1 Department of Pediatrics (Cardiology), Baylor College of Medicine, Houston, TX
77030, USA
2 Program in Cardiovascular Sciences, Baylor College of Medicine, Houston, TX
77030, USA
3 Department of Biochemistry and Molecular Biology, MD Anderson Cancer Center
and Program in Genes and Development, Graduate School of Biomedical Sciences,
University of Texas, Houston, TX 77030, USA
4 Alkek Institute of Biosciences and Technology, Texas A and M System Health
Science Center, Houston, TX 77030, USA
5 Center for Cardiovascular Development, Baylor College of Medicine, Houston, TX
77030, USA
6 Department of Human and Molecular Genetics, Baylor College of Medicine,
Houston, TX 77030, USA
7 CEINGE Biotecnologie Avanzate S.C. ar. I., via Communale Margherita,
482-80145, Naples, Italy
8 Division of Cardiology, Second University of Naples, Naples, Italy
* Author for correspondence (e-mail: elindsay{at}bcm.tmc.edu)
Accepted 13 September 2005
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SUMMARY |
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Key words: Tbx1, DiGeorge syndrome, 22q11DS, Pharyngeal epithelia, Mouse
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Introduction |
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Tbx1 has been identified as a human disease gene. Mutational
analysis has revealed that TBX1 mutation causes a disease phenotype
that is essentially identical to that associated with the relatively common
genetic disorder known as 22q11 deletion syndrome (22q11DS)
(Yagi et al., 2003), which
includes DiGeorge syndrome and velocardiofacial syndrome. 22q11DS is caused by
gene haploinsufficiency and it is the most common microdeletion syndrome,
occurring in
1:4000 live births (Botto
et al., 2003
; McDonald-McGinn
et al., 1997
; Ryan et al.,
1997
; Wilson et al.,
1994
). 22q11DS is caused by an
3 Mb heterozygous deletion in
22q11.2 that includes TBX1 and
40 other genes
(http://www.ensembl.org).
One of the cardinal features of 22q11DS and of TBX1 mutation is
congenital heart disease, the most common defects being cardiac outflow tract
defects and aortic arch abnormalities. In particular, interrupted aortic arch
type B (IAA-B) is mainly caused by this genetic defect
(Lewin et al., 1997
;
Rauch et al., 1998
), making it
one of the most etiologically homogeneous cardiovascular defects known. Other
common cardiovascular defects are aberrant origin of the right subclavian
artery (ARSA) and the right aortic arch (RAA). In mice, heterozygous
inactivation of Tbx1 causes the same aortic arch abnormalities as
those seen in patients (Jerome and
Papaioannou, 2001
; Lindsay et
al., 2001
; Merscher et al.,
2001
), while Tbx1 loss of function causes a much more
severe phenotype that is only rarely seen in patients. Therefore, the
Tbx1+/ mouse is a genetically accurate model of the
22q11DS cardiovascular phenotype and, in particular, of fourth PAA-derived
aortic arch abnormalities.
In mice, Tbx1 is required for fourth PAA formation and growth. We
have shown that 100% of Tbx1+/ embryos have
hypoplastic fourth PAAs at E10.5, whereas, at term, 30-50% have fourth
PAA-derived cardiovascular defects (specifically, IAA-B, RAA and ARSA),
according to whether the left, right, or both fourth PAAs are affected
(Lindsay and Baldini, 2001).
The penetrance of these defects in E10.5 and term embryos varies with genetic
background (Lindsay and Baldini,
2001
) (this study). Although it is not usually possible to
ascertain the embryonic origin of these cardiovascular abnormalities in
humans, it is likely that they have the same embryological basis as in
mice.
The Tbx1 expression domains of mid-gestation mouse embryos have
been described (Chapman et al.,
1996; Garg et al.,
2001
; Jerome and Papaioannou,
2001
; Lindsay et al.,
2001
; Merscher et al.,
2001
; Vitelli et al.,
2002a
). Briefly, they comprise pharyngeal endoderm, mesoderm and
ectoderm, mesenchyme of arches III and IV, sclerotome, otocyst and head
mesenchyme. Tbx1 is not expressed in neural crest cells that
infiltrate the pharyngeal arches and cardiac outflow tract. When the fourth
PAA forms at around E9.75, Tbx1 is most highly expressed in the
endoderm of the fourth arch and pouch. Fgf8 is also expressed in
pharyngeal endoderm at this time, and we have hypothesized that Tbx1 may
regulate fourth PAA development by activating signals from pharyngeal
endoderm, perhaps via Fgf8, that are directed towards the underlying
mesenchyme surrounding the PAAs (Vitelli
et al., 2002b
). In support of this hypothesis, we have shown that
Tbx1 and Fgf8 interact genetically in fourth PAA development
(Vitelli et al., 2002b
), and a
more recent study has shown that Fgf8 expression is required in
surface ectoderm for fourth PAA development
(Macatee et al., 2003
).
