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35.Clinical outcome and monitoring of minimal residual disease in patients with acute lymphoblastic leukemia expressing the MLL ENL fusion gene.

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Research Article
Clinical outcome and monitoring of minimal residual disease in patients
with acute lymphoblastic leukemia expressing the MLL/ENL fusion gene
Loredana Elia,1 Sara Grammatico,1 Francesca Paoloni,2 Marco Vignetti,1 Angela Rago,1 Natalia Cenfra,1
Sergio Mecarocci,1 Marco Mancini,1 Matteo Luciani,3 Francesco Di Raimondo,4 Giovanni Cazzaniga,5
Mabel Matarazzo,1 Maria Luisa Moleti,1 Lidia Santoro,6 Gianluca Gaidano,7 Robin Foà,1
Franco Mandelli,2 and Giuseppe Cimino1
We analyzed 12 MLL/ENL positive ALL patients consecutively diagnosed between 1999 and 2009. The MLL/
ENL fusion was identified in 4/150 (2.6%), 8/993 (0.8%), and 0/70 of pediatric, adult, and elderly patients,
respectively. Eight patients had a WBC count >50 3 109/L. Ten cases had an evaluable immunophenotyping. A
B or T precursor ALL occurred in 7 and 3 patients, respectively. Eleven/12 patients (92%) achieved CR. At 48
months, overall survival and event-free survival rates were 73.3% and 67%, respectively. At CR, a parallel RTPCR evaluation of the MLL/ENL expression was available in 5 cases. Of these latter, 2 tested MLL/ENL-negative
and 3 positive. The minimal residual disease molecular monitoring showed that MLL/ENL status did not correlate with outcome. In fact, all the 2 PCR-negative and 1 of the 3 PCR-positive cases relapsed. Further, a MLL/
ENL expression, not preceding a relapse, was detected several times during the follow-up of five long-survivors.
In conclusion, also in adults, the MLL/ENL fusion identifies a rare leukemic entity with a favorable prognosis.
The observed inconsistency between the clinical cure and the presence of detectable MLL/ENL transcript suggests the existence of a MLL/ENL-expressing ‘‘preleukemia’’ stem cells, similar to what demonstrated for the
C 2011 Wiley-Liss, Inc.
AML1/ETO-positive leukemia setting. Am. J. Hematol. 86:993–997, 2011. V
Introduction
The mixed-lineage leukemia (MLL) gene at 11q23, also
named ALL-1/HRX/Hrtx1 [1–3], represents one of the most
frequent target of translocations in human acute leukemia
(AL). MLL encodes a complex protein and, characteristically, participates in a great number of chromosomal translocations resulting in many different MLL fusions and, as a
consequence, in a heterogeneous spectrum of proteins. To
date, 73 different translocations and 54 partner genes have
been cloned. In addition, a less frequent type of MLL rearrangement does not involve partner genes but rather partial
duplication of the MLL N-terminal segments. The most frequent partner genes are AF4, AF9, and ENL originated by the
t(4;11)(q21–q23), t(9;11)(p22;q23), and t(11;19)(q23;p13.3),
respectively. Altogether, these latter alterations account for
60% of AL cases with 11q23 abnormalities [4]. This biological heterogeneity was recently confirmed by Stam et al. [5]
who demonstrated, in a relatively numerous and heterogeneous group of infants, the existence of specific gene
expression signatures associated with the different MLL
translocations characterizing infant ALLs.
At the clinical level, because of the relative rarity of these
rearrangements and their prevalence in infants, the outcome of patients with MLL rearrangements is still not firmly
established. Although the t(4;11) translocation is generally
considered an adverse prognostic factor especially in
infants, the outcome of patients harboring the other more
rare MLL alterations, such as the MLL/ENL fusion, is uncertain. In childhood ALL, Rubnitz et al. [6] demonstrated that
MLL/ENL gene was associated with an excellent prognosis
both in T- and in B-precursor ALL. These findings were not
fully confirmed by Pui et al. [7] in a larger multicentric retrospective study, in which they showed only a trend to a better clinical outcome for noninfant patients with the MLL/ENL
alteration. As concerns adults, no data are available about
the frequency and the clinical outcome of this rare genetically defined leukemic subtype.
