TEL-AML1 Fusion Transcript in Relapsed Childhood Acute Lymphoblastic Leukemia

TEL-AML1 fusion precedes differentiation to pre-B cells in childhood acute lymphoblastic leukemia

The TEL-AML1 gene fusion results from a karyotypically cryptic t(12;21) translocation, the most common genetic abnormality in pediatric acute lymphoblastic leukemia (ALL). The presence of the TEL-AML1 fusion in utero, its protracted latency to overt leukemia, and secondary loss of the untranslocated TEL suggest it is an initiating event. Sequences of the TEL-AML1 genomic breakpoint and the immunoglobulin heavy chain (IgH) and/or T-cell receptor (TCR) gene rearrangements were characterized in four pediatric pre-B ALL patients. Analysis of these markers in relapsed patients revealed that immunophenotypically and cytogenetically distinct, and clonally unrelated antigen receptor leukemic cell populations harbored the same initiating TEL-AML1 molecular abnormality. Furthermore, TEL-AML1-positive cells persisted during remission even in the absence of detectable clone-specific IgH and TCR markers. We demonstrate that the TEL-AML1 translocation can occur in vivo during B-cell development before rearrangement of the IgH and TCR genes. We propose, in some cases, that the TEL-AML1 translocation occurs in a stem or B progenitor cell, and that recurrent TEL-AML1-positive pre-B ALL represents a de novo-transformed population that retains the same diagnostic initiating event.

Slower molecular response to treatment predicts poor outcome in patients with TEL/AML1 positive acute lymphoblastic leukemia: prospective real-time quantitative reverse transcriptase-polymerase chain reaction study

BACKGROUND

The translocation t(12;21)(p13;q22), which produces the TEL/AML1 fusion gene, is the most frequent chromosomal abnormality in patients with childhood acute lymphoblastic leukemia (ALL) and generally is associated with a favorable prognosis. Furthermore, real-time quantitative-polymerase chain reaction (RQ-PCR)-based detection of TEL/AML1 represents an accurate technique for the reproducible assessment of minimal residual disease (MRD).

METHODS

The authors employed RQ-reverse transcriptase-PCR (RQ-RT-PCR) technology to analyze MRD levels in 57 newly diagnosed patients with TEL/AML1 positive ALL in a prospective study.

RESULTS

On Day + 33, a particularly important time point in terms of outcome prediction based on MRD monitoring, 75% of patients reached negativity, 13% of patients were positive at very low levels (< 10(-4); i.e., 1 or more leukemic cell per 10(4) normal cells), and another 13% of patients were positive at the level of 10(-2) to 10(-4) cells. No patient showed MRD levels > or = 10(-2) cells at this time. The data demonstrate that patients with TEL/AML1 positive ALL had a better response to induction chemotherapy on Day + 33 compared with a group of unselected patients with ALL (P = 0.0001). However, four patients with TEL/AML1 positive ALL developed relapse disease. Remarkably, these children were positive for MRD on Day + 33 at a level between 10(-2) cells and 10(-4) (n = 3 patients) and at < 10(-4) (n = 1 patient). Kaplan-Meier analysis of disease free survival showed the statistical significance of this distribution (MRD positive vs. MRD negative; log-rank P = 0.0016).

CONCLUSIONS

The authors conclude that, although the TEL/AML1 positive leukemias generally are associated with a favorable outcome, MRD positivity assessed by RQ-RT-PCR analysis at the end of induction therapy represents a significantly negative prognostic feature.

Recent advances in pediatric acute lymphoblastic and myeloid leukemia

Acute leukemia is the most common form of childhood cancer and is the primary cause of cancer-related mortality in children. In the United approximately 3250 cases are diagnosed annually in children and adolescents younger than 20 years, of whom 2400 have acute lymphoblastic leukemia (ALL). Treatment results in childhood ALL continue to improve, and the expected current cure rates approach 75 to 80% of all children with ALL, including T-ALL and mature B-cell ALL, the two variants that, not too long ago, had a considerably poorer prognosis compared with the common form of BpALL. The most significant new development in the past 2 years has been the development of further evidence for fetal origin of childhood leukemias, and additional evidence to support the notion that postnatal events modulating the events of immune-mediated elimination of these leukemic clones play a major role in the eventual development of clinical disease. Other epidemiologic developments include (1) increased appreciation of the role of drug-metabolizing enzymes, both in determining the predisposition to leukemia and response to therapy; and (2) both clinical observations and gene expression studies seeming to identify a new approach to the evaluation and treatment of children with MLL (11q23) rearrangements. A most remarkable new development in the induction therapy of childhood leukemia and lymphoma in the United States is the use of urate oxidase for prevention of tumor lysis syndrome and the associated uric acid nephropathy. Drug resistance, determined either on leukemic blast cells in vitro or by studies of MRD, is being looked at critically in an effort to improve the treatment results further. Consolidation with HDMTX has gained wider popularity with the realization that effective CNS prophylaxis can be achieved with intrathecal therapy plus HDMTX for consolidation. In contrast to ALL, the progress in the therapy of acute myeloid leukemia (AML) lags behind, with cure rates of approximately 40 to 50%. There is no convincing evidence for substitution of daunorubicin with other anthracyclines, nor evidence for using high-dose cytarabine during induction in childhood AML. Rather, a 3 + 10 regimen with total daunorubicin 180 mg/m2 and cytarabine 100 to 200 mg/2 for 10 days appears to yield the best results. The most important component of the postremission chemotherapy continues to be several courses of high-dose cytarabine. The results from the MRC 10, LAME 89/91 studies and the recent BFM 93 trial with high-dose cytarabine and mitoxantrone suggest that there may be some benefit to including this combination in the postremission phase of AML. Despite these improvements in chemotherapy, allogeneic BMT from a matched family donor remains the best option for most patients (excluding Down syndrome, APL, and possibly those with inv16). Newer prognostic markers of interest include FLT3/ITD and minimal residual disease at the end of induction therapy.

Soluble l-selectin (sCD62L) in relapsed childhood acute lymphoblastic leukaemia

Soluble l-selectin (sCD62L) plasma concentrations at diagnosis and outcome were investigated in 193 children at first relapse of acute lymphoblastic leukaemia (ALL) after treatment according to the Berlin-Frankfurt-Münster relapsed ALL multicentre trials, ALL-REZ BFM 95 and 96. sCD62L was low (< fifth paediatric reference percentile) in 63 (33%) and high (> 95th percentile) in 36 (19%) children, and was independent of remission duration, sex, BCR-ABL fusion or extramedullary disease. High sCD62L was associated with circulating blasts and T-cell phenotype. More initial adverse events occurred in children with high and low levels of sCD62L (23 out of 99) than in those with normal levels (9 out of 94, P = 0.018). Among 75 worst-prognosis patients (risk groups S3/S4, isolated bone marrow relapse occurring less than 6 months after elective cessation of front-line therapy, or T-cell phenotype with bone marrow involvement), 27 had low sCD62L and decreased event-free survival (EFS) probability (PEFS5 = 0.09 at 5 years) and duration (219 d) compared with normal sCD62L (29 out of 75, PEFS5 = 0.24, 640 d, P = 0.01). Low (44 out of 118), normal (72 out of 118), and high (19 out of 118) sCD62L non-S3/S4 patients fared similarly (average PEFS5 = 0.45, 1369 d; P = 0.5). Low sCD62L may be a marker of malignant blasts replacing normal sCD62L-producing haematopoietic cells. In children with first relapse of ALL and worst prognosis, plasma sCD62L may be useful for risk-adapted stratification.

