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 Previous Article | Table of Contents | Next Article 
Blood, Vol. 94 No. 10 (November 15), 1999:
pp. 3567-3575
The MAL Gene Is Expressed in Primary Mediastinal Large B-Cell Lymphoma
By
Christiane Copie-Bergman,
Philippe Gaulard,
Leïla Maouche-Chrétien,
Josette Brière,
Corinne Haioun,
Miguel A. Alonso,
Paul-Henri Roméo, and
Karen Leroy
From the Département de Pathologie and EA 2348, the Service
d'Hématologie Clinique, AP-HP, Hôpital Henri Mondor,
Créteil, France; INSERM U 474, Créteil, France; the Service
d'Anatomie et de Cytologie Pathologiques, Hôpital Laennec,
Paris, France; the Centro de Biologia Molecular "Severo Ochoa,"
Universidad Autonoma de Madrid and Consejo Superior de Investigaciones
Cientificas, Cantoblanco, Madrid, Spain.
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ABSTRACT |
Primary mediastinal large B-cell lymphoma (PMBL) appears to be a
distinct clinicopathologic entity among diffuse large B-cell lymphomas
(DLBLs). To find molecular alterations associated with this disease, we
compared the mRNAs expressed in 3 PMBLs and 3 peripheral DLBLs by
differential display-reverse transcription (DDRT) and identified a mRNA
specifically expressed in PMBLs. Sequence analysis showed that this
mRNA is encoded by the MAL gene, the expression of which
was shown to be restricted to the T-cell lineage during hematopoiesis.
MAL gene expression was demonstrated by Northern blot and reverse
transcription-polymerase chain reaction (RT-PCR) in 8 of
12 PMBLs. However, there was little or no MAL gene expression in 8 peripheral DLBLs. Immunohistochemical analysis evidenced expression of
MAL protein in tumoral B cells restricted to the PMBL subtype. Finally,
Southern blot studies did not demonstrate rearrangement of the MAL
gene. Altogether, our results indicate that MAL expression is recurrent
in PMBLs, providing further evidence that PMBL represents a distinct
entity among DLBLs. Because MAL protein is located in
detergent-insoluble glycolipid-enriched membrane (GEM) domains involved
in lymphocyte signal transduction, abnormal expression of MAL protein
in the B-lymphoid lineage may have significant implications in PMBL lymphomagenesis.
© 1999 by The American Society of Hematology.
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INTRODUCTION |
DIFFUSE LARGE B-CELL lymphomas (DLBLs)
account for 30% to 40% of adult non-Hodgkin's lymphomas and
constitute a heterogeneous group of lymphoid neoplasms with a wide
spectrum of morphological and clinical features, treatment response,
and prognosis.1 The genetic basis of this heterogeneity
remains poorly understood, and identification of new molecular markers
has been the focus of numerous studies in the past few years. In the
Revised European-American classification of Lymphoid neoplasms (REAL
classification), DLBLs encompass 3 major histological subtypes, namely
centroblastic, immunoblastic, and anaplastic large B-cell
lymphomas.1 This morphological subclassification may help
in prognosis stratification, but its usefulness is limited by a poor
interobserver reproducibility.2 At the molecular level,
several genes have been implicated in the pathogenesis of these
lymphomas. The most frequent genetic lesions (30% to 40%) are
rearrangements of the LAZ3/Bcl6 gene, which encodes a zinc finger
transcription factor that participates in B-cell
differentiation.3,4 Besides these frequent genomic alterations, approximately 20% of DLBLs harbor a t(14;18) involving the Bcl2 gene, 20% of cases have mutations of the p53 gene, and a
small percentage of DLBLs display rearrangements and/or mutations of
the c-myc gene.5-7 Recently, a comparative genomic
hybridization analysis associated with a candidate gene approach
evidenced amplification of rel, myc, Bcl2, gl1, cdk4, and
mdm2 genes, providing additional genetic information on
DLBLs.8
Among DLBLs, primary mediastinal large B-cell lymphoma (PMBL) was
individualized as a distinct subtype in the REAL
classification.1 These lymphomas may be distinguished
from peripheral DLBLs on clinical, morphological, and immunophenotypic
features. Clinically, they are characterized by a female predominance
and a median age at diagnosis in the fourth decade. Patients present
with a prominent mediastinal tumoral mass, commonly associated with
symptoms of airway compromise and superior vena cava syndrome. The
tumor mass is usually bulky (>10 cm) and extension remains most
frequently localized to the adjacent intrathoracic structures.
