The majority of myeloid-antigen-positive (My+) childhood B-cell precursor acute lymphoblastic leukaemias express TEL-AML1 fusion transcripts

Emerging molecular subtypes and therapies in acute lymphoblastic leukemia

Tremendous strides have been made in the molecular and cytogenetic classification of acute lymphoblastic leukemia based on gene expression profiling data, leading to an expansion of entities in the recent International Consensus Classification (ICC) of myeloid neoplasms and acute leukemias and 2022 WHO Classification of Tumours: Haematolymphoid Tumors, 5th edition. This increased diagnostic and therapeutic complexity can be overwhelming, and this review compares nomenclature differences between the ICC and WHO 5th edition publications, compiles key features of each entity, and provides a diagnostic algorithmic approach. In covering B-lymphoblastic leukemia (B-ALL), we divided the entities into established (those present in the revised 4th edition WHO) and novel (those added to either the ICC or WHO 5th edition) groups. The established B-ALL entities include B-ALL with BCR::ABL1 fusion, BCR::ABL1-like features, KMT2A rearrangement, ETV6::RUNX1 rearrangement, high hyperdiploidy, hypodiploidy (focusing on near haploid and low hypodiploid), IGH::IL3 rearrangement, TCF3::PBX1 rearrangement, and iAMP21. The novel B-ALL entities include B-ALL with MYC rearrangement; DUX4 rearrangement; MEF2D rearrangement; ZNF384 or ZNF362 rearrangement, NUTM1 rearrangement; HLF rearrangement; UBTF::ATXN7L3/PAN3,CDX2; mutated IKZF1 N159Y; mutated PAX5 P80R; ETV6::RUNX1-like features; PAX5 alteration; mutated ZEB2 (p.H1038R)/IGH::CEBPE; ZNF384 rearranged-like; KMT2A-rearranged-like; and CRLF2 rearrangement (non-Ph-like). Classification of T-ALL is complex with some variability in how the subtypes are defined in recent literature. It was classified as early T-precursor lymphoblastic leukemia/lymphoma and T-ALL, NOS in the WHO revised 4th edition and WHO 5th edition. The ICC added an entity into early T-cell precursor ALL, BCL11B-activated, and also added provisional entities subclassified based on transcription factor families that are aberrantly activated.

Impact of immunophenotypic characteristics on genetic subgrouping in childhood acute lymphoblastic leukemia: Tokyo Children's Cancer Study Group (TCCSG) study L04-16

Immunophenotyping was performed in 1044 consecutive childhood acute lymphoblastic leukemia (ALL) patients enrolled in the Tokyo Children's Cancer Study Group L04-16 trial, revealing novel findings associated with genetic abnormalities. In addition to TCF3-PBX1 and MEF2D fusions, the CD10⁽⁺⁾ subtype of KMT2A-MLLT3-positive ALL frequently exhibited the cytoplasmic-μ⁽⁺⁾ pre-B ALL immunophenotype. Although ETV6-RUNX1 was significantly correlated with myeloid antigen expression, more than half of patients expressed neither CD33 nor CD13, while the CD27⁽⁺⁾ /CD44^(-) immunophenotype was maintained. Expression of CD117 and CD56 in B-cell precursor-ALL was limited to certain subtypes including ETV6-RUNX1 and KMT2A-MLLT3. Besides BCR-ABL1, CRLF2, hyperdiploidy, and hypodiploidy, CD66c was also expressed in Ph-like kinase fusion-, PAX5 fusion-, and DUX4 fusion-positive ALL, but not in MEF2D fusion-positive ALL, indicating constant selectivity of CD66c expression. In T-ALL, SIL-TAL1-positive patients were likely to exhibit a more mature immunophenotype. Expression of CD21 and CD10 was not rare in T-ALL, while lack of CD28 was an additional feature of early T-cell precursor-ALL. Considering the immunophenotype as a prognostic maker, MEF2D fusion-positive ALL with CD5 expression may be associated with a poorer prognosis in comparison with those lacking CD5 expression. In cases with characteristic marker expression, the presence of certain fusion transcripts could be predicted accurately.