Results of minimal residual disease (MRD) evaluation and MRD-based treatment stratification in childhood ALL

The study of minimal residual disease (MRD) as a 'surrogate' marker of molecular response to treatment has drawn great interest because of the potential of tailoring treatment and the possibility of gaining insight into the nature of a cure. Polymerase chain reaction-based (PCR-based) detection of MRD by immunoglobulin (Ig) and T-cell receptor (TCR) gene rearrangements can be applied in more than 90-95% of cases of childhood acute lymphoblastic leukaemia (ALL). Accordingly, several retrospective studies of MRD in childhood ALL have used one of the different PCR approaches for the detection of antigen-receptor gene rearrangements. The promising results on the predictivity of MRD evaluation at the end of induction treatment has raised the need of a new definition of remission. Until now, most PCR-based MRD studies have used semiquantitative methods for the detection of Ig and TCR gene rearrangements. The introduction of real-time quantitative PCR (RQ-PCR) has resulted in the improvement of sensitivity and specificity and has given better quality control of the MRD data. There is an urgent need to incorporate MRD data in clinical studies, properly designed to address treatment questions. In this context several ongoing co-operative study groups have adopted an MRD-based risk group classification to explore whether a better tailored treatment would result in further improvement in cure rates for children with ALL.

Immature CD34+CD19- progenitor/stem cells in TEL/AML1-positive acute lymphoblastic leukemia are genetically and functionally normal

One important question in stem cell biology of childhood acute lymphoblastic leukemia (ALL) is whether immature CD34+CD19- cells are part of the leukemic cell clone. CD34+CD19- cells from the bone marrow of 9 children with TEL/AML1-positive ALL were purified by flow sorting and subjected to reverse transcriptase-polymerase chain reaction (RT-PCR), fluorescence in situ hybridization, and methylcellulose cultures. In 3 of 8 patients analyzed by RT-PCR, no TEL/AML1-positive cells could be detected in the CD34+CD19- cell fraction. Altogether, the percentage of TEL/AML1-positive cells was low: 1.6% (n = 8; SD 2.2%) by nested real-time RT-PCR and 2.5% (n = 5; SD 2.6%) by fluorescence in situ hybridization. This correlated with the percentage of contaminating CD19+ leukemic cells in the CD34+CD19- cell fraction in 6 control sorts (mean 4.6%, SD 3.6%), indicating that the low levels of leukemic cells detected in the CD34+CD19- cell fraction could be attributed to sorter errors. Methylcellulose cultures in 3 patients provided further evidence that CD34+CD19- cells represent a candidate normal cell population. The clonogenicity of the CD34+CD19- cell fraction was similar to normal progenitors, including growth of primitive granulocyte, erythroid, macrophage, megakaryocyte colony-forming units. Each of 92 colonies from cultures with CD34+CD19- cells tested negative for TEL/AML1. In conclusion, our data support the hypothesis that the leukemia in TEL/AML1-positive childhood ALL originates in a CD19+ lymphoid progenitor. This has many therapeutic implications, eg, for purging of autologous stem cell products, flow cytometric monitoring of minimal residual disease, and targeting immunotherapy against the leukemic cell clone.

TEL/AML1-positive pediatric leukemia: prognostic significance and therapeutic approaches

This article presents the most recent insights into the biology, prognostic significance, and therapeutic approaches to TEL/AML1-positive leukemia. The TEL/AML1 fusion gene, also known as ETV6 /CBFA2, is the most commonly occurring gene rearrangement in pediatric acute lymphoblastic leukemia (ALL). Considerable controversy exists over its prognostic significance with currently available therapies. Differences in outcome may be explained by the differing intensities of various chemotherapy regimens, individual host responses to chemotherapy, or the hypothesis that relapsed TEL/AML1-positive leukemia represents an outgrowth of a secondary leukemia that shares a common initiating event with the first. Incorporating knowledge of this gene rearrangement into treatment decisions serves as a paradigm for translating molecular discoveries into clinically meaningful data to direct patient care and improve outcome.

Antigen expression patterns reflecting genotype of acute leukemias

Multi-parameter flow cytometry, molecular genetics, and cytogenetic studies have all contributed to new classification of leukemia. In this review we discuss immunophenotypic characteristics of major genotypic leukemia categories. We describe immunophenotype of: B-lineage ALL with MLL rearrangements, TEL/AML1, BCR/ABL, E2A/PBX1 translocations, hyperdiploidy, and myc fusion genes; T-ALL with SCL gene aberrations and t(5;14) translocation; and AML with AML1/ETO, PML/RARalpha, OTT/MAL and CBFbeta/MYH11 translocations, trisomies 8 or 11 and aberrations of chromosomes 7 and 5. Whereas some genotypes associate with certain immunophenotypic features, others can present with variable immunophenotype. Single molecules (as NG2, CBFbeta/SMMHC and PML/RARalpha proteins) associated with or derived from specific translocations have been described. More often, complex immunophenotype patterns have been related to the genotype categories. Most known associations between immunophenotype and genotype have been defined empirically. Therefore, these associations should be validated in independent patient cohorts before they can be widely used for prescreening of leukemia. Progress in our knowledge on leukemia will show how the molecular-genetic changes modulate the immunophenotype as well as how the expressed protein molecules further modulate cell behavior.

Cytogenetics and molecular genetics of acute lymphoblastic leukemia

An important factor in the diagnosis of acute lymphoblastic leukemia (ALL) is that karyotype is an independent prognostic indicator, with an impact on the choice of treatment. Outcome is related to the number of chromosomes. For example, high hyperdiploidy (51-65 chromosomes) is associated with a good prognosis, whereas patients with near haploidy (23-29 chromosomes) have a poor outcome. The discovery of recurring chromosomal abnormalities in the leukemic blasts of patients with ALL has identified a large number of genes involved in leukemogenesis. Certain specific genetic changes are related to prognosis. The ETV6/AML1 fusion arising from the translocation (t12;21) (p13;q22) has been associated with a good outcome; the BCR/ABL fusion of (t9;22)(q34;q11), rearrangements of the MLL gene, and abnormalities of the short arm of chromosomes 9 involving the tumor suppressor genes p16INK4A have a poor prognosis. Unfortunately, the classification of patients into prognostic groups based on cytogenetics is not always as predicted. Even when other clinically based risk factors are taken into account, some patients with good-risk cytogenetic features will relapse. In the search for new measures of prognosis, it has recently emerged that the level of minimal residual disease following induction therapy can be a reliable predictor of outcome in ALL.

Childhood acute lymphoblastic leukemia

As cure rates in childhood acute lymphoblastic leukemia reach 80%, emphasis is increasingly placed on the accurate identification of drug-resistant cases, the elucidation of the mechanisms involved in drug resistance and the development of new therapeutic strategies targeted toward the pivotal molecular lesions. Pharmacodynamic and pharmacogenomic studies have provided rational criteria for individualizing therapy to enhance efficacy and reduce acute toxicity and late sequelae. Currently, assessment of the early response to treatment by measurement of minimal residual disease (MRD) is the most powerful independent prognostic indicator. MRD is affected by both the drug sensitivity of leukemic cells and the pharmacodynamic and pharmacogenetic properties of the host cells. Rapid advances in biotechnology and bioinformatics should ultimately facilitate the development of molecular diagnostic assays that can be used to optimize antileukemic therapy and elucidate the mechanisms of leukemogenesis. In the interim, prospective clinical trials have provided valuable clues that are further increasing the cure rate of childhood acute lymphoblastic leukemia.

