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Next Article 
Blood, Vol. 91 No. 11 (June 1), 1998:
pp. 3995-4019
REVIEW ARTICLE
Clinical Significance of Cytogenetic Abnormalities in Adult Acute
Lymphoblastic Leukemia
By
Stefan Faderl,
Hagop M. Kantarjian,
Moshe Talpaz, and
Zeev Estrov
From the Departments of Bioimmunotherapy and Leukemia, The University
of Texas M.D. Anderson Cancer Center, Houston.
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INTRODUCTION |
ACUTE LYMPHOBLASTIC leukemias (ALL) are
characterized by clonal proliferation, accumulation, and tissue
infiltration of neoplastic cells. They are mainly regarded as childhood
diseases, with an early incidence peak at 2 to 5 years of age, where
they represent about 80% of the childhood leukemias in the United
States, and occur with an incidence of up to 30 cases per 1 million
population per year.1 The age-adjusted incidence of ALL in
adults (usually defined as 15 years of age and older) amounts to about
one third of that in children.1 However, ALL has a bimodal
distribution, with a second peak around age 50 and a low but steady
rise in incidence with increasing age.2
Improvements in cytogenetic techniques have yielded significant insight
as to the importance of cytogenetic abnormalities in the
pathophysiology and prognosis of hematologic malignancies. Heim and
Mitelman3 reported an overall increase in the number of
reported cases of cancer with cytogenetic alterations from 3,844 in
1983 to more than 22,000 in 1994. Of all neoplasms, leukemias have been
by far the most intensively investigated and account for more than 60%
of all listed chromosomal aberrations, including more than 3,000 cases
of ALL.
The majority of cases of ALL demonstrate an abnormal karyotype, either
in chromosome number (ploidy) or as structural changes such as
translocations, inversions, or deletions. These changes were detected
in only half of ALL patients in the first banding studies.3
The scantiness of information gained from chromosomal findings in ALL
has been, in large part, due to technical difficulties. Chromosome
studies in ALL exhibit poor morphology; chromosomes tend to spread
poorly, and appear blurred and fuzzy with indistinct margins, making
banding studies challenging or even impossible.4,5 Improvements in spreading and banding techniques have resulted in
higher rates of detection, and most studies now report chromosomal changes in 60% to 85% of ALL cases.6-10 The Third
International Workshop on Chromosomes in Leukemia (TIWCL)
found the majority of cytogenetic changes in cases of B-precursor ALL,
with only 39% occurring in T-cell ALL.6,9 Williams et
al4 used a direct technique of bone marrow (BM) chromosomal
analysis developed particularly for studies in ALL, which paid
attention to sampling and processing steps using specific flaming
techniques and modified G-banding procedures. They identified clonal
karyotypic abnormalities in 94% to 98% of cases of ALL. Such improved
techniques also detected nonrandomly occurring cytogenetic
abnormalities in cases with hyperdiploid chromosome numbers (>50)
that had previously been classified as normal in
karyotype.11 These results showed a high prevalence of
clonal chromosomal abnormalities in ALL, as was shown for acute
nonlymphoblastic leukemias by Yunis,12 who used high-resolution banding techniques. These and similar studies underscore the significant yield achieved with thorough cytogenetic studies in ALL.
That cytogenetic abnormalities confer important prognostic information
in ALL was first reported by Secker-Walker et al13 in 1978 in a series of childhood ALL. The investigators reported better
clinical outcomes in cases with hyperdiploid karyotypes than in those
with hypodiploidy or pseudodiploidy, and these findings were confirmed
in the follow-up study14 and by other
researchers.8,15-17 The TIWCL examined 330 newly diagnosed
ALL patients (172 adults and 157 children) and found that chromosomal
abnormalities distinguished high-risk from low-risk patients. Complete
remission (CR) rates, remission durations, as well as
disease-free-survivals (DFS) were significantly affected by the
karyotypic abnormalities.6 Among adult patients the highest
likelihood of cure (21% to 30%) was projected in patients with
chromosome numbers of >50, or 47 to 50, with 6q , or with a normal
karyotype.18 As with children, karyotypes in adults were
significant independent predictors of remission duration or DFS, after
considering covariates such as age, leukocyte count at presentation, or
French-American-British (FAB) morphology.19 Secker-Walker
et al10 reported the prognostic effect of certain
structural rearrangements, such as t(9;22), to be independent of other
single variables.
Most studies on karyotypic abnormalities and their clinical
significance have been performed in childhood ALL.15,20-24
Adult ALL may show nonrandom chromosomal abnormalities similar to those found in childhood ALL, but their distribution and, possibly, their
biological significance are different. Few studies have addressed these
issues in adult ALL.6-8,18,25,26 This review focuses on the
most important chromosomal abnormalities found in adult ALL and their
prognostic and therapeutic implications.
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NUMERICAL CHROMOSOME ABNORMALITIES |
Numerical chromosome abnormalities, either alone or in association with
structural changes, are found in about half of ALL cases. Several
ploidy groups have been identified (Table1).15,27 These include low hyperdiploidy (modal number 47 to 50), high or massive hyperdiploidy (>50), hypodiploidy (46 and
lower), pseudodiploidy (normal number of chromosomes, but with
associated structural changes), as well as gain or loss of a single
chromosome as the sole karyotypic change. Sole numerical aberrations
are less frequent than the numerical aberrations in combination with
structural changes, which occur in about 40% to 70% of
cases.5,7
Hyperdiploidy
Chromosome numbers of 47 or more are found in up to one fourth of adult
ALL cases, making the hyperdiploid group one of the most frequent
ploidy groups (Table 1). Hyperdiploid chromosome numbers, particularly
those with >50 chromosomes, are more frequent in children than in
adults. Chromosome numbers cluster around 47 and between 51 to 55 so
that a bimodal distribution becomes evident, similar to the
distribution in childhood ALL.15 A low hyperdiploidy group
with 47 to 50 chromosomes can be distinguished from a massive
hyperdiploidy group with >50 chromosomes. Disomies and, particularly,
trisomies or tetrasomies, are frequent and are dominant in the group
with >50 chromosomes.
Although various chromosomes are involved in hyperdiploidy, certain
karyotypes seem to prevail. In a series of 31 patients, Ankathil et
al28 found that hyperdiploid karyotypes were mainly characterized by trisomies of chromosomes 8, 18, 19, and 21. The TIWCL
study also showed, in both hyperdiploidy groups, gains in chromosomes 4 and 8, as well as 21 and 18, to be more frequent. Additional
chromosomes 6, 10, and 14 appeared almost exclusively in the massive
hyperdiploidy group.6 The Groupe Français de Cytogénétique Hématologique (GFCH) found in the group
of >50 chromosomes mainly chromosomes 4, 6, 8, 10, 14, 17, and 21 and in the 47 to 50 group mainly chromosomes 5, 8, 10, and 21.7 About half of all adult ALL cases with hyperdiploidy of a modal number
of >50 can be expected to show additional chromosomal rearrangements, with translocation t(9;22) being the most common.7,9 In
contrast, the TIWCL found relatively few structural rearrangements in
both hyperdiploidy groups. This difference reflects the impact of
advances in technology and expertise over little more than a decade on the yield of cytogenetic studies. In addition, the technical quality of
hyperdiploid metaphase spreads is frequently substandard, and structural aberrations are still not reliably identified in a significant number of cases.29
In the TIWCL study, almost all cases with >50 chromosomes were of
precursor B-cell type and associated with favorable prognostic features
such as low white blood cell count (WBC) at presentation, low lactate
dehydrogenase (LDH) levels, and FAB L1 or L2 morphology.6
In children with ALL, chromosomal numbers of >50 have been reported
to have better response duration as well as median survival time (MST),
whereas numbers between 47 and 50 conferred an intermediate prognosis.
In this context, the combination of trisomies of both chromosomes 4 and
10 in children with hyperdiploid ALL identified a subgroup of patients
with an extremely favorable DFS, and likelihood of cure with
antimetabolite-based chemotherapy.30
No such favorable constellation could be identified in adult ALL, where
the impact of hyperdiploidy on prognosis appeared less significant
(Table 2). Although the TIWCL showed higher CR rates, as well as increased CR duration and MST in this ploidy group
for both children and adults, children still survived significantly longer than adults for modal numbers >50 or 47 to 50. The GFCH study
on 443 adult ALL patients found a favorable prognosis in patients with
hyperdiploidy of >50 chromosomes without the Philadelphia chromosome
(Ph), as well as in patients with tetraploidy.7 The UKALL
XA trial observed better outcomes with hyperdiploidy which added
prognostic significance to age, sex, and WBC, unless these variables
were combined.10 Campbell et al31 showed a survival advantage for adult patients with only numerical abnormalities and particularly for those with chromosome numbers more than 50. However, other studies could not confirm the association of
hyperdiploidy with good outcome in adult ALL. Fenaux et al8
found no significant differences in CR rate or duration between
cytogenetic groups in 73 adult patients, except for a slight increase
in CR duration with normal karyotypes, and worse outcomes with the Ph
abnormality. This contrasted with their findings in pediatric patients,
where a significant difference in response to treatment was found
between children with hyperdiploidy and those with hypodiploidy.
Walters et al,32 in a series of 91 adult patients, could
not demonstrate superior outcome with hyperdiploidy.
The less favorable prognosis in adult ALL with hyperdiploid karyotype,
compared with children, may be explained by the higher prevalence of
associated unfavorable structural changes. The GFCH study, for example,
reported the additional presence of Ph in 11 of 30 patients with
hyperdiploid (>50) karyotypes.7 The UKALL XA trial showed
that the prognostic impact of structural changes overrode the
prognostic significance of ploidy groups. Poor-risk structural
rearrangements (such as Ph) confer a bad outcome even if they occur in
otherwise good-risk ploidy groups.10
Hypodiploidy
Modal chromosome numbers of 45 and less are rare, particularly the
nearly haploid numbers of 24 to 36. In most series of adult patients,
hypodiploid chromosome numbers were found in 2% to 8% (Table 1).
Sandberg33 compiled, from various references, 26 patients
with near-haploid or severe hypodiploid leukemias, including seven
adults. The most common losses involved chromosomes 1, 5, 6, 10, 11, 18, 19, 21, and 22. Interestingly, these were the same chromosomes
involved in the hyperdiploid karyotypes.6,7,33 The reason
for this preference is not known. Near-haploid cases almost always have
only numerical changes, whereas cases with modal numbers of 30 to 44 show frequent structural aberrations, mainly
translocations.6,34 In nearly all hypodiploid cases analyzed by Rieder et al,9 additional structural
abnormalities were identified (half of them involving the Ph).
How massive chromosome losses contribute to leukemogenesis is unclear.
