Investigations on the role of SLP-65 and other B cell signaling proteins in human leukemia
SLP-65 is an adaptor protein preferentially expressed in B cells with a central role in BCR and pre-BCR mediated signal transduction. By providing multiple binding sites for other cellular effector molecules it assembles various proteins into one signaling complex. Therefore SLP-65 is involved in the control of different signaling pathways important for B cell development, survival and activation. SLP-65 KO mice demonstrated the critical role of SLP-65 in B cell development. Based on the fact that 5-10 of the SLP-65 KO mice develop pre-B cell leukemia most similar to childhood precursor B cell acute lymphoblastic leukemia, SLP-65 was analyzed in human tumor samples. SLP-65 expression was investigated by RT-PCR, immunoblotting and flow cytometry in 38 samples of precursor B acute lymphoblastic leukemia. In half of the cases SLP-65 protein was reduced or absent. Interestingly, aberrant SLP-65 mRNA splicing was detected in the tumor samples. Alternative exons containing pre-mature stop codons were included in the aberrantly spliced SLP-65 mRNA. These exons were shown to contain pre-mature stop codons leading to nonsense mediated decay of the aberrantly spliced SLP-65 mRNA. In addition, a genetic polymorphism involved in mediating exon recognition was identified. Three of ten patients were deficient for the transcription factor pax-5 involved in regulating SLP-65 expression and one of those patients had aberrant pax-5 mRNA’s lacking the sequences for the transactivation domain. Thus, loss of pax-5 expression could be another possibility to explain the SLP-65 deficiency in addition to aberrant splicing. Our data indicate that a block in pre-B cell differentiation, due to a somatic deficiency in key signaling elements combined with proliferation capacity and active RAG 1/2 enzymes may be the primary cause of pre-B ALL.
SLP-65, BLNK, Adaptorprotein, BSAP, pax-5, acute lymphoblastic leukemia, B lymphocyte, tumorsuppressor
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