Significance of CD66c expression in childhood acute lymphoblastic leukemia

Upon analyzing 696 childhood B-cell precursor acute lymphoblastic leukemia (BCP-ALL) cases, we identified the characteristics of CD66c expression. In addition to the confirmation of strong correlation with BCR-ABL positivity and hyperdiploid, we further observed that CD66c is frequently expressed in CRLF2-positive (11/15, p<0.01 against chimeric gene-negative) as well as hypodiploid cases (3/4), whereas it is never expressed in ETV6-RUNX1, MLL-AF4, MLL-AF9, MLL-ENL, and E2A-PBX1-positive cases. Although the expression of CD66c itself is not directly linked to the prognosis, the accompanying genetic abnormalities are important prognostic factors for BCP-ALL, indicating the importance of CD66c expression in the initial diagnosis of BCP-ALL.

New markers for minimal residual disease detection in acute lymphoblastic leukemia

To identify new markers for minimal residual disease (MRD) detection in acute lymphoblastic leukemia (ALL), we compared genome-wide gene expression of lymphoblasts from 270 patients with newly diagnosed childhood ALL to that of normal CD19⁺CD10⁺ B-cell progenitors (n = 4). Expression of 30 genes differentially expressed by ≥ 3-fold in at least 25% of cases of ALL (or 40% of ALL subtypes) was tested by flow cytometry in 200 B-lineage ALL and 61 nonleukemic BM samples, including samples containing hematogones. Of the 30 markers, 22 (CD44, BCL2, HSPB1, CD73, CD24, CD123, CD72, CD86, CD200, CD79b, CD164, CD304, CD97, CD102, CD99, CD300a, CD130, PBX1, CTNNA1, ITGB7, CD69, CD49f) were differentially expressed in up to 81.4% of ALL cases; expression of some markers was associated with the presence of genetic abnormalities. Results of MRD detection by flow cytometry with these markers correlated well with those of molecular testing (52 follow-up samples from 18 patients); sequential studies during treatment and diagnosis-relapse comparisons documented their stability. When incorporated in 6-marker combinations, the new markers afforded the detection of 1 leukemic cell among 10(5) BM cells. These new markers should allow MRD studies in all B-lineage ALL patients, and substantially improve their sensitivity.

Prognosis of children with mixed phenotype acute leukemia treated on the basis of consistent immunophenotypic criteria

BACKGROUND

Mixed phenotype acute leukemia (MPAL) represents a diagnostic and therapeutic dilemma. The European Group for the Immunological Classification of Leukemias (EGIL) scoring system unambiguously defines MPAL expressing aberrant lineage markers. Discussions surrounding it have focused on scoring details, and information is limited regarding its biological, clinical and prognostic significance. The recent World Health Organization classification is simpler and could replace the EGIL scoring system after transformation into unambiguous guidelines.

DESIGN AND METHODS

Simple immunophenotypic criteria were used to classify all cases of childhood acute leukemia in order to provide therapy directed against acute lymphoblastic leukemia or acute myeloid leukemia. Prognosis, genotype and immunoglobulin/T-cell receptor gene rearrangement status were analyzed.

RESULTS

The incidences of MPAL were 28/582 and 4/107 for children treated with acute lymphoblastic leukemia and acute myeloid leukemia regimens, respectively. In immunophenotypic principal component analysis, MPAL treated as T-cell acute lymphoblastic leukemia clustered between cases of non-mixed T-cell acute lymphoblastic leukemia and acute myeloid leukemia, while other MPAL cases were included in the respective non-mixed B-cell progenitor acute lymphoblastic leukemia or acute myeloid leukemia clusters. Analogously, immunoglobulin/T-cell receptor gene rearrangements followed the expected pattern in patients treated as having acute myeloid leukemia (non-rearranged, 4/4) or as having B-cell progenitor acute lymphoblastic leukemia (rearranged, 20/20), but were missing in 3/5 analyzed cases of MPAL treated as having T-cell acute lymphobastic leukemia. In patients who received acute lymphoblastic leukemia treatment, the 5-year event-free survival of the MPAL cases was worse than that of the non-mixed cases (53+/-10% and 76+/-2% at 5 years, respectively, P=0.0075), with a more pronounced difference among B lineage cases. The small numbers of MPAL cases treated as T-cell acute lymphoblastic leukemia or as acute myeloid leukemia hampered separate statistics. We compared prognosis of all subsets with the prognosis of previously published cohorts.

CONCLUSIONS

Simple immunophenotypic criteria are useful for therapy decisions in MPAL. In B lineage leukemia, MPAL confers poorer prognosis. However, our data do not justify a preferential use of current acute myeloid leukemia-based therapy in MPAL.

