B Cell Linker Protein (BLNK) Is a Selective Target of Repression by PAX5-PML Protein in the Differentiation Block That Leads to the Development of Acute Lymphoblastic Leukemia

Abstract

PAX5 is a transcription factor that is required for the development and maintenance of B cells. Promyelocytic leukemia (PML) is a tumor suppressor and proapoptotic factor. The fusion gene PAX5-PML has been identified in acute lymphoblastic leukemia with chromosomal translocation t(9;15)(p13; q24). We have reported previously that PAX5-PML dominantnegatively inhibited PAX5 transcriptional activity and impaired PML function by disrupting PML nuclear bodies (NBs). Here we demonstrated the leukemogenicity of PAX5-PML by introducing it into normal mouse pro-B cells. Arrest of differentiation was observed in PAX5-PML-introduced pro-B cells, resulting in the development of acute lymphoblastic leukemia after a long latency in mice. Among the transactivation targets of PAX5, B cell linker protein (BLNK) was repressed selectively in leukemia cells, and enforced BLNK expression abrogated the differentiation block and survival induced by PAX5-PML, indicating the importance of BLNK repression for the formation of preleukemic state. We also showed that PML NBs were intact in leukemia cells and attributed this to the low expression of PAX5-PML, indicating that the disruption of PML NBs was not required for the PAX5-PML-induced onset of leukemia. These results provide novel insights into the molecular mechanisms underlying the onset of leukemia by PAX5 mutations.PAX5 is a member of the highly conserved paired box (PAX) 2 domain family of transcription factors. PAX5 is expressed exclusively from the pro-B to mature B cell stage and is downregulated during terminal differentiation into plasma cells (1). PAX5 is indispensable for B lineage commitment by the transcriptional activation of B lineage-specific genes (2), such as CD19 (3), CD79A (4), and B cell linker protein (BLNK) (5), and its target disruption has been shown to cause B lymphoid maturation arrest at the pro-B cell stage (6). Previous studies have identified the PAX5 gene as the most frequent target of somatic mutations in childhood and adult B-progenitor acute lymphoblastic leukemia (ALL), being altered in 38.9% and 34% of cases, respectively (7,8), and these findings further emphasized the essential role of PAX5 in the proper development of B cells. Somatic mutations consist of partial or complete hemizygous deletions, homozygous deletions, partial or complete amplifications, point mutations, or fusion genes (7). These aberrations in the PAX5 gene are considered to impair PAX5 function and play a role in blocking B cell differentiation. PAX5 fusion proteins such as PAX5-TEL, PAX5-ENL, PAX5-PML, and PAX5-C20S have been shown previously to have dominant-negative effects on PAX5 transcriptional activity and have been suggested to be mainly responsible for the differentiation disorder of ALL with these fusion genes (9 -12). Consistently, a previous study has reported that PAX5 haploinsufficiency cooperated with the constitutive activation of STAT5 to initiate ALL in mice (13). However, the oncogenicity of PAX5 mutations, including fusion genes, has yet to be demonstrated...

Leukemia multiclass assessment and classification from Microarray and RNA-seq technologies integration at gene expression level

“…On the other hand, BLNK gene encodes for a cytoplasmic adaptor that plays a critical role in B-cell development [60]. Deficiency in this protein has been identified in some cases of pre-B acute lymphoblastic leukemia [61, 62]. In fact, the somatic loss of BLNK and concomitant mutations lead to a constitutive activation of Jak/STAT5 pathway, resulting in the generation of pre-B-cell leukemia [63].…”

Screening and identification of key candidate genes and pathways in myelodysplastic syndrome by bioinformatic analysis

“…IFI44L and IRF4 have been introduced previously. BLNK (B Cell Linker Protein), which is a selective target of repression, can result in the development of ALL by differentiation block of PAX5-PML protein (Imoto et al, 2016). The results of Nakayama et al (2009) are consistent with this and somatic cell loss and mutation of BLNK lead to the generation of pre-B-cell leukemia.…”

Copy number profiling of adult relapsed B‐cell precursor acute lymphoblastic leukemia reveals potential leukemia progression mechanisms

“…In addition to IKZF1, deletions in other B cell development genes such as VPREB1 have been associated with a poor outcome . BLNK has shown to play important roles in tumor suppression and B cell differentiation in B‐ALL cell cultures and mouse models …”