In this study, we analyzed the clinic–biologic features of a
series of children and adults with MLL/ENL positive ALL
cases consecutively diagnosed and monitored for minimal
residual disease (MRD) at our institution. We observed a
particularly favorable prognosis for patients with ALL
expressing the MLL/ENL gene fusion. In addition, we provide novel data on the molecular monitoring of MRD, showing a persisting expression of the fusion transcript for a
long time in cured patients.
Materials and Methods
Patients and treatment. Between November 1999 and September
2009, we identified 12 ALL patients with the MLL/ENL fusion transcript.
Four cases were children and were diagnosed among a group of 150
pediatric (<18 years) patients consecutively seen at the Bambino Gesù
pediatric hospital of Rome (104 patients) and at the Hematology pediatric unit of University ‘‘Sapienza’’ of Rome (46 patients); eight cases
were adults and were identified among a group of 1475 adult (>18
years) patients entered into the GIMEMA LAL 0496 (no. of patients:
111), 2000 (no. of patients: 571), and 0904 (no. of patients: 311) that
were opened to enrolment, respectively, from October 1996 to December 1999, January 2000 to September 2004, and from October 2004 to
l December 2009. In addition, 70 elderly cases (>60 years) were
screened for this genetic alteration without identifying any case. The
detailed demographic characteristics of these patients are listed in
Table I. The diagnosis of ALL was based on standard morphologic and
cytochemical evaluation and on immunophenotypic criteria. Cytogenetic
1
Department of Cellular Biotechnologies and Hematology, ‘‘Sapienza’’ University, Rome; 2GIMEMA Data Center, GIMEMA Foundation, Rome; 3Hematology Unit, ‘‘Bambino Gesù’’ Hospital, Rome; 4Department of Biomedical
Sciences, Ferrarotto Hospital, University of Catania, Italy; 5M. Tettamanti,
Research Center, Pediatric Clinic University of Milan, Bicocca, Monza, Italy;
6
Hematology Unit, ‘‘S.G. Moscati’’ Hospital, Avellino; 7Division of Hematology, ‘‘Amedeo Avogadro’’ University of Eastern Piemonte, Novara, Italy
Conflict of interest: Nothing to report
*Correspondence to: Giuseppe Cimino, Department of Cellular Biotechnology and Hematology, University ‘‘Sapienza,’’ Rome, Via Benevento 6, 00161
Rome, Italy. E-mail: cimino@bce.uniroma1.it
Received for publication 26 July 2011; Accepted 29 July 2011
Am. J. Hematol. 86:993–997, 2011.
Published online 4 August 2011 in Wiley Online Library (wileyonlinelibrary.com).
DOI: 10.1002/ajh.22161
C 2011 Wiley-Liss, Inc.
V
American Journal of Hematology
993
http://wileyonlinelibrary.com/cgi-bin/jhome/35105
research article
TABLE I. Clinic–Biological Characteristics, Treatment, Therapeutic Response, and Clinical Courses of the 12 MLL–ENL-Positive ALL Patients
Paz.
Sex
Age
WBC (3109/L)
Immunophenotype
Karyotype
Treatment
Response
Relapse
Follow-up
1
2
3
4
5
6
7
8
9
10
11
12
M
M
F
M
F
M
F
M
M
F
M
M
38.7
44.7
32.8
59.4
32.3
19.6
45.1
16.2
5.9
13.5
0.2
4.9
117.0
35.0
79.9
69.0
155.0
66.8
89.7
17.3
184.0
2.9
707.0
20.7
Pro-B
Pro-B
T-ALL
Pro-B
n.a.