However, Tbx1 is expressed in many tissues potentially involved in
the development of these arteries.
To understand the developmental and genetic mechanisms governing the formation, growth and remodeling of the fourth PAAs, it is necessary to understand the role of the individual tissues. This can be approached using tissue-specific, gene-dosage reduction. To this end, we have used a panel of Cre drivers (Table 1) that induce recombination in different tissues of the pharyngeal apparatus to delete one copy of the Tbx1 gene. Results clearly indicate that the critical tissue for early fourth PAA development is the pharyngeal epithelia. We believe that the use of multiple Cre drivers, with unique but partially overlapping patterns of Cre recombination, strengthens the major conclusions of our study because it permits the confirmation of results obtained with individual Cre drivers, which are likely to have some inherent variability due to minor differences in the onset and extent of recombination.
|
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Materials and methods |
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Generation of new mouse lines
We generated two stable transgenic Fgf15-Cre lines. For the first line, we
cloned an Hsp68 minimal promoter and Cre recombinase downstream of a 4.2-kb
XhoI-BamHI fragment from the upstream promoter region of
Fgf15 (J.V. and Y.F., unpublished). This fragment, in combination
with the Hsp68 minimal promoter and a lacZ reporter, drives
expression specifically in pharyngeal ectoderm and endoderm (not shown). To
analyze Fgf15-Cre-induced recombination, we bred a single founder
(TgFgf15HspCre) with R26R reporter mice and analyzed embryos with the genotype
TgFgf15HspCre; R26R by X-gal staining. For the second transgenic line, we used
the endogenous Fgf15 promoter instead of Hsp68. This promoter fragment extends
from base pair 520 to 80 (considering the first base of the
Fgf15 trascription start codon (ATG) as base pair 1). The promoter
fragment, in combination with Cre recombinase was cloned downstream of a 4-kb
XhoI-NheI fragment from the upstream promoter region of
Fgf15 (J.V. and Y.F., unpublished). Cre-induced recombination from
this second transgenic line was analyzed by breeding a single founder
(TgFgf15Cre) with R26R reporter mice and analyzing embryos with the genotype
TgFgf15Cre; R26R by X-gal staining. To generate Tbx1 conditional
mutants, we bred TgFgf15HspCre or TgFgf15Cre mice with
Tbx1flox/flox mice and analyzed the phenotype of embryos
with the genotypes TgFgf15HspCre; Tbx1flox/+ or
TgFgf15Cre; Tbx1flox/+, respectively.
X-gal staining, histology and RNA in situ hybridization
To visualize ß-gal activity, paraformaldehyde-fixed embryos were
stained using X-gal substrate, according to standard procedures. Stained
embryos were photographed as whole-mount specimens and then embedded in
paraffin wax and cut into 10 µm histological sections. Sections were
counterstained with Nuclear Fast Red. RNA in situ hybridization experiments
were performed on 10 µm embryo sections, according to a published protocol
(Albrecht et al., 1997).
Labeled sense and antisense probes were prepared by reverse transcription of
DNA clones in the presence of 35S-UTP (MP Biomedical).
![]() |
Results |
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To determine whether heterozygous loss of Tbx1 in endothelial
precursors causes fourth PAA abnormalities, we bred Tie2-Cre or
Mesp1Cre/+ mice, both of which express Cre in endothelial
precursors (Kisanuki et al.,
2001; Saga et al.,
1996
), with Tbx1flox/flox mice and analyzed
the fourth PAAs of conditional mutants at E10.5 by intracardiac ink injection.
No fourth PAA abnormalities were seen in Tie2-Cre;
Tbx1flox/+ embryos (data not shown), or
Mesp1Cre/+; Tbx1flox/+ embryos (reported fully
below), indicating that loss of Tbx1 in endothelial precursors does
not cause the Tbx1+/ haploinsufficiency
phenotype.
Mesodermal expression of Tbx1 is not required for fourth PAA development
Nkx2.5cre-induced recombination occurs in pharyngeal
mesoderm, specifically in the core mesoderm of arches I-III
(Fig. 2B, part a), the cardiac
outflow tract and the secondary heart field
(Fig. 2B, part b), as well as
in pharyngeal endoderm (Fig.
2B, part c, Fig.
3B') and ectoderm (Fig.
3B'). Tbx1 is also expressed in these tissues
(Fig. 2A). However, there is
little overlap between Tbx1 expression and
Nkx2.5cre-induced recombination in the pharyngeal endoderm
at E10.5 (compare Fig. 2A part
c with 2B part c).
