Advertisement

A Mammalian patched Homolog Is Expressed in Target Tissues of sonic hedgehog and Maps to a Region Associated with Developmental Abnormalities (∗)

  • Heidi Hahn
    Affiliations
    From the (1) Human Genetics Section, Laboratory of Viral Carcinogenesis, Frederick, Maryland 21702 and the
    Search for articles by this author
  • Jeffrey Christiansen
    Affiliations
    (3) Centre for Molecular and Cellular Biology, University of Queensland, St. Lucia 4072, Australia, the
    Search for articles by this author
  • Carol Wicking
    Affiliations
    (3) Centre for Molecular and Cellular Biology, University of Queensland, St. Lucia 4072, Australia, the
    Search for articles by this author
  • Peter G. Zaphiropoulos
    Affiliations
    (4) Department of Bioscience, Center for Nutrition and Toxicology, Karolinska Institute, S-171 57 Huddinge, Sweden, and the
    Search for articles by this author
  • Abirami Chidambaram
    Affiliations
    (2) Intramural Research Support Program, Scientific Applications International Corporation Frederick, National Cancer Institute-Frederick Cancer Research and Development Center, Frederick, Maryland 21702, the
    Search for articles by this author
  • Bernard Gerrard
    Affiliations
    (2) Intramural Research Support Program, Scientific Applications International Corporation Frederick, National Cancer Institute-Frederick Cancer Research and Development Center, Frederick, Maryland 21702, the
    Search for articles by this author
  • Igor Vorechovsky
    Affiliations
    (4) Department of Bioscience, Center for Nutrition and Toxicology, Karolinska Institute, S-171 57 Huddinge, Sweden, and the
    Search for articles by this author
  • Allen E. Bale
    Affiliations
    (5) Department of Genetics, Yale University School of Medicine, New Haven, Connecticut 06520
    Search for articles by this author
  • Rune Toftgard
    Affiliations
    (4) Department of Bioscience, Center for Nutrition and Toxicology, Karolinska Institute, S-171 57 Huddinge, Sweden, and the
    Search for articles by this author
  • Michael Dean
    Correspondence
    To whom correspondence should be addressed. Tel.: 301-846-5931; Fax: 301-846-1909;
    Affiliations
    From the (1) Human Genetics Section, Laboratory of Viral Carcinogenesis, Frederick, Maryland 21702 and the
    Search for articles by this author
  • Brandon Wainwright
    Affiliations
    (3) Centre for Molecular and Cellular Biology, University of Queensland, St. Lucia 4072, Australia, the
    Search for articles by this author
  • Author Footnotes
    ∗ This work was supported by a grant from the Australian National Health and Medical Research Council (to B. W. and C. A. W.) and grants from the Swedish Cancer Fund, Swedish Radiation Protection Institute, and Edvard Welanders Stiftelse (to P. G. Z., I. V., and R. T.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore by hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.The nucleotide sequence(s) reported in this paper has been submitted to the GenBank ™/EMBL Data Bank with accession number(s) U43148.
Open AccessPublished:May 24, 1996DOI:https://doi.org/10.1074/jbc.271.21.12125
      Drosophila patched is a segment polarity gene required for the correct patterning of larval segments and imaginal discs during fly development and has a close functional relationship with hedgehog. We have isolated a complete human PATCHED cDNA sequence, which encodes a putative protein of 1296 amino acids, and displays 39% identity and 60% similarity to the Drosophila PATCHED protein. Hydropathy analysis suggests that human PATCHED is an integral membrane protein with a pattern of hydrophobic and hydrophilic stretches nearly identical to that of Drosophila patched. In the developing mouse embryo, patched is initially detected within the ventral neural tube and later in the somites and limb buds. Expression in the limb buds is restricted to the posterior ectoderm surrounding the zone of polarizing activity. The results show that patched is expressed in target tissues of sonic hedgehog, a murine homolog of Drosophila hedgehog suggesting that patched/hedgehog interactions have been conserved during evolution. Human PATCHED maps to human chromosome 9q22.3, the candidate region for the nevoid basal cell carcinoma syndrome. Patched expression is compatible with the congenital defects observed in the nevoid basal cell carcinoma syndrome.

