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Short Communications |





**

From the Departments of Pathology,*
Pediatrics,
Orthopaedic
Surgery,
and
Otolaryngology,¶
Center for Human Molecular
Genetics, University of Nebraska Medical Center, Omaha, Nebraska; the
Departments of Pathology
and
Hematology-Oncology,**
St. Jude Childrens
Research Hospital, Memphis, Tennessee; the Department of
Pediatrics,

University of
Tennessee, College of Medicine, Memphis, Tennessee; and the Department
of Pathology,||
Memorial Sloan-Kettering Cancer Center,
New York, New York
| Abstract |
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| Introduction |
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| Materials and Methods |
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ALK Immunohistochemistry
ALK immunostaining was performed using monoclonal mouse anti-human antibody ALK-1 (DAKO, Carpinteria, CA) at a dilution of 1:25 and polyclonal rabbit anti-human ALK-114 at a dilution of 1:200 after heat-induced epitope retrieval. Antibody detection was achieved using a modified avidin-biotin peroxidase complex method.
Cytogenetic Analysis
A 0.25-cm3 sterile, representative tissue sample from case 1 was submitted for cytogenetic analysis. Standard culture and harvesting procedures were performed, as described previously.5
Fluorescence in Situ Hybridization (FISH)
FISH studies using 2p23 (ALK) breakpoint spanning and flanking probes (Vysis, Inc., Downers Grove, IL) were executed on metaphase preparations of case 1 and on cytological touch preparations of both cases according to the manufacturers instructions. Examination for the presence of split signals and image preparation were performed as previously described.6
RT-PCR Analysis and Rapid Amplification of cDNA Ends (RACE) Analysis
Total RNA was extracted from fresh or frozen tissue using Trizol reagent (Life Technologies, Inc., Gaithersburg, MD) or RNA Stat-60 (Tel-Test, Inc., Friendswood, TX). To exclude the presence of a TPM-ALK fusion transcript, RT-PCR studies were performed on both cases using a consensus TPM3/TPM4-FWD primer and an ALK-REV primer (5'-GGAAAAGACAATTGATGAC and 5'-GCAGTAGTTGGGGTTGTAGTC, respectively). Methodological details of the RT-PCR assays were essentially as previously described.6 The PCR products were electrophoresed in a 2% NuSieve agarose gel (FMC Bioproducts, Rockland, ME) and visualized by ethidium bromide staining.
RACE studies were performed using the 5'RACE system from Life Technologies, Inc. (Rockville, MD) according to the manufacturers instructions. Briefly, after annealing of an ALK-specific reverse primer (ALK/RT, 5'-TTCAGGCAGCGTCTTCACAG), 5 µg of RNA from each case was used for cDNA synthesis by SuperScript II reverse transcriptase (Life Technologies, Inc., Rockville, MD). The RNA was then degraded with RNase and the cDNA purified using a Glass MAX Spin Cartridge. Subsequently, the purified cDNA was tailed with dCTP using TdT and amplified by PCR using the manufacturers Abridged anchor primer as forward primer and a nested ALK-specific reverse primer (ALK/TK2, 5'-GGCTTGGGTC GTTGGGCATTC). Reamplification of the primary PCR product was completed using the manufacturers Abridged universal anchor primer and a nested ALK-specific reverse primer (ALK-3', 5'-CGAGGT GCGGAGCTTGCTCAGC). The products were analyzed on agarose gels. Sharp product bands were subjected to direct sequencing, whereas nondiscrete products were cloned and multiple independent clones were sequenced.
After completion of the RACE studies, a hemi-nested RT-PCR method was used to confirm the presence of a CLTC-ALK fusion gene in both cases. Thirty-five thermal cycles were used for the first-round PCR with CLTC-FWD primer (5'-TTAGATGCTTCAGAATC ACTG) and ALK-REV primer at the following temperatures: 95°C for 1 minute, 60°C for 1 minute, 72°C for 1 minute, with a final extension of 72°C for 10 minutes. Second-round PCR was performed with CLTC-FWD primer and ALK-3' primer using the same PCR conditions described for the first round. The amplified fragments were identified by gel electrophoresis and ethidium bromide staining, followed by direct sequencing of the product band. The integrity of the tumor RNAs was assessed by an independent amplification using primers for the ubiquitously expressed hypoxanthine phosphoribosyl-transferase (HPRT) or phosphoglycerate kinase (PGK) genes. A negative control devoid of a template and a negative control lacking reverse transcriptase were included in all RT-PCR studies.
| Results |
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Cases 1 and 2 were composed of spindle-shaped myofibroblasts
arranged focally in fascicular or storiform-like patterns, admixed with
lymphocytes and plasma cells. The presence of vimentin, smooth muscle
actin, muscle-specific actin, and desmin immunoreactivity and/or
prominent, dilated rough endoplasmic reticulum and abundant thin
microfilaments forming dense bodies beneath cell membranes
ultrastructurally, confirmed the myofibroblastic nature of the lesions
(not shown). Case 1 exhibited strong granular cytoplasmic
immunoreactivity for both ALK-1 and ALK-11 in 70 to 80% of the
myofibroblasts (Figure 1A)
. Case 2 was
not immunoreactive for ALK-1, but did show faint-to-moderate
immunoreactivity for ALK-11 localized to the cytoplasm of many
myofibroblasts (not shown). In our anecdotal experiences, as well as
those of a number of other investigators (S. W. Morris,
unpublished observations), the ALK-1 monoclonal antibody is
substantially less sensitive for detection of low-level ALK fusion
protein expression than the ALK-11 polyclonal antibody; the discordant
staining of case 2 observed with these two antibodies likely reflects
these sensitivity differences, given that our FISH and RT-PCR studies
clearly revealed this case to contain ALK rearrangement and
fusion (below).
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Case 1 exhibited the following complements:
