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Amplification of AML1 on a duplicated chromosome 21 in acute lymphoblastic
leukemia: a study of 20 cases
L Harewood1,4, H Robinson1, R Harris1, M Jabbar Al-Obaidi1, GR Jalali1, M Martineau1, AV Moorman1, N Sumption1,
S Richards2, C Mitchell3 and CJ Harrison1on behalf of the Medical Research Council Childhood and Adult Leukaemia
Working Parties
1
Leukaemia
2
Research Fund Cytogenetics Group, Cancer Sciences Division, University of Southampton, Southampton, UK;
Clinical Trial Service Unit, Radcliffe Infirmary, Oxford, UK; and 3Paediatric Oncology, John Radcliffe Hospital, Oxford, UK
This study identifies multiple copies of the AML1 gene on a
duplicated chromosome 21, dup(21), as a recurrent abnormality
in acute lymphoblastic leukemia (ALL). Clusters of AML1
signals were visible at interphase by fluorescence in situ
hybridization (FISH). In metaphase, they appeared tandemly
duplicated on marker chromosomes of five distinct morphological types: large or small acrocentrics, metacentrics, submetacentrics or rings. The markers comprised only chromosome 21
material. Karyotypes were near-diploid and, besides dup(21), no
other established chromosomal changes were observed. A total
of 20 patients, 1.5 and o0.5% among consecutive series of
childhood and adult ALL respectively, showed this phenomenon. Their median age was 9 years, white cell counts were low
and all had a pre-B/common immunophenotype. Although this
series is not the first report of this abnormality, it is the largest,
permitting a detailed description of the variety of morphological
forms that duplicated chromosome 21 can assume.
Leukemia (2003) 17, 547–553. doi:10.1038/sj.leu.2402849
Keywords: amplified AML1; acute lymphoblastic leukemia;
duplicated chromosome 21
Introduction
The AML1 (CBFA2) gene, located in the chromosomal band
21q22, has recently attracted a lot of interest in terms of its role
in leukemogenesis. AML1 is involved in a number of chromosomal translocations in leukemia. Of particular note in acute
lymphoblastic leukemia (ALL) is the t(12;21)(p13;q22), in which
AML1 fuses to the TEL (ETV6) gene. This translocation accounts
for approximately 25% of B-lineage ALL in children.1–4
Additional copies of AML1 have often been observed in acute
leukemia. Acquired trisomy 21, a frequent finding in childhood
ALL, itself produces one extra copy of the gene. Structural
rearrangements involving duplication of the long arm of
chromosome 21, dup(21), have also been described.5–14
Fluorescence in situ hybridization (FISH) techniques showed
that extra copies of the AML1 gene were present on these
abnormal chromosomes.6,9–14 In one of two patients, for whom
the amplified regions were shown to extend beyond the AML1
gene in both centromeric and telomeric directions, the
amplification included the ERG and ETS2 genes.10
Amplification of AML1 has been reported in acute myeloid
leukemia (AML), where structural rearrangements resulted in
partial gains of chromosome 21. Among seven cases, two had
evolved from a prior myelodysplasia.15–18 Point mutations19–22
and cryptic chromosome 21 rearrangements associated with
Correspondence: CJ Harrison, Leukaemia Research Fund Cytogenetics
Group, Cancer Sciences Division, University of Southampton, MP 822
Duthie Building, Southampton General Hospital, Southampton SO16
6YD, UK; Fax: 44 (0)23 8079 6432
4
Current address: Medical Research Council Human Genetics
Unit, Edinburgh, UK
Received 12 September 2002; accepted 12 November 2002
deletions of AML123,24 have also been reported in myeloid
disorders. Collectively, these observations point to possible
dosage effects of AML1 in the pathogenesis of leukemia. Further
studies in larger groups of patients are needed in order to
elucidate the role of AML1 and to help identify the specific
chromosomal regions and interacting genes involved in this
process.
The series of patients described here revealed the existence of
a distinct subgroup of older children with ALL and amplification
of the AML1 gene, arising from partial duplication of the long
arm of chromosome 21, including the 21q22 region.
