Leukemia (2014) 28, 1799–1803 & 2014 Macmillan Publishers Limited All rights reserved 0887-6924/14 www.nature.com/leu CONCISE REVIEW MYD88 and beyond: novel opportunities for diagnosis, prognosis and treatment in Waldenstro¨m’s Macroglobulinemia O Landgren and N Tageja Waldenstro¨m’s Macroglobulinemia (WM) is a rare disease of the elderly with a median age of 63–68 years at diagnosis. Despite recent progress in biological insights and therapeutics, WM remains clinically challenging to diagnose and is difficult to manage with significant morbidity and lack of established curative therapies. Recently, the use of whole-genome sequencing has helped to identify a highly recurrent somatic mutation, myeloid differentiation factor 88 [MYD88] L265P in WM. This has fueled major interest in the field and as newer evidence accumulates, it is clear that that discovery of MYD88 L265P mutation may represent an important breakthrough in understanding the pathogenesis of WM and lymphoproliferative disorders. Recent scientific work in this field has also guided the identification of new targets such as CXCR4 and PI3K-delta that may have major implications in the future treatment of WM. This review discusses the role of MYD88 L265P mutations as well as targets beyond MYD88 in the setting of pathogenesis and development of future rational therapeutic trials focusing on patients diagnosed with WM. Leukemia (2014) 28, 1799–1803; doi:10.1038/leu.2014.88 INTRODUCTION transduces signals to the NF-kB transcription factors in response to Waldenstro¨m’s Macroglobulinemia (WM) is a rare hematological IL-1R1 signaling. MYD88 has a modular structure with a Toll/IL-1R malignancy with a reported age-adjusted incidence rate of 3.4 per (TIR) domain at its COOH terminus and a death domain at its 12 11 million among men and 1.7 per million among women in the NH2 terminus. Wesche et al. and Muzio et al. used United States of America that increases geometrically with age.1 coimmunoprecipitation experiments to show that the TIR Clinically and pathologically, WM represents an intermediate stage domain of MYD88 interacts with the TIR domains of IL-1R1 and between lymphoproliferative disorders and plasma cell dyscrasias IL-1RAP. MYD88 binds the serine–threonine kinases IRAK1 and with characteristics that may lie anywhere between the two IRAK2, mammalian homologs of Drosophila Pelle in the Toll spectrums.2 While the molecular pathogenesis of WM is not fully pathway, via a heterotypic death domain-mediated interaction understood,2,3 the crucial role of nuclear factor-kappa B (NF-kB) and thus acts as a pure adaptor linking the IL-1R1 to downstream 13 activation in tumorigenesis has been reported for at least half a IRAK kinases. Subsequent work by Adachi et al. found MYD88 to decade.4 More recently, whole-genome sequencing (WGS) in WM be essential for induction of NF-kB and MAPK signaling in allowed the identification of a somatic variant (T-C) at position response to IL-18, a cytokine structurally related to IL-1beta. 38182641 in chromosome 3p22.2 that results in an amino-acid Deficiency of MYD88 in mice causes defects in T-cell proliferation change from leucine to proline (L265P) in the MYD88 gene.5 as well as induction of acute phase proteins and cytokines in 13 Myeloid differentiation factor 88 (MYD88) is a crucial adaptor response to IL-1. Increases in interferon-gamma production and protein in the interleukin (IL)-1, IL-18 and Toll-like receptors (TLRs) natural killer cell activity in response to IL-18 are also abrogated. signaling pathway that directs the assemblage of a multisubunit Absence of MYD88 in humans causes life-threatening pyogenic 14 15 signaling complex that includes IL-1R-associated kinase (IRAK) 1 bacterial infections. Finally, Medzhitov et al. using over- 16 and IRAK4, leading up to the activation of Janus kinase–signal expression studies and Kawai et al. using MYD88-deficient transducer and activator of transcription (JAK–STAT) 3 signaling mice showed that MYD88 is critical for TLR signaling. Taken and NF-kB pathways.6,7 This paper summarizes recent evidence together, these studies defined MYD88 as a crucial adaptor for that has provided useful insights into the role of MYD88 and its signaling through the IL-1/IL-18/Toll receptor superfamily. We now alterations in the pathogenesis of WM and the potential know that following stimulation by IL-1R or TLR, MYD88 is applicability in diagnosis, prognosis, therapeutics as well as recruited to the activated receptor complex as a homodimer that 17,18 response assessment. It also addresses recent discovery work complexes with IRAK 4 to activate IRAK 1. This activates the showing a role for CXC chemokine receptor 4 (CXCR4) and PI3K- tumor necrosis factor-associated factor 6 leading to NF-kB 17,18 delta as potential new therapeutic targets in WM.8,9 activation via IkB-alpha phosphorylation (Figure 1). THE MYD88 PATHWAY AND FUNCTIONAL ROLE MYD88 IN LYMPHOMA, WM AND IGM MGUS MYD88 was first identified