Characterization of immunophenotypic aberrancies in 200 cases of B acute lymphoblastic leukemia

Morphologic distinction of leukemic lymphoblasts in B acute lymphoblastic leukemia (B-ALL) from their nonneoplastic counterparts in bone marrow (hematogones) can be difficult. Thus, the presence of aberrant antigen expression detectable by flow cytometry may be critical for diagnosis of B-ALL and detection of minimal residual disease. The current study examined the immunophenotype of B-lineage leukemic lymphoblasts in 200 consecutive, unique, pretreatment patient specimens. We found that all cases of B-ALL exhibited multiple immunophenotypic aberrancies by which they can be distinguished from hematogones. The most frequent aberrancies were uniform or a spectrum of expression of terminal deoxynucleotidyl transferase and CD34, underexpression of CD45, overexpression of CD10, underexpression of CD38, and underexpression of CD20. Asynchronous coexpression of CD34 and CD20 was also frequently observed. Of the 200 cases, 86.5% expressed myeloid-associated antigens, and 19.0% expressed 3 or more. Of 200 cases, 9.0% aberrantly expressed T cell-associated antigens. There were significant differences in antigen-expression patterns between adult and pediatric B-ALL. Specific aberrancies correlate with recurrent cytogenetic abnormalities in B-ALL.

A subtype of childhood acute lymphoblastic leukaemia with poor treatment outcome: a genome-wide classification study

BACKGROUND

Genetic subtypes of acute lymphoblastic leukaemia (ALL) are used to determine risk and treatment in children. 25% of precursor B-ALL cases are genetically unclassified and have intermediate prognosis. We aimed to use a genome-wide study to improve prognostic classification of ALL in children.

METHODS

We constructed a classifier based on gene expression in 190 children with newly diagnosed ALL (German Cooperative ALL [COALL] discovery cohort) by use of double-loop cross-validation and validated this in an independent cohort of 107 newly diagnosed patients (Dutch Childhood Oncology Group [DCOG] independent validation cohort). Hierarchical cluster analysis with classifying gene-probe sets revealed a new ALL subtype, the underlying genetic abnormalities of which were characterised by comparative genomic hybridisation-arrays and molecular cytogenetics.

FINDINGS

Our classifier predicted ALL subtype with a median accuracy of 90.0% (IQR 88.3-91.7) in the discovery cohort and correctly identified 94 of 107 patients (accuracy 87.9%) in the independent validation cohort. Without our classifier, 44 children in the COALL cohort and 33 children in the DCOG cohort would have been classified as B-other. However, hierarchical clustering showed that many of these genetically unclassified cases clustered with BCR-ABL1-positive cases: 30 (19%) of 154 children with precursor B-ALL in the COALL cohort and 14 (15%) of 92 children with precursor B-ALL in the DCOG cohort had this BCR-ABL1-like disease. In the COALL cohort, these patients had unfavourable outcome (5-year disease-free survival 59.5%, 95% CI 37.1-81.9) compared with patients with other precursor B-ALL (84.4%, 76.8-92.1%; p=0.012), a prognosis similar to that of patients with BCR-ABL1-positive ALL (51.9%, 23.1-80.6%). In the DCOG cohort, the prognosis of BCR-ABL1-like disease (57.1%, 31.2-83.1%) was worse than that of other precursor B-ALL (79.2%, 70.2-88.3%; p=0.026), and similar to that of BCR-ABL1-positive ALL (32.5%, 2.3-62.7%). 36 (82%) of the patients with BCR-ABL1-like disease had deletions in genes involved in B-cell development, including IKZF1, TCF3, EBF1, PAX5, and VPREB1; only nine (36%) of 25 patients with B-other ALL had deletions in these genes (p=0.0002). Compared with other precursor B-ALL cells, BCR-ABL1-like cells were 73 times more resistant to L-asparaginase (p=0.001) and 1.6 times more resistant to daunorubicin (p=0.017), but toxicity of prednisolone and vincristine did not differ.

INTERPRETATION

New treatment strategies are needed to improve outcome for this newly identified high-risk subtype of ALL.

FUNDING

Dutch Cancer Society, Sophia Foundation for Medical Research, Paediatric Oncology Foundation Rotterdam, Centre of Medical Systems Biology of the Netherlands Genomics Initiative/Netherlands Organisation for Scientific Research, American National Institute of Health, American National Cancer Institute, and American Lebanese Syrian Associated Charities.

