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Gene Rearrangements Demonstrates the Need for Multiple Primer Sets
From the Department of Pathology and Microbiology, University of Nebraska Medical Center, Nebraska Medical Center, Omaha, Nebraska
| Abstract |
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gene rearrangements (TCR
GR) in T-cell lymphoproliferative disorders. The reported efficacy of TCR
GR protocols ranges from 60% to greater than 90%. Laboratories reporting a lower detection rate tend to use a limited set of primers. The goal of our study was to provide TCR
GR data to demonstrate the molecular biological basis for needing multiple primer sets targeting all gene segments. Sixty cases with a confirmed histological diagnosis of a T-cell lymphoproliferative disorder and TCR
GR were identified in our lymphoma registry from 1995 to 2001. DNA was obtained from fresh/frozen tissue, cell lysates, or paraffin-embedded tissue. Variable (V
) region gene segments were identified using denaturing gradient gel electrophoresis, which was used to select the cases in the study. Capillary electrophoresis using fluorescent-labeled joining (J
) region primers was performed to identify J
segments. Sixty cases contained a total of 98 TCR
GR, as some cases have more than one rearrangement. The most frequent gene segment combination involved the V
18 and J
1/2 segments. If a single primer set directed at these two segments were used for clinical diagnosis, that pair of primers would only diagnose 67% of cases as positive for TCR
GR. Our gene segment distribution data emphasize the importance of using a comprehensive set of V
and J
primers for an optimal detection rate of TCR
GR. Protocols with limited numbers of primers should be reconsidered. | Introduction |
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In most T-cell lymphomas, diagnostic assays in T-cell receptor gene rearrangements targeting the ß and
genes are most useful. Rearrangements of the T-cell receptor
(TCR
) chain gene are often analyzed in T-cell lymphoproliferative disorders by polymerase chain reaction (PCR), due to the relative structural simplicity of the gene. The TCR
chain gene is located on the short arm of chromosome 7 and has 2 constant, 5 joining, and 14 variable region segments. Of the 14 variable region segments, 11 are functional and have been described as rearranged in T-cell lymphoproliferative disorders1
, 2, 3, 4, 5
There is limited data regarding the distribution of gene segments in rearrangements of the TCR
chain gene. Two previous studies by Theodorou et al4
and Födinger et al6
have demonstrated that most variable region (V
) rearrangements occur within the V
18 subgroup (Group I) and most joining region (J
) rearrangements involve the J
1/2 segment. This high frequency may have prompted some laboratories to devise TCR
PCR assays that use only single primer sets for the V
18 and J
1/2 segments. However, both studies have demonstrated that some T-cell lymphoproliferative disorders involve other V
and J
segments that would not be identified with a single V
18 and J
1/2 primer set. Despite this data, there is a heterogeneous group of primer sets currently used by many laboratories.
A recent multi-center study by Arber et al7
involving 21 participating laboratories, specifically addressed the use of different primer sets in TCR
PCR testing and compared sensitivity rates. Based on their findings, 25% of laboratories used a single primer set directed at the V
18 and J
1/2 segments. Since the survey was sent only to members of the Association for Molecular Pathology, the percentage of all laboratories using a single primer set is unknown. This study found a 77.9% overall detection rate by TCR
PCR and noted a significant difference in true positive results among laboratories that used multiple primer sets (84%) versus those that used only a single primer set (61.4%) directed against the V
18 and J
1/2 segments. Given the variation in TCR
primer sets used by laboratories and in the sensitivity results reported by Arber et al,7
we sought to determine the distribution of involved V
and J
segments for the purpose of supporting the utilization of complete primer sets in TCR
PCR testing.
| Materials and Methods |
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GR(s) identified by denaturing gradient gel electrophoresis (DGGE), diagnostic material for morphological review, a T-cell phenotype by immunohistochemical analysis, and adequate DNA amplification on re-analysis. Cases were classified according to the World Health Organization classification system.8
The cases consisted of 38 peripheral T-cell lymphomas, 10 anaplastic large-cell lymphomas, six cases of mycosis fungoides, three T-cell large granular lymphocyte proliferations, two lymphomatoid papulosis cases, and one hepatosplenic T-cell lymphoma.
