Evaluation of potential regulatory elements identified as DNase | hypersensitive sites in the CFTR gene Marios Phylactides’, Rebecca Rowntree’, Hugh Nuthall', David Ussery”, Ann Wheeler
Trang 1Evaluation of potential regulatory elements identified as DNase | hypersensitive sites in the CFTR gene
Marios Phylactides’, Rebecca Rowntree’, Hugh Nuthall', David Ussery”, Ann Wheeler’ and Ann Harris’
‘Paediatric Molecular Genetics, Institute of Molecular Medicine, Oxford University, John Radcliffe Hospital, UK;
Center for Biological Sequence Analysis, Biocentrum DTU, Technical University of Denmark, Lyngby, Denmark
The cystic fibrosis transmembrane conductance regulator
(CFTR) gene shows a complex pattern of expression, with
temporal and spatial regulation that is not accounted for by
elements in the promoter One approach to identifying the
regulatory elements for CFTR is the mapping of DNase I
hypersensitive sites (DHS) within the locus We previously
identified at least 12 clusters of DHS across the CFTR gene
and here further evaluate DHS in introns 2, 3, 10, 16, 17a, 18,
20 and 21 to assess their functional importance in regulation
of CFTR gene expression Transient transfections of enhan-
cer/reporter constructs containing the DHS regions showed
that those in introns 20 and 21 augmented the activity of the
CFTR promoter Structural analysis of the DNA sequence
at the DHS suggested that only the one intron 21 might be caused by inherent DNA structures Cell specificity of the DHS suggested a role for the DHS in introns 2 and 18 in CFTR expression in some pancreatic duct cells Finally, regulatory elements at the DHS in introns 10 and 18 may contribute to upregulation of CFTR gene transcription by forskolin and mitomycin C, respectively These data support
a model of regulation of expression of the CFTR gene in which multiple elements contribute to tightly co-ordinated expression in vivo
Keywords: CFTR; regulation; DNase I hypersensitive sites
The cystic fibrosis transmembrane conductance regulator
(CFTR) gene shows a tightly regulated pattern of temporal
and spatial expression though the elements responsible for
this remain poorly characterized We previously identified
DNase I hypersensitive sites (DHS) across 400 kb flanking
the CFTR gene in order to locate potential regulatory
elements [1-5] These DHS lie 5’ to the gene at —79.5 and
—20.9 kb with respect to the translational start site; in
introns 1, 2, 3, 10, 16, 17a, 18, 20 and 21; and 3’ to the gene
at +5.4to +7.4and + 15.6 kb (Fig 1) DHS are often, but
not always, associated with regulatory elements in chrom-
atin As we have identified multiple clusters of DHS, it is
possible that not all of these represent important regulatory
elements for the CFTR gene The aim of this work was to
evaluate the regions of the CFTR gene containing the DHS
to identify those containing important regulatory elements
In yitro analyses of the DHS regions have included
evaluation in enhancer/reporter gene constructs where
luciferase activity is driven by the CFTR basal promoter
and DNA flanking the DHS is inserted into the enhancer
site of the vector The results suggest that in addition to the
DHS in intron | (at 185 + 10 kb) which was previously
shown to increase CFTR promoter activity [2], the DHS in
intron 20 (at 4005 + 4 kb) also augments promoter activity
Correspondence to A Harris, Paediatric Molecular Genetics, Institute
of Molecular Medicine, Oxford University, John Radcliffe Hospital,
Oxford, OX3 9DS, UK Fax: + 44 1865 222626,
E-mail: aharris@)molbiol.ox.ac.uk
Abbreviations CFTR, cystic fibrosis transmembrane conductance
regulator; DHS, DNase I hypersensitive sites
(Received 30 August 2001, revised 8 November 2001, accepted 16
November 2001)
and the DHS in intron 21 (at 4095 + 7.2 kb) has modest enhancer activity
The majority of the DHS were initially identified in the Caco2 colon carcinoma cell line [5] We have now looked for tissue-specific regulatory elements by analysing chromatin structure at these DHS in two pancreatic adenocarcinoma cell lines Capan1 [6] and NP31 [7] and an airway epithelial cell line Calu3 [8], all of which express CFTR, to see if any showed cell type specificity The pancreatic lines show a different predominance of DHS than the Caco2 cell line, while the intensity of DHS in Calu3 chromatin is very weak Finally we evaluated the effect of known activators of CFTR transcription, including forskolin [9] and mitomy- cin C [10] on the DHS
