Assignment of the Vascular Endothelial Growth Factor Gene to Human Chromosome 6p21.3
Background Vascular endothelial growth factor (VEGF) is an endothelial cell–specific growth factor and a regulator of physiological and pathological angiogenesis. Four different proteins are produced by alternative splicing of a unique transcript generated from a single-copy gene. Knowledge of the chromosomal location of the VEGF gene would help in determining a linkage to any known human congenital syndrome and/or to known chromosomal rearrangements in tumors.
Methods and Results A human chromosome mapping panel was used to assign the VEGF gene to human chromosomes by polymerase chain reaction using VEGF-specific oligonucleotide primers. Amplified DNA fragments were fractionated on a 1% agarose gel. A single band of the expected size was obtained only from the DNA of those hybrid cell lines that contained the human chromosome 6. Three YAC clones containing the VEGF gene were obtained by screening the ICI Diagnostics library. In situ hybridization was then used to locate the VEGF gene in the 6p21.3 region.
Conclusions The location of the VEGF gene in the 6p21.3 region is a potential starting point for a linkage study. In addition, the isolation of YAC clones containing the VEGF gene will contribute to the construction of the physical map of this chromosomal region.
Vascular endothelial growth factor (VEGF) is a growth factor for endothelial cells in vitro; it elicited an angiogenic response in several in vivo assays.1 2 In addition, VEGF is able to increase the permeability of capillary vessels to different macromolecules. VEGF consists of a family of polypeptide isoforms, generated from a single-copy gene by alternative splicing of the primary transcript.3 VEGF is secreted by intact cells as a dimer of the four isoforms of 121, 165, 189, and 206 amino acids. All forms, except homodimeric VEGF121, can bind heparin in vitro and bind heparan derivatives of the extracellular matrix in vivo. VEGF produced from several cell types binds to the two endothelial cell–specific receptors Flt-1 and Flk-1/KDR.4 5 VEGF mRNA is expressed in a variety of normal adult tissues and during embryonic development, as well as in many tumors. VEGF mRNA expression is upregulated by oxygen deprivation in vitro in several cell types as well as by ischemia in vivo.6 Intravenous administration of VEGF results in enhanced collateral vessel formation in the rabbit ischemic hind limb.7 8
Polymerase Chain Reaction Amplification Analysis and Chromosomal Mapping Panel
The hamster×human hybrid cell lines were characterized for their human chromosome content.9 Oligonucleotide primers (sense, GTGGTGAAGTTCATGGATGTCTA; antisense, TTGGTGAGGTTTGATCCGCATAA) were chosen from the published VEGF genomic sequence to amplify a 1119-bp portion of the human VEGF gene.3 Three hundred nanograms of DNA from each cell line was amplified in 25 μL of 50 mmol/L KCl, 1.5 mmol/L MgCl2, 10 mmol/L Tris-HCl (pH 8.5), 20 μg/mL gelatin, 0.2 mmol/L each dNTP, 0.2 μmol/L each oligonucleotide, and 0.5 units Ampli-Taq polymerase (Perkin-Elmer Cetus). Reactions were performed for 35 rounds of amplification as follows: 45 seconds at 94°C for denaturation, 40 seconds at 60°C for annealing, and 2 minutes at 72°C for extension. Amplified DNA fragments were fractionated on a 1% agarose gel in TBE (53 mmol/L Tris-base, 53 mmol/L boric acid, 1.2 mmol/L EDTA). DNA was then transferred onto nylon Hybond-N membranes (Amersham Inc) according to the manufacturer’s instructions. Hybridization reactions were performed at 65°C as previously described.9 The VEGF probe corresponding to a 0.6-kb EcoRI-BamHI cDNA fragment spanning the entire human VEGF121 isoform was provided by Dr H. Weich, Braunschweig, Germany.