Extraction Protocols
DNA from 10 fresh tissue samples, 14 frozen tissue samples, and 36 formalin-fixed, paraffin-embedded tissue samples was used for the analyses. DNA extraction procedures for each sample type have been previously described.5, 9
Briefly, fresh and frozen tissue samples were homogenized and 0.5 µg DNA was used for PCR following proteinase K (Sigma, St. Louis, MO) digestion, phenol/chloroform (Amresco, Solon, OH) extraction, and ethanol precipitation procedures. For peripheral blood samples, 2.0 to 5.0 µl of cell lysate (estimated 0.5 to 1.0 µg) from a mononuclear cell fraction isolated from a Ficoll-Hypaque (Accurate-Chemical, Westbury, NY) gradient and digested with proteinase K was used for PCR.10
For formalin-fixed, paraffin-embedded tissue, 5- to 10-µm microtome sections were cut and paraffin was dissolved with xylene (Sigma). The tissue was then washed with 100% ethanol (Sigma) and placed in a dry incubator at 50°C to evaporate residual alcohol. A proteinase K digestion step was performed overnight at 37°C to produce a tissue lysate. Proteinase K was then inactivated by heating to 95°C for 10 minutes before using lysate estimated to contain 0.5 µg of DNA.
V
Segment Identification
The V
gene segment analysis was previously performed by DGGE (CBS Scientific, Del Mar, CA) during clinical testing of the cases using multiple GC-clamped V
primers (V
2, V
9, V
10, and V
11) and J
primers (J
P1/2, J
2, and J
P), as described.5
Primer sequences are listed in Table 1
. Four separate reaction mixes were prepared with each one containing a different GC-clamped V
primer to identify the V
segment combined with all J
primers (Figure 1)
. The PCR cycles consisted of a 9-minute denaturing step at 94°C, followed by 45 PCR cycles (94°C for 75 seconds, 66°C for 75 seconds, and 72°C for 10 seconds with a 1-second additive extension per each cycle). A final extension step at 72°C was held for 7 minutes. The GC-clamped products were separated using a modified DGGE procedure using an 8% polyacrylamide gel (Amresco) with a 30 to 60% urea-formamide gradient, and stained with ethidium bromide solution (Fisher, Pittsburgh, PA).5, 11, 12
DNA from T-cell lines (Jurkat, HSB2, and CEM) served as positive controls and a reaction mix with no DNA was used as a negative control.5
Only samples with discrete bands and staining intensity similar to the controls were regarded as positive. DNA integrity was assessed using a multiplex ß-hemoglobin gene assay.9, 13
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Segment Identification
segment analysis was performed in duplicate assays using capillary electrophoresis (CE) in an ABITM Prism 310 Genetic Analyzer (Perkin Elmer Applied Biosystems, Foster City, CA).14, 15
Fluorescent-labeled J
primer sets (J
1/2, J
P 1/2, and J
P) were coupled with a set of V
primers (V
2, V
3, V
9, V
10, and V
11) in a single multiplex PCR tube using a Hybaid Omnigene thermal cycler (National Labnet). The PCR reagents contained 0.6 µmol/L of fluorescent-labeled J
primers, 0.6 µmol/L of V
primers, 100 µmol/L of each dNTPs, 1.5 mmol/L MgCl2, 10 mmol/L Tris-HCl, 0.01% gelatin (Perkin Elmer ABI) and 1.25 units of Platinum Taq polymerase (Invitrogen, Carlsbad, CA). Primer sequences are listed in Table 2
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| Results |
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GRs among the 60 cases analyzed, as many cases had biallelic or more rearrangements, a characteristic of TCR
GR.4, 5, 14, 16
Twenty-five cases had a single clonal rearrangement, 32 cases had two clonal rearrangements, and three cases had three clonal rearrangements. The cases with three rearrangements, which may represent the development of subclones, included two peripheral T-cell lymphomas and a large granular lymphocyte proliferation/leukemia. All cases included had concordant results on the number of TCR
rearrangements between DGGE and CE.