MATERIALS AND METHODS
Cell culture The following cell lines were analysed: the colon carcinoma Caco? [6], the pancreatic adenocarcinomas Capan1 [6] and NP31 [7] and the Calu3 lung adenocarcinoma [8]
Transient transfection assays Transient expression constructs were generated using the pGL3 Basic vector (Promega) A 787-bp fragment (245) [2] spanning the CFTR basal promoter (from —820 to
—33 with respect to the ATG translational start codon) was cloned into Nhel and BglII sites of the promoter multiple cloning site of pGL3B in the correct orientation for driving transcription of the luciferase gene The regions spanning the DHS identified in intron 2 (P1.8, ac000111: 46769-48558), 10 (BI.5, ac000111: 111755
113258 and H2.1, ac000111: 120380-122527), 16-17a
Trang 2
20kb
Fig 1 Diagram of the CFTR locus showing DNase I hypersensitive sites Numbers immediately above the line denote exons Numbers above the arrows denote the individual DHS as defined previously [1,2,4,5]
(E4.5, ac000111: 147607 to acOQ0061 : 3003, including
intron 17a) 20 (Bg2.6, acQ00061 : 35259-37869) and 21
(PE1.9, acO00061: 49458-51381) were cloned into the
BamHI restriction site in the ‘enhancer’ segment of the
vector The orientation of each fragment with respect to
the 245 promoter fragment was verified and further
experiments carried out on those orientated 5’—3’ with
respect to the vector backbone
In all transfection experiments the pGL3B-245 constructs
were cotransfected with one-quarter or one-tenth the
amount of DNA of pCMVBGal as a transfection control,
using FuGene 6 (Roche) Luciferase and B-galactosidase
assays were carried out by standard procedures using
Promega reporter lysis buffer and luciferase assay reagent
and a luminescent B-galactosidase detection kit (Clontech)
Luminescence was measured in a calibrated Turner TD 20e
luminometer Each transfection experiment was carried out
a minimum of four times with individual constructs being
assayed in triplicate in each experiment Results are
expressed as relative luciferase activity, with the pGL3B-
245 CFTR promoter construct activity equal to 1, corrected
for transfection efficiency as measured by B-galactosidase
activity Statistical analysis was performed using nonpaired
t-tests assuming unequal variance (Welch) using software
available at http://www.graphpad.com
DNA structure determination
Structural analysis of the CFTR sequence was made using
two contigs that cover the entire coding region (GenBank
accession numbers acQ00111 and acQ00061) DNA struc-
tural atlases were constructed as described previously [11]
DNase | hypersensitivity assays
Chromatin from a panel of cell types was probed for
DNase I hypersensitive regions by standard methods [2]
Nuclei were treated in parallel aliquots by digestion with
DNase I (01, 0, 20, 40, 80 and 160 U of DNase I;
FPLCpure, Pharmacia; 1 U is equal to approximately 0.3
kunitz units for approximately 10-15 min) Sample 01 was
kept on ice whilst the remaining samples were incubated at
37 °C To ensure that each batch of DNase I digested
chromatin was adequately digested, they were evaluated
with the RA 2.2 probe that detects a constitutive DHS in the
% globin locus [12] Probes for the DHS in introns 1
(185 + 10 kb), H4.0; introns 2 (296 + 4.4 kb) and 3
(405 + 0.7 kb), F34L; intron 10(1716 + 13.2 kb, + 13.7 kb and + 23 kb), 116/117 and BT1.2; intron 16 and 17a (3120 + 3kb and 3271 + 0.7 kb) E1.9; intron 20 (4005 +
4 kb) EB1.4 and intron 21 (4095 + 7.2 kb) H2.2 were described previously [1,5] All cell types studied were tested for CFTR mRNA expression by RT-PCR at the time of isolation of nuclei for chromatin analysis [1]
Treatment with activators of CF7R transcription Caco2, Capanl and Calu3 cell lines were treated with Forskolin (10 um in dimethylsulfoxide, 8 h), mitomicin C (0.25 um, 4 h) by addition to the culture medium Nuclei were then immediately processed for DNase I hypersensi- tivity assays as above For all experiments, RNA was extracted from an aliquot of cells to evaluate CFTR mRNA expression by RT-PCR at the time of isolation of nuclei for chromatin analysis [1]
