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 Previous Article | Table of Contents | Next Article 
Blood, Vol. 94 No. 1 (July 1), 1999:
pp. 225-232
Isochromosome 17q in Blast Crisis of Chronic Myeloid Leukemia and in
Other Hematologic Malignancies Is the Result of Clustered Breakpoints
in 17p11 and Is Not Associated With Coding TP53 Mutations
By
Thoas Fioretos,
Bodil Strömbeck,
Therese Sandberg,
Bertil Johansson,
Rolf Billström,
Åke Borg,
Per-Gunnar Nilsson,
Herman Van Den Berghe,
Anne Hagemeijer,
Felix Mitelman, and
Mattias Höglund
From the Departments of Clinical Genetics, Oncology, and Medicine,
the Division of Hematology, Lund University Hospital, Sweden; and the
Department of Human Genetics, University of Leuven, Belgium.
 |
ABSTRACT |
An isochromosome of the long arm of chromosome 17, i(17q), is the
most frequent genetic abnormality observed during the disease progression of Philadelphia chromosome-positive chronic myeloid leukemia (CML), and has been described as the sole anomaly in various
other hematologic malignancies. The i(17q) hence plays a presumably
important pathogenetic role both in leukemia development and
progression. This notwithstanding, the molecular consequences of this
abnormality have not been investigated in detail. We have analyzed 21 hematologic malignancies (8 CML in blast crisis, 8 myelodysplastic
syndromes [MDS], 2 acute myeloid leukemias, 2 chronic lymphocytic
leukemias, and 1 acute lymphoblastic leukemia) with i(17q) by
fluorescence in situ hybridization (FISH). Using a yeast artificial
chromosome (YAC) contig, derived from the short arm of chromosome 17, all cases were shown to have a breakpoint in 17p. In 12 cases, the
breaks occurred within the Smith-Magenis Syndrome (SMS) common deletion
region in 17p11, a gene-rich region which is genetically unstable. In
10 of these 12 cases, we were able to further map the breakpoints to
specific markers localized within a single YAC clone. Six other cases
showed breakpoints located proximally to the SMS common deletion
region, but still within 17p11, and yet another case had a breakpoint
distal to this region. Furthermore, using chromosome 17 centromere-specific probes, it could be shown that the majority of the
i(17q) chromosomes (11 of 15 investigated cases) were dicentric, ie,
they contained two centromeres, strongly suggesting that i(17q) is
formed through an intrachromosomal recombination event, and also
implicating that the i(17q), in a formal sense, should be designated
idic(17)(p11). Because i(17q) formation results in loss of 17p
material, potentially uncovering the effect of a tumor suppressor on
the remaining 17p, the occurrence of TP53 mutations was studied
in 17 cases by sequencing the entire coding region. In 16 cases, no
TP53 mutations were found, whereas one MDS displayed a
homozygous deletion of TP53. Thus, our data suggest that there
is no association between i(17q) and coding TP53 mutations, and
that another tumor suppressor gene(s), located in proximity of the SMS
common deletion region, or in a more distal location, is of
pathogenetic importance in i(17q)-associated leukemia.
© 1999 by The American Society of Hematology.