Analysis of Individual TCR
Rearrangements
Based on a total of 98 gene rearrangements, we first analyzed the number of individual rearrangements involving each V
and J
segment (Table 3)
. As expected from previous TCR
GR studies,6, 17
the Group I V
18 and J
1/2 genes were the most commonly rearranged segments. For the V
segments, the V
18 segments were involved in 55 (56%) individual rearrangements, the V
9 in 22 (23%) rearrangements, the V
10 in 17 (17%) rearrangements and the V
11 in 4 (4%) rearrangements. For the J
segments, most rearrangements occurred in the J
1/2 segments (81 of 98, 83%), followed by the J
P-1/2 segments (14 of 98, 14%), and the J
P segment (3 of 98, 3%).
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and J
primers used for analysis. The results of the V
and J
segment pairs are listed in Table 3
18 and J
1/2 was the most frequently involved segment pair. However, slightly less than half (47%) of all individual TCR
GR involved the V
18 and J
1/2 combination, which supports the conclusion that using only these two primers is insufficient for clinical testing purposes. The data in Table 3
and J
segments are not uncommon, although rearrangements involving either the rare V
11 and J
P segments constitute only 7% percent of the total.
Analysis of Case Positivity Detection Rates with Various Primer Sets
To further investigate the efficacy of different primer sets, we determined the case positivity rate for detecting at least one rearrangement (Table 4)
. Case positivity, or qualitative sensitivity, is defined as the percentage of cases detected with TCR
GR using a specific set of primers. We first analyzed the case positivity rate if only the V
18 and J
1/2 primers were used, and found only a 67% case positivity detection rate. The sensitivity rate of 61.4% reported by Arber et al7
for laboratories that only use the V
18 and J
1/2 primer pair is consistent with our TCR
GR distribution data. Distribution data for the segments used in the TCR
GR was not analyzed in the multi-center study.7
Even if all of the J
primers were used with the V
18 primers, the detection rate only increases to 75% (data not shown), which demonstrates the requirement for using V
9, V
10, and V
11 primers.
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primers were added and coupled with a single J
1/2 primer. When V
9, V
10, and V
11 primers are cumulatively added to the primer set, the detection rate increases to 73%, 80%, and 82%, respectively. When the entire V
primer set covering the V
segments (111) is used and as additional J
primers are added to J
1/2, the detection rate increases to 97% (J
P1/2) and 100% (J
P1/2 and J
P). Since only cases with a known TCR
GR were selected in this study, the detection rate with the complete V
and J
primer set is by definition 100%. | Discussion |
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GR protocols that use only primers to the V
18 and J
1/2 gene segments are insufficient for diagnostic testing purposes. In comparison to previous studies that reported TCR
gene segment distribution data,4, 6
our larger study of 60 T-cell lymphoproliferative disorders shows a lower overall incidence of V
18 rearrangements and a higher incidence of V
9 and V
10 rearrangements. Our J
segment data are similar to these previous studies, and together these studies show a very small percentage of rearrangements involve either the V
11 or J
P segment. This is the first work that demonstrates the case detection results one can expect with different combinations of TCR
primers. In addition, we provide the molecular basis that explains why the presence of biallelic TCR
GR provides partial success for protocols with limited primer sets.
If the V
and J
segments are determined for all of the rearrangements, less than half (47%) of individual rearrangements involved the V
18 and J
1/2 segments. The reason why sensitivity rates for TCR
GR assays which use only a single V
18 and J
1/2 primer set are higher than the frequency of individual gene segments used is due to the frequent occurrence of biallelic rearrangements in T-cell lymphoproliferative disorders. In our study, 53% of cases exhibited biallelic rearrangements and 5% of cases had three rearrangements. Some studies have found higher incidences of biallelic rearrangements.16, 18
Laboratories that use limited primer sets are actually achieving higher detection rates because biallelic TCR
GR occur, often with one rearrangement involving the V
18 or J
1/2 segments. Among the 33 cases with biallelic or more rearrangements, nearly all (30 of 33, 91%) had a V
18 and J
1/2 in one of the rearrangements. However, cases with a monoallelic and rare biallelic TCR
GR involving the other gene segments will be missed. In a previous study by Födinger et al,6
eight of 30 (27%) patient samples did not have a TCR
GR involving the V
18 or J
1/2 gene segments. Our study also demonstrates a high false negative (33%) rate would occur if only these two primers are used. Given the importance of identifying a clonal T-cell population in establishing a diagnosis of a T-cell lymphoproliferative disorder, a false negative result could potentially lead to an erroneous diagnosis.