      INTRODUCTION

      The concept of diffusible morphogens has attracted widespread attention in developmental biology, since it provides a useful model to explain the patterning of tissues along a particular axis in an organism. According to this paradigm, position-specific cell fates are acquired due to the response of individual cells to different concentrations of a long range signal, which is secreted by a distinct inducing tissue (reviewed in (
      • Johnson R.L.
      • Tabin C.
      )).
      Drosophila patched (ptc)
      The abbreviations used are: ptc
      patched
      NBCCS
      nevoid basal cell carcinoma syndrome
      PCR
      polymerase chain reaction
      YAC
      yeast artificial chromosome
      dpc
      days post-coitus
      kb
      kilobase pair(s)
      ORF
      open reading frame.
      is a segment polarity gene required for the correct patterning of larval segments and imaginal discs during fly development(
      • Nakano Y.
      • Guerrero I.
      • Hidalgo A.
      • Taylor A.
      • Whittle J.R.
      • Ingham P.W.
      ,
      • Hooper J.E.
      • Scott M.P.
      ). Based on genetic studies, patched is a component of the signaling pathway of the morphogen hedgehog(
      • Basler K.
      • Struhl G.
      ,
      • Capdevila J.
      • Estrada M.P.
      • Sanchez-Herrero E.
      • Guerrero I.
      ,
      • Ingham P.W.
      • Taylor A.M.
      • Nakano Y.
      ). Since Patched is a putative membrane-spanning protein, and is expressed in hedgehog-responsive cells, it has been proposed to be the hedgehog receptor(
      • Ingham P.W.
      • Taylor A.M.
      • Nakano Y.
      ). In vertebrates, several hedgehog homologs have been i dentified. The best characterized of them, sonic hedgehog, has been implicated in the dorsal-ventral patterning of neural tube(
      • Roelink H.
      • Augsburger A.
      • Heemskerk J.
      • Korzh V.
      • Norlin S.
      • Ruiz i Altaba A.
      • Tanabe Y.
      • Placzek M.
      • Edlund T.
      • Jessell T.M.
      • et al.
      ,
      • Roelink H.
      • Porter J.A.
      • Chiang C.
      • Tanabe Y.
      • Chang D.T.
      • Beachy P.A.
      • Jessell T.M.
      ), in the differentiation of somites (
      • Johnson R.L.
      • Laufer E.
      • Riddle R.D.
      • Tabin C.
      ) and in the establishing of the anterior-posterior axis of the limb bud(
      • Riddle R.D.
      • Johnson R.L.
      • Laufer E.
      • Tabin C.
      ). The biochemical basis of hedgehog signaling in vertebrates remains poorly understood and has been hampered largely by the lack of a proven receptor for the molecule.

      EXPERIMENTAL PROCEDURES

      Cosmid Isolation

      Cosmids used in this study were isolated from a human chromosome 9-specific genomic cosmid library (LL09NC01“P”, Biomedical Sciences Division, Lawrence Livermore National Laboratory, Livermore, CA) by screening with the YAC clone ICI-2ef8 (United Kingdom Human Genome Mapping Project Resource Centre). This clone contains the microsatellite marker D9S287, which has been localized to chromosome 9q22.3(
      • Povey S.
      • Armour J.
      • Farndon P.
      • Haines J.L.
      • Knowles M.
      • Olopade F.
      • Pilz A.
      • White J.A.
      • Kwiatkowski D.J.
      ). The isolation of YAC DNA and hybridization was performed as described elsewhere(
      • Vorechovský I.
      • Vetrie D.
      • Holland J.
      • Bentley D.R.
      • Thomas K.
      • Zhou J.N.
      • Notarangelo L.D.
      • Plebani A.
      • Fontán G.
      • Ochs H.D.
      • Hammarström L.
      • Sideras P.
      • Smith C.I.F.
      ). The localization of the cosmids was confirmed by hybridization to YAC ICI-2ef8 resolved by means of pulsed-field gel electrophoresis (data not shown). The 96-well plate format of the cosmid clones that contain PTC is 42H11, 96F9, 218A8, 226G7.

