Axl Oncogene

The present invention relates to oncogenes in general, and particularly relates to an oncogene which codes for a receptor having tyrosine kinase activity.

Receptor tyrosine kinases represent a class of proteins that transduce signals from the extracellular milieu into the cytoplasm by binding peptide growth factors. These growth factors and their cognate receptors are involved in regulating cellular growth and differentiation. Alteration of either growth factors or their receptors can result in neoplastic transformation and altered development. See M. Cross and T. Dexter, Cell 64:271 (1991). There are several mechanisms by which growth factor receptors can be rendered transforming. Retroviral transduction of proto-oncogenes can result in truncation and mutation of the normal version of the gene. This has been shown for the EGF receptor (v-erbB) (J. Downward et al., Nature 307:521 (1984)), CSF-1 receptor (v-fms) (C. Sherr, Blood 75:1 (1990)) and ros (v-ros) (H. Matsushime et al., Mol. Cell. Biol. 6:3000 (1986)). Overexpression of an otherwise normal receptor, with or without the concomitant application of ligand, can also result in neoplastic transformation as shown for the IGF-1R (M. Kaleko et al., Mol. Cell. Biol. 10:464 (1990)), EGFR (P. Di Fiore et al., Cell 51:1063 (1987); T. Velu et al., Science 238:1408 (1987)), CSF-1R (M. Roussel et al., Nature (London) 325:549 (1987)), eph (Y. Maru et al., Oncogene 5:445 (1990)) and neu (P. Di Fiore et al., Science 237:178 (1987)). Furthermore, structural rearrangement, as seen with ret, trk, and met, may also activate the transforming capacity of receptor kinases. See M. Grieco et al., Cell 60:557 (1990); D. Martin-Zanca et al., Nature (London) 319:743 (1986); M. Park et al., Cell 45:895 (1986).

In addition to the transforming activity of altered or overexpressed receptor tyrosine kinases, studies have demonstrated that the normal cellular homologues of these receptors function to regulate cell growth and differentiation. Examples include the trk receptor kinase originally isolated by genomic transfection of human colon carcinoma DNA into NIH/3T3 cells. D. Martin-Zanca et al., supra. Although trk is not generally involved in colon cancer, recent work has demonstrated that this gene encodes a subunit of the nerve growth factor receptor which plays a critical role in neural development. See D. Kaplan et al., Science 252:554 (1991); R. Klein et al., Cell 65:189 (1991). Furthermore, the met proto-oncogene, originally isolated by transfection of 3T3 cells with genomic DNA from a tumorigenic human osteogenic sarcoma cell line MNNG-HOS (C. Cooper et al., Nature (London) 311:29 (1984)), has been demonstrated to be the cell-surface receptor for hepatocyte growth factor, a potential growth factor for a broad spectrum of cell types as well as a mediator of liver regeneration. D. Bottaro et al., Science 251:802 (1991). In addition, the CSF-1 receptor mediates the pleiotrophic effects of its cognate ligand, colony-stimulating factor-1 (CSF-1). Together these two molecules stimulate the proliferation and differentiation of cells of the macrophage lineage. C. Sherr, supra.

One oncogene which has been of considerable interest is neu (also known as "her2"). Methods for the detection of neu gene expression and products are disclosed in PCT Application WO 89/10412. Methods of treating tumor cells by inhibiting the function of a growth factor receptor such as HER2 receptor are disclosed in PCT Application WO 89/06692. Methods of detecting point mutations in the neu gene are disclosed in U.S. Pat. No. 4,935,341. Genes in this family are thought to be activated by the alteration of a single nucleotide in the normal cellular DNA sequence (the protooncogene), resulting in activation of the oncogene. This point mutation was initially seen in a rat neuroblastoma induced by transplacental exposure to a carcinogen.

In an effort to determine genes involved in the progression of chronic myelogenous leukemia (CML) to acute phase leukemia, we previously reported the identification of a transforming gene in the DNAs of two patients with CML. See E. Liu et al., Proc. Natl Acad. Sci. U.S.A. 85:1952 (1988). We term this gene axl (anexelekto, anexelekto=axl, Greek for "uncontrolled"). The present invention is based upon our ongoing interest in investigating this gene.

Described herein is the molecular cloning and characterization of the axl gene. These data show that axl is a receptor tyrosine kinase with a structure unique amongst tyrosine kinases. Our data indicate that the axl protein has tyrosine kinase activity and is capable of transforming NIH/3T3 cells. Furthermore, axl's transforming capacity results from overexpression of axl mRNA rather than structural mutation.

In accordance with the foregoing, disclosed is an isolated DNA sequence encoding a mammalian axl receptor which exhibits axl oncogene activity. More particularly, such DNA sequences are DNA sequences selected from the group consisting of: (a) isolated DNA which encodes a human axl oncogene; (b) isolated DNA which hybridizes to isolated DNA of (a) above and which encodes an axl receptor; and (c) isolated DNA differing from the isolated DNAs of (a) and (b) above in nucleotide sequence due to the degeneracy of the genetic code, and which encodes an axl receptor. Recombinant DNA comprising vector DNA and a DNA as described above, along with host cells containing such recombinant DNA and capable of expressing the same, are also disclosed.

Also disclosed is an isolated and purified axl receptor selected from the group consisting of mammalian axl receptor proteins and soluble extracellular fragments thereof having axl receptor binding activity. More particularly, such receptors are axl receptors coded for by a DNA sequence selected from the group consisting of: (a) DNA which encodes a human axl oncogene; (b) DNA which hybridizes to isolated DNA of (a) above and which encodes an axl receptor; (c) DNA differing from the DNAs of (a) and (b) above in nucleotide sequence due to the degeneracy of the genetic code, and which encodes an axl receptor; and (d) DNA which encodes a soluble extracellular fragment of an axl receptor protein encoded by a DNA of (a), (b), or (c) above, which fragment has axl receptor binding activity.

Also disclosed are soluble mammalian axl receptor which exhibits axl receptor binding activity. Such receptors may comprise a mammalian axl receptor having a deleted transmembrane region, or the extracellular portion of a mammalian axl receptor protein.

Also disclosed are recombinant DNA constructs coding for chimeric proteins, along with the coded-for chimeric proteins. The chimerics protein comprise an an axl receptor extracellular portion having axl receptor binding activity operatively associated an effector portion capable of generating a detectable signal upon binding of a ligand to said axl receptor extracellular portion.

Also disclosed herein are antibodies, such as monoclonal antibodies, which specifically binds to the axl receptor, along with hybridomas for making such monoclonal antibodies.

Also disclosed is an assay for detecting a tumor. The assay comprises the steps of, first, contacting cells to antibodies capable of specifically binding the extracellular domain of the axl receptor, and then determining the extend of binding of said antibodies to said cells.

The term "oncogene", as used herein, refers to a genetic sequence whose expression within a cell induces that cell to become converted from a normal cell into a tumor cell.