Oshimura et al35 proposed multipolar mitosis with subsequent misdivision as a possible explanation. Conceivably, a
near-haploid karyotype allows expression of recessive genes that would
otherwise have been under the dominance of their allelic counterparts,
with loss of regulatory control of growth and differentiation of
lymphoid cells.33 Many ALL patients also have a second
hyperdiploid population with 52 to 56 chromosomes, twice the
near-haploid number, at some time in their disease course, possibly
arising by secondary endoreduplication.34 In these
instances hyperdiploidy may be an expression of clonal evolution from a
near-haploid stem line.33
ALL associated with a hypodiploid karyotype is usually of a precursor
B-cell type, although in the TIWCL series, 20% of hypodiploid cases
had T-cell ALL.6 Median initial WBC and percentage of blasts were higher than with diploid or hyperdiploid karyotype, and
morphology was predominantly FAB L2.6 Hypodiploid adult patients had the worst 3-year survival in the UKALL XA trial, and
hypodiploidy was shown to have prognostic significance independent of
other important variables (including the combination of age, sex, and
WBC).10 Like hyperdiploidy, hypodiploidy appeared to have
more effect on outcome in children than in adults, where data are still
limited.25 Nevertheless, hypodiploidy conferred poor
prognosis in most studies of adult ALL, comparable to other poor-risk
chromosomal translocations such as t(4;11) and t(1;19) (Table
2).7,10,18
Pseudodiploidy
A normal chromosome number with structural changes is the most
frequently found abnormal karyotype in adult ALL, and provides for the
most heterogeneous group of patients (Table 1). The GFCH showed a
pseudodiploid karyotype in 59% of ALL patients, 60% of whom had
recurrent translocations (Ph in 40%). This group was also
characterized by the highest peripheral WBC counts.7 The TIWCL study found that, compared to other karyotype groups, the group
with pseudodiploidy had increased ratios of adults to children, and of
L2 to L1 morphology, as well as high initial leukocyte and blast
counts.36 The pseudodiploid group was also notable for
having the highest percentage of T-cell ALL.37 Most cases had structural changes, with only a few combining structural and numerical aberrations. Chromosomes frequently involved were 1, 6, 9, and 14 but included every autosome with the exception of 4, 16, 18, and
21.36 In the UKALL XA trial, pseudodiploidy was observed in
association with translocation t(9;22), with abnormalities of 6q or 9p,
or, less often, with translocations t(4;11) and t(1;19).10 In some studies, initial leukemic cell burden was high in pseudodiploid cases, which was reflected by increased leukocyte counts and LDH levels.6,14 The poor prognosis associated with
pseudodiploidy (Table 2) is likely a reflection of structural
rearrangements and other poor-risk features. More accurate
identification of underlying specific structural abnormalities may soon
make the pseudodiploid group redundant in a prognostically useful
classification.
Single Chromosomal Gains or Losses
Nonrandom single chromosome gains or losses occur frequently in
ALL,38 although their incidence is lower than in myeloid leukemias.34,39-42 Rarely are they the sole karyotypic
abnormality.38 The mechanisms by which such changes
contribute to leukemogenesis are unknown. Heim and
Mitelman43 offered two possible explanations: a dose
effect, with abundance of certain gene products resulting in abnormal
proliferation or differentiation, or a duplication of a small genetic
defect with oncogenic potential. Most reports on single chromosome
gains or losses have been published in childhood ALL, mainly with
trisomy 8, monosomy 20, and trisomy 21.34,42,44-46 Only in
10% to 20% of these were the trisomy or monosomy the only karyotypic
anomaly, and no particular distinguishing characteristics could be
observed.
The TIWCL demonstrated trisomy 21 as the most frequent chromosomal gain
in ALL, and further studies confirmed its relatively high incidence,
but mainly as part of other cytogenetic changes such as
hyperdiploidy.36,47 Trisomy 8 as the only karyotypic change
is a frequent occurrence, mainly in myeloid leukemias.48 Garipidou et al44 estimated its incidence in ALL to be 1%
to 2%. It is indicative of a poor prognosis in acute myeloid leukemias (AML), but its prognostic significance in either adults or children with ALL is not established.49
Trisomy 4 has been observed in a broad range of hematologic
malignancies,38,50 but is rare in ALL51-53 and,
as the sole abnormality, tends to be associated with myeloid leukemias,
while no such association existed when there were additional cytogenetic changes.
The first report of trisomy 5 in ALL, by Sandberg et al,54
was of a 26-year-old male patient with B-lineage ALL. Nagesh Rao et
al55 described a 24-year-old woman with T-lymphoblastic lymphoma and trisomy 5 as the sole cytogenetic abnormality. Their review of the literature found only five cases in addition to the case
reported by Sandberg et al: 3 AML, 1 non-Hodgkin's lymphoma, and 1 case of Hodgkin's disease. Chen et al56 delineated certain regions on chromosome 5, such as 5p13 and 5q11-31, that were supposedly more specific to ALL.
Monosomies of chromosomes 5 and 7 through deletion or losses of
chromosomal material are frequently found in AML and myelodysplastic syndromes. However, their occurrence and significance is not well established in ALL. Dabaja et al57 analyzed 468 adult
patients with ALL and found abnormalities of chromosome 5 in 3 and of
chromosome 7 in 31 patients; Ph was an associated abnormality in one
third. This association translated into lower CR and 3-year survival rates compared with patients who did not have abnormalities of chromosomes 5 or 7. However, when patients with 5 and 7, but without Ph were considered, clinical outcome was not different. The
GFCH reported 45 cases with monosomy 7 among 443 adult patients. It was
the sole abnormality in only one patient with T-cell ALL.7 Rieder et al9 reported only 1 case out of 100 with loss of chromosome 7 as the sole cytogenetic abnormality, confirming the rarity
of this karyotype.
Monosomy 20 rarely presents as the single anomaly in ALL. Most studies
refer to pediatric patients but occasionally describe this finding in
adults, where it correlated with FAB L1 morphology, although this
association was not consistent.45,58
Overall, the significance of monosomies or trisomies as isolated
karyotypic changes is unclear, and no specific disease characteristics have been established. Furthermore, reports of single chromosomal gains
or losses in adults were too sporadic to draw conclusions of clinical
relevance.
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STRUCTURAL CHROMOSOME ABNORMALITIES |
More than 30 different nonrandomly occurring rearrangements are
presently known in ALL (Table3).34 The GFCH found structural abnormalities in 78% of
cases distributed across all ploidy groups.7 Translocations
constituted the most common changes. They were found in 30% to 37% of
adult cases, with the t(9;22) translocation being the most
frequent.7,33,37 The TIWCL showed significant differences
between chromosome groups regarding central nervous system (CNS)
involvement, leukocyte and blast counts, FAB morphology, and
immunophenotype.37 In adults, translocation t(4;11) and the
Ph karyotypes were associated with higher leukocyte and blast counts
than were other chromosomal rearrangements. Translocations t(8;14) and
t(4;11), as well as 14q+, correlated with a higher risk of CNS
involvement. Both children and adults with chromosomal translocations
had worse survival than those with normal karyotypes. In children the
presence of cytogenetic abnormalities other than translocations was
associated with outcome similar to normal karyotypes, but in adults all
structural abnormalities adversely influenced survival.
Translocation t(9;22)(q34;q11)
In 1960 Nowell and Hungerford59 discovered the Ph as a
distinct chromosomal abnormality in chronic myeloid leukemia (CML). It
was the first chromosomal abnormality to be associated with a specific
malignant disease in humans, and became a karyotypic hallmark of CML.
