeneous IgH products in order that a polyclonal pattern may be obtained on gel electrophoresis.2 Conversely, our data revealed that monoclonality could be scored reliably when DNA from a monoclonal B cell process is diluted at 1:1000 (0.05 ng) in placental DNA, but only at 1:50 (1 ng) in polyclonal B cell DNA.
An important distinguishing characteristic of the pseudomonoclonal bands is the absence of reproducibility in duplicate reactions. Our results confirm the observations of other investigators who have shown pseudomonoclonal bands of different molecular weight in parallel reactions of multiple aliquots of DNA from a polyclonal sample.21 The inconstant nature of these bands is presumably reflective of the stochastic nature of the amplified IgH segments in the few B cells within the DNA sample. This phenomenon may explain the detection of a false positive monoclonal rearrangement in a case of colonic adenocarcinoma.22
Using the heminested IgH PCR assay, we generated nonreproducible oligoclonal bands in 2/5 (40%) cases, a pseudomonoclonal band in 1/5 (20%) cases, and reproducible polyclonal bands in 2/5 (40%) cases of H. pylori-associated chronic active gastritis. In the pseudomonoclonal case, we were able to abolish the single discrete bands in subsequent reactions by increasing the number of whole tissue sections from which DNA was extracted, while maintaining the same overall concentration of DNA per reaction (50 ng). Although the non-nested assay also yielded nonreproducible pseudomonoclonal bands in the hyperplastic lymphoid tissues evaluated, the nonreproducible bands were initially detected at lower starting template concentrations than were obtained with the heminested assay. These results indicate that the heminested assay is more prone to the generation of pseudomonoclonal bands, especially at lower starting B cell template copy numbers.
Our studies show detection of pseudomonoclonal bands in polyclonal B cell populations diluted in placental DNA at a higher sensitivity (1:2001:1000) than that of a monoclonal B cell population serially diluted with polyclonal B cell DNA (1:50). This is due in part to the fact that serial dilutions of the polyclonal B cells in placental DNA substantially reduce the number of B cell targets available for amplification, thus generating an artifactual oligoclonal B cell situation. By contrast, a monoclonal population serially diluted within a polyclonal background yields a scenario where the abundant polyclonal B cell targets effectively compete with the monoclonal population during amplification, and decrease the observers ability to discriminate the monoclonal population in an increasingly polyclonal background.
Nevertheless, the occurrence of pseudomonoclonal bands cannot be attributed entirely to the effect of limiting dilutions of polyclonal B cells. Based on the estimate of 6 pg of DNA per diploid cell,23 we calculate that the lowest detection limit of our IgH PCR assay is 10 to 20 B cell genome equivalents. In other words, a limiting dilution down to one B cell will not be detectable by our assay, and hence will not yield a pseudomonoclonal band. An additional contributing factor to the detection of pseudomonoclonal or pseudo-oligoclonal bands is the existence of geographically restricted microclonal B cell populations within the specimen. In this regard, others have shown that non-reproducible pseudomonoclonal bands can also be generated in DNA samples obtained from specimens containing abundant lymphocytic aggregates.19, 24
We recommend that for reliable assessment of IgH PCR-based clonality studies using DNA obtained from fixed, paraffin-embedded tissue extracted samples, no less than 50 ng of DNA should be used as the starting quantity of template DNA, of which B cells should account for at least 5 ng (10% of the total cells). We also recommend that multiple (at least two) aliquots of a DNA sample be subjected to IgH PCR analysis, and that specimens that produce pseudomonoclonal bands be categorized as indeterminate. Conversely, monoclonality may be scored if multiple aliquots and serial dilutions of a specimen yield one or two discrete and reproducible band(s) on IgH PCR analysis. In rare cases, reproducible pseudomonoclonal bands may be detected in reactive processes, particularly in microdissected material. It is worthwhile in these cases to use sequence-specific methods such as single strand conformational polymorphism analysis, denaturation gradient gel electrophoresis, or direct DNA sequencing to determine the relatedness of IgH PCR products from duplicate aliquots of the same sample. In light of the diagnostic significance of the assignment of monoclonality in a lymphoproliferative process, we believe that it is necessary to use clinical and histopathological parameters in conjunction with the results of molecular studies to ensure diagnostic accuracy.
| Footnotes |
|---|
Performed at ARUP Institute for Clinical and Experimental Pathology, Salt Lake City, UT 84108.
