US 5,532,340 AGrant
Tumor Suppressor Protein PRB2, Related Gene Products, and DNA Encoding Therefor
Issue Date:1996-07-02
•2 Claims
•2 Drawing Sheets
Abstract
The invention provides a tumor suppressor protein of the retinoblastoma family (pRb2) which binds to the E1A transforming domain and to DNA encoding for the pRb2 protein.
Metadata
Assignee
- Temple University of The Commonwealth System of Higher Education
Inventor
- Antonio Giordano
Application Information
Application Number:US 4292642
Filing Date:1995-04-25
Priority Date:1993-08-12
Art Unit:184
Classifications
IPC:
C07K 1447
Field of Search:
5309305143502202;12
Patent Drawings (2 sheets)
Description
FIELD OF THE INVENTION
This invention relates to a tumor suppressor protein (pRb2) of the retinoblastoma family which binds to the E1A transforming domain. The invention also concerns DNA encoding for pRb2 and related gene products.
BACKGROUND OF THE INVENTION
Many types of human cancer are now believed to be caused by an imbalance of growth regulators within a cell. A decrease in negative control growth regulators and/or their deactivation can cause a cancerous condition. Further, an increase in positive control growth regulators can also cause a cancerous condition.
Since the identification of the first tumor suppressor gene, much effort in cancer research has been focused on the identification of new tumor suppressor genes and their involvement in human cancer. Many types of human cancers are thought to develop by a loss of heterozygosity of putative tumor suppressor genes not yet identified (Lasko et al., Annu. Rev. Genetics, 25, 281-296 (1991)) according to Knudson's "two-hit" hypothesis (Knudson, Proc. Natl. Acad. Sci. USA, 68, 820-823 (1971)).
One of the most studied tumor suppressor genes is the retinoblastoma susceptibility gene (rb), whose gene product (pRb) has been shown to play a key role in the regulation of cell division. In interphasic cells, pRb contributes to maintaining the quiescent state of the cell by repressing transcription of genes required for the cell cycle through interaction with transcription factors, such as E2F (Wagner et al., Nature, 352,189-190 (1991); Nevins, Science, 258, 424-429 (1992); and Hiebert et al., Genes Develop., 6, 177-185 (1992)). The loss of this activity can induce cell transformation as evidenced by the reversion of the transformed phenotype in pRb cells after replacement of a functional pRb (Huang et al., Science 242 1563-1565 (1988); Bookstein et al., Science, 247 712-715 (1990); and Sumegi et al., Cell Growth Differ., 1 247-250 (1990)).
Upon entrance into the cell cycle, pRb seems to be phosphorylated by cell cycle-dependent kinases (Lees et al., EMBO J. 10 4279-4290 (1991); Hu et al., Mol. Cell. Biol., 12 971-980 (1992); Hinds et al., Cell, 70 993-1006 (1992); Matsushime et al., Nature, 35 295-300 (1992)) which is thought to permit its dissociation from transcription factors and, hence, the expression of genes required for progression through the cell cycle. Noteworthily, the association of pRb with cell cycle regulators like cyclins and cell cycle-dependent kinases suggests a universal character to its function.
However, pRb involvement in human cancer has been restricted to a limited number of tumor types suggesting that .this hypothetically universal function may be exerted by other gene products in a cell type-specific manner. Consistently, knock out of the rb gene in mice affects only specific cell types and after several days of embryonic development (Jacks et al., Proc. Natl. Acad. Sci. USA, 68 820-823 (1992); Lee et al., Nature, 359 288-294 (1992); Clarke et al., Nature, 359 328-330 (1992)).
The ability of several transforming proteins from human DNA tumor viruses to activate cell proliferation has been a useful tool for the identification of cellular factors involved in the regulation of the cell cycle. Negative regulators of cell growth may thus be effective targets for inactivation by these viral proteins, as it occurs with the product of the retinoblastoma gene.
Adenovirus E1A, SV40 T antigen, and papillomavirus E7 are three viral proteins which have been found to bind to pRb. This binding is responsible for the release of transcription factors required for the expression of cell cycle genes (Nevins, Science, 258 424-429 (1992); Bandara et al., Nature, 351 494-497 (1991)).