A. Genetic Engineering Techniques

The production of cloned genes, recombinant DNA, vectors, transformed host cells, proteins and protein fragments by genetic engineering is well known. See, e.g., U.S. Pat. No. 4,761,371, to Bell et al. at Col. 6 line 3 to Col. 9 line 65; U.S. Pat. No. 4,877,729, to Clark et al. at Col. 4 line 38 to Col. 7 line 6; U.S. Pat. No. 4,912,038, to Schilling at Col. 3 line 26 to Col. 14 line 12; and U.S. Pat. No. 4,879,224, to Wallner at Col. 6 line 8 to Col. 8 line 59.

A vector is a replicable DNA construct. Vectors are used herein either to amplify DNA encoding axl and/or to express axl. An expression vector is a replicable DNA construct in which an axl oncogene is operably linked to suitable control sequences capable of effecting the expression of axl in a suitable host. The need for such control sequences will vary depending upon the host selected and the transformation method chosen. Generally, control sequences include a transcriptional promoter, an optional operator sequence to control transcription, a sequence encoding suitable mRNA ribosomal binding sites, and sequences which control the termination of transcription and translation.

Amplification vectors do not require expression control domains. All that is needed is the ability to replicate in a host, usually conferred by an origin of replication, and a selection gene to facilitate recognition of transformants.

Vectors comprise plasmids, viruses (e.g., adenovirus, cytomegalovirus), phage, and integratable DNA fragments (i.e., fragments integratable into the host genome by recombination). The vector replicates and functions independently of the host genome, or may, in some instances, integrate into the genome itself. Expression vectors should contain a promoter and RNA binding sites which are operably linked to the gene to be expressed and are operable in the host organism.

DNA regions are operably linked or operably associated when they are functionally related to each other. For example, a promoter is operably linked to a coding sequence if it controls the transcription of the sequence; or a ribosome binding site is operably linked to a coding sequence if it is positioned so as to permit translation.

Transformed host cells are cells which have been transformed or transfected with vectors containing an axl oncogene constructed using recombinant DNA techniques. Transformed host cells ordinarily express axl, but host cells transformed for purposes of cloning or amplifying axl oncogene DNA need not express axl.

Suitable host cells include prokaryote, yeast or higher eukaryotic cells such as mammalian cells and insect cells. Cells derived from multicellular organisms are a particularly suitable host for synthesis of the axl oncogene receptor by recombinant means, and mammalian cells are particularly preferred. Propagation of such cells in cell culture has become a routine procedure (Tissue Culture, Academic Press, Kruse and Patterson, editors (1973)). Examples of useful host cell lines are VERO and HeLa cells, Chinese hamster ovary (CHO) cell lines, and WI138, BHK, COS-7, CV, and MDCK cell lines. Expression vectors for such cells ordinarily include (if necessary) an origin of replication, a promoter located upstream from the axl oncogene to be expressed and operatively associated therewith, along with a ribosome binding site, an RNA splice site (if intron-containing genomic DNA is used), a polyadenylation site, and a transcriptional termination sequence.

The transcriptional and translational control sequences in expression vectors to be used in transforming vertebrate cells are often provided by viral sources. For example, commonly used promoters are derived from polyoma, Adenovirus 2, and Simian Virus 40 (SV40). See, e.g., U.S. Pat. No. 4,599,308.

An origin of replication may be provided either by construction of the vector to include an exogenous origin, such as may be derived from SV 40 or other viral (e.g. Polyoma, Adenovirus, VSV, or BPV) source, or may be provided by the host cell chromosomal replication mechanism. If the vector is integrated into the host cell chromosome, the latter is often sufficient.

Rather than using vectors which contain viral origins of replication, one can transform mammalian cells by the method of cotransformation with a selectable marker and the axl oncogene. Examples of suitable selectable markers are dihydrofolate reductase (DHFR) or thymidine kinase. This method is further described in U.S. Pat. No. 4,399,216 (Applicant specifically intends that the disclosure of all patent references cited herein be incorporated herein by reference).

Other methods suitable for adaptation to the expression of an axl oncogene in recombinant vertebrate cell culture include those described in M-J. Gething et al., Nature 293, 620 (1981); N. Mantei et al., Nature 281, 40; A. Levinson et al., EPO Application Nos. 117,060A and 117,058A.

Host cells such as insect cells (e.g., cultured Spodoptera frugiperda cells) and expression vectors such as the baculovirus expression vector (e.g., vectors derived from Autographa californica MNPV, Trichoplusia ni MNPV, Rachiplusia ou MNPV, or Galleria ou MNPV) may be employed in carrying out the present invention, as described in U.S. Pat. Nos. 4,745,051 and 4,879,236, to Smith et al. In general, a baculovirus expression vector comprises a baculovirus genome containing the gene to be expressed inserted into the polyhedrin gene at a position ranging from the polyhedrin transcriptional start signal to the ATG start site and under the transcriptional control of a baculovirus polyhedrin promoter.

Prokaryote host cells include gram negative or gram positive organisms, for example Escherichia coli (E. coli) or Bacilli. Higher eukaryotic cells include established cell lines of mammalian origin as described below. Exemplary host cells are E. coli W3110 (ATCC 27,325), E. coli B, E. coli X1776 (ATCC 31,537), E. coli 294 (ATCC 31,446). A broad variety of suitable prokaryotic and microbial vectors are available. E. coli is typically transformed using pBR322. Promoters most commonly used in recombinant microbial expression vectors include the betalactamase (penicillinase) and lactose promoter systems (Chang et al., Nature 275, 615 (1978); and Goeddel et al., Nature 281, 544 (1979)), a tryptophan (trp) promoter system (Goeddel et al., Nucleic Acids Res. 8, 4057 (1980) and EPO App. Publ. No. 36,776) and the tac promoter (H. De Boer et al., Proc. Natl. Acad. Sci. USA 80, 21 (1983)). The promoter and Shine-Dalgarno sequence (for prokaryotic host expression) are operably linked to the axl oncogene, i.e., they are positioned so as to promote transcription of axl oncogene messenger RNA from the DNA.

Eukaryotic microbes such as yeast cultures may also be transformed with axl oncogene vectors. see, e.g., U.S. Pat. No. 4,745,057. Saccharomyces cerevisiae is the most commonly used among lower eukaryotic host microorganisms, although a number of other strains are commonly available. Yeast vectors may contain an origin of replication from the 2 micron yeast plasmid or an autonomously replicating sequence (ARS), a promoter, an axl oncogene, sequences for polyadenylation and transcription termination, and a selection gene. An exemplary plasmid is YRp7, (Stinchcomb et al., Nature 282, 39 (1979); Kingsman et al., Gene 7, 141 (1979); Tschemper et al., Gene 10, 157 (1980)). Suitable promoting sequences in yeast vectors include the promoters for metallothionein, 3-phosphoglycerate kinase (Hitzeman et al., J. Biol. Chem. 255, 2073 (1980) or other glycolytic enzymes (Hess et al., J. Adv. Enzyme Reg. 7, 149 (1968); and Holland et al., Biochemistry 17, 4900 (1978)). Suitable vectors and promoters for use in yeast expression are further described in R. Hitzeman et al., EPO Publn. No. 73,657.