ETV6/AML1 fusion by FISH in adult acute lymphoblastic leukemia

Dual-color interphase fluorescence in situ hybridization (FISH) with ETV6 and AML1 probes was used for the first time on a series of 159 adult patients with acute lymphoblastic leukemia (ALL), for detection of the t(12;21)(p13;q22) translocation. Seven patients (4.4%) were found, with 50-100% of positive cells, of whom one of two tested, proved negative for the fusion product by RT-PCR. Two of them, aged 43 and 50 years, are the oldest patients so far confirmed to have the translocation. Three who relapsed at 10, 11 and 24 months, suggest that adults may not enjoy the good short-term prognosis reported for t(12;21)-positive children. Thirty-one-negative cases had signal numbers differing from the two expected for each gene. In 15 cases these results were consistent with the karyotype. In nine cases with uninformative cytogenetics, the numbers were consistent with those for centromeres and indicated a hidden aneuploidy. Loss of ETV6 genes in two cases and AML1 amplification in three others were not suspected from the cytogenetics. In conclusion, FISH proved to be reliable in defining ETV6/AML1 positivity in this group of patients as well as providing valuable insights into negative cases.

Monitoring minimal residual disease using chromosomal translocations in childhood ALL

Clonal chromosomal abnormalities have been identified in approximately 80% of childhood ALL. In most instances the genes disrupted by these abnormalities have been identified, thus providing important insights into disease pathogenesis and normal cellular physiology. Polymerase chain reaction (PCR) amplification of fusion transcripts resulting from chromosomal translocations has emerged as a sensitive and reproducible method to monitor minimal residual disease (MRD) in childhood ALL. The measure of the initial response to therapy in patients who have achieved complete remission by morphological standards can dissect clinical heterogeneity within the genetically homogeneous childhood ALL subgroup. Moreover, MRD monitoring can be applied to predict impending relapses early. Despite notable progress with this method, several critical issues must be resolved before MRD determinations can be routinely considered in clinical decision making. This chapter will focus on the main progress and common pitfalls in the PCR detection of chromosomal translocations applied to clinical studies.

Comprehensive genotypic analysis of leukemia: clinical and therapeutic implications

Over the past several years, the application of a spectrum of cytogenetic and molecular diagnostic techniques has dramatically improved our understanding of the pathophysiology of leukemia. These techniques include chromosomal translocations visualized by G-banding techniques, fluorescence in-situ hybridization, spectral karyotyping, comparative genomic hybridization, loss of heterozygosity analysis, and characterization of point mutations by DNA sequence analysis. We will review the application of these techniques, update novel findings utilizing these techniques over the past year as they apply to specific leukemias, and review the clinical and therapeutic implications of these findings.

Loss at 12p detected by comparative genomic hybridization (CGH): association with TEL-AML1 fusion and favorable prognostic features in childhood acute lymphoblastic leukemia (ALL). A multi-institutional study

BACKGROUND

Genetic aberrations provide prognostic information in childhood ALL. The proportion of patients with detectable aberrations can be increased by combining G-banding with comparative genomic hybridization (CGH).

PROCEDURE

We studied 79 children with ALL by CGH and G-banding, and explored the relationship of these findings to clinical features and outcome.

RESULTS

CGH revealed DNA copy number changes in 57 patients (72%), 9 of whom had normal karyotype by G-banding. Gains were more frequent than losses, and changes of whole chromosomes more frequent than partial aberrations. Two frequent partial losses were found; at 9p and 12p. The 9 patients with loss at 12p were studied for the deletion of TEL (ETV6) gene and the fusion of TEL and AML1 genes by fluorescent in situ hybridization (FISH). Eight out of the 9 children with loss at 12p harbored the TEL-AML1 translocation and all 9 had the deletion of a nontranslocated TEL allele. All 9 had precursor-B phenotype and L1 morphology, and 8/9 had WBC below 50 x 10(9)/liter. All children were treated according to Nordic ALL protocols, had a good response to treatment based on day 15 bone marrow morphology, and 7 out of the 9 survived in continuous complete remission (median follow-up 74 months).

CONCLUSIONS

CGH is a valuable tool in screening for genetic aberrations in childhood ALL. DNA copy number losses detected at 12p associate with TEL-AML1 fusion as well as with favorable prognostic features.

Piceatannol, a hydroxylated analog of the chemopreventive agent resveratrol, is a potent inducer of apoptosis in the lymphoma cell line BJAB and in primary, leukemic lymphoblasts

The stilbene phytochemicals resveratrol and piceatannol have been reported to possess substantial antitumorigenic and antileukemic activities, respectively. Although recent experimental data revealed the proapoptotic potency of resveratrol, the molecular mechanisms underlying the antileukemic activity have not yet been studied in detail. In the present study, we show that resveratrol, as well as the hydroxylated analog piceatannol, are potent inducers of apoptotic cell death in BJAB Burkitt-like lymphoma cells with an ED50 concentration of 25 microM. Further experiments revealed that treatment of BJAB cells with both substances led to a concentration-dependent activation of caspase-3 and mitochondrial permeability transition. Using BJAB cells overexpressing a dominant-negative mutant of the Fas-associated death domain (FADD) adaptor protein to block death receptor-mediated apoptosis, we demonstrate that resveratrol- and piceatannol-induced cell death in these cells is independent of the CD95/Fas signaling pathway. To explore the antileukemic properties of both compounds in more detail, we extended our study to primary, leukemic lymphoblasts. Interestingly, piceatannol but not resveratrol is a very efficient inducer of apoptosis in this ex vivo assay with leukemic lymphoblasts of 21 patients suffering from childhood lymphoblastic leukemia (ALL).

Novel cryptic, complex rearrangements involving ETV6-CBFA2 (TEL-AML1) genes identified by fluorescence in situ hybridization in pediatric patients with acute lymphoblastic leukemia

In childhood B-lineage acute lymphoblastic leukemia (ALL), the most common genetic change, the ETV6-CBFA2 (TEL-AML1) fusion resulting from the cryptic t(12;21)(p13;q22) is associated with a favorable outcome. Therefore, it is important to identify patients with this translocation so that they can receive appropriate treatment. To identify new partner breakpoints for ETV6 and CBFA2, we selected 30 patients with childhood ALL in whose leukemic cells a t(12;21) had been detected by RT-PCR. Conventional cytogenetics revealed that 12p abnormalities were present in 10 patients and that other random abnormalities were present in another 15, including 9 with a numerical or structural abnormality of chromosome 21. Normal karyotypes were observed in the leukemic blasts of five patients. Interphase fluorescence in situ hybridization (FISH) confirmed the RT-PCR finding of the t(12;21) in each patient and detected the loss of the wild-type ETV6 allele in 14 (47%) patients. Metaphase cells from only 20 patients were available for additional FISH analysis. In 13 patients, the expected fusion signal of t(12;21) was observed on der(21)t(12;21), and the reciprocal CBFA2 signal was observed on der(12)t(12;21). However, in six patients with the ETV6-CBFA2 fusion on chromosome 21, the reciprocal CBFA2 signal was observed not on 12p13 but on 4q21, 4q27, 8q24, 11q24, 14q11.2, or 16p13.1. In four of these six patients, we found interstitial insertions of part of CBFA2. In another patient, the ETV6-CBFA2 fusion was observed on 4q21 rather than on 21q. Thus, seven (35%) of the 20 patients with a t(12;21) revealed complex rearrangements. Our findings also indicate the importance of analyzing metaphase chromosomes in identifying cryptic and complex rearrangements involving ETV6 and CBFA2.