Specifically, Nkx2.5cre recombination is most prominent in
the floor of the pharynx, whereas Tbx1 expression is most prominent
in the lateral pharynx and developing pouches. Furthermore, in earlier embryos
(E9), both Tbx1 expression and Nkx2.5Cre-induced
recombination in the mesoderm is patchy (compare
Fig. 3A' with 3B'). Xu et al. showed that Nkx2.5Cre;
Tbx1flox/ embryos have the same outflow tract phenotype
as Tbx1/ embryos do, but that the aortic
arch phenotype is much milder (Xu et al.,
2004). In that study, the effect of deleting one copy of
Tbx1 in the Nkx2.5 expression domain on fourth PAA formation
was not tested. To do this, we analyzed conditional mutant embryos
(Nkx2.5Cre/+; Tbx1flox/+) at E10.5 by
intracardiac ink injection at E10.5. None of the twelve conditional mutants
had fourth PAA abnormalities (Fig.
2B, part d), suggesting that Nkx2.5cre-induced
recombination does not occur in the cells that require Tbx1 for
fourth PAA formation and growth. In order to achieve more extensive
recombination in pharyngeal mesoderm, we used an alternative mesoderm Cre
driver.
|
|
To investigate the cause of reduced penetrance of the fourth PAA phenotype, we analyzed Cre-induced recombination at earlier embryonic stages. Recombination was first detectable in pharyngeal endoderm, and to a lesser extent in surface ectoderm, at E8.5 (Fig. 4B', Table 1). At this stage and at E9 (Fig. 3D'), recombination was patchy. Therefore, reduced penetrance of the fourth PAA phenotype in Foxg1Cre conditional mutants may be due to the late activation of Cre-induced recombination of the Tbx1-floxed allele in some cells. As with Nkx2.5Cre and Mesp1Cre, Foxg1Cre induces recombination in mesodermally derived tissues of the pharyngeal apparatus, but, unlike the former Cre drivers, Foxg1Cre also induces extensive recombination in pharyngeal endoderm and surface ectoderm, indicating that it is the recombination in these latter tissues that causes the fourth PAA hypoplasia in conditional mutants.
To dissect further the tissue requirement of Tbx1 in fourth PAA
development, we generated two transgenic Fgf15Cre driver lines that are
specific for pharyngeal epithelia. For the first line, we used a 4.2-kb
enhancer fragment located 4 kb upstream from the transcription start site
of the Fgf15 gene that contains putative tissue-specific regulatory
elements (J.V. and Y.F., unpublished). This fragment, in combination with an
Hsp68 minimal promoter can drive the expression of a lacZ reporter in
pharyngeal ectoderm and endoderm in transgenic embryos (not shown). We used
this enhancer fragment and an Hsp68 promoter in combination with Cre
recombinase to generate a stable transgenic line. A second stable transgenic
line was generated using a closely related enhancer fragment (
4 kb
XhoI-NheI) in combination with the endogenous Fgf15
promoter and Cre recombinase. Cre-induced recombination was analyzed by
crossing transgenic founders with R26R mice. A similar pattern of pharyngeal
epithelial-specific Cre-induced recombination was seen from founders of both
transgenic lines, although, from the TgFgf15HspCre founder, we observed some
embryos (
20%) with variable levels of ectopic Cre-induced recombination.
From the TgFgf15Cre founder, no ectopic recombination was seen in any of the
23 TgFgf15Cre; R26R embryos analyzed. All illustrations are from this latter
transgenic line. Cre-induced recombination was first seen in pharyngeal
endoderm at E9 (Fig. 4C', Table 1), in a more anterior
position to the Tbx1 endodermal expression that is observed at a
similar stage (compare Fig.
3A' with Fig.
4C'). From E9.25 (Fig.
3E), Cre-induced recombination encompassed all surface ectoderm
and pharyngeal endoderm (Fig.
2E, part c; Fig.
3E'), between and including arch I and arch IV. As no
recombination was seen in the core arch mesoderm
(Fig. 2E, part a) or arch
mesenchyme (Fig. 2E, part c;
Fig. 3E'), we conclude
that recombination is epithelial specific.