      Library Screening

      Human cDNA clones were isolated from a fetal brain cDNA library in the λ ZAPII phage vector (Stratagene), using standard procedures. The probes were labeled with [32P]dCTP by random priming (Rediprime, Amersham Corp.). Positive clones were rescued using the 704 helper phage/pBluescript excision system (Rapid Excision Kit, Stratagene) and sequenced. Mouse genomic clones were isolated from a 129SV λ FixII library (Stratagene). Phage DNA was cut with EcoRI and hybridized with PTC-specific probes. Mouse cDNA clones were isolated from an 11.5 dpc mouse embryo (Swiss male) library constructed in λ gt10. Hybridization was performed at 55°C. Positive clones were subcloned into pBluescript II SK (Stratagene) digested with NotI.

      Sequencing

      Templates for sequencing were prepared from overnight cultures of rescued cDNA clones and/or EcoRI cosmid fragments subcloned in pBluescript KS(+) using a plasmid purification kit (Qiagen). Sequencing was performed with the Taq Dyedeoxy Terminator Cycle Sequencing kit (Applied Biosystems) according to the manufacturer's instructions. Sequencing reactions were resolved on an Applied Biosystems Inc. model 373A automated sequencer. Sequence analysis was performed using the GCG software. BLAST searches were performed with the NCBI network service.

      Northern Hybridization

      Expression of human PTC mRNA was examined by Northern hybridization of human tissue blots (Clontech) using cDNA probes labeled with [32P]dCTP. Hybridization solution contained 5 × SSPE, 10 × Denhardt's solution, 100 mg/ml denatured sheared herring sperm DNA, 50% formamide, and 2% SDS. Washes were performed at 60°C with 2 × SSC and 0.1% SDS.

      Chromosomal Localization

      The chromosomal localization of human PTC was identified by PCR analysis of DNA panels obtained from human-hamster hybrid cells. The panel consisted of both whole chromosome 9 hybrids and deletion hybrids of 9q22.3. The primers used were PTC1 (5′-TTG CAT AAC CAG CGA GTCT 3′) and PTC2 (5′-CAA ATG TAC GAG CAC TTC AAGG). Murine Ptc was mapped by means of interspecific backcross mapping. The panels were provided by the Jackson Laboratory (Bar Harbor, ME) and are the BSB panel from a cross (C57BL/6J × Mus spretus) × C57BL/6J and a similar BSS panel made up of DNA from the reciprocal backcross (C57BL/6JEi × SPRET/Ei) × SPRET/Ei(
      • Rowe L.B.
      • Nadeau J.H.
      • Turner R.
      • Frankel W.N.
      • Letts V.A.
      • Eppig J.T.
      • Ko M.S.H.
      • Thurston S.J.
      • Birkenmeier E.H.
      ). Mapping was performed by means of SSCP (single strand conformation polymorphism) analysis with the primers W18F3 (5′-CTG TCA AGG TGA ATG GAC 3′) and W18R3 (5′-GGG GTT ATT CTG TAA AAGG 3′). PCR reactions were performed in the presence of [32P]dCTP. The samples were resolved on a 6% acrylamide gel (2.6% cross-linking) at 4°C at 70 watts within 1.5 h. Genetic linkage was performed by segregation analysis.