B. Axl Oncogenes

Axl oncogenes of the present invention include those coding for proteins homologous to, and having essentially the same biological properties as, the human axl oncogenes disclosed herein, and particularly the human axl oncogenes disclosed herein as SEQ ID NO: 1 and SEQ ID NO: 3. This definition is intended to encompass natural allelic variations in the axl oncogene. Cloned genes of the present invention can be of any species of origin, including mouse, rat, rabbit, cat, porcine, and human, but are preferably of mammalian origin. Thus, DNA sequences which hybridize to DNA which encodes human axl and which code on expression for an axl receptor protein are also an aspect of this invention. Conditions which will permit other DNA sequences which code on expression for an axl receptor protein to hybridize to the DNA sequence of the human axl oncogenes disclosed herein can be determined in accordance with known techniques. For example, hybridization of such sequences may be carried out under conditions of reduced stringency, medium stringency or even stringent conditions (e.g., conditions represented by a wash stringency of 35-40% Formamide with 5x Denhardt's solution, 0.5% SDS and 1x SSPE at 37.degree. C.; conditions represented by a wash stringency of 40-45% Formamide with 5x Denhardt's solution, 0.5% SDS, and 1x SSPE at 42.degree. C.; and conditions represented by a wash stringency of 50% Formamide with 5x Denhardt's solution, 0.5% SDS and 1x SSPE at 42.degree. C., respectively, to DNA encoding the axl oncogene disclosed herein in a standard hybridization assay. See J. Sambrook et al., Molecular Cloning, A Laboratory Manual (2d Ed. 1989)(Cold Spring Harbor Laboratory)). In general, sequences which code for axl receptor protein and hybridize to the axl oncogenes disclosed herein will be at least 75% homologous, 85% homologous, and even 95% homologous or more with the sequence of the human axl oncogenes disclosed herein. Further, DNA sequences which code for axl receptor proteins, or sequences which code for a receptor protein coded for by a sequence which hybridizes to the DNA sequences which code for human axl oncogenes disclosed herein, but which differ in codon sequence from these due to the degeneracy of the genetic code, are also an aspect of this invention. The degeneracy of the genetic code, which allows different nucleic acid sequences to code for the same protein or peptide, is well known in the literature. See, e.g., U.S. Pat. No. 4,757,006, to Toole et al. at Col. 2, Table 1.

Knowledge of the axl nucleotide sequence as disclosed herein can be used to generate hybridization probes which specifically bind to the axl gene to determine the presence of axl gene amplification or overexpression. The hybridization probes may be cDNA fragments or oligonucleotides, and may be labelled with a detectable group as discussed hereinbelow. Pairs of probes which will serve as PCR primers for the axl oncogene or a portion thereof may be used in accordance with the process described in U.S. Pat. Nos. 4,683,202, and 4,683,195.

C. Axl Receptors and Proteins

As noted above, the present invention provides isolated and purified axl receptors, such as mammalian (or more preferably human) axl receptor proteins, and soluble extracellular fragments thereof which have axl receptor binding activity. Such receptors can be purified from host cells, as described above, which express the same, in accordance with known techniques, or even manufactured synthetically. Specific examples of such receptors include the protein having SEQ ID NO: 2, the protein having SEQ ID NO: 4, and the soluble extracellular fragments thereof.

Soluble mammalian axl receptors which exhibits axl receptor binding activity can be made by deleting the hydrophobic transmembrane region of the native axl receptor. With respect to SEQ ID NO: 1 herein (also referred to herein as axl.sup.+), the region coding for the transmembrane domain of the receptor protein is from nucleotide number 1500 to nucleotide number 1574. With respect to SEQ ID NO: 2 herein (also referred to herein as axl.sup.-), the region coding for the transmembrane domain of the receptor protein is from nucleotide number 1473 to nucleotide number 1547. By "deletion" is meant removal or alteration of a sufficient portion of the hydrophobic transmembrane domain to render the coded-for receptor soluble in an aqueous solution. In addition, soluble axl receptors can be made from the extracellular portion of a mammalian axl receptor protein as disclosed herein, with the extracellular portion comprising the portion coded for by the domain 5' to the transmembrane domain of the axl oncogene.

Soluble axl receptors may be used in a variety of ways. They may be used for therapeutic purposes, administered to block or stimulate the function of the in situ axl protein, along the same lines disclosed in PCT Application No. WO9015870 with respect to the IL-7 receptor. They may be provided in an aqueous solution, solubilzed therein, to screen for axl receptor binding ligands, or immobilized on a solid support such as dextrose or silica to provide an affinity column which can be used to bind axl receptor binding ligands. The soluble receptor can be labelled with a detectable group such as an alkaline phosphatase in the manner described by J. Flanagan and P. Leder, Cell 63, 185-194 (1990), to seek out a membrane-bound ligand for the axl receptor in an immunohistochemical procedure. Other detectable groups can be employed, and the term "labelled" is used herein to refer to the conjugating or covalent bonding of any suitable detectable group, including enzymes (e.g., horseradish peroxidase, .beta.-glucuronidase, alkaline phosphatase, and .beta.-D-galactosidase), fluorescent labels (e.g., fluorescein, luciferase), and radiolabels (e.g. .sup.14 C, .sup.131 I, .sup.3 H, .sup.32 P, and .sup.35 S) to the compound being labelled. Techniques for labelling various compounds, including proteins, peptides, and antibodies, are well known. See, e.g., Morrison, Methods in Enzymology 32b, 103 (1974); Syvanen et al., J. Biol. Chem. 284, 3762 (1973); Bolton and Hunter, Biochem. J. 133, 529 (1973).

The axl oncogenes disclosed herein can be used to construct chimeric proteins which are useful for, among other things, detecting the binding of axl receptor ligands. Such chimeric proteins can be made with a recombinant DNA construct which codes for the chimeric protein. The construct comprises (a) an an axl receptor extracellular portion having axl receptor binding activity operatively associated with (b) an effector portion capable of generating a detectable signal upon binding of a ligand to the axl receptor extracellular portion. Suitable effector portions are the enzymatic domain of a membrane bound kinase, such as the enzymatic portion of an epidermal growth factor receptor. The chimeric protein is expressed in a suitable host cell in accordance with the techniques described above so that, when an axl receptor ligand is contacted to the host cell (or a membrane fraction thereof), a detectable signal such as enzymatic activity is generated through activation of the effector portion.

Antibodies which specifically bind to the axl receptor (i.e., antibodies which bind to a single antigenic site or epitope on the axl receptor) are useful for a variety of diagnostic and therapeutic purposes. Such antibodies may be polyclonal or monoclonal in origin, but are preferably of monoclonal origin. The antibodies are preferably IgG antibodies of any suitable species, such as rat, rabbit, or horse, but are generally of mammalian origin. Fragments of IgG antibodies which retain the ability to specifically bind the axl receptor, such as F(ab').sub.2, F(ab'), and Fab fragments, are intended to be encompassed by the term "antibody" herein. The antibodies may be chimeric, as described by M. Walker et al., Molecular Immunol. 26, 403 (1989). Preferred are antibodies which specifically bind to the extracellular domain of the axl receptor.