Materials and methods
Patients
The 20 patients in this study emerged from a series of children
and adults with a diagnosis of ALL entered to one of the Medical
Research Council (MRC) ALL treatment trials, UKALL XI or ALL
97 for children aged 1–18 years inclusive, or UKALL XII for
adults aged 15–55 years. White blood cell count (WBC),
immunophenotyping and morphological classification were
carried out in the local referral centers and these data, including
follow-up information, were collected by the Clinical Trial
Service Unit (CTSU), Oxford. In the current childhood trial, ALL
97, no distinction was made between pre-B and common ALL.
Cytogenetics
Cytogenetic analysis of diagnostic bone marrow or peripheral
blood samples was carried out in the regional cytogenetics
laboratories. The G-banded slides were reviewed at the
Leukaemia Research Fund UK Cancer Cytogenetics Group
(UKCCG) Karyotype Database in Acute Leukaemia (Database).25 Karyograms were generated using MacKtype software
(Applied Imaging International, UK) and the karyotypes were
described according to the International System of Human
Cytogenetic Nomenclature.26
FISH
Interphase FISH screening is routinely carried out on all trial
patients for the prognostically significant abnormalities, TEL/
AML1 or BCR/ABL fusion and rearrangements of the MLL gene.
This is performed on the fixed cell suspensions used for
cytogenetic analysis. The dual color probe kit, LSIs TEL/AML1
ES (Vysis, UK), designed to detect the translocation,
t(12;21)(p13;q22), was hybridized to cell suspensions according
to the manufacturer’s instructions. This kit contains a probe
spanning the AML1 gene at 21q22, labeled with Spectrum
Amplification of AML1 on duplicated chromosome 21
L Harewood et al
548
Orange, and a second probe covering exons 1–4 of the TEL
gene, labeled with Spectrum Green.
To confirm the presence of the AML1 gene within the
duplicated chromosome 21 regions, two exon-specific cosmid
probes together covering the entire gene were hybridized
individually onto interphase and, wherever possible, metaphase
cells from each patient. Cosmid ICRF c103 CO664 contains
exons 1–5 of AML1 and cosmid H11086 contains exons 6 and
7. These probes were directly labeled with Spectrum Red using a
Nick Translation kit (Vysis, UK). Slides with selected metaphases
were co-denatured with 0.04 mg of labeled probe in 10 ml of
Hybrisol VII solution (Appligene-Oncor, Qbiogene, UK) at 721C
for 2 min and hybridized at 371C overnight. They were then
washed in 0.4x SSC with 0.3% IGEPAL (Sigma, UK), followed by
2 SSC with 0.1% IGEPAL at room temperature. Cells were
examined using a Zeiss Axioskop fluorescence microscope fitted
with appropriate filters (Karl Zeiss, Germany).
Whole chromosome paints (wcp) (STAR*FISH, Cambio, UK)
were applied to metaphases where possible to resolve the Gbanded karyotypes. Multiplex-FISH (M-FISH) was performed on
patients 3956, 4178, 5607 and 5754 using the SpectraVysion 24
color chromosome painting kit (Vysis, UK). These probes were
hybridized according to the manufacturers’ instructions.
FISH images from interphase and metaphase cells were
captured using an Olympus BX microscope with a cooled
coupled device (CCD) camera (Photometrics, USA) and
analyzed using MacProbe and M-FISH software (Applied
Imaging International Ltd, UK) as appropriate.
Results
A total of 20 patients with multiple copies of the AML1 signal
were identified during a routine interphase FISH screening
program using the LSIs TEL/AML1 ES probe (Figures 1a and b).
In all, 13 patients were part of a consecutive series of 870
children, aged between 1 and 18 years, from which we
estimated the incidence of this abnormality to be 1.5%. Among
adults over the age of 19 years, the incidence was o0.5% (one
out of 229 patients). Patients with one or two additional signals
corresponding to extra copies of chromosome 21 were
excluded. The karyotypes, clinical details and follow-up data
for all 20 patients are shown in Table 1. All patients were
negative for TEL/AML1 fusion, for BCR/ABL fusion and for
rearrangements of the MLL gene.