      In Situ Hybridization

      Whole mount in situ hybridization on mouse embryos and subsequent sectioning was performed as described previously(
      • Christiansen J.H.
      • Dennis C.L.
      • Wicking C.A.
      • Monkley S.J.
      • Wilkinson D.G.
      • Wainwright B.J.
      ). The mouse Ptc probe was a 706-base pair NotI/PstI cDNA fragment, from the 5′-end of the gene, subcloned in pBluescriptII SK. The probe was linearized with SacII, the overhang blunted by incubation with 5 units/mg Klenow at 22°C for 15 min, and antisense RNA synthesized by transcribing with T7 RNA polymerase.

      RESULTS AND DISCUSSION

      Cloning of a Human PTC Homolog

      Cosmids used in this study were isolated from a human chromosome 9-specific genomic cosmid library using the YAC clone ICI-2ef8. This clone contains the microsatellite marker D9S287, which has been localized to chromosome 9q22.3(
      • Povey S.
      • Armour J.
      • Farndon P.
      • Haines J.L.
      • Knowles M.
      • Olopade F.
      • Pilz A.
      • White J.A.
      • Kwiatkowski D.J.
      ). Sequencing of a 1.8-kb EcoRI fragment of cosmid 42H11 yielded an open reading frame with significant homology to three consecutive stretches of the Drosophila Ptc protein. Using the 1.8-kb EcoRI fragment as a probe we have isolated the complete human and partial mouse PTC cDNA sequences.
      The sequence of human PTC consists of an open reading frame of 4242 nucleotides flanked by 87 and 2238 nucleotides on the 5′- and 3′-untranslated regions, respectively (Fig. 1). The open reading frame of human PTC cDNA encodes for a putative protein of 1414 amino acids. The first AUG codon is located 357 bases into the reading frame and has a moderate match for the translational start consensus sequence in vertebrates (GAGGCTAUGT in PTC versus GCCGCCAUGG(
      • Kozak M.
      )). Assuming that this codon encodes for the first amino acid of the protein, human PTC consists of 1296 amino acids with a relative molecular weight (Mr) of 145 × 103. It shows 39% identity and 60% similarity to its Drosophila counterpart. The 3′-un translated region contains a canonical polyadenylation signal (AATAAA) as well as mRNA destabilizing ATTTA motifs. These are localized 1030 nucleotides and 167, 372, and 1144 nucleotides after the termination codon, respectively.
      Figure thumbnail gr1a
      Figure 1:Sequence of the human PTC cDNA. The sequence of PTC is shown including the open reading frame and flanking 5′ and 3′ sequences. The open reading frame (ORF) is at +1 to +3870. The first ATG in the ORF is in bold type. The poly(A) signal and mRNA destabilizing signals are underlined.
      Figure thumbnail gr1b
      Figure 1:Sequence of the human PTC cDNA. The sequence of PTC is shown including the open reading frame and flanking 5′ and 3′ sequences. The open reading frame (ORF) is at +1 to +3870. The first ATG in the ORF is in bold type. The poly(A) signal and mRNA destabilizing signals are underlined.
      Hydropathy analysis (
      • Kyte J.
      • Doolittle R.F.
      ) of the entire open reading frame of human PTC predicts the presence of eight main hydrophobic stretches (Fig. 2). Distribution of the hydrophobic blocks is remarkably well conserved between species indicating that human PTC, like its Drosophila counterpart, is an integral membrane protein.
      Figure thumbnail gr2
      Figure 2:Hydropathy plot of PTC proteins. The hydropathy of the predicted ORF of human PTC of 1414 amino acids was analyzed by the modified method of Kyte and Doolittle(
      • Nakano Y.
      • Guerrero I.
      • Hidalgo A.
      • Taylor A.
      • Whittle J.R.
      • Ingham P.W.
      ,
      • Kyte J.
      • Doolittle R.F.
      ). A, human PTC; B, Drosophila Ptc.

