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(American Journal of Pathology. 2004;165:1875-1882.)
© 2004 American Society for Investigative Pathology

Transgenic Mice Expressing a Ligand-Inducible Cre Recombinase in Osteoblasts and Odontoblasts

A New Tool to Examine Physiology and Disease of Postnatal Bone and Tooth

Jung-Eun Kim, Kazuhisa Nakashima and Benoit de Crombrugghe

From the Department of Molecular Genetics, The University of Texas M.D. Anderson Cancer Center, Houston, Texas


   Abstract
Top
 Abstract
Materials and Methods
 Results
 Discussion
 References
 
The skeleton supports body structures in vertebrates and helps maintain calcium homeostasis throughout life. Disruption of genes involved in mammalian bone formation has often led to embryonic lethality, hence preventing study of these genes’ role in adult animals. To develop a usable tool for such study, we generated transgenic mice in which a 2.3-kb mouse Col1a1 proximal promoter, which is active in all osteoblasts, drives a transgene coding for a polypeptide consisting of Cre recombinase fused to a mutated ligand-binding domain of the estrogen receptor. In this Col1a1-CreERT2 mouse line, expression patterns of the transgene and of the resulting Cre-mediated DNA recombination are analyzed by crossing with ROSA26 reporter mice and by measurement of ß-galactosidase activity and X-gal staining. Exposure to 4-hydroxytamoxifen induced Cre-mediated recombination in osteoblasts in virtually all bones and in odontoblasts in teeth of both embryos and postnatal mice. The generation of these transgenic mice provides a new and important tool with which to study the function of specific genes in bone and tooth physiology and diseases in intact animals after birth.

Composed of cartilage and bone, the vertebrate skeleton not only supports body structures but also helps maintain calcium homeostasis throughout life. Manipulating mammalian genomes by disrupting them by homologous recombination1-4 in embryonic stem cells has provided insights into gene functions in skeletal development and bone-related diseases. However, many questions related to the structure of genetic pathways that regulate cellular differentiation and function remain to be clarified. In particular, dissection of genetic pathways that control embryonic skeletal development is often hampered by the disruption of some critical genes that function in early embryonic decisions, thereby leading to embryonic or perinatal lethality and preventing study of the role of these genes in postnatal bone development and physiology. To overcome this difficulty, gene-targeting methods using site- and time-specific recombination based on the Cre/loxP system have been used to delete particular genes in specific tissues and stages of development.5-7 Thus, the generation and characterization of transgenic mice expressing Cre recombinase under the control of a tissue- and stage-specific promoter is a prerequisite to studying skeletogenesis.

Type I collagen, a major constituent of many mammalian tissues,8 is synthesized by osteoblasts and odontoblasts and by various types of fibroblasts and mesenchymal cells. Previous reports identified an osteoblast-specific enhancer element in the mouse Col1a1 gene.9,10 Indeed, transgenic mice harboring a 2.3-kb proximal fragment of mouse or rat Col1a1 promoter showed high activity of the transgene in bone-forming cells, osteoblasts, and in the dentin-forming cells, odontoblasts in teeth, very low activity in tendons, and no activity in other tissues.10-14

In the present study, we generated transgenic mouse lines in which a 2.3-kb Col1a1 osteoblast- and odontoblast-specific promoter drives a transgene coding for a polypeptide consisting of Cre recombinase fused to a mutated ligand-binding domain (LBD) of the estrogen receptor (ER). This CreER fusion polypeptide becomes active only after administration of the synthetic estrogen antagonist 4-hydroxytamoxifen (4-OHT). The Cre recombinase is inactive in the absence of 4-OHT. To generate these mice, we have used the CreERT2 recombinase, which contains a G400V/M543A/L544A triple mutation in the ER LBD and is more sensitive to 4-OHT than is the mutant ER LBD with a single G521R substitution.15,16 These transgenic mice also harbor the ubiquitously active ROSA26 locus, in which the LacZ gene is preceded by a transcriptional stop cassette flanked by loxP sites. By measuring ß-galactosidase (ß-gal) activity and assessing X-gal staining, we found that in this transgenic mouse line, referred to as Col1a1-CreERT2, recombination of loxP sites could be induced by 4-OHT in both osteoblasts and odontoblasts, and that osteoblast- and odontoblast-specific gene deletion could be achieved both during embryonic development and after birth. Use of these mice provides a novel experimental approach to studying the role of specific genes in the physiology and disease of bone and tooth in intact animals after birth.