Since the AML1 probe of the LSIs TEL/AML1 ES kit is
B500 kb size and extends beyond the 50 and 30 ends of the
AML1 gene, exon-specific probes to AML1 were applied in
order to verify that the gene itself was amplified. Although these
cosmids spanned more than one exon of AML1 (ICRF c103
Figure 1
(a) LSIs TEL/AML1 ES probe on interphase nuclei from patient 2776. The AML1 signals are in red and the TEL signals are green. (b)
LSIs TEL/AML1 ES probe on a metaphase from patient 3368. The normal chromosome 21 has a single red signal and the dup(21) has tandemly
repeated signals along part of its length. The normal and dup(21) are painted with wcp 21 (yellow). (c) Cosmid ICRF c103 CO664 covering exons
1–5 and (d) cosmid H11086 covering exons 6 and 7 of AML1 hybridized to metaphases from patient 4623.
Leukemia
Table 1
Patient no.
Sex/age
(years)
WBC 109/l
EFS
(months)
OS
(months)
AML1
signals/cell
Form of
dup(21)
Karyotype
M/10
F/7
M/11
M/20
F/13
F/7
M/14
M/7
F/12
M/8
F/14
F/14
F/13
M/5
M/8
M/9
F/6
F/14
M/8
M/9
2
17
11
6
3
15
19
3
2
4
1
3
3
8
3
2
14
1
4
2
36
19
27
3+
26+
27+
28+
4
19+
17
18+
17+
1+
13+
13
10+
7+
2+
2+
1+
60+
51+
32
3+
26+
27+
28+
18
19+
20+
18+
17+
1+
13+
13+
10+
7+
2+
2+
1+
6+
4–6
6+
5+
5+
4–7
6+
6–10
5+
6+
7+
6–8
4–9
5–8
6+
4–7
4–6
4–6
4–6
5–7
M
R
M
AL
AS
R
SM
AL
R
AL
SM
AL
SM
SM
R
R
AL
AS
R
M
48,XY,+X,+14,ider(21)(q10)dup(21)(q?)
47,XX,+X,der(21)r(21)(q?)dup(21)(q?)
46,XY,t(1;16)(q23;p13),ider(21)(q10)dup(21)(q?)/51,idem,+X,+3,+10,+14,+21
46,XY,del(7)(p1?5),t(8;22)(q1?1;q13),dup(21)(q?)
45,XX,7,del(12)(p12),dup(21)(q?)
45,XX,dup(8)(p?),11,der(15)t(11;15)(?;q24),der(21)r(21)(q?)dup(21)(q?)
46,XY,der(21)dup(21)(q?)/46,idem,del(5)(q?),der(15)t(5;15)(q?;q?)
45,XY,dic(8;16)(p1;p1),del(13)(q1?4),dup(21)(q?)/46,idem,t(Y;13)(q1;q1?4),+dic(8;16)(p1;p1)b
47,XX,add(7)(q2),+10,der(21)r(21)(q?)dup(21)(q?)/47,idem,del(12)(p13)
46,XY,dup(21)(q?)
46,XX,t(12;16)(q24;p11),del(15)(q24q26),t(17;20)(p1?3;q11),der(21)dup(21)(q?)
46,XX,del(7)(q22),t(14;22)(q32;q11),dup(21)(q?)b
46,XX,der(21)dup(21)(q?)/46,idem,del(16)(q1)
47,XY,12,der(21)dup(21)(q?),+mar1,+mar2
45,Y,t(X;15)(q2?1;q2?4),dic(12;17)(p1;p1),der(21)r(21)(q?)dup(21)(q?)
46,XY,der(21)r(21)(q?)dup(21)(q?)
46,XX,dup(21)(q?)/47,idem,+X
46,XX,del(9)(p22),dup(21)(q?)
