Review on BLNK (B-cell linker), with data on DNA, on the protein encoded, and where the gene is implicated

Hendriks, RW

Yuvaraj, S

Atlas of Genetics and Cytogenetics in Oncology and Haematology

I-Revues

 
 
 
   
Gene Section 
Review 
 
Atlas Genet Cytogenet Oncol Haematol. 2013; 17(2) 106 
INIST-CNRS 
 
OPEN ACCESS JOURNAL 
Atlas of Genetics and Cytogenetics 
in Oncology and Haematology 
BLNK (B-cell linker) 
Saravanan Yuvaraj, Rudi W Hendriks 
Department of Pulmonary Medicine, Erasmus MC, Rotterdam, The Netherlands (SY, RWH) 
 
Published in Atlas Database: August 2012 
Online updated version: http://AtlasGeneticsOncology.org/Genes/BLNKID804ch10q24.html 
DOI: 10.4267/2042/48492 
This work is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 2.0 France Licence. 
© 2013 Atlas of Genetics and Cytogenetics in Oncology and Haematology 
 
Identity 
Other names: AGM4, BASH, bca, BLNK-S, LY57, 
SLP-65, SLP65 
HGNC (Hugo): BLNK  
Location: 10q24.1 
Note 
BLNK is a cytoplasmic linker or adaptor protein that 
plays a critical role in B cell development in human 
and mice. 
DNA/RNA 
Description 
The gene spans over 79 kb in human and is encoded 
in 17 exons on the long arm of chromosome 10. 
Transcription 
The transcript is about 1829 bps in human and about 
2097 in mice and UTR regions are present in exons 1, 
2 and 17. 
Protein 
Description 
BLNK is a 65 kDa protein of 456 amino acids. 
Translation of the BLNK begins at the ATG site 
located in exon 2 and ends at exon 17. The amino-
terminal region of BLNK is rich in tyrosines and 
contains a highly conserved leucine zipper that is 
responsible for the recruitment of BLNK to the 
plasma membrane. The mid-portion of the molecule 
contains a proline-rich region that has efficient SH3-
domain binding properties. Finally the C-terminus 
consists of an SH2-domain that allows interaction 
with other tyrosine phosphorylated proteins. 
Phosphorylated BLNK provides docking sites for 
several proteins that induces Ig L chain 
recombination and differentiation. 
Expression 
BLNK is expressed exclusively in hematopoietic 
cells, mainly in B cells and myeloid cells. 
Localisation 
It has been shown that the highly conserved N 
terminus of BLNK comprises a domain for 
constitutive membrane association, which is essential 
for its function. Membrane association depends on a 
leucine zipper motif in the N-terminal domain. The 
BLNK N-terminus can be functionally replaced by a 
heterologuous membrane localization signal and can 
be transferred to other proteins to confer membrane 
localization. To connect the pre-B cell receptor (pre-
BCR) or the BCR with downstream signaling 
elements, BLNK must be localized at the plasma 
membrane where these receptors reside. A 
stimulation-independent and constant association of 
BLNK with the Cbl-interacting protein of 85 kDa 
(CIN85) is requisite for BLNK phosphorylation. 
Function 
BLNK lacks intrinsic enzymatic function, but 
regulates the assembly and localization of signaling 
complexes. Such molecules are known as adapters or 
linkers and have the capacity to regulate the 
availability of a substrate to an enzyme and create a 
scaffold for bridging signaling cascades. 
Phosphorylated BLNK (by the spleen tyrosine kinase 
SYK) provides docking sites for several proteins 
including the BTK, GRB2, VAV, NCK and PLCγ2 
(Figure 1). SYK and its substrate BLNK are proximal 
signal transducer elements of the BCR. When the in 
vivo interactome of BLNK in resting and stimulated 
B cells was determined by mass spectrometry, it was 
found that BLNK orchestrated a complex signal 
network of ~30 proteins that was predominantly 
based on dynamic interactions. 
Pre-BCR signaling leads to activation of SYK, which 
then phosphorylates BLNK and thereby regulates pre-
BLNK (B-cell linker) Yuvaraj S, Hendriks RW 
 