Acute Lymphoblastic Leukemia: Diagnosis and Detection of Minimal Residual Disease Following Therapy

Flow cytometric immunophenotyping (FCI) is an important diagnostic modality in the evaluation of patients who have suspected or known acute lymphoblastic leukemia (ALL). It enables rapid identification, quantification, and immunophenotypic characterization of leukemic blasts, permitting accurate and timely diagnosis. Beyond facilitating the classification of ALL into fundamental diagnostic categories, FCI may anticipate recurrent cytogenetic and molecular abnormalities. FCI permits the detection of leukemic blasts after therapy at a level lower than that achievable by conventional microscopic examination. Flow cytometric detection of minimal residual disease is among the strongest prognostic factors in patients who have ALL and may provide an opportunity for more precise risk-adapted therapies.

Frequent but nonrandom expression of myeloid markers on de novo childhood acute lymphoblastic leukemia

The expression of the myeloid markers CD13, CD33, and CD15 in two hundred and eighty-three cases of de novo childhood acute lymphoblastic leukemia (ALL) is examined. The expression of at least one marker is a frequent event which is noted in 64% and 74% of B- and T-lineage ALL cases, respectively. Certain patterns of myeloid antigen expression can be recognized including: no expression of CD13, CD33, and CD15 in mature B-ALL, significantly higher levels of CD13 and CD33 and significantly lower levels of CD15 in TEL-AML1-positive B cell precursor ALL, no expression of CD13 and CD33 in E2A-PBX1-positive B cell precursor ALL cases and common T-ALL (double positive for CD4 and CD8), and no expression of CD13 in MLL-AF4-positive B cell precursor ALL cases. Although the numbers in some ALL subtypes are small, these patterns are consistent with nonrandom expression of myeloid markers in de novo childhood ALL.

Cytogenetic patterns inETV6/RUNX1-positive pediatric B-cell precursor acute lymphoblastic leukemia: A Nordic series of 245 cases and review of the literature

Between 1992 and 2004, 1,140 children (1 to<15 years) were diagnosed with B-cell precursor acute lymphoblastic leukemia (ALL) in the Nordic countries. Of these, 288 (25%) were positive for t(12;21)(p13;q22) [ETV6/RUNX1]. G-banding analyses were successful in 245 (85%); 43 (15%) were karyotypic failures. The modal chromosome numbers, incidence, types, and numbers of additional abnormalities, genomic imbalances, and chromosomal breakpoints in the 245 karyotypically informative cases, as well as in 152 previously reported cytogenetically characterized t(12;21)-positive ALLs in the same age group, were ascertained. The most common modal numbers among the 397 cases were 46 (67%), 47 (16%), 48 (6%), and 45 (5%). High-hyperdiploidy, triploidy, and tetraploidy were each found in approximately 1%; none had less than 40 chromosomes. Secondary chromosomal abnormalities were identified by chromosome banding in 248 (62%) of the 397 ALLs. Of these, 172 (69%) displayed only unbalanced changes, 14 (6%) only balanced aberrations, and 26 (10%) harbored both unbalanced and balanced abnormalities; 36 (15%) were uninformative because of incomplete karyotypes. The numbers of secondary changes varied between 1 and 19, with a median of 2 additional aberrations per cytogenetically abnormal case. The most frequent genomic imbalances were deletions of 6q21-27 (18%), 8p11-23 (6%), 9p13-24 (7%), 11q23-25 (6%), 12p11-13 (27%), 13q14-34 (7%), loss of the X chromosome (8%), and gains of 10 (9%), 16 (6%), and 21 (29%); no frequent partial gains were noted. The chromosome bands most often involved in structural rearrangements were 3p21 (2%), 5q13 (2%), 6q12 (2%), 6q14 (2%), 6q16 (2%), 6q21 (10%), 6q23 (6%), 6q25 (3%), 9p13 (2%), 11q13 (2%), 11q23 (2%), 12p11 (6%), 12p12 (7%), 12p13 (25%), 21q10 (6%), and 21q22 (6%). Considering that the t(12;21) is known to arise in utero and that the postnatal latency period is protracted, additional mutations are most likely necessary for overt ALL. The frequently rearranged chromosome regions may harbor genes of importance for the transformation and/or progression of an initial preleukemic t(12;21)-positive clone.