Monoclonal antibodies which bind to axl are made by culturing a cell or cell line capable of producing the antibody under conditions suitable for the production of the antibody (e.g., by maintaining the cell line in HAT media), and then collecting the antibody from the culture (e.g., by precipitation, ion exchange chromatography, affinity chromatography, or the like). The antibodies may be generated in a hybridoma cell line in the widely used procedure described by G. Kohler and C. Milstein, Nature 256, 495 (1975), or may be generated with a recombinant vector in a suitable host cell such as Escherichia coli in the manner described by W. Huse et al., Generation of a Large Combinatorial Library of the Immunoglobulin Repertoire in Phage Lambda, Science 246, 1275 (1989).

An antibody as described above which is capable of inhibiting axl receptor function may be used for therapeutic purposes or in vitro diagnostic purposes, along the lines described in R. Hudziak et al., PCT Appln WO 89/06692. Inhibition can be achieved with an antibody which blocks receptor function (i.e., an antagonist), or with an antibody that stimulates receptor function (i.e., an agonist) through downregulation. Antibodies which specifically bind the extracellular domain of the axl receptor may be conjugated to thericin aA chain in the manner described by Vitetta et al., Science 238, 1098 (1987).

Diagnostic assays for detecting a tumor generally comprise the steps of, first, contacting cells to antibodies capable of specifically binding the axl receptor, particularly the extracellular domain of the axl receptor, and determining the extend of binding of said antibodies to said cells. The antibody is preferably labelled, as discussed above, to facilitate the detection of binding. Any suitable immunoassay procedure may be employed, such as radioimmunoassay, immunofluorescence, precipitation, agglutination, complement fixation, and enzyme-linked immunosorbent assay. When the cells to be tested remain within the body of a mammal, the antibodies are labelled with a radioactive detectable group and administered to the mammal, and the extend of binding of the antibodies to the cells is observed by external scanning for radioactivity. Further, cells may be screened with a set of two different antibodies, one member of the set selectively binding axl.sup.- receptors and the other member of the set selectively binding axl.sup.+ receptors, to diagnose the differential levels of expression of these two receptors. As discussed above, while any type of antibody may be employed for the foregoing diagnostic purposes, monoclonal antibodies are preferred.

The examples set forth below are for illustrative purposes only, and are not to be taken as limiting of the present invention.

Amino acid sequences disclosed herein are presented in the amino to carboxy direction, from left to right. The amino and carboxy groups are not presented in the sequence.

Nucleotide sequences are presented herein by single strand only, in the 5' to 3' direction, from left to right.

I. MATERIALS AND METHODS

Identification of a transforming gene in CML cells. Transfections and nude mouse tumorigenicity assays were performed as described previously. See E. Liu et al., Proc. Natl Acad. Sci. U.S.A. 85:1952 (1988). The cell lines AF6295 and AF3642 were derived from secondary nude mouse tumors arising from transfection of DNA from blast crisis and chronic phase CML patients, respectively. Tumors were isolated from nude mice and digested in the presence of trypsin-EDTA. A portion of these cells was then plated in plastic tissue culture flasks.

Isolation of cosmids containing eDNA clones. DNA fragments for cosmid cloning were generated by partial MboI digests of genomic DNA from a secondary nude mouse tumor cell line, AF6295. The restricted DNA fragments were size selected on sodium chloride gradients and cloned in the c2RB cosmid vector as previously described. P. Bates, in Enzymology, vol 153. Academic Press, New York, N.Y. (1987). Following ligation, recombinants were packaged with Gigapack gold (Stratagens, LaJolla, Calif.) according to manufacturer's recommendations and used to infect E. coli strain W46-4. The cosmid library was screened using the Blur8 probe which contains human Alu repetitive sequences (W. Jelinek et al., Proc. Natl. Acad. Sci. U.S.A. 77:1398 (1980)) resulting in the identification of two Alu positive clones, 1-1 and 4-2.

Unique human exon fragments were identified by differential screening of Southern blots of cosmid clones 1-1 and 4-2. Poly-A.sup.+ RNA from both AF6295 and untransformed NIH/3T3 cells was obtained as previously described. See F. Ausubel et al., Current Protocols in Molecular Biology. Greene Publishing Associates and Wiley-Interscience, New York, N.Y. (1989). First strand cDNA reactions were performed in a 50 ml final volume of 0.6 mM dCTP, 1 mM each of dATT, dGTT, dTTP, 10 mM DTT, 20 U RNasin, 100 mg/ml random hexamers (Pharmacia, Piscataway, N.J.), 100 mCi .sup.32 P dCTP [3000 Ci/mM (Dupont/NEN)], 1x reverse transcriptass buffer (50 mM Tris, pH 8.3, 50 mM KCl, 10 mM MgCl.sub.2, 1 mM DTT, 1 mM EDTA, 100 mg/ml bovine serum albumin), 20 U AMV reverse transcriptase (Promega, Madison, Wis.) and 200 ng poly-A.sup.+ mRNA. The mixture was incubated at 42.degree. C. for 45 minutes and denatured at 95.degree. C. for 5 minutes. Unincorporated label was separated on a Sepharose G-50 column. The resulting probes were used for Southern hybridization (see, e.g., F. Ausubel et al., supra) using sheared and denatured mouse or human DNA at 100 mg/ml and salmon sperm DNA at 100 mg/ml for blocking. In addition, Southerns blots were also hybridized with Blur8 to identify fragments from the cosmid clones containing Alu repeat sequences. A single 2.8 kb EcoRI/BamHI fragment (pcc-1) in cosmid clone 1-1 was found to be free of Alu and mouse sequences while containing transcribed exons from the putative human transforming gene.

Production of cDNA libraries. cDNA libraries were constructed in .lambda.gt10 using poly-A.sup.+ RNA from AF6295. Using pcc-1 as probe, several cDNAs were identified. Sequence analysis of these cDNAs indicated these clones were short (approximately 1 kb in size), and did not contain poly-A.sup.+ tails. A full-length cDNA, clone 1-4, was then isolated from a size-selected, oligo-dT primed cDNA library constructed as described (U. Gubler and B. Hoffman, Gene 25:263 (1983)) except as follows: 4 mg of Poly A.sup.+ RNA was reverse transcribed using the Superscript M-MuLV (Bethesda Research Laboratories, Gaithersburg, Md.). After second strand synthesis, cDNA products were fractionated on a 0.8% agarose gel in 1xTAE [0.04M Tris-acetate/0.002M EDTA]. Products in the range of 2.7 to 6 kb were electrophoresed onto NA-45 paper (Schleicher and Schuell, Keene, N.H.) and then eluted with 2.times.250 ml Elution buffer [1M NaCl/50 mM arginine, free base]. The eluate was extracted three times with phenol:chloroform (1:1) and ethanol precipitated. cDNA products were ligated in EcoRI cut .lambda.gt10 arms (Stratagene, La Jolla, Calif.), packaged into phage heads with Gigapack Gold (Stratagene, La Jolla, Calif.) and then plated on C600Hfl E. coli. Recombinants were screened with an axl specific cDNA. Positive clones were subcloned into pBluescript II KS (Stratagene, La Jolla, Calif.) for subsequent sequence analysis.