T-ALL
Pre-pre B
Pre-B
T-ALL
Pre-pre B
Pre-pre B
n.a.
46XY t(11;19)(q23;p13)
n.e.
46,XX
46 XY, t(11;19)(q23;p13)
46 XX, t(11;19)(q23;p13)
46 XY, t(11;19)(q23;p13)
46,XX
n.a
n.a
n.a
n.a
n.a
GIMEMA LAL0904
GIMEMA LAL0904
GIMEMA LAL0904
GIMEMA LAL0904
GIMEMA LAL2000
GIMEMA LAL2000
GIMEMA LAL0904
GIMEMA LAL0496
AIEOP LAL2000
AIEOP LAL2000
INTERFANT99
AIEOP LAL2000
CR
CR
CR
CR
ID
CR
CR
CR
CR
CR
CR
CR
YES (2.9 mos.)
NO
NO
NO
–
NO
YES (1.6 mos.)
YES (9.9 mos.)
NO
NO
NO
NO
Dead
Alive in CR
Alive in CR
Alive in CR
Dead
Alive
Dead
Alive in CR
Alive in CR
Alive in Cr
Alive in CR
n.e., not evaluable; n.a., not available.
Figure 1. RT-PCR amplification of MLL/ENL fusion gene: assay strategy, results
of dilution experiments, size, and nature of amplified products.
and molecular studies of all cases were performed at our Institution.
All patients gave informed consent for both treatment and diagnostic
procedures.
Cytogenetic analysis. Cytogenetic analyses were performed on BM
cells after 24 hr of unstimulated culture. GTG-banded karyotypes were
reviewed and defined according to the ISCN 2005 criteria.
Molecular detection of MLL–ENL fusion transcript. Total RNA was
extracted from cells cryopreserved in guanidium isothiocyanate according to the method of Chomczynsky and Sacchi [8]. The quality of RNA
was assessed on an ethidium bromide-stained 1% agarose gel containing 2.2 M formaldehyde.
In vitro reverse transcription of 1 lg total RNA to cDNA was performed using the following specific cDNA primers 12–13 nucleotides
(nt) long and located 10–100 nt downstream of the most 30 PCR
primer: ENL 5 137 L12 50 -tctccacgaagt-30 ; MLL 5 417L13 50 tttggtctctgat-30 [9]. The molecular detection of the MLL/ENL fusion was
performed using a multiplex reverse transcription-PCR assay previously
described that allow the detection of the most common fusion transcript
of ALL [10]. In all the reaction mixtures, the specific pairs of primer
E2A1075U21 (50 -ttctcgtccagcccttctacc-30 ) and E2A1883L22 (50 ttttcctcttctcgccgtttca-30 ), at the amplification step, and E2A1173U19 (50 ctacgacgggggtctccac-30 ) and E2A1844L19 (50 -aggttccgctctcgcactt-30 ),
at the nested reaction, to amplify the E2A as control gene, were
added.Negative controls without DNA template were included for all
PCR reaction mixtures.
Molecular monitoring of MRD. The PCR strategy used to monitor
MLL/ENL expression is illustrated in Fig. 1. The individual pairs of primers and parameters of PCR amplification were those previously
described elsewhere [9,10]. As shown in Fig. 1, due to the different
breakpoint location within the MLL gene bcr, this strategy allowed to
detect multiple specific bands of 330, 250, 150, and 120 bp, respectively. To avoid the probability to amplify several fusion products and to
better define the MLL breakpoint locations, we decide to monitor MRD
by different single PCR reactions, each one containing the following different pairs of primers: (1) MLL:3730U20 (50 -ggaccgccaagaaaagaagt30 ) and ENL:81L22 (50 -caccatccagtcgtgagtgaac-30 ) as first PCR step
994
Figure 2. The molecular monitoring of MRD of patient no. 12 at diagnosis and
different time points during follow-up using the following distinct pairs of primers:
MLL:3730U20 (50 -ggaccgccaagaaaagaagt-30 ) and ENL:81L22 (50 -caccatccagtcgt
gagtgaac-30 ) as first PCR step and MLL:3751U20 (50 -ggaccgccaagaaaagaagt-30 )
and ENL:30L19 (50 -gcgatgccccagctctaac-30 ) for the nested PCR. M, molecular
marker; D, diagnosis.