Accepted for publication March 14, 2000.
| References |
|---|
|
|
|---|
This article has been cited by other articles:
![]() |
A. Bagg Malleable Immunoglobulin Genes and Hematopathology - The Good, the Bad, and the Ugly: A Paper from the 2007 William Beaumont Hospital Symposium on Molecular Pathology J. Mol. Diagn., September 1, 2008; 10(5): 396 - 410. [Abstract] [Full Text] [PDF] |
||||
![]() |
E. Yakirevich, C. L. Jackson, P. A. Meitner, D. MacKenzie, R. Tavares, L. Robinson-Bostom, R. A. DeLellis, and M. B. Resnick Analysis of T-Cell Clonality Using Laser Capture Microdissection and High-Resolution Microcapillary Electrophoresis J. Mol. Diagn., September 1, 2007; 9(4): 490 - 497. [Abstract] [Full Text] [PDF] |
||||
![]() |
S. Lassmann, U. V. Gerlach, K. Technau-Ihling, M. Werner, and P. Fisch Application of BIOMED-2 Primers in Fixed and Decalcified Bone Marrow Biopsies: Analysis of Immunoglobulin H Receptor Rearrangements in B-Cell Non-Hodgkin's Lymphomas J. Mol. Diagn., November 1, 2005; 7(5): 582 - 591. [Abstract] [Full Text] [PDF] |
||||
![]() |
Y. Sandberg, E. J. van Gastel-Mol, B. Verhaaf, K. H. Lam, J. J.M. van Dongen, and A. W. Langerak BIOMED-2 Multiplex Immunoglobulin/T-Cell Receptor Polymerase Chain Reaction Protocols Can Reliably Replace Southern Blot Analysis in Routine Clonality Diagnostics J. Mol. Diagn., October 1, 2005; 7(4): 495 - 503. [Abstract] [Full Text] [PDF] |
||||
![]() |
K. Ahrens, R. Braylan, N. Almasri, R. Foss, and L. Rimsza IgH PCR of Zinc Formalin-Fixed, Paraffin-Embedded Non-Lymphomatous Gastric Samples Produces Artifactual "Clonal" Bands Not Observed in Paired Tissues Unexposed to Zinc Formalin J. Mol. Diagn., August 1, 2002; 4(3): 159 - 163. [Abstract] [Full Text] [PDF] |
||||
![]() |
A. Bagg, R. M. Braziel, D. A. Arber, K. E. Bijwaard, and A. Y. Chu Immunoglobulin Heavy Chain Gene Analysis in Lymphomas: A Multi-Center Study Demonstrating the Heterogeneity of Performance of Polymerase Chain Reaction Assays J. Mol. Diagn., May 1, 2002; 4(2): 81 - 89. [Abstract] [Full Text] [PDF] |
||||
![]() |
R. Taylor, J.-A. Vergilio, M. Shapiro, D. Raizen, J. Hunt, C. McGrath, A. Rook, and A. Bagg T Cell Receptor {gamma}-Chain Gene Polymerase Chain Reaction to Diagnose Central Nervous System Involvement by Cutaneous T Cell Lymphoma J. Mol. Diagn., May 1, 2002; 4(2): 118 - 120. [Abstract] [Full Text] [PDF] |
||||
![]() |
D. E. Sabath, B. L. Wood, S. J. Kussick, S. Bohling, and R. S. Mitchell PCR Methods for Determining B Cell Clonality J. Mol. Diagn., November 1, 2000; 2(4): 217 - 218. [Full Text] |
||||
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |