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Blood, Vol. 96 No. 1 (July 1), 2000:
pp. 24-33
REVIEW ARTICLE
Biological and therapeutic aspects of infant
leukemia
Andrea Biondi,
Giuseppe Cimino,
Rob Pieters, and
Ching-Hon Pui
From Centro Ricerca M. Tettamanti, Clinica Pediatrica,
Università Milano-Bicocca, Monza, Italy; Dipartimento di
Biotecnologie Cellulari ed Ematologia, Università degli Studi
"La Sapienza," Rome, Italy; University Hospital Rotterdam/Sophia
Children's Hospital, Rotterdam, The Netherlands; St. Jude Children's
Research Hospital and University of Tennessee, Memphis, College of
Medicine, Memphis, TN.
 |
Introduction |
Leukemias diagnosed in the first 12 months of life are
characterized by an equal distribution of lymphoid and myeloid subtypes and account for 2.5% to 5% of acute lymphoblastic leukemias (ALLs) and 6% to 14% of acute myeloid leukemias (AMLs) of
childhood.1,2 In contrast to an excess of boys among older
children with leukemia, there is a slight female predominance among
infants with this disease.3-5 Infant leukemias display
unique biological and clinical features that have provided important
insights into the mechanisms governing normal and aberrant hemopoiesis
in the fetus and young children, as well as reasons for the increased
rates of treatment failure in infants as compared with older
children. This review summarizes recent progress in understanding
the biology of infant leukemias and the prospects for better treatment.
| |
Epidemiology |
The risk of leukemia in children, as in cancer patients
in general, reflects a complex interplay between inherited
predisposition, exogenous exposures to agents with leukemogenic
potential, and chance events. Infant leukemias afford unique
investigative models for the study of leukemogenesis. Despite the fact
that such leukemias arise very early in life, the leukemogenic
contribution of abnormal alleles (transmission of parental mutations)
is generally assumed to be small. Familial clustering is not seen in
the infant leukemias, and constitutional predisposing alleles have not
been identified. However, new germinal mutations in 1 parent could
affect a single predisposed offspring if the alterations occur
downstream in the spermatogenetic/oogenic pathway; reciprocal
translocations that target cells of the developing hemopoietic system
could also play a role.6 Infant leukemias have been
associated with Down syndrome,7 with Turner syndrome, and
with trisomy 9.8 In contrast to other cases, some infant
leukemias associated with Down syndrome undergo spontaneous
remission.7 These proliferations have distinguishing morphologic, immunophenotypic, and cytogenetic features.9
Infants with acute lymphoblastic leukemia (ALL) or acute myeloid
leukemia (AML) usually have acquired ALL1/MLL/HRX gene fusions
as the major consistent genetic abnormality (see
"ALL1/MLL/HRX cloning, structure, and function: clues to
pathogenesis?").
Epidemiological and molecular genetic studies demonstrate that most, if
not all, cases of infant leukemia arise in utero. First, infant
leukemias exhibit fetal-type DJH joining sequences in the
immunoglobulin gene.10 Second, the very early onset of some
cases with 11q23 chromosomal rearrangements strongly suggests a
prenatal leukemogenic event.11 In fact, a report of fetal death due to AML with an ALL1/MLL/HRX rearrangement provided
direct evidence of oncogenesis in utero.12 Third, molecular
studies of identical twins with leukemias harboring
ALL1/MLL/HRX-rearranged genes have corroborated suggestions
that ALL1/MLL/HRX rearrangement is acquired in utero. Indeed,
the detection of an identical clonal, nonconstitutional rearrangement
of the ALL1/MLL/HRX gene in the leukemic cells of both twins
provided evidence for a single clonal event in utero in 1 twin,
generating leukemic clonogenic progeny that metastasized to the other
twin through placental anastomoses or by crossing the maternal
circulation and reaching the second twin.13-17 Finally, the
same clonotypic ALL1/MLL/HRX-AF4 genomic fusion sequences have
recently been backtracked to neonatal blood spots from individuals who
were diagnosed with ALL at ages 51 months to 2 years.18 The
fetal origin of leukemia has also been established in older children
(up to 14 years old) with T-cell ALL or B-precursor cell ALL with
TEL-AML1 fusion by studies of monozygotic twin
pairs19-21 or neonatal blood spots (Guthrie
cards).22 One interpretation of this finding is that many
childhood leukemias are initiated by a mutagenic event in utero. The
presence of ALL1/MLL/HRX fusion in a susceptible cell type
appears sufficient to induce leukemia, whereas with other genetic
alterations, additional postnatal mutations are required. It is also
likely that ALL1/MLL/HRX fusion occurs more frequently during
fetal development, accounting for the high incidence of this genetic
abnormality in infants with leukemia.