 
 
 
 
Atlas Genet Cytogenet Oncol Haematol. 2013; 17(2) 107 
B cell differentiation (Figure 1). BLNK is required 
for the downregulation of the pre-BCR surrogate light 
chain component λ-like (λ5 in mice), thereby 
terminating pre-BCR expression. It has been shown 
in mice that Blnk-mediated signals induces activation 
of Ig κ light chain recombination by induction of the 
expression of the transcription factors Aiolos, Irf-4, 
Foxo3a and Foxo1. Deficiency of Blnk in mice 
results in an increased proliferative response of pre-B 
cells to IL-7 in vitro, indicating that Blnk is crucially 
involved in the termination of IL-7 driven 
proliferation. BLNK inhibits IL-7R signaling through 
direct binding to Jak3 in a Btk-independent manner 
and also by downregulating IL-7R expression (Figure 
1). In the absence of Btk mainly Ig λ L chain 
germline transcription and recombination is impaired, 
whereas in Btk/BLNK double-deficient pre-B cells, 
both κ and λ L chain germline transcripts are severely 
reduced. These data suggests a dual role of BLNK in 
the downregulation of surrogate light chain 
expression and the activation of Igκ germline 
transcription. 
Homology 
The SLP76 family of adapters has three members 
namely SLP76, BLNK (SLP65) and Clnk/MIST. 
Implicated in 
Pre-B cell tumors 
Disease 
Deficiency of BLNK has been found in ~50% of 
childhood precursor-B cell acute lymphocytic 
leukemia (ALL) cases. The loss of BLNK protein 
was found to be due to defective splicing, leading to 
premature stop codons. Moreover, in precursor-B 
ALL that where positive for the BCR-ABL 
translocation the activity of the fusion protein, the 
BCR-ABL1 kinase, was linked to the expression of 
the same aberrant BLNK transcripts. Because other 
expression profiling studies with a large number of 
patients reported a low frequency it is not clear 
whether the loss of BLNK is a common 
leukemogenic event. Nevertheless, the reported 
findings indicate that loss of BLNK and the 
accompanying pre-B cell differentiation arrest may be 
one of the primary causes of precursor-B ALL. In a 
study of 28 pre-B ALL and 27 B cell lymphomas, 
BLNK expression was found to be defective in seven 
and five cases, respectively. BLNK deficiency 
correlated with RAG1/RAG2  
expression and unremitting VH gene rearrangement 
activity BLNK deficiency resulted in perpetual V(D)J 
recombinase activity in pre-B-lymphoblastic 
leukemia and B-cell lymphoma cells. 
Blnk-deficient mice spontaneously develop pre-B cell
leukemia with a similar phenotype as found in  
humans. In contrast, mice deficient for Btk, do not 
develop pre-B cell leukemia. Nevertheless, Btk and 
Blnk cooperate as tumor suppressors whereby Btk 
exerts its tumor suppressor function independently of 
its kinase activity. Combined deficiency of Blnk and 
Btk result in a more complete arrest at the pre-B cell 
stage and a higher incidence of pre-B cell leukemia 
than compared with single deficient mice, suggesting 
that the developmental block is one of the tumor-
promoting factors.  
In addition, expression of the pre-BCR is also 
essential for the development of leukemia, because 
mice which are Rag-deficient or which cannot 
express the Ig µ H chain on the surface (e.g. becaus  
of deficiency of the pre-BCR component λ5 or due to 
a disruption of the membrane exon of the Ig µ H 
chain constant region) are arrested at the pro-B cell 
stage, but do not develop leukemia. Furthermore, 
Blnk has a specific function that suppresses 
malignant transformation because in Btk/Plcγ2 and 
Irf-4/Irf-8 double-deficient mice with a nearly 
complete arrest at the large pre-B cell stage no 
leukemias have been reported. The absence of Blnk 
results in sustained expression of the pre-BCR and 
the IL-7R in large cycling pre-B cells (Figure 1). 
Since IL-7R signaling induces c-Myc this results in 
constitutively high levels of c-Myc. At this stage, 
Foxo transcription factors that normally suppress c-
Myc-driven lymphomagenesis via direct activation of 
p19 Arf are not properly activated because of the 
absence of BLNK. Furthermore, IL-7R signaling and 
aberrant pre-BCR signaling represses Bcl6 and 
thereby induces p19Arf expression. Subsequently, 
Blnk-deficient pre-B cells acquiring sporadic 
alterations in the p19Arf-Mdm2-p53 are selected to 
become malignant. Furthermore, expression of p19Arf  
in Blnk-deficient leukemias possibly promotes 
survival under metabolic stress. The Jak3/Stat5 
signaling pathway is constitutively activated in pre-B 
leukemia cells derived from Blnk-deficient mice, 
mostly due to autocrine production of IL-7.  
In the leukemia cells, exogenously expressed Blnk 
inhibited autocrine Jak3/Stat5 signaling, resulting  
p27(kip1) induction, cell-cycle arrest, and apoptosis. 
Blnk-inhibition of Jak3 was dependent on the binding 
of Blnk to Jak3. 
Btk/Slp65 double-deficient mice that carried a 
functional B cell receptor transgene (3-83µδ), 
developed B-cell leukemias expressing both the 3-
83µδ BCR and the pre-BCR components λ5/VpreB. 
Interestingly, these leukemias were found at similar 
frequencies in mice concomitantly deficient for Rag1 
or the non-homologous end-joining factor DNA-
PKcs. It was therefore concluded that malignant 
transformation of Btk/Slp65 double-deficient pre-B 
cells is independent of deregulated V(D)J 
recombination activity in the mouse. 
 
