140-HELD-420 denotes that this patient was on 140 mg/d of ibrutinib prior to hospitalization that was held upon admission; he experienced worsening hypoxia after ibrutinib was held and required mechanical ventilation, following which he was restarted on 420 mg/d of ibrutinib and showed rapid improvement in oxygenation.

Key Points Using a sensitive method, the MYD88 (L265P) mutation is detectable in all patients with Waldenström’s macroglobulinemia, therefore representing a hallmark of the disease. MYD88 (L265P) is also found in a substantial proportion of patients with IgM-MGUS.

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.

The somatically acquired V600E mutation of the BRAF gene has been recently described as a molecular marker of hairy cell leukemia (HCL). We developed an allele-specific PCR for this mutation and studied 62 patients with HCL, 1 with HCL variant, 91 with splenic marginal zone lymphoma, 29 with Waldenström macroglobulinemia, and 57 with B-cell chronic lymphoproliferative disorders. The BRAF V600E mutation was detected in all HCL cases and in only 2 of the remaining 178 patients. These 2 subjects had B-cell chronic lymphoproliferative disorders that did not fulfill the diagnostic criteria for HCL. Despite the positive PCR finding, the mutation could not be detected by Sanger sequencing in these 2 cases, suggesting that it was associated with a small subclone. We conclude that the BRAF V600E mutation is present in all patients with HCL and that, in combination with clinical and morphologic features, represents a reliable molecular marker for this condition.

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.

Summary Ibrutinib is associated with response rate of 90% and median progression‐free survival (PFS) in excess of 5 years in Waldenström macroglobulinaemia (WM) patients. CXCR4 mutations are detected in 30–40% of patients with WM and associate with lower rates of response and shorter PFS to ibrutinib therapy. Both frameshift (CXCR4FS) and nonsense (CXCR4NS) CXCR4 mutations have been described. The impact of these mutations on outcomes to ibrutinib have not been evaluated in WM patients. We studied consecutive patients with a diagnosis of WM, on ibrutinib therapy, for the presence of CXCR4FS and CXCR4NS mutations and evaluated the differences in response and PFS between groups. Of 180 patients, 68 patients (38%) had CXCR4 mutations; 49 (27%) had CXCR4NS and 19 (11%) had CXCR4FS mutations. In multivariate models, patients with CXCR4NS had lower odds of major response (Odds ratio 0·25, 95% confidence interval [CI] 0·12–0·53; P < 0·001) and worse PFS (Hazard ratio 4·02, 95% CI 1·95–8·26; P < 0·001) than patients without CXCR4 mutations. CXCR4FS was not associated with worse major response or PFS rates than patients without CXCR4 mutations. Our results suggest different response and PFS rates to ibrutinib for WM patients with CXCR4NS and CXCR4FS, and advocate in favour of CXCR4 mutational testing as well as CXCR4‐directed therapy.

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