      Chromosomal Localization of PTC

      Chromosomal localization of human PTC on 9q22.3 was confirmed by PCR analysis of chromosome 9 hybrids, and deletion hybrids of 9q22.3, human-hamster hybrid DNA panels. The primers used (PTC1 and PTC2) were derived from a sequence of a 1.8-kb EcoRI fragment of cosmid 42H11. Primer PTC1 is derived from an exon sequence and PTC2 from an intron sequence. All DNA hybridization and cDNA sequencing data suggest that human PTC is a single copy gene (data not shown). Murine Ptc maps to a short region of chromosome 13, close to the murine Facc locus (no recombination out of 188 meioses). This region contains the mouse mutations flexed tail (f) and purkinje cell degeneration (pcd), and it is syntenic with human 9q22-q31. Both f and pcd involve abnormal development of cells of the bone or brain and could be allelic to Ptc.

      Expression of PTC

      Northern blot analysis revealed five distinct PTC transcripts in all human tissues examined. Expression of these transcripts appears to be differentially regulated (Fig. 3). During mouse embryogenesis, expression of Ptc is first detected at E 8.0 dpc in ventral neuroepithelial tissue in two separate domains along the midline (Fig. 4A). Expression persists in ventral neural cells through to 9.5 dpc (Fig. 4, B-D), and transcripts are also detected in lateral mesenchyme surrounding the neural tube (Fig. 5A). Ptc transcription is detected in the somites soon after the time of their appearance (Fig. 4C) and follows a rostro-caudal gradient of expression (Fig. 4, C, G, and H). Somite expression is restricted to epithelial cells within the medial aspects of each somite (Fig. 5B). Expression of Ptc is also detected in the posterior ectoderm of each limb bud from 10.0 dpc to 12.5 dpc (Fig. 4, E-H, and 5C). This region corresponds to surface ectoderm that covers the zone of polarizing activity. Other sites of Ptc expression during this period include the inner surfaces of the branchial arches which flank the oropharyngeal region, cells surrounding the placodes of the vibrissae and the genital eminence (data not shown). No staining was observed with a Ptc sense probe (data not shown).
      Figure thumbnail gr3
      Figure 3:Expression of PTC in selected adult human tissues. Northern blots (2 μg of poly(A)+/lane) were hybridized with 32P-labeled 1.5-kb cDNA probe corresponding to the nucleotides 691-2228 of human PTC ORF. K, kidney; Li, liver; Lu, lung; B, brain; H, heart; P, placenta; M, skeletal muscle; Pa, pancreas.
      Figure thumbnail gr4
      Figure 4:Expression of Ptc during murine embryogenesis. A, expression is first detected at ∼8 dpc in neuroepithelium on either side of the neural groove. B, 8.25 dpc embryo showing expression in the ventral neural tube and lateral mesenchyme. C, 8.5 dpc embryo (after turning) showing expression as in B, but also with expression in the somites. D, neural expression of Ptc continues at 9.25 dpc. E, by 9.5 dpc, expression is detected in the posterior limb bud. F-H, expression continues in the posterior limb through to 12.5 dpc in both the forelimb (fl) and hindlimb (hl).
      Figure thumbnail gr5
      Figure 5:Details of Ptc expression during murine embryogenesis. A, section taken through the neural tube of a 9.5 dpc embryo. Ptc expression is detected in the ventral neural tube (nt) and the surrounding lateral mesenchyme. Note the absence of Ptc expression in the notochord (arrow). B, section through the tail at 11.0 dpc. Expression of Ptc is detected in the ventral neural tube (nt) and within epithelial cells along the medial edge of each somite (s). C, in the limb-bud (lb), expression of Ptc is restricted to posterior ectoderm.