A conserved motif found in the three viral proteins allows for interaction and complex formation with pRb (Moran, Curr. Op. Gen. Dev., 3 63-70 (1993)). In the case of the adenovirus E1A protein, this motif is located in the transforming domain 2, which is required for growth activation. The pRb-related product p107 also binds in this region (Egan et al., Mol. Cell. Biol., 8 3955-3959 (1988); Whyte et al., Cell, 56 67-75 (1989)) .
Domain 2 is also the site of interaction of an additional E1A-binding protein, p130 (Giordano et al., Oncogene, 1 481-485 (1991)). This has led to the suggestion that p130 has a structural relationship to pRb and p107 (Moran, Curr. Op. Gen. Dev., 3 63-70 (1993)).
The E1A-binding domain in pRb and p107 is a conserved region termed the "pocket region" (Kaelin et al., Mol. Cell. Biol,, 10 3761-3769 (1990); Ewin et al., Cell, 66 1155-1164 (1991)), and it is thought to play a primary role in the function of these proteins. The pocket is structurally formed by two regions A and B, which are conserved in pRb and p107 and separated by non-conserved spacers of different sizes in pRb and p107.
In addition to pRb and p107, there are other cellular E1A-binding proteins that have been identified by co-immunoprecipitation experiments using antibodies to E1A. These cellular proteins include the major polypeptides p300, p130, p60/cyclin A, and several other minor forms (Yee, et al., Virology 147 142-153 (1985); Harlow et al., Mol. Cell. Biol. 1 1579-1589 (1986); Giordano, et al., Cell 58 981-990 (1989); Giordano et al. Science 253 1271-1275 (1991)). Binding to the N-terminal region has been shown to be exclusive to p300 (Egan et al., Mol. Cell. Biol., 8 3955-3959 (1988); Whyte et al., Cell, 56 67-75 (1989); Stein et al., J. Virol., 64 4421-4427 (1990)), and pRb2 consistently failed to bind to this region. Both domains 1 and 2 of the E1A protein have been shown to be necessary for the E1A binding of the following set of proteins: pRb, p107, p60/cyclin A, and p130 (Egan et al., Mol. Cell. Biol., 8 3955-3959 (1988); Whyte et al., Cell, 56 67-75 (1989); Giordano et al. Science 253 1271-1275 (1991)). Furthermore, the E1A-928 mutant has been previously shown to bind to p107 and p60/cyclin A, but not to pRb and p130.
The association of pRb with transcription factors, such as E2F, occurs by interactions at the pocket region (Raychaudhuri et al., Genes Develop., 5 200-1207 (1991)) and, recently, p107 has also been shown to exert such a binding profile (Cao et al., Nature, 355 176-179 (1992)). Moreover, the pocket region is found mutated in several human cancers where a lack of function of the pRb protein is thought to be involved in the acquisition of the transformed phenotype (Hu et al., EMBO J., 9 1147-1153 (1990)); Huang et al., 1990).
There is a need for identification and sequencing of new rb-related genes that may have an involvement in cell growth inhibition. Genes related to rb and their protein products that also have tumor suppressor activity in specific cell types are needed. However, identification and sequencing of such new genes and their protein products would be surprising in view of the amount of previous research in this area.
BRIEF DESCRIPTION OF THE FIGURES
FIG. 1 is a schematic representation of wild type and mutant forms of the E1A protein. The mutant forms are pm928/961, dl2-36, dl38-67 and dl73-120. The nature of each mutation is set forth schematically.
FIG. 2 is an SDS-PAGE gel showing binding of the pRb2 protein to a fusion construct of a wild-type E1A protein and a Glutathione-S-Transferase (GST) protein (lane 2) and to E1A mutant constructs fused to a GST protein: dl2-36 (lane 3), dl38-67 (lane 4), dl73-120 (lane 5), and pm928/961 (lane 6). GST protein with no E1A fused was included as a control (lane 1), which showed no binding with the pRb2 protein.
SUMMARY OF THE INVENTION
The present invention provides a recombinant DNA cloning vehicle comprising a cDNA sequence comprising the human pRb2 gene cDNA sequence. A preferred cDNA sequence is a sequence according to SEQ ID NO:1. The cDNA comprises a sequence coding for the amino acid sequence according to SEQ ID NO:2.
In another embodiment the present invention provides a protein essentially having an amino acid sequence according to SEQ ID NO:2. Preferably, the protein corresponding to SEQ ID NO:2 is not phosphorylated.