46,XY,t(8;11)(p2?1;q21),del(11)(q21),der(21)r(21)(q?)dup(21)(q?)/47,idem,+Xb
46,XY,ider(21)(q10)dup(21)(q?)b
Amplification of AML1 on duplicated chromosome 21
L Harewood et al
2423
2776
3131
3368a
3527
3743
3745
3956
3970
4134
4135
4178
4237
4279
4405
4444
4623
5601
5607
5754
Karyotypes and Clinical Details of Patients with dup(21)
Previously reported as patient 38.28
Confirmed by M-FISH.
M: metacentric; SM: submetacentric; AL: large acrocentric; AS: small acrocentric; R: ring chromosome.
a
b
549
Leukemia
Amplification of AML1 on duplicated chromosome 21
L Harewood et al
550
Figure 2
Partial G-banded karyograms showing examples of the different forms of dup(21). Normal chromosome 21 on the left, dup(21) on the
right: (a) a large acrocentric (patient 3956), (b) a small acrocentric (patient 5601), (c) a metacentric (patient 3131), (d) a submetacentric (patient
3745), and (e) a ring chromosome (patient 2776).
CO664 contains exons 1–5 and H11086 contains exons 6 and
7), the finding of discrete signals with both probes in each
patient suggested that the entire gene was amplified (Figures 1c
and d). The approximate number of copies of AML1 signals is
shown in Table 1. This number varied between patients and also
from cell to cell within the same sample, the close association of
signals in many nuclei making accurate enumeration impossible.
In metaphase cells, the AML1 signals, either discrete or in
clusters, were observed in tandem duplication on a distinctive
marker chromosome (Figures 1b, c and d). The marker was
proved to be composed entirely of chromosome 21 material in
the 14 cases tested with wcp 21 (Figure 1b) and the four tested
by M-FISH, indicating that at least part of it had been duplicated.
The results from the AML1-specific probes confirmed that the
duplication clearly included the chromosomal region 21q22,
where the AML1 gene is located. Since the extent of the
duplication and the number of copies of the AML1 signal were
difficult to determine, the abnormalities were described in the
karyotypes as dup(21)(q?) (Table 1).
The marker assumed one of five distinct morphological forms
in G-banded cells (Table 1 and Figure 2): a small acrocentric F
dup(21)(q?) (two patients); a large acrocentric F dup(21)(q?)
(five patients); a metacentric resembling an isochromosome
Fider(21)(q10)dup(21)(q?) (three patients); a submetacentric F
der(21)dup(21)(q?) (four patients); or a ring chromosome F
der(21)r(21)(q?)dup(21)(q?) (six patients), each with a variable
number of dark staining Giemsa-positive bands along its length.
From microscopic observation of the G-banded metaphases,
it was sometimes difficult to determine the true form of
the marker between metaphases from an individual patient,
raising the possibility that it may change from one type to
another.
The karyotypes of all 20 patients were abnormal with some
similar features. The stemlines were all near-diploid with 45–48
chromosomes, 12 of them being pseudodiploid. In all patients,
the dup(21) replaced one copy of the normal chromosome 21,
being the sole karyotypic change in the stemline in six of them.
In association with the interphase FISH observations described
above, there was no cytogenetic evidence of any other
established chromosomal changes. Five patients showed the
gain of an X chromosome as a secondary change. Chromosomes
7, 8, 12, 15 and 16 (p arm) were involved in accompanying
structural abnormalities in three or more patients.
The children in this series were older, being outside the usual
distribution for childhood ALL of 2–5 years. Their median age
was 9 years with a range from 5 to 14 years, with nine of them
being more than 10 years old. The only adult patient with
dup(21) was aged 20 years. The patients all had a low WBC,
pre-B or common immunophenotypes and L1 morphology. As
far as prognosis is concerned, follow-up time was too short for
conclusions to be drawn. Of the six patients who relapsed, one
had an event-free survival (EFS) of o4 months and an overall
Leukemia
survival of only 18 months, while the remainder relapsed
between 13 and 36 months.