In 1970 Propp and Lizzi60 reported a 53-year-old patient
with ALL who had the classic Ph in a high percentage of marrow cells.
It is now well established that a t(9;22) translocation can be observed
in up to 95% of patients with CML, in about 1% to 2% of patients
with AML, as well as in up to 5% of children and 15% to 30% of
adults with ALL, making it the most common ALL-associated chromosomal
abnormality in the latter group.61-64
The Ph is a shortened chromosome 22 that results from a reciprocal
translocation between the long arms of chromosomes 9 and 22 (Fig 1).62 This transposes the
large 3 segment of the c-ABL gene from chromosome 9 to the 5
part of the BCR gene on chromosome 22, creating a hybrid
BCR-ABL gene that is transcribed into a chimeric
BCR-ABL mRNA.65,66 The breakpoint in the
ABL region can occur anywhere in a 300-kb intron, but is
usually 5 of ABL exon a2. Deletions of ABL exon a2 and
in-frame joining at the mRNA level of 5 BCR sequences to the
ABL exon a3 have also been described.67 In almost
all cases of CML, as well as in about half of adult Ph+
ALL, the ABL gene from chromosome 9 transposes into the major breakpoint cluster region (M-bcr) on chromosome 22, spanning exons 12 to 16 (historically named b1 to b5), giving rise to BCR-ABL fusion transcript mRNAs with a b2a2 or b3a2 junction. The fusion mRNAs
translate into a fusion protein of 210 kD called
p210BCR-ABL.68 In very rare cases of CML or
AML, but in about 50% of adult Ph+ ALL and in 80% of
childhood Ph+ ALL, the breakpoint on chromosome 22 falls 5
of the M-bcr, within a long intron segment separating the alternative
exon e2 from e2 called the minor breakpoint cluster region
(m-bcr).69 Splicing out exons e1 and e2 creates an e1a2
junction in the BCR-ABL transcript, translating into a smaller
BCR-ABL fusion protein called p190BCR-ABL. Both
p210BCR-ABL and p190BCR-ABL have significantly
increased tyrosine phosphokinase activity compared to the normal human
c-ABL protein, p145. Saglio et al70 described a novel
position for a breakpoint in the BCR gene that is located in a
3 direction from the M-bcr region, between exons e19 and e20
(historically c3 and c4), creating a new fusion transcript with an
e19a2 junction. This transcript contains a significantly larger part of
the BCR gene and codes for the p230BCR-ABL fusion
protein. In other cases, complex variant translocations involve three
or more chromosomes, mask the Ph marker, or a fusion gene is created by
the insertion of ABL sequences into a normal appearing
chromosome 22.
View larger version (23K):
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| Fig 1.
Translocation t(9;22)(q34;q11) and fusion products. The
Ph is a shortened chromosome 22 that results from the transposition of
3-ABL segments on chromosome 9 to 5-BCR segments on
chromosome 22. Whereas breakpoint locations on chromosome 9 appear
rather constant 5 of ABL exon a2, several breakpoint cluster
regions have been identified along the BCR gene on chromosome
22. Depending on which breakpoints are involved on chromosome 22, differently sized segments from the BCR gene are joined
together with the 3-sequences of the ABL gene. The
translocation thus results in fusion mRNA molecules of different length
(e1a2, b2a2, b3a2, e19a2) and, subsequently, chimeric protein products
with variable molecular weights and presumably function (p190, p210,
p230) (see text for details).
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Traditional karyotypic studies underestimate the incidence of the
BCR-ABL fusion gene, and Ph cytogenetics with
positive molecular tests for the BCR-ABL fusion gene have been
documented.62,71-73 Molecular tools for the detection of
the BCR-ABL fusion gene include fluorescent in situ
hybridization (FISH),74 polymerase chain reaction
(PCR),75 and pulsed-field gel
electrophoresis,76 which are complementary to cytogenetic analysis.73 Southern blot analysis, as it is applied in
CML, is inadequate as a single diagnostic tool for Ph+ ALL,
since it detects p210BCR-ABL but not
p190BCR-ABL which is found in 50% to 80% of
Ph+ ALL, because breakpoints on the BCR gene occur
outside the M-bcr region in an area too large to be reliably
recognized.71
The variability of the breakpoint locations within the BCR
gene, and the fact that p190BCR-ABL is predominantly
associated with ALL, while p210BCR-ABL is most consistently
associated with CML suggests a distinction between true de novo ALL
(m-bcr, p190BCR-ABL) from CML in lymphoid blastic phase
(M-bcr, p210BCR-ABL). There is also a difference in the
frequency of expression of p190BCR-ABL between pediatric
and adult ALL (80% v 50% of Ph+ ALL
cases).77,78 In vitro studies showing that
p190BCR-ABL is a more active tyrosine kinase than is
p210BCR-ABL also suggest that these entities are different
on the basis of their breakpoint location.77,79,80 The
relation among these disease groups is still not well
understood.81 Melo61 reported that the location
of the breakpoint in the BCR gene (and/or possibly in
the ABL gene) determined the disease phenotype. The hybrid gene
product p190BCR-ABL may function preferentially during
lymphoid and monocytic differentiation, while p210BCR-ABL
contains additional BCR-sequences that appear to affect pathways common
to both ALL and CML precursors. However, p190BCR-ABL
transcripts are not confined to primary acute leukemias. Using quantitative reverse transcription-PCR (RT-PCR) for
p210BCR-ABL and p190BCR-ABL mRNA, Van Rhee et
al,82 in a study of adult leukemias, detected p190BCR-ABL mRNA in 88% (14 of 16) of patients with CML in
chronic phase, 100% (10 of 10) of patients with CML in lymphoid
blastic phase, and 100% (10 of 10) of cases with
p210BCR-ABL-positive ALL. However, the
p190BCR-ABL/ p210BCR-ABL ratio was 10 times
greater in ALL than CML, whereas the ratio was similar in CML at
diagnosis, in chronic phase, and in lymphoid blastic phase.
