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Clinical relevance of MYD88 mutations in B-NHL
constitutively recruits IRAK1 for the myddosome and, together with IRAK4, was found to be essential for sur- vival of activated B-cell (ABC) diffuse large B-cell lym- phoma (DLBCL) cell lines with MYD88(L265P).2,7,8 In addition, IRAK1 was shown to be co-immunoprecipitat- ed with MYD88 in chronic lymphocytic leukemia (CLL) cells with MYD88(L265P) and stimulation of IL-1R and TLR induced a 5-fold to 150-fold increase of cytokine secretion compared to that of CLL cells with wildtype MYD88.9 However, Ansell et al.7 identified that in Waldenström macroglobulinemia (WM) cell lines, the myddosome complex consisted of IRAK4, TRAF6, and MYD88, but not IRAK1. The authors hypothesized that this difference in complex formation was instigated by the heterozygous nature of MYD88(L265P) in WM and the homozygous nature in DLBCL, which was strength- ened by the finding that downstream signaling of TAK1 phosphorylation was highest in the DLBCL cell line with homozygous MYD88(L265P).7 Furthermore, the stabiliz- ing effect of heat shock protein 110 (HSP110) on the myd- dosome complex, due to interference with the proteaso- mal degradation of MYD88, is stronger in ABC-DLBCL cell lines with MYD88(L265P) than in those with wild- type MYD88.10 As MYD88(L265P) constitutively activates the NF-κB pathway, it is regarded as an important onco- genic driver in B-NHL.2,7-12
B-cell receptor signaling
release of NF-κB subunits.4,5,14,15
BTK is an integral protein in the BCR signaling cascade
and has been found to be preferentially complexed to MYD88 in WM cells with MYD88(L265P) and not in MYD88 wildtype cells. Inhibition of BTK resulted in a decrease of the formation of this MYD88-BTK complex, but lacked effect on IRAK4/IRAK1 activity and vice versa, indicating a potential necessity of dual inhibition of IRAK and BTK for WM with MYD88(L265P).16-18 MYD88 is fre- quently mutated in patients who also harbor a mutation in the 196 tyrosine residue in the ITAM domain of CD79B (NM_000626) and these patients seem to benefit most from BTK-inhibition treatment.19 The exact conse- quence of these double mutations in B-NHL is unclear, but Phelan et al.8 recently provided new insight into the mechanism of combined MYD88 and BCR-pathway acti- vation as they identified a MYD88-TLR9-BCR (My-T- BCR) supercomplex. This supercomplex is generated by constitutive trafficking of the BCR towards endolyso- somes that contain TLR9 and interacts with mTOR and the CBM complex, thereby promoting lymphomagenesis by activation of the mTOR and NF-κB pathways. Its pres- ence was demonstrated in cell lines and biopsies of ABC- DLBCL, primary DLBCL of the central nervous system, and lymphoplasmacytic lymphoma and correlated with responsiveness to BTK inhibition. On the other hand, the supercomplex was not identified in CLL or mantle cell lymphoma, suggesting a different mechanism of BCR sig- naling in these entities. Therefore, the My-T-BCR super- complex could potentially be used as a biomarker for pre- dicting the efficacy of BTK inhibitors, as a classifier of B- NHL subtypes, or as a novel therapeutic target via inhibi- tion of TLR9.8
Autocrine signaling
As described, increased formation of the myddosome complex with IRAK1, as well as activation of the BCR pathway, caused by interactions of BTK with MYD88(L265P), CD79B mutations, and the My-T-BCR supercomplex, result in constitutive activation of the NF- κB pathway. NF-κB not only activates the transcription of genes involved in cell survival and proliferation, but also results in autocrine signaling with IL-6 and IL-10. One consequence of this autocrine signaling loop is the phos- phorylation of Janus kinase 1 (JAK1) and, subsequently, signal transducer and activator of transcription 3 (STAT3) with the assembly of a STAT3/STAT3 complex. This complex increases transcription of genes involved in sev- eral signaling cascades, including the PI3K/AKT/mTOR, E2F/G2M cell-cycle checkpoint, JAK/STAT, and NF-κB pathways. In addition, STAT3 activity represses the pro- apoptotic type I interferon (IFN) signaling pathway by downregulating IFN-regulatory factor 7 (IRF7), IRF9, STAT1, and STAT2 expression.2,3,20
Another consequence of IL-6 signaling is the aberrant expression of hematopoietic cell kinase (HCK), as identi- fied in primary WM cells and B-NHL cell lines.21 Increased levels of HCK promote lymphomagenesis, as HCK knock- down in B-NHL cell lines reduces survival and lowers the activity of the BCR, PI3K/AKT, and MAPK/ERK (extracel- lular signal-regulated kinases) pathways. Furthermore, BTK- and HCK-inhibition treatment of ABC-DLBCL and WM cells with MYD88(L265P) decreased HCK expres- sion, whereas mutant HCK(T333M) (NM_002110.4) attenuated this effect. These findings suggest that HCK is
In addition to the canonical NF-κB pathway, the BCR pathway plays an important role in B-cell survival and proliferation and oncogenesis of B-NHL with MYD88 mutations (Figure 1). In normal physiology, stimulation of the BCR activates NF-κB, as well as the phosphoinositide 3-kinase (PI3K)/AKT/mammalian target of rapamycin (mTOR), and nuclear factor of activated T cells (NFAT) pathways. After antigen recognition by the BCR, Lck/Yes-related novel protein tyrosine kinase (LYN) is released from its inactive state through dephosphoryla- tion of the C-terminal regulatory tyrosine by cluster of differentiation 45 (CD45) or an exogenous ligand for the Src-homology 2 (SH2) and SH3 domains of LYN, such as CD19. Activated LYN consecutively phosphorylates the immunoreceptor tyrosine-based activation motif (ITAM) domains of the coupled CD79A and CD79B het- erodimers. These double-phosphorylated ITAM domains provide a docking site for the SH2 domains of spleen tyrosine kinase (SYK), which is activated by autophos- phorylation or through transphosphorylation by LYN. LYN and SYK then activate Bruton tyrosine kinase (BTK) by phosphorylation, which is recruited to the membrane through interaction between the pleckstrin homology (PH) domain of BTK and phosphatidylinositol-3, 4, 5- triphosphate (PIP3) of the PI3K pathway or through inter- action between the SH2 domain of BTK with the B-cell linker protein (BLNK) adapter molecule that also recruits phospholipase Cg2 (PLCg2) to the membrane.13 BTK acti- vates PLCg2, initiating activation of the NF-κB pathway through formation of CBM complex, consisting of cas- pase recruitment domain family member 11 (CARD11), BCL10, and mucosa-associated lymphoid tissue lym- phoma translocation protein 1 (MALT1). In addition, BTK activates the MAPK and PI3K pathways14 and PLCg2 trig- gers the NFAT pathway through calcineurin. The CBM complex subsequently attracts TRAF6, TAK1, and TAB, and promotes the degradation of IκB, which leads to the
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