46,XX,t(2;17)(p23;q23),add(16)(q24)[5]/92,idemx2[1]/46,XX[4]
(Figure 1B)
. In case 2, material suitable for cytogenetic analysis was
not available.
FISH
Case 1 metaphase cell FISH confirmed the presence of a 2;17
translocation involving the ALK locus (Figure 1C)
. Case 1
and 2 interphase cell FISH revealed a split of one set of the two-color
probe signals, indicating a disruption of the 2p23 breakpoint
(ALK) on one chromosome 2 homologue in 52 and 46% of
the cells, respectively (Figure 1D)
.
RT-PCR and RACE Analyses
RT-PCR analysis was negative for a TPM3-ALK or
TPM4-ALK fusion gene product in both cases (data not shown).
Rather, 5' RACE analysis of case 1 revealed an ALK fusion
with the clathrin heavy chain gene (CLTC) localized
to 17q23.7
This CLTC-ALK fusion incorporates 1634 residues
of the 1675-amino acid clathrin heavy chain. Hemi-nested RT-PCR
analysis confirmed the presence of a CLTC-ALK fusion gene
product in case 1 and demonstrated the identical fusion product in case
2 (Figure 1, E and F)
. The ALK and CLTC
breakpoints in these IMTs were identical to those recently reported in
CLTC-ALK fusions in ALCL.8
| Discussion |
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Here, we describe a case of IMT cytogenetically exhibiting a 2;17 translocation similar to that reported recently by Griffin and colleagues.2 Additional studies including FISH and immunohistochemistry confirmed that this tumor, as well as a separate IMT lesion included in the current study, featured ALK rearrangements and ALK expression. Initially, RT-PCR studies were performed to determine whether previously defined fusion transcripts involving ALK and TPM3 or TPM4 characterized either of the IMTs but these were negative. These findings prompted us to perform 5' RACE studies in an effort to identify the fusion partner of ALK in these two cases. These studies revealed a fusion transcript involving ALK and the clathrin heavy chain gene (CLTC) in both tumors, not previously described in IMT.
Clathrin is the major protein constituent of the coat that surrounds the cytoplasmic face of the organelles (coated vesicles) mediating selective protein transport.14 Clathrin coats are involved in receptor-mediated endocytosis, localization of resident membrane proteins to the trans-Golgi network, and transport of proteins to the lysosome/vacuole.15-17 Recently, clathrin has also been immunodetected in the mitotic spindle, suggesting a novel role for clathrin in mitosis or a novel regulatory mechanism for localization of clathrin in mitotic cells.18
Clathrin is a three-legged molecule, termed a triskelion, composed of heavy and light chains. Two clathrin heavy chain genes exist in the genome, clathrin heavy chain gene (CLTC), which has been localized to 17q23,7 and clathrin heavy chain polypeptide-like gene (CLTCL) that has been localized to 22q11.2.19 In the present IMT cases, the fusion point in the CLTC transcript was close to its 3' end, thus conserving nearly all of the clathrin heavy chain, including the motifs responsible for triskelion assembly. Presumably, the clathrin moiety in this CLTC-ALK fusion promotes constitutive activation and relocalization of the ALK kinase domain from its normal position at the inner surface of the cell membrane in neural cells to the cytoplasm of myofibroblastic cells.
The sequence of the CLTC-ALK fusion transcript identified in the IMTs in this study is identical to that recently described in ALCL. The partner gene in this ALK fusion was originally inadvertently misidentified as CLTCL,8 possibly because of changes in the designation of the corresponding GenBank accession NM[lowhy]004859. The CLTC nucleotides adjacent to the CLTC-ALK fusion junction are unlike CLTCL allowing distinction between the two genes. Although cytogenetic studies were not available in the report by Touriol and colleagues,8 it is likely that the lymphomas they studied contain a t(2;17)(p23;q23). Analogous to the CLTC-ALK fusion-positive ALCL, the ALK immunostaining in our IMT cases was confined to the cytoplasm of the tumor cells and was characterized by a granular appearance.
CLTC-ALK is the second known fusion oncogene that
transforms, in vivo, both mesenchymal and lymphoid human
cell lineages. A TPM3-ALK fusion oncogene identical to that
observed in some ALCL was identified by Lawrence and
colleagues3
in two IMT cases. The most common
ALK fusion gene partner in ALCL, detected in
80% of
cases, is nucleophosmin (NPM). The NPM-ALK
fusion gene results from a t(2;5)(p23;q35).20
Interestingly, an NPM-ALK fusion oncogene has not yet been
identified in IMT.
In conclusion, the CLTC-ALK fusion oncogene represents a novel mechanism of ALK activation in IMT and demonstrates that, similar to ALCL, the fusion partners of the ALK gene in IMT are diverse. ALK protein expression is an independent predictor of survival and serves as a useful biological marker of a specific disease entity within the spectrum of ALCL. Additional studies are warranted to determine whether ALK protein expression is likewise associated with specific clinicopathological traits in IMT.
| Acknowledgements |
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| Footnotes |
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Supported by National Cancer Institute grants NCI-P30 CA36727, the John A. Wiebe Jr. Childrens Health Care Fund and National Childhood Cancer Foundation (to J. A. B.), National Cancer Institute grant NCI CA69129 and CORE grant CA21765 (to S. W. M.), the American Lebanese Syrian Associated Charities, the St. Jude Childrens Hospital (to S. W. M.), and the Fundaçao de Amparo à Pesquisa do Estado de Sao Paulo (to G. W. B. C.).
G. W. B. C. is presently at the Department of Medicine, Universidade Federal de Sao Paulo, Sao Paulo, Brazil.
Accepted for publication April 20, 2001.
| References |
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