Discussion
This study presents a series of 20 patients with ALL, each with an
abnormal marker chromosome of variable morphology, replacing one copy of a normal chromosome 21. The markers were
composed entirely of chromosome 21 material, with multiple
copies of the AML1 gene duplicated in tandem along their
length. Previous reports have described similar markers, and
those in which the same or comparable techniques were used in
their identification, are shown in Table 2 (patients 1–11). Probes
to the AML1 gene were used in all 11 cases, while the
involvement of chromosome 21 was proved by whole chromosome painting for patients 1–8, by spectral karyotyping (SKY) for
patient 9 and by comparative genomic hybridization for patients
10 and 11.6,9–12,14 Two earlier reports described markers that
were defined by cytogenetics as triplication or quadruplication
of chromosome 21.5,7 They resembled the large acrocentrics of
our own and other series.9,11,12,14 Since wcp 21 was only used
on one of them7 and they had not been tested with probes to
AML1, they were not included in Table 2.
Ours is clearly the largest series so far reported and is the first
to identify the incidence of dup(21) at approximately 1.5%
among children. In two smaller series, the incidence appeared
to be slightly higher. Two patients (6 and 7, Table 2) were found
in a consecutive series of 107 patients14 and a further two
patients (10 and 11, Table 2) were observed in a series of 109.9
There is no mention of this latter series being consecutive, but
the majority of patients came from a single center. It is however
noteworthy that they were collected over a 25-year period,
whereas our cases, which were UK wide in origin, had been
collected in less than 6 years.
Abnormal numbers of AML1 signals, but on an apparently
normal chromosome 21, have been reported before.27–29
Patients 12–15 (Table 2) were from a recent study in which
four of 41 childhood ALL patients were reported to have AML1
amplification with signals repeated in a tandem fashion on a
chromosome 21, which showed no evidence of duplication.
Patient 16, a young adult, showed the same phenomenon, with
five AML1 signals per interphase nucleus and tandem copies of
AML1 also on an apparently normal chromosome 21. No details
are provided of the five patients with amplified AML1 and a
normal karyotype from the series of Najfeld;27 therefore they
have not been included in Table 2. The four patients reported by
Mikhail et al 29 formed a group that appeared clinically distinct
from our series in that they were all 6 years old or less and three
had WBC in excess of 50 109/l. Karyotypes were normal in
three of them, although one harbored a t(12;21) translocation.
Although these observations may indicate that this group of
cases are distinct from those with an obviously abnormal
Table 2
Patient
no.
1
2
Patient no.
in paper
Sex/age
(years)
WBC 109/l
EFS
(months)
OS
(months)
Diagnosis
AML1
Signals/cell
110
210
M/15
M/6
4
5
33+
NA
33+
NA
L1/pre-B
L2/common/pre-B
5
4–5
36,10
111
211
114
214
113
512
19
29
129
2329
3129
4129
3628
F/11
F/10
M/11
M/17
F/19
F/8
?/12
M/12
F/6
M/3
F/2
F/6
F/6
F/17
23
1
6
1
10
1
2
4
26
100
78
2
56
2
65
14+
7+
10+
12+
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
14+
7+
10+
12+
NA
NA
NA
NA
NA
NA
NA
NA
NA
B-ALL
Common
Common
Pre-B
Pre-B
Pre-B
B-lineage
Pre-B
Pre-B
Pre-B
Pre-B
Pre-B
Early pre-B
Common
5
6
7
8
6–8
5–10
6+
10–15
6
3 or 4
3 or 4c
3 or 4
4
5
Karyotype
46,X,Y,add(21)(q22),+mar
53,XY,+X,+Y,inv(3)(p14q21),add(4)(q35),+9,+17,+21,+21,+add(21)(q22)/54,idem,
+add(21)(q22)/53,idem,inv(3)(p14q21),+mar1,+mar2/54,idem,+21a
46,XX,21,+mar
46,XX,der(21)
46,X,+X,inv(Y)(p11.2q12),+10,20,der(21)
46,XY,add(1)(q25),add(21)(q21)
46,XX,del(7)(p14p21),21,+mar
47,XX,+X,del(21)(q22),der(21)
+X,+10,del(11)(q23),qdp(21)(q11.1q22)b
46,XY,del(18)(p11),der(21)
48,XX,20,+der(21),+2mar
46,XY
46,XX
45,XX,19
46,XX
46,XX
a
Has only two copies of AML1 on the add(21); the other additional signals correspond to extra copies of chromosome 21.