The types of exposures that give rise to leukemogenic somatic genetic
changes in utero can be assessed with greater precision in infants than
in older children. Thus, maternal alcohol consumption, but not smoking,
during pregnancy has been correlated with an increased risk of infant
leukemia, especially AML.23,24 It was postulated that
ethanol induces microsomal enzymes, such as cytochrome P450, which in
turn activate precarcinogens.25 Most studies have shown
that an increased incidence of high birth weights and a low incidence
of low birth weights correlate with higher rates of infant ALL and
AML.26-31 It was suggested that high levels of insulinlike
growth factor-1 might produce large babies and contribute
to leukemogenesis, an interesting theory that remains to be
proved.26 An increased maternal consumption of DNA
topoisomerase-II-inhibitor-containing foods, such as specific fruits
and vegetables that contain quercetin; soybeans (genistein); tea,
cocoa, and wine (catechins); and caffeine have all been related to an
increased risk of infant AML.32-34
ALL1/MLL/HRX gene rearrangements are common in secondary acute
leukemia (usually with monoblastic or myelomonoblastic morphology) arising after exposure to an epipodophyllotoxin or an
anthyacycline, both of which inhibit topoisomerase-II.35,36
This observation supports the theory that exposure of pregnant women to
substances that inhibit topoisomerase-II might be a critical event in
the development of leukemia in infancy. Anticancer drugs, quinolone antibiotics, flavonoids, catechins, podophylline resin, benzene metabolites, and estrogens can all inhibit topoisomerase-II in vivo or
in vitro and may be considered potential mutagens in the induction of
acute leukemias with rearranged ALL1/MLL/HRX
genes.37 Exposures of the pregnant mother and fetus to
dietary, medical, and environmental chemicals that interact with
topoisomerase-II may be order-of-magnitude lower in functional doses
than in exposures to drugs used in cancer chemotherapy, even though the
first class of agents are as biologically active as the latter
class.38 It has been proposed that interindividual
pharmacogenetic-based differences in metabolism between these types of
chemicals might play an important role in dose-response relationships
that modulate the risk of pediatric leukemia. Thus,
ALL1/MLL/HRX rearrangement has been correlated with low NAD(P)H
quinone oxidoreductase (NQO1) activity,39 the quinone
moiety being shared by many topoisomerase-II-inhibiting drugs and other
chemicals.40-42 Some of the different maternal exposures
during pregnancy that have been implicated in the genesis of infant
leukemia could therefore operate via the quinone metabolic pathway.
The possible role of parental genetic susceptibility factors in
modulating the effects of parental carcinogen exposure was recently
suggested by studies of glutathione-S-transferase (GST) genes
polymorphisms in parents of diseased infants. Among the parents of
infants with leukemias lacking ALL1/MLL/HRX gene
rearrangements, the frequencies of single and double GST genes (class
M-GSTM; class T-GSTT) deletions were significantly higher than
expected. The deletion of both GSST1 and GSTM1 genes in
either parent may therefore affect the risk of infant leukemia through
a pathway independent of the ALL1/MLL/HRX gene.43
Whether parental preconceptional or in utero exposure to radiation
increases the risk of infant leukemia remains controversial. One report
suggests that there might have been a transient increase in infant
leukemia in northern Greece in association with radioactive fallout
from the Chernobyl accident.44 However, the European
Childhood Leukemia-Lymphoma Incidence Study failed to show any increase
in the incidence of childhood leukemia as a consequence of this
event.45 Likewise, in a subsequent study, German
investigations were not able to correlate an increased incidence of
infant leukemia with ionizing radiation from the
accident.46
| |
ALL1/MLL/HRX cloning, structure, and function: clues to
pathogenesis? |
The ALL1/MLL/HRX gene, located at cytogenetic band 11q23, is
consistently altered in infant acute leukemia, being rearranged in more
than 60% to 70% of cases.1,2 This gene was identified in
1991 and completely cloned and characterized in 1992. Somatic cell-hybrids or fluorescent in situ hybridization (FISH) was used to
map the chromosomal 11q23 breakpoints into a region between the
CD3 and the porhobilinogen deaminase genes. Subsequently, a
yeast artificial chromosome (YAC), containing the CD3 gene was cloned and shown by FISH analysis to span the t(4;11), t(6;11), t(9;11), and t(11;19) chromosomal translocation
breakpoints.47 When a similar YAC was used, a DNA insert
was obtained in which Southern blot analysis detected rearranged bands
in leukemic cells from patients with t(1;11), t(4;11), t(6;11),
t(9;11), t(10;11), and del(11q23) abnormalities. Breakpoints were
clustered in a small region of  8 kilobases (kb) within a gene named
ALL1 by Cimino et al48 because rearrangements were
first identified in a patient with ALL. Other investigators completed
characterization of the gene and added the designation MLL
because the gene could be altered in myeloid or lymphoid leukemias, and
the designations HRX or Hrtx1 to indicate homology with
the trithorax (trx) gene of
Drosophila.47,49-51 ALL1/MLL/HRX spans
approximately 90 kb of DNA, encodes a major transcript of 15 kb, and
consists of 36 exons, ranging in size from 65 to 4249 base pairs. The
protein product consists of more than 3910 amino acids containing 3 regions homologous to sequences of the Drosophila trx gene,
including cysteine-rich regions that can fold into 6 zinc-finger-like
domains and a highly conserved 200-amino acid SET domain located at
the carboxyl-terminal end.49-56
In Drosophila, the trx gene acts to spatially maintain
restricted expression pattern of the Antennapoedia and
Bithorax complexes during fruit fly development.
trx activates transcription of multiple genes of the 2 complexes and, thus, counteracts the activity of Polycomb group
(PcG) genes, which repress the transcription of the same genes.
Gene-targeting studies demonstrated that ALL1/MLL/HRX is also a
positive regulator of Hox genes in mice.57
Hox expression is shifted posteriorly in ALL1/MLL/HRX
heterozygous (+/ ) embryos and completely abolished in
ALL1/MLL/HRX homozygous null (/) embryos. Shifts
in Hox expression are also observed in mice with targeted
mutations in PcG.58,59 More recently, Yu et
al60 showed that mice with deletions of the
ALL1/MLL/HRX gene had altered maintenance rather than
activation of the Hox gene.
The ALL1/MLL/HRX gene product possesses 2 other regions that
could be directly or indirectly involved in the control of gene transcription. These are (1) a region that is similar to the AT hook of
high-mobility-group-I proteins and that binds to AT-rich regions of
the minor groove of the DNA and (2) a cysteine-rich region homologous
to the mammalian DNA methyltransferase double helix, which by favoring
conformational DNA changes, facilitates the action of other regulatory
genes.49-51
To date, at least 16 different fusion partner genes involved in
chromosomal translocations with ALL1/MLL/HRX have been
characterized (Table 1).61-77
Additionally, internal duplications within the amino-terminal part of
ALL1/MLL/HRX and specific deletions of exon 8 have been
detected in leukemic blast cells of some leukemia patients
78-80
The impressive heterogeneity of ALL1/MLL/HRX recombination
raises difficult questions as to how fusion proteins cause leukemia and
the role of partner genes in activating the ALL1/MLL/HRX gene and determining the leukemic phenotype. Thus, several investigators have searched for structural similarities among the different ALL1/MLL/HRX partner genes. With the exception of the homology shown by AF9 with ENL, by AF10 with
AF17, by AFX with AF6q21, and by MSF
with hCDCrel, sequence analysis did not reveal structural or functional similarities. Thus, it is unlikely that these fusion partners could play a role in the function of the hybrid gene by simply
providing transcriptional modulation (activation or repression)
domains. By contrast, the active functional contribution of partner
genes in determining the oncogenic capacityof the resulting hybrid gene
is strongly suggested by several observations. All partner genes are
fused in frame to ALL1/MLL/HRX, generating a full-length fusion
protein, whereas terminal deletions of ALL1/MLL/HRX have not
been identified in leukemic cells. In addition, by testing a series of
ALL1/MLL/HRX-ENL mutants to investigate the
participation of several conserved sequence motifs in the oncogenic
activity of the fusion product, Slany et al81 recently
demonstrated that the DNA binding motifs of ALL1/MLL/HRX, as
well as the transcriptional transactivation activity of ENL,
are required for in vitro immortalization of murine myeloid cells.
Finally, knock-in mice expressing an ALL1/MLL/HRX-AF9 fusion gene under the control of
the natural ALL1/MLL/HRX promoter developed AML, while mice
expressing an ALL1/MLL/HRX/Myc tag fusion gene remained
free of leukemia.82
Despite extensive studies, it is still unclear as to how fusion
products participate in leukemogenesis. One possibility is that they
might supply a dimerization domain, which could activate the
ALL1/MLL/HRX chimeric genes. This hypothesis is supported, first, by the observation that several ALL1/MLL/HRX partner
genes possess structures involved in the protein-protein interactions and, second, by the finding that the novel self-fusion genetic mechanism mentioned above leads to internal duplication of the amino-terminal part of the ALL1/MLL/HRX gene, which
functionally could be equivalent to a dimer of the NH2 portion of the
ALL1/MLL/HRX. Since the RNA polymerase elongation factor
ELL and the transcriptional coactivator CBP (gene for
CREB binding protein) are 2 ALL1/MLL/HRX fusion partners
involved in transcriptional regulation, an interaction of the
ALL1/MLL/HRX fusion genes with the RNA polymerase-II
transcription machinery has been proposed.