      The expression pattern of Ptc points to a close relationship between Ptc and the hedgehog family of morphogens. This relationship was originally established in Drosophila(
      • Ingham P.W.
      • Taylor A.M.
      • Nakano Y.
      ). In vertebrates, the best characterized hedgehog homolog, sonic hedgehog, has been implicated in the induction of the floorplate and motor neurons within the ventral neural tube (
      • Jessell T.M.
      • Dodd J.
      ,
      • Yamada T.
      • Pfaff S.L.
      • Edlund T.
      • Jessell T.M.
      ) as well as in the differentiation of sclerotome within the somites(
      • Pourquie O.
      • Coltey M.
      • Teillet M.A.
      • Ordahl C.
      • Le Douarin N.M.
      ). In the limb bud, sonic hedgehog expression in the mesenchymal “zone of polarizing activity” triggers antero-posterior patterning of the limb(
      • Riddle R.D.
      • Johnson R.L.
      • Laufer E.
      • Tabin C.
      ). Our data show that vertebrate PTC is expressed in all major target tissues of sonic hedgehog, such as the ventral neural tube, somites, and tissues surrounding the zone of polarizing activity of the limb bud. The striking spatial complementarity and temporal coincidence of the sonic hedgehog and Ptc expression patterns suggest that both genes might be members of a common signaling pathway. After the completion of this work, Goodrich et al.(
      • Goodrich L.V.
      • Johnson R.L.
      • Milenkovic L.
      • McMahon J.A.
      • Scott M.P.
      ) published the sequence of a mouse Ptc gene with an expression pattern essentially identical to that described here.
      The localization of PTC in the region containing the nevoid basal cell carcinoma syndrome (NBCCS) gene is intriguing. NBCCS is an autosomal dominant disorder, which predisposes affected individuals to basal cell carcinomas of the skin, medulloblastomas, and various other tumors(
      • Gorlin R.J.
      ). Recent genetic studies have placed the gene for the nevoid basal cell carcinoma syndrome to chromosome 9q22.3, between the markers Fanconi anemia complementation group A (
      • Farndon P.A.
      • Morris D.J.
      • Hardy C.
      • McConville C.M.
      • Weissenbach J.
      • Kilpatrick M.W.
      • Reis A.
      ) and D9S287(
      • Pericak-Vance M.
      ). Several lines of evidence suggest that PTC is a candidate gene for the nevoid basal cell carcinoma syndrome. Ptc expression is compatible with the congenital defects commonly found in NBCCS patients. Frequent symptoms in newborns and infants are developmental anomalies of the spine and ribs(
      • Gorlin R.J.
      ). These malformations could be due to a PTC deficiency, expression of which coincides spatially and temporally with the development of the neural tube and of the somites. In addition, Ptc expression in the surface ectoderm surrounding the zone of polarizing activity is consistent with limb abnormalities often observed in the patients with NBCCS(
      • Gorlin R.J.
      ). PTC expression in all adult tissues points to a pleiotropic role of PTC in adult signal transduction pathways. Defects in these signaling pathways could account for the symptoms that develop postnatally(
      • Levan t S.
      • Gorlin R.
      • Fallet S.
      • Johnson D.
      • Fantasia J.
      • Bale A.
      ,
      • Bale A.E.
      • Gailani M.R.
      • Leffell D.J.
      ).