RESULTS
Transient transfections DNA fragments of between 1.5 and 4.5 kb were cloned into the enhancer site of pGL3B-245, containing 787 bp of the CFTR basal promoter, and contructs assayed for enhancer activity following transient transfection into Caco2 colon carcinoma cells and MCF7 breast carcinoma cells (Fig 2)
A 2.6-kb region of DNA encompassing the DHS region in intron 20 showed a 4.4-fold enhancement of luciferase activity over the promoter only construct in Caco2 cells (P < 0.0001) A region of 1.9 kb of DNA encompassing the DHS region in intron 21 showed a 1.5-fold enhancement
of luciferase activity in Caco2 (P < 0.005); no other DHS region contained sequences that enhanced luciferase expression in Caco2 cells No DHS region enhanced CFTR promoter activity in MCF7 cells that do not express CFTR In fact, many of the constructs showed reduced luciferase expression (P < 0.0007 to p < 0.002) in comparison to the CFTR promoter only construct in MCF cells
Structure determination The regions encompassing each DHS as described in the transient assays were evaluated for structural motifs which might cause inherent DNase I hypersensitivity Of
Trang 3
5
š 45
@ 35
augment the activity of the CFTR promoter in $ 2
Caco2 cells The bar chart shows the luciferase - 1.5
activities for each construct relative to 5 1
pGL3B-245 (CFTR promoter only construct) 2 ae
in Caco2 and MCF-7 cells Luciferase activi- $
ties were normalized for transfection efficien- 5 a A: v` ®: q9 N
cies by cotransfection with pCMV/B Each bar x Y yo @ &o se ws oS
is the average of at least four transfection OY cŸ gy cŸ gy’ b f `
experiments, with each sample assayed in Co OY oY oY v3 `?
triplicate, and standard errors of the mean are S S R 8 £ £
particular interest are two structural parameters, the
‘DNase I sensitivity’ model of Brukner et al [14] and the
presence of alternating pyrimidine (Y) purine (R) tracts of
10 bp in length or longer The results for the DNA sequence
in ac0Q00061 including the regions around the DHS 20
(Bg2.6) and DHS 21 (PE1.9) are shown in Fig 3 Predicted
DHS (based on the structural properties of the naked DNA
sequence) are shown in lane C, where the darker blue
regions represent predicted hypersensitive sites For example
there are two predicted hypersensitive sites, which lie just 3’
to the Bg2.6 region that encompasses the DHS 20 These
two predicted DHS also correspond to long YR tracts (blue bands in lane D) Note that these regions do not easily correlate with areas expected to melt readily or with higher
AT content (red regions in lanes E and F) DHS 2/3 (P1.8), DHS 10a,b (B1.5), DHS 10C (H2.1), DHS 16/17a (E45), and DHS 20 (Bg2.6) did not contain regions expected to be hypersensitive to DNase I, based on either the Brukner DNase I sensitivity model or the presence of YR tracts Only DHS21 (PE1.9) encompasses a region predicted to be sensitive to DNase I, based on structural properties of the DNA sequence alone
Cystic Fibrosis gene, Exons 17a to 3’ end (GenBank: ACO00061) 82,512 bp
17al7b Exon 15
A)
=n
Wee
HINH LÍ
B) Annotations:
A) Annotations
Fragments encompassing DHS
[I:I:ÍÍ
ae 1 |
| PEL9 II †
C) DNase I Sensitivity E) Stacking Energy
ny ì lavz
3
F) Percent AT hil ae m = |
mn
D) (¥YR)5
Fig 3 DNA atlas of ac000061 showing a region of predicted DNase I hypersensitivity close to the DHS in intron 21, around 51 kb The lanes are as annotated in the figure Lanes A and B are based on annotations from the GenBank file, and the DNase hypersensitivity sites marked in black are the experimentally determined DHS regions Lane C (‘DNase I sensitivity’) is based on the trinucleotide model of Brukner ef ai [14] smoothed over
a 330-bp window, and lane D is the density of YR tracts of at least 10 bp in length, smoothed over a 165-bp window Lane E is the stacking energy,
in kcal‘mol', based on the dinucleotide model of Ornstein et al [20]; on this scale, the red regions are expected to melt more readily Lane F is the
AT content, ranging from 20% (turquoise) to 80% (red).