In a further embodiment the present invention provides a host cell line transformed by the cDNA of the cloning vehicle described above, which host cell line expresses the cDNA from the cloning vehicle to produce a protein. Preferably the cDNA has a sequence according to SEQ ID NO:1 and the protein produced has a sequence according to SEQ ID NO:2.
DETAILED DESCRIPTION OF THE INVENTION
The pRb2 protein is a previously unsequenced, uncharacterized member of the pRb family of tumor suppressor proteins. This protein was designated as pRb2 since it binds to Adenovirus E1A protein in a manner similar to pRb and p107. Thus, the cDNA sequence coding for pRb2 was designated as the pRb2 gene.
Polymerase chain reaction (PCR) using probes derived from domains 1 and 2 of the rb gene was utilized to identify the pRb2 cDNA sequence as follows.
Synthetic degenerate oligonucleotides were designed based on conserved amino acid sequences flanking the spacers in pRb and p107. These oligonucleotides were designated as primer A and primer B and used as PCR primers to isolate and clone the pRb2 cDNA sequence.
Primer A is an oligonucleotide containing 18 nucleotides coding for the amino acid sequence Phe--Tyr--Lys--Val--Ile--Glu (SEQ ID NO:3). The 5' end of primer A also contains nine nucleotides which form a BamHI restriction site.
Primer B is an oligonucleotide containing 18 nucleotides coding for the amino acid sequence Gln--Asp--Leu--His--Arg--Asp (SEQ ID NO:4). The 5' end of primer B also contains nine nucleotides which form a HindIII restriction site.
Each of the 18-mer primers A and B correspond to conserved portions in the pocket regions of pRb and p107. The two restriction sites (BamHI for primer A, and HindIII for primer B) were used to conveniently subclone the amplified PCR fragments into a commercially available vector (pBluescript, Stratagene, La Jolla, Calif.).
The PCR product was used as a probe for the screening of cDNA libraries from human 293 and HeLa cells. From the screening several positive clones were identified. These clones were sequenced and analyzed for a clone containing full length cDNA.
One of the HeLa cDNA clones contained a putarive initiation codon which is compatible with the Kozak initiation sequence (Kozak, J. Mol. Biol., 196 947-950 (1987)). This clone showed a unique open reading frame ending in a termination codon 3,249 base pairs downstream (see SEQ ID NO:1). The complete sequence included 55 base pairs upstream of the open reading frame which did not contain any putative initiation site, and a 3' noncoding region ending in a poly A tail. The open reading frame encoded a polypeptide of 1,082 amino acids (SEQ ID NO:2) with a predicted molecular mass of approximately 120 kD. This cDNA clone was designated rb2 and, hence, the encoded protein pRb2.
The sequence of protein pRb2 (SEQ ID NO:2) as compared to the pRb and p107 protein sequences shows a high level of identity, 53% with respect to p107, and 32% with respect to pRb. This suggests a closer relationship of pRb2 to p107. A partial comparisons of the amino acid sequences of these three proteins shows that the pocket region is clearly conserved in pRb2, mainly at the level of the domains A and B. This suggests that pRb2 has properties similar to pRb and p107, such as the formation of cell cycle-associated protein complexes which are known to occur via the pocket region. This suggests that pRb2 would be involved in the cell cycle machinery. Moreover, the high identities found in the C and N-terminal portions between pRb2 and p107 suggest a role for these regions in a function of .p107 and pRb2 which may differentiate them from pRb.
The pRb2 cDNA clone was transcribed into an RNA segment in vitro by a T7 RNA polymerase capping reaction on the linearized pBluescript-pRb2. The resulting transcription product (RNA segment) was extracted with a phenol/chloroform solution and precipitated in an ethanol solution.
The transcription product was used as a substrate for in vitro translation into a protein by using a rabbit reticulocyte lysate (Promega, Biotec, Madison, Wis.) and .sup.35 S-methionine as a radioactive label (Pelham et al., Eur. J. Biochem., 67 248-256 (1976)). Several truncated forms of the protein were produced. The largest pRb2 protein form migrated to approximately 120 kD by SDS-PAGE. The most prominent of the bands corresponds to a protein form which migrated to around 90 kD, and a third protein form was found to migrate to 85 kD.