TEL-AML1 promotes development of specific hematopoietic lineages consistent with preleukemic activity

The t(12;21)(p13;q22) translocation is the most common chromosomal abnormality yet identified in any pediatric leukemia and gives rise to the TEL-AML1 fusion product. To investigate the effects of TEL-AML1 on hematopoiesis, fetal liver hematopoietic progenitor cells (HPCs) were transduced with retroviral vectors expressing this fusion protein. We show that TEL-AML1 dramatically alters differentiation of HPCs in vitro, preferentially promoting B-lymphocyte development, enhancing self-renewal of B-cell precursors, and leading to the establishment of long-term growth factor-dependent pre-B-cell lines. However, it had no effect on myeloid development in vitro. Further experiments were performed to determine whether TEL-AML1 also demonstrates lineage-specific activity in vivo. TEL-AML1-expressing HPCs displayed a competitive advantage in reconstituting both B-cell and myeloid lineages in vivo but had no effect on reconstitution of the T-cell lineage. Despite promoting these alterations in hematopoiesis, TEL-AML1 did not induce leukemia in transplanted mice. Our study provides a unique insight into the role of TEL-AML1 in leukemia predisposition and a potential model to study the mechanism of leukemogenesis associated with this fusion.

Complex karyotypes in childhood acute lymphoblastic leukemia: cytogenetic and molecular cytogenetic study of 21 cases

Cytogenetic and molecular cytogenetic analysis of 79 childhood acute lymphoblastic leukemias (ALL) revealed chromosomal abnormalities in 76 (96%). Complex karyotypes (a finding of three and more chromosomal aberrations in a karyotype) were identified in 21 (26.6%) out of 79 patients. In 11 patients, complex karyotypes have included common recurrent chromosomal abnormalities, such as translocation t(12;21) in seven cases, t(9;22) in two cases, one case with t(2;1;19) and another one with translocation involving 11q23. In 10 patients, miscellaneous abnormalities were detected. Five patients displayed hyperdiploidy (47 approximately 57 chromosomes), three patients complex karyotypes with deletions of 9p, one patient with two new complex translocations t(2;4;12;13) and t(7;11;20), and the last patient with dic(12;21). The evaluation of the frequency of the chromosomal breaks (>5 per chromosome) showed that chromosomes 2, 4, 5, 7, 9, 12, 13, and 21 were most frequently affected. Survival analysis revealed statistically significant unfavorable event-free survival (EFS) (P=0.013) and decreased overall survival in the group with complex karyotypes (n=21) compared with the other cases (n=58). The evaluation of overexpression profile revealed increased occurrence of double CD13/CD33 positivity in patients with common recurrent chromosomal abnormalities (in 70% of cases); no such cases were registered in the other group (P<0.01).

A fluorescence in situ hybridization study of TEL-AML1 fusion gene in B-cell acute lymphoblastic leukemia (1984-2001)

Acute lymphoblastic leukemia (ALL) is associated with recurrent chromosomal abnormalities. The cryptic translocation t(12;21)(p13;q22), which leads to the TEL-AML1 fusion gene, is the most common abnormality in childhood B-cell ALL. The aim of this retrospective study was to determine the incidence of TEL-AML1 fusion in childhood and adult B-cell ALL using interphase fluorescence in situ hybridization (I-FISH) and its association with additional changes. FISH, using dual-color extra-signal (ES) DNA probe specific for the TEL and AML1 genes, was performed either on blast cells suspensions stored in liquid nitrogen immediately after Ficoll or on leukemic cells preserved in fixative solution at -20 degrees C after short-term culture. No TEL-AML1 fusion was observed in the 26 ALL adults. The fusion was found among 19.6% of the 57 ALL children, additional changes being identified by conventional cytogenetics in 80% of the cases. A deletion of the untranslocated TEL was observed in 36.3% of the ALL with the TEL-AML1 fusion. The coexpression of myelocytic and B-lymphoid antigens was found in 3 of the 11 of TEL-AML1 fusion positive-ALL. Our results (frequency of TEL-AML1 fusion in children and of the deletion of the untranslocated TEL allele, mean age of the patients and white blood cell count) are within the range observed by others. Structural chromosomal abnormalities other than the t(12;21) are frequent and may play a role in the prognosis of these patients.

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.

