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From the Departments of Pathology,*
Biochemistry,
and Clinical
Pathology
and the Cancer Research
Institute,§
College of Medicine, The Catholic
University of Korea, Seoul, Korea, and the Department of
Pathology,||
Chung Ang University College of Medicine,
Seoul, Korea
| Abstract |
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| Introduction |
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Fas is widely expressed in normal and neoplastic cells4 but the expression of this protein does not necessarily predict susceptibility to killing.9 This can reflect the presence of inhibiting mechanisms of Fas-mediated apoptosis. Fas-mediated apoptosis can be blocked by several mechanisms, including the production of soluble Fas,10 the lack of cell-surface Fas expression,11-13 the overexpression of inhibitory proteins in signal transduction pathways such as Fas associated phosphatase-114 and FLICE-inhibitory protein (FLIP),15 and the mutation of the primary structure of Fas.16-25
The consequences of the Fas gene mutations have been well demonstrated in germline mutation models of this gene.16-21 Mice bearing the Fas gene mutation (lpr) have an abnormality of mature T-cell deletion in the peripheral tissues, resulting in lymphadenopathy, splenomegaly, and systemic autoimmune disease.16 Germline mutations of the Fas gene in human also results in autoimmune lymphoproliferative syndrome (ALPS), which is characterized by an increase in double-negative T cells and profound lymphadenopathy,17-21 as observed in lpr mice. Most of the Fas mutations in ALPS were point mutations in the death domain, were heterozygous, and showed a dominant negative phenotype.17-19 Interestingly, the lpr mice have been reported to have spontaneous development of plasmacytoid tumors26 and some ALPS patients have been reported to have malignancies,19-21 including multiple tumor development in one patient.19 Although it is not clear if the tumors that occurred in ALPS patients arose as a result of Fas mutations, it is conceivable that Fas mutation might influence tumor development in these patients.
The key role of the Fas system in negative growth regulation has been studied mostly within the immune system1 and somatic mutations of Fas gene in cancer patients have been described solely in lymphoid-lineage malignancies, including multiple myelomas,22 childhood T-cell lymphoblastic leukemias,23 adult T-cell leukemias,24 and non-Hodgkin's lymphomas.25 Therefore, resistance against Fas-mediated apoptosis may lead to a longer survival of affected tumor cells and might contribute to tumorigenesis of these lymphoid-lineage malignancies.
There is mounting evidence that disruption of the Fas system occurs frequently in nonlymphoid malignancies as well.9-12,27,28 To date, however, somatic mutations of Fas gene, one of the possible mechanisms that mediate the disruption of the Fas system, have not yet been reported in nonlymphoid malignancies, including cutaneous malignant melanoma (MM). In addition, previous loss of heterozygosity (LOH) studies have suggested that loss of one or more putative tumor suppressor genes at chromosome 10q may be involved in the development of cutaneous MM.29-32 One of the candidate genes in this region is Fas located at chromosome 10q24.1.3 In the present study, to characterize the potential apoptosis-resistant pathway of the Fas system in human cutaneous MM, we analyzed a series of 44 cutaneous MMs for somatic mutations of the Fas gene.
| Materials and Methods |
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Paraffin-embedded tissues of human cutaneous MM were obtained from 44 surgically treated patients. All MMs analyzed showed vertical growth pattern. Diagnosis of each case had been confirmed by dermatopathologists morphologically and, if necessary, by immunohistochemistry and electron microscopy.