Normal axl cDNA clones were isolated as described above using a size-selected cDNA library constructed from poly-A+ RNA from the normal human diploid fibroblast cell IMR-90. Recombinant phage were screened using a PCR probe generated from clone 1-4 DNA using primers PJA-14 (bp 2090-2109), GCCCACTCAGATGCTAGTGA and PJA-17 (bp 2569-2588) CAAGGCCTTCAGTGTGTTCT. Positive clones were subcloned into pBluescript II KS for subsequent sequence analysis.

DNA sequencing. Complete sequence from both strands was determined by the dideoxy termination method (F. Sanger et al., Proc. Natl. Acad. Sci. U.S.A. 74:5463 (1977)) using the modified T7 polymerase (Sequenase; United States Biochemical, Cleveland, Ohio). Overlapping fragments for sequence analysis were generated by digestion with exonuclease III (S. Henikoff, in R. Wu (ed) Methods in Enzymology, vol 155. (Academic Press, New York, N.Y. 1987)) or by priming with sequence-specific oligonucleotides. Sequences were compared with the BESTFIT DNA sequence analysis program [University of Wisconsin Genetics Computer Group (J. Devereux et al., Nucl. Acids Res. 12:387 (1984))].

RMA-POR analysis of axl expression. RNA (100 ng of mRNA or 1 mg of total) was denatured at 95.degree. C. for 3-5 minutes in the presence of 100 pmol random hexamers (Pharmacia) and cooled on ice for 2 minutes. The RNA was reverse transcribed in a 20 ml final volume of 1x PCR buffer [10 mM Tris, pH 8.3, 50 mM KCl, 1.5 mM MgCl.sub.2, 0.001% gelatin], 625 mM of each dNTP, 20 U RNasin (Promega), 10 mM DTT and 200 U M-MuLV Superscript reverse transcriptase (BRL). The reaction was incubated 10 minutes at room temperature, 45 minutes at 42.degree. C. and 5 minutes at 95.degree. C. One microliter of cDNA product was PCR amplified in a 50 ml final volume of 10 mM Tris, pH 8.3, 50 mM KCl, 1.5 mM MgCl.sub.2, 200 mM of each dNTP 0.001% gelatin, 0.5 mM each primer and 1.25 U Taq polymerase (Promega) for 30 cycles under the following conditions: 5 minute at 94.degree. C. 1 minute 55.degree. C. 1.5 minutes at 72.degree. C. for 1 cycle; 1 minute 94.degree. C. 1 minute 55.degree. C. 1.5 minutes 72.degree. C. for 28 cycles; 1 minute 94.degree. C. 10 minutes 60.degree. C. Differential PCR (A. Neubauer et al., Nucl. Acids Res. 18:993 (1990)) was performed using .alpha.-actin as a reference. Ten percent of this reaction was electrophoresed on a 12% polyacrylamide gel and stained with ethidium bromide. Primer sequences are as follows: PJA-2 (bp 1847-1866), GGTGGCTGTGAAGACGATGA; PJA-3 (bp 2149-2130), CTCAGATACTCCATGCCACT; PJA-8 (bp 1320-1339), GAGGTGACCCTGGAGCTGCA; JB-2 (bp 1490-1471), AGGAGTTGAAGGTCCCTTCA; 5' .alpha.-actin, CCTTCCTGGGCATGGAGTCCT (SEQ ID NO: 5); 3' .alpha.-actin, GGAGCAATGATCTTGATCTTC (SEQ ID NO: 6).

Chromosomal localization and fluorescence in situ chromosomal hybridization. A Southern blot of DNA from human/hamster somatic cell hybrids (BIOS, New Haven, Conn.) was probed with random-primer labelled cDNA clones 12-1, 1-4 and genomic clone pcc-1 under conditions recommended by the manufacturer. The filter was washed twice in 2x SSC, 0.5% SDS for 10 minutes at room temperature, once in 1x SSC, 1.0% SDS for 15 minutes at 65.degree. C. twice in 0.1xSSC, 1.0% SDS for 15 minutes at 65.degree. C. and exposed overnight at -70.degree. C.

For in situ hybridization, human metaphase cells were prepared from phytohemagglutinin-stimulated peripheral blood lymphocytes. The axl probe was cosmid 1-1 containing a 41 kb genomic fragment cloned in the cosmid vector c2RB. The procedure used for fluorescence in situ hybridization is a modification of the method described by P. Lichter et al., Human Genet. 80:224 (1988). Biotin-labelled probes were prepared by nick-translation using Bio-11-dUTP (Enzo Diagnostics, New York, N.Y.). Chromosomal DNA was denatured by immersion of the slides in 70% formamide-4xSSC, pH 7.0 at 70.degree. C. for two minutes. Prior to hybridization, unlabelled, sonicated, total human genomic DNA (150 mg/ml) was added to the hybridization mixture (50% formamide, 1xSSC, 10% dextran sulfate, pH 7.0) containing the labelled probe (50-100 ng). The hybridization mixture was heated at 75.degree. C. for 5 minutes, then incubated at 37.degree. C. for 5-10 minutes to promote partial reannealing. After hybridization, the slides were washed in 50 % formamide-4xSSC (3 washes, 5 minutes each) at 40.degree. C. followed by three washes in 4xSSC at 40.degree. C. (3 minutes each).

Detection reagents were prepared in 4xSSC, 0.1% TritonX-100, 1% BSA and the washes were performed in 4xSSC, 0.1% TritonX-100 at 40.degree. C. (3 washes, 3 minutes each). For the detection of the labelled probe, slides were incubated with 5 mg/ml fluorescein isothiocyanate (FITC)-conjugated avidin at 37.degree. C. for 30 minutes (Vector Laboratories, Burlingame, Calif.), followed by washes. Metaphase cells were counterstained with 4,6-diamidino-2-phenylindole-dihydrochloride (DAPI, Sigma, St. Louis, Mo.; 200 ng/ml in 2xSSC, 5 minutes, room temperature) and mounted in 20 mM Tris-HCl, pH 8.0, 90% glycerol containing 2.3% DABCO antifade (Sigma). The slides were examined using a Zeiss microscope equipped with epifluorescence optics.