Morphologically, the tumors consist of clear large cells and exhibit a
diffuse growth pattern associated with a variable degree of sclerosis. Thymic remnants may be found within the tumor. These lymphomas display
a particular immunophenotype
CD19+CD20+CD79a+CD10 CD21,
with variable expression of CD23 and CD30, and absence of expression of
cytoplasmic or surface Ig, despite Ig genes
rearrangements.1,9-12 The Ig and
CD21 immunophenotype bears similarity to the thymic
medullary B cells, and it was postulated that these lymphomas may arise
from this particular subset of B cells.13,14
Specific genetic alterations involved in the pathogenesis of PMBLs are
presently unknown. Alterations of the c-myc gene consisting of
major rearrangements, and mutations or small rearrangements in the
5' noncoding region were reported in a substantial proportion of
cases in 2 studies (3 of 6 and 3 of 16 cases,
respectively).15,16 Other reported molecular alterations
include rearrangement of the Bcl6 gene (1 of 16 cases) and missense
point mutations of the p53 gene (3 of 16 cases).16 A
comparative genomic hybridization approach performed in a series of 26 PMBLs demonstrated frequent gains of chromosomal material involving
chromosomes 2p, 9p, 12q, and Xq and amplification of the proto-oncogene
rel in 2 cases.17 Hence, in addition to distinct
clinicopathologic features, PMBLs generally lack the molecular
alterations involved in peripheral DLBLs.
To identify molecular alterations associated with this disease, we
compared the mRNAs expressed in PMBLs and peripheral DLBLs by
differential display-reverse transcription (DDRT). We identified MAL
mRNA as being differentially expressed between these 2 entities and
further confirmed by immunohistochemistry the specific expression of
MAL protein in PMBL neoplastic cells.
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MATERIALS AND METHODS |
Tissue specimens and cell lines.
Tumor samples from 12 patients with PMBL and 8 patients with peripheral
DLBL were collected from the archives of 2 departments of Pathology
(Hôpital Henri Mondor [Créteil, France] and Hôpital Laennec [Paris, France]). The clinical and pathologic features of all
cases are summarized in Table 1. All
patients with PMBL presented with an initial prominent bulky
mediastinal mass. Representative tumor samples from PMBLs were
obtained at mediastinoscopy for 9 cases and from supraclavicular and
axillary lymph nodes for 3 cases. Biopsy specimens of peripheral DLBLs
consisted of peripheral lymph nodes in all cases. Disseminated B-cell
lymphomas with bulky mediastinal involvement were excluded. The
morphologic features were assessed on hematoxylin-eosin-stained
sections of Bouin's or formalin-fixed, paraffin-embedded tissue.
Expression of B- and T-cell-associated differentiation antigens were
evaluated on deparaffinized tissue sections using the
alkaline-phosphatase/anti-alkaline-phosphatase (APAAP) procedure with
the CD3 , L26/CD20, and Ber-H2/CD30 antibodies (Dako SA, Glostrup,
Denmark).18 All cases had a
CD20+CD3 immunophenotype and
occasionally expressed CD30 after microwave antigen retrieval.
Diagnosis of PMBLs and peripheral DLBLs were established in each case
by standard clinical, histological, and immunohistochemical criteria,
and lymphomas were classified according to the REAL and updated Kiel
classifications.1,19
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Table 1.