and MLL:3751U20 (50 -ggaccgccaagaaaagaagt-30 ) and ENL:30L19 (50 gcgatgccccagctctaac-30 ) for the nested PCR; (2) MLL:3955U24 (50 agcactctctccaatggcaatagt-30 ) and ENL:81L22 (50 -caccatccagtcgtgagtgaac-30 ) for the first PCR step and MLL:3996U24 (50 -agcagatggagtccacaggatcag-30 ), and ENL:30L19 (50 -gcgatgccccagctctaac-30 ) for the
nested PCR. Each master reaction contains the primers to amplify the
E2A as control gene: E2A 1075U21 and E2A1883L22 at the amplification step and E2A1075U21 and E2A1883L22 at the nested reaction. A
representative example of a molecular monitoring of MRD is illustrated
in Fig. 2. Five microliters of diluted cDNA reaction were added to the
amplification-mixture-reaction that contained 1.5 U of AmpliTaq-Gold
polymerase (Applied Biosystems, Foster City, CA). The first PCR consisted of an initial activation cycle at 958C for 15 min, followed by 30
cycles of PCR amplification (annealing at 588C for 30 sec, elongation
at 728C for 1 min, and denaturation at 958C for 30 sec). For the nested
PCR, 1 ll of the first reaction mixture was transferred to 24 ll of the
nested-mixture-reaction with 1.5 U of AmpliTaq-Gold polymerase and
amplified for 25 cycles according to the same amplification conditions
of the first PCR step. Afterward, 15 ll of the nested PCR reaction was
electrophoresed in a 1.5% agarose gel for 60 min at 100 V. Sensitivity
of RT-PCR reactions was 1–1024 normal cells, as assessed by serial
dilution experiments diluting RNA isolated from BM cells collected at diagnosis from individual MLL/ENL positive cases in RNA extracted from
normal BM mononuclear cells and subjecting to RT-PCR amplification
as described earlier (Fig. 1). Negative controls (all reagents and water)
were included in all PCR experiments. To rule out the possibility of
cDNA contaminations of RNA samples, all tests with positive results
were repeated mixing all reagents without RNA.
Immunoglobulin and T-cell receptor gene rearrangements (Ig/TCR)
were detected on diagnostic specimen of pts. no. 9, 10, 11, and 12, as
described elsewhere [10]. In brief, on the basis of the junction region of
the identified rearrangements, patient-specific primers were designed
to obtain at least two sensitive markers (1 3 1024) and used for the
American Journal of Hematology
research article
real-time quantitative PCR analysis of Ig/TCR at fixed time points during induction treatment according to international multicenter trial
AIEOP-BFM ALL 2000 for childhood acute lymphoblastic leukemia [11].
Direct sequencing. A representative sample of RT-PCR amplified
was analyzed by direct sequencing on both strands for each fragment,
by using the ABI Prism 3130 (Applied Biosystems, Foster City, CA) to
confirm the MLL/ENL positivity. The sequences obtained were compared to the wild-type sequence of MLL gene and ENL gene (accession no. L04284 e NM 005934 respectively), by using GenBank of
National Center for Biotechnology Information BLAST (NCBI; Fig. 1).
Treatment and criteria of response. The infant case was treated
according to the Interfant 99 protocol [12], the three pediatric patients
according to AIEOP LAL 2000 [11] treatment (intermediate risk stratification), while the eight adults were treated according to the GIMEMA
LAL0496 [13], LAL2000, and LAL0904 trials, depending on which of
these three protocols were open to enrolment at the time of the diagnosis of each patients. Only one pediatric patient, who persisted Ig/TCR
positive after the induction treatment underwent hematopoietic stemcell transplantation (HSCT) in first Complete Remission (first CR).