      Acknowledgments

      We thank Gary Smythers of the Frederick Biomedical Supercomputing Center for assistance in the sequence analysis, Stan Cevario for primer synthesis, Andy Greenfield for the mouse embryo cDNA library, and Toshiya Yamada, Patrick Tam, and Leszek Wojnowski for helpful discussions regarding the data. The chromosome-specific gene library LL09NC01 was constructed at the Biomedical Sciences Division, Lawrence Livermore National Laboratory Gene Library Project sponsored by the United States Department of Energy.

      REFERENCES

        • Johnson R.L.
        • Tabin C.
        Cell. 1995; 81: 313-316
        • Nakano Y.
        • Guerrero I.
        • Hidalgo A.
        • Taylor A.
        • Whittle J.R.
        • Ingham P.W.
        Nature. 1989; 341: 508-513
        • Hooper J.E.
        • Scott M.P.
        Cell. 1989; 59: 751-765
        • Basler K.
        • Struhl G.
        Nature. 1994; 368: 208-214
        • Capdevila J.
        • Estrada M.P.
        • Sanchez-Herrero E.
        • Guerrero I.
        EMBO J. 1994; 13: 71-82
        • Ingham P.W.
        • Taylor A.M.
        • Nakano Y.
        Nature. 1991; 353: 184-187
        • Roelink H.
        • Augsburger A.
        • Heemskerk J.
        • Korzh V.
        • Norlin S.
        • Ruiz i Altaba A.
        • Tanabe Y.
        • Placzek M.
        • Edlund T.
        • Jessell T.M.
        • et al.
        Cell. 1994; 76: 761-775
        • Roelink H.
        • Porter J.A.
        • Chiang C.
        • Tanabe Y.
        • Chang D.T.
        • Beachy P.A.
        • Jessell T.M.
        Cell. 1995; 81: 445-455
        • Johnson R.L.
        • Laufer E.
        • Riddle R.D.
        • Tabin C.
        Cell. 1994; 79: 1165-1173
        • Riddle R.D.
        • Johnson R.L.
        • Laufer E.
        • Tabin C.
        Cell. 1993; 75: 1401-1416
        • Povey S.
        • Armour J.
        • Farndon P.
        • Haines J.L.
        • Knowles M.
        • Olopade F.
        • Pilz A.
        • White J.A.
        • Kwiatkowski D.J.
        Ann. Hum. Genet. 1994; 58: 177-250
        • Vorechovský I.
        • Vetrie D.
        • Holland J.
        • Bentley D.R.
        • Thomas K.
        • Zhou J.N.
        • Notarangelo L.D.
        • Plebani A.
        • Fontán G.
        • Ochs H.D.
        • Hammarström L.
        • Sideras P.
        • Smith C.I.F.
        Genomics. 1994; 21: 517-524
        • Christiansen J.H.
        • Dennis C.L.
        • Wicking C.A.
        • Monkley S.J.
        • Wilkinson D.G.
        • Wainwright B.J.
        Mech. Dev. 1995; 51: 341-350
        • Kozak M.
        J. Biol. Chem. 1991; 266: 19867-19870
        • Kyte J.
        • Doolittle R.F.
        J. Mol. Biol. 1982; 157: 105-132
        • Jessell T.M.
        • Dodd J.
        Harvey Lect. 1990; 86: 87-128
        • Yamada T.
        • Pfaff S.L.
        • Edlund T.
        • Jessell T.M.
        Cell. 1993; 73: 673-686
        • Pourquie O.
        • Coltey M.
        • Teillet M.A.
        • Ordahl C.
        • Le Douarin N.M.
        Proc. Natl. Acad. Sci. U. S. A. 1993; 90: 5242-5246
        • Goodrich L.V.
        • Johnson R.L.
        • Milenkovic L.
        • McMahon J.A.
        • Scott M.P.
        Genes & Dev. 1996; 10: 301-312
        • Gorlin R.J.
        Medicine (Balt.). 1987; 6 6: 98-113
        • Farndon P.A.
        • Morris D.J.
        • Hardy C.
        • McConville C.M.
        • Weissenbach J.
        • Kilpatrick M.W.
        • Reis A.
        Genomics. 1994; 23: 486-489
        • Pericak-Vance M.
        Ann. Hum. Genet. 1995; 59: 347-365
        • Levan t S.
        • Gorlin R.
        • Fallet S.
        • Johnson D.
        • Fantasia J.
        • Bale A.
        Nat. Genet. 1996; 12: 85-87
        • Bale A.E.
        • Gailani M.R.
        • Leffell D.J.
        J Invest Dermatol. 1994; 103: 126S-130S
        • Rowe L.B.
        • Nadeau J.H.
        • Turner R.
        • Frankel W.N.
        • Letts V.A.
        • Eppig J.T.
        • Ko M.S.H.
        • Thurston S.J.
        • Birkenmeier E.H.
        Mamm. Genome. 1994; 5: 253-274