First report of MYD88^L265P somatic mutation in IgM-associated light-chain amyloidosis

To the editor:

A mutation in myeloid differentiation factor gene, MYD88, leading to constitutive activation of the nuclear factor κB pathway was shown to be oncogenically active in 29% of patients with activated B-cell type diffuse large B-cell lymphoma.¹  The most common somatic variant of MYD88 mutation is substitution of leucine by proline at position 265 (L265P)¹  because of a single nucleotide change (T→C) in chromosome 3p22.2.²  Subsequently, MYD88^L265P was discovered in 86% to 100% of patients with Waldenström macroglobulinemia (WM)/lymphoplasmacytic lymphoma,^2-4  10% to 87% of immunoglobulin M (IgM) monoclonal gammopathy of undetermined significance (MGUS),^2-6  36% to 38% of primary central nervous system lymphoma,^7,8  6% to 21% of splenic marginal zone lymphoma,^3,4,6  9% of gastric mucosa associated lymphoid tissue lymphomas, and 2.9% to 4% of those with chronic lymphocytic leukemia.^3,9 MYD88 is an adaptor protein that drives tumor growth by increased Toll-like receptor and interleukin-1 receptor signaling upon activation of its Toll interleukin-1 receptor domain. This leads to homodimerization and activation of interleukin-1 receptor associated kinase 1 and 4,¹  as well as transforming growth factor-β associated kinase-1.¹⁰ 

A risk stratification model in patients with IgM MGUS has predicted an increased 5- and 10-year cumulative incidence of progression to WM or other lymphoproliferative disorders (LPDs) in those harboring MYD88^L265P mutation, compared with wild-type variants (5- and 10-year risk of 15% and 45% vs 2% and 14%, respectively).¹¹  Although primary systemic amyloidosis is associated with clonal plasma cell proliferative disorders or B-cell LPDs,¹²  the frequency of an activating MYD88 mutation has not been studied in this population thus far. In a study by Jiménez et al,⁶  patients with light chain (AL) amyloidosis were reported to be negative for MYD88^L265P 6. However, isotype of the monoclonal protein was not reported. The objective of our study was to detect the frequency of activating MYD88 mutation in IgM amyloidosis cases and identify clinicopathologic correlations, if any.

We report data on 15 bone marrow tissue specimens, which were obtained from 14 patients with archival tissue available from the transplant database in Mayo Clinic after approval by the Institutional Review Board. All samples were examined by standard morphology and flow cytometric evaluation of B cells and plasma cells compartments. All available clinical and pathologic data were reviewed.

DNA was extracted from CD138⁺ or CD19⁺CD138⁺ sorted cells isolated from the bone marrows of IgM amyloidosis patients using the Gentra PureGene DNA Isolation Kit (Qiagen, Valencia, CA). Real-time allele-specific oligonucleotide polymerase chain reaction was performed using qBiomarker Somatic Mutation Assay for MYD88_85940 (SABiosciences-Qiagen, Hilden, Germany) according to the manufacturer’s protocol. The reverse transcription polymerase chain reaction was analyzed using CFX96 real-time thermal cycler (Bio-Rad, Hercules, CA). To obtain a ΔΔC_t range for wild-type alleles, the assay was performed on DNA from 10 MYD88^WT controls. The cutoff for wild-type vs mutant MYD88 was a ΔΔC_t value of 0.003.¹⁰ 