Therapeutic responses were evaluated at the end of induction treatment in all cases. Hematologic complete remission (HCR) was defined
as the normalization of peripheral blood count and less than 5% blasts
in the bone marrow (BM) with normal cellularity. Relapse was defined
as the reappearance of leukemic cells in the bone marrow (>5%
blasts) and/or reappearance of clinical evidence of the disease.
As concerns the molecular monitoring of the MLL/ENL fusion gene,
molecular remission was characterized by the absence, on ethidium
bromide-stained electrophoresis gel, of the specific MLL/ENL amplification band detected at diagnosis in the presence of RNA integrity, as
evaluated by minigel visualization and successful amplification of the
control gene.
The PCR analysis of Ig/TCR rearrangements was considered negative if samples tested negative (i.e., an Ig/TCR rearrangement detection
reduced 1023 respect to the diagnostic level) using two sensitive
markers (1 3 1024).
Statistical analysis. Overall survival (OS) was defined as the time
from the diagnosis to death from any cause. Event-free survival (EFS)
was defined as the time from the diagnosis until death, resistant disease, or relapse, considering as event the first which occurred. Median
follow-up time was estimated by reversing the codes for the censoring
indicator in a Kaplan–Meier analysis. Analyses were performed by SAS
v.9.1.3 statistical software.
Results
Using our multiplex RT-PCR assay [10], we identified the
MLL/ENL fusion gene in 4 of 150 children (2.6%), in 8 of
993 adults (0.8 %), and in none of the 70 elderly ALL
cases (0%). The diagnostic clinical and biological features
and the clinical outcome of all the 12 MLL/ENL-pos. ALL
patients are reported in Table I. Four patients were females
and eight were males. The median age was 25.9 years
(range, 0.2–59.4 years). At diagnosis, the median WBC
count was 74.5 3 109/l (range, 2.9–707.0 3 109/l), and
eight patients had a WBC count >50 3 109/l. Immunophenotypic characterization was available in 10 cases. B-precursor ALLs were present in seven cases (i.e., pro-B 5 3
cases; pre-pre B 5 3 cases; pre-B 5 1 case), while a T
phenotype was detected in the remaining three patients.
An evaluable karyotype was available in 624 of the 993
(63%) adult patients with ALL included in this study. A
t(11;19)(q23;p13.3) was detected in four of the eight
patients with a MLL–ENL fusion (Table I), while it was never
detected in any of the MLL–ENL negative cases. Cytogenetic alterations at the 11q23 cytogenetic band were
detected in two additional patients who presented the following karyotype: 46,XX,der(11)t(11;?)(q23;?) [14]/46,XX
[8] and 44,XY,-3,-9,add(11)(q23) [6]/46,XY [15]. At the molecular analysis, both these two patients tested MLL–ENL
negative but BCR–ABL positive.
The median follow-up of this cohort of patient at the time
of the analysis was 34.4 months.
HCR was achieved in 11 (92%) of 12 patients. One
patient died during induction for infection. Three of the 11
American Journal of Hematology
Figure 3.
(A) Actuarial OS and (B) EFS of the MLL/ENL-positive ALL patients.
responders relapsed at 1.6, 2.9, and 9.9 months from CR,
respectively. Of these relapsing patients, two died because
of disease progression and one was, at the time of the
analysis, still alive. All relapsed patients were adults. OS
and EFS estimations at 48 months were 73.3% (C.I. 95%:
56.6–95.1) and 66.7% (C.I. 95%: 51.1–87), respectively
(Fig. 3).