TEL/AML1 rearrangement and the prognostic significance in childhood acute lymphoblastic leukemia in Hong Kong

The TEL/AML1 rearrangement has been implicated as an independent good prognostic factor in pediatric acute lymphoblastic leukemia (ALL). We examined TEL/AML1 using nested reverse-transcription polymerase chain reaction (RT-PCR) and correlated TEL/AML1 with cytogenetics and immunophenotypes in 75 consecutively analyzed Chinese children with ALL in Hong Kong. TEL/AML1 was detected in 17.9% (12/67) B-lineage ALL at diagnosis but not in 8 T-ALL children or in 34 adults with ALL. E2A/PBX1, MLL/AF4, and BCR/ABL were not found in TEL/AML1+ patients. Coexpression of cross-lineage antigens was associated with TEL/AML1 gene fusion (p = 0.032), with CD13 in 80% (4/5) TEL/AML1+ cohort. Chromosomal abnormalities were demonstrated in 50% of the TEL/AML1+ ALL; however, a cryptic t(12;21) was not detected in these cases. Hyperdiploidy of 47-48 chromosomes was encountered in 25%. Deletion of 12p resulting in the loss of the normal allele of TEL and nonspecific del(6q) were noted in 8% (1/12) and 25% (3/12) of the TEL/AML1+ children, respectively. Rapid clearance of TEL/AML1 was noted in 50% of the patients on completion of the induction therapy; however, 16.7% (2/12) TEL/AML1+ ALL relapsed at a mean of 48.6 months from diagnosis (25 months off-therapy). The incidence of relapses of TEL/AML1+ ALL was comparable to that at diagnosis in B-lineage ALL (14.3% [2/14] vs. 17.9% [12/67], p > 0.05). The relapse rate in TEL/AML1+ ALL was similar to that of TEL/AML1- ALL (16.7% [2/12] vs. 20.6% [13/63], p > 0.05). The duration of first complete remission in TEL/AML1+ ALL was significantly longer as compared to TEL/AML1- ALL (mean [range] in month: 48.6 [47.2 - 50] vs 14.6 [2.9 - 42.3], p < 0.0001). Irrespective of TEL/AML1 rearrangement, the probabilities of the five-year overall survival and the event-free survival of patients were comparable (overall survival: 100% vs. 72.3%, p = 0.166 and event-free survival: 60% vs. 56.2%, p = 0.343). Our data would not suggest a less aggressive treatment regimen for TEL/AML1+ ALL.

Characterization of additional genetic events in childhood acute lymphoblastic leukemia with TEL/AML1 gene fusion: a molecular cytogenetics study

TEL/AML1 gene fusion that results from a cryptic t(12;21) is the most common genetic aberration in childhood B-lineage acute lymphoblastic leukemia (ALL). While the translocation may initiate the leukemic process, critical secondary genetic events are currently believed to be pivotal for leukemogenesis. We investigated 12 cases of childhood ALL with TEL/AML1 gene fusion by fluorescence in situ hybridization (FISH) and comparative genomic hybridization (CGH) and documented additional or secondary genetic changes in seven patients (58%). Three patients showed extra copies of chromosome 21 including a case in which the trisomy 21 (+21) clone was distinct from the one harboring TEL/AML1 gene fusion. Interestingly, one patient without +21 showed amplification of the AML1 gene on chromosome 21q, supporting the contention that AML1 amplification may be an important additional genetic event. Gene expression study by semi-quantitative reverse-transcription polymerase chain reaction (RT-PCR) in two of these four patients showed an increase in AML1 transcripts that paralleled the increase in gene copy number. Deletion of the normal TEL allele was detected in two patients, with one of them showing loss of chromosome 12 together with duplication of the der(12)t(12;21). Finally, one patient showed duplication of the fusion signal. Our findings confirm that additional or secondary genetic changes including AML1 amplification are commonly encountered in childhood ALL with TEL/AML1 gene fusion, which are envisaged to play significant roles in disease progression.

Acute lymphoblastic leukaemia

In acute lymphoblastic leukaemia (ALL) the karyotype provides important prognostic information which is beginning to have an impact on treatment. The most significant structural chromosomal changes include: the poor-risk abnormalities; t(9;22)(q34;q11), giving rise to the BCR/ABL fusion and rearrangements of the MLL gene; abnormalities previously designated as poor-risk; t(1;19)(q23;p13), producing the E2A/PBX1 and rearrangements of MYC with the immunoglobulin genes; and the probable good risk translocation t(12;21)(p13;q22), which results in the ETV6/AML1 fusion. These abnormalities occur most frequently in B-lineage leukaemias, while rearrangements of the T cell receptor genes are associated with T-lineage ALL. Abnormalities of the short arm of chromosome 9, in particular homozygous deletions involving the tumour suppressor gene (TSG) p16(INK4A), are associated with a poor outcome. Numerical chromosomal abnormalities are of particular importance in relation to prognosis. High hyperdiploidy (51-65 chromosomes) is associated with a good risk, whereas the outlook for patients with near haploidy (23-29 chromosomes) is extremely poor. In view of the introduction of risk-adjusted therapy into the UK childhood ALL treatment trials, an interphase FISH screening programme has been developed to reveal chromosomal abnormalities with prognostic significance in childhood ALL. Novel techniques in molecular cytogenetics are identifying new, cryptic abnormalities in small groups of patients which may lead to further improvements in future treatment protocols.

Origins of "late" relapse in childhood acute lymphoblastic leukemia with TEL-AML1 fusion genes

Approximately 20% of childhood B-precursor acute lymphoblastic leukemia (ALL) has a TEL-AML1 fusion gene, often in association with deletions of the nonrearranged TEL allele. TEL-AML1 gene fusion appears to be an initiating event and usually occurs before birth, in utero. This subgroup of ALL generally presents with low- or medium-risk features and overall has a very good prognosis. Some patients, however, do have relapses late or after the cessation of treatment, at least on some therapeutic protocols. They usually achieve sustained second remissions. Posttreatment relapses, or even very late relapses (5-20 years after diagnosis), in childhood ALL are clonally related to the leukemic cells at diagnosis (by IGH or T-cell receptor [TCR] gene sequencing) and are considered, therefore, to represent a slow re-emergence or escape of the initial clone seen at diagnosis. Microsatellite markers and fluorescence in situ hybridization identified deletions of the unrearranged TEL allele and IGH/TCR gene rearrangements were analyzed; the results show that posttreatment relapse cells in 2 patients with TEL-AML1-positive ALL were not derived from the dominant clone present at diagnosis but were from a sibling clone. In contrast, a patient who had a relapse while on treatment with TEL-AML1 fusion had essentially the same TEL deletion, though with evidence for microsatellite instability 5(') of TEL gene deletion at diagnosis, leading to extended 5(') deletion at relapse. It is speculated that, in some patients, combination chemotherapy for childhood ALL may fail to eliminate a fetal preleukemic clone with TEL-AML1 and that a second, independent transformation event within this clone after treatment gives rise to a new leukemia masquerading as relapse. (Blood. 2001;98:558-564)