Hybridization analysis. Northern and Southern analysis was performed according to standard protocols (see F. Ausubel et al., supra). RNAs were isolated from cell lines by the methods of Glisin et. al. (Biochemistry 13:2633 (1974)) or Chomczynski and Sacchi (Anal. Biochem. 162:156 (1987)). Radiolabelled probes were generated by the random priming technique or PCR (A. Feinberg and B. Vogelstein, Anal. Biochem. 132:6 (1983); R. Jansen and F. Ledley, Gene. Anal. Techn. 6:79 (1989)) using primers PJA-2 and PJA-3 (see above); PJA-8 (see above) and PJA-9 (bp 1890-1871), CTGACCTCGTGCAGATGGCA; PJA-11 (bp 646-627), TGAGCTTGGCAGCTCAGGTT and PJA-12 (bp 248-267), GCAGGCTGAAGAAAGTCCCT; PJA-14 (bp 2090-2109), GCCCACTCAGATGCTAGTGA and PJA-17 (bp 2569-2588), CAAGGCCTTCAGTGTGTTCT.

Transfection and tumorigenicity assays A 100 mm dish of NIH/3T3 cells containing approximately 5.times.10.sup.5 cells was fed with fresh media 4-24 hours prior to transfection. Five micrograms of plasmid DNA was precipitated with 3M sodium acetate in the presence of 30 mg high molecular weight salmon sperm DNA in a 100 ml volume. The resulting precipitate was resuspended in 450 ml H.sub.2 O and incubated overnight at 37.degree. C. with gentle rocking. On the following day, 50 ml 2.5M CaCl.sub.2 was added dropwise while bubbling air through a plugged pipette. This solution was then added dropwise to 500 ml of 2xHBS (280 mM NaCl, 50 mM HEPES, 1.5 mM Na.sub.2 HPO.sub.4. 12H.sub.2 O, pH 7.05) while bubbling air through a plugged pipette. This solution was allowed to stand for 20-30 minutes at room temperature without agitation. The fine precipitate was then exposed to 3T3 cells for 8 hours at 37.degree. C. The media was removed and 2 mls 15% glycerol (v/v) in phosphate buffered saline (PBS) was added to cells for 90 seconds. Cells were washed with PBS and fed with fresh media. After 24-48 hours, cells were split 1:4 into 100 mm dishes containing 600 mg/ml G418 (total drug). Emergent colonies were then pooled to produce the non-clonal cell lines used in subsequent analysis (TF14A & TF14B-transfected with axl.sup.+ sense construct; TF15A & TF15B-transfected with axl.sup.+ anti-sense construct; TF16A & TF16B-transfected with pLXSN vector).

For analysis of transformation, half of the pooled G418 resistant cells were passaged once, grown to confluence and scored for focus formation after 2-3 weeks. The remainder of these cells were expanded and injected into the flanks of athymic nude mice at concentrations of 1.times.10.sup.5 and 5.times.10.sup.5 cells per site. Tumor formation was scored 50 days after injection.

Baculovirus expression. The baculovirus transfer vector, pBlueBac (Invitrogen, San Diego, Calif.) contains a .beta.-galactosidase expression cassette which permits recombinant plaque screening by addition of the chromagenic substrate X-gal (5-bromo-4-chloro-3-indolyl-.beta.-D-galactoside; 0.15 mg/ml) to the agarose overlay. The cDNA insertion site of pBlueBac is an Nhel site. An axl cDNA segment with full-length coding sequence and Nhel compatible ends was produced by cutting the axl cDNA clone 1-4 at the translation start site with Ncol. The resulting 3' overhangs were filled in using the Klenow fragment of DNA pol I plus dNTPs. After heat inactivation, the fragment was digested with EcoR1, ligated into Smal/EcoR1 digested pBluescriptII-KS plasmid and then used to transform E. coli. The resulting plasmid contains an Spel site 12 bp upstream of the axl ATG start site and an Xbal site 265 bp downstream of the translation stop site. This Spel-Xbal fragment was cloned into Nhel-digested pBlueBac. The resulting axl transfer vector and wild type (WT) AcNPV DNA were co-transfected into Sf9 cells (M. Summers et al., in Bulletin No. 1555. Texas Agriculture Experiment Station and Texas A&M University, College Station, Tex. (1987)). Subsequent plaque selection yielded the recombinant virus axl-B1Bac. Positives were evaluated for the presence of axl sequences with PCR using primer pairs PJA-5 (bp 951-970), TCAGACGATGGGATGGGCAT and PJA-6 (bp 1006-1025), CACGGATGCTTGCGAGGTGA; or PJA19 (bp 2755-2774), ATGTCCTCTGCCCTTCCACA; and PJ-7 (bp 2827-2843), TCAGGCACCATCCTCCT.

Immunoblot analysis of axl expression. For analysis of baculovirus expression, Sf9 cells were grown at 27.degree. C. in monolayers (2.times.10.sup.6 cells/60 mm dish) in TMN-FH media (Invitrogen) and infected with WT or axl-B1Bac virus at a multiplicity of infection greater than 10. After 1, 2, and 3 days the dishes were placed on ice and the cell monolayers were rapidly rinsed with PBS, lysed and then scraped in 0.3 ml of RIPA buffer (50 mM Tris-HCl pH 7.4, 150 mM NaCl, 5 mM EDTA, 1% Triton-X 100, 1% sodium deoxycholate, 0.1% sodium dodecylsulfate (SDS), 100 mM Na.sub.3 VO.sub.4, 0.5 mM PMSF, and 2.5 mM p-nitrophenylphosphate). After addition of 0.15 ml of 3x Laemmli gel sample loading buffer (30 mM Tris-HCl, pH 7.8, 9% SDS,15% glycerol, 6% 2-mercaptoethanol and 0.05% bromophenol blue) and boiling 5 minutes at 100.degree. C. 50 ml aliquots of the samples were electrophoresed on 9% SDS-polyacrylamide gels. The proteins were electrotransfered from the gel onto PVDF membrane (Immobilon-P, Millipore, Bedford, Mass.). The membrane was treated for 30 min with 150 mM NaCl, 10 mM Tris-HCl pH 7.5, 0.05% Tween-20 (TBST) containing 1% bovine serum albumin. The blot was then incubated for 1 hr with anti-phosphotyrosine monoclonal antibody (PT-66; Sigma) diluted 1:2000 in TBST. Using a duplicate blot, 1 mM phenylphosphate was included with the anti-phosphotyrosine antibody to block specific binding to phosphotyrosine. The blot was rinsed three times for 5 minute with TBST and then incubated with antimouse IgG-alkaline phosphatase conjugate (A-5153; Sigma) diluted 1:2000 in TBST for 45 min. After 3 rinses in TBST, the blot was incubated with the chromagenic substrates (0.33 mg/ml NBT, 0,165 mg/ml BCIP) in alkaline buffer (100 mM Tris pH 9.5, 100 mMNaCl, 10 mM MgCl.sub.2) for 5-20 min. Color development was stopped by rinsing with deionized water. Analysis of tyrosine phosphorylated proteins in the axl-transformed 3T3 cells was performed according to previous protocols (B. McCune and H. Earp, J. Biol. Chem. 264:15501 (1989)).

