Next-generation sequencing has revealed recurring somatic mutations in Waldenström macroglobulinemia (WM), including MYD88 (95%-97%), CXCR4 (30%-40%), ARID1A (17%), and CD79B (8%-15%). Deletions involving chromosome 6q are common in patients with mutated MYD88 and include genes that modulate NFKB, BCL2, Bruton tyrosine kinase (BTK), and apoptosis. Patients with wild-type MYD88 WM show an increased risk of transformation and death and exhibit many mutations found in diffuse large B-cell lymphoma. The discovery of MYD88 and CXCR4 mutations in WM has facilitated rational drug development, including the development of BTK and CXCR4 inhibitors. Responses to many agents commonly used to treat WM, including the BTK inhibitor ibrutinib, are affected by MYD88 and/or CXCR4 mutation status. The mutation status of both MYD88 and CXCR4 can be used for a precision-guided treatment approach to WM.

Purpose Ibrutinib is active in previously treated Waldenström macroglobulinemia (WM). MYD88 mutations ( MYD88) and CXCR4 mutations ( CXCR4) affect ibrutinib response. We report on a prospective study of ibrutinib monotherapy in symptomatic, untreated patients with WM, and the effect of CXCR4 status on outcome. Patients and Methods Symptomatic, treatment-naïve patients with WM were eligible. Ibrutinib (420 mg) was administered daily until progression or unacceptable toxicity. All tumors were genotyped for MYD88 and CXCR4. Results A total of 30 patients with WM received ibrutinib. All carried MYD88, and 14 (47%) carried a CXCR4. After ibrutinib treatment, median serum IgM levels declined from 4,370 to 1,513 mg/dL, bone marrow involvement declined from 65% to 20%, and hemoglobin level rose from 10.3 to 13.9 g/dL ( P < .001 for all comparisons). Overall (minor or more than minor) and major (partial or greater than partial) responses for all patients were 100% and 83%, respectively. Rates of major (94% v 71%) and very good partial (31 v 7%) responses were higher and time to major responses more rapid (1.8 v 7.3 months; P = 0.01) in patients with wild-type CXCR4 versus those with CXCR4, respectively. With a median follow-up of 14.6 months, disease in two patients (both with CXCR4) progressed. The 18-month, estimated progression-free survival is 92% (95% CI, 73% to 98%). All patients are alive. Grade 2/3 treatment-related toxicities in > 5% of patients included arthralgia (7%), bruising (7%), neutropenia (7%), upper respiratory tract infection (7%), urinary tract infection (7%), atrial fibrillation (10%), and hypertension (13%). There were no grade 4 or unexpected toxicities. Conclusion Ibrutinib is highly active, produces durable responses, and is safe as primary therapy in patients with symptomatic WM. CXCR4 status affects responses to ibrutinib.

Key Points• HCK transcription and activation is triggered by mutated MYD88, and is an important determinant of pro-survival signaling.• HCK is also a target of ibrutinib, and inhibition of its kinase activity triggers apoptosis in mutated MYD88 cells.Activating mutations in MYD88 are present in ∼95% of patients with Waldenström macroglobulinemia (WM), as well as other B-cell malignancies including activated B-cell (ABC) diffuse large B-cell lymphoma (DLBCL). In WM, mutated MYD88 triggers activation of Bruton tyrosine kinase (BTK). Ibrutinib, a pleiotropic kinase inhibitor that targets BTK, is highly active in patients with mutated MYD88. We observed that mutated MYD88 WM and ABC DLBCL cell lines, as well as primary WM cells show enhanced hematopoietic cell kinase (HCK) transcription and activation, and that HCK is activated by interleukin 6 (IL-6). Over-expression of mutated MYD88 triggers HCK and IL-6 transcription, whereas knockdown of HCK reduced survival and attenuated BTK, phosphoinositide 3-kinase/ AKT, and mitogen-activated protein kinase/extracellular signal-regulated kinase signaling in mutated MYD88 WM and/or ABC DLBCL cells. Ibrutinib and the more potent HCK inhibitor A419259, blocked HCK activation and induced apoptosis in mutated MYD88 WM and ABC DLBCL cells. Docking and pull-down studies confirmed that HCK was a target of ibrutinib. Ibrutinib and A419259 also blocked adenosine triphosphate binding to HCK, whereas transduction of mutated MYD88 expressing WM cells with a mutated HCK gatekeeper greatly increased the half maximal effective concentration for ibrutinib and A419259. The findings support that HCK expression and activation is triggered by mutated MYD88, supports the growth and survival of mutated MYD88 WM and ABC DLBCL cells, and is a direct target of ibrutinib. HCK represents a novel target for therapeutic development in MYD88-mutated WM and ABC DLBCL, and possibly other diseases driven by mutated MYD88. (Blood. 2016;127(25):3237-3252)