In order to evaluate whether Tbx1 dosage reduction specifically in pharyngeal endoderm and ectoderm was sufficient to recapitulate the Tbx1+/ haploinsufficiency phenotype, we bred founders from both transgenic lines with Tbx1flox/flox mice and analyzed the phenotype of conditional mutants (TgFgf15HspCre; Tbx1flox/+ or TgFgf15Cre; Tbx1flox/+) at E10.5 by intracardiac ink injection. Fifty percent of TgFgf15HspCre conditional mutants had hypoplasia of one or both fourth PAAs (n=14, not shown). From founder TgFgf15Cre, which gave no embryos with ectopic recombination, we obtained similar results, although at a lower penetrance (20%, n=34, Fig. 2E, part d). Together, these data indicate that Tbx1 dosage reduction in pharyngeal epithelia is sufficient to cause fourth PAA hypoplasia.
|
Tbx1 expression in non-epithelial tissues does not contribute to early fourth PAA development
Reduced penetrance of fourth PAA hypoplasia in TgFgf15Cre and
Foxg1Cre conditional mutants may occur because Tbx1 is
required before these Cre alleles are fully activated (after E9), or because
Tbx1 expression in non-epithelial tissues, although insufficient to
ensure normal fourth PAA formation, has an additive effect. To begin to
address these possibilities, we used the Hoxa3Cre driver,
which induces recombination in all pharyngeal tissues caudal to pharyngeal
arch II. Hoxa3Cre-induced recombination begins in the
caudal part of embryos before E8 (not shown). At E8.25, recombination extends
into the pharyngeal region, where it is weak and confined to the surface
ectoderm (Fig. 4D,D'). By
E8.5, recombination extends into the pharyngeal mesoderm and endoderm
(Fig. 4E,E'), and from
late E9, it includes all of the pharyngeal tissues
(Fig. 3F,F',
Fig. 2F). We bred
Hoxa3cre/+ mutants with Tbx1flox/flox
mutants and analyzed the phenotype of conditional (Hoxa3Cre/+;
Tbx1flox/+) mutants by intracardiac ink injection at E10.5.
Fifty percent of conditional mutants had fourth PAA hypoplasia (n=17,
Fig. 2F, part d). Thus, even
though Hoxa3Cre recombines in all pharyngeal tissues, the
penetrance of fourth PAA hypoplasia was similar to that obtained with other
Cre drivers that recombine in the pharyngeal epithelia
(Foxg1Cre and TgFgf15Cre). This suggests that
Tbx1 expression in non-epithelial tissues does not have an additive
effect on fourth PAA growth at this stage. Our results also suggest that the
requirement for Tbx1 expression begins before
Hoxa3Cre recombination in pharyngeal tissues occurs.
![]() |
Discussion |
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![]() ![]() ![]() ![]() ![]() ![]() ![]() |
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The recent finding of TBX1 mutations in patients with a classical clinical presentation but without a 22q11.2 deletion strongly supports the extensive evidence gathered from mouse studies that Tbx1 is the major gene involved in 22q11DS. In humans, the most common genetic cause of IAA-B is 22q11DS and Tbx1 mutation causes similar defects. Thus, Tbx1 gene dosage is critical for fourth PAA development, making the Tbx1+/ mouse an excellent model in which to study the molecular basis of aortic arch artery abnormalities. In order to understand the mechanism by which Tbx1 dosage reduction affects fourth PAA development it is important to determine where Tbx1 is required for this function. It is likely that fourth PAA development requires the interaction of different tissues. We have previously hypothesized that Tbx1 in pharyngeal endoderm may provide molecular instructions for the formation, growth and remodeling of the PAAs. However, Tbx1 is expressed in many tissues that could contribute to fourth PAA development, including, as we have shown here, pharyngeal ectoderm and precursors of fourth PAA endothelial cells. We therefore considered the Cre-driver strategy to be an effective way to address systematically the tissue requirement for Tbx1 in fourth PAA development. Furthermore, by using multiple Cre drivers with different but partially overlapping patterns of recombination, we have been able to reaffirm results obtained with individual Cre drivers. Thus, we have found fourth PAA defects in embryos where Tbx1 expression was conditionally reduced via three Cre drivers, all of which are robustly expressed in pharyngeal epithelia (TgFgf15Cre, Foxg1Cre, Hoxa3Cre), whereas we did not find these abnormalities with Cre drivers that were not expressed in pharyngeal epithelia (Mesp1Cre), or where it was only partially expressed (Nkx2.5Cre). The most compelling data were obtained with the TgFgf15Cre driver, which expresses exclusively in pharyngeal epithelia and thereby demonstrates conclusively that Tbx1 is required in this tissue for early fourth PAA development. We thereby also demonstrate for the first time that Tbx1 function in fourth PAA development is cell non-autonomous. We exclude that Tbx1 in mesoderm contributes to early phases of fourth PAA development because of the early expression of the mesoderm-specific Cre driver Mesp1Cre.
The Cre driver strategy used here has also given us an insight into the time requirement for Tbx1 for fourth PAA development. Specifically, we have shown that Tbx1 is expressed in surface ectoderm and pharyngeal endoderm from E8.25 (7 somites), whereas the earliest time in which a uniform Cre-induced recombination occurred in both pharyngeal epithelia (via the Hoxa3Cre driver) was at E9 (12-19 somites). As the fourth PAA defects resulting from Hoxa3Cre-induced deletion of Tbx1 occurred at a reduced penetrance (50%), this suggests that there is a critical requirement for Tbx1 between E8 and E9. This requirement may continue after E9. Such an early time requirement for Tbx1 was unexpected, as the fourth PAAs do not develop until 18-24 hours later at E9.75, and it may indicate that Tbx1 is involved in patterning the caudal arches.