Clinical and Pathologic Characteristics of PMBLs (Cases
No. 1 Through 12) and Peripheral DLBLs (Cases No. 13 Through 20)
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Cell lines used in the present study included Jurkat T-cell line,
B-cell lines at various stages of differentiation (Raji, Ramos, RL,
697, and RS 4:11), erythroleukemic cell lines (K562 and HEL), and an
epithelial cell line (Hela). Hematopoietic cell lines were maintained
in RPMI 1640 medium, and Hela cell line was maintained in Dulbecco's
modified Eagle medium, supplemented with 10% fetal calf serum. Cell
lines were grown at 37°C in 5% CO2.
DDRT-polymerase chain reaction (DDRT-PCR).
DDRT was performed as described.20 Total RNAs were
extracted from frozen tumor samples of 3 patients presenting with
peripheral DLBL and 3 patients presenting with PMBL using the TRIZOL
reagent (GIBCO-BRL Life Technologies, Cergy-Pontoise, France). After
poly(A) enrichment on oligod(T) cellulose, 50 ng mRNAs were
reverse-transcribed with 200 U Superscript II reverse transcriptase
(GIBCO-BRL) in the presence of 50 pmol anchored T12MN primers (in which
M represents A, C, or G and N is T, A, C, or G) in 20 µL RT buffer
(50 mmol/L Tris, pH 8.3, 75 mmol/L KCl, 3 mmol/L MgCl2, 10 mmol/L dithiothreitol [DTT]) containing 20 µmol/L dNTP
for 50 minutes at 37°C. After heat inactivation of the RT at
95°C for 5 minutes, subsequent PCR amplification was performed
using 1 µL of the cDNA with 50 pmol of the appropriate T12NM primer
and 10 pmol of arbitrary decamer. The PCR reaction was performed with 2 U Taq DNA polymerase (Perkin Elmer Applied Biosystems, Courtaboeuf,
France) and 0.075 µL [ -33P]dATP (1,000 Ci/mmol) in 20 µL PCR buffer (10 mmol/L Tris, pH 8.3, 50 mmol/L KCl,
1.25 mmol/L MgCl2) containing 2 µmol/L dNTP. The cycling
parameters were as follows: 94°C for 4 minutes, and then 40 cycles
of denaturation (94°C for 15 seconds), annealing (42°C for 1 minute), and elongation (72°C for 30 seconds), followed by an
elongation step at 72°C for 7 minutes. The amplified cDNAs were
separated on a 6% polyacrylamide sequencing gel, which was then dried
and exposed to a Kodak X-OMAT AR film (Eastman Kodak, Rochester,
NY) overnight. Differential bands were excised from the
dried sequencing gel, boiled in 100 µL H2O for 15 minutes, precipitated with ethanol, and suspended in 10 µL of water.
One fourth of the recovered cDNA was used for reamplification in a 40 µL reaction volume using the same primer set and PCR conditions as
used in the mRNA display, except a higher concentration of dNTP (20 µmol/L). Reamplified cDNA fragments were subcloned into pBS SK plasmid.
DNA sequencing.
Plasmid DNA and PCR products were Taq cycle sequenced using the Applied
Biosystems PRISM ready reaction Dye-dideoxy Terminator and Dye-Primer
sequencing kits, and samples were run on an ABI 373A DNA sequencer
(Applied Biosystems, Foster City, CA).
Northern blot analysis.
Total RNAs were extracted using TRIZOL (GIBCO-BRL) according to the
manufacturer's instructions. RNAs extracted from lymphomas or cell
lines (15 µg) were denatured for 10 minutes at 68°C and run in
1% agarose gel containing 2 mol/L formaldehyde in 10 mmol/L phosphate
buffer, pH 7. RNAs were transferred on Hybond-N+ membranes (Amersham
Pharmacia Biotech, Orsay, France) and cross-linked by UV irradiation.