YAC Isolation and Fluorescent In Situ Hybridization Analysis
A human YAC library was screened to isolate VEGF-specific clones. The ICI library10 was screened at the YAC screening center DIBIT-HSR, Milan, Italy, by polymerase chain reaction analysis using the primers described above. Human metaphase chromosome preparations, probe labeling, and in situ hybridization were performed as previously described.11
Chromosome Mapping With Somatic Cell Hybrid Panel
A human chromosome mapping panel was used to assign the VEGF gene to human chromosome 6. A human VEGF-specific fragment was amplified from 300 ng of cell hybrid DNA template. A single band of the expected size (1119 bp) was obtained only from the DNA of the hybrid cell lines containing chromosome 6 (Table⇓). Southern blot hybridization of the polymerase chain reaction products with a VEGF-specific probe confirmed the identity of the reaction product (data not shown).
In situ hybridization was used to determine regional location of the VEGF gene. Three independent YAC clones (10CC7, 9DE2, and 28FD11) containing the VEGF gene were obtained by screening of the ICI library.10 Metaphase chromosome spreads were prepared from phytohemagglutinin-stimulated whole blood by standard techniques. Total DNA from YAC clones 10CC7 and 28FD11 was labeled with biotin-16-dUTP by nick translation. A probe concentration of 5 ng/mL was used, with 300 ng of human Cot-1 competitor DNA/μL. Hybridization to all four chromatids in the 6p21.3 region was observed in all 10 metaphase spreads that were analyzed for each YAC probe used.
The Figure⇓ shows a typical hybridization of the YAC clone 28FD11 to the 6p21.3 region of both chromatids of both chromosomes.
The VEGF primary protein sequence shares homology with another angiogenic factor, placenta growth factor (PlGF).9 12 Both VEGF and PlGF contain the eight-cysteine-residue motif characteristic of platelet-derived growth factor. These factors are active as homodimers PlGF/PlGF and VEGF/VEGF and heterodimers PlGF/VEGF. The VEGF homodimer binds to both Flt-1 and KDR/Flk-1 receptors.4 5 The PlGF homodimer binds only to the Flt-1 receptor.5 It has been hypothesized that the two receptors also could form heterodimers and homodimers.12 Presumably, the three protein forms could bind with different affinity to the three endothelium-specific receptor isoforms, thereby eliciting specific signal transduction cascades. Mutations in the polypeptide chain could impair either dimer formation or binding to the receptor.
Flt-1 and Flk-1/KDR deficiencies cause in utero death of mouse embryos between 8.5 and 9.5 days postcoitum.13 14 The analysis of homozygous Flk-1/KDR null mutant embryos reveals a defect in the development of hemopoietic and endothelial cells.13 Organized blood vessels are not observed either in the embryo or in the yolk sac, and hemopoietic progenitors are severely reduced. These results suggest that Flk-1/KDR is essential for yolk sac blood-island formation and vasculogenesis in the mouse embryo.13 Mouse embryos homozygous for a targeted mutation in the Flt-1 locus produced endothelial cells in both extraembryonic regions but assembled these cells into abnormal vascular channels; such embryos died in utero at midsomite stages.14 These data suggest that the Flt-1 signaling pathway may regulate normal endothelial cell-cell or cell-matrix interaction during vascular development.14
The human VEGF gene was mapped to determine whether associated mutations could possibly correspond to any known human congenital syndrome and/or to known chromosomal rearrangements in tumors.
Several diseases whose genetic defect is unknown have been mapped to the human 6p21.3 band.15 Among these, we draw attention to the atrial septal defect secundum type and hemocromatosis. Both Flt-1 and KDR/Flk-1 receptor genes have recently been mapped on chromosomes 4 and 13, respectively.16 17 We have mapped the mouse Plgf gene to chromosome 12, one cM from D12Mit5, and the human PlGF gene to 14q24 using both FISH and genetic crosses. The comparative analysis of the phenotypes of transgenic mice and mapping of human disease genes could help in understanding the role of these growth factors and their receptors in normal and pathological angiogenesis.
This work was supported by grants from the Associazione Italiana Ricerca sul Cancro (AIRC), Progetto Finalizzato “Applicazioni Cliniche della Ricerca Oncologica”; the CNR; and the Ministero della Sanità, Istituto Superiore di Sanità “Progetto A.I.D.S. 1994-Roma-Italia” to Dr Persico and in part by grants from the AIRC and Telethon Program to Dr Rocchi.
- Received January 9, 1996.
- Revision received January 31, 1996.
- Accepted February 1, 1996.
- Copyright © 1996 by American Heart Association
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