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INTRODUCTION |
ISOCHROMOSOME 17q [i(17q)] is the most
common isochromosome in hematologic malignancies and has been described
both as a primary and a secondary aberration. i(17q) is a frequent secondary chromosomal aberration in the accelerated phase or blast crisis of chronic myeloid leukemia (CML), indicating that this abnormality plays an important role in the disease
progression.1,2 The i(17q) is also found in 1.4% to 2.4%
of acute myeloid leukemias (AML), chronic myeloproliferative disorders
(CMD), myelodysplastic syndromes (MDS), acute lymphoblastic leukemias
(ALL), and chronic lymphoproliferative disorders with clonal chromosome
aberrations.3
Isochromosome formation is generally assumed to be the result of a
break or misdivision of the centromere, resulting in two mirror image
arms attached to a single centromere. In the case of i(17q) formation,
this would lead to loss of the entire short arm and gain of the entire
long arm. However, cytogenetic4-6 and recent molecular
genetic studies7,8 on constitutional and acquired
isochromosomes suggest that the breakpoints, in the few cases studied,
occur in the pericentromeric region. Furthermore, primitive
neuroectodermal tumors (PNET) often show an i(17q)9,10 and
data from loss of heterozygosity (LOH) studies are consistent with a
clustering of breakpoints in 17p11 close to the
centromere.8 This region coincides with the Smith-Magenis
Syndrome (SMS) common deletion region, a genetically unstable gene-rich
region frequently deleted in SMS patients.11-13
Whether i(17q) in hematologic malignancies also is the result of a
breakpoint within the pericentromeric region of 17p has not been
determined, although a few cytogenetic and fluorescence in situ
hybridization (FISH) studies, using chromosome 17 centromere-specific probes, have shown that i(17q) in some cases is
dicentric.4-6,14,15 These findings indicate that the i(17q)
in hematopoietic disorders also could be the result of a breakpoint in
17p, close to the centromere.
Given the frequent occurrence of i(17q) in hematologic malignancies,
and the fact that i(17q) sometimes is found as the sole karyotypic
abnormality suggesting a possible role as a primary genetic alteration
in leukemogenesiswe investigated 21 cases of hematologic neoplasms
characterized by an i(17q) using a yeast artificial chromosome (YAC)
contig from the proximal region of chromosome 17 (17p11) and FISH on
metaphase chromosomes. Because i(17q) formation results in loss of 17p
material, potentially unmasking the effect of a tumor suppressor gene
(TSG), we also studied the occurrence of TP53 mutations in 17 cases by sequencing the entire coding region.
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MATERIALS AND METHODS |
Patients.
Twenty-one patients with hematologic malignancies8 CML in blast
crisis (BC), 8 MDS, 2 AML, 1 ALL, and 2 chronic lymphocytic leukemias
(CLL)with an i(17q) as a sole or a secondary chromosomal aberration,
and where material in fixative was available, were selected for FISH
and TP53 mutational analyses (Table
1). In addition, six patients with CML in accelerated or blastic phase without chromosome 17 abnormalities were studied. All patients had been
cytogenetically analyzed, either at the Department of Clinical
Genetics, Lund University Hospital (Sweden) or at the Department of
Human Genetics, University of Leuven (Belgium).
Cytogenetic studies.
Bone marrow and/or peripheral blood cells were cultured and
cytogenetically analyzed according to standard procedures. The remaining cell pellets were stored at  20°C in fixative
(methanol:acetic acid, 3:1, vol/vol). Description of karyotypes and
criteria for clonality followed the recommendation of ISCN
(1995).16
Probes and FISH analysis.
The following 10 YACs were used: 845d2, 935a6, 828b9, 951g11, 52b10,
481h11, 912d7, 961f10, 427g11 (TP53), and 436g12
(TP53). The YAC clones were obtained from CEPH (Paris,
France) and were selected on the basis of their reported
genetic and physical mapping position on chromosome 17 (refs 17 and 18, http://carbon.wi.mit.edu:8000/cgi-bin/contig/phys_map, and
http://www.cephb.fr/infoclone.html). All clones, except 427g11 and
436g12, are contained within the whole contig 17.3 (WC17.3) constructed
at the Whitehead Institute for Genomic Research (MIT, Boston, MA), and
have been mapped within or adjacent to the Charcot Marie Tooth (CMT1A)
and SMS loci in 17p1117,18 (Fig
1). YACs 427g11 and 436g12 contain TP53 (verified by amplifying
the entire coding region of TP53). Hybridization of individual
YAC clones to normal lymphocyte metaphase cells showed that 845d2
mapped to 17q11, whereas the localization of 427g11 and 436g12 to
17p13, and of the remaining seven YAC clones to 17p11, was confirmed. For identification of the entire chromosome 17, a whole chromosome painting probe (wcp17), obtained by inter-Alu PCR from a somatic cell
hybrid containing human chromosome 17 (NA10498; NIGMS Human Genetic
Mutant Cell Repository, Camden, NJ), was used. The probe D17Z1
(American Type Culture Collection [ATCC], Manassas, VA ) was used as
a chromosome 17 centromere-specific (cen17) probe, and for
identification of chromosome arm 17p, a commercially available partial
chromosome paint 17 probe (pcp17) was used (ALTechnologies, Arlington,
VA).
