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From the Departments of Pathology and Laboratory Medicine, Tumor Cell Biology, Hematology-Oncology, Biostatistics, and Genetics, St Jude Children's Research Hospital, Memphis; and the Department of Pathology, and the Division of Hematology-Oncology, the Department of Pediatrics, University of Tennessee, Memphis, College of Medicine, Memphis, TN.
Although abnormalities involving the short arm of chromosome 12 (12p) are one of the most frequently observed rearrangements in childhood acute lymphoblastic leukemia (ALL), little is known about the frequency of different structural abnormalities and their relationship to the status of the ETV6 (also named TEL) gene in this region. Of 815 children with newly diagnosed ALL, 94 (11.5%) had a total of 104 cytogenetic 12p abnormalities. Loss of genetic material was observed in 67 (64%) of these abnormalities. Cases with 12p alterations had a much lower frequency of hyperdiploidy greater than 50 (7%) than did the ALL population in general, but these cases had a similar distribution of immunophenotype and similar 5-year event-free survival (70% ± 5% SE v 64% ± 2%, P = .64). Rearrangement of the ETV6 gene was identified in 36 (56%) of 64 cases evaluated. The ETV6-CBFA2 (TEL-AML1) fusion transcript was found in 25 (66%) of 38 cases evaluated, and all but one of these showed ETV6 rearrangement. Importantly, ETV6 rearrangement was associated with a favorable prognosis (5-year event-free survival: 89% ± 6% v 60% ± 1%, P < .01). We conclude that most but not all 12p cytogenetic abnormalities in childhood ALL involve ETV6, and that rearrangement of ETV6 is associated with a favorable treatment outcome.
CHROMOSOME 12p abnormalities are identified by conventional cytogenetics studies in 8% to 11% of acute lymphoblastic leukemia (ALL) cases.1-3 Most 12p abnormalities involve translocations, such as dic(9; 12) (p11; p11),4-6 dic(7; 12)(p11; p11-12),6,7 t(12; 13)(p13; q14),6-8 t(2; 12)(q14; p13),1,6 and t(12; 17)(p13; q21).9 Approximately one fourth of the 12p abnormalities are deletions.3
Most ALL cases with a 12p abnormality have a B-lineage immunophenotype, although a few T-lineage cases have been reported.1,2 Abnormalities of 12p, most notably the dic(9; 12)(p11; p12), are generally associated with a favorable prognosis,5,10,11 although one report described an increased risk of central nervous system (CNS) relapse.2
ETV6, also known as TEL, is one of the gene(s) involved in 12p abnormalities, and has recently drawn intense investigation.12,13 This gene encodes an ETS-like putative transcription factor and was initially identified by its fusion with the platelet-derived growth factor receptor beta (PDGFR The frequency of ETV6 involvement in pediatric ALL with 12p abnormalities has not been described, nor has its effect, if any, on clinical characteristics and prognosis. To address these issues, we studied the largest cohort of patients with 12p abnormalities reported to date.
Patients
Cytogenetic Evaluation
Blast Cell Phenotyping Blast cell surface antigens were detected by standard indirect immunofluorescence assays with monoclonal antibodies to lymphoid-associated antigens. Blast cells were also tested for surface and cytoplasmic immunoglobulins (sIg and cIg) and for formation of heat-stable rosettes with sheep erythrocytes. Depending on their reactivity patterns, cells were classified as T (CD7+, CD5+, E-rosette±), B (sIg+), pre-B (cIg+), early pre-B (cIg-, sIg-, T-, HLA-DR+, CD19+, CD10±), or common B-lineage (sIg-, CD19+, DR+, CD10+, but unknown cIg status).31Southern Blot Analysis DNA was extracted from frozen marrow samples obtained at diagnosis, using standard techniques.22 For Southern analysis, 5 to 10 µg of DNA were digested with the restriction enzymes BamHI and HindIII, size-fractionated on a 0.8% agarose gel, and blotted onto nylon membranes. All blots were hybridized with a 32P-labeled probe consisting of the 0.6-kb Sac I/BamHI fragment of the ETV6 cDNA.22Reverse Transcriptase Polymerase Chain Reaction (RT-PCR) RNA was extracted (Purescript kit; Gentra, Research Triangle, NC) and RT-PCR performed as previously described.32 Samples were analyzed for the presence of both derivative fusion products using previously reported primer pairs.22 Following amplification, PCR products were separated by electrophoresis, transferred to nylon membrane, and hybridized to 32P-end-labeled oligonucleotide probes specific for CBFA2 or ETV6. All cases apparently negative for the der(21) products were reamplified using a more 5' oligonucleotide, as described.22FISH Of the 25 cases with germline ETV6 by Southern analysis, only 11 cases (Nos. 11, 12, 16, 26, 45, 49, 57, 58, 68, 74, and 83) had frozen cells/cytogenetics pellets available for FISH analysis; 4 (Nos. 45, 57, 58, and 68) exhibited cytogenetic evidence of 12p13 involvement. Three cases with ETV6 rearrangement (Nos. 42, 46, and 69) were also subjected to the FISH assay as positive controls, and four normal donors were subjected as negative controls.Statistical Analysis Event-free survival was estimated by the method of Kaplan and Meier,34 with standard errors calculated by the method of Peto and coworkers.35 Statistical comparisons of event-free survival distributions were made by the log-rank test, stratified by study.36 The distributions of categorically defined presenting features were compared between the groups with and without ETV6 rearrangement using Fisher's exact test.