Prehybridization and hybridization were performed in Church buffer (140 mmol/L NaH2PO4, 360 mmol/L
Na2HPO4, pH 7, 7% sodium dodecyl sulfate
[SDS], and 1 mmol/L EDTA) with random-primed, -32P-labeled probe. The MAL probe was obtained by PCR
amplification of a fragment of MAL cDNA (nt 62 to 585, according to the
nucleotide sequence published21). Control hybridizations
were performed using a probe specific for glyceraldehyde-phosphate
deshydrogenase (GAPDH) mRNA.
RT-PCR analysis.
One microgram of total RNA was reverse transcribed to cDNA using 200 U
Superscript Plus (GIBCO-BRL) and 300 ng random primers in 20 µL RT
buffer containing 0.5 mmol/L dNTP. MAL cDNA was amplified together with
an internal standard, consisting of S14 ribosomal protein cDNA, in 1 tube. The PCR reactions were performed in 2 steps: the first step
consisted of the amplification of MAL cDNA with 5 pmol of sense primer
(5'CTTGCCCGACTTGCTCTTCA3') and antisense primer
(5'GGGGGGGTGGTTGTTTTCTT3') and 0.5 U Taq Gold DNA
polymerase (Perkin Elmer) in 20 µL PCR buffer containing 0.2 mmol/L
dNTP. Thermocycling was performed as follows: 12 minutes at 95°C
and 8 cycles of 95°C for 30 seconds, 60°C for 30 seconds, and
72°C for 1 minute +2 seconds per cycle. The second
step consisted of the addition of 10 µL PCR buffer containing 0.2 mmol/L dNTP, 2.5 pmol MAL sense and antisense primers, 5 pmol of S14
ribosomal protein sense primer (5'GGCAGACCGAGATGAATCCTCA3')
and antisense primer (5'CAGGTCCAGGGGTCTTGGTCC3'), and 0.5 U
Taq Gold DNA polymerase (Perkin Elmer). The second thermocycling was
performed as follows: 12 minutes at 95°C and 26 cycles of 95°C
for 30 seconds, 60°C for 30 seconds, and 72°C for 1 minute +2
seconds per cycle. Ten microliters of the PCR products
was run on a 2% agarose gel in 1× TBE buffer (100 mmol/L Tris,
90 mmol/L Boric acid, and 1 mmol/L EDTA, pH 8.3). Samples containing
distilled water and Jurkat cDNA were used as negative and positive
controls, respectively.
DNA extraction and Southern blot analysis.
DNA from tumor samples of 6 PMBLs and 3 peripheral DLBLs was extracted
by proteinase K digestion, phenol/chloroform extraction, and ethanol
precipitation.22 After digestion with EcoRI, DNA fragments were electrophoresed on 0.8% agarose gels in 1× TAE buffer (40 mmol/L Tris-acetate, 1 mmol/L EDTA, pH 8.3) and transferred onto nylon N+ membrane (Amersham). Prehybridization and hybridization were performed in Quick Express Hyb Solution (Clontech, Palo Alto, CA) with -32P probes, which were labeled
by random priming according to the manufacturer instructions
(GIBCO-BRL). The first probe consisted of an 1.5-kb
EcoRI-HindIII fragment of the MAL gene derived from a
plasmid containing the MAL first exon and 5.5 kb of upstream sequences.23 The second probe was obtained by PCR
amplification of a fragment of MAL cDNA (nt 62 to 585, according to the
nucleotide sequence published21).
Immunohistochemistry.
Immunohistochemistry was performed on paraffin sections using the APAAP
method and an anti-MAL monoclonal antibody directed against amino acids
114-123 of the human MAL protein.18,24 Rabbit antimouse Igs
and APAAP complexes were obtained from Dako. Paraffin sections from
normal kidney were used as positive control.
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RESULTS |
Identification of MAL mRNA through differential screening of lymphomas
mRNAs.