Of all the ALL1/MLL/HRX motifs present in all the fusion
proteins, the AT hook region is the best characterized, and several lines of evidence suggest that this region has an important role in
targeting and regulating transcriptional units for normal hematopoietic growth and differentiation. In this respect, it has been suggested that
the Trx protein maintains target genes in a transcriptionally active
state by an epigenetic mechanism that probably involves chromatin
remodeling. Although ALL1/MLL/HRX has not been shown to remodel
chromatin, the carboxy-terminal SET domain of ALL1/MLL/HRX interacts with hSNT5/INI1, a component of the SNF/SWI complex, a
chromatin-remodeling system.83,84 Importantly, the SET
domain is lost when the aminoterminus of the ALL1/MLL/HRX and the
carboxyl-terminal partner residues fuse to form ALL1/MLL/HRX fusion
protein. The loss of this domain may explain the down-regulation of the
ARP1 gene in embryonic stem cells from ALL1/MLL/HRX
double-knockout mice, as ARP1 was recently identified and
characterized as a target of ALL1/MLL/HRX.85
Finally, Adler et al86 have shown that the GADD34
gene, which encodes a DNA damage-inducible factor, is another target
of ALL1/MLL/HRX. These authors also showed that, while ALL1/MLL/HRX protein interacts directly with GADD34, resulting in a
significant increase in apoptosis after treatment with ionizing
radiation, the coexpression of 3 different ALL1/MLL/HRX fusion proteins
(ie, ALL1/MLL/HRX-ENL, ALL1/MLL/HRX-AF9, and ALL1/MLL/HRX-ELL) had an
antiapoptotic effect, abrogating GADD34-induced apoptosis. The
authors also observed a difference between wild-type and leukemic ALL1/MLL/HRX fusion proteins, leading them to postulate a gain of
function for ALL1/MLL/HRX compared with the wild-type protein, suggesting that the inhibition of apoptosis may be relevant to leukemogenesis.86
With regard to the precise function of the wild-type ALL1/MLL/HRX
protein, Hess et al,87 after examining the effects of the
haploinsufficiency or absence of ALL1/MLL/HRX on the in vitro differentiation of yolk sac progenitor cells, concluded that
ALL1/MLL/HRX is required for the generation of normal numbers of
hematopoietic progenitors and their proper differentiation, especially
along the granulocytic and monocytic lineages.
| |
Possible mechanisms of aberrant ALL1/MLL/HRX1 recombination |
Chromosomal translocations leading to oncogene activation are common
events in the pathogenesis of leukemia, but the molecular basis for
this process is still incompletely understood. ALL1/MLL/HRX1 offers a useful model for elucidating such mechanisms. First, the gene
is altered by promiscuous chromosomal recombination with a variety of
partner genes in various subsets of acute leukemias, including some
childhood and adult acute lymphoid or myeloid leukemias, secondary
leukemias associated with prior exposure to drugs that target
topoisomerase-II (etoposide, tenoposide, and anthracyclines), and,
especially, infant leukemias.88-92 Second, several DNA
motifs implicated in DNA-recombination mechanisms have been recently identified and localized within the ALL1/MLL/HRX1 breakpoint
cluster region (bcr). These include (1) recombinase signal sequences
(heptamers and nonamers); (2) scaffold attachment regions (SARs); (3)
high-affinity topoisomerase-II-binding sites, including a strong site
in exon 9; and (4) Alu sequences.93-95 By comparing
ALL1/MLL/HRX rearrangements in de novo versus therapy-related
acute leukemias, Broeker et al93 identified
statistically significant differences in the breakpoint distribution
between the 2 groups. In particular, they found that in therapy-related
acute leukemias, the breakpoints clustered in the telomeric portion of
the ALL1 bcr, which is characterized by the presence of SARs
and high-affinity topoisomerase-II binding sites, in contrast to cases
of de novo leukemias, whose breakpoints in most instances clustered in
the centromeric or 5' bcr. On the basis of these observations,
the authors suggested that the mechanisms of translocation in de novo
and treatment-related leukemias secondary to treatment with
topoisomerase-II inhibitors might be different.93 This
conclusion has important implications for attempts to understand the
etiology and pathogenesis of infant leukemias. Molecular analyses of
ALL1/MLL/HRX rearrangements in infant twins showed that these genetic aberrations arise during fetal hemopoiesis in
utero.13 Epidemiologic evidence has also indicated that
certain conditions during pregnancy, such as exposure to drugs,
alcohol, and pesticides, are associated with an increased risk of
infant leukemia.96,97
Therefore, one mechanism leading to ALL1/MLL/HRX translocations
might be chromosomal breakage induced by topoisomerase-II inhibitors
within the ALL1/MLL/HRX gene, while another could be represented by mistakes in DNA-repair mechanisms. This hypothesis is
supported by recent observations by Aplan et al98 showing that topoisomerase-II-inhibiting drugs cleave double-stranded DNA at a
site in ALL1/MLL/HRX exon 9 both in vivo and in vitro. More
recently, Gillert et al99 implicated "error-prone
repair" as the DNA-repair process leading to ALL1/MLL/HRX translocations.