Determinants of outcome after intensified therapy of childhood lymphoblastic leukaemia: results from Medical Research Council United Kingdom acute lymphoblastic leukaemia XI protocol

The single most important prognostic determinant in childhood acute lymphoblastic leukaemia (ALL) is effective therapy and changes in therapy may influence the significance of other risk factors. The effect of intensified therapy on the importance of currently recognized phenotypic and genotypic determinants of outcome was assessed in 2090 children enrolled on the Medical Research Council United Kingdom acute lymphoblastic leukaemia XI (MRC UKALL XI) protocol. Treatment allocation was not determined by risk factors. Multivariate analysis confirmed the dominant influence on prognosis of age, sex and presenting white cell count (WCC). After allowing for these features, blast karyotype, d 8 marrow blast percentage and remission status at the end of induction therapy were the only remaining significant predictors of outcome. Organomegaly, haemoglobin concentration, French--American--British type, body mass index, presence of central nervous system disease at diagnosis, immunophenotype and presence of TEL/AML1 fusion gene (examined in a subset of 659 patients) either had no significant effect on outcome or were significant only in univariate analysis. Among karyotype abnormalities with an independent influence on prognosis, high hyperdiploidy (> 50 chromosomes) was shown to be favourable, whereas near haploidy (23--29 chromosomes), presence of the Philadelphia chromosome, t(4;11) and abnormalities affecting the short arm of chromosome 9 [abn (9p)] were adverse risk factors. Early responders to therapy, determined by residual marrow infiltration after 8 d of induction therapy, had a good outcome, while the small proportion of patients who did not achieve a complete remission by the end of induction therapy had a poor outcome. A third block of late intensification was shown to improve event-free survival by 8% at 5 years. The effect of these risk factors was not significantly different between those randomized to the third intensification block and those not randomized to a third block.

Einsiedel H, Geilen W, Seifert G, Eckert C, Henze G, Seeger K. Specific Reverse Transcription-PCR Quantification of Vascular Endothelial Growth Factor (VEGF) Splice Variants by LightCycler Technology

Background: Overexpression of vascular endothelial growth factor (VEGF) is associated with increased angiogenesis, growth and invasion in solid tumors, and hematologic malignancies. The expression of isoforms of VEGF, which mediate different effects, can be discriminated by splice-variant-specific quantitative reverse transcription-PCR (RT-PCR), but current methods have only modest sensitivity and precision and suffer from heteroduplex formation.

Methods: We used a real-time RT-PCR assay on the LightCycler system. Applicability for detection of different VEGF mRNAs and total VEGF message was tested on seven healthy tissues (each pooled from healthy donors) and seven correlated malignant tissues. Results were normalized to β2-microglobulin mRNA. Amplification of VEGF splice variants was performed exclusively with variant-specific reverse primers, whereas forward primer and fluorescent probe were common to obtain similar RT-PCR kinetics.

Results: Highly specific detection of VEGF splice variants was achieved with minor intra- and interassay variation (&lt;0.22 threshold cycle). Total VEGF expression was higher in malignant tissues. In healthy tissues, the mRNA encoding diffusible variants VEGF121 and VEGF165 constituted on average 78% (SD = 9.3%) of the total VEGF message, and the cell-adherent variant VEGF189 constituted on average 22% (SD = 5.4%). In contrast, in malignant tissues VEGF121 and VEGF165 accounted for 94% (SD = 7.6%) and VEGF189 only 6% (SD = 3.7%).

Conclusions: Because of the ability for quantification of VEGF splice variants with high specificity, sensitivity, and reproducibility, this new LightCycler assay is superior to conventional semiquantitative competitive RT-PCR and immunological assays and may contribute to better understanding of VEGF-mediated angiogenesis.

Characterization of cytokine, growth factor receptor, costimulatory and adhesion molecule expression patterns of bone marrow blasts in relapsed childhood B cell precursor all

Relapse of childhood acute lymphoblastic leukaemia (ALL) comprises a leading challenge of investigation. Characterization of leukaemic cells regarding their potency to express growth factors and surface molecules can provide insight into their aberrant biology. Thus, we analyzed bone marrow blasts from 10 children with relapsed B cell precursor ALL. The gene and protein expression of essential haematopoietic growth factors (IL-2, IL-4, IL-7, IL-10, IL-15, IFN-gamma, G-CSFR), their corresponding receptors as well as the expression pattern of adhesion molecules (ICAM-1, CD58) and costimulatory proteins (CD40, CD40L, B7.1, B7.2, CD28, MHC-I and II) was analyzed by RT-PCR and flow cytometry. Constitutive gene expression was found for IL-7, IL-10, IL-15 and IFN-gamma and their corresponding receptors. Flow-cytometric analysis showed that IL-10R, IL-7Ralpha, IL-4Ralpha and the gamma(c)chain are constitutively expressed, and that some cells bear the G-CSFR. IL-10 and IL-15 protein-producing leukaemic cells were easily detectable. The neoplastic cells mainly lack B7.1, and ICAM-1 is mostly decreased. Furthermore, high CD40, and, surprisingly, CD40L expression could be found. These studies show that ALL cells are likely to be sensitive to many growth factors and some factors are produced by the neoplastic cell itself. The secretion of IL-10 by leukaemic cells, and the absence or downregulation of conventional adhesion and costimulatory molecules might represent an effective mechanism of escape of immune surveillance in relapsed ALL.

The TEL-AML1 fusion accompanied by loss of the untranslocated TEL allele in B-precursor acute lymphoblastic leukaemia of childhood

The TEL-AML1 fusion which results from the t(12;21) rearrangement in childhood B-precursor acute lymphoblastic leukaemia (B-precursor ALL) is often accompanied by loss of the untranslocated TEL allele. From 32/109 children with B-precursor ALL screened for these abnormalities, we found evidence for del 12p, including the loss of the untranslocated TEL allele, to be the secondary event to take place in the leukaemic cells from those patients positive for these abnormalities. This suggests that the initial or predisposing event is the generation of a TEL-AML1 fusion, followed by the promoting event of a deletion of a gene(s) on 12p. A striking characteristic of the leukaemic cells in 61% of the patients showing t(12:21). was the substantial evolution of the primary clonal line containing the reciprocal TEL-AML1 fusion. We were able to show loss of normal TEL in the same patients by interphase fluorescence in situ hybridisation (FISH) analysis and reverse-transcriptase polymerase chain reaction (RT-PCR).

Insights into the biologic and molecular abnormalities in adult acute lymphocytic leukemia

The last 3 decades have seen much progress in the treatment and outcome of patients with ALL. Unfortunately, the success that has been achieved in children with ALL has not yet been translated into adult patients. Insight into the biologic and molecular abnormalities in ALL may, however, provide the necessary clues that allow a clearer understanding of the crucial differences in the behavior of ALL in different groups of patients. As the molecular basis of the disease is deciphered, new targets are discovered that may prove useful for therapeutic interventions in the future.