   Materials and Methods
Top
 Abstract
 Materials and Methods
Results
 Discussion
 References
 
Construction of the Col1a1-CreERT2 Transgene

pGS-CreERT2 plasmid DNA15 containing CreERT2 cDNA and pJ2300LacZ plasmid DNA10 containing a 2.3-kb promoter fragment of the gene for the {alpha}1 chain of mouse type I collagen (Col1a1) were used to construct the Col1a1-CreERT2 transgene. A 2.9-kb AvrII/SalI fragment with CreERT2 cDNA from pGS-CreERT2 and a 2.3-kb NotI/XbaI Col1a1 promoter fragment from pJ2300LacZ were subcloned into the NotI/SalI site of the pBluscript II KS(+) vector (Stratagene, La Jolla, CA).

Generation and Identification of Col1a1-CreERT2 Transgenic Mice

To remove vector sequences for pronuclear injection, a 5.2-kb fragment of the Col1a1-CreERT2 transgene was digested with NotI and SalI. The Col1a1-CreERT2 fragment excised from the vector backbone was microinjected into the pronuclei of fertilized B6D2 F1 oocytes. Founder mice were genotyped using tail genomic DNA by polymerase chain reaction (PCR) with the Cre-specific primers, 5'-ATCCGAAAAGAAAACGTTGA-3' and 5'-ATCCAGGTTACGGATATAGT-3'. The size of the amplified product was ~700 bp for the Col1a1-CreERT2.

Analysis of Expression Level of Cre Transgenes by Reverse Transcriptase (RT)-PCR

Total RNA from limbs of embryos at 13.5 days post coitum (dpc), 16.5 dpc, and 18.5 dpc, and from limbs of 3-day-old pups was prepared using TRIzol reagent (Invitrogen Inc., Carlsbad, CA) according to the manufacturer’s instructions. Briefly, each limb was homogenized in 1 ml of TRIzol reagent and incubated for 5 minutes at room temperature. After adding 0.2 ml of chloroform, the mixtures were separated by centrifugation into a phenol-chloroform phase, an interphase, and an upper aqueous phase. RNA was precipitated from the aqueous phase by mixing it with 0.5 ml of isopropanol. For RT-PCR analysis, cDNA was synthesized from 1 µg of total RNA by using the First Strand cDNA Synthesis kit for RT-PCR (Roche Applied Science, Indianapolis, IN). PCR was performed according to standard procedure by using the Cre-specific primers. Control PCR was performed with the mouse glyceraldehyde-3-phosphate dehydrogenase (GAPDH) primers, 5'-TGAAGGTCGGTGTGAACGGATTTGGC-3' and 5'-CATGTAGGCCATGAGGTCCACCAC-3'.

Test of Cre Recombinase Activity by 4-OHT Treatment in Vivo

After crossing Col1a1-CreERT2 male transgenic mice with female ROSA26 reporter (R26R) mice,17 offspring were genotyped from the yolk sac or pup tail genomic DNA by PCR using Cre-specific and LacZ-specific primers for CreERT2 and R26R alleles, respectively. LacZ PCR was performed with the LacZ-specific primers, 5'-GCATCGAGCTGGGTAATAAGGGTTGGCAAT-3' and 5'-GACACCAGACCAACT-GGTAATGGTAGCGAC-3'. 4-OHT (Sigma-Aldrich, St. Louis, MO) used for testing the activity and ligand dependency of the Col1a1-CreERT2 transgene was prepared as suspensions of 2.5, 5.0, 7.5, or 10.0 mg/ml 4-OHT in autoclaved sunflower oil (Sigma-Aldrich).

For analysis of embryonic activity of Cre recombinase, pregnant female mice were injected intraperitoneally with 1 mg of 4-OHT for 3 consecutive days starting at 12.5, 13.5, or 14.5 dpc. Injected mice were sacrificed 48 hours after the final injection, and the embryos were processed for whole-mount X-gal (5-bromo-4-chloro-3-indoyl ß-D-galactopyranoside; Brinkmann, Westbury, NY) staining. Control embryos whose dams were treated with sunflower oil instead of 4-OHT were used for comparison to confirm the activity and ligand dependency of Cre recombinase. To analyze the postnatal activity of the conditional Cre recombinase, pups were injected intraperitoneally with 0.25, 0.5, 0.75, or 1.0 mg of 4-OHT for 5 consecutive days starting when they were 12 days old. Forty-eight hours after the final injection, mice were killed and various tissue samples were processed for measurement of ß-galactosidase (ß-gal) activity and X-gal staining.