After isolation, the 120 kD pRb2 protein and its 90 kD and 85 kD truncated forms were tested for E1A protein binding properties. The E1A binding results were compared to the E1A binding properties of the pRb and p107 proteins. Both pRb and p107 proteins bind to the adenovirus E1A protein through their respective pocket regions. Demonstration of E1A protein binding by the pRb2 protein would indicate that the latter protein has a key role in the regulation of cell division. The E1A binding capacity of pRb2 was thus determined as follows.
Wild type and mutant forms of the E1A protein were obtained. The nature of the mutations are set forth in FIG. 1. A binding assay was performed to test the binding of the wild type and mutant E1A proteins to in vitro-translated pRb2 protein (120 kD and truncated 90 kD and 85 kD proteins) precleared of translation solution. Each of the three in vitro translated main forms of the pRb2 protein bound to E1A.
An E1A deletion mutation involving the N-terminal portion of E1A (dl2-36) did not affect binding of the pRb2 to E1A. The binding of pRb2 was also not affected by deletion mutations involving the transforming domain 1 of E1A (E1A mutants dl38-67 and dl73-120). This suggests that binding of pRb2 to E1A does not take place via these regions of E1A. However, the ability of the pRb2 protein to bind was almost completely abolished when an E1A mutant protein containing a double point mutation in the transforming domain 2 of E1A was used (E1A mutant pm928/961, in which Cys was substituted for Gly at position 124, and Lys for Glu at position 135). Therefore, the transforming domain 2 of E1A is required for binding to pRb2. This suggests that the E1A-binding capacity of pRb2 is involved, at least in part, in the transforming activity of E1A.
Although the pRb2 protein is similar in molecular weight to the p130 protein and they both have similar binding profiles to E1A wild type and mutant proteins the two proteins are not identical. The p130 protein is phosphorylated on Ser and Thr residues while pRb2 is unphosphorylated. Moreover, p130 exists in more than one phosphorylated form.
A cloning vector designated as pBluescript-pRb2 which contained the pRb2 cDNA was deposited with the ATCC, Rockville, Md. on Aug. 11, 1993 and was given the ATCC number 75521.
An E. coli bacterial strain designated as E. coli pBluescript-pRb2 which contained a plasmid containing the pRb2 cDNA was deposited with the ATCC, Rockville, Md. on Aug. 11, 1993 and given the ATCC number 69383.
It is well within the skill of those in the genetic engineering art to use the nucleotide sequence of SEQ ID NO:1 or related sequences encoding for the pRb2 protein of the present invention to produce pRb2 protein via microbial processes. Using the nucleotide sequence of SEQ ID NO:1 to produce pRb2 is made easier for one of ordinary skill by utilizing the pBluescript-pRb2 cloning vector according to the invention.
Fusing the nucleotide sequences encoding for the pRb2 protein into an expression vector and transforming or transfecting into hosts, either eukaryotic (yeast or mammalian cells) or prokaryotic (bacterial cells), are standard procedures used in producing other well-known proteins, e.g., insulin, interferons, human growth hormone, and the like. Similar procedures, or obvious modifications thereof, can be employed to prepare pRb2 proteins by microbial means or mammalian tissue-culture technology in accord with the subject invention.
Those of ordinary skill in the art will appreciate the fact that the cDNA fragment set forth in SEQ ID NO:1 is only one DNA segment coding for pRb2. Other equivalent DNA segments of substantial similarity will immediately be envisioned which will code for pRb2. The present invention also includes such equivalent DNA sequences.
Moreover, substitution of equivalent amino acids in SEQ ID NO:2 would not be expected to affect the pRb2 protein's activity. These amino acid substitutions would be envisioned by those of ordinary skill in the art. Such equivalent amino acid sequences are also included within the present invention.
The following non-limiting examples are provided to illustrate the invention.
EXAMPLES
Example 1
Obtaining pRb2 cDNA and Amino Acid Sequences
A. Synthesis of PCR Primers
Primers A and B were synthesized using standard oligonucleotide synthesis techniques and purified.
Primer A contained 18 nucleotides coding for the polypeptide sequence according to SEQ ID NO:3. The 5'-end was of nine additional nucleotides that formed a BamHI restriction site. Primer B contained 18 nucleotides coding for the polypeptide sequence according to SEQ ID NO:4. The 5'-end was of nine additional nucleotides which formed a HindIII restriction site.