Malignant cells were selectively procured from hematoxylin and eosin-stained sections using a 30G1/2 hypodermic needle (Becton Dickinson, Franklin Lakes, NJ) affixed to a micromanipulator, as described previously.33 We also microdissected infiltrating lymphocytes from the slides and used them for corresponding normal DNA. This microdissection technique used in this study has been proven to be precise and effective for procurement of tumor cells without normal cell contamination.33
Single-Strand Conformation Polymorphism (SSCP) Analysis for Mutation and LOH
Genomic DNA each from normal lymphocytes or tumor cells was
amplified with primer pairs covering the entire coding region and parts
of the promoter region of Fas gene (Table 1)
. Oligonucleotide primers were designed
with the program Oligo (National Biosciences, Plymouth, MN) using
sequences obtained from GenBank (accession No. M67454). Each polymerase
chain reaction (PCR) was performed under standard conditions in a
10-µl reaction mixture containing 1 µl of template DNA, 0.5
µmol/L of each primer, 0.2 µmol/L of each deoxynucleotide
triphosphate, 1.5 mmol/L MgCl2, 0.4 units of
Taq polymerase, 0.5 µCi of
[32P]dCTP (Amersham, Buckinghamshire, UK), and
1 µl of 10x buffer. The reaction mixture was denatured for 1 minute
at 94°C and incubated for 40 cycles (denaturing for 40 seconds at
94°C, annealing for 40 seconds at variable temperatures as described
in Table 1
, and extending for 40 seconds at 72°C). Final extension
was continued for 5 minutes at 72°C. After amplification, PCR
products were denatured 5 minutes at 95°C at a 1:1 dilution of sample
buffer containing 98% formamide/5 mmol/L NaOH and were loaded onto a
SSCP gel (FMC Mutation Detection Enhancement system, Intermountain
Scientific, Kaysville, UT) with 10% glycerol. After electrophoresis,
the gels were transferred to 3-mm Whatman paper and dried and
autoradiography was performed with X-OMAT film (Eastman Kodak,
Rochester, NY). For the detection of mutations, DNAs showing mobility
shifts were cut out from the dried gel and reamplified for 35 cycles
using the same primer set. Sequencing of the PCR products was
carried out using the cyclic sequencing kit (Perkin-Elmer, Foster
City, CA) according to the manufacturer's recommendation.
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Immunohistochemistry
Rabbit antibody for human Fas (C-20, Santa Cruz Biotechnology, Santa Cruz, CA) was used to detect Fas on paraffin-embedded tissue sections. Immunohistochemical procedures were performed as described previously.36 Tumors were interpreted as positive for Fas by immunohistochemistry when at least weak to moderate cytoplasmic staining was seen in greater than 30% of the neoplastic cells. The Fas immunostaining was judged to be antibody-specific by several criteria, including use of normal rabbit sera at the same dilution produced no consistent immunostaining of any cells; intensity of the signal diminished as the dilution of antibody was increased; and preincubating antibody with blocking peptide of Fas (Santa Cruz Biotechnology) abrogated the positive immunostaining. The results were reviewed independently by three pathologists.
| Results |
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Using the microdissection technique we successfully procured tumor
cells from histological sections of 44 MMs, as shown in Figure 1, A and B
. Genomic DNA was isolated and
analyzed for potential mutations in all nine exons of the
Fas gene by PCR-SSCP analysis. Enrichment and direct
sequence analysis of aberrantly migrating bands led to the
identification of mutations in 3 of 44 samples (6.8%) (Table 2
and Figure 2A
). None of the normal samples showed
evidence of mutations by SSCP (Figure 2A)
, indicating the mutations
detected in the MM specimens had risen somatically.
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Allelic Status
Because missense mutations in the death domain of Fas in patients with ALPS have been suggested to affect receptor function in a dominant-negative fashion,17-19 we examined the allelic status of Fas in tumors carrying missense mutations. Overall, 31 of 44 cases (70%) were informative for at least one of the four polymorphic markers, and 11 of 31 (35%) informative cases showed LOH with one or more markers. The heterozygosity rates of the two polymorphic markers in exon 3 and 7 (primers 3A and 7) were too low for LOH study, whereas 30 of 44 (68%) cases showed heterozygosity with one or both of the two polymorphic markers in the promoter region (primers PA and PB) of the Fas gene.
In the three cases with the Fas gene mutations, one (case 5)
showed LOH with marker PB (Figure 2D
and Table 2
). Another mutation
case (case 20) was not informative for the polymorphic markers (Table 2)
. The remaining mutation case (case 9) was heterozygous for marker
PA, but did not show LOH (Table 2)
. Interestingly, however, SSCP
pattern of case 9 at the mutation sites (exon 9) showed only aberrant
bands of mutant allele without those of the wild-type allele (Figure 2A)
, and sequencing analysis also revealed only mutation sequence
without wild-type one (Figure 2C)
, indicating either homozygous
mutation or hemizygous mutation with allelic loss. Therefore, although
SSCP analysis at the polymorphic sites did not provide direct evidence
of the second allele status in this mutation cases, the SSCP patterns
at mutation sites and sequencing analysis suggested that the second
allele of Fas in case 9 had been also altered. In cases
without Fas mutation, 10 of 29 (34%) informative cases
showed evidence of allelic loss (Table 2)
.
Immunohistochemistry
We demonstrated Fas expressions in MMs by immunohistochemistry.
The MMs analyzed showed immunoreactivity for Fas in 26 of 44 cases
(59%). Fas immunostaining, when present, was cytoplasmic and along the
cell membranes; nuclei were clearly negative (Figure 3)
. All three MMs with Fas
mutations showed positive immunostaining for Fas.