From these considerations and from analogy with the involvement of the
ALL1/MLL/HRX gene in treatment-related leukemia, it was
suggested that the critical leukemogenic event(s) occurring in utero
might similarly involve prenatal exposure to topoisomerase-II inhibitors as represented by several natural and medicinal substances. That infant leukemias and topoisomerase-II-related secondary leukemias show the same biased distribution of ALL1/MLL/HRX
breaks100 lends credence to this hypothesis. A substantial
list of candidate leukemogenic agents are under investigation in
international case-control epidemiological studies.
| |
Why are infant leukemias so different? |
Clinical and biological features
ALL of infancy is associated with a high leukocyte count at
presentation, hepatosplenomegaly, and central nervous system (CNS) involvement.1,101,102 The immunophenotype is usually that
of immature B-lineage precursors and is characterized by a lack of CD10
expression and the coexpression of myeloid-associated antigens. A high
frequency of myeloperoxidase-gene expression typifies infant ALL.103 These findings suggest that the classic form of
infant ALL originates in a stem cell that has not fully committed to lymphoid differentiation. This hypothesis is supported by the observation that multipotential stem and progenitor cells prime the
commitment and differentiation of several different hematopoietic lineages.104 The frequency of ALL1/MLL/HRX gene
rearrangements is very high, possibly as high as 75% when studied with
molecular techniques,1,2 as would be predicted from the
frequency of the t(4;11), the translocation most often
involved in the generation of this fusion gene.
These 3 characteristics lack of CD10 expression, expression of
myeloid-associated markers, and ALL1/MLL/HRX gene
rearrangementsare correlated with one another, and their presence is
inversely related to the age of the
infant.2,102,105-111 For example,
ALL1/MLL/HRX rearrangement is associated with 90% of the
CD10 cases, contrasted with only 20% of the
CD10+ cases.109,110,112 Among infants, a lack
of CD10 expression, coexpression of myeloid-associated markers,
ALL1/MLL/HRX rearrangement, and age of less than 6 months are
associated with a poor prognosis.2,101,102,105-107,109-115 The event-free survival of infants with CD10
B-precursor-cell ALL is only about 25%, as compared with 50% to 55%
for those with the CD10+ phenotype.102,106,113
Similar estimates apply to cases with an ALL1/MLL/HRX
rearrangement: 10% to 20% as compared with 50% in cases with
ALL1/MLL/HRX in a germ-line
configuration.102,107,109-111,115 Two
studies105,111 have analyzed the relation between
myeloid-associated antigen expression and outcome in infant ALL: the
event-free survival in cases expressing the antigens was 0% to 10%,
compared with about 60% in the other cases. Finally, age itself can be
used as a prognostic factor in infant ALL. Infants younger than 6 months of age have a worse outcome (10% to 20% event-free survival)
than do infants between 6 and 12 months of age at diagnosis (40% to 45% event-free survival).102,106,107,111
The above-mentioned high-risk factors are closely interrelated. In
several analyses, the ALL1/MLL/HRX rearrangement emerged as an
important adverse prognostic factor.109,114-117 In one
large multivariate analysis, it was shown that ALL1/MLL/HRX
rearrangement, age, CD10, and white blood cell count were
all independent prognostic factors.115 Two
other small studies showed that the ALL1/MLL/HRX rearrangement
was of prognostic relevance independent of the white blood cell
count.109,114 Recent studies by the Childrens Cancer Group
(CCG) suggested that the t(4;11) is the only
ALL1/MLL/HRX-related translocation associated with a dismal
outcome.107,112,118 In a combined Pediatric Oncology Group
(POG)-St. Jude study, infant ALL cases with ALL1/MLL/HRX-ENL
fusion due to the t(11;19) had an extremely poor
prognosis.119 However, the adverse outcome cannot be
attributed solely to the t(4;11) or the t(11;19), as children 1 to 9 years old with this abnormality have a reasonably favorable
prognosis.119-121 Thus, other factors must contribute to the generally poor treatment results obtained in infants. In a recent
study by the Berlin-Frankfurt-Münster (BFM) group, a poor early-treatment response to prednisone was found to be the strongest predictor of outcome in infant ALL, even in the subgroup with the
t(4;11).122 Hence, a poor early-prednisone response is
being used as the sole criterion for allogenic, hematopoietic stem cell transplantation in the Interfant '99 study of infant ALL, conducted by
a consortium of European and US investigators.