Several studies reported on the association between breakpoint location
and clinical features and prognosis in Ph+
ALL.79,80 Secker-Walker et al80 studied 113 adult patients with Ph+ ALL and found no significant
difference between those with an M-bcr breakpoint and those with a
different breakpoint location for age, immunophenotype, or outcome.
Only the peripheral WBC count was significantly higher with an M-bcr
breakpoint. Likewise, Kantarjian et al77 found no
significant clinical, laboratory, or karyotypic differences in patients
with p210BCR-ABL versus p190BCR-ABL ALL.
Patients with either abnormality had similar incidences of older age,
organomegaly, anemia, leukocytosis, thrombocytopenia, and blastosis.
Although patients with p190BCR-ABL disease showed a trend
for longer CR duration, overall survival was not influenced by the
expression of either p190BCR-ABL or
p210BCR-ABL. When maintenance therapy was not given after
CR, some patients with p210BCR-ABL disease demonstrated a
"second chronic phase" CML-like hematologic picture in the
blood.77
In patients treated with bone marrow transplantation (BMT) for
Ph+ ALL, a different clinical behavior was reported for
Ph+ ALL expressing p190BCR-ABL compared with
p210BCR-ABL. In a study of 36 patients (29 adults) with
Ph+ ALL, Radich et al83 showed that detection
of BCR-ABL by PCR after BMT correlated with a high risk of
relapse, and that the expression of p190BCR-ABL was
associated with a higher risk of relapse than was the expression of
p210BCR-ABL. All of the relapsed patients were adults.
Adult patients with Ph+ ALL tended to be older, had higher
WBC and blast counts and, in some studies, more frequent
lymphadenopathy and splenomegaly than Ph
patients.84,85 Secker-Walker et al80 found that
44% of ALL patients older than 50 years were Ph+. Almost
all cases of Ph+ ALL had a pre-B
phenotype,7,8,36,84 and other immunophenotypes were
rare.7,36,86,87 Increased expression of CD10 ("common ALL antigen" or CALLA) was found with Ph+ ALL, and in
CALLA+ cases BCR-ABL identified a group of patients
with short remission duration and poor DFS.73,84 Myeloid
markers were present in 40% to 65% of Ph+ ALL
cases.6,86
In children, Ph+ ALL has a dismal prognosis even with
intensive chemotherapy programs that have improved survival in other cytogenetic subgroups.15,18,88,89 A similarly poor
prognosis was reported in adults where, despite high remission rates
comparable to those of Ph ALL, remission duration and
survival times were short (Table4).25,54,73,84,90 Therefore, patients with Ph+
ALL are suitable candidates for innovative and intensified strategies. In younger patients, allogeneic BMT with a related or unrelated donor,
or peripheral blood stem cell transplantation during first CR are
indicated, although relapse rates tend to be high ranging from 40% to
80%, and disease recurrence after BMT has a particularly devastating
prognosis in adults.83,91 In cases where no match can be
found or marrow transplantation is not an option for other reasons,
high-dose ara-C containing regimens can achieve considerable response
rates.84 Other treatment alternatives include interferon, adoptive immunotherapy, and antibody-based therapies.83
Unfortunately, to date, no satisfying track-record of durable responses
has been established with any of these approaches.
Abnormalities of the Short Arm of Chromosome 9
Loss or mutation of 9p21-22.
In 1983 Kowalczyk and Sandberg92 described 7 children with ALL, 5 with deletions of the short arm of chromosome 9 (9p) and 2 with total loss of chromosome 9. In all cases segment
9p21-pter was missing. Compared with other children with ALL, the group had distinct clinical features such as older age, prominent
lymphadenopathy and splenomegaly, and high WBC count and percentage of
blasts. Four of the seven children had T-cell ALL. Median survival time for children with 9p abnormalities was 1 year shorter than for children without these anomalies. In 1985 Chilcote et al93
reported 65 patients with ALL (41 children and 24 adults). They singled out 8 patients (4 children and 4 adults) with "lymphomatous"
features and showed that 6 of them (3 children and 3 adults) had loss
of bands p21-p22 on the short arm of chromosome 9, whereas these bands
were missing in only 1 out of the remaining 57 cases. The mechanisms
involved included deletions, unbalanced translocations, or loss of the
entire chromosome. The outcome for these patients was poor, as in the
Kowalczyk and Sandberg study. The investigators hypothesized a possible suppressor gene in or near 9p21-22 that is
involved in the control of proliferation of lymphoid precursors. Finally, in 1987 Pollack and Hagemeijer94
described 32 patients (18 adults and 14 children) with a 9p
karyotype, including 20 with ALL (10 adults and 10 children). Among
these, 3 adults showed 9p as the sole karyotypic anomaly, whereas
none of the children had 9p without other chromosomal changes. On
average, the karyotypes in children appeared more complex than in
adults. However, no association of 9p with T-cell ALL or
"lymphomatous" features was seen. Other studies have confirmed
that 9p anomalies are more likely in ALL than in other hematological
malignancies, with a reported frequency of 7% to 13%, and no apparent
difference in childhood versus adult ALL.92,93,95-97 An
association with high-risk clinical features such as older age, higher
leukocyte and blast counts, bulky disease, increased incidence of CNS
disease, or T-cell immunophenotype was found in some, but not in all
studies.
