Frequent Loss of the SLP-65 Adapter Protein in Childhood Acute B-Precursor Lymphoblastic Leukemia (ALL) with TEL/AML1 Rearrangement.

The adaptor protein SLP-65 plays an essential role during B cell differentiation. A crucial consequence of SLP-65 deficiency in mice is a high incidence of pre-B-cell leukemia, suggesting a tumor suppressor role for SLP-65 in pre-B-cells. While the link between SLP-65 deficiency and leukemia development is established in mice, experiments mainly using microarrays for gene expression profiling suggested normal expression of SLP-65 in human precursor B-cell ALL. This analysis however does not discriminate between normal and aberrant SLP-65 transcripts with the latter being unable to generate functional protein. To examine the correlation between SLP-65 deficiency and childhood precursor B-cell ALL, we determined SLP-65 expression in 119 precursor B-cell ALL samples by both RNA and protein methods. The expression of SLP-65 was compared to clinical and laboratory findings, cytogenetics as well as to the outcome data within this uniformly treated cohort of patients. Loss of slp-65 protein was significantly associated with the occurrence of the TEL/AML1 rearrangement (p=0.026) but not with any other clinical or cytogenetic feature. We found a profound disconnection between slp-65 mRNA and protein expression in 38 out of the 119 leukemic samples pointing to a posttranscriptional regulation of slp-65 (Table). To confirm that SLP-65 transcript expression does not automatically correlate with its protein expression, we analyzed a panel of human cell lines derived from precursor B-cell ALL patients. The cell lines HPB-NULL and BV-173 showed a deficiency in SLP-65 protein expression, although SLP-65 transcripts can easily be detected in both lines. Together, the data suggest that SLP-65 expression might be regulated at the posttranscriptional level and that the presence of SLP-65 transcripts does not necessarily lead to SLP-65 protein and function. In one particular patient, we found a truncated slp-65 transcript and the predicted slp-65 protein lacks its SH2 domain. We tested whether this SLP-65 protein lacking the SH2 domain is functional in pre-B cells. To this end, we transfected murine SLP-65 −/− pre-B cells with retroviral constructs for either wild-type (wt SLP-65) or truncated SLP-65 (SLP-65delSH2) and analysed pre-BCR downregulation, Ca2+ release and pre-B cell differentiation. The results showed that, in contrast to wt SLP-65, SLP-65delSH2 failed to induce any effects in the performed experiments. Together with previous findings showing that SLP-65-deficient mice develop pre-B cell leukemia, the data suggest that SLP-65 acts as a tumor suppressor that limits pre-B cell proliferation by inducing differentiation.