In AML, age and the presence of 11q23 abnormalities have no clear
adverse prognostic impact.1,111,123,124 Infant AML is characterized by myelomonoblastic or monoblastic morphology, a high
percentage of CNS involvement, and a high leukocyte count. ALL1/MLL/HRX rearrangements are found in about 60% of infant
AML cases.111,125 The prognostic factors that define infant
AML are not clearly defined. In a St. Jude study, high presenting
leukocyte counts and male sex were the only 2 independent adverse
prognostic factors.111 The association of high leukocyte
counts with a poor outcome is not unexpected; however, the basis for
the predictive strength of male sex is uncertain. Lie et
al126 have also found male gender to be a predictor of a
poor outcome. The prognostic impact of M4 and M5 morphology is
controversial, probably owing to the use of different treatment
regimens.111,127 In a recent St. Jude study, the presence
of the t(9;11)(p22;q22) conferred a favorable prognosis.128
Clearly, additional studies are needed to ascertain the prognostic
factors in infant AML.
Drug-resistance profile
Age, immunophenotype, and ALL1/MLL/HRX rearrangement reflect
or cause differences in drug-resistance factors. These can be pharmacokinetic factors that determine the amount of drugs to which the
leukemic cells are exposed or differences in cellular pharmacodynamics
that determine the sensitivity of the cells to the drugs. There are no
data suggesting that pharmacokinetic resistance might explain the
poorer outcome of infant ALL; infants simply do not show increased
clearance of antileukemic drugs.
Nonetheless, some studies suggest differences at the cellular level.
Kumagai et al129 showed that leukemic cells from infants with 11q23 rearrangements grow better on stromal layers in vitro than
do cells from other cases. Uckun et al130 similarly showed that cells from infants with ALL1/MLL/HRX-rearranged ALL are
more readily recovered from severe combined immunodeficiency (SCID) mice than are cells from children with other types of ALL. In related
studies, Kersey at al131 found that leukemic cells with the
t(4;11) are more resistant to stress-induced death than are other
B-lineage blast cells, while Pieters et al132 showed that cells from infants with ALL are significantly more resistant in vitro
to prednisolone and L-asparaginase than are cells from older children.
In vitro resistance to these drugs is a strong adverse prognostic
factor.133-135 Of considerable therapeutic interest, the leukemic cells of infants with ALL are significantly more sensitive to
cytarabine than are cells from older children.132 In
addition, B-cell precursors that lacked CD10 expression were resistant
to prednisone and L-asparaginase but showed significant sensitivity to
cytarabine, in contrast to cases with CD10 expression. A study by the
Dana-Farber Cancer Institute Consortium showed that high-dose cytarabine given immediately after remission induction is a feasible strategy and might benefit infants with ALL.136 Recent data
from a German study of adult ALL patients showed that the long-term survival of adults with pro-B ALL with the t(4;11) has increased to
about 40% with the introduction of high-dose cytarabine/mitoxantrone consolidation therapy.137 Finally, Reiter et
al108 reported that infants with ALL more frequently show a
poor in vivo response to prednisone than do older children. In their
study, age lost its prognostic value in a multivariate analysis because
of its association with a poor prednisone response in vivo. A recent update of the BFM 1986 and 1990 studies confirmed the
prognostic significance of the steroid response: the 6-year event-free
survival for infants with a good prednisone response was 58% versus
only 16% for infants with a poor prednisone response.122
Differences between infants and older children
Rapid changes in physiologic processes govern drug disposition in
infants, especially neonates. First, the total body water content as a
percentage of total body weight decreases from 75% in the newborn
period to 60% at 1 year and 55% by adulthood; extracellular water as
a percentage of total body water decreases from 45% in the neonate to
20% in the adult.138 Second, many drugs bind less avidly
to serum proteins in the neonate than in the adult, leading to an
increase in the unbound fraction (presumably the active drugs) and
potentially enhanced pharmacologic responses in the former age
group.139 Third, the activity of many P-450 enzymes is low
during infancy.140,141 This decreased metabolic activity could result in reduced cytotoxic effects of antineoplastic drugs that
require bioactivation (eg, cyclophosphamide) or in enhanced cytotoxicity of those that undergo inactivation (eg, vincristine and
daunorubicin). Fourth, renal tubular function and the glomerular filtration rate reach adult levels by 7 months and 5 months of age,
respectively142,143; hence, any drugs that depend on renal function for clearance will have increased systemic exposure and pharmacologic effects in infants.
There are, in addition, many important anatomical differences by age.
For example, the volume of the CNS relative to body surface area or
body weight is much larger in young children than in adults. While the
CNS volume in infants approaches 80% to 90% of the adult value by age
4 to 6 years, body surface area does not reach adult values until
approximately 16 to 18 years of age.144 Indeed,
Bleyer144 demonstrated that the dosage of intrathecal chemotherapy should be based on age rather than body surface area to
avoid undertreatment in young children. Another major age difference is
the ratio of body weight (kg) to body surface area (m2);
for example, the ratio for neonates is 18, which is lower than the
ratio of 25 in 5-year-olds, which in turn is a lower value than the
value of 40 in adults.145 Thus, if drug dosage were based
solely on body weight for all age groups, infants would receive a
substantially lower dosage by body surface area than would other
children. Whether the dosage of any given drug in infants should be
based on body surface area or body weight remains in question. This
uncertainty is well illustrated by the empirical approach to drug
dosing in different clinical trials of infant leukemias, some based
entirely on body weight, others on body surface area, and still others
on body surface area adjusted by age (ie, proportionally lower in young infants).