Measurement of Tissue ß-Gal Activity

Tissue ß-gal activity was assessed in 18-day-old pups by using the Galacto-Light Plus systems (Applied Biosystems, Bedford, MA). Dissected tissues from tail, skin, limb, liver, and calvaria were homogenized in 1 ml of extraction buffer (0.1 mol/L potassium phosphate, 1 mol/L dithiothreitol, and 10% Triton X-100). After centrifuging the samples at 13,000 rpm for 10 minutes, 10-µl extracts of each sample were used for the assay, following the instructions of the manufacturer. Protein concentrations were determined with the Bio-Rad protein assay (Bio-Rad Laboratories, Hercules, CA). ß-Gal activity was normalized to the protein concentration to determine the number of relative light units per microgram of protein.

X-Gal Staining and Histological Analysis

X-Gal staining in whole-mount embryos at various stages of development or in various tissue samples from postnatal mice was performed as described previously.10 Briefly, whole embryos or dissected tissue fragments from pups were fixed for 45 to 60 minutes in 0.1 mol/L phosphate buffer (pH 7.5) with 5 mmol/L EGTA (pH 8.0), 2 mmol/L MgCl2, 0.2% glutaraldehyde, and 0.8% formaldehyde, and rinsed three times for 30 minutes each with 0.1 mol/L phosphate buffer (pH 7.3) containing 2 mmol/L MgCl2, 0.2% Nonidet P-40, and 0.1% sodium deoxycholate. Samples were then stained with the same rinsing solution supplemented with 1 mg/ml of X-gal, 5 mmol/L potassium ferrocyanide, and 5 mmol/L potassium ferricyanide. For embryos older than 16.5 dpc or postnatal pups, skin was removed for fixation and stained. After staining, the embryos or pup tissues were washed in 1x phosphate-buffered saline (PBS, pH 7.8) with 10 mmol/L EDTA (pH 8.0) and were photographed. For histological analysis, X-gal stained samples were fixed again in 4% formaldehyde/1x PBS, pH 7.8, at 4°C overnight and bone samples were decalcified in 0.25 mol/L EDTA/0.1 mol/L NaPB, pH 6.5, for 2 to 3 weeks. Then, all embryo and pup samples were dehydrated, embedded in paraffin, and sections of 12 µm were cut and counterstained with nuclear fast red.


   Results
Top
 Abstract
 Materials and Methods
 Results
Discussion
 References
 
Generation of Col1a1-CreERT2 Transgenic Mice

Founders of two types of transgenic mice were successfully generated. In the first type, the transgene consisted of the 2.3-kb mouse Col1a1 promoter linked to CreER DNA with a G521R mutation in the LBD of the ER. The second type harbored the CreERT2 transgene (Figure 1A) , which consisted of the same promoter linked to CreER with a triple mutation G400V, M543A, and L544A in the ER LBD. Although in both types of mice recombinase activity was specifically induced in osteoblasts and odontoblasts by 4-OHT, this study focuses on the characterization of transgenic mice harboring the CreERT2 transgene because the CreERT2 recombinase achieves higher activity with 4-OHT than the CreERT recombinase. As assessed using RT-PCR, Cre mRNA expression in 13.5, 16.5, and 18.5-dpc embryos, and in 3-day-old pups was consistent with activation of the Col1a1 promoter at the time of osteoblasts differentiation and bone formation (Figure 1B) .



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  		Figure 1. Generation of the Col1a1-CreERT2 transgene. A: Construction of the Col1a1-CreERT2 transgene for osteoblast-specific expression of ligand-dependent Cre recombinase. The Col1a1 promoter cassette contains a 2.3-kb proximal fragment of the mouse type I collagen promoter. The ERT2 LBD of the ER contains the G400V/M543A/L544A triple mutation. B: RT-PCR analysis of Cre mRNA expression in limbs of embryos at 13.5, 16.5, and 18.5 dpc and of 3-day-old pups. Mouse GAPDH PCR shows that the same amount of cDNA was used in each sample. Wt and TG indicate wild-type and Col1a1-CreERT2 transgenic embryo or pup, respectively.