B. PCR Amplification of a Human 293 cDNA Library
A lambda-ZAPII cDNA library was obtained from reverse transcription of RNA from human 293 cells using standard techniques. The cDNA library was amplified via PCR with Primers A and B from Example 1A above. The PCR was performed using a GeneAmp.TM. kit (Perkin Elmer Cetus, Norwalk, Conn.) according to the instructions of the manufacturer. Briefly, thirty cycles including a one minute denaturization at 94.degree. C., one minute annealing at 37.degree. C., and two minutes extension at 68.degree. C., were followed by fifteen minute extension. This resulted in PCR amplification of a 1 kb fragment in addition to pRb and p107 segments.
C. Subcloning and Nucleotide Sequencing the 1kb Fragment
The amplified 1 kb fragment was subcloned into a pBluescript vector (Stratagene). After subcloning, nucleotide sequencing was performed using the dideoxy method of the Sequenase kit (United States Biochemicals). The nucleotide sequence of the 1 kb fragment revealed some homology with pRb and p107 cDNAs.
D. Probing cDNA Libraries From 293 and HeLa Cells
The 1 kb fragment was utilized as a probe to screen additional cDNA libraries. Lambda-ZAPII cDNA libraries from human 293 and HeLa cells (Stratagene), respectively were screened using the 1 kb fragment labeled with .alpha.-.sup.32 P-CTP by the random primer method (Boehringer Mannheim). Briefly, lambda-ZAP phage was adsorbed to Escherichia coli BB4 strain bacteria and plated in agar medium. Nitrocellulose filters were hybridized to the PCR probe in a high stringency protocol which included the pre-hybridization mixture: 5.times. SSPE, 10.times. Denhardt's solution, 150 .mu.g/ml herring sperm DNA, 50% formamide and 2% SDS; a hybridization mixture adding 10.sup.6 cpm/ml of the 1 kb PCR probe to the pre-hybridization mixture; and, three washes of twenty minutes each at 42.degree. C. with 0.2.times. SSC and 0.1% SDS.
E. Analyzing the Positive Clones From the Probing
From the probing procedures of Example 1D several positive clones were located. In vivo excision (Stratagene) was performed on the several positive Lambda-ZAP clones. pBluescript vectors containing cDNA clones were obtained. The cDNA clones were reproduced and nucleotide sequencing of each cDNA clone was performed as described above in Example 1C. The sequencing results analyzed for the full length cDNA including the 1 kb fragment.
One of the HeLa cDNA clones contained a putative initiation codon which was compatible with the Kozak initiation sequence. This clone showed a unique open reading frame ending in a termination codon 3,249 base pairs downstream (see SEQ ID NO:1). The complete sequence included 55 base pairs upstream of the open reading frame which did not contain any putative initiation site, and a 3' non-coding region ending in a poly A tail. The open reading frame encoded a polypeptide of 1,082 amino acids (SEQ ID NO:2) with a predicted molecular mass of approximately 120 kD. This cDNA clone was designated rb2 and, hence, the encoded protein pRb2. The pBluescript vector cloned with the pRb2 gene was designated as pBluescript-pRb2.
The sequence of protein pRb2 (SEQ ID NO:2, which is derived from the corresponding cDNA sequence) as compared to pRb and p107 protein sequences showed a high level of identity, 53% with respect to p107, and 32% with respect to pRb. This suggests a closer relationship of pRb2 to p107. Partial comparisons of these three protein sequences show that the pocket region is clearly conserved in pRb2, mainly at the level of the domains A and B.
Example 2
E1A Binding of In Vitro-Translated pRb2
A. Transcription and Translation of pBluescript-pRb2
The Example 1E pBluescript-pRb2 cDNA clone was transcribed into an RNA segment in vitro by a T7 RNA polymerase capping reaction on the linearized pBluescript-pRb2. The resulting transcription product (RNA segment) was extracted with a phenol/chloroform solution and precipitated in an ethanol solution.