The polyclonal anti-axl antiserum was prepared by immunizing rabbits with gel-purified baculovirus-expressed axl protein. The antigen in the form of Coomassie-stained gel slice was processed and administered in primary and (after one month) boost injections (E. Harlow and D. Lane, Antibodies: a laboratory manual (Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y. 1988)). The anti-axl serum was collected 25 days following the boost injection. Immunoblot analysis was performed with the anti-axl antiserum and pre-immune serum diluted 1:3000 in TBST. Anti-rabbit IgG alkaline phosphatase conjugate was used as the secondary antibody.

II. RESULTS

Cloning of the transforming axl gene. In our previous study, using a sensitive NIH/3T3 transfection/tumorigenicity assay, we identified a transforming gene in the DNA from the peripheral leukocytes of two CML patients that lacked ras mutations (E. Liu, supra). The resulting tumors arising from secondary transfection of tumor DNA gave rise to two explant tumor cell lines (AF6295 and AF3642) which harbored the same unique transforming human sequences. Several cosmid clones bearing human Alu repeats were isolated from a cosmid library derived from the DNA of one such explant tumor cell line, AF6295 (see Materials and Methods). Exon mapping identified a genomic fragment, pcc-1, that contained transcribed human sequences but was devoid of Alu repeats and mouse exons. Using pcc-1 as a probe on Northern blots, two axl mRNA species of 5.0 and 3.4 kb were identified in the explant 3T3 cell lines from the CML transfectants, but not in untransformed NIH/3T3 cells or in 3T3 cells transformed by a mutant H-ras oncogene (data not shown). Using pcc-1 as a probe, several axl-specific cDNA clones were isolated from cDNA libraries derived from AF6295 poly-A.sup.+ RNA. All hybridized to the same 5.0 and 3.4 kb transcripts in the nude mouse tumor explant cell lines on Northern blot analysis. Sequence analysis of these cDNAs showed sequences in common with pcc-1. Since the original cDNA clones were short and did not contain poly-A.sup.+ tails, a size-selected cDNA library was screened in order to identify full-length cDNAs. Several clones were isolated and found to contain 3.2 kb inserts consistent in size with one of the two axl mRNA species. One of these clones, 1-4, was subcloned and sequenced.

Sequence analysis of axl indicates it is a novel receptor tyrosine kinass. The sequence of 1-4 (SEQ ID NO: 1) predicted a protein of 894 amino acids (SEQ ID NO: 2) with two in-frame methionine residues that are candidate initiating amino acids based on Kozak consensus rules (M. Kozak, Nucleic Acid Res. 15:8125 (1987)). Furthermore, an in-frame termination codon lies 170 bp upstream from the first methionine codon. This methionine marks the beginning of a potential signal sequence of 32 amino acids See G. von Heijne, J. Mol. Biol. 173:243 (1984). A search of the EMBL, Genbank, and NBRF Protein databases indicated that axl is a novel protein tyrosine kinass with homology to many known kinases in particular eph, eck, elk, ros, trk, insulin receptor, insulin-related receptor (IRR), IGF-1 receptor, and sevenless. See P. Bates, in Enzymology, vol 153. Academic Press, New York, N.Y. (1987); Y. Ebina et al., Cell 40:747-758; H. Hirai et al., Science 238:1717 (1987); V. Lhotak et al. Mol. Cell. Bio. 11:2496 (1991); R. Lindberg and T. Hunter, Mol. Cell. Biol 10:6316 (1990); D. Martin-Zanca et al., Mol. Cell. Bio. 9:24 (1989); H. Matsushime et al., Mol. Cell. Biol. 6:3000 (1986); P. Shier et al., J. Biol. Chem 264:14605 (1989); A. Ullrich et al., EMBO-J. 5:2503 (1986). The predicted peptide at amino acids 543-569 contained the consensus sequence GxGxxG . . . (15-20 a.a) . . . AxKxM which functions as the Mg.sup.+2 -ATP binding site for tyrosine kinases. H. Hanks et al., Science 241:42 (1988). At approximately 100 amino acids carboxyl-terminal to the Mg.sup.+2 -ATP binding domain in axl lies the sequence DLAARN . . . (34 a.a) . . . PVKWIAIE which resembles, but is distinct from a consensus sequence specific to tyrosine kinases, DLAARN . . . P(I/V) (K/R)W(T/M)APE. H. Hanks et al., supra. Furthermore, there are two sites at amino acids 779 and 821 in the carboxyl-terminus of axl that are similar to the consensus sequence [(E/D)(E/D)(E/D) (E/D)(E/D)Y(M/V)PMXX, where Y=phosphotyrosine] for a phoshphatidylinositol 3-kinase (PtdIns3-kinase) binding site. L. Cantley et al., Cell 64:281 (1991). These observations suggest that axl is a tyrosine kinase that interacts intracellularly with PtdIns 3-kinase.

The amino acids 448 to 472 in axl comprise a hydrophobic region consistent with a transmembrane domain. Within the putative extracellular domain, sequences (a.a. 224-428) amino-terminal to the transmembrane region of axl encode two fibronectin type III (FNIII) repeats. T. Peterson et al., Proc. Natl. Acad. Sci. U.S.A. 80:137 (1983); K. Skorstengaard et al., Eur. J. Biochem. 161:441 (1986). Amino terminal to the FNIII repeats in axl (a.a. 37-212) are two immunoglobulin-like (IgL) repeats. A. Williams and A. Barclay, Ann. Rev. Immunol. 6:381 (1988). Highly conserved cysteine and tryptophan residues are characteristic of IgL domains. In addition, the extracellular domain of axl contains six consensus sites for N-linked glycosylation, NxT/S, predicting that the mature axl protein is glycosylated. The juxtaposition of IgL and FNIII repeats in the extracellular region of axl has not been reported for any known receptor tyrosine kinase, thus making axl unique amongst this class of proteins.

Biologic activity of axl. Using cDNA clone 7-1 as a probe for Southern hybridization with nude mouse tumor DNAs, we detected human axl sequences in the appropriate primary and secondary tumors indicating that axl was the transforming gene passaged with each cycle of transfection (data not shown).

To confirm axl's transforming potential, we constructed retroviral expression vectors containing the full-length axl cDNA (clone 1-4). In these constructs, transcription of axl is under the control of an MoMSV long terminal repeat whereas the neomycin phosphotransferase gene, which confers resistance to the drug G418, is driven by an SV40 promoter. A. Miller and G. Rosman, BioTechniques 7:980 (1989). NIH/3T3 cells were transfected with the axl sense (pL1-4S) or anti-sense (pL1-4AS) constructs or vector alone (pLXSN) and then assayed for transformation in a standard focus formation assay (i.e., without G418 selection). No foci were observed in the focus formation assay. Furthermore, colonies arising after G418 selection did not appear transformed. However, when the G418 resistant colonies were passaged once and allowed to reach confluence, foci emerged in the cells transfected with the pL1-4S (axl.sup.+ sense construct) but not the pL1-4AS (axl.sup.+ anti-sense construct) or the pLXSN (vector). This suggests that axl alone is neccessary but insufficient for transformation in this system and that either a second genetic event or selection for cells overexpressing axl is required for axl to be transforming. Northern analysis was performed on RNA isolated from the initial pooled G418 resistant cells (cell lines TF14A, TF14B, TF15A, TF15B, TF16A and TF16B) and transformed foci cloned from cells transfected with the sense axl cDNA (subclones TF14B1, 14B2, 14B3, 14B7, and 14B10). Expression of the human axl gene was seen in cells transfected by sense (TF14A&B) and anti-sense (TF15A&B) constructs albeit at low levels. However, the axl transformed subclones expressed high levels of axl RNA (data not shown). Thus, the second event in axl transformation appears to be a selection for cells expressing increased levels of axl message indicating that overexpression of axl is necessary for 3T3 transformation.