 
Analysis of Ligand-Dependent Cre Recombinase Activity in Col1a1-CreERT2;R26R Double-Transgenic Mice

To determine the ability of Cre to induce recombination specifically in bone, male Col1a1-CreERT2 transgenic mice were crossed with female mice of the ROSA26 reporter strain (R26R).17 In latter strain, the LacZ gene, which is inserted in the ubiquitously expressed ROSA locus, is preceded by a transcriptional stop cassette flanked by loxP sites. Thus, in Col1a1-CreERT2;R26R double-transgenic mice, ß-galactosidase (ß-gal) should be expressed in bones when Cre recombinase is activated by administration of 4-OHT. In 18-day-old offspring that had been injected with 4-OHT, we detected high ß-gal activity in the tail, limb, and calvaria (Figure 2) . The level of ß-gal activity strongly depended on the concentration of 4-OHT injected and was higher after five injections than after three injections (data not shown). No ß-gal activity was detected in the liver and only a low level of activity in the skin of the double-transgenic mice regardless of treatment with 4-OHT. In other control mice containing only the Col1a1-CreERT2 or only the R26R allele allele, little or no ß-gal activity was detected. In some tissues, low-level endogenous galactosidase activity could be present.



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  		Figure 2. Inducible Cre-mediated ß-gal activity in 18-day-old Col1a1-CreERT2;R26R double-transgenic mice. Pups harboring both the Col1a1-CreERT2 transgene and the ROSA26-LacZ locus had been injected intraperitoneally with 0.75 mg of 4-OHT for 5 consecutive days starting at 12 days of age and then killed 48 hours later. The indicated tissues were homogenized and assayed for ß-gal activity using a chemiluminescence assay as described in Materials and Methods. Wt indicates wild-type mice. Enzymatic activity is presented as mean relative light units (RLU) divided by the protein concentration.

 
X-Gal staining was also examined in the same tissues that showed high levels of ß-gal activity (ie, tail, limb, and calvaria). Strong X-gal staining occurred in all of the long bones of the forelimb and hindlimb (data not shown) and in the ribs, vertebrae, and calvaria (Figure 3; A1 to A4 ). In contrast, uninjected 18-day-old double-transgenic control pups showed weak staining, mainly at sites corresponding to endogenous galactosidase activity (Figure 3; B1 to B4 ). Other tissues (eg, skin, liver, and kidney), whether exposed to 4-OHT or not, revealed no X-gal staining (data not shown).



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  		Figure 3. LacZ expression induced by 4-OHT in whole-mount transgenic embryos and postnatal mice. A: Eighteen-day-old pups containing the Col1a1-CreERT2;R26R double transgene had been injected intraperitoneally with 1 mg of 4-OHT for 5 consecutive days starting at 12 days of age and were killed 48 hours later. B: Control pups were not injected with 4-OHT. X-Gal staining was strong in the bones of the skull (A1), calvaria (A2), digits (A3), and axial skeleton (A4) of 4-OHT-injected pups but weak in the corresponding bones of control pups (B1–B4). C: Female R26R mice were mated with male Col1a1-CreERT2 mice, injected intraperitoneally three times with 1 mg of 4-OHT or sunflower oil starting at 14.5 dpc, and then killed 48 hours later; the 18.5 dpc embryos were stained with X-gal. Only the Col1a1-CreERT2;R26R double-transgenic embryos from 4-OHT-treated dams show LacZ expression in bone (arrowheads). Higher-magnification images of the cranial view of the skull (D1 and E1), caudal view of the skull (D2 and E2), forelimbs (D3 and E3), vertebrae (D4 and E4), and ribs (D5 and E5) of 18.5 dpc double-transgenic embryos whose dams were treated with 4-OHT or sunflower oil, respectively.

 
To analyze patterns of Cre recombinase activity during embryonic development, we repeated the crossbreeding of male Col1a1-CreERT2 mice and female R26R mice. The pregnant mice were then given intraperitoneal injections of 1 mg of 4-OHT starting at 12.5, 13.5, or 14.5 dpc for 3 consecutive days. Injected female mice were sacrificed at 48 hours after the final injection. The pattern of X-gal staining of skeletal elements in the Col1a1-CreERT2;R26R double-transgenic embryos whose dams were treated with 4-OHT was similar at different stages of embryonic development (data not shown). X-Gal staining was very strong in all bones of these embryos, but no staining was observed in other tissues except the intestine (Figure 3C , top). Magnified images of the skull, forelimb, vertebra, and ribs, which stained strongly, indicated the presence of ligand-inducible Cre recombinase in all bones of these embryos (Figure 3; D1 to D5) . In contrast, in control embryos whose dams were treated with sunflower oil instead of 4-OHT, no X-gal staining was observed in bone or any other tissues except the intestine [Figure 3, C (bottom row) and E1 to E5]. The staining of the intestine was present in all embryos regardless of 4-OHT treatment because of endogenous galactosidase activity.

