Cellular Expression of Antiapoptotic BCL-2 Oncoprotein in Newly Diagnosed Childhood Acute Lymphoblastic Leukemia: A Children's Cancer Group Study

Hypermethylation of the spleen tyrosine kinase promoter in T-lineage acute lymphoblastic leukemia

Sequence analysis of the noncoding first exon (exon 1) of the Syk gene demonstrated the presence of a previously cloned CpG island (GenBank #Z 65706). Transient transfection analysis in Daudi cells demonstrated promoter activity (18-fold increase over parental luciferase plasmid) for a 348 bp BstXI-BsrBI fragment containing this island. This region exhibits a high GC content (approximately 75%), contains several SP1 binding sites and a potential initiator sequence, but lacks a strong TATA consensus. Bisulfite sequencing and methylation-specific PCR (MSP) of this region demonstrated that the Syk promoter CpG island was largely unmethylated in B-lineage leukemia cell lines, control peripheral blood cells, human thymocytes and CD3(+) T lymphocytes. However, dense methylation was seen in four T-lineage leukemia cell lines, Jurkat, H9, Molt 3 and HUT 78. MSP screening of leukemia cells from six T-lineage acute lymphoblastic leukemia (ALL) patients demonstrated methylation of the Syk promoter CpG island in one T-lineage ALL patient. Promoter methylation was correlated with reduced to absent expression of Syk mRNA and SYK protein in the T-lineage leukemia cell lines. Treatment of the leukemia lines Ha and Molt 3, with the methylation inhibitor, 5-aza-2'-deoxycytidine (5-aza-CdR) resulted in increased Syk mRNA expression. The presence of a methylated promoter sequence in these T-lineage leukemia cell lines and in one T-lineage patient suggests a potential role for SYK as a tumor suppressor in T-ALL.

Spleen tyrosine kinase (Syk) deficiency in childhood pro-B cell acute lymphoblastic leukemia

The cytoplasmic spleen tyrosine kinase (SYK) is a key regulator of signal transduction events, apoptosis and orderly cell cycle progression in B-lineage lymphoid cells. Although SYK has not been linked to a human disease, defective expression of the closely related T-cell tyrosine kinase ZAP-70 has been associated with severe combined immunodeficiency. Childhood CD19(+)CD10(-) pro-B cell acute lymphoblastic leukemia (ALL) is thought to originate from B-cell precursors with a maturational arrest at the pro-B cell stage and it is associated with poor prognosis. Since lethally irradiated mice reconstituted with SYK-deficient fetal liver-derived lymphohematopoietic progenitor cells show a block in B-cell ontogeny at the pro-B to pre-B cell transition, we examined the SYK expression profiles of primary leukemic cells from children with pro-B cell ALL. Here we report that leukemic cells from pediatric CD19(+)CD10(-) pro-B cell ALL patients (but not leukemic cells from patients with CD19(+)CD10(+) common pre-pre-B cell ALL) have markedly reduced SYK activity. Sequencing of the reverse transcriptase-polymerase chain reaction (RT-PCR) products of the Syk mRNA in these pro-B leukemia cells revealed profoundly aberrant coding sequences with deletions or insertions. These mRNA species encode abnormal SYK proteins with a missing or truncated catalytic kinase domain. In contrast to pro-B leukemia cells, pre-pre-B leukemia cells from children with CD19(+)CD10(+) common B-lineage ALL and EBV-transformed B-cell lines from healthy volunteers expressed wild-type Syk coding sequences. Examination of the genomic structure of the Syk gene by inter-exonic PCR and genomic cloning demonstrated that the deletions and insertions in the abnormal mRNA species of pro-B leukemia cells are caused by aberrant splicing resulting in either mis-splicing, exon skipping or inclusion of alternative exons, consistent with an abnormal posttranscriptional regulation of alternative splicing of Syk pre-mRNA. Our findings link for the first time specific molecular defects involving the Syk gene to an immunophenotypically distinct category of childhood ALL. To our knowledge, this is the first discovery of a specific tyrosine kinase deficiency in a human hematologic malignancy.

Increased BAX Expression Is Associated With an Increased Risk of Relapse in Childhood Acute Lymphocytic Leukemia

Studies in cell lines have indicated that expression of the BCL-2 family of proteins is an important determinant of chemotherapy-induced apoptosis; however, the level of expression of these proteins in childhood acute lymphoblastic leukemia (ALL) has not been extensively reported. Using quantitative Western blotting we have determined the level of expression of BCL-2, BAX, MCL-1, and BCL-X in lymphoblasts from 47 children with ALL (33 at presentation only, 4 at relapse only, and 10 at both presentation and on relapse). Results were determined as a ratio to actin as an internal control. BCL-2, BAX, and MCL-1 were detected in all samples. BCL-XL was only detected in 6 cases (4 at presentation and 2 at relapse) and BCL-XS in none. No correlation was found between expression and white blood cell count, age at diagnosis, gender, or blast karyotype. BCL-2 levels and the BCL/BAX and MCL-1/BAX ratios were found to be significantly higher in B-lineage as compared with T-lineage disease (P < .003, .02, and .02, respectively). No consistent pattern of change in expression was noted in the 10 cases studied at both presentation and relapse. Kaplan-Meier analysis showed a significant correlation between high BAX expression and an increased probability of relapse (P< .05 by the log rank test), suggesting that chemosensitivity in leukemic blasts may be regulated by factors that override the BCL-2 pathway.