We compared the mRNAs expressed in tumor samples of 3 PMBL patients
(patients no. 9, 10, and 12) with the mRNAs expressed in tumor samples
of 3 other patients with peripheral DLBL (patients no. 19, 13, and 20).
Total RNAs extracted from frozen tissue samples were reverse
transcribed using T12GC anchor primer. Subsequent PCR amplification of
the cDNAs were performed using T12GC anchor primer and OPA 18 arbitrary
decamer (5'AGGTGACCGT3'). Comparative analysis of the PCR
products identified a band that was present in 3 of 3 PMBLs but not in
the 3 peripheral DLBLs (Fig 1). This band
was eluted from the gel, reamplified by PCR with the set of primers
used in the corresponding DDRT experiment, and sequenced. This sequence
was compared with the nucleotide databases and proved to correspond to
the 3' end of MAL mRNA.
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| Fig 1.
PCR differential screening. mRNA from tumor samples of 3 patients with peripheral DLBLs (patients no. 19, 13, and 20) and 3 patients with PMBLs (patients no. 9, 10, and 12) were reverse
transcribed and amplified by PCR using the T12GC anchor primer and the
OPA 18 arbitrary primer. Amplified cDNAs were run side by side on a 6%
sequencing gel. The arrow indicates the band seen only in amplified
cDNAs corresponding to patients with PMBL.
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Northern blot analysis of MAL expression.
To confirm that the differential product observed corresponded to a
differential mRNA, we studied MAL gene expression in tumor samples of
patients with PMBL or peripheral DLBL by Northern blot analysis. As
shown in Fig 2A, hybridization with MAL
cDNA showed high expression of MAL 1.1-kb transcripts in 2 PMBLs (cases
no. 11 and 12) and very low or undetectable expression in 4 peripheral DLBLs (cases no. 15, 17, 18, and 16). In addition, we tested MAL expression in a panel of human hematopoietic cell lines, including B-cell lines at various stages of differentiation. MAL mRNAs were detected in the Jurkat T-cell line, but were absent in the B-cell lines
(Raji, RL, 697, RS 4:11, and Ramos), erythroleukemic cell lines (HEL
and K562), and epithelial cell line (Hela) studied (Fig 2B). These
results showed that MAL is not expressed in B-cell lines and thus
confirmed that MAL expression is restricted to the T-cell lineage
during hematopoiesis.
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| Fig 2.
Northern blot analysis of MAL expression. Fifteen
micrograms of total RNAs extracted from PMBLs, peripheral DLBLs, and
human cell lines was loaded per lane. Hybond N+ membranes were
hybridized with a PCR-derived MAL cDNA fragment (upper panel),
stripped, and rehybridized with a GAPDH probe (lower panel) to check
the RNA amounts loaded and transferred to the membrane. (A) RNAs from
Jurkat T-cell line, 2 PMBLs (no. 11 and 12), and 4 peripheral DLBLs
(no. 15, 17, 18, and 16). (B) RNAs from human cell lines. Hela is
nonhematopoietic. HEL and K562 are erythroleukemic cell lines. Jurkat
is a T-cell line. Raji and Ramos are derived from
Burkitt's lymphoma. RL bears the t(14;18) translocation
associated with follicular lymphoma. 697 is a pre-B-cell line and RS
4:11 a pro-B-cell line.
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Analysis of MAL expression by RT-PCR.
The small quantity of biopsy material obtained at mediastinocopy did
not yield enough RNA to perform Northern blot analysis in a large
series of lymphomas. Therefore, we studied MAL expression in 12 PMBLs
and 8 peripheral DLBLs by RT-PCR analysis. As shown in
Fig 3, a specific MAL PCR product of 520 bp
was detected in 8 of 12 PMBLs (cases no. 2, 3, 5, 6, 7, 10, 11, and 12)
and in only 2 of 8 peripheral DLBLs (cases no. 15 and 20). In these 2 cases, the level of MAL expression appeared lower than that observed in
PMBLs. Efficient amplification of the S14 internal control was detected
in all cases. This analysis confirmed the differential expression of
MAL mRNA in PMBLs compared with peripheral DLBLs.
