Blood, 15 March 2002, Vol. 99, No. 6, pp. 2274-2275
CORRESPONDENCE
To the editor:
Deletions of the derivative chromosome 9 do not account for the
poor prognosis associated with Philadelphia-positive acute
lymphoblastic leukemia
The Philadelphia (Ph) translocation t(9;22)(q34;q11) is found in
15% to 25% of adults and 3% to 5% of children with acute lymphoblastic leukemia (ALL)1 and identifies patients with a particularly poor prognosis.2 The Ph translocation is
also the hallmark of chronic myeloid leukemia (CML), a biphasic disease arising in the stem cell compartment, that inevitably progresses to a
terminal blastic phase.3,4 The molecular consequence of
the Ph translocation is the formation of a BCR-ABL fusion
gene.5 In the majority of CML patients and approximately
one third of patients with ALL the translocation breakpoint in
BCR is found within a region spanning exons 12-16 termed the
major breakpoint cluster region (M-bcr). This results in a
p210BCR-ABL fusion protein.6 In about two
thirds of patients with ALL and in rare cases of CML, the
BCR breakpoint occurs further upstream between exons e2' and
e2, in a region termed the minor breakpoint cluster region
(m-bcr) resulting in a p190BCR-ABL protein.
We and others have recently reported deletions adjacent to the t(9;22)
translocation junction on the derivative chromosome 9.7-11
The deletions spanned several megabases, and usually resulted in loss
of sequences from both chromosome 9 and 22 sides of the junction.
Deletions were detected in 10% to 15% of CML patients and were
associated with rapid onset of blast crisis and a significantly shorter
survival,8,10,11 raising the possibility that similar deletions may contribute to the aggressive clinical course of Ph-positive ALL.
Here we present results from a study of 67 Ph-positive ALL patients (2 children and 65 adults). Bone marrow samples were screened for
deletions of the derivative chromosome 9 by fluorescence in situ
hybridization (FISH). The hybridization patterns expected in nondeleted
and deleted Ph-positive metaphases with major or minor BCR
breakpoints for the 3 probe systems used are shown in Figure
1A. Using the triple-probe system (Vysis,
Downers Grove, IL, as described in Sinclair et al8 and
Huntly et al11), loss of a blue signal on the derivative
chromosome 9, indicating deletion of 9q sequences, was observed in 1 of
67 patients (Figure 1B). Forty-eight of 49 patients analyzed using the
D-FISH system12 (QBiogene, Middlesex, United Kingdom)
displayed both red and green signals on the derivative chromosome 9 indicating no overt loss of chromosome 9 or 22 material from the
derivative chromosome 9 (data not shown). In the single patient in whom
loss of 9q sequences had been observed using the triple-probe system,
D-FISH demonstrated that the deletion involved loss of both chromosome
9 and 22 sequences (Figure 1C). The frequency of deletions in patients
with Ph-positive ALL (1/67), was therefore significantly lower than the
frequency of deletions in our study of CML11 (39/253,
2: P = .002), suggesting that derivative
chromosome 9 deletions are rare in Ph-positive ALL cases and do not
account for the aggressive clinical course of this disease.
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| Figure 1.
FISH analysis of bone marrow metaphases from Ph-positive ALL patients.
(A) Structure of ABL and BCR genes with
composition of the indicated probe systems together with expected
hybridization patterns using all 3 probe systems on Ph-positive
metaphases. Chromosome 9 homologues are shown on left (normal
9 and derivative 9, respectively) while chromosome 22 homologues are
shown on right (normal 22 and Ph, respectively). (B-D) FISH analysis of
Ph-positive bone marrow metaphases from ALL patient with deletion of
derivative chromosome 9. Each panel shows a metaphase together with an
image of the expected hybridization signals (inset). (B) Triple-probe
analysis. The blue ASS signal is absent from the derivative chromosome
9 indicating a deletion of chromosome 9 sequences from this chromosome.
(C) D-FISH analysis with absence of colocalized signal on der(9)
revealing deletion of 9q and 22q sequences from this chromosome. (D)
MBCR/ABL analysis. Lack of a BCR signal on the Ph chromosome indicates
an m-bcr breakpoint. This signal does not appear on the
derivative chromosome 9 consistent with a deletion of this region.
Images were captured using an epifluorescence microscope (Axioplan 2, Zeiss, United Kingdom), and SmartCapture 2001 software (Digital
Scientific, United Kingdom).
|
|
In patients without a deletion, the triple-probe system can distinguish
M-bcr and m-bcr breakpoints by virtue of
different signals on the derivative chromosome 9 (Figure 1A). In the
current series 54 of 66 patients lacking a deletion had an
m-bcr breakpoint while the remaining 12 patients had an
M-bcr breakpoint. However, if a patient has a deletion of
the derivative chromosome 9, then no hybridization signals will be
present on this chromosome and so breakpoint position cannot be
assessed using the triple-probe system. The MBCR/ABL system (QBiogene)
was therefore used to study the single patient with a deletion. As
shown in Figures 1A and D, the green BCR signal was absent from the Ph
chromosome consistent with an m-bcr breakpoint. This result
demonstrates that derivative chromosome 9 deletions are not restricted
to patients with an M-bcr breakpoint.
Several mechanisms may account for the rarity of chromosome 9 deletions
in Ph-positive ALL relative to CML. First, deletions are more common in
patients with variant Ph translocations,8,10,11 and
whereas variant translocations occur in 10% of patients with CML they
are rare in ALL.13 None of the 67 patients presented here
had a variant Ph translocation and this may account in part for the
paucity of deletions.
Second, it is possible that a deletion is more likely to accompany a
translocation with an M-bcr breakpoint. Since
M-bcr breakpoints occur in the vast majority of patients
with CML but in only a minority of patients with Ph-positive ALL, this
could account for the rarity of deletions in the latter disease.
Although the numbers are small our data are consistent with this idea
since deletions were observed in only one of the 54 patients with an m-bcr breakpoint compared with 25 of 212 CML patients with a
classical Ph translocation11 (P = .036).
Third, the rarity of deletions in Ph-positive ALL may reflect features
of the target cell in which the translocation occurs. CML results from
transformation of a multipotent stem cell,3 whereas ALL
more often results from transformation of a committed B-cell
progenitor.14 Lymphoid cells undergo antigen receptor rearrangements that require accurate joining of double stranded DNA
breaks15 and may therefore employ mechanisms that minimize the concomitant occurrence of large deletions.
Alistair G. Reid, Brian J. P. Huntly, Eveline Hennig, Dietger Niederwieser, Lynda J. Campbell, Nick Bown, Nick Telford, Helen Walker, Colin D. Grace, Michael W. Deininger, Anthony R. Green, and Elisabeth P. Nacheva
Correspondence: Elisabeth P. Nacheva, University of Cambridge,
Department of Haematology, Cambridge Institute of Medical Research,
Hills Rd, Cambridge, CB2 2XY, United Kingdom; e-mail:
en{at}mole.bio.cam.ac.uk
Acknowledgments
Supported by the Leukemia Research Fund and the Kay Kendall
Leukaemia Fund.
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