as a myeloid differentiation primary Ngo et al.19 implicated TLR signaling deregulation in lympho- response gene in 1990.10 Subsequent work by Muzio et al.11 and magenesis by discovering MYD88 mutations in 39% of cases with Wesche et al.12 identified MYD88 as a proximal adaptor of the activated B-cell (ABC) diffuse large B-cell lymphoma (DLBCL). IL-1R signaling pathway and went on to show that MYD88 Using high-throughput mRNA resequencing of ABC cell lines Multiple Myeloma Section, Metabolism Branch, Center for Cancer Research, National Cancer Institute, National Institutes of Health, Bethesda, MD, USA. Correspondence: Dr O Landgren, Multiple Myeloma Section, Metabolism Branch, Center for Cancer Research, National Cancer Institute, National Institutes of Health, 9000 Rockville Pike, Building 10/Room 13N240, Bethesda 20892, MD, USA. E-mail: [email protected] Received 16 December 2013; revised 13 January 2014; accepted 15 January 2014; accpted article preview online 27 February 2014; advance online publication, 16 May 2014 MYD88 and beyond: opportunities for WM O Landgren and N Tageja 1800 use of inhibitors that block MYD88 and IRAK activity, Yang et al.21 observed that MYD88 signaling was supportive of WM growth and survival. Overexpression of MYD88 L265P promoted enhanced survival of WM cells versus WT MYD88, consistent with a gain-of- function mutation as reported by Ngo et al.19 in ABC DLBCL cells. MYD88 was preferentially complexed to phosphorylated BTK in both L265P-expressing WM cell lines, whereas little complexing was observed in MYD88 WT cells. Importantly, knockdown of MYD88 or the use of a MYD88 inhibitor abrogated BTK activity in L265P-expressing cell lines, whereas overexpression of MYD88 L265P showed more robust BTK activity. The use of ibrutinib, an inhibitor of BTK kinase activity, resulted in decreased MYD88-BTK complexing in MYD88 L265P-expressing cells.21 These studies by Ngo et al.19 and Yang et al.21 show that IRAK and BTK independently direct downstream NF-kB activation, and combined use of IRAK and BTK inhibitors leads to synergistic tumor cell killing in MYD88 L265P-expressing WM cells. These studies would suggest a model wherein MYD88 L265P triggers Figure 1. Stimulation of Toll-like receptor 4 facilitates the activation NF-kB via dual but independent pathways which signal through BTK and/or IRAK1/IRAK4. of two pathways: the MYD88 (myeloid differentiation primary- 5 response protein 88)-dependent and MYD88-independent Translating the preclinical work to bedside, Treon et al. pathways. The MYD88-dependent pathway involves the early phase conducted an important study where they reported MYD88 of nuclear factor-B (NF-B) activation, which leads to the production L265P to be a commonly recurring mutation in patients with WM. of inflammatory cytokines. The MYD88-independent pathway Forty nine of 54 patients (91%) with WM and 3 of 3 patients activates interferon (IFN)-regulatory factor (IRF3) and involves the (100%) with non–IgM-secreting LPL had MYD88 L265P expres- late phase of NF-B activation, both of which lead to the production sion by whole-genome sequencing, and confirmed on Sanger of IFN and the expression of IFN-inducible genes. ask1, apoptosis sequencing. This high frequency of the MYD88 L265P somatic signal-regulating kinase 1; ERK, extracellular signal-regulated kinase; Fks, glucan synthase; IKK, inhibitor of k-B kinase; IRAK, interleukin-1 mutation in patients with WM and IgM MGUS has been receptor-associated kinase; IRF, interferon regulatory transcription confirmed with subsequent studies using Sanger, polymerase factor; JNK, c-Jun N-terminal kinase; MEKK3, mitogen- chain reaction (PCR), and allele-specific PCR (AS-PCR) assays activated protein kinase kinase kinase 3; NF-kB, nuclear factor-kB; (Table 1).22–27 TLR, Toll-like receptor; Tpl2, tumor progression Locus 2; TRAF, TNF Whereas the knowledge on this subject is constantly evolving, receptor-associated factor; TRIF, TIR-domain-containing adapter- the present evidence points to MYD88 L265P as an early inducing interferon-b. oncogenic event for the development of WM. Our group used Sanger sequencing to assess the status of MYD88 L265P expression in nine patients with IgM MGUS.25 These patients and sequence analyses of MYD88-coding region in 382 biopsied were extensively worked up with immunohistochemical analyses, tumor samples, a single amino-acid substitution, L265P, was flow cytometry, molecular (immunoglobulin gene rearrangement) observed in 29% of the biopsies. This mutation was lacking in analysis, imaging and laboratory and clinical examinations to rule other subtypes of DLBCL. Subsequent copy number variation out WM and confirm the diagnosis of IgM MGUS. MYD88 L265P analysis implied that the tumor cells might be preferentially expression was found in five of them (56%);25 all five patients had selecting this allele, indicating the biological potency of the L265P clonal plasma cells and clonal lymphocytes in bone marrow mutant.