There are only limited pharmacokinetic and pharmacodynamic data on
individual antileukemic drugs and on the tolerance to these agents in
infants. An early report suggested that infants with leukemia are more
susceptible to severe vincristine neurotoxicity than are
children.146 In a subsequent study of remission-induction therapy with vincristine (1.5 mg/m2), prednisone,
L-asparaginase, and intrathecal methotrexate, 7 of the 9 patients with
a body surface area less than 0.5 m2 developed vincristine
neurotoxicity, which was severe in 4.147 It was uncertain
whether L-asparaginase or methotrexate contributed to increased
toxicity by altering hepatic function or whether the infant nervous
system is more sensitive to vincristine. Because infants have a large
body surface area relative to body weight, the authors proposed that
the drug doses in infants should be calculated on the basis of body
weight (in kilograms), with dosages normalized from those of body
surface area (dividing by 30). This conversion effectively lowers the
final dosage and has proved adequate for vincristine treatment in
infants. Limited data suggest that dosing of teniposide, etoposide, and
cytarabine based on body surface area would yield similar systemic
exposure in infants and adults.145 By contrast, normalized
dosing of doxorubicin by body weight was more likely to achieve similar
systemic exposure in these 2 age groups. The study also showed that
methotrexate clearance tended to be lower in infants, but there was no
need to reduce dosage, as methotrexate was better tolerated in these patients. Thus, uniform rules for dosage adjustment of all antileukemic agents used in infants is inappropriate; additional pharmacokinetic and
pharmacodynamic studies are needed in infants younger than 2 months of age.
| |
Have treatment results for infant leukemias improved? |
Acute lymphoblastic leukemia
Contemporary treatment for childhood ALL has cured approximately
80% of patients in some clinical trials,148 but results for infant ALL are still suboptimal. A variety of treatment regimens have been tested in infants, generally yielding event-free survival rates of 20% to 35% (Table
2).101,102,106,122,132,136,149-154
(See also A. Biondi, unpublished data, 1999.) In several recent
clinical trials, high-dose methotrexate, high-dose cytarabine, and
intensive consolidation/reinduction therapy appear to have improved
clinical outcome,106,112,122,136,153 but these results
should be viewed as preliminary because of the small numbers of
patients enrolled, the lack of randomization, and the disproportionate
numbers of cases with high-risk disease (ie, ALL1/MLL/HRX-AF4
fusion). Moreover, the efficacy of any treatment component is affected
by the overall therapeutic strategy. Hence, while clinical trials
incorporating high-dose methotrexate with or without cytarabine have
generally yielded improved results, 1 study with a similar therapeutic
strategy resulted in an inferior outcome, partly because of an increase in remission deaths from infection or gastrointestinal complications due to combination treatment with etoposide and high-dose
cytarabine.102 Likewise, excessive toxicities and
treatment-related deaths, presumably due to high-dose daunorubicin in
very young infants, were encountered in a POG study, despite
encouraging overall results.153 Both studies underscore the
need for pharmacokinetic and pharmacodynamic studies to ensure optimal
dosing in infants.
Neuropsychological abnormalities are well-recognized complications of
cranial irradiation, especially in very young children. Severe
neurological deficits and learning disabilities were observed in 4 of
8, in 9 of 11, and in 2 of 4 long-term survivors of infant ALL who had
received cranial irradiation in 3 separate
studies.101,106,136 By contrast, in another study, infants
who did not receive cranial irradiation showed normal
neuropsychological development when tested at 5 years of
age.155 In virtually all studies, attempts have been made
to reduce neuropsychological complications by reducing the dosage of
cranial irradiations, delaying the radiotherapy until the child was
more than 1 year old, or avoiding irradiation altogether (Table 2). To
date, most investigators favor eliminating irradiation in all infants
with ALL, even in those with CNS leukemia at diagnosis, relying instead
on intensive systemic and intrathecal treatments. Several observations
support this approach. First, in the early studies of the CCG, cranial
irradiation had no impact on treatment outcome.101 Second,
in the recent CCG-1883 trial, which did not include cranial
irradiation, the cumulative risk of isolated CNS relapse was only
3% ± 2%,112 despite a 14.2% prevalence of CNS
leukemia at diagnosis. Third, in a POG study that employed only
intrathecal treatment of the CNS for all patients, failure rates were
similar in infant cases with and without CNS leukemia at
diagnosis.152 In fact, none of the 21 patients with CNS leukemia at diagnosis had CNS involvement at the time of relapse.