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| Fig 1.
Summary of the FISH results. Each case number (1-21) is
indicated to the left. For clarity, the cases were ordered depending on
their breakpoint location, allowing further assignment into two
different groups (I and II; indicated to the right). The symbols used
to designate the result of a FISH experiment are indicated at the
bottom right of the figure. The approximate locations of the YAC clones
used, in relation to selected markers and genes on the physical map, is
based on the results reported by Chen et al, 1997,18 and
public available databases (see Materials and Methods). The
CMT1A locus and SMS common deletion region, together with the
repetitive sequence elements (CMT1A-REP, and SMS-REP) contained within
these regions, are indicated below the physical map. At the bottom, an
ideogram of chromosome 17 is depicted, showing the approximate location
of the region investigated. *Case 7 showed a split signal using YAC
935a6, with one signal on each q arm (see results and Fig 2C). For
abbreviations used, see text.
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YAC probe preparations and FISH analyses were performed essentially as
described previously.19 In brief, after amplification of
total yeast DNA using Alu primers, probes were generated by labeling
with biotin- or digoxigenin-conjugated dUTP (Boehringer Mannheim,
Mannheim, Germany), using Amersham's Mega prime kit (Amersham, UK).
Probes were purified on a Sepharose CL-6B column (Pharmacia, Uppsala,
Sweden), and 100 ng of each probe was coprecipitated with Cot 1 DNA
(Bethesda Research Laboratories [BRL], Gaithersburg, MD)
and Sonicated salmon DNA. Cell pellets were resuspended in fresh
fixative. After spreading, the slides were kept at 60°C overnight,
and subsequently treated in 2X saline sodium citrate (SSC)
at 60°C for 1 to 2 hours. Slides were then treated in 10 µg/mL
pepsin in 10 mmol/L HCl for 5 to 10 minutes at 37°C, washed in 1X
phosphate-buffered saline (PBS), followed by dehydration. The
chromosomes were denatured in 2X SSC and 70% formamide at 70°C for
varying amounts of time, typically between 15 seconds and 2 minutes.
The previously precipitated probes were resuspended in hybridization
solution (50% formamide, 50 mmol/L Na2HPO4, 2X SSC, and 10% dextrane sulfate), denatured at 70°C for 10 minutes, prehybridized at 37°C for 1 hour, applied to each slide at a volume of 10 µL, and then covered by a coverslip. Hybridizations were performed overnight in a humidified sealed chamber, and the slides were
washed at 70°C in 0.4X SSC for 2 minutes. Biotinylated probes were
detected with avidin-Cy3 (Amersham, Amersham Place, UK) and digoxigenin-labeled probes were visualized using one layer of sheep
antidigoxigenin-fluorescein isothiocyanate (FITC; Boeringer Mannheim,
Mannheim, Germany), at a concentration of 1 and 5 µg/mL, respectively. The hybridization signals were analyzed in a Cytovision Ultra System (Applied Imaging, Sunderland, UK), using a cooled charged
coupled device (CCD) camera. Whenever possible, and in the great
majority, at least 10 metaphases were analyzed.
Interphase FISH and TP53 mutational analyses.
To evaluate the size of the clones characterized by an i(17q), FISH
analysis was performed on interphase cells using a pool of two YACs
containing TP53 (427g11 and 436g12) and one YAC localized to
17q11 (845d2). To reduce the false-positive background rate, only
nuclei with at least two signals for the 845d2 YAC were scored for the
presence of one or two copies of the TP53 locus. At least 100 interphase nuclei were studied in all samples, including two normal controls.
DNA was extracted from remaining fixative or from stored peripheral
and/or bone marrow samples using standard methods. Sufficient amounts
of DNA were obtained from 17 cases, and 200 ng DNA was used in each
polymerase chain reaction (PCR) reaction to amplify the entire coding
region of the TP53 gene in seven or eight different fragments.