Complete karyotype not provided.
c
Extra copies of AML1 associated with TEL/AML1 fusion.
? indicates gender of patient not provided; superscripts represent reference numbers; NA: not available.
b
Amplification of AML1 on duplicated chromosome 21
L Harewood et al
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Previously Published Cases with dup(21) and/or extra copies of AML1
551
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Amplification of AML1 on duplicated chromosome 21
L Harewood et al
552
marker, it is perhaps more realistic to think of them as different
ends of a spectrum that encompasses all degrees of duplication
of segments of chromosome 21, the end result of which, among
others, is amplification of at least the AML1 gene.
In our series, we only used probes to the AML1 gene. Other
studies have however shown amplification of regions involving
genes both centromeric and telomeric to AML1 on the abnormal
chromosomes.6,10,27 The size of the duplicated segments
suggests that this is probably true for the majority of patients
with dup(21).
Although with hindsight it is easy to suspect the presence of a
duplicated chromosome 21 in a karyogram that contains a
distinctive marker and from which one copy of a normal
chromosome 21 is missing, the abnormality was in fact
discovered by chance at interphase, while screening for the
TEL/AML1 fusion. Abnormal numbers of AML1 signals in
interphase nuclei led to the metaphase investigations, which
located them on the marker. This was also true for patients 6, 7,
10 and 11 (Table 2).9,14 Patient 9 (Table 2) is an exception. The
dup(21) was identified firstly by SKY and the presence of AML1
was confirmed subsequently.12
The markers in the 20 patients reported here could be
classified into five distinct types with some overlapping features.
Five of our patients had large acrocentric dup(21), with signal
numbers for AML1 in interphase cells, varying from five to 10
between them. The markers in patients 4, 5, 6, 9 and 11 (Table 2)
also appeared as large acrocentrics with between 4 and 8 copies
of AML1 in interphase cells.9,11,12,14 In two of our patients, the
markers were small acrocentrics, each having 4–6 AML1
interphase signals. They appear to compare with patient 2 in
Table 2. Seven cases in our series appeared to be either large
metacentric or submetacentric chromosomes with from 4 to 9
AML1 interphase FISH signals. Patients 3, 7 and 10 (Table 2)
also had metacentric markers. Patient 3 had five AML1 signals in
interphase, two of which were sited on each arm of the marker
in metaphase. Patient 10 had the highest number of signals
reported for this or any other series with from 10 to 15 signals at
interphase that were ‘tandemly placed in two sites’ on the
‘der(21)’. Patient 8 was different from the other reported cases. A
single AML1 signal was observed on one arm of the marker,
with duplicated signals on the other arm. In addition, the AML1
signal was deleted from the other chromosome 21. This is the
only patient described in which both chromosomes 21 are
abnormal.13
Our report is the first to describe the dup(21) in the form of a
ring. These chromosomes, which were observed in six patients,
had from 4 to 7 AML1 signals at interphase. Patient 1 (Table 2)
also appeared to have a marker of a unique form, not identified
in our series. It was described as ‘dicentric’ in four cells and as
‘telomeric’ in four others, while the pattern of uneven
hybridization of certain probes along its length was said to
confirm its dicentric origin.10 The marker in patient 3 (Table 2)
was described in more detail in an earlier publication.6 Here
also there was uneven hybridization of probes, but this time on
the two arms of a metacentric marker, causing the authors to
suggest that it may have arisen from secondary breakage of a
chromosome 21 that originated as a ring. In our own series, the
difficulties we encountered in classifying the markers of any one
patient into a single cytogenetic type may be an indication that
they have the inherent instability ascribed to ring chromosomes
and may indeed have evolved from them. This could explain the
variation in size between the different forms of the markers,
although, interestingly, the relative size of the marker was not
universally paralleled by an increase in the number of AML1
signals.