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| Fig 3.
RT-PCR analysis of MAL expression in PMBLs and peripheral
DLBLs. One microgram of RNA extracted from 12 PMBLs and 8 peripheral
DLBLs was reverse transcribed and the cDNAs were coamplified with MAL
and S14 primers. PCR products were run on a 2% agarose gel stained
with ethidium bromide. Specific amplification of MAL and S14 cDNAs
produced 520- and 141-bp bands, respectively. Positive control
represented by serial dilutions of Jurkat cDNA and a negative control
without template (T ) are included.
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MAL protein expression in PMBLs and peripheral DLBLs.
To examine whether MAL protein was expressed in tumoral B cells and to
rule out the possibility that MAL mRNA expression observed in PMBLs was
due to intratumoral reactive T cells, we analyzed the expression of MAL
protein by immunohistochemistry on paraffin sections of tumor samples.
The results are shown in Table 2. MAL
protein was detected in neoplastic cells in 7 of 12 PMBLs (cases no. 2, 5, 6, 7, 10, 11, and 12). Two cases were negative, although internal
positive controls, represented by small lymphoid cells, consistent with
reactive T cells, were present. Three cases were not interpretable
because of the absence of internal positive control. The staining
pattern was associated with the surface membrane and there was granular
cytoplasmic positivity, with accentuation in the Golgi area in most
neoplastic cells (Fig 4B and D). The majority of positive PMBLs (5/7) displayed more than 50% positive neoplastic cells. Two PMBLs (cases no. 5 and 11) demonstrated only 10%
to 20% positive neoplastic cells; however, the possibility of partial
antigenic denaturation due to Bouin's fixative cannot be ruled out in
these 2 cases. In comparison, all peripheral DLBLs tested were
negative, although internal positive control were present in all cases
(Fig 4F).
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| Fig 4.
Morphological features and immunostaining for MAL of
PMBLs and peripheral DLBLs. (A) PMBL of centroblastic subtype with a
clear-cell component (case no. 10). (B) Case no. 10 stained for the MAL
protein showing surface membrane and granular cytoplasmic
immunoreactivity with accentuation in the Golgi area of neoplastic
cells. (C) PMBL of centroblastic polymorphic subtype with a anaplastic
large-cell component (case no. 2). (D) Staining for MAL of case no. 2 showing membrane and striking paranuclear dot-like positivity in
neoplastic cells (inset). (E) Peripheral DLBL of centroblastic
multilobated subtype (case no. 17). (F) Staining for MAL of case no. 17 showing that the tumor cells lack expression of the MAL protein.
Positive internal control are present, represented by small lymphoid
cells, consistent with reactive T cells. (A, C, and E,
hematoxylin-eosin stain; B, D, and F, immunohistochemical staining of
paraffin section with the anti-MAL antibody, APAAP method.)
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The different methods used to detect MAL expression in PMBLs and
peripheral DLBLs yielded comparable results. Among the 8 RT-PCR-positive PMBLs, 7 were positive by immunohistochemistry and 1 case (case no. 3) was not interpretable. Two cases of peripheral DLBLs
(cases no. 15 and 20) showed low levels of MAL mRNA expression by
RT-PCR analysis, but the neoplastic cells remained negative by
immunohistochemistry. In these 2 cases, the faint signal observed upon
RT-PCR analysis is presumably related to the presence of intratumoral
reactive T cells.
Southern blot analysis.