Summary CXCR4WHIM somatic mutations are distinctive to Waldenstrom Macroglobulinaemia (WM), and impact disease presentation and treatment outcome. The clonal architecture of CXCR4WHIM mutations remains to be delineated. We developed highly sensitive allele-specific polymerase chain reaction(AS-PCR) assays for detecting the most common CXCR4WHIM mutations (CXCR4S338X C>A and C>G) in WM. The AS-PCR assays detected CXCR4S338X mutations in WM and IgM monoclonal gammopathy of unknown significance (MGUS) patients not revealed by Sanger sequencing. By combined AS-PCR and Sanger sequencing, CXCR4WHIM mutations were identified in 44/102 (43%), 21/62 (34%), 2/12 (17%) and 1/20 (5%)untreated WM, previously treated WM, IgM MGUS and marginal zonelymphoma patients, respectively, but no chronic lymphocytic leukaemia, multiple myeloma, non-IgM MGUS patients or healthy donors. Cancer cellfraction analysis in WM and IgM MGUS patients showed CXCR4S338X mutations were primarily subclonal, with highly variable clonal distribution(median 35·1%, range 1·2–97·5%). Combined AS-PCR and Sangersequencing revealed multiple CXCR4WHIM mutations in many individual WM patients, including homozygous and compound heterozygous mutations validated by deep RNA sequencing. The findings show thatCXCR4WHIM mutations are more common in WM than previously revealed, and are primarily subclonal, supporting their acquisition after MYD88L265P in WM oncogenesis. The presence of multiple CXCR4WHIM mutations within individual WM patients may be indicative of targeted CXCR4 genomic instability.

Key Points BTKCys481 mutations, including multiple mutated variants within individual patients are common in ibrutinib-progressing WM patients. BTKCys481 mutations were associated with mutated CXCR4 in WM patients progressing on ibrutinib.

• Transcription profiles associated with mutated MYD88, CXCR4, ARID1A, abnormal cytogenetics including 6q2, and familial WM are described.• Mutated CXCR4 profiles show impaired expression of the tumor suppressor response induced by MYD88 L265P and also G-protein/MAPK inhibitors.Whole-genome sequencing has identified highly prevalent somatic mutations including MYD88, CXCR4, and ARID1A in Waldenström macroglobulinemia (WM). The impact of these and other somatic mutations on transcriptional regulation in WM remains to be clarified. We performed next-generation transcriptional profiling in 57 WM patients and compared findings to healthy donor B cells. Compared with healthy donors, WM patient samples showed greatly enhanced expression of the VDJ recombination genes DNTT, RAG1, and RAG2, but not AICDA. Genes related to CXCR4 signaling were also upregulated and included CXCR4, CXCL12, and VCAM1 regardless of CXCR4 mutation status, indicating a potential role for CXCR4 signaling in all WM patients. The WM transcriptional profile was equally dissimilar to healthy memory B cells and circulating B cells likely due increased differentiation rather than cellular origin. The profile for CXCR4 mutations corresponded to diminished B-cell differentiation and suppression of tumor suppressors upregulated by MYD88 mutations in a manner associated with the suppression of TLR4 signaling relative to those mutated for MYD88 alone. Promoter methylation studies of top findings failed to explain this suppressive effect but identified aberrant methylation patterns in MYD88 wild-type patients. CXCR4 and MYD88 transcription were negatively correlated, demonstrated allele-specific transcription bias, and, along with CXCL13, were associated with bone marrow disease involvement. Distinct gene expression profiles for patients with wild-type MYD88, mutated ARID1A, familial predisposition to WM, chr6q deletions, chr3q amplifications, and trisomy 4 are also described. The findings provide novel insights into the molecular pathogenesis and opportunities for targeted therapeutic strategies for WM. (Blood. 2016;128(6):827-838)