Trang 4Tissue specificity of DHS elements
In previous experiments we evaluated DHS in the Caco2
colon carcinoma cell line and performed a preliminary
screen for these DHS in other cell lines [5] To look for
tissue-specific DHS in the pancreatic duct, the pancreatic
cell lines Capan! and NP31 were evaluated further DHS in
airway epithelial cells were investigated further in the airway
carcinoma cell line Calu3
Of particular interest were the DHS in introns 2 and 18,
which were strongly evident in Capanl in comparison to
Caco2 chromatin (Fig 4) The DHS in introns 2 and 3 were
detected as subbands of 4.5 and 3.4 kb, respectively, when
Caco2 chromatin was hybridized with the F34L probe
(Fig 4B) In contrast, in Capan! cells the DHS in intron 2
(4.5-kb fragment) is much more evident and the DHS in
intron 3 is not detectable (Fig 4A) The DHS in introns 16,
17, 18 are detected with a single probe (E1.9) and they
appear as subfragments of the 24-kb genomic fragment at 5,
7 and 17 kb, respectively In Caco? cells the DHS in introns
16 and 17 are of similar intensity but the DHS in intron 18 is
less evident (Fig 5B) In contrast, the DHS in intron 18 is
more prominent than those in introns 16 and 17 (Fig 5A) in
chromatin from Capan1 cells The DHS in introns 1, 10a,b
(very weak) and 20 were also evident in Capan| cells (data
not shown) Evaluation of DHS in another pancreatic
adenocarcinoma cell line NP-31 revealed the DHS in introns
2, 10c, 17a, and 20 though other DHS were either extremely
weak or nondetectable (data not shown)
Fig 4 The DHS in intron 2 (296 + 4.4 kb) is prominent in Capanl
pancreatic adenocarcinoma cells Southern blots of DNase I digested
(A) Capanl and (B) Caco2 chromatin cleaved with BamHI and
hybridized with the F34L probe In each panel, lanes | (stored on ice)
and lanes 2 (incubated at 37 °C) show chromatin not treated with
DNase I and lanes 3-6 show chromatin prepared from nuclei with
increasing amounts of DNase I (20, 40, 80 and 160 U, respectively)
A 1-kb ladder (Life Technologies) was used for size markers
9 4— 24 kb
‹.4-!1:tb q— 7kb 4— 5kb
Extensive analysis of chromatin from the Calu3 cell line revealed a paucity of DHS, with only the DHS in introns
1, 16,17, 18 and 20 being detectable (data not shown) In addition to the 4005 + 4 kb DHS detected in intron 20 in chromatin from Caco2, two novel intron 20 DHS were seen
in Calu3 chromatin, detected as 3.8 and 3.3-kb subfrag- ments of the 24.5-kb genomic fragment detected by the EB1.4 probe These correspond to DHS at 4005 + 6.7 kb and 4005 + 7.2 kb (Fig 6)
Activation of CF7R expression Chromatin was extracted from untreated Caco2 and Capan! cells or after incubation with forskolin or mitomy- cin C and then digested with DNase I To ensure that control and drug-treated samples of chromatin were equally digested they were evaluated with the RA 2.2 probe that detects a constitutive DNase I hypersensitive site in the
a globin locus [12] Subsequently the intensity of the DHS in introns 1, 2/3, 10a,b, 16/17/18 and 20 were compared in drug-treated and control samples on Southern blots of chromatin The intensity of the signal derived from the genomic band and the DHS-derived band were determined using a phosphorimager and IMAGEQUANT 5.12 software (Molecular Dynamics) In all cases where preliminary data showed increased intensity of a DHS, the experiment was repeated to show that it was a consistent observation The only DHS that consistently showed an increase in intensity following exposure to activators of CFTR transcription were DHS10a, b, following forskolin activation in Caco2 cells (Fig 7A) and DHS 18 in Capanl cells following mitomycin C activation (Fig 7B) The bar charts show the intensity ratios of the DNase I derived subfragments to genomic fragments for forskolin/mitomicin C treated and control chromatin processed simultaneously Hence if the drug treatment were having no effect on the intensity of the DHS then the two bars in each pair would be of the same height For both panels A and B the increasing amounts of DNase show a proportionate increase in the intensity of the DHS fragments in the control samples In Fig 7A the forskolin-treated chromatin shows a relative increase in the intensity of the DHS10a (1716 + 13.2 kb) appearing at
20 U of DNase I but being more evident after 40 U of DNase I (ratio forskolin-treated/control = 1.7: 1.15) The effect of mitomycin C on DHS 18 (3600 + 7 kb) in Capan cells is shown in Fig 7B where ratios of genomic/DNase I
Fig 5 The DHS in intron 18 3600 + 7 kb)
is prominent in Capan1 pancreatic adeno- carcinoma cells Southern blots of DNase I
—— 2 digested (A) Capanl and (B) Caco2 chromatin + 17kb cleaved with BamHI and hybridized with the 4q— 7kb CE1.9 probe In each panel, lanes | (stored on 4— 5kb ice) and lanes 2 (incubated at 37 °C) show
chromatin not treated with DNase I and lanes 3-6 show chromatin prepared from nuclei with increasing amounts of DNase I (20, 40, 80 and 160 U, respectively) A 1-kb ladder (Life Technologies) was used for size markers.