BLNK (B-cell linker) Yuvaraj S, Hendriks RW 
 
 
 
 
 
Atlas Genet Cytogenet Oncol Haematol. 2013; 17(2) 108 
 
Figure 1. Function of Blnk in pre-BCR signaling in the mouse. A. Pre-BCR activation results in the activation of Syk tyrosine 
kinase, which together with the Src family kinase Lyn phosphorylates several downstream signaling molecules. This provides signals 
for proliferation and survival by PI3K-dependent activation of Pkb/Akt and by synergy with the MAP kinase pathway downstream of 
the IL-7R. In addition, pre-BCR signaling also leads to Blnk-Btk-PLCγ complex formation that is essential for downregulation of 
proliferative signals. Blnk acts as (1) a negative regulator of Jak3 and (2) through inhibition of Pkb/Akt activation. Activation of FoxO 
is associated with downregulation of surrogate light chain (SLC) and IL-7R expression and the activation of the Ig kappa locus for 
gene recombination. B. When in pre-B lymphoma cells Blnk expression is low or absent, proliferative signals are maintained because 
of (1) ongoing Jak/Stat signaling and (2) Pkb/Akt activation, leading to FoxO phosphorylation and degradation, thereby precluding 
proper maturation and downregulation of pre-BCR and IL-7R expression, resulting in Syk-dependent pre-B cell proliferation (3). 
BLNK (B-cell linker) Yuvaraj S, Hendriks RW 
 
 
 
 
 
Atlas Genet Cytogenet Oncol Haematol. 2013; 17(2) 109 
Other disease 
Note 
In human, mutations in SLP65 or BTK result in 
defective pre-B cell proliferation and an almost 
complete arrest of B cell development at the pro-B t  
pre-B cell transition, associated with the 
immunodeficiency disorder agammaglobulinemia. In 
contrast, mice deficient for Slp65 or Btk show only a 
partial arrest at the large cycling pre-B cell stage, 
while a nearly complete block is present in Btk/Slp65 
double deficient mice. 
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This article should be referenced as such: 
Yuvaraj S, Hendriks RW. BLNK (B-cell linker). Atlas Genet 
Cytogenet Oncol Haematol. 2013; 17(2):106-109. 


BLNK (B-cell linker)

S Yuvaraj

RW Hendriks

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BLNK (B-cell linker)

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