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| Discussion |
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Although functional studies have not yet been performed, some of the mutations identified in the present study are likely to disrupt or alter the normal function of Fas. To date, loss-of-function mutations of Fas in lpr mice,16 ALPS patients,17-21 and some lymphoid malignancies,22-25 have been identified in the promoter and exons 2, 3, 4, 6, 7, 8, and 9. However, most of the mutations have been detected in exon 9, which encodes death domain. The death domain is evolutionarily highly conserved and has been shown to be necessary and sufficient for the transduction of an apoptotic signal.1-3 In the current study, all three Fas mutations were identified in this conserved area, suggesting that the mutations might disrupt death signaling.
In multiple myeloma, Landowski et al22 identified an identical point mutation in two different patients that generates an amino acid substitution at 253. Mutations of codon 248 and 256 of Fas were also identified in non-Hodgkin's lymphomas.25 Furthermore, two ALPS patients were reported to have a 2-bp deletion that generates an unrelated amino acid sequence beginning at residue 254.18 Of the 3 Fas gene mutations in MMs, 2 mutations generated amino acid substitution at residues 250 and 251, which are close to the Fas mutations described above, indicating that this area may be a potential hot spot in the Fas coding sequence.
Most of the patients with ALPS carry a heterozygous mutation in the
Fas gene.17-19
In the ALPS, the affected Fas
protein seemed to work in a dominant-negative fashion, and T
lymphocytes from these patients did not die on
activation.17-19
Binding of FasL to Fas protein induces
trimerization of Fas protein and FADD/MORT-1 and Daxx, the adapter
proteins of Fas, bind to the trimerized Fas cytoplasmic region (death
domain). Then, FADD/MORT-1 and Daxx transmit apoptotic
signals.37,38
In our study, one Fas mutation
(case 20) seemed to have hemizygous mutations without allelic deletion
(Table 2)
. Therefore, in this case it is possible that the hemizygously
mutated Fas protein(s) may bind with other normal Fas protein(s) to
construct a structurally abnormal Fas trimer, which might have a defect
in binding to the adapter proteins. In contrast, case 5 showed evidence
of alterations of both alleles (Table 2)
and case 9 showed only
aberrant bands of mutant allele without those of the wild-type allele
on SSCP of exon 9 (Figure 2A)
, indicating potential biallelic
inactivation of the Fas gene in these cases. These biallelic
inactivations of the Fas gene may also lead to abnormal
construction of Fas-trimer, but the functional difference between
monoallelic and biallelic inactivations of Fas gene
alterations in the tumorigenesis of MM remains unknown at this stage.
Previous reports have identified Fas protein expression in about half of primary MMs and cultured MM cell lines.27,28,39 In agreement with these reports, we observed Fas protein expression in 26 of 44 MMs (59%). The Fas gene mutations in 3 MMs, all of which showed Fas expression by immunohistochemistry, might be involved in the mechanisms of Fas resistance of those tumors. Other MMs with positive Fas expression may have another strategies, including the expression of bcl-240 and FLIP,15 to mediate the Fas resistance. In the MMs that were not shown to express Fas protein, loss or down-regulation of the protein may be another way to avoid Fas-mediated apoptosis.
Several lines of evidence suggest that the loss of Fas function can enhance lymphoid tumor development. For example, lymphomatogenesis driven by the Eµ-myc transgene was shown to be markedly accelerated in lpr mice compared to wild-type mice, confirming a causal, rather than correlative, role for Fas loss in tumor development.41 Spontaneous development of B-cell lymphoid tumors in lpr mice also indicated that Fas gene mutation plays a key role in tumorigenesis.26 Moreover, somatic Fas gene mutations in human cancers have been found exclusively in lymphoid malignancies.22-25 These are well correlated with the facts that ALPS patients and lpr mice have shown phenotypical abnormalities only in the lymphoid system.16-21 However, we were able to find somatic mutations of Fas gene in cutaneous MM, one of the nonlymphoid malignancies, potentially extending the concept of loss of Fas function to the pathogenesis of nonlymphoid malignancies as well and it is possible that Fas gene mutations may occur widely in nonlymphoid malignancies. Clearly, therefore, studies are now needed that attempt to find the potential Fas gene mutations in other nonlymphoid malignancies.
| Footnotes |
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Supported by a grant from the Basic Research Program of the Korea Science and Engineering Foundation (981-0709-073-2).
Accepted for publication March 2, 1999.
| References |
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