Clinical and demographic characteristics of each patient has been summarized in Table 1. Ten out of 14 patients (71%) were positive for the MYD88^L265P mutation. Median age of the study population was 60 years (range 48-70 years). Four MYD88^L265P-positive patients (patients 1, 7, 8, and 10) had a prior diagnosis of WM. Three patients (patients 3, 9, and 13) had biclonal gammopathy on immunofixation. Among MYD88^L265P-positive patients, bone marrow examination revealed clonal plasma cells in 8 patients (patients 1, 2, 3, 5, 7, 8, 9, and 10), clonal B lymphocytes in 9 patients (patients 1, 2, 4, 5, 6, 7, 8, 9, and 10), and both clonal B lymphocytes and plasma cells in 7 patients (patients 1, 2, 5, 7, 8, 9, and 10). Of note, 9/10 patients with MYD88^L265P had clonal B lymphocytes in bone marrow, as opposed to 1/4 patient with MYD88^WT. Conventional karyotype analysis showed normal karyotype in all patients. Cytogenetic analysis by fluorescent in situ hybridization was done in 8 out of 14 patients. Among MYD88^L265P-positive patients (5 evaluable patients), 1 patient had trisomy 9, monosomy 13, and deletion of IgH variable region; 1 had normal fluorescent in situ hybridization cytogenetics; and 3 had insufficient plasma cells for analysis. Among MYD88^WT patients (3 evaluable patients), 2 patients had t(11;14) and 1 had insufficient plasma cells for analysis. Median serum M-spike in MYD88^L265P and MYD88^WT patients was 1.15 g/dL (range 0.24-2.3 g/dL) and 0.55 g/dL (range 0.1-0.9 g/dL), respectively. Cardiac involvement was seen in 1 out of 10 patients with MYD88^L265P and 2 out of 4 patients with MYD88^WT. Nervous system involvement (peripheral and/or autonomic) was seen in 4 out of 10 patients with MYD88^L265P and 0 out of 4 patients with MYD88^WT.

Estimated median-follow up of surviving patients was 77.9 months (95% confidence interval [CI], 26.3-120.3 months). Median overall survival (OS) and progression-free survival (PFS) from diagnosis for the entire cohort was 95.8 months (95% CI, 41.2 to not reached [NR]) and 48.2 months (95% CI, 27.8-120.3), respectively. Median OS in patients with MYD88^L265P and MYD88^WT was 83.0 months (95% CI, 11.9-108.6) and NR (95% CI, 16.4-NR), respectively, 5-year OS rates being 60% (95% CI, 24% to 87%) and 75% (95% CI, 24% to 97%), respectively. Median PFS in patients with MYD88^L265P and MYD88^WT was 43.3 months (95% CI, 11.9-75.7) and 120.3 months (95% CI, 14.0-120.3), respectively. Statistical significance was not achieved for comparison between the 2 groups, likely because of small sample size. Clinical characteristics and outcomes of the entire cohort has been summarized in Table 2.

IgM-associated AL amyloidosis is a distinct clinical entity, constituting 5% to 7% of all AL amyloidosis cases.^13,14  They are usually characterized by older age at onset, higher incidence of peripheral neuropathy and lymphadenopathy, with no survival difference from non-IgM cases.¹⁴  A study by Landgren and Staudt showing the presence of MYD88^L265P by Sanger sequencing in 56% of patients with IgM MGUS had demonstrated the presence of clonal plasma cells and clonal lymphocytes in all patients positive for this mutation.⁵  In our study, all but 1 MYD88^L265P-positive patient had clonal B lymphocyte in marrow, likely indicating association of MYD88^L265P mutation with LPDs. MYD88^L265P-positive patients had a lower rate of cardiac involvement (1/10; 10%), compared with an expected rate of 32% to 40%^14,15  in patients with IgM amyloidosis. On the other hand, a higher incidence of amyloid neuropathy (4/10; 40%) was seen in these patients. One MYD88^L265P-positive patient (patient 4) had multiple cranial nerve palsies, an unusual manifestation of amyloidosis. In patients with WM, MYD88^L265P mutation was shown to be associated with a higher disease burden in bone marrow and peripheral blood, poor response to therapy, and inferior OS.^16,17  It is unclear whether an activating MYD88 mutation is a driver for increased amyloid deposition by the plasma cell clone and whether presence of these mutations in patients with B-cell LPDs entails an increased risk for development of systemic IgM amyloidosis. Given the ease of detection of MYD88^L265P mutation, studies should focus on assessing its frequency in larger cohorts of B-cell LPDs and correlating with clinical characteristics, including the development of systemic amyloidosis. A novel oligonucleotide, IMO-8400, which acts by inhibiting activated Toll-like receptor signaling pathway in patients with MYD88^L265P mutation, is currently being tested in patients with WM/lymphoplasmacytic lymphoma (#NCT02092909) and activated B-cell type diffuse large B-cell lymphoma (#NCT02252146). Preclinical studies in mice have shown antiproliferative activity of inhibitors of MYD88 dimerization (ST2825),¹⁸  which could create novel therapeutic avenues for IgM amyloidosis in the future.