The smallest segment lost included 9p21. Specific deletions of 9p21
have also been described in a variety of human, rodent, and simian
cancer cell lines, pointing at 9p21 as a possible location for a tumor
suppressor gene.98,99 Detailed molecular analyses of this
location, searching for a possible tumor suppressor gene, have been
performed with a multitude of different probes.100 Trent et
al101 have localized the interferon gene cluster to 9p21,
and the assignment by Carrera et al102 of the
methyladenosine phosphorylase (MTAPase) gene, coding for an essential
enzyme in the purine salvage pathways, to the same region assumed new
clinical relevance. Using molecular analysis, the interferon gene
cluster was shown to be deleted in 43% of leukemia-derived cell lines and in 29% of primary leukemia samples,95,103 whereas
MTAPase deficiency could be demonstrated in 10% of leukemias and, in
particular, in 38% of cases of T-cell ALL.104 Kamb et
al105 and Nobori et al106 analyzed 9p21
deletions in detail and localized two previously cloned genes to the
9p21 segment: p16INK4A and the structurally
homologous gene, p15INK4B, located only 25 kb
centromeric to p16INK4A. They named these genes
Multiple Tumor Suppressor Gene 1 (MTS1) and Multiple
Tumor Suppressor Gene 2 (MTS2), respectively, as they
could demonstrate high frequencies of homozygous deletions in a variety
of human tumor cell lines. Kamb et al105 also studied four
leukemia cell lines and found deletions within 9p21 in one. Nobori et
al106 analyzed 14 leukemia cell lines by PCR and found MTS1 deletions in 9 of them (64%), with deletions of the
MTAPase and interferon genes in 29% and 50% of cases, respectively.
Both MTS1 and MTS2 encode for proteins that inhibit the
cyclin-dependent kinases CDK4 and CDK6 and play a crucial role in cell
cycle progression,100,107,108 which makes them ideal
candidates for a putative tumor suppressor gene at 9p21.
The main mechanism for p16INK4A inactivation is
biallelic deletions, with p15INK4B gene codeletions
in most but not all cases (Table5).108 Current data suggest that
p16INK4A is the primary target of such deletions.
This is underscored by the fact that in most studies
p15INK4B deletions occur only in
p16INK4A-deleted cases, whereas the converse does
not.109-112 This has also been shown for
interferon- gene deletions.113 The
frequency of homozygous deletions of the
interferon- gene is lower than that for
p16INK4A in one published report.103
Point mutations within p16INK4A are rare (Table 5).
Quesnel et al113 analyzed p16INK4A gene
deletions by Southern blot in 63 ALL patients (61 adults) and found 9 cases with homozygous deletions of p16INK4A (3 T-cell ALL, 6 precursor B-cell ALL). Single-stranded conformation polymorphism (SSCP) analysis of exons 1 and 2 of
p16INK4A was performed in 88 cases of ALL,
including the 63 cases mentioned. Only one missense mutation at codon
49 was detected. Inactivation of p16INK4A occurred
mainly through deletions of both copies of the gene, whereas deletion
of one allele and point mutation of the other (as is frequently
observed with the tumor suppressor gene p53) were
rare.97,112,114,115
More recently Herman et al116 and Batova et
al117 presented evidence for an additional mechanism of
gene inactivation. They showed that the p15INK4B
gene was preferentially hypermethylated at a 5-CpG island. This was
demonstrated in 17 of 45 children with T-cell ALL at diagnosis and in 7 of 32 at relapse. Methylation of the p16INK4A gene
was rare; it occurred in only 2 of 49 patients at diagnosis and none at
relapse. Methylation correlated with loss of transcription, and the
preferred methylation site at p15INK4B in ALL
lended strong support for a role of p15INK4B
inactivation in the pathogenesis of ALL. Whether a similar mechanism is
operative in adult ALL patients remains to be investigated.
Disagreement still exists about whether 9p21 abnormalities correlate
with immunophenotypic or prognostic features. Strong correlations with
T-cell ALL have been reported by some
investigators96,109,110,118 but not by
others.111,112 Quesnel et al113 found at least
one poor prognostic factor, such as bulky disease or high WBC count, in
patients with ALL and p16INK4A gene deletions, and
most of these patients relapsed. However, no strong association with
T-cell lineage was evident. Likewise, Fizzotti et al119
reported a significant relationship between p16INK4A/p15INK4B gene deletions and
leukemic cell mass and WBC count, but no difference by immunophenotype.
The GFCH found 9p21 abnormalities in 15% of ALL patients.7
The abnormalities had no effect on prognosis and no significant
correlation with T or B lineage. Nevertheless, abnormalities of
chromosome 9p21 with deletions of the
p16INK4A/p15INK4B region are highly
specific to lymphoid tumors and have been among the most consistent
genetic defects found in ALL to date.
t/dic(9;12)(p11-12)(p11-13).
This rare group of chromosomal abnormalities represents, for the most
part, unbalanced translocations with loss of part of the short arms of
chromosomes 9 and 12. Considerable breakpoint variation exists on 9p11,
where the translocation can fall anywhere in a 300-kb
segment.34 This abnormality was first discovered in 1985 by
Heerema et al,22 who described a child with the karyotype 45, XY, 9, 12, +der(12)t(9;12)(q1?;p13). In 1987 Carroll et al120 reported eight children with precursor-B-cell ALL and
t/dic(9;12)(p11-13;p11-12); seven were hypodiploid, with loss of both
chromosomes 9 and 12 and a remaining der(12)t/dic(9;12), whereas the
eighth child retained two normal chromosomes 9, with one normal
chromosome 12 and a der(12)t/dic(9;12). In four children this
abnormality was the only cytogenetic change. Since then other dicentric
translocations involving the short arms of chromosomes 9 and 12 have
been described. Larger series were published in 1992 by the United
Kingdom Cancer Cytogenetics Group121 and by Mahmoud et
al.122 In the latter series 15 of 2,303 children carried a
t/dic(9;12). There was a striking association with a
pre-B/early-pre-B-cell ALL phenotype, male gender, and an excellent
prognosis. In 1995 a dic(9;12) study group analyzed additional 14 cases
of dic(9;12)(p11-13;p11-12),123 including 11 cases of ALL
with a precursor-B phenotype (5 children and 6 adults). The ALL
patients were characterized by young age (median age 15 years, all but
one under 25 years), a median WBC count of 6.3 × 109/L,
no CNS involvement, predominantly FAB L1 morphology, early pre-B-cell
phenotype, positivity for CD10, and an excellent prognosis. Almost all
achieved CR, with DFS and overall survivals (OS) of 93% and 97%,
respectively, at a median follow-up of 61 months. In 41% of cases
dic(9;12) was the sole cytogenetic abnormality, in which cases the
dicentric chromosomes resulted in hypodiploidy. Additional numerical
abnormalities were found in 34%, with trisomy 8 being the most common.