We then performed histological analysis of different bones of the Col1a1-CreERT2;R26R double-transgenic embryos and postnatal mice after administration of 4-OHT. For embryos whose dams had been treated with 4-OHT, coronal sections of the skull revealed strong X-gal staining in osteoblasts of the calvaria and the jaw (Figure 4, A1 and A2) , and sections of long bones showed strong staining in osteoblasts but not chondrocytes (Figure 4, A3 and A4) . At 18.5 dpc after 4-OHT administration, ~61% of the osteoblasts stained with X-gal indicating gene excision at the ROSA locus by Cre recombinase. In contrast, no staining of osteoblasts was detectable in oil-treated control embryos (Figure 4B) . No X-gal staining occurred in other organs such as heart, lung, liver, and kidney of embryos whose dams were treated with 4-OHT (Figure 4C) . In histological analysis of bones in postnatal mice, the staining pattern was similar to that seen in the embryos. X-Gal staining was clearly seen in the long bones (Figure 4D) , whereas none was visible in osteoblasts of control mice (Figure 4E) or in other organs such as skin, liver, and kidney of pups treated with 4-OHT (Figure 4F) . There was also no X-gal staining in brain, eyes, intestine, bladder, blood vessels, muscle, cartilage, and hematopoietic cells (data not shown). These results indicated that Cre recombination activated in a ligand-dependent manner occurred in osteoblasts under the control of the 2.3-kb Col1a1 promoter.



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  		Figure 4. Histological analysis of bones of Col1a1-CreERT2;R26R double-transgenic embryos and pups. Analysis was performed after whole-mount staining with X-gal and counterstaining with nuclear fast red. A and B: Histological results for 18.5 dpc Col1a1-CreERT2;R26R embryos whose dams had been treated with 4-OHT or sunflower oil embryos, respectively. A1, A2, B1, and B2: Coronal sections of the skull. Arrowheads indicate strong X-gal staining in osteoblasts of calvaria and jaw. A3 and B3: Longitudinal sections of the femur. A4 and B4: Higher-magnification images corresponding to the boxed areas in A3 and B3, respectively. In embryos whose dams had been treated with 4-OHT, no X-gal staining was detected in the heart (C1), lung (C2), liver (C3), or kidney (C4). D and E: Histological results of longitudinal section of femur from 18-day-old Col1a1-CreERT2;R26R pups with or without treatment with 4-OHT, respectively. X-Gal staining shows osteoblasts of femur. D2, D4, E2, and E4: Higher-magnification images corresponding to of the boxed areas in D1, D3, E1, and E3, respectively. B and BM in D3 and E3 indicate bone and bone marrow cavity, respectively. In pups treated with 4-OHT, no X-gal staining was detected in the skin (F1), liver (F2), or kidney (F3).

 
X-Gal-staining cells were also detected in teeth of 4-OHT-treated Col1a1-CreERT2;R26R double-transgenic embryos and pups. Odontoblasts in molars and incisors showed strong X-gal staining in 18.5 dpc Col1a1-CreERT2;R26R double-transgenic embryos whose dams had been treated with 4-OHT (Figure 5A) but not in control embryos (Figure 5B) . X-Gal staining of odontoblasts also occurred in 18-day-old pups treated with 4-OHT (Figure 5C) but not in control pups (Figure 5D) .



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  		Figure 5. Histological analysis of coronal sections of molars and incisors of Col1a1-CreERT2;R26R double-transgenic 18.5 dpc embryos (A and B) and 18-day-old mice (C and D). Staining was performed with X-gal and counterstaining with nuclear fast red. X-Gal staining was strong in the odontoblasts of molar and incisor. A and C: Histological results for the Col1a1-CreERT2;R26R embryos and pups treated with 4-OHT, respectively. B and D: Histological results for the Col1a1-CreERT2;R26R embryos and pups not treated with 4-OHT, respectively. Columns 2 and 4 are higher magnification images corresponding to columns 1 and 3, respectively.