Minimal residual disease studies are beneficial in the follow-up of TEL/AML1 patients with B-precursor acute lymphoblastic leukaemia

The t(12;21)(p13;q22) translocation has been identified as the most common chromosomal abnormality in childhood acute lymphoblastic leukaemia (ALL). Initially, several investigators reported an excellent prognosis in paediatric leukaemias with this translocation, but other studies showed a 20% incidence in relapsed ALL. We performed an extensive analysis of 90 ALL patients. In 17 (19%) cases a TEL/AML1 fusion was found. However, this group was not representative as it included a high number of relapsed patients compared with the normal incidence in B-precursor ALL [54 in continuous complete remission (CCR) and 36 relapsed patients] and only a slightly better prognosis for TEL/AML1-positive patients was found (not significant) (four relapses in 17 TEL/AML1-positive patients vs. 32 relapses in 73 TEL/AML1-negative patients). Comparison of known prognostic factors (age, sex, ploidy, white blood cell count and immunophenotype) between relapsed TEL/AML1-positive and TEL/AML1-positive patients in CCR did not reveal differences, except that the white blood cell count was significantly higher in the relapsed group (P = 0.001). Time between diagnosis and relapse was not different for the relapsed TEL/AML1-positive group vs. the relapsed TEL/AML1-negative group. In 11 TEL/AML1-positive patients, the minimal residual disease (MRD) level at the end of induction therapy was quantified in a limiting dilution assay using IGH or TCRD junctional regions as polymerase chain reaction (PCR) targets. In all four relapsed patients, the level of MRD at the end of induction therapy was high (range 0.24-1.2%), whereas in all seven CCR patients, the MRD level was extremely low (0.02 to < 0.001%). In agreement with previous studies in which MRD levels at the end of induction therapy were found to be the strongest risk factor independent of other risk factors, in the present study we show that the MRD level remains a risk factor independent of the presence of a TEL/AML1 fusion gene.

ETV6/CBFA2 fusions in childhood B-cell precursor acute lymphoblastic leukemia with myeloid markers

A translocation resulting in a fusion of ETV6 (TEL) gene at 12p13 and CBFA2 (AML1) gene at 21q22 is variably reported in 16-36% of cases of childhood acute lymphoblastic leukemia (ALL). This t(12;21)(p13;q22) is not detectable by conventional cytogenetic methods and was reported to be associated with B-cell precursor ALL with presumed favorable prognosis. We have examined 18 cases of well characterized childhood B-cell precursor ALL with cytogenetic, immunophenotypic, and clinical data for the presence of the t(12;21) using fluorescence in situ hybridization (FISH). Fourteen of the 18 cases (78%) were positive for fusion ETV6/CBFA2. One of seven adult ALL patients was positive (12% of cells positive in this 21 year old patient). By contrast, no evidence of t(12;21) by FISH was noted in two childhood T-ALL cases and 10 normal bone marrow samples. Twelve of the 14 positive childhood cases had CD13 and/or CD33 expression (myeloid markers) while only one of the four negative cases was CD13 and CD33 positive. Eight of 12 cases positive for t(12;21), and with conventional cytogenetic data, had structural and/or numerical chromosome abnormalities other than the detected t(12;21). One case had relapse with gradual increase in percentage of cells positive for t(12;21) and development of an isochromosome 21 carrying the fusion signals. The data reveal a strong association of t(12;21) with B-cell precursor ALL, especially with myeloid marker expression.

Analysis of ETV6/AML1 abnormalities in acute lymphoblastic leukaemia: incidence, alternative spliced forms and minimal residual disease value

The t(12;21)(p13;q22) translocation, resulting in the fusion of the ETV6 and AML1 genes, occurs in 20-25% of paediatric B-lineage acute lymphoblastic leukaemias (ALL). The identification of the fusion product has important prognostic and therapeutic implications as the translocation has been associated with a favourable clinical outcome. The aim of this study was threefold: (i) to assess the frequency and clinical association of the fusion gene in patients with and without a cytogenetically detectable chromosome 12 and/or 21 abnormality or failed cytogenetic results, (ii) to characterize alternative forms of ETV6/AML1 transcripts, and (iii) to use ETV6/AML1 as a molecular marker for the investigation of minimal residual disease (MRD). ETV6/AML1 fusion was detected in 22 (39%) of 56 cases studied by reverse transcriptase polymerase chain reaction (RT-PCR). ETV6/AML1 fusion was found in nine out of 16 (56%) cases with a cytogenetically visible chromosome 12 abnormality, but also in nine out of 29 patients (31%) without a chromosome 12 abnormality or patients with failed cytogenetics (four out of 11 patients, 36%), making this the most common cytogenetic abnormality in childhood ALL. Alternatively spliced ETV6/AML1 forms were investigated in 14 of the positive patients. Exon 5 of ETV6 was fused in frame to all AML1 exons, except exon 4. Fusion to exon 6 of AML1 resulted in one amino acid change. The presence of ETV6/AML1 was associated with a lower white blood cell count (Student's t-test; P = 0.009) and common (c)ALL phenotype (chi(2) test; P > 0.001), but no better disease-free survival. Our study shows that (i) RT-PCR is the most effective approach for the detection of t(12;21) in childhood ALL, (ii) the association of ETV6/AML1 and chromosome 12 and/or 21, seen in 56% of our cases, further confirms existing data, (iii) overall survival of patients with t(12;21) was not better than other cytogenetics groups, and (d) MRD analysis using ETV6/AML1 fusion is specific, but not sensitive enough to avoid false negative results.

Familial cryptic translocation with del 4q34-->qter and dup 12pter-->p13 in sibs with tracheal stenosis: clinical, classical and molecular cytogenetic studies and CGH analyses from archival placental tissues evidencing tertiary trisomy 4 in one abortion specimen

We report on two retarded half-sibs of different sex and seemingly normal karyotype who had the same syndrome of minor anomalies, heart defect and a distal tracheal stenosis, and who shared a healthy mother. These findings raised suspicions of a cryptic chromosome translocation. A translocation t(4;12)(q34;p13), balanced in the mother and unbalanced in the sibs with loss of terminal 4q and gain of terminal 12p regions, was verified by FISH using whole chromosome painting, subtelomeric and YAC probes. Clinical features could be explained by partial monosomy 4q and partial trisomy 12p. Tracheal stenosis was interpreted as a consequence of the same developmental disturbance leading to esophageal atresia and tracheo-esophageal fistula. It was attributed to the 4q deletion in which esophageal atresia as also respiratory difficulties and airway obstructions had been described. Paraffin-embedded placental tissues were available from three of the five abortions of the mother allowing DNA extraction and comparative genome hybridization (CGH). Two of the abortion specimens had the same der(4)t(4;12)(q34;p13) unbalanced translocation as identified in the sibs. In the third abortion specimen, suspicious of triploidy because of partial hydatidiform mole, CGH uncovered a tertiary trisomy 4 resulting from a 3:1 segregation of the translocation chromosomes and their homologs during maternal meiosis I. Differences in CGH results using DNA generated directly or after DOP-PCR were explained by DNA fragmentation in paraffin-embedded tissues and unequal amplification. Am. J. Med. Genet. 94:271-280, 2000.