There is a paucity of data on allogeneic hematopoietic stem-cell
transplantation in infants with ALL. Two limited collaborative group
studies have yielded dismal results: only 2 of 11 and none of 3 patients survived.102,112 The experience of the Fred
Hutchinson Cancer Research Center is more encouraging, with 7 of 9 infants alive in first remission for 2 to 11 years after allogeneic
transplantation.156 Additional studies are clearly needed
to determine the role of hematopoietic stem-cell transplantation in
high-risk infant cases.
Acute myeloid leukemia
While infants with ALL are treated on separate protocols in most
clinical trials, those with AML receive essentially the same therapy as
older children in virtually all
studies.1,126,157-166 Infants with acute
monoblastic leukemia are sometimes treated with
epipodophyllotoxin-containing regimens,167 apparently
because of the increased sensitivity of their leukemic cells to this
class of agents.168,169 In most clinical trials of AML
therapy, event-free survival rates are similar for infants and older
children.126,157-160,164,165 In the BFM 1983 and 1987 trials, children younger than 2 years had an
inferior treatment outcome, as compared with older
children.166 However, in multivariate analyses, age lacked
independent significance after adjustment for the FAB M5 or M7
subtypes, hyperleukocytosis, and an unfavorable karyotype. In the POG
8498 study, children younger than 2 years had a more favorable
outcome than did older children.161 The inclusion of
children 1 to 2 years of age made it difficult to determine the
prognosis for infants younger than 12 months.
Since treatment outcome generally does not differ by age group in
childhood AML, there is no compelling reason to develop separate trials
for infant AML, with the following exception. Infants with
megakaryoblastic leukemia and the t(1;22) (p13;q13) appear to have a
particularly poor prognosis170,171 and may be candidates
for innovative experimental therapy or perhaps allogeneic hematopoietic
stem-cell transplantation. Although the latter procedure has yielded
long-term survival in some infant cases,3,172,173 its
relative efficacy compared with contemporary intensive chemotherapy is
unknown owing to the lack of randomized studies.
Ongoing clinical trials
Currently, 2 large international prospective studies for the
treatment of infant ALL are under way. One is a collaborative US study
conducted by the POG and CCG. The other is a large international effort, Interfant '99, by European and US study groups. The POG/CCG trial tests the feasibility and efficacy of intensive therapy. Infants
with an ALL1/MLL/HRX rearrangement are eligible for allogeneic hematopoietic stem-cell transplantation. The Interfant '99 protocol is
based on a so-called hybrid form of therapy, consisting of elements
from both ALL and AML treatments administered on an ALL-like schedule
and combining both low-dose and high-dose cytarabine. Only patients
with a poor initial response to prednisone are eligible for
hematopoietic stem-cell transplantation. No CNS or total-body irradiation is used, and anthracyclines, epipodophyllotoxins, and
alkylating agents are either avoided or used only sparingly. Both
studies will prospectively analyze whether age, immunophenotype, leukocyte count, initial response to therapy, and ALL1/MLL/HRX rearrangement have independent prognostic value.
Conclusion
In most cases of infant ALL and AML, the discovery of
ALL1/MLL/HRX gene involvement opened new opportunities for
molecular diagnosis and monitoring, molecular epidemiology, and studies to unravel basic biologic mechanisms. Continued molecular
investigations are needed to gain further insight into the basic
differences between leukemias in infants and older children. Current
therapy for infant ALL and AML is inadequate. Although intensification of chemotherapy and wider use of allogeneic hematopoietic stem-cell transplantation could improve this situation, there remains an urgent
need to develop novel therapies by exploiting the unusual biologic
properties of leukemic progenitor cells expressing the abnormal
ALL1/MLL/HRX gene product.
| |
Acknowledgments |
We thank E. Paccagnini for secretarial support and J. Gilbert for
editorial review.
| |
Footnotes |
Submitted September 20, 1999; accepted February 24, 2000.
Supported by Fondazione M. Tettamanti, Associazione Italiana Ricerca
sul Cancro (AIRC), and MURST (A.B.); by the Foundation Pediatric
Oncology Center Rotterdam and grants 94-679, 95-921, and 97-1564 from
the Dutch Cancer Society (R.P.); and by a Center of Excellence Grant
from the State of Tennessee, by the American Lebanese Syrian Associated
Charities (ALSAC), and by grants CA51001, CA21765, CA36401, CA78224,
CA20180, and CA60419 from the National Institutes of Health (C-H.P).
Reprints: Andrea Biondi, Centro Ricerca M. Tettamanti, Clinica
Pediatrica, Università Milano-Bicocca, Ospedale S. Gerardo, Via
Donizetti, 106, 20052-Monza (MI) Italy; e-mail: fondazione.tettamanti{at}galactica.it.
The publication costs of this
article were defrayed in part by
page charge payment. Therefore,
and solely to indicate this fact,
this article is hereby marked
"advertisement"
in accordance with 18 U.S.C.
section 1734.
| |
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Introduction
Epidemiology