Leukemia
Mikhail et al 29 demonstrated that the detection of extra AML1
signals by FISH was associated with overexpression of AML1
measured by real-time quantitative RT-PCR, both in the cases
listed in Table 2 and those with additional signals resulting from
trisomy 21. It is of interest to consider whether amplification
within a single chromosome, either visible at the cytogenetic
level or not, has the same effect as an acquired gain of
chromosome 21, and to what extent the structure and
expression of the gene are altered in these different situations.
Although point mutations of the runt domain of the AML1 gene
have been observed in myeloid disorders,19–22 there was no
evidence of point mutations in exons 3–5 of AML1 (encoding
the runt domain) in five ALL patients tested (patients 1–3, 6 and
7).10,14 Although these studies could not rule out point
mutations in other exons, it was presumed to be unlikely. This
led the authors to suggest that it may be the amplification of
wild-type AML1 that is important in leukemogenesis. It could be
that additional copies of AML1 result in dysregulation of the
normal AML1 gene product. Further studies using DNA
microarray techniques will provide information on the expression of AML1 in relation to other leukemia-associated genes
both on chromosome 21 and elsewhere within the genome.
The karyotypes in our series were near-diploid, with the
majority being pseudodiploid, which was consistent with most
of the previously reported karyotypes in Table 2. In six patients
in our series and in two previously reported, the dup(21) was the
sole karyotypic change, at least in the stemline,6,11 suggesting
that it may occur as a primary genetic event. Apart from dup(21),
no known established numerical or structural chromosomal
change was present. Gain of an X chromosome, observed in five
cases, was also seen in four reported cases,6,11–13 while
chromosome Y was involved in a translocation with chromosome 13 in one of our cases and respectively gained, lost or
inverted in three reported cases.10,11
Among the 11 patients previously reported, the clinical
features were similar to those observed in our series. Followup was too short to say anything conclusive about the prognostic
implications of this abnormality. From the screening of our own
adult series, in conjunction with the 42 adult ALL patients in the
study of Penther et al,14 there were no patients over the age of
21 years identified with this abnormality. This adds further
evidence to that already established for the ETV6/AML1
translocation,28 that abnormalities involving chromosome 21
in ALL appear to be more common in children than in adults.
In this report, we have made a significant contribution to the
number of cases described with a dup(21) and associated
amplification of the AML1 gene, increasing the number of
reported cases to 31. We have confirmed that the abnormal
chromosome may assume a variety of shapes, including the
previously unidentified form of a ring. The important number of
cases included in our series has permitted a realistic estimate of
the incidence of this abnormality to be 1.5% in childhood ALL.
Our observations reinforce the importance of chromosome 21 in
leukemogenesis. It seems likely that the amplification of AML1,
and possibly of other genes within close proximity, directly
contributes to the etiology of acute lymphoblastic leukemia.
Acknowledgements
We thank the Leukaemia Research Fund for financial support
and the UK cytogenetics laboratories that contributed samples to
this study: Belfast, Birmingham, Cambridge, Cardiff, Croydon,
Dundee, Edinburgh, Great Ormond Street Children’s Hospital,
London, Manchester, Newcastle, Norwich, Royal Marsden
Amplification of AML1 on duplicated chromosome 21
L Harewood et al
553
Hospital, London, Salisbury, Sheffield, and University College
Hospital, London. Cosmids ICRF c103 CO664 and H11086 were
kindly donated by Dr Anne Hagermeijer, EU Concerted Action
and Dr Roland Berger, Paris, respectively. We are grateful to Dr
John Crolla and his team, Wessex Regional Genetics Laboratory,
Salisbury for the growing and preparation of the AML1 specific
cosmids as part of an ongoing collaboration. We thank Dr Helena
Kempski for helpful discussions.
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