MYD88 mutations are present in 95% of Waldenstrom Macroglobulinaemia (WM) patients, and support diagnostic discrimination from other IgM-secreting B-cell malignancies. Diagnostic discrimination can be difficult among suspected wild-type MYD88 (MYD88 ) WM cases. We systematically reviewed the clinical, pathological and laboratory studies for 64 suspected MYD88 WM patients. World Health Organization and WM consensus guidelines were used to establish clinicopathological diagnosis. Up to 30% of suspected MYD88 WM cases had an alternative clinicopathological diagnosis, including IgM multiple myeloma. The estimated 10-year survival was 73% (95% confidence interval [CI] 52-86%) for MYD88 versus 90% (95% CI 82-95%) for mutated (MYD88 ) WM patients (Log-rank P < 0·001). Multivariate analysis only showed MYD88 mutation status (P < 0·001) as a significant determinant for overall survival. Diffuse large B-cell lymphoma (DLBCL) was diagnosed in 7 (15·2%) and 2 (0·76%) of MYD88 and MYD88 patients, respectively (Odds ratio 23·3; 95% CI 4·2-233·8; P < 0·001). Overall survival was shorter among MYD88 patients with an associated DLBCL event (Log-rank P = 0·08). The findings show that among suspected MYD88 WM cases, an alternative clinicopathological diagnosis is common and can impact clinical care. WM patients with MYD88 disease have a high incidence of associated DLBCL events and significantly shorter survival versus those with MYD88 disease.

Acquired ibrutinib resistance due to BTK mutations occurs in B-cell malignancies, including those with MYD88 mutations. BTK mutations are usually subclonal, and their relevance to clinical progression remains unclear. Moreover, the signaling pathways that promote ibrutinib resistance remain to be clarified. We therefore engineered BTK and BTK expressing MYD88-mutated Waldenström macroglobulinemia (WM) and activated B-cell (ABC) diffuse large B-cell lymphoma (DLBCL) cells and observed reactivation of BTK-PLCγ2-ERK1/2 signaling in the presence of ibrutinib in only the former. Use of ERK1/2 inhibitors triggered apoptosis in BTK-expressing cells and showed synergistic cytotoxicity with ibrutinib. ERK1/2 reactivation in ibrutinib-treated BTK cells was accompanied by release of many prosurvival and inflammatory cytokines, including interleukin-6 (IL-6) and IL-10 that were also blocked by ERK1/2 inhibition. To clarify if cytokine release by ibrutinib-treated BTK cells could protect BTK MYD88-mutated malignant cells, we used a Transwell coculture system and showed that nontransduced BTK MYD88-mutated WM or ABC DLBCL cells were rescued from ibrutinib-induced killing when cocultured with BTK but not their BTK-expressing counterparts. Use of IL-6 and/or IL-10 blocking antibodies abolished the protective effect conferred on nontransduced BTK by coculture with BTK expressing WM or ABC DLBCL cell counterparts. Rebound of IL-6 and IL-10 serum levels also accompanied disease progression in WM patients with acquired BTK mutations. Our findings show that the BTK mutation drives ibrutinib resistance in MYD88-mutated WM and ABC DLBCL cells through reactivation of ERK1/2 and can confer a protective effect on BTK cells through a paracrine mechanism.