Behrendt et al123 described an interesting case of relapsed
ALL with both dic(9;12) and t(21;21)(q21;q22). Translocations involving
21q22 occur in secondary leukemia patients after treatment with
topoisomerase II inhibitors, and the patient had been previously treated with VP16.
Comparable data for adult patients have to be largely extrapolated from
the available information in children or adolescents. This chromosomal
abnormality is rare in adults, and little is known about its effect on
prognosis.
Abnormalities Involving 11q23
Abnormalities of 11q23 are among the most frequent cytogenetic
abnormalities in a variety of adult hematopoietic malignancies. They
also occur in 60% to 70% of acute leukemias in
infants.124-126 Their frequency in older children and
adults with ALL is lower (up to 10%).124-126 When there
has been previous therapy with topoisomerase II inhibitors, frequencies
as high as 80% can be observed.127,128
The common molecular denominator is the disruption of a gene located at
band q23 of chromosome 11. In 1991 Ziemin-van der Poel et
al129 and others identified this gene at 11q23 as
the "mixed-lineage leukemia," or "myeloid-lymphoid
leukemia," gene (MLL, also referred to as
ALL-1, HRX, or HTRX1).129-131 The gene contains at least 21 exons and spans about 100 kb. It encodes a protein
of 3,968 amino acids with a molecular weight of 431 kD. It contains two
central zinc-finger domains and a 210-amino acid C-terminal and shows
significant homology with the Drosophila trithorax
protein.130,131 At the N-terminal are a methyltransferase domain and three AT-hook motifs through which the protein can bind to
AT-rich regions of the minor groove of the DNA double helix.132
The function of the MLL protein is not entirely clear, but it appears
to act as a transcription factor in the regulation of differentiation
pathways by direct interaction with DNA or with other DNA-binding
proteins.133,134 In 1993 Thirman et al135 identified a single complementary DNA (cDNA) probe from the MLL gene that could detect rearrangements with a breakpoint at band 11q23
when DNA from leukemia patients was digested with a single enzyme. The
breakpoints cluster within an 8.3 kb-region between exons 5 and 11. This is a comparatively restricted area that can be identified by
Southern blot analysis of genomic DNA probed with a small cDNA fragment
that overlaps the cluster region.
More than 20 reciprocal chromosomal loci are known to participate in
11q23 translocations, the most common are 4q21, 9p22, 19p13, and 1p32,
and many partner genes have been identified
(Table 6).133,136 With the
exception of AF10 (10p12-14) and AF17 (17q21), as well
as ENL (19p13) and AF9 (9p22), no close relation exists among the various partner genes on a structural or functional basis.134 The breakpoints on the reciprocal chromosomes are
also scattered over larger segments (in the case of 4q21, over at least 38 kb) than is the case with MLL, giving rise to fusion
transcripts that vary in length and exon composition.137
Janssen et al125 used RT-PCR to show fusion transcripts
containing MLL and AF4, the gene located at 4q21. They
demonstrated that a large number of differently sized fusion
transcripts was probably resulting from alternative splicing events.
Overall they identified eight different MLL-AF4 versions. The
fusion genes contain nearly identical 5 portions of MLL
transposed to various partner genes. The resultant chimeric proteins
contain the N-terminal portions of the MLL protein, including the
AT-hook motifs and methyltransferase domain, but not the C-terminal
segments including the central zinc-finger motifs.133,138
Several mechanisms for MLL gene rearrangements are possible
including aberrant V-D-J recombination, homologous recombination between Alu-repeats, and topoisomerase-II-mediated nonhomologous recombinations.133 Truncation or loss of function of the
MLL gene alone may be the crucial event in leukemogenesis, with
partner genes assuming a minor role and being interchangeable. However, Rogaia et al139 recently showed that fusion of the
MLL gene with eps15 (on chromosome 1, band p32) in AML
altered the cellular compartmentalization of the fusion protein,
providing a putative mechanism for activation of MLL in 11q23
recombinations and suggesting a more active role of the fusion partner
genes in leukemogenesis.
t(4;11)(q21;q23).
The most common translocation involving 11q23 is translocation
t(4;11)(q21;q23), first described by Oshimura et al140 in 1977. It is observed in more than 60% of infants with ALL, 2% of
children with ALL, and 3% to 6% of adults with
ALL.7,9,18,37 It is invariably associated with young age
(generally under 2 years), female sex, and high WBC counts, and is
frequently associated with organomegaly and involvement of sanctuary
sites such as the CNS.33,90,125,141 The immunophenotype is
of the early pre-B or pre-B-cell type, is positive for TdT, HLA-DR, and
CD19 with rearrangements of the Ig heavy-chain (IgH) genes, and is
variably CD10. Cells frequently coexpress myeloid
antigens, such as CD13, CD15, or CD33. Janssen et al125
found a significant association of MLL-AF4 recombinations in
particular with CDw65, compared with early pre-B-cell ALL without this
genetic translocation. This association emphasizes an important
characteristic of 11q23 abnormalities suggesting that the transforming
event originates at the stage of a pluripotential progenitor cell with
the capacity to differentiate to both lymphoid and myeloid
lineages.142 In fact, 11q23 abnormalities can frequently be
detected in biphenotypic or acute myeloid leukemias.128,143
The clinical outcome for both adults and children with the
t(4;11)(q21;q23) translocation is poor (Table
7).33
The GFCH demonstrated 11q23 abnormalities in 7% of adult patients with
ALL.7 Half of these patients had a t(4;11), 50% of whom
had coexpression of myeloid markers: their CR rate was 75%, with a
median DFS of 7 months and no DFS at 3 years. This was slightly worse
than for the whole 11q23 group, in whom the CR rate was 72% and the
median DFS was 8 months, but the 3-year DFS was 26%. No differences in
DFS were noted with translocations as opposed to deletions of 11q23.
Janssen et al125 analyzed MLL-AF4 rearrangements by
PCR in a series of 46 patients with early pre-B-ALL (34 adults, 12 children). They demonstrated fusion transcripts in 39% of the patients
(14 adults, 4 children). Using intensified treatment strategies which included BMT in 4 adult cases, they reported 9 of 19 ALL patients with
MLL-AF4 to be in remission for up to 54 months (median CR duration [CRD], 26 months), with 7 of the 9 showing PCR negativity for minimal residual disease.