Primer sequences for the TP53 gene are given in
Table 2. Primers for the microsatellite
markers D17S379, D17S1322, and D17S1326 were
obtained from Research Genetics, Inc (Huntsville, AL). PCR was
performed in 50-µL reactions with 1.5 mmol/L MgCl2, and
200 µmol/L of each dNTP in PCR buffer II (Perkin Elmer, Branchburg, NJ). Depending on the primer pairs used, the DNA was amplified for 30 cycles at 93°C to 96°C for 30 to 45 seconds, 55°C to
62°C for 30 to 90 seconds, and 72°C for 50 to 90 seconds,
followed by a 5- to 10-minute final extension at 72°C. One of the
primers of each primer pair contained an M13 sequence (Table 2),
allowing direct sequencing of the PCR product using the Dye Primer
Cycle Sequencing Ready Reaction 21 M13 kit (Perkin Elmer) on an
ABI 373 Sequencer (Applied Biosystems, Foster City, CA)
with 4.75% denaturing acrylamide gels.
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RESULTS |
Cytogenetic analysis.
The cytogenetic findings are summarized in Table 1. Among the eight CML
BC, the i(17q) was the only additional structural aberration in five.
Seven of the eight MDS, as well as the two CLL, displayed an i(17q) as
the sole acquired cytogenetic aberration. The ALL had several numerical
and structural changes in addition to the i(17q), whereas one of the
two AML had one additional chromosomal change.
i(17q) is the result of clustered breakpoints in 17p11.
The i(17q) in all 21 cases was shown to be the result of a breakpoint
within 17p. The results from the FISH analyses are schematically summarized in Fig 1. Cases 1-18 were ordered and divided into two
groups depending on their breakpoint location (I and II; Fig 1). Cases
1-6 (group I) showed no signal on i(17q) using the most proximal YAC
(935a6), whereas the pcp17 probe clearly revealed a signal, consistent
with the presence of 17p material (Fig 2A and B). Case 7 (included in group II, see below) showed an unexpected pattern of hybridization on i(17q) using YAC 935a6; a split YAC signal
with one signal on each q arm was observed (Fig 2C), whereas YAC 828b9
clearly was absent (not shown). This could indicate that an inversion,
with the breakpoint localized within the 935a6 YAC, had taken place
before the formation of the i(17q). In cases 8-17 (group II), the
i(17q) was positive for YAC 828b9 (Fig 2D), but negative for the more
telomeric YAC 481h11 (Fig 2E). The two YACs 52b10 and 951g11, located
in between 828b9 and 481h11 (Fig 1), showed an inconsistent
intra-individual hybridization pattern in cases 8-17 (not shown). In
some metaphases, a weak signal was present on i(17q), whereas some
metaphases lacked a signal. These YACs, as well as YAC 912d7, contain
parts of repetitive sequence elements (SMS-REP) which are present in
the SMS common deletion region (Fig 1). Thus, the most likely
explanation for the weak hybridization signals observed when using
these YACs is a weak crosshybridization to the most proximally located
SMS-REP (SMS-REPP). The breakpoint in case 18 (group II) was located
between YAC 481h11 and 912d7 (not shown). Only case 19 had a breakpoint
telomeric to the SMS common deletion region, because YAC 961f10 was
contained within the i(17q) (Fig 2F). In cases 20 and 21, the YAC 828b9 was present on i(17q), but we were unable to further map the breakpoint in these two cases because of a lack of material.
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| Fig 2.