The strategy and sensitivity of our RT-PCR assay, used
to monitor MLL/ENL expression at prefixed times during the
patient’s follow-up, is shown in Fig. 1. As previously
reported in the ‘‘Materials and Methods’’ section, due to the
splicing of the MLL exon 10, the simultaneous use of the
two pairs of primers, in patients with breakpoints falling at
MLL intron 10, gave origin to four amplification products of
330, 250, 150, and 120 bp, respectively. As shown by
sequencing analysis of these products, the 330 bps band
included
the
MLL
(NCBI
Reference
Sequence:
NM_005933.3) ex 7–9 (nucleotides 3,964–4,234) and ENL
(NCBI Reference Sequence: NM_005934.3) ex 2; the 250
bp resulted by the fusion between the MLL ex 7–8 (nucleotides 3,964–4,109) and the ENL ex 2; the 150 bps product
included the MLL ex 9–10 (nucleotides 4,168–4,292) fused
to the ENL ex2; and the 120 bps was due to the fusion of
the MLL ex 9 (nucleotides 4,168–4,239) and of the ENL ex
2. Therefore, to avoid the probability to amplify several
fusion products, we decided to monitor MRD by different
single PCR reactions, each one containing distinct pair of
primers. A representative example of this strategy used to
monitor MRD is illustrated in Fig. 2, in which is reported the
molecular monitoring achieved in patient no 12 of Table I.
The results of MRD molecular monitoring in the 11 cases
in HCR are reported in Fig. 4. At the time of CR, five cases
had a parallel evaluation of MLL/ENL expression. A PCRnegative status was achieved in two of the five tested
patients, while the remaining three cases persisted MLL/
ENL positive. Both MLL/ENL negative patients relapsed at
2.9 and 1.9 months from CR, respectively. One patient died
because of disease progression at 14 months from diagnosis, whereas, the other case, who went out of protocol
995
research article
Figure 4. Molecular monitoring of MRD of the 11 MLL/ENL-positive ALL patients
who achieved a HCR of disease. Patient no. 5 of Table I, who died during the
induction treatment, was not included. R, relapse of disease; D, dead; HSCT, hematopoietic stem cell transplantation; dot line, overt disease; continuous line, CCR
status.
because of the refusal of the consolidation treatment, was
still alive at the time of analysis. Among the other three
persistent PCR positive patients, one was alive in first HCR
at third year, one relapsed 9.9 months after CR and died
because of infection, while the remaining child, who persisted positive at the evaluation of Ig/TCR molecular
marker, underwent to HSCT in first HCR and was alive and
well at the fifth year. All the six cases who did not have a
molecular evaluation of MRD at time of HCR were alive in
first CR at 15 months, 3, 6, 7, 8, and 9 years, respectively.
Among these latter, pt. no 3, tested negative for MLL–ENL
expression at 9 and 15 months, while all the other five
patients became long-survivors but showed repeated MLL/
ENL RT-PCR positivity during their clinical follow-up. The
three children included in this group of five cases resulted
Ig/TCR negative at early time points during induction–
consolidation treatments.
Repeated MLL/ENL RT-PCR positivity was also detected
in the fourth child (pt. 12) who underwent to an allogeneic
HSCT in first HCR that induced a long-term continuous CR
lasting for 5 years (Figs. 2 and 4).
Discussion
Genetic alterations are believed to identify distinct leukemic subtypes and play an ever increasing role in guiding
classification, prognosis, and therapeutic strategies in
patients with AL. Among these, MLL/ENL is a fusion gene
that defines a rare leukemic subtype with putative distinct
clinic–biological features and outcome. It represents 1%
of successfully karyotyped pediatric ALL cases [6,7], while
its frequency in adult ALL is not yet established.
In this study, taking advantage of the routine use in our
laboratory of a multiplex RT-PCR assay to detect the most
common genetic alterations of ALL cases, including the
MLL/ENL gene fusion, we provide novel data regarding the
frequency and clinical outcome of the MLL/ENL genetically
defined leukemic subtype both in children and in adults.