To study if MAL gene expression in tumoral B cells was due to genomic
rearrangements, DNAs extracted from tumor samples of PMBLs and
peripheral DLBLs were digested with EcoRI and subjected to
Southern blot analysis. The blot was first hybridized with a 1.5-kb
EcoRI-HindIII fragment located 3.5 kb upstream of MAL first exon, showing the same 17-kb DNA fragment in all lymphomas tested
(Fig 5B). The blot was stripped and
subsequently hybridized with a second probe obtained by PCR
amplification of a fragment of MAL cDNA spanning exons 2, 3, and 4 and
thus exploring the 3' part of the MAL gene. In peripheral DLBLs
as well as in PMBLs, this probe hybridized with the expected 5- and
2.6-kb DNA fragments (Fig 5C). These experiments did not demonstrate
any rearrangement of the MAL gene in PMBLs. Furthermore, quantitative
analysis of the signals correlated with the amount of DNA loaded in
each lane (data not shown), thereby ruling out the possibility of an
overexpression related to gain of chromosomal material.
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| Fig 5.
Southern blot analysis. Genomic DNAs extracted from 6 PMBLs (no. 12, 6, 7, 11, 10, and 5) and 3 peripheral DLBLs (no. 15, 19, and 14) were subjected to EcoRI digestion, agarose gel
electrophoresis, Nylon N+ membrane transfer, and hybridization with 2 MAL probes. (A) The physical map of the MAL gene and localization of
the 2 probes are represented at the top of the figure. The boxes
represent the exons, which are not drawn to scale for the clarity of
the figure. (B) A 1.5-kb EcoRI-HindIII fragment (probe
a) exploring the 5' part of the MAL gene showed the same 17-kb
DNA fragment in all samples. (C) A PCR-generated fragment of MAL cDNA
(probe b) spanning exons 2, 3, and 4 showed 2 DNA fragments of 5 and
2.6 kb. The faint bands observed for case no. 5 in (B) and (C) are due
to the lower amount of DNA loaded on the gel. However, longer
autoradiographic expositions demonstrated the same DNA fragments in
this sample.
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DISCUSSION |
Using a differential screening method, we identified a transcript
expressed in PMBLs, which proved upon sequencing to correspond to the
3' end of MAL mRNA. We demonstrated by Northern blot and RT-PCR
the recurrent and differential expression of the MAL gene in PMBLs
among DLBLs and identified the MAL protein at the surface membrane and
in the Golgi area of PMBL neoplastic B cells.
The MAL gene was first described as differentially expressed during
T-cell development and was subsequently shown to be expressed in
thymus, thyroid, kidney, and brain.21,24-26 During
hematopoiesis, MAL gene expression has been shown in leukemic T-cell
lines, T-cell clones, and peripheral blood lymphocytes, and our study
is the first report describing MAL expression in B
cells.21,23 It has been postulated that PMBLs arise from B
cells at a terminal stage of differentiation.14 Because MAL
expression has been mostly studied in B-cell lines derived from bone
marrow precursors or germinal center B cells, the possibility that MAL
expression in PMBLs is related to terminal differentiation cannot be
formally excluded. Our results could support the hypothesis that PMBLs arise from a specific subset of resident thymic medullary B cells expressing MAL. An immunohistochemical analysis of serial paraffin sections of a neonatal thymus with anti-MAL and anti-CD20 antibodies demonstrated that MAL protein expression was restricted to the thymic
cortex in the T-cell areas with very few positive cells in the medulla,
where the population of B cells is concentrated (data not shown). The
absence of superimposed staining pattern suggests that MAL expression
is not a common feature of normal thymic medullary B cells.
Furthermore, MAL expression is found in PMBL biopsy samples issued both
from the mediastinum and peripheral lymph nodes. These data indicate
that MAL gene expression is not related to the anatomic site of origin
of these lymphomas. Thus, MAL gene expression in PMBLs could be related
to genomic alterations. Southern blot analysis did not show MAL gene
rearrangements or gain of chromosomal material. This latter result is
in agreement with comparative genomic hybridization studies of PMBLs
that demonstrated gains of chromosomal material involving the
short arm of chromosome 2 but not the long arm, where the MAL gene
was mapped.16,27 However, ponctual mutations,
microdeletions, or more distant rearrangements of the gene cannot be
excluded, and further experiments are needed to elucidate the origin of
MAL expression in PMBL neoplastic B cells.