Amplification of AML1 gene is present in childhood acute lymphoblastic leukemia but not in adult, and is not associated with AML1 gene mutation

The AML1/CBFA2/RUNX1 gene is the target of many recurrent translocations seen in different leukemia subtypes. The t(12;21)(p13;q22) is the most frequent translocation observed in childhood B acute lymphoblastic leukemia (ALL), occurring in 20% to 25% of cases. In adult ALL this rearrangement is scarce. Another route of AML1deregulation could be point mutations in the runt domain. We now report on AML1amplification in two cases of childhood ALL, found in a series of 107 consecutive children with B-lineage ALL analyzed by fluorescence in situ hybridization (FISH). A parallel analysis of 42 adult B-ALL failed to detect any AML1 rearrangement by FISH. The two patients with AML1 amplification were further analyzed using molecular techniques. SSCP analysis did not detect any mutation. Furthermore, direct sequencing of the cDNA did not reveal any mutation. In conclusion, AML1amplification seems to be observed only in childhood ALL and is not associated with AML1 gene mutation. Other mechanisms, such as gene dosage effects could be hypothesized.

Determination of myeloid antigen expression on childhood acute lymphoblastic leukaemia cells: discrepancies using different monoclonal antibody clones

Prospective clinical studies including large numbers of patients have led to the conclusion that co-expression of myeloid antigens in childhood acute lymphoblastic leukaemia (My+ ALL) does not have prognostic significance. However, reports of the frequency of My+ ALL in children vary widely across laboratories using different mAb clones and staining and analysing procedures. Taking two commonly accepted thresholds of positivity for myeloid antigens (20 and 30%), we analysed the immunoreactivity of the most widely employed mAb clones against CD13 (SJ1D1, L138 and My7) and CD33 (My9, P67.6 and D3HL60) and compared the proportions of My+ ALL detected by these clones in childhood ALL. The correlation between myeloid antigen expression and the presence of the t(12;21) translocation was analysed concomitantly in the same samples. The percentage of ALL cases positive for myeloid markers varied significantly depending on the mAb clone and the positive threshold. Among patients with B-ALL, the proportion of CD13+ ALL was significantly lower using SJ1D1 than using L138 or My7, while the proportion of CD33+ ALL was significantly higher for My9 than for P67.6 or D3HL60. Analysis of the co-expression of CD13 and CD33 on B-ALL cells using combinations of mAb clones showed that this frequency was either underestimated by the SJ1D1/D3HL60 or overestimated by the L138/P67.6 and My7/My9 combinations. A correlation between CD13/CD33 positivity and the t(12;21) translocation was uniformly observed in B-ALL patients for a positive threshold of 30%, whereas SJ1D1/D3HL60 detected no correlation between t(12;21) and CD13/CD33 positivity when the threshold was lowered to 20%. These data show that the mAb clones commonly used to detect the CD13 and CD33 surface antigens have variable immunoreactivity against childhood ALL cells, which may partly explain the conflicting reports concerning the prognostic significance of myeloid antigen expression in paediatric ALL and its association with different translocations. The present findings may also be of clinical importance for therapeutic choices.

Determination of myeloid antigen expression on childhood acute lymphoblastic leukaemia cells: discrepancies using different monoclonal antibody clones

Prospective clinical studies including large numbers of patients have led to the conclusion that co-expression of myeloid antigens in childhood acute lymphoblastic leukaemia (My+ ALL) does not have prognostic significance. However, reports of the frequency of My+ ALL in children vary widely across laboratories using different mAb clones and staining and analysing procedures. Taking two commonly accepted thresholds of positivity for myeloid antigens (20 and 30%), we analysed the immunoreactivity of the most widely employed mAb clones against CD13 (SJ1D1, L138 and My7) and CD33 (My9, P67.6 and D3HL60) and compared the proportions of My+ ALL detected by these clones in childhood ALL. The correlation between myeloid antigen expression and the presence of the t(12;21) translocation was analysed concomitantly in the same samples. The percentage of ALL cases positive for myeloid markers varied significantly depending on the mAb clone and the positive threshold. Among patients with B-ALL, the proportion of CD13+ ALL was significantly lower using SJ1D1 than using L138 or My7, while the proportion of CD33+ ALL was significantly higher for My9 than for P67.6 or D3HL60. Analysis of the co-expression of CD13 and CD33 on B-ALL cells using combinations of mAb clones showed that this frequency was either underestimated by the SJ1D1/D3HL60 or overestimated by the L138/P67.6 and My7/My9 combinations. A correlation between CD13/CD33 positivity and the t(12;21) translocation was uniformly observed in B-ALL patients for a positive threshold of 30%, whereas SJ1D1/D3HL60 detected no correlation between t(12;21) and CD13/CD33 positivity when the threshold was lowered to 20%. These data show that the mAb clones commonly used to detect the CD13 and CD33 surface antigens have variable immunoreactivity against childhood ALL cells, which may partly explain the conflicting reports concerning the prognostic significance of myeloid antigen expression in paediatric ALL and its association with different translocations. The present findings may also be of clinical importance for therapeutic choices.