Examples of FISH analyses of i(17q). YAC and pcp17 probes
appear as red signals (Cy3), whereas wcp17 and cen17 probes appear as
green signals (FITC). When red (Cy3) and green (FITC) signals are
located adjacent to each other a yellow signal is obtained. Chromosomes
were counterstained using DAPI, yielding a blue color. (A) Case 1 hybridized with a combination of YAC 935a6 and wcp17 showing the
absence of a YAC signal on i(17q) and presence of a signal on the
normal chromosome 17. (B) Case 1 hybridized with a combination of pcp17
and cen17 showing two separate centromere 17 signals on the i(17q),
with 17p material present in between. (C) Case 7 hybridized with a
combination of YAC 935a6 and wcp17 showing a split YAC signal on
i(17q), with one signal on each q arm. (D) Case 11 hybridized with a
combination of YAC 828b9 and wcp17 showing the presence of a YAC signal
on i(17q). (E) Case 11 hybridized with a combination of YAC 481h11 and
wcp17 showing the absence of YAC signal on i(17q) and presence of a
signal on the normal chromosome 17. (F) Case 19 hybridized with a
combination of YAC 961f10 and wcp17 showing the presence of YAC signal
on i(17q).
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The majority of the i(17q) contain two centromeres.
A total of 15 cases could conclusively be analyzed for the presence of
one or two chromosome 17 centromeres on the i(17q) (Fig 1). In nine
cases, two separate cen17 signals, with 17p material present in
between, were obtained when using the cen17/pcp17 combination (see, eg,
Fig 2B). In cases 6 and 16, no separation of the two cen17 signals was
observed, but the signal intensity was roughly two times stronger on
the i(17q) than on the normal chromosome 17, consistent with the
presence of two centromeres on i(17q). In four cases (cases 3, 4, 13, and 18), no separation of the two cen17 signals was observed, nor was
the cen17 signal on the i(17q) stronger than on the normal chromosome
17. This finding may be due to a combination of suboptimal
hybridization efficiency and presence of condensed chromosomes, not
giving rise to stronger or separate cen17 signals. An alternative
explanation could be that the mechanism, by which these i(17q) were
formed, is different from the other i(17q). Nevertheless, the great
majority of i(17q) (11 of 15 cases) were clearly dicentric.
FISH analysis of CML BC without structural changes of chromosome 17.
Given the clustering of breakpoints in 17p11 in a region, which due to
the presence of repetitive sequences has been shown to be genetically
unstable and deleted in patients with SMS,17,18 we
investigated whether CML BC without structural changes of chromosome 17 may harbor submicroscopic deletions within this region. Six cases were
analyzed using YAC 481h11, but no clearly absent or diminished signals
were observed.
i(17q) is not associated with mutations in the TP53 gene.
No coding TP53 mutations were identified in any of the 17 cases
investigated (Table 1). In 13 cases, the entire coding region of the
TP53 gene was sequenced. Because of a lack of material, exons 2 and 3 were not sequenced in case 1, and for the same reason no data
were obtained from exons 2, 3, and 7 and from exons 2-5 in cases 3 and
13, respectively. A previously described polymorphism in exon 4 (http://www.iarc.fr/p53/poly.htm), Arg72Pro (CGC  CCC), was
found in case 1, whereas case 19 showed a noncoding polymorphism in
intron 9 [exon 9 (+12) T C]. In case 4, no PCR products
were obtained when using the primer pairs specific for exons 4-11, whereas amplification of two microsatellite markers located in 17q
(D17S1322 and D17S1326), and one in 17p13
(D17S379), yielded fragments of expected sizes (data not
shown). These results are consistent with a homozygous deletion of the
TP53 gene.
To rule out the possibility that the low occurrence of TP53
mutations was the result of a high admixture of normal cells, we
performed interphase FISH to estimate the percentage of cells containing an i(17q). The size of the clones was shown to be relatively large (range, 52% to 95%, mean 82%; Table 1), making it highly unlikely that the lack of identified TP53 mutations was due to a large contamination of normal cells. Two normal peripheral blood samples were investigated to determine the false-positive background rate. A total of 349 and 154 interphase nuclei were analyzed in each
sample, revealing a background rate of 7% and 6%, respectively.
