while the specification concludes with claims particularly pointing out and distinctly claiming the subject matter regarded as forming the present invention , it is believed that the invention will be better understood from the following preferred embodiments of the invention taken in conjunction with the accompanying drawings . fig1 in two parts , a and b , shows the histological analysis of fgf - 9 null mouse embryos compared to littermate control ; both at 14 . 5 days of gestation . l , lung ; a , atria ; v , ventricle . top line , the full length fgf receptor with three ig - like domains . the major alternative splicing pathways will express either ig - like domain iiib or iiic . the stippled region beginning in ig - like domain iii is the sequence subject to alternative splicing . middle line , short form of the fgf receptor expressing ig - like domains ii and iii . bottom line , secreted form of the fgf receptor expressing ig - like domains ii and iiia . fig3 is a bar chart which shows the binding of 125 i - labeled fgf - 9 or fgf - 1 to soluble fgf receptors . in order to further illustrate the invention , the following detailed examples are disclosed although it will be appreciated that the invention is not limited to these specific examples or the details therein . fgf - 9 and fgfr3 are co - expressed in brain and kidney . the detailed expression pattern of fgf - 9 in other tissues is not known . in vitro , fgf - 9 is a ligand for fgfr2 and fgfr3 . these preliminary observations suggest that in vivo , fgf - 9 is also a physiological ligand for these receptors . to evaluate the developmental role of fgf - 9 , the fgf - 9 gene was disrupted in mice . fgf - 9 -/- mice are used to determine whether the activity of fgfr2 or fgfr3 is affected in tissues that co - express r2 + r3 or r + r9 proteins . mice containing a β - galactosidase gene inserted into the fgf - 9 locus can be used to identify sites of fgf - 9 expression . the phenotypes in fgf - 9 -/- mice are distinguished from those seen in fgfr3 -/- mice . these distinct phenotypes indicate that physiologically fgf - 9 is signaling through another fgfr , believed to be fgfr2 . several preliminary studies have been pursued prior to the knockout of fgf - 9 . these studies as set forth below define the fgf - 9 gene structure ( c ) and chromosomal location ( a ) and suggest that no known mouse mutant for fgf - 9 exists . ( a ) chromosomal mapping of fgf - 9 in the mouse . before proceeding with the knockout of fgf - 9 , the chromosomal location of fgf - 9 was determined to rule out any known candidate mouse mutations . by comparing regions of synteny between mouse and human chromosomes one can also estimate the map position of human fgf - 9 . this helps to identify human genetic diseases that may involve the fgf - 9 gene . the fgf - 9 locus was genetically mapped using recombinant inbred ( ri ) mouse strains [ 27 ]. using the partial murine fgf - 9 cdna as a probe , taq i restriction fragment length polymorphisms ( rflps ) have been identified between the c57bl / 6j and dba strains of mice . dna from 26 bxd ri strains , obtained from the jackson laboratory , bar harbor , me ., were cut with the restriction enzyme taq i and probed with the fgf - 9 cdna . the data obtained from this analysis localized fgf - 9 to mouse chromosome 14 at the ctla - 6 locus . this region of chromosome 14 is interesting because it contains two other loci involving possible developmental phenotypes : ds ( disorganized ) and tg737 ( resulting in polycystic kidney disease ) [ 28 , 29 ]. to more precisely map the fgf - 9 gene in this region , dna from an interspecific backcross between m . musculus and c3h has been similarly analyzed . fgf - 9 shows no recombinants with tf737 indicating that these genes are within 1 cm of each other . however , it is believed that fgf - 9 and tg737 are unlikely to be allelic . no candidate human diseases currently map to the syntenic region on human chromosome 13 . ( b ) expression patterns of fgf - 9 during development and in the adult ( specific result 1 ). rnase protection analysis of embryonic rna demonstrates that fgf - 9 is expressed in day 12 . 