The detection rates for different primer sets in the literature is difficult to evaluate, since case selection methods are variable as is the gold standard used to compare results. Using a single V
18 and J
1/2 primer set, Gutzmer et al19
found a 59 to 72% detection rate among 22 patients with cutaneous T-cell lymphoma. The detection rate varied due to the different protocols that were used, including using a thermocycler versus a LightCycler for amplification and polyacrylamide gel electrophoresis versus melting curve analysis for detection. In a study by Dippel et al,20
which included 21 patients with advanced stage cutaneous T-cell lymphoma, a 76% sensitivity rate was reported using primers to the V
18, J
1/2, and J
P1/2 gene segments.
In comparison, Lamberson et al21
found a 90% sensitivity rate using multiple primer sets in three separate multiplex reactions. The findings by Luo et al22
and Vega et al16
also support the trend of a higher detection when complete primer sets are used. They found a 92% and 98% sensitivity rate respectively, in T-cell lymphomas using a large primer set directed at all V
and J
segments. Since 5 to 10% of T-cell lymphomas will lack a detectable TCR
GR,4
these are excellent results. Our data and the results of these studies emphasize the need for complete primer sets for an optimal detection rate. Many protocols have been described with complete V
and J
primer sets5, 14, 16, 17, 22, 23, 24, 25, 26, 27
that laboratories could choose in their labs. However, since some T-cell lymphomas lack TCR
GR, the use of a TCR-ß assay is recommended after a negative TCR
assay, when lymphoma is still suspected.
Given the literature findings, it is surprising that many laboratories continue to use only a single primer set or limited primer sets. Perhaps one concern may be attributed to a previous report of potential false positive results associated with V
9 and J
P primers.28
TCR
GR that involve the less frequently involved V
and J
segments should always be interpreted with caution, particularly if the PCR reactions are performed in separate tubes. Amplification with all primers in a single tube helps avoid false positive results with rare V
or J
segments. If only a small number of normal cells with a particular sized segment are amplified, a true polyclonal distribution may not be apparent and faint bands or small spikes could potentially be interpreted as positive. Combining all primers together in capillary electrophoresis helps in defining the polyclonal background and the significance of small peaks. Information concerning the number of T cells present within the specimen analyzed should be taken into account. In addition, duplicate assays help to identify reproducible peaks and eliminate false positive results.
It is critical to compare the peak intensity with the control samples with a known percentage of clonal cells when making a determination of a clonal population. Capillary electrophoresis is our preferred method due to the decreased turnaround time and labor costs. Using the capillary electrophoresis technique, only well-defined spikes with a peak height that is greater than twice that of the polyclonal background peaks are presently interpreted as positive in our laboratory, as compared to a 2% positive control.14
Lee et al29
have described the presence of pseudo-spikes in histologically benign lymphoid tissues, although none of these samples had a maximal peak to polyclonal background ratio that exceeded 1.37 and most were below 1.0. In our experience, we have not identified a significant false positive rate associated with these primers when strict criteria for a true clonal population are used. Similarly, in a comparative study of 21 laboratories by Arber et al,7
there was no significant difference in false positive results with any T
primer group.
In conclusion, pathologists should reassess TCR
protocols in their laboratories or reference laboratories to ensure that multiple primer sets are used, as many laboratories do not report the primer sets used in the procedure part of their report. Clearly, it is unacceptable to have a false negative rate of up to 33% when protocols can be modified at minimal cost to include primers to the uncommon T
gene segments.
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| Acknowledgments |
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| Footnotes |
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Supported by USPHS grant CA 36727.
Presented at the 2002 United States and Canadian Academy of Pathology Meeting.
Accepted for publication December 20, 2002.
| References |
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