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DISCUSSION |
In the present study, we show that i(17q) is the result of a breakpoint
in the pericentromeric region of the short arm of chromosome 17 and
that most breakpoints occur proximally to, or within, a previously
delineated region, the SMS common deletion region in
17p11.11-13 Using a YAC contig from this region and FISH on
metaphase chromosomes, we mapped the breakpoints in 19 of 21 hematologic malignancies characterized by an i(17q). In 12 cases, the
breaks occurred within the SMS common deletion region and the great
majority of the breaks (10 of 12 cases) were localized within or
adjacent to YAC 828b9 (Fig 1). Six cases showed breakpoints located
proximally to the SMS common deletion region, but still within 17p11,
and one case had a breakpoint distal to the SMS common deletion region.
We were also able to show that the majority of the i(17q) chromosomes
(11 of 15 investigated cases) contained two centomeres, lending further
support to previous results from a limited number of cases using
conventional C-banding techniques4-6 and centromere 17-specific FISH probes.14,15 Hence, i(17q) should
formally be designated idic(17)(p11).16 It has been
proposed that i(17q) may arise through a break in the short arm
followed by joining of the two chromatids containing the
centromeres.6 It is also possible that i(17q) could occur
through a nonreciprocal translocation between the two chromosomes 17, with breakpoints in 17p11 in each homolog. This would require a gain of
a chromosome 17 before the translocation event because one normal
chromosome 17 is present in most instances. In four cases, the i(17q)
did not appear to have two centromeres. Although we believe that the
reason for this is methodological, we cannot exclude the possibility
that these i(17q) were formed through a different mechanism, eg, a nonreciprocal translocation with breakpoints in 17p11 and 17q11 of each
homolog. Again, however, this would require a nondisjunctional event
leading to a gain of a chromosome 17 before the translocation. Therefore, we believe that i(17q) arises through an intrachromosomal recombination event. Interestingly, it has been shown that the SMS
common deletion region, the region to which most of our breakpoints mapped, contains three copies of SMS-REPs, and that a recombination event between the distal (SMS-REPD) and proximal repeat (SMS-REPP) most
likely is responsible for the interstitial deletions observed in SMS
patients.18 It is possible that the same mechanism is responsible for the chromosomal disruption and subsequent formation of
an i(17q) in hematologic malignancies.
Another tumor type that also frequently shows an i(17q) is PNET. Using
LOH and interphase FISH studies, Scheurlen et al8 mapped
the breakpoints in nine cases that had been found to have monoallelic
deletions encompassing nearly the complete short arm of chromosome 17, suggesting the presence of an i(17q). All nine tumors had breakpoints
proximal to, or within, the SMS common deletion region, and in five
tumors they were able to determine the chromosomal breakpoint between
two adjacent microsatellite markers. The most frequent breakpoint was
found between the markers D17S71 and D17S805 (the
latter marker is present in YAC 828b9; Fig 1). Thus, it seems as if the
breakpoints in hematologic malignancies are quite similar to the ones
observed in PNET. Also, we did not detect any clear-cut differences
between the localization of the chromosomal breakpoints and the type of
hematologic malignancy in our series. This suggests that 17p harbors a
"promiscuous" gene which, if structurally rearranged or otherwise
deregulated, is pathogenetically important in a wide variety of neoplasms.
The i(17q) formation leads to a loss of genetic material located
distally to the breakpoints described above, whereas material located
proximally is duplicated. It is presently not known if the loss or the
gain, or perhaps a combination of the two, is the pathogenetically
important event. It is also unknown if the observed clustering of
breakpoints within the SMS common deletion region merely is a result of
its genetic instability, and that any gene(s) located distally to the
breakpoint on the short arm of chromosome 17 could be a candidate TSG,
or if gene(s) located in the breakpoint region could become deleted or
altered in their expression or structure due to the chromosomal
disruption. Although speculative, one of our cases (case 7; Fig 2C) may
suggest that the latter mechanism indeed is possible. Using YAC 935a6,
we observed a split YAC signal on i(17q) with one signal on each q arm,
indicating that an inversion most likely had taken place before the
isochromosome formation. This inversion could delete or alter the
structure of critical gene(s) within the SMS common deletion region.
