Exploring Droplet Digtial PCR Detection of MYD88L265P mutation Burden for the Evaluation of Efficacy in Patients with WM : A Convenient and Accurate Supplementary Approach to IMWG-11 Criteria
Authors: Xue Hua , Li Wenjie , Zhao Songying , Wang Jing , Guo Huimei , Liu Jia , Zhang Jiangbo , Xu Jianmei , Liu Yao 
Published: December 2024

Introduction
Serum IgM concentration is a critical factor in assessing the effectiveness of Waldenstrom's macroglobulinemia (WM) treatment and currently serves as the standard for evaluating the burden of WM. IWWM-11 was tasked with evaluating a simplified response assessment for WM based on IgM criteria. While the IgM concentration does not correlate with tumor burden, it may lead to an underestimation of disease burden after treatment. There may be unresolved issues with the IWWM-11 Criteria. Currently, numerous unresolved issues persist in current methods for assessing tumor burden, such as high cost, repeated invasive procedures, and poor sensitivity