Cytogenetic Findings Table 1 lists the clinical and cytogenetic features of the 94 cases with 12p abnormalities. The modal number was 45 in 15 cases (16%), 46 in 54 (57%), 47 to 50 in 18 (19%), and greater than 50 in 7 (8%). Aberrations affecting 12p were the sole chromosomal abnormality in 21 of the cases (22%). The other 73 cases had additional aberrations comprising mainly random numerical and structural chromosomal abnormalities; the few recurrent aberrations included deletions in 6q (n = 7), 9p (n = 5), 11q (n = 5), and 13q (n = 4). Four cases had common recurrent translocations other than those involving the 12p region: a pre-B case with a der(19)t(1; 19)(q21; p13) (patient 56), an early pre-B case with a t(17; 19)(q22; p13) (patient 88), and two T-cell cases, one with a t(10; 14)(q24; q11) (patient 11) and one with a t(7; 14)(p15; q32) (patient 83).
Detection of ETV6 Rearrangements and ETV6-CBFA2 Fusion ETV6 rearrangements were detected by Southern blot analysis in 36 (56%) of the 64 cases with 12p abnormalities for which frozen cells were available for testing. Of the 38 available cases evaluated by RT-PCR, 25 (66%) expressed the ETV6-CBFA2 chimeric transcript. Tables 1 and 2 list the frequency and correlation of ETV6 gene rearrangements and ETV6-CBFA2 fusion transcripts among the subgroups of 12p abnormalities. ETV6-CBFA2 chimeric transcript was undetected in only one (No. 52) of the 25 cases with ETV6 rearrangement that were tested. Notably, 12 of the 25 tested cases with an assigned breakpoint at 12p13 showed no ETV6 rearrangements.
FISH All 11 cases with germline ETV6 as well as 4 normal controls showed two red/green signals by FISH, indicating the lack of ETV6-CBFA2 fusion. Notably, case No. 74, who was shown to be positive for the fusion transcript by RT-PCR but lacked rearrangement by Southern, was among the negative cases by FISH analysis. Three positive controls (Nos. 42, 46, and 69) showed the ETV6 rearrangement by all three methods (Southern, RT-PCR, and FISH [split signals]).Clinical Characteristics and Outcome In the 94 children with 12p abnormalities, the median age was 4.7 years (range, 0.64 to 18.2 years) and the median initial leukocyte count was 14.3 × 109/L (range, 1.2 to 357 × 109/L). The only infant studied (patient 76) presented with a dic(9; 12), a 6 × 109/L leukocyte count, a pre-B phenotype, and ETV6-CBFA2 fusion transcript; this patient is alive, 36 months postdiagnosis.
Our pediatric ALL population had an 11.5% rate of leukemic cell 12p rearrangement by conventional cytogenetics. The frequency and heterogeneity of these chromosomal abnormalities suggest that the 12p region is subjected to diverse genetic alterations that contribute to the leukemogenic process in childhood ALL. In the 94 cases that had 12p changes, a total of 104 structural chromosomal abnormalities were observed in the 12p11-p13 region, including translocations (n = 69), deletions (n = 29), inversions (n = 5), and isochromosome 12q (n = 1). The 69 translocations involved 20 different reciprocal partner chromosomes and 43 distinct breakpoints. The most frequent exchanges involved 9p11 (n = 8); 7p11 (n = 4); 1q22-23 and 13q14 (n = 3); and 2q21, 4q21, 8p21, and 10q22 (n = 2 each). The sex chromosomes and chromosomes 19 and 20 were not involved.
Submitted October 16, 1996;
accepted July 31, 1997.
The publication costs of this article were defrayed in part by page
charge payment. This article must therefore be hearly marked
``advertisment'' in accordance with 18 U.S.C. section 1734 solely to
indicate this fact. We thank the cytogenetic technologists for their excellent technical assistance; M. Griffith, W.P. Conn, W.K. Williams, and A. Curcio-Brint for technical expertise; R. Seshadri for statistical programming; and S. Naron for editorial assistance.
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