The only known transcriptional target of Tbx1 that could
potentially mediate a cell non-autonomous function is Fgf10
(Xu et al., 2004). We have
proposed that Fgf10 may interact with Tbx1 in the secondary
heart field to regulate the proliferation of myocyte precursors fated to the
outflow tract (Xu et al.,
2004
). However, a similar interaction cannot be invoked for the
regulation of fourth PAA development because Fgf10 is not expressed
in pharyngeal endoderm or ectoderm at the relevant developmental stage. Three
other genes have been proposed to interact with Tbx1 Vegf,
chordin (Chrd) and Fgf8 all of which are
co-expressed with Tbx1 in pharyngeal endoderm.
Vegf has been proposed to be an upstream regulator of
Tbx1 in PAA development (Stalmans
et al., 2003). However, at E10.5, Vegf mutants were
reported to have an enlargement of the right dorsal aorta and a local
narrowing of an otherwise apparently well-grown fourth and sixth PAA. This
phenotype is quite different to that seen in Tbx1+/
mutants, which is characterized by overall hypoplasia of the fourth PAAs,
which is often severe, but does not extend to the sixth PAAs. Therefore,
Vegf is likely to exert its effect on PAA development via a different
mechanism.
Chrd-null mice have a phenotype that is strikingly similar to that
of Tbx1/ mutants, including similar fourth
PAA-derived cardiovascular defects and cardiac outflow tract defects
(Bachiller et al., 2003).
Tbx1 expression was reported to be reduced in
Chrd/ embryos at E9, suggesting that the
gene may lie upstream of Tbx1 in the regulation of pharyngeal
development. However, neither the development of the fourth PAA in
Chrd/ embryos at mid-gestation nor the
phenotype of Chrd+/ mice have been reported, so we
do not know by what mechanism mutation of Chrd affects fourth PAA
development.
Fgf8 is strongly expressed in the pharyngeal epithelia of
mid-gestation embryos (Fig.
1C), and we have demonstrated that the two genes interact
genetically in fourth PAA development
(Vitelli et al., 2002b).
Recently, a Tbx1-responsive enhancer has been identified in the
5' region of the Fgf8 gene
(Hu et al., 2004
), but
Fgf8 has not yet been demonstrated to be a direct transcriptional
target of Tbx1. Currently, Fgf8 is the only gene other than
Tbx1 for which a tissue-specific requirement in fourth PAA
development has been demonstrated (Macatee
et al., 2003
), intriguingly in pharyngeal ectoderm. Therefore, our
finding that Tbx1 is also expressed in pharyngeal ectoderm, albeit
transiently, raises the question as to whether the two genes may interact in
pharyngeal ectoderm to regulate fourth PAA development. However, several lines
of evidence from this study and from the study of Macatee et al. suggest that
Tbx1 and Fgf8 operate through different mechanisms in
ectoderm. In the study of Macatee et al., conditional mutagenesis was used to
ablate Fgf8 specifically in pharyngeal ectoderm, which resulted in a
range of fourth PAA-derived cardiovascular defects in conditional mutants
(Macatee et al., 2003
).
However, ectodermal expression of Fgf8 is robust in
Tbx1/ mutants, therefore it is unlikely that
Tbx1 regulates Fgf8 expression in the ectoderm. In addition,
extensive apoptosis of neural crest cells was observed in arch IV of
Fgf8 conditional mutants, which could account for the fourth PAA
growth failure, as suggested by the authors, but we have not detected
increased apoptosis in arch IV of Tbx1+/ mutants
(Vitelli et al., 2002b
). We do
not yet know the effect of endoderm-specific deletion of Fgf8 on
fourth PAA development, thus it is still possible that Fgf8 and
Tbx1 interact in the endoderm and affect fourth PAA development
through a mechanism other than cell death.
The Tbx1/ phenotype demonstrates that the
gene is required for formation of the PAAs, thus it is reasonable to think
that fourth PAA hypoplasia in Tbx+/ embryos is a
milder consequence of a dosage-sensitive role of Tbx1 during the
formation of the arteries. There is a precedent for endodermal induction of
vessel formation from classical embryology and anatomical studies, which
showed that the earliest intraembryonic vessels arise close to endoderm. More
recent studies, performed with the aid of endothelial-specific molecular
markers, have confirmed the overall requirement of endoderm for early
vasculogenesis, and several endodermally expressed genes have been shown to be
involved in this process in various species, specifically, Fgf2
(Riese et al., 1995),
Vegf122 (Cleaver and
Krieg, 1998
), Gdf6
(Hall et al., 2002
),
Ihh (Byrd et al.,
2002
; Dyer et al.,
2001
), Shh (Vokes et
al., 2004
) and Hhex
(Hallaq et al., 2004
).