Trang 5
24.5 kb
6.5 kb
‹ &6 38kb
W kL.x—.-
Fig 6 Novel DHS in intron 20 at 4005 + 6.7 kb and 4005 + 7.2 kbin
Calu3 chromatin Southern blot of DNase I digested Calu3 chromatin
cleaved with BamHI and hybridized with the EB1.4 probe In each
panel, lanes | (stored on ice) and lanes 2 (incubated at 37 °C) show
chromatin that had not been treated with DNase I and lanes 3-6 show
chromatin prepared from nuclei with increasing amounts of DNase I
(20, 40, 80 and 160 U, respectively) A 1-kb ladder (Life Technologies)
was used for size markers
treated fragment are greater than in the control chromatin,
most prominently at 20 U (ratio, 0.81 : 0.28)
DISCUSSION
Our current model for tissue specific and temporal regula-
tion of the CFTR gene predicts that co-operation of many
different regulatory elements may contribute to CFTR
expression in the chromatin environment in vivo DHS are
often, though not always, associated with regulatory
elements We have previously identified DHS both 5’ and
3’ to the CFTR gene and within at least nine introns The aim
of the current experiments was to evaluate the role of
individual intragenic DHS in regulation of CFTR expres-
sion The first series of experiments evaluated potential
enhancer activity of the intragenic DHS in transient
transfection of reporter/enhancer constructs We have
shown previously that the 185 + 10 kb DHS in intron |
augments CF7R promoter activity in transient transfections
of Caco? cells [2] and that removal of 32 bp at the predicted
core of the DHS abolished this activity [15] Analysis of the
DHS in introns 2, 3, 10, 16, 17a, 20 and 21 showed that the
4005 + 4kb DHS in intron 20 and the 4095 + 7.2 kb
DHS in intron 21 both augmented the activity of the
CFTR promoter in Caco? cells Neither construct affected
CFTR promoter activity in MCF7 cells that do not express
A Effect of forskolin on DHS10a (1716 + 13.2 kb)
in Caco2 cells
@ Forskolin
2¢e
E
SE
ow
os
Pe
oe
DNase!
B Effect of Mitomycin C on DHS18 (3600 + 7 kb)
in Capan1 cells 1.4
= «
gi '?
: = 0.8 B Mitomycin C |
Ẵ E04
È 02
0
0 0 20 40 80 160
DNase |
Fig 7 Effect of activation of CF7R transcription on DHS The charts show the effect of (A) forskolin on DHS10a (1716 + 13.2 kb) in Caco2 cells and (B) mitomycin on DHS 18 (3600 + 7 kb) in Capanl cells Charts show the ratio of the fragment intensities of the DHS-derived subfragment to the genomic fragment on phosphorimages of Southern blots in control and forskolin/mitomycin C-treated cells
CFTR These data suggest that while the DHS in introns 20 and 21 may contain tissue-specific enhancer sequences, the remaining DHS are not associated with enhancer function Due to the inherent limitations of transient transfection assays, further in vivo analysis will be required to evaluate the role of the intron 20 and 21 DHS in CFTR expression in chromatin
The DNA sequence within the intragenic DHS was evaluated to search for specific motifs that might cause inherent DNase I hypersensitivity based on bent or easily melted DNA Generation ofa DNA atlas for each of the two contigs covering the CFTR gene (ac000111 and ac000061) enabled the prediction of DNase sensitivity and YR steps that predict ease of melting Although there are many areas
of predicted sequence-based DNase I hypersensitivity within the gene, the only one that corresponds to the DHS that we have evaluated here is that in intron 21 (The region of the intron | at 185 + 10 kb also shows some inherent DNase I sensitivity.) These data suggest that the DHS that we have observed, with the exception of that in intron 21, are not structural artefacts induced by DNA sequence alone Our model for regulation of expression of the CFTR gene would predict that individual differentiated cell types would show a specific set of DHS that might differ from other cell types The Caco2 intestinal carcinoma cell line that we used
Trang 6initially to search for DHS due to its high level of
endogenous CFTR transcription exhibits at least 12 DHS
or clusters of DHS lying 5’, within the gene and 3’ Several of
these DHS have only been seen in Caco2 chromatin and
fewer DHS are evident in the other cell lines that we have