5 mouse embryos . it is also expressed in adult brain and kidney . preliminary in situ hybridization experiments were done using partial cdna probes . rabbit polyclonal antibodies were made against biologically active recombinant fgf - 9 ( recombinant protein was sent to caltag , inc . for injection into rabbits ). these antibodies can be used to immunolocalize fgf - 9 in both embryonic and adult tissues . anti - peptide antibodies also were made and tested . a second targeting construct ( see below ) was made in which the β - galactosidase gene was inserted into the first exon of the fgf - 9 gene . this allows chimeric mice , or mice which have transmitted the mutant allele through the germline , to be analyzed for β - galactosidase expression patterns . this methodology has been used to study the patterns of several genes including the fgf - 3 / int - 2 gene [ 23 , 30 ]. ( c ) fgf - 9 gene structure . a p1 containing the fgf - 9 gene was obtained from a 129 / sv library . dna fragments have been subcloned , and three exons coding for the mature fgf - 9 protein have been mapped . these clones have been used to construct gene - targeting vectors . ( d ) generation of mice lacking fgf - 9 ( specific result 2 ). to assess the role of fgf - 9 in development and to determine whether it interacts with fgfr3 in a genetically defined pathway , a null allele for fgf - 9 was introduced into the mouse germline using homologous recombination in embryonic stem cells . fgf - 9 is targeted by making a deletion including the 3 &# 39 ; half of exon 1 . successful targeting of this exon , which contains the initiation methionine , resulted in a null allele for fgf - 9 . a targeting construct containing a pol - 2 neo selection cassette [ 31 ], and two herpes virus thymidine kinase genes has been constructed . two electroporations were carried out . the first electroporation yielded no homologous recombinants out of 240 colonies screened . the second electroporation has yielded four targeted cell lines out of 200 colonies screened . these cells were introduced into mouse blastocysts to generate chimeras , which were bred to determine germline transmission . as discussed above , in vitro studies show that fgf - 9 can activate both fgfr2c and fgfr3c . because these receptors appear to be co - expressed in developing glia , fgf - 9 null mice and fgf - 9 / fgfr3 double null mice may allow a role of fgf signaling in glial development to be uncovered . similarly , fgf - 9 / fgfr3 double mutants may uncover a kidney phenotype even when no phenotype is seen in the single knockout animals . the methodologies for these procedures , though difficult , are well established [ 32 , 33 ]. in our laboratory we have successfully targeted the fgfr3 gene . the initial analysis of fgf - 9 recombinant embryonic stem cell clones uses dna blot hybridizations . after germline transmission of the fgf - 9 targeted allele is achieved , heterozygous mice are bred to homozygosity and the offspring examined for a phenotype . targeting a β - galactosidase gene to the fgf - 9 locus . to determine in situ where and when fgf - 9 is expressed , a β - galactosidase gene was inserted into the fgf - 9 genomic locus . like the original fgf - 9 targeting construct , the β - galactosidase / fgf - 9 &# 39 ; construct generates a 0 . 5 kb deletion beginning 50 bp downstream of the atg in exon 1 and extending into intron sequence ; the construct contains 5 kb of 5 &# 39 ; homologous sequence , 3 . 