Adult precursor B-ALL with BCR/ABL gene rearrangements displays a unique immunophenotype based on the pattern of CD10, CD34, CD13 and CD38 expresssion

The Philadelphia chromosome (Ph+) reflects a balanced reciprocal translocation between the long arms of chromosomes 9 and 22 [t(9;22)(q34;q11.2] involving the BCR and ABL genes. At present, detection of BCR/ABL gene rearrangements is mandatory in precursor-B-ALL patients at diagnosis for prognostic stratification and treatment decision. In spite of the clinical impact, no screening method, displaying a high sensitive and specificity, is available for the identification of BCR/ABL+ precursor-B-ALL cases. The aim of the present study was to explore the immunophenotypic characteristics of precursor B-ALL cases displaying BCR/ABL gene rearrangements using multiple stainings analyzed by quantitative flow cytometry in order to rapidly (<1 h) identify unique phenotypes associated with this translocation. From the 82 precursor-B-ALL cases included in the study 12 displayed BCR/ABL gene rearragements, all corresponding to adult patients, four of which also displayed DNA aneuploidy. Our results show that BCR/ABL+ precursor B-ALL cases constantly displayed a homogeneous expression of CD10 and CD34 but low and relatively heterogeneous CD38 expression, together with an aberrant reactivity for CD13. In contrast, this unique phenotype was only detected in three out of 70 BCR/ABL cases. Therefore, the combined use of staining patterns for CD34, CD38 and CD13 expression within CD10-positive blast cells is highly suggestive of BCR/ABL gene rearrangements in adults with precursor B-ALL.

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.

Immunophenotyping of leukemia

Modern immunophenotyping of hematological malignancies by flow cytometry is assisted by a wide array of easily accessible monoclonal antibodies, by antibodies conjugated to diverse fluorochromes, and by reliable techniques for cell membrane permeabilization. Simultaneous assessment of multiple surface and intracellular markers at diagnosis reduces the number of cells required, helps the identification of the malignant cells and determines the degree of immunophenotypic heterogeneity of the malignant cell population. A few critical markers are sufficient to establish the lineage association in the majority of cases of acute and chronic leukemias and lymphomas. More extensive immunophenotyping can provide information about the cells' stage of differentiation, assess the expression of prognostically important features, and determine clonality. The identification of leukemia-associated immunophenotypes can be used for monitoring minimal residual disease during therapy. The presence of cells expressing these phenotypes in patients who are in clinical remission is associated with an increased risk of relapse.

The incidence of clonal T-cell receptor rearrangements in B-cell precursor acute lymphoblastic leukemia varies with age and genotype

B-cell precursor acute lymphoblastic leukemias (BCP-ALLs) are increasingly treated on risk-adapted protocols based on presenting clinical and biological features. Residual molecular positivity of clonal immunoglobulin (IG) and T-cell receptor (TCR) rearrangements allows detection of patients at an increased risk of relapse. If these rearrangements are to be used for universal follow-up, it is important to determine the extent to which they are informative in different BCP-ALL subsets. We show thatIGH V-D-J rearrangements occur in 89% of 163 BCP-ALL, with no significant variation according to age or genotype (BCR-ABL, TEL-AML1, MLL-AF4, and E2A-PBX1). In contrast,TCRG rearrangements, which occur in 60% of patients overall, are frequent in BCR-ABL and TEL-AML1, are less so in MLL-AF4, and are virtually absent in infants aged predominantly from 1 to 2 years and in E2A-PBX1 ALLs. Incidence of the predominant TCRD Vδ2-Dδ3 rearrangement decreases with age but is independent of genotype. These differences are not due to differential recombination activating gene activity, nor can they be explained adequately by stage of maturation arrest. Analysis of MLL-AF4 BCP-ALL is in keeping with transformation of a precursor at an early stage of ontogenic development, despite the adult onset of the cases analyzed. We postulate that the complete absence of TCRG rearrangement in E2A-PBX1 cases may result from deregulated E2A function. These data also have practical consequences for the use ofTCR clonality for the molecular follow-up of BCP-ALL.

