Southern blot analysis revealed no rearrangement in Mbcr1, and direct sequencing of the PCR product confirmed it to be the ALL-type mbcr1 fusion mRNA with the first exon of the BCR gene fused to ABL exon a2.
Alternative chimeric proteins, p210BCR-ABL and p190BCR-ABL, are produced that are characteristic of chronic myelogenous leukemia and acute lymphoblastic leukemia, respectively.
This genomic structure is of interest because of its analogy to the organization of the ABL gene and because this part of the gene is not affected by the breakpoints occurring in Ph1-positive acute lymphoblastic leukemia (ALL).
Analysis of the functions of these new molecules may provide insight into mechanisms by which oncogenic abl proteins participate in the etiology of CML and ALL.
Two-thirds of patients with Philadelphia (Ph) chromosome-positive acute lymphoblastic leukaemia (ALL) have a breakpoint in the minor breakpoint cluster region (m-bcr) of the BCR gene, which results in an e1a2 transcript and a P190BCR-ABL fusion protein.
We have now used the same technique, reverse transcription/polymerase chain reaction amplification of ABL-BCR transcripts, to study nine patients with Philadelphia (Ph) chromosome-positive acute lymphoblastic leukemia (ALL); seven expressed the P190 and two the P210 type of BCR-ABL fusion protein.
In the present study, three chronic myelogenous leukemia (CML) patients with variant Philadelphia (Ph) chromosomes (complex types), two CML patients with a masked Ph, one case with Ph positive acute lymphocytic leukemia (ALL), and one with Ph positive acute myelocytic leukemia (AML) were analyzed by standard cytogenetic techniques (G-banding), Southern blot studies, and fluorescence in situ hybridization (FISH) procedures using probes from portions of the bcr and abl genes.
Approximately five percent of pediatric acute lymphoblastic leukemias (ALL) contain a translocation (9;22)(q34;q11) which results in rearrangement of the bcr and abl genes.
We therefore sought evidence for analogous point mutations in the ABL gene in patients with Ph-negative, BCR-negative CML (n = 25), Ph-negative ALL (n = 18) and in Ph-positive CML in transformation (n = 28).
In Philadelphia chromosome (Ph1)-positive leukemias such as chronic myelogenous leukemia (CML) and Ph1-positive acute lymphoblastic leukemia (ALL), both of which express bcr-abl fused gene products (P210bcr-abl or P190bcr-abl protein kinase) with augmented tyrosine kinase activities, herbimycin A markedly inhibited the in vitro growth of the Ph1-positive ALL cells and the leukemic cells derived from CML blast crisis.
The ABL oncogene is consistently rearranged and activated as a consequence of the translocation t(9;22) that gives rise to the Philadelphia chromosome in chronic myeloid leukemia and in some cases of acute lymphoblastic leukemia.
The findings suggested two distinct subtypes of ALL: one defined by t(9;22)(q34;q11) and expression of P185BCR-ABL tyrosine kinase and one with variant karyotypes and no P185BCR-ABL expression.
The role of ABL on the Philadelphia chromosome in acute lymphoblastic leukemia is only now beginning to be understood, but is likely to be similar, and a new ABL species has already been identified by several groups.
Thus, the order of loci on chromosome 22 is centromere----BCR2, BCR4, and IGL----BCR1----BCR3----SIS, possibly eliminating BCR2 and BCR4 loci as candidate targets for juxtaposition to the ABL gene in the acute lymphoblastic leukemia Ph1 chromosome.