The transcription product RNA segment was used as a substrate for in vitro translation into a protein by using a rabbit reticulocyte lysate (Promega, Biotec, Madison, Wis.) and .sup.35 S-methionine as a radioactive label (Pelham et al., Eur. J. Biochem., 67 248-256 (1976)). Several truncated forms of the protein were produced. The largest form migrated to approximately 120 kD by SDS-PAGE. The most prominent of these bands migrated to around 90 kD, and a third one was found to migrate to 85 kD.
B. Obtaining Wild-Type and Mutant E1A Proteins
Wild type and mutant forms of the E1A protein were obtained. The mutant forms were pm928/961, dl2-36, dl38-67 and dl73- 120. The nature of the mutations are set forth in FIG. 1. The wild type and mutant forms of E1A were sub-cloned into pGEX-2T and expressed in E. coli as GST-fusion proteins. The E1A proteins were then isolated from E. coli cultures by standard techniques.
C. Binding Assay for pRb2 and E1A Proteins
A binding assay was performed to test the binding of the wild type and mutant E1A proteins of Example 2B with the in vitro-translated pRb2 proteins of Example 2A precleared of translation solution. The pRb2 protein (120 kD, 90 kD, and 85 kD) was precleared with glutathione-sepharose and GST-glutathione-sepharose beads in NETN buffer containing 1 mM DTT, 1 mM PMSF, and 10 .mu.g/ml leupeptine, at 4.degree. C.
Two .mu.g of each E1A protein were incubated with precleared pRb2 for one hour at 4.degree. C., and glutathione-sepharose beads were added and incubated for an additional hour. Proteins were resolved using SDS-PAGE according to standard protocols and a Fuji phosphoimage analyzer system was used to develop the protein signal.
The results of the binding assay are set forth in FIG. 2, which is an SDS-PAGE showing binding to wild-type E1A (lane 2) and to E1A mutant constructs: dl2-36 (lane 3), dl38-67 (lane 4), dl73-120 (lane 5), and pm928/961 (lane 6). GST with no E1A fused was included as a control (lane 1). The in vitro-translated product resulting from a rabbit reticulocyte translation reaction with no exogenous RNA was included as a control, which did not give any signal (not shown).
Each of the three in vitro translated main forms of the pRb2 protein (120 kD, 90 kD, and 80 kD) bound to E1A. A deletion mutation involving the N-terminal portion of E1A (dl2-36) did not affect pRb2 binding. E1A binding to pRb2 was not affected by dl38-67 or dl73-120 deletion mutations, both involving the transforming domain 1 of E1A. This suggests that binding of pRb2 to E1A does not take place in these regions. However, binding was almost completely abolished when an E1A-fusion protein containing a double point mutation in the transforming domain 2 of E1A was used (E1A mutant pm928/961, in which Cys was substituted for Gly at position 124, and Lys for Glu at position 135). Therefore, the transforming domain 2 of E1A is required for binding to pRb2. This suggests that the E1A-binding capacity of pRb2 is involved, at least in part, in the transforming activity of E1A.
For comparative purposes the pRb and p107 proteins were obtained and a binding assay performed with the wild type and mutant E1A proteins. The pRb2 protein showed similar binding characteristics to the pRb and p107 proteins.
Since the pRb2 protein binds the E1A protein in manner similar to the pRb protein, the pRb2 protein is a useful diagnostic tool for identifying cell infected with adenovirus E1A or a related DNA virus producing oncoproteins related to the E1A protein. Because of the binding capacity of pRb2, it can also be administered to cells infected with adenovirus E1A, where it may act as a cell growth suppressor to reverse the effects of the E1A oncoprotein. This reversal of E1A protein effects could restore the balance of cell growth in a retinoblastoma cancer tumor. Thus, pRb2 may be a useful tumor suppressor agent, for treating cancers such as retinoblastoma interocular cancer. Further, pRb2 may be a useful research tool for binding and identifying other DNA tumor virus oncoproteins which have sequences related to the E1A protein.
All references cited with respect to synthetic, preparative and analytical procedures are incorporated herein by reference.
The present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be made to the appended claims, rather than to the foregoing specification, as indicating the scope of the invention.
Claims
I claim:
1. An isolated and substantially purified protein comprising the amino acid sequence SEQ ID No: 2.
2. A protein according to claim 1, which is not phosphorylated.
Patent Citations (1)
| Patent | Date | Inventor | Cited By |
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| US4675285 | 1987-06-01 | Clark et al. |
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