Clinical implications of recurring chromosomal and associated molecular abnormalities in acute lymphoblastic leukemia

Comprehensive study of the major chromosomal/molecular abnormalities in children and adults with acute lymphoblastic leukemia (ALL) has demonstrated prognostic utility for many of these anomalies, to the extent that cytogenetic and molecular genetic evaluations are now required for optimal clinical management of newly diagnosed cases. For example, the t(12;21)/TEL-AML1 (ETV6-CBFA2) or hyperdiploid karyotypes each identifies subgroups of children who can be cured with well-tolerated chemotherapy based primarily on drugs with few long-term toxicities, such as L-asparaginase and antimetabolites. By contrast, the t(1;19)/E2A-PBX1 identifies a subtype of ALL that responds much better to more intensive regimens that rely on genotoxic drugs. At the extreme end of the risk spectrum, the t(4;11)/MLL-AF4 and t(9;22)/BCR-ABL almost always confer a dire prognosis in both children and adults with ALL, who warrant high-dose chemotherapy and hematopoietic stem cell rescue to sustain or even induce first remission. Such chromosomal/molecular markers are being incorporated into risk classification schemes, as they convey prognostic information that cannot be gleaned from conventional risk factors such as immunophenotype, presenting age, and the initial circulating leukemic blast cell count. The most exciting prospect is the discovery of drugs that inhibit specific oncogenes, as illustrated by the BCR-ABL tyrosine kinase inhibitor STI-571.

Integration of molecular methods for detection of balanced translocations in the diagnosis and follow-up of patients with leukemia

Molecular methods are emerging as important tools for the diagnosis and stratification of patients with leukemia. Together with rearrangements in immunoglobulin and T-cell receptor genes, balanced translocations are the most important genetic lesions amenable to molecular diagnosis. Moreover, many publications have identified significant differences in the prognosis of acute leukemia patients with such balanced translocations. Because not all balanced translocations can be diagnosed by cytogenetic techniques, reverse-transcriptase polymerase chain reaction (RT-PCR)-based methods are increasingly employed. This method has the added advantage that it can also be used to monitor for minimal residual disease (MRD). A disadvantage is that the multitude of balanced translocations in leukemia would make efforts to detect all lesions at diagnosis by such standard techniques extremely labor-intensive. Furthermore, difficulties in optimizing semiquantitative PCR assays have limited the utility of these methods for MRD. Recent advances in the design of multiplex PCR, which detects a number of genetic aberrations simultaneously, may improve the diagnostic process. Accurate quantitation of the fusion transcript for balanced translocations has become possible by use of fluorogenic probes and real-time PCR. Together, such methodologies may constitute a novel platform for the integration of molecular methods in clinical decision-making.

Relapse in childhood acute lymphoblastic leukemia is associated with a decrease of the Bax/Bcl-2 ratio and loss of spontaneous caspase-3 processing in vivo

Dysfunction of the p53/Bax/caspase-3 apoptosis signaling pathway has been shown to play a role in tumorigenesis and tumor progression, ie the development of acquired drug resistance. Low expression of the apoptosis inducer Bax correlates with poor response to therapy and shorter overall survival in solid tumors. In the present study, we analyzed the p53/Bax/caspase-3 pathway in a paired and an unpaired sample series of children with acute lymphoblastic leukemia (ALL) at initial diagnosis and relapse. The data demonstrate that both Bax expression levels and the Bax/Bcl-2 ratio are significantly lower in samples at relapse as compared with samples at initial diagnosis (P=0.013, Wilcoxon signed rank test (paired samples); P=0.0039, Mann-Whitney U test (unpaired samples)). The loss of Bax protein expression was not a consequence of Bax frameshift mutations of the G8 tract and could not be attributed to mutations of the p53 coding sequence (exons 5 to 8) which were detected to a similar extent in de novo ALL samples and at relapse. Analysis of the downstream effector caspase-3 showed loss of spontaneous caspase-3 processing at relapse. Whereas nine out of 14 (64%, paired samples) or 37 out of 77 (48%, unpaired samples) ALL patients at initial diagnosis displayed spontaneous in vivo processing of caspase-3, this was completely absent in patients at relapse (paired samples) or detected in only one out of 34 patients at relapse (2.9%, unpaired samples). We therefore conclude that in ALL relapse a severe disturbance of apoptotic pathways occurs, both at the level of Bax expression and caspase-3 activation.

Pediatric Acute Lymphoblastic Leukemia: Challenges and Controversies in 2000

This article discusses ways in which pediatric patients with acute lymphoblastic leukemia (ALL) can be stratified to receive intensive and less intensive therapies in order to decrease morbidity and mortality. Specifically, the focus may shift away from current intensive therapies for ultra low-risk patients and away from transplantation for certain patients at relapse. In contrast, infants with ALL comprise an ultra high-risk population in need of specialized approaches.

In Section I Dr. Lange describes the need to identify ultra low-risk children. Groups around the world have improved the outcome of children with ALL by identifying the basic “total therapy” model of the 1970s and stratifying treatment according to risk of relapse. Current first-line treatment cures about 85% of children with standard-risk ALL and 70% of children with high-risk disease. However, all children receive anthracyclines, alkylating agents, or moderate- to high-dose antimetabolite infusions. While randomized clinical trials prove that these intensifications reduce relapses, they also show that half of all children with ALL can be cured with the modest therapy of the 1970s and early 1980s. The patients curable with lesser therapy may be considered an ultra low-risk group. Attempts to use age, gender, white count, morphology, and karyotype to identify the ultra low-risk group of patients with a 90-95% cure rate with minimal therapy have failed. An expanded repertoire of tools such as pharmacogenetic profiling, PCR measurement of minimal residual disease and microarray technology may make this goal achievable in this decade.

In section II Dr. Chessells addresses the management of children with relapsed ALL. The chance of successful re-treatment with conventional chemotherapy for relapse depends on the duration of first remission and the site of relapse. Bone marrow transplantation from a histocompatible sibling or other suitable donor, which is widely accepted as the treatment of choice for children with a first remission of &lt; 24 months, is associated with a high risk of relapse. Bone marrow transplantation for later bone marrow relapse improves leukemia-free survival but has significant short-term and long-term toxicities. The challenges are to develop more effective treatment for early relapse and to identify those children with relapsed ALL who are curable with chemotherapy or, failing this, those children who would be candidates for bone marrow transplantation in third remission.

In Section III Dr. Felix addresses the problem of infant ALL. ALL of infancy is clinically aggressive, and infants continue to have the worst prognosis of all pediatric patients with ALL. High white blood cell count, younger age, bulky extramedullary disease, and CNS disease at diagnosis are unfavorable characteristics. These features occur with MLL gene translocations. The probability of an MLL gene translocation and the probability of poor outcome both are greatest in younger infants. Specialized intensive chemotherapy approaches and bone marrow transplantation in first remission for this disease may lead to improved survival.

Refined recognition of pediatric patients with ALL who need more and less intensive therapies is necessary to increase survival and decrease toxicities.

Acute lymphoblastic leukemia in children

As the overall long-term event-free survival rate in children with acute lymphoblastic leukemia approaches 80%, emphasis is being placed on risk-directed therapy so that patients are neither overtreated nor undertreated. It has become apparent that a risk assignment system based on primary genetic abnormalities is inadequate by itself. For example, leukemias with the MLL-AF4 or BCR-ABL fusion gene are, in fact, heterogeneous diseases. Many require allogeneic hematopoietic stem-cell transplantation; some, if the patient is of favorable age and has a low presenting leukocyte count, can be cured with chemotherapy alone. Measurement of early responses to therapy and extent of minimal residual disease can greatly improve the accuracy of risk assessment. Consideration of the variable effects of therapy on the prognostic significance of specific genetic abnormalities is also important. Therefore, TEL-AML1 fusion confers a favorable prognosis in some protocols of chemotherapy but not in others. Studies to identify genetic polymorphisms with pharmacokinetic and pharmacodynamic significance promise to guide further refinement of treatment strategies. This will allow maximization of anticancer effects without induction of unacceptable toxicity in individual patients.