Our data confirm the rarity of this genetic entity, and we
noticed that its frequency is inversely correlated with
patient’s age, decreasing from 4.6% to 0.8% and to 0% in
children, adults, and elderly patients, respectively. This frequency might be slightly underestimated, because our
strategy did not detect the recently described very rare
MLL/ENL fusions in which breakpoints fell at exons 4 or 6
of the ENL gene [14]. However, these novel MLL/ENL
996
fusions are very rare and are not restricted to ALL cases.
Furthermore, our cytogenetic data did not show any additional patients with t(11;19) alteration in the MLL/ENL negative population. Using a RT-PCR assay similar to our strategy, but coupled with a long distance inverse PCR, the GMALL study group reported a 6% frequency of MLL/ENL
positivity in a group of 184 adults with a pro-B ALL, a leukemic subtype accounting for approximately one-fifth of all
the ALL cases [16]. However, as also demonstrated by the
present study, MLL/ENL positive ALL are not restricted to
the pro-B immunophenotype, being detected also in other
B or T precursor ALLs [6,7].
As concerns prognosis, we showed that, similar to children, adults with MLL/ENL positive ALL have a favorable
clinical outcome after conventional intensive chemotherapy
treatment without consolidations with transplants. In fact,
although 8/12 patients in the present series presented
hyperleukocytosis, 11 patients achieved a CR. Three cases
relapsed and only two died. OS and EFS estimations at 48
months were 73.3% and 66.7%, respectively. It is interesting to note that the favorable prognosis reported for MLL–
ENL positive patients of the present series counteract the
adverse clinical outcome of the ALL cases harboring other
MLL fusion proteins. We have recently updated the
GIMEMA experience in the treatment of adult patients with
MLL/AF4 positive ALL showing that, despite the use of hematopoietic stem-cell transplantations as consolidation
treatments, the prognosis of these patients remains very
poor with OS and EFS rates at 36 months of 32.9% and
28%, respectively [17].
All the three relapses that we observed in the present series of patients occurred in adult cases. However, the small
number of patients included in the present study did not
allow us to conclude about a possible different prognosis
between children and adults.
An additional novel issue of the present study is the data
obtained in monitoring MRD. In our patients, the PCR status at response evaluation did not correlate with the clinical outcome. In addition, the persistence or reappearance
of an RT-PCR positivity for MLL/ENL did not predict a clinical relapse of disease, being detected several times during the follow-up of long-survivor patients. These observations demonstrate the existence of an inconsistency
between the clinical cure and the persistence at the molecular level of the MLL/ENL fusion transcript. This finding
parallels to that of other MLL-positive ALL cases. We previously reported the transient reappearance of the MLL/
AF4 transcript, not related to a disease relapse, in a longsurvivor ALL patient [18]. A similar observation was also
done in few our additional MLL/AF4-positive ALL cases
during their MRD monitoring (data not shown). Thus, similarly to what has been demonstrated for t(8;21) AML, in
which the AML1/ETO transcript was detected in long-term
remission patients [15,19], it is possible to suggest the
persistence in cured patients of normal nonleukemic precursor bone marrow cells expressing the MLL/ENL fusion
gene. These cells may represent a preleukemic clone that
following additional transforming events have given origin
to overt leukemia.
In conclusion, we provided data showing that, also in
adults, the MLL/ENL fusion identifies a rare genetically
determined ALL entity with an extremely favorable prognosis. In addition, similar to what demonstrated for the AML1/
ETO positive leukemia setting, the observed inconsistency
between the clinical cure and the presence of detectable
MLL/ENL transcript, both at therapeutic response evaluation and during follow-up of long-term CR patients, suggests the presence of a MLL–ENL-expressing ‘‘preleukemia’’ stem cell.
American Journal of Hematology
research article
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patients, leukemia, lymphoblastic, disease, minimax, outcomes, expressive, enl, monitoring, clinical, residual, mll, genes, fusion, acute
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