The MAL gene encodes a highly hydrophobic integral transmembrane
protein of 17 kD that belongs to the proteolipid group of proteins.23 MAL protein was found in detergent-insoluble
glycolipid-enriched membrane (GEM) microdomains of T lymphocytes,
myelin-forming cells, and both polarized kidney and thyroid epithelial
cells.24,28,29 GEM domains are lipid rafts enriched in
glycosylphosphatidylinositol (GPI)-anchored proteins and other proteins
involved in signal transduction, including Src-family kinases and
heterotrimeric guanosine triphosphate-binding proteins (G
proteins).30 The essential role of rafts in signal
transduction in the lymphoid lineage was recently highlighted by
several reports. Upon stimulation, activated T-cell receptor and
additional signal-transducing molecules are recruited to GEM domains,
and disruption of raft structure inhibits the early stages of T-cell
receptor signaling.31 Furthermore, Viola et
al32 demonstrated that the costimulatory effect of CD28 in
resting T cells is mediated to a large extent by its effect on raft
redistribution. In addition, molecules participating in B- and T-cell
activation, ie, CD44, CD45/lyn (B cells), and CD45/lck/CD4 (T
cells), were shown to cofractionate in the GEM
domains. 33,34
Several studies suggest that MAL is involved in both membrane
trafficking and signaling. MAL was identified in trans-Golgi network-derived transport vesicles in epithelial cells and ectopic expression of MAL in a heterologous cell system induces extensive vesiculation of GEM.28,35 Besides its role as an element of the machinery for apical sorting in polarized epithelial cells, involvement of MAL in membrane signaling was suggested by
coimmunoprecipitation experiments that demonstrated a specific
association of MAL with the GPI-anchored CD59 protein and the Lck
tyrosine kinase in GEM microdomains of T lymphocytes.36,37
It was proposed that MAL could act as a transmembrane linker protein
that mediates the interactions between GPI-anchored proteins and the
Src-like kinases. It is tempting to speculate that MAL abnormal
expression in B cells may interfere with GEM specialized functions and
have significant implications in cell growth. In support of this
hypothesis, caveolin transmembrane proteins, which are chief structural
elements of GEM domains in many cell types, were linked to cell
proliferation and oncogenic transformation in fibroblast and epithelial
cell systems.38-43
In conclusion, our study shows recurrent expression of the MAL gene in
PMBLs and suggests that PMBLs may not only have distinct clinicopathologic features, but also harbor particular molecular alterations. These results further support the idea that
PMBL is a distinct lymphoma subtype among DLBLs, as proposed in the REAL classification. By analogy to the role of caveolins in tumor cell
growth and in view of the putative functions of MAL in GEM vesiculation
and signal transduction, it is tempting to speculate that abnormal
expression of MAL protein in the B-lymphoid lineage may be related to
PMBL lymphomagenesis. Further studies are needed to provide additional
insights in the role of MAL in PMBLs.
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ACKNOWLEDGMENT |
The authors thank Marie-Laure Boulland, Nadine Martin, and Marie-Claude
Labastie for their technical assistance.
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FOOTNOTES |
Submitted March 31, 1999; accepted July 16, 1999.
Supported by the Institut National de la Santé et de la Recherche
Médicale (INSERM), the Fondation de France, and the Association pour la Recherche contre le Cancer (ARC).
The publication costs of this
article were defrayed in part by
page charge payment. This article
must therefore be hereby marked
"advertisement"
in accordance with 18 U.S.C. section
1734 solely to indicate this fact.
Address reprint requests to Karen Leroy, MD, PhD, INSERM U
474, Hôpital Henri Mondor, 51 av du Mal de Lattre de
Tassigny, 94010 Créteil, France.
| |
REFERENCES |
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TOP
ABSTRACT
INTRODUCTION