2 kb of 3 &# 39 ; homologous sequence and 2 thymidine kinase genes for negative selection . in the new construct , a β - galactosidase expression cassette is inserted in frame with fgf - 9 sequence starting 50 bp from the atg in exon 1 . a pol - 2 neo selection cassette flanked by lox p sites [ 34 ] is inserted 3 &# 39 ; of β - galactosidase . sm - 1 embryonic stem cells were transfected with this targeting vector , selected in the presence of geneticin ( antibiotic g418 ) and fiau [ 1 -( 2 &# 39 ;- deoxy - 2 &# 39 ;- fluoro - β - d - arabinofuranosyl )- 5 - iodouracil ], and screened for homologous recombination as described above . the lox - p sites allow the optional removal of the pol - 2 cassette via expression of crerecombinase either in embryonic stem cells or by pronuclear injection into heterozygous oocytes [ 35 , 37 ] or by mating . this allows analysis of β - galactosidase expression driven by the endogenous fgf - 9 promoter without adjacent pol - 2 promoter elements which may alter β - galactosidase expression . ( e ) determine sequence motifs in fgf receptor 3 that allow fgf - 9 binding ( specific result 3 ). iodinated fgf - 9 binds soluble fgfr3 ectodomain but not soluble fgfr1 ectodomain ( fig3 ). to map the region ( s ) in the fgfr binding domain that is required for ligand binding specificity , chimeric soluble receptors are constructed and assayed for fgf - 9 binding activity . pilot experiments indicate that sequence elements within ig - like domain 2 may confer fgf - 9 binding specificity to fgfr3 . this is in contrast to fgf - 2 and 7 in which specificity is determined by sequences within ig - like domain iii . the knowledge gained from these experiments facilitates the development of reagents that specifically can mimic or antagonize an fgf ligand . this type of structure - function information is useful in designing drugs that modulate fgf - fgf receptor binding and in understanding their mechanism of action . the specificity of such drugs allows diseases that involve fgfs to be treated with minimal side effects . the experimental approaches to investigate the primary sequence requirements for ligand binding specificity involve deletion mutagenesis , construction of chimeric receptors and point mutagenesis . the following set of experiments use soluble receptors in which the extracellular region is fused in frame to alkaline phosphatase . the soluble fgf receptor - ap binding assay [ 3 , 39 ] is used for screening purposes , because of the ease in expression the soluble receptor in cos - 7 cells and in performing the binding assay . informative mutant proteins identified in the primary screen are reconstructed into full - length receptors and are assayed for biologic activity in growth factor - dependent lymphoid cells . deletion mutagensis . amino terminal deletions of fgf receptor 1 and 3 remove ig - like domain i , the region between ig - like domain i and ii and ig - like domain ii ( fig1 ). this series of constructs tests the functional importance of ig - like domain i and the acidic amino acid domain and define a minimal functional binding domain . binding to these constructs is assayed with 125 i - fgf - 9 . chimeric receptor mutagensis . a series of constructs create chimeric proteins between extracellular regions of fgfrs 1 and 3 . specific constructs replace sequences from fgfr 1 with homologous sequences from fgfr 3 . these constructs show an increased affinity for fgf - 9 as fgfr 1 sequence is replaced by fgfr 3 sequence . assembly is by pcr , pcr mutagenesis and pcr ligation , and is confirmed by dna sequencing . chimeric scanning mutants . to compliment the above experiments and to define critical sequence elements required for ligand binding specificity , a series of chimeric scanning mutants are constructed . these chimeric proteins replace smaller regions of fgfr 1 with homologous sequence derived from fgfr 3 . when a short peptide sequence is responsible for ligand binding specificity , then a subset of this series of chimeric receptors shows an increased affinity for fgf - 9 . such regions are further analyzed by alanine scanning mutagenesis ( introduction of alanine point mutations ), a method that has been used successfully to define the binding properties of human growth hormone [ 38 ]. ( f ) the significance of fgf - 9 and mice lacking the fgf - 9 gene . the pathophysiological significance of fgf - 9 is poorly defined . however , its expression has been detected in 37 percent of primary human mammary tumors ( out of a sample of 103 consecutive tumor resections ) [ 40 ]. fgf - 9 was discovered as a growth factor activity in human glioma - derived cell lines ( fgf - 9 was originally named glial activating factor ( gaf ) [ 41 ]. thus fgf - 9 may have a role in the development of progression of human cancers . gaf / fgf - 9 was found to be mitogenic for oligodendrocytes , 3t3 cells and pc - 12 cells , but not for endothelial cells [ 41 ]. the lack of activity towards endothelial cells is interesting and consistent with recently published data [ 42 ], showing that recombinant fgf - 9 is a poor ligand for fgfr1 , which is the only known fgf receptor expressed on endothelial cells . in the adult , fgf - 9 is expressed in the central nervous system ( cns ) and the kidney . a recent in situ hybridization study demonstrates that fgf - 9 is expressed in the adult cns in specific neuronal nuclei in a pattern that differs from that of others fgfs [ 43 ]. its expression pattern in the embryo is unknown . however , preliminary data indicates that it is expressed in precartilaginous condensations where it may activate fgfr3 . the data ( table 1 ) [ 42 ] demonstrates that fgf - 9 efficiently activates the &# 34 ; b &# 34 ; and &# 34 ; c &# 34 ; splice forms of fgfr3 and the &# 34 ; c &# 34 ; splice form of fgfr2 . fgf - 9 thus becomes the first selective ligand for fgfr3b ( fgf - 1 activates all fgf receptors ). it is of interest to determine whether this specific ligand - receptor pair is physiologically relevant . the role of fgf - 9 in chondrogenesis can be examined in the mice lacking the fgf - 9 gene as described herein , and examining chondrogenesis in day 16 to 18 mouse embryos . because fgf - 9 , fgfr2 and fgfr3 are prominently expressed in the cns of adult mice , and fgf - 9 and fgfr 3 are also prominently expressed in the kidney , it is possible that fgf - 9 , fgfr 2 and fgfr 3 form physiologically or developmentally relevant ligand - receptor pairs in these tissues . a phenotype in mice lacking the fgf - 9 gene can identify sites where signaling between fgf - 9 and fgfr2 and / or fgfr3 is important . although no human diseases have thus far been associated with fgf - 9 , the extremely high degree of conservation between the mouse and human genes does suggest an essential developmental or physiological role in humans . the genetic mapping of the fgf - 9 gene can facilitate linking this gene to potential human diseases . table i__________________________________________________________________________ligand * fgfr fgf - 1 fgf - 2 fgf - 3 fgf - 4 fgf - 5 fgf - 6 fgf - 7 fgf - 8 fgf - 9__________________________________________________________________________1b 100 59 . 9 34 . 3 15 . 6 3 . 8 4 . 6 6 . 3 3 . 5 3 . 51c 100 103 . 9 0 . 3 102 . 3 59 . 0 54 . 9 0 . 3 0 . 7 21 . 12b 100 9 . 0 44 . 6 14 . 9 5 . 0 5 . 4 80 . 6 3 . 8 7 . 32c 100 63 . 9 4 . 2 94 . 3 25 . 0 60 . 7 2 . 5 16 . 1 89 . 23b 100 1 . 2 1 . 5 1 . 0 1 . 0 0 . 9 1 . 2 0 . 9 41 . 53c 100 107 . 2 0 . 6 69 . 1 11 . 8 8 . 8 1 . 0 40 . 5 95 . 64δ 100 94 . 6 7 . 2 64 . 7 6 . 3 42 . 0 1 . 7 69 . 4 60 . 3__________________________________________________________________________ * fgf receptor activation by fgfs 1 - 9 . relative mitogenic activity of baf . sub . 3 cell lines expressing the indicated fgfr ( 26 , 38 , 99 ). mitogeni activity ( calculated as the average of the ratio of . sup . 3 hthymidine incorporated at 312 and 1250 pm fgf to 0 pm fgf ) was normalized to that for fgf1 for each receptorexpressing cell line . fgfrs 1 - 3 are three iglik domain forms , fgfr4δ is the 2 iglike domain form . various other examples will be apparent to the person skilled in the art after reading the present disclosure without departing from the spirit and scope of the invention . it is intended that all such other examples be included within the scope of the appended claims .
a unique animal model is disclosed which is useful for studying the role of fgf - 9 activity in cardiovascular diseases . to provide this animal model , the fgf - 9 gene is disrupted in mice , that is , null mutation in the mouse fgf - 9 gene is engineered . this knockout mouse was produced by deleting the sequences immediately downstream of the initiation methionine in exon 1 . analysis of the fgf - 9 null embryos demonstrates perinatal lethality with apparent pathology in lung and cardiac tissue .