However, it is less clear whether endodermal induction of vessel formation
continues into later stages of mouse embryogenesis.
We propose a model whereby Tbx1 regulates fourth PAA formation by activating signals from the pharyngeal endoderm, via Fgf8 and/or other extracellular signaling systems. These signals are directed either towards the mesenchyme surrounding the nascent vessels, or towards the dorsal aortae, from which the PAAs may sprout and which at the time of fourth PAA formation are in close contact with pharyngeal endoderm (Fig. 1A').
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ACKNOWLEDGMENTS |
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REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Abu-Issa, R., Smyth, G., Smoak, I., Yamamura, K. and Meyers, E.
N. (2002). Fgf8 is required for pharyngeal arch and
cardiovascular development in the mouse. Development
129,4613
-4625.
Albrecht, U., Eichele, G., Helms, J. A. and Lu, H. C. (1997). Visualization of gene expression patterns by in situ hybridization. In Molecular and Cellular Methods in Developmental Toxicology (ed. G. P. Daston), pp.23 -48. New York: CRC Press, Inc.
Bachiller, D., Klingensmith, J., Shneyder, N., Tran, U.,
Anderson, R., Rossant, J. and De Robertis, E. M. (2003). The
role of chordin/Bmp signals in mammalian pharyngeal development and DiGeorge
syndrome. Development
130,3567
-3578.
Botto, L. D., May, K., Fernhoff, P. M., Correa, A., Coleman, K.,
Rasmussen, S. A., Merritt, R. K., O'Leary, L. A., Wong, L. Y., Elixson, E. M.
et al. (2003). A population-based study of the 22q11.2
deletion: phenotype, incidence, and contribution to major birth defects in the
population. Pediatrics
112,101
-107.
Byrd, N., Becker, S., Maye, P., Narasimhaiah, R., St-Jacques, B., Zhang, X., McMahon, J., McMahon, A. and Grabel, L. (2002). Hedgehog is required for murine yolk sac angiogenesis. Development 129,361 -372.[Medline]
Chapman, D. L., Garvey, N., Hancock, S., Alexiou, M., Agulnik, S. I., Gibson-Brown, J. J., Cebra-Thomas, J., Bollag, R. J., Silver, L. M. and Papaioannou, V. E. (1996). Expression of the T-box family genes, Tbx1-Tbx5, during early mouse development. Dev. Dyn. 206,379 -390.[CrossRef][Medline]
Cleaver, O. and Krieg, P. A. (1998). VEGF
mediates angioblast migration during development of the dorsal aorta in
Xenopus. Development
125,3905
-3914.
Clouthier, D. E., Hosoda, K., Richardson, J. A., Williams, S.
C., Yanagisawa, H., Kuwaki, T., Kumada, M., Hammer, R. E. and Yanagisawa,
M. (1998). Cranial and cardiac neural crest defects in
endothelin-A receptor-deficient mice. Development
125,813
-824.
Dyer, M. A., Farrington, S. M., Mohn, D., Munday, J. R. and
Baron, M. H. (2001). Indian hedgehog activates hematopoiesis
and vasculogenesis and can respecify prospective neurectodermal cell fate in
the mouse embryo. Development
128,1717
-1730.
Frank, D. U., Fotheringham, L. K., Brewer, J. A., Muglia, L. J.,
Tristani-Firouzi, M., Capecchi, M. R. and Moon, A. M. (2002).
An Fgf8 mouse mutant phenocopies human 22q11 deletion syndrome.
Development 129,4591
-4603.
Garg, V., Yamagishi, C., Hu, T., Kathiriya, I. S., Yamagishi, H. and Srivastava, D. (2001). Tbx1, a digeorge syndrome candidate gene, is regulated by sonic hedgehog during pharyngeal arch development. Dev. Biol. 235, 62-73.[CrossRef][Medline]
Guris, D. L., Fantes, J., Tara, D., Druker, B. J. and Imamoto, A. (2001). Mice lacking the homologue of the human 22q11.2 gene CRKL phenocopy neurocristopathies of DiGeorge syndrome. Nat. Genet. 27,293 -298.[CrossRef][Medline]
Hall, C. J., Flores, M. V., Davidson, A. J., Crosier, K. E. and Crosier, P. S. (2002). Radar is required for the establishment of vascular integrity in the zebrafish. Dev. Biol. 251,105 -117.[CrossRef][Medline]
Hallaq, H., Pinter, E., Enciso, J., McGrath, J., Zeiss, C.,
Brueckner, M., Madri, J., Jacobs, H. C., Wilson, C. M., Vasavada, H. et
al. (2004). A null mutation of Hhex results in abnormal
cardiac development, defective vasculogenesis and elevated Vegfa levels.