We next tested the tumorigenic capacity of cells transfected with the genomic axl gene (AF6295) or axl cDNA (clones TF14B1, 14B2, 14B3, 14B7 and 14B10). Injection of either AF6295 or clones TF14B1, TF14B2, TF14B3, TF14B7, or TF14B10, all of which express high levels of axl message, resulted in the emergence of tumors within 9 days. There appeared to be a rough correlation between a short tumor latency with increased expression of axl RNA and protein further supporting the notion that axl overexpression drives 3T3 transformation. In contrast, the initial axl transfectants (TF14A, TF14B, TF15A, TF15B), vector-transfected NIH/3T3 cells and untransformed NIH/3T3 cells did not form tumors within an incubation period of 50 days except for one of the clones transfected with vector alone (TF16A). This clone formed a tumor after 43 days and therefore represents background tumor formation.

Tyrosine kinase activity of axl. To test whether axl is a tyrosine kinass, we analyzed the phosphotyrosine profile of cells transformed by either genomic axl or an axl cDNA. Using an anti-phosphotyrosine antibody, a novel 140 kd tyrosine phosphorylated protein was seen only in cells transfected with axl and not untransformed or ras-transformed NIH/3T3 cells (data not shown). Furthermore, the level of pp140 correlated with the level of axl RNA in these cells.

To further test the tyrosine kinase activity of axl, recombinant axl protein was expressed in Sf9 insect cells using the baculovirus expression system. In a Coomassie stained polyacrylamide gel of cellular proteins, a novel protein of approximately 104 kd was induced in cells infected with a recombinant baculovirus containing axl cDNA (1-4) which was not present in wild-type infected or uninfected cells. The molecular weight of this recombinant protein corresponds roughly to the predicted size (95 kd) of the unglycosylated axl protein minus the signal peptide as deduced from the amino acid sequence using the PEPTIDESORT program [Genetics Computer Group of the University of Wisconsin (J. Devereux et al., Nucl. Acids Res. 12:387 (1984))]. Using an anti-phosphotyrosine monoclonal antibody in Western blot analysis, we detected a similar 104 kd protein in axl baculovirus-infected cells but not wild-type baculovirus infected or uninfected cells (FIG. 6B). In addition, we also detected a 120 kd protein which we suspect represents a partially glycosylated form of axl.

Cloning and characterization of c-axl. Many oncogenes have been shown to be activated by genomic alterations. We accordingly investigated whether rearrangements or deletions of axl were present in the original patient DNAs from which axl was isolated as compared to DNAs from normal peripheral leukocytes and from the nude mouse tumor lines harboring the human axl oncogene. Using probes corresponding to the kinase, transmembrane, and extracellular domains of the cloned axl cDNA in Southern blot analyses, no rearrangements of axl were detected in either the DNAs from the original CML patients or the nude mouse tumors as compared to DNAs from normal blood (data not shown). Furthermore, Northern blot analysis of axl transcripts from AF6295 and several normal and transformed cell types which also express axl mRNA (A549, A431, IMR-90, HeLa and BG-9) revealed the identical 3.4 and 5.0 kb transcripts making rearrangements and major deletions unlikely.

In order to determine the mechanism by which axl was activated, we cloned cDNAs of the normal axl homologue (c-axl). Using RNA from the normal human fibroblast IMR-90, we cloned a normal axl cDNA (clone 4-9) which appears identical to the transforming clone, 1-4. However, another cDNA clone isolated from the normal fibroblast library, 6-2 (SEQ ID NO: 3), predicted an altered version of axl (SEQ ID NO: 4) which lacked amino acids 429-437 (GQAQPVHQL) and contained a G to A transition at base 1170 resulting in the substitution of a leucine for a glutamic acid. We will refer to the predicted axl transcript from 1-4 as "axl.sup.+ " and that from 6-2 as "axl.sup.- ". Reverse polymerase chain reaction analysis of RNA with oligonucleotide primers flanking amino acids 429-437 showed both species of axl message present in the nude mouse tumor cell lines as well as other human cell lines and a primary human fibroblast suggesting that axl.sup.+ and axl.sup.- are splice variants from the same gene. However, the "axl.sup.+ " species was overexpressed relative to the "axl.sup.- " species at an average ratio of 17:1 in the nude mouse tumor cells lines as compared to normal human fibroblasts which exhibited a ratio of 2:1.

Transforming capacity of axl.sup.-. To test the transforming capacity of the axl.sup.- cDNA, we expressed clone 6-2 in NIH/3T3 cells using the pLXSN retroviral expression vector. Using the same assay as with 1-4, expression of 6-2 also resulted in the emegence of foci after a single passage of G418 resistant colonies. These data suggest that the activation of axl's transforming capacity is due to overexpression and not to structural changes.

Expression of axl. Using both Northern blot and differential polymerase chain reaction (PCR) analysis on cDNA (F. Ausubel et al., supra, A. Neubauer et al., Nucl. Acids Res. 18:993 (1990)), variable expression of axl was seen in most human cell lines tested including cell lines of epithelial, mesenchymal and hematopoietic origins (Table 1).

In addition, axl was expressed in normal, non-transformed cells including primary fibroblast and breast epithelium. The near ubiquitous expression of axl suggests an important normal cellular function for this receptor kinase.

Chromosomal localization of axl. Various cDNA fragments of axl were hybridized to a Southern blot of DNAs from human/hamster somatic cell hybrids (BIOS, New Haven, Conn.) to determine the chromosomal position of axl. All results indicated that axl is located on human chromosome 19. To determine the location of the axl gene using an independent method, we used fluorescence in situ chromosomal hybridization of a biotin-labelled axl cosmid clone, 1-1, to normal human metaphase chromosomes. Of 25 cells examined, specific labelling was observed on one (4 cells), two (4 cells), three (11 cells) or all four (6 cells) chromatids of chromosome 19 homologues; the signal was localized to the long arm of this chromosome, at band q13.2. Similar results were obtained in a second hybridization experiment using this probe.

The foregoing examples are illustrative of the present invention, and are not to be construed as limiting thereof. The invention is defined by the following claims, with equivalents of the claims to be included therein.