 

   Discussion
Top
 Abstract
 Materials and Methods
 Results
 Discussion
References
 
Considerable progress has recently been made in understanding the role of transcription factors, signaling molecules, and extracellular matrix components in the formation of bones during embryonic development.18 These studies have relied in large part on the identification of genes mutated in specific human skeletal diseases19,20 and on the generation of mouse embryos and mice with targeted mutations in genes with major roles in bone formation.21,22 However, our understanding of the role of these factors in the physiology and homeostasis of bones after birth and in adult animals is much less complete. Often, mice with a gene that is essential for bone formation but that has been inactivated die during embryonic development or perinatally. Thus, embryonic or perinatal lethality because of disruption of these genes has prevented our studying their role in bone physiology in adult animals.

Gene-targeting technology based on the Cre/loxP system has been used as a tool to delete genes in specific tissues or at specific stages of development.5-7 In this system, the bacteriophage P1 Cre recombinase excises specific DNA fragments by recognizing loxP sites.23 Therefore, the generation and characterization of mouse strains expressing Cre recombinase in specific cell types have provided an essential tool for studying the function of specific genes in specific tissues.5

A transcriptional enhancer previously identified in the mouse and rat Col1a1 gene directs expression of reporter genes selectively in osteoblasts and odontoblasts of transgenic mice.10-13 This promoter has recently been used to generate mice expressing Cre recombinase in osteoblasts.11 Because this promoter is activated during embryonic development when osteoblast differentiation starts, inactivation of specific genes will occur during embryonic development and eventually affect bone formation and development.11 In this study, to examine the function of specific genes on bone and tooth physiology and homeostasis after birth, we have generated transgenic mice that express an inducible Cre recombinase (CreERT2) in osteoblasts and odontoblasts using the same 2.3-kb mouse Col1a1 promoter. Our results showed that the Cre-ERT2 gene was expressed specifically in osteoblasts and odontoblasts and that the Cre recombinase was active only after systemic administration of 4-OHT. On the basis of our results with the R26R reporter mouse strain, we estimate Cre-ERT2 gene was active in most osteoblasts and odontoblasts of 18.5 dpc embryos. This appears also to be the case 3 weeks after birth although the penetration of X-gal into bone and tooth tissues may be less complete at that time. Furthermore, our results are consistent with a previous report that the 2.3-kb fragment of Col1a1 promoter directs strong expression of reporter genes in osteoblasts and odontoblasts.10

The mouse strain we generated will provide opportunities to examine the consequences of the ablation of a number of specific genes after birth. For instance, null mutations in Runx221,22 or Osterix,24 which are transcription factors required for osteoblast differentiation, cause lethality in the immediate perinatal period. When conditional alleles for the genes of these proteins become available, their postnatal inactivation should generate essential information about their role after embryonic development is completed and should provide a proof of principle about their function during steady-state bone remodeling. Similar experiments with genes for components of signaling pathways (eg, the Wnt/ß-catenin, the bone morphogenetic protein, and the TGF-ß pathway) known to affect bone formation will also be possible. The same approach might also be used with genes for bone extracellular matrix components. Furthermore, this approach may be important to understand better the chronic diseases of bone (eg, osteoporosis) that characteristically occur late in life. Thus, we believe that the Col1a1-CreERT2 transgenic mice we have generated will become an important tool with which to study bone and tooth physiology and diseases.


   Acknowledgements
 
We thank Mr. Chad Smith and Dr. Jan Parker-Thornburg of the University of Texas M.D. Anderson’s Genetically Engineered Mouse Facility for performing the pronuclear injections; and Dr. John Clifford of M.D. Anderson Cancer Center for providing plasmid pSG-CreERT2.


   Footnotes
 
Address reprint requests to Benoit de Crombrugghe, M.D., Department of Molecular Genetics, Unit 011, The University of Texas M. D. Anderson Cancer Center, 1515 Holcombe Blvd., Houston, TX 77030. E-mail: bdecromb{at}mdanderson.org

Supported by the National Institutes of Health (grant R01 AR49072 to B.d.C.).

Current address of Kazuhisa Nakashima, D.D.S., Ph.D., is Department of Molecular Pharmacology Medical Research Institute, Tokyo Medical and Dental University 3-10 Kanda-Surugadai 2-chrome Chiyoda-Ku, Tokyo 101-0062. Japan.

Accepted for publication August 13, 2004.


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 Materials and Methods
 Results
 Discussion
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