Development 131,5197
-5209.
Hebert, J. M. and McConnell, S. K. (2000). Targeting of cre to the Foxg1 (BF-1) locus mediates loxP recombination in the telencephalon and other developing head structures. Dev. Biol. 222,296 -306.[CrossRef][Medline]
Hu, T., Yamagishi, H., Maeda, J., McAnally, J., Yamagishi, C.
and Srivastava, D. (2004). Tbx1 regulates fibroblast growth
factors in the anterior heart field through a reinforcing autoregulatory loop
involving forkhead transcription factors. Development
131,5491
-5502.
Iida, K., Koseki, H., Kakinuma, H., Kato, N., Mizutani-Koseki,
Y., Ohuchi, H., Yoshioka, H., Noji, S., Kawamura, K., Kataoka, Y. et al.
(1997). Essential roles of the winged helix transcription factor
MFH-1 in aortic arch patterning and skeletogenesis.
Development 124,4627
-4638.
Jerome, L. A. and Papaioannou, V. E. (2001). DiGeorge syndrome phenotype in mice mutant for the T-box gene, Tbx1. Nat. Genet. 27,286 -291.[CrossRef]
Kisanuki, Y. Y., Hammer, R. E., Miyazaki, J., Williams, S. C., Richardson, J. A. and Yanagisawa, M. (2001). Tie2-Cre transgenic mice: a new model for endothelial cell-lineage analysis in vivo. Dev. Biol. 230,230 -242.[CrossRef][Medline]
Kume, T., Deng, K. and Hogan, B. L. (2000).
Murine forkhead/winged helix genes Foxc1 (Mf1) and Foxc2 (Mfh1) are required
for the early organogenesis of the kidney and urinary tract.
Development 127,1387
-1395.
Kume, T., Jiang, H., Topczewska, J. M. and Hogan, B. L.
(2001). The murine winged helix transcription factors, Foxc1 and
Foxc2, are both required for cardiovascular development and somitogenesis.
Genes Dev. 15,2470
-2482.
Lewin, M. B., Lindsay, E. A., Jurecic, V., Goytia, V., Towbin, J. A. and Baldini, A. (1997). A genetic etiology for interruption of the aortic arch type B. Am. J. Cardiol. 80,493 -497.[CrossRef][Medline]
Lindsay, E. A. and Baldini, A. (2001). Recovery
from arterial growth delay reduces penetrance of cardiovascular defects in
mice deleted for the DiGeorge syndrome region. Hum. Mol.
Genet. 10,997
-1002.
Lindsay, E. A., Vitelli, F., Su, H., Morishima, M., Huynh, T., Pramparo, T., Jurecic, V., Ogunrinu, G., Sutherland, H. S., Scambler, P. J. et al. (2001). Tbx1 haploinsufficiency in the DiGeorge syndrome region causes aortic arch defects in mice. Nature 410,97 -101.[CrossRef][Medline]
Liu, C., Liu, W., Palie, J., Lu, M. F., Brown, N. A. and Martin,
J. F. (2002). Pitx2c patterns anterior myocardium and aortic
arch vessels and is required for local cell movement into atrioventricular
cushions. Development
129,5081
-5091.
Macatee, T. L., Hammond, B. P., Arenkiel, B. R., Francis, L.,
Frank, D. U. and Moon, A. M. (2003). Ablation of specific
expression domains reveals discrete functions of ectoderm- and
endoderm-derived FGF8 during cardiovascular and pharyngeal development.
Development 130,6361
-6374.