examined Among other cell types, such as pancreatic and
airway epithelial cells we have not found consistent profiles
of DHS For example the Capanl and NP31 pancreatic
adenocarcinoma cell lines that both express CFTR mRNA
(the former at very low levels) show different DHS Features
Of the Capanl line were the strong DHS in intron 2 and
intron 18 Although NP31 showed the intron 2 DHS that in
intron 18 was not evident and the DHS in intron 10 at
1716 + 23 kb was strong The role of these DHS in CFTR
expression in the pancreas warrants further evaluation In
the airway cell line Calu3, that expresses a high level of
CFTR mRNA, very few DHS were evident This could be a
genuine feature of this cell line or be due to only a small
percentage of cells in the culture expressing high levels of
CFTR, which would then only contribute a small part of the
chromatin sample making DHS hard to detect One
disadvantage of analysing carcinoma cell lines is that some
of the DHS we observe may be the result of these lines
showing aberrant gene expression following tumorigenesis,
rather than normal endogenous CFTR expression How-
ever, it is not possible to obtain sufficient chromatin from
primary cells from pancreas and airway epithelium to
evaluate DHS
Our model for regulation of CFTR transcription also
predicts that agents that activate CFTR transcription would
act at certain regulatory elements associated with DHS but
not others, depending on their properties and role in CFTR
transcription It is known that chemicals which increase
intracellular cAMP cause an increase in CFTR protein
expression in cell membranes and activation of chloride
secretion The increased CFTR protein has been shown in
part to be the result of transcriptional activation of CFTR
[9] It is known that cAMP response elements are present in
the CFTR promoter [16-18] but also in other predicted
regulatory elements [4] Here we have shown that fors-
kolin (an inducer of intracellular cAMP) reproducibly
enhances the DHS in intron 10 of the CFTR gene at
1716 + 13.2 kb in CaCO, cells Analysis of the sequence
around this DHS has shown a cluster of CREB and
CREB-related motifs between acO00111 : 111 936-112 125
which are undergoing further analysis to evaluate their
potential role in CFTR expression
Noncytotoxic doses of mitomycin C, a DNA cross-
linking reagent have been shown to preferentially alter the
expression of inducible genes [19] Mitomycin C was also
shown to induce CFTR mRNA and protein levels in colon
carcinoma cells lines [10] We have shown that activation of
the Capanl pancreatic adenocarcinoma cells by a low dose
of mitomycin C reproducibly enhanced the intensity of the
DHS in intron 18 in comparison to nonactivated cells It is
possible that activation of a potential regulatory element
sited at this DHS plays a role in pancreatic expression of
CFTR This would be consistent with our data on the cell-
specific expression of this DHS
The data presented here confirm the complexity of the
regulation of expression of the CFTR gene Elements within
the CFTR promoter are known to be inadequate to explain
the tissue-specific and temporal regulation of CFTR We
have previously shown that the DHS at 185 + 10 kb in intron | of the CFTR gene augments intestinal expression of the gene in vivo, both in human colon carcinoma cells and in transgenic mice [15] It is probable that several other DHS that we have identified contain regulatory elements that have specific roles in co-ordinating CFTR expression in vivo
In the present studies we have evaluated a number of the intragenic DHS to define which might be involved in specific tissues or regulatory pathways and which might merely reflect structural motifs within the CFTR gene Further
in vivo evaluation is warranted to fully understand the CFTR regulatory mechanisms
ACKNOWLEDGEMENTS
We thanks Dr Gabriel Capella for the NP31 cell line and Drs Nathalie Mouchel, David Smith and Sytse Henstra for contributions This work was funded by the Cystic Fibrosis Trust and Vaincre La Mucovisci- dose D U is funded by the Danish Research Foundation A W was
in receipt of a Wellcome Trust vacation scholarship
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