These results emphasize that survival can be improved substantially by
applying intensive therapy to cytogenetic groups that had been
previously defined as poor risk by their response to standard
treatments. Although the data in adult ALL with translocation t(4;11)
are scarce, risk-adapted therapy might benefit this group as it did in
children.33
t(11;19)(q23;p13).
Translocation t(11;19)(q23;p13) shares similar clinical and prognostic
features with t(4;11). It results in a fusion of the ENL gene
(also called LTG19 or MLLT1) from chromosome 19 to the 5 end of MLL on 11q23.144 The translocation is
associated with younger age and high WBC count, as well as IgH
gene rearrangements in blast cells and coexpression of lymphoid and
myelomonocytoid antigens.
Abnormalities Involving 19p13
t(1;19)(q23;p13).
The two known translocations involving band p13 of chromosome 19 are
t(1;19)(q23;p13) and its rarer variant t(17;19)(q21-22;p13). Translocation t(1;19) was first reported in 1984 by Carroll et al,145 who found that some leukemias with a
pre-B-cell ALL phenotype (cytoplasmic Ig [clg]-positive and surface
Ig [slg]-negative) carried this abnormality. It was confirmed in
subsequent studies as one of the most common recurring translocations
in childhood ALL, with a frequency of 5% to 6% overall, and of 25%
in pre-B-cell ALL.15,90,146 It is also seen in 1% of
childhood cases of early pre-B-cell ALL (clg,
slg) and occasionally in ALL patients with a
transitional pre-B phenotype (clg+, µ heavy-chain Igs
detectable on cell surface).147-150 It is present in less
than 5% of adult ALL cases.7,8,151
The translocation can be observed in two principal forms. In its
unbalanced form, 19, +der(19)t(1;19), two normal chromosomes 1 are
present. Shikano et al152 suggested that this anomaly may result from loss of the der(1)t(1;19)(q23;p13.3) by nondisjunction or
asynchronous replication, with replacement of the normal chromosome 1 by a second copy, which ultimately resulted in trisomy for 1q23  1qter. Abnormalities of chromosome 1, such as complete or partial trisomies for the long arm, are known to arise during clonal evolution and can be observed in recurrent hematologic
malignancies.153-155 The other form of the translocation is
a balanced rearrangement, a simple reciprocal translocation without net
loss or gain of genetic material. Unbalanced translocations appear to
be more common than balanced rearrangements (about 75% are
unbalanced). No differences in clinical presentation or prognosis
exist.34,153
At a molecular level, the breakpoint on chromosome 19 has been mapped
to a tightly clustered region on band p13.2-p13.3. This segment
contains a gene, E2A, which encodes the two transcription factors E12 and E47 via alternative splicing of precursor
mRNA.156 E12 and E47 are ubiquitous helix-loop-helix
(HLH)-containing proteins that bind to the E-box element in the  light-chain DNA-enhancer region.157 They are considered
essential for normal lymphopoiesis and regulation of B-cell
development.158 Breakpoints in the E2A gene occur
almost exclusively in a 3.5-kb intron segment between exons 13 and
14.159 The breakpoint region on chromosome 1q23 appears to
be more dispersed and lies within an intron of at least 50 kb in
size.159 It disrupts the homeobox-containing "pre-B-cell leukemia" gene (PRL, also referred to as PBX1),
which is transcriptionally silent in lymphoid cells. The genomic
organization of PBX1 has not yet been detailed in
entirety.158 The chromosomal rearrangement fuses the 5
sequences of the E2A gene with 3 sequences of PBX1. The resulting fusion transcript is a chimeric transcription factor which preserves the activation domain of E2A but has its
DNA-binding domain and HLH dimerization domain replaced by
PBX1.159 E2A-PBX1 appears to function as a
potent transcriptional activator.160,161 Despite the size
of the breakpoint location on 1q23, an unvarying portion of
PBX1 is fused with E2A as most mRNAs generated by the translocation have the same sequence, suggesting that the site-specific fusion between E2A and PBX1 is pathogenetically
important.157 However, differential splicing seems to occur
at the mRNA level, leading to the production of two E2A-PBX1 chimeric
proteins that differ in their extreme carboxyl-terminal
end.158 Several groups have detected E2A-PBX1
fusion transcripts that differ in their mRNA
sequences.162,163 Their exact derivation, as well as their clinical significance, are unknown. Other members of the PBX
family with homeodomains that are nearly identical to that of
PBX1, namely PBX2 and PBX3, have been
identified, but their role in lymphoid cell transformation is
unknown.164
A strong association exists between t(1;19) and pre-B-ALL, especially
in children,145 where the translocation is present in 25%
to 30% of cases.15,33,90 Among 73 adults and 101 children with ALL, Fenaux et al8 found that all patients with
t(1;19) (1 adult and 7 children) had a pre-B-ALL phenotype. No other
translocation demonstrated a comparably close relationship to a
specific immunophenotype in this study. Raimondi et al147
found that most cases with pre-B-ALL and t(1;19) were pseudodiploid,
and hyperdiploidy with more than 50 chromosomes was virtually absent.
The immunophenotype in the majority of cases was positive for CD10,
CD19, and clg. Borowitz et al165 analyzed the surface
marker expression of t(1;19) ALL in more detail. They found 22 cases
with t(1;19) among 697 patients with pre-B-ALL. Twenty of them were
characterized by an identical, complex phenotype with homogenous
expression of CD10, CD19, and CD9, complete absence of CD34, and
partial absence of CD20. All 12 cases analyzed with molecular studies
showed E2A-PBX1 abnormalities. The same complex phenotype was
seen in only 8% of children with pre-B-ALL without t(1;19). Pui et
al153 later added positivity for CD22 and negativity for
CD21 to the characteristic surface-marker profile.
In rare cases, t(1;19) is seen in clg, early-pre-B-ALL.
Involvement of either E2A or P |
Introduction
References