Rapid and sensitive minimal residual disease detection in acute leukemia by quantitative real-time RT-PCR exemplified by t(12;21) TEL-AML1 fusion transcript

Because previous PCR-based methodologies for detection of minimal residual disease (MRD) in leukemia patients have been too cumbersome to allow for widespread clinical usefulness, we have employed a real-time quantitative PCR (RQ-PCR) system to develop an MRD assay for t(12;21). We initially determined the expression of the different alternatively spliced TEL-AML1 mRNAs found in t(12;21) breakpoint variants I and II. We then optimized PCR primers for the RQ-PCR system and, using the t(12;21)+ REH cell line in spiking experiments, found a linear detection of TEL-AML1 over at least five logs. Moreover, 1 malignant cell in a background of 1,000,000 normal cells could be detected. The expression of the GAPDH, ABL, and beta(2)-microglobulin (beta2M) housekeeping genes were then compared in normal donors and in leukemic patients, and the very stably expressed beta2M was selected as an internal reference gene, allowing us to compensate for variation in RNA quality and day-to-day variation. In 12 samples from t(12;21)-positive patients at diagnosis, the levels of the TEL-AML1 fusion transcripts were found to vary up to 14-fold after normalization to beta2M. Interestingly, in samples obtained from seven patients at diagnosis, during induction chemotherapy, or relapse, the level of TEL-AML1 in peripheral blood (PB) and bone marrow (BM) was found to differ only by threefold, suggesting that MRD may be evaluated in PB samples in most patients. We conclude that this assay could set new standards for t(12;21) MRD detection with its accuracy, its high throughput, and its short turnover time for samples. Genes Chromosomes Cancer 26:355-365, 1999.

Cytogenetics and molecular genetics of childhood leukemia

Childhood leukemia is the commonest form of childhood cancer and represents clonal proliferation of transformed hemopoietic cells as a result of genetic changes. Molecular characterization of these changes, in particular chromosomal translocations, has yielded a wealth of information on the mechanisms of leukemogenesis. These findings have also allowed the development of sensitive assays for the identification of underlying molecular defects, which is applicable to disease diagnosis and to monitor response to treatment. Genetic alterations in childhood leukemia are powerful prognostic indicators. TEL-AML1 fusion and hyperdiploidy >50 chromosomes are associated with a good prognosis in childhood acute lymphoblastic leukemia, whereas BCR-ABL fusion and MLL rearrangements are associated with a poor prognosis. Hence cytogenetic and molecular genetic classification of childhood leukemia will significantly improve the ability of clinicians to predict therapeutic response and prognosis, which paves the way for risk stratification based on clinical and genetic features. Finally, deciphering of genetic lesions in leukemia has allowed elucidation of the molecular basis of current treatment, as typified by the success of all-trans retinoic treatment in acute promyelocytic leukemia, and has identified targets for novel therapeutic approaches. It is envisaged that efforts in characterization of molecular defects in childhood leukemia will ultimately be translated into better clinical outcome for patients.

Molecular diagnostics in the treatment of leukemia

The molecular characterization of childhood leukemias directly affects our treatment strategies. Acute lymphoblastic leukemia patients with the TEL-AML1 fusion have a favorable prognosis, whereas those with the E2A-PBX1 fusion require more intensive therapy to obtain a good outcome. Acute lymphoblastic leukemia patients whose leukemic lymphoblasts contain the MLL-AF4 or the BCR-ABL fusion are often candidates for allogeneic hematopoietic stem cell transplantation during first remission. Among acute myeloid leukemia patients, AML1-ETO and CBFbeta-MYH11 fusions are associated with a favorable response, especially when the chemotherapy regimen includes high-dose cytarabine. Patients with acute promyelocytic leukemia who carry the PML-RAR alpha fusion respond to all-trans retinoic acid and have an excellent outcome after treatment with all-trans retinoic acid in combination with anthracyclines. Several novel therapeutic agents targeted to molecular lesions of leukemic cells are under investigation.

Fish analysis at diagnosis in acute lymphoblastic leukemia

Many recurrent cytogenetic abnormalities have been described in acute lymphoblastic leukemia (ALL). With improvement of survival, it became evident that some of these chromosomal changes had a major prognostic value: hyperdiploidy and t(12;21) are associated with very long disease free survival, whereas t(9;22) and 11q23 rearrangements correlate with a poor outcome. Because of this prognostic impact, detection of these four specific abnormalities has to be accurate and rapid at diagnosis. In order to reliably analyze ALL patients, we used fluorescence in situ hybridization (FISH). We selected specific probes enabling the detection of the structural rearrangements [t(9;22), t(12;21) and 11q23 rearrangements] directly on interphase cells. Detection of hyperdiploidy was performed using comparative genomic hybridization (CGH). This technique enables to characterize chromosomal gains and losses without requiring metaphase obtention. We showed that this FISH-based approach was complementary to conventional cytogenetics, and should be systematically used in ALL at diagnosis.

Biology and clinical significance of the TEL/AML1 rearrangement

The accurate identification of chromosomal abnormalities in patients with leukemia is essential for diagnosis and treatment assignment. Recent technical improvements in the detection of such aberrations have demonstrated that the previously unrecognized chromosomal translocation t(12;21) is the most prevalent structural aberration in childhood acute lymphoblastic leukemia. Both genes involved, translocation ets like gene (or ETV6) on chromosome 12 and acute myeloid leukemia 1 gene (or CBF alpha) on chromosome 21 had been identified for several years previously, which facilitated the rapid development of molecular diagnostic assays and their implementation in therapy trials. Although first described less than four years ago, the TEL/AML1 story is an excellent example of how close collaboration between physicians and molecular biologists is mandatory for achieving general insights into the molecular pathogenesis of leukemia and for further improvements in diagnosis and in monitoring response to chemotherapy.

Genetic abnormalities and drug resistance in acute lymphoblastic leukemia

Recent advances in cytogenetics and molecular genetics have made it possible to identify an array of genomic abnormalities with prognostic and therapeutic significance. Hyperdiploidy > 50 chromosomes and ETV6-CBFA2 fusions have been used to identify low-risk cases, and BCR-ABL and MLL-AF4 to define high-risk leukemias. Despite their clinical utility, the risk classification system based on these findings lack absolute precision and should be complemented with other variables, the most important of which is the early blast cell response to remission induction therapy. Studies of tumor suppressor genes and proto-oncogenes in the BCL2 family genes may unravel the mechanisms of leukemia cell progression and the development of drug resistance, leading to innovative therapies. As the cure rates for childhood acute lymphoblastic leukemia (ALL) approach 80%, precise methods of risk assessment are needed to permit better selection of treatment that is neither excessive nor inadequate for individual patients. Because one or more genetic abnormalities underlie every case of leukemia, a risk assignment system based on primary genetic abnormalities has great intuitive appeal. Even though over 90% of childhood ALL cases can be readily classified according to numerical or gross structural chromosomal abnormalities, molecular analyses are essential to identify therapeutically relevant, submicroscopic genetic lesions not visible by karyotyping. This review focuses mainly on recent advances in genetic studies that have contributed to therapeutic advances or that hold promise for the future.