The incidence of clonal T-cell receptor rearrangements in B-cell precursor acute lymphoblastic leukemia varies with age and genotype

B-cell precursor acute lymphoblastic leukemias (BCP-ALLs) are increasingly treated on risk-adapted protocols based on presenting clinical and biological features. Residual molecular positivity of clonal immunoglobulin (IG) and T-cell receptor (TCR) rearrangements allows detection of patients at an increased risk of relapse. If these rearrangements are to be used for universal follow-up, it is important to determine the extent to which they are informative in different BCP-ALL subsets. We show thatIGH V-D-J rearrangements occur in 89% of 163 BCP-ALL, with no significant variation according to age or genotype (BCR-ABL, TEL-AML1, MLL-AF4, and E2A-PBX1). In contrast,TCRG rearrangements, which occur in 60% of patients overall, are frequent in BCR-ABL and TEL-AML1, are less so in MLL-AF4, and are virtually absent in infants aged predominantly from 1 to 2 years and in E2A-PBX1 ALLs. Incidence of the predominant TCRD Vδ2-Dδ3 rearrangement decreases with age but is independent of genotype. These differences are not due to differential recombination activating gene activity, nor can they be explained adequately by stage of maturation arrest. Analysis of MLL-AF4 BCP-ALL is in keeping with transformation of a precursor at an early stage of ontogenic development, despite the adult onset of the cases analyzed. We postulate that the complete absence of TCRG rearrangement in E2A-PBX1 cases may result from deregulated E2A function. These data also have practical consequences for the use ofTCR clonality for the molecular follow-up of BCP-ALL.

Quantitative multiparametric immunophenotyping in acute lymphoblastic leukemia: correlation with specific genotype. I. ETV6/AML1 ALLs identification

The t(12;21)(p13;q22) fusion gene is the most frequent genetic lesion described in precursor B cell acute lymphoblastic leukemia (ALL) of childhood occurring in a quarter of cases. This gene rearrangement is associated with a good outcome presenting a high response rate to chemotherapy. In spite of its potential clinical relevance, the t(12;21) translocation usually goes undetected with conventional cytogenetic procedures. In the present study we utilized an objective flow cytometric approach (multiparametric quantitative analysis) for the phenotypic characterization of this type of ALL. We studied a total of 74 precursor B-ALL children, including 21 t(12;21)+ and 53 t(12;21)- cases. Our results show that the t(12;21)(p13;q22)+ ALLs display a higher intensity of CD10 (P = 0.0016) and HLADR (P = 0.005) expression together with lower levels of the CD20 (P = 0.01), CD45 (P = 0.01), CD135 (P = 0.003) and CD34 (P = 0.03) antigens as compared to the t(12;21) cases. Moreover, as regards CD34 expression, we observed a more heterogeneous antigen expression within individual patients with higher coefficients of variation (median of 202 vs 88, P = 0.0001). A multi-variate analysis disclosed that with the immunophenotypic approach used identification of t(12;21)+ cases can be achieved with a sensitivity of 86% and a specificity of 100%. We conclude that childhood precursor B-ALL carrying the t(12;21) translocation display characteristic phenotypic features which could provide a rapid, simple, sensitive and specific screening method to select for those cases that should undergo confirmatory molecular analysis.

An investigation of the t(12;21) rearrangement in children with B-precursor acute lymphoblastic leukaemia using cytogenetic and molecular methods

The t(12;21) is the commonest recurrent translocation in childhood acute lymphoblastic leukaemia (ALL), the presence of which has been suggested to be a good prognostic feature. We have studied 22 childhood cases of B-precursor ALL with this rearrangement, and have found no significant differences in event-free survival between these and a control group of patients with similar phenotypes. Using a variety of cytogenetic and molecular techniques, we have confirmed a strong association with co-expression of myeloid markers, frequent deletions of the short-arm of the untranslocated chromosome 12 homologue and duplication of the derivative chromosome 21. Intragenic deletion of the untranslocated ETV6 gene in 3/12 informative patients points to the likelihood of this gene being a target for deletion.

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.