Several genes and ESTs have been mapped to the SMS common deletion
region,17,18,20 among them the human homolog
(LLGL1)21 of the drosophila TSG D-lgl and a
possible transcription factor, ZNF179,21,22 that
could be of pathogenetic relevance. In all but two of our cases (18 and
19), the breakpoints were located proximally to LLGL1,
consistent with a heterozygous loss of this gene. In principle, any
gene within the SMS common deletion region could, however, be
important, because chromosomal disruptions have been shown to be
associated with concomitant deletions in many
instances,19,23-25 and large insert (YAC) clones, which do not detect small deletions or rearrangements, were used in the present
study. Given the genetic instability of the SMS common deletion region,
we also searched for possible submicroscopic interstitial deletions in
six CML BC lacking chromosome 17 abnormalities. Using the YAC 481h11,
which is located in proximity to LLGL1, no clearly absent or
diminished signals were observed, indicating that larger deletions
within the SMS common deletion region is not a generally occurring
event in CML BC without chromosome 17 alterations.
The TSG TP53 at 17p13 has been shown to be mutated in several
tumor types, including hematologic malignancies. As a consequence of
the i(17q) formation, one copy of this gene will be lost, an event that
could unmask the effect of an existing mutation in the other allele. In
CML BC, the reported incidence of TP53 mutations is variable
and somewhat contradictory. Several types of TP53 gene
alterations have been described.26 When considering only coding TP53 point mutations, some studies have reported a
relatively high incidence (17% to 28%),26-28 whereas
others have found a low one (0% to 15%),29-34 with the
average frequency being 11% (20 of 175). However, only a few of these
studies included cytogenetic data,26,29,33,35 and when
ascertaining cases of CML BC with i(17q), in which TP53
mutational analysis had been performed, and excluding data on cell
lines, we were able to identify 18 cases. Of these, a coding
TP53 mutation was detected in four
(22%).26,29,33,35 In our material, none of the seven
investigated CML BC with i(17q) harbored any coding TP53 mutation.
TP53 mutations have also been described at varying frequencies
(0% to 25%) in MDS,36 AML,37,38
ALL,39,40 and CLL.39,40 In MDS and AML,
TP53 mutations have been shown to occur predominantly in cases
with a complex karyotype including a 17p deletion or 17.36,41,42 As to the occurrence of coding
TP53 mutations in MDS, AML, ALL, and CLL associated with
i(17q), we could identify only seven cases in the
literature.33,41,43 None of these showed any mutation,
which is consistent with our results where 0 of 9 cases harbored
TP53 mutations. One MDS (case 4) displayed a homozygous
deletion of TP53, which could indicate that another closely
located TSG could be of pathogenetic importance. The existence of such
a TSG, located distally to TP53, has indeed been suggested in
several other tumor types, eg, breast and ovarian
carcinomas.44-47
Overall, we found no coding TP53 mutations in any of the cases
investigated by sequence analysis. Hence, it seems clear that the
formation of an i(17q) does not serve to uncover the effect of an
already existing TP53 mutation on the normal-appearing
chromosome 17. Whether the pathogenetically important gene(s) in
i(17q)-associated leukemia is located in proximity of the SMS common
deletion regionas could be suggested by the observed clustering of
breakpoints in this regionor in a more distal region, remains to be established.
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ACKNOWLEDGMENT |
The authors thank Margareth Isaksson for expert technical assistance.
| |
FOOTNOTES |
Submitted October 19, 1998; accepted February 19, 1999.
Supported by grants from the Swedish Cancer Society, the Children's
Cancer Fund of Sweden, the Swedish Society of Medicine, the IngaBritt
and Arne Lundberg Foundation, and the Belgian Program and
Inter-university Poles of Attraction initiated by the Belgian State,
Prime Minister's Office, Science Policy Programming.
The publication costs of this
article were defrayed in part by
page charge payment. This article
must therefore be hereby marked
"advertisement"
in accordance with 18 U.S.C. section
1734 solely to indicate this fact.
Address reprint requests to Thoas Fioretos, MD, PhD, Department of
Clinical Genetics, University Hospital, S-221 85 Lund, Sweden; e-mail:
Thoas.Fioretos{at}klingen.lu.se.
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