McDonald-McGinn, D. M., LaRossa, D., Goldmuntz, E., Sullivan, K., Eicher, P., Gerdes, M., Moss, E., Wang, P., Solot, C., Schultz, P. et al. (1997). The 22q11.2 deletion: screening, diagnostic workup, and outcome of results; report on 181 patients. Genet. Test. 1,99 -108.[Medline]
Merscher, S., Funke, B., Epstein, J. A., Heyer, J., Puech, A., Min Lu, M. M., Xavier, R. J., Demay, M. B., Russell, R. G., Factor, S. et al. (2001). TBX1 is responsible for cardiovascular defects in Velo-Cardio-Facial/DiGeorge Syndrome. Cell 104,619 -629.[CrossRef][Medline]
Molin, D. G., DeRuiter, M. C., Wisse, L. J., Azhar, M., Doetschman, T., Poelmann, R. E. and Gittenberger-de Groot, A. C. (2002). Altered apoptosis pattern during pharyngeal arch artery remodelling is associated with aortic arch malformations in Tgfbeta2 knock-out mice. Cardiovasc. Res. 56,312 -322.[CrossRef][Medline]
Moses, K. A., DeMayo, F., Braun, R. M., Reecy, J. L. and Schwartz, R. J. (2001). Embryonic expression of an Nkx2-5/Cre gene using ROSA26 reporter mice. Genesis 31,176 -180.[CrossRef][Medline]
Rauch, A., Hofbeck, M., Leipold, G., Klinge, J., Trautmann, U., Kirsch, M., Singer, H. and Pfeiffer, R. A. (1998). Incidence and significance of 22q11.2 hemizygosity in patients with interrupted aortic arch. Am. J. Med. Genet. 78,322 -331.[CrossRef][Medline]
Riese, J., Zeller, R. and Dono, R. (1995). Nucleo-cytoplasmic translocation and secretion of fibroblast growth factor-2 during avian gastrulation. Mech. Dev. 49, 13-22.[CrossRef][Medline]
Ryan, A. K., Goodship, J. A., Wilson, D. I., Philip, N., Levy, A., Seidel, H., Schuffenhauer, S., Oechsler, H., Belohradsky, B., Prieur, M. et al. (1997). Spectrum of clinical features associated with interstitial chromosome 22q11 deletions: a European collaborative study [see comments]. J. Med. Genet. 34,798 -804.[Abstract]
Saga, Y., Hata, N., Kobayashi, S., Magnuson, T., Seldin, M. F.
and Taketo, M. M. (1996). MesP1: a novel basic
helix-loop-helix protein expressed in the nascent mesodermal cells during
mouse gastrulation. Development
122,2769
-2778.
Saga, Y., Miyagawa-Tomita, S., Takagi, A., Kitajima, S.,
Miyazaki, J. and Inoue, T. (1999). MesP1 is expressed in the
heart precursor cells and required for the formation of a single heart tube.
Development 126,3437
-3447.
Soriano, P. (1999). Generalized lacZ expression with the ROSA26 Cre reporter strain. Nat. Genet. 21, 70-71.[CrossRef][Medline]
Stalmans, I., Lambrechts, D., De Smet, F., Jansen, S., Wang, J., Maity, S., Kneer, P., von der Ohe, M., Swillen, A., Maes, C. et al. (2003). VEGF: a modifier of the del22q11 (DiGeorge) syndrome? Nat. Med. 9,173 -182.[CrossRef][Medline]
Verrou, C., Zhang, Y., Zurn, C., Schamel, W. W. and Reth, M. (1999). Comparison of the tamoxifen regulated chimeric Cre recombinases MerCreMer and CreMer. Biol. Chem. 380,1435 -1438.[CrossRef][Medline]
Vitelli, F., Morishima, M., Taddei, I., Lindsay, E. A. and
Baldini, A. (2002a). Tbx1 mutation causes multiple
cardiovascular defects and disrupts neural crest and cranial nerve migratory
pathways. Hum. Mol. Genet.
11,915
-922.
Vitelli, F., Taddei, I., Morishima, M., Meyers, E. N., Lindsay,
E. A. and Baldini, A. (2002b). A genetic link between Tbx1
and fibroblast growth factor signaling. Development
129,4605
-4611.
Vokes, S. A., Yatskievych, T. A., Heimark, R. L., McMahon, J.,
McMahon, A. P., Antin, P. B. and Krieg, P. A. (2004).
Hedgehog signaling is essential for endothelial tube formation during
vasculogenesis. Development
131,4371
-4380.
Wilson, D. I., Cross, I. E., Wren, C., Scambler, P. J., Burn, J. and Goodship, J. (1994). Minimum prevalence of chromosome 22q11 deletions. Am. J. Hum. Genet. 55, A975.
Winnier, G. E., Kume, T., Deng, K., Rogers, R., Bundy, J., Raines, C., Walter, M. A., Hogan, B. L. and Conway, S. J. (1999). Roles for the winged helix transcription factors MF1 and MFH1 in cardiovascular development revealed by nonallelic noncomplementation of null alleles. Dev. Biol. 213,418 -431.[CrossRef][Medline]
Xu, H., Morishima, M., Wylie, J. N., Schwartz, R. J., Bruneau,
B. G., Lindsay, E. A. and Baldini, A. (2004). Tbx1 has a dual
role in the morphogenesis of the cardiac outflow tract.
Development 131,3217
-3227.
Yagi, H., Furutani, Y., Hamada, H., Sasaki, T., Asakawa, S., Minoshima, S., Ichida, F., Joo, K., Kimura, M., Imamura, S. et al. (2003). Role of TBX1 in human del22q11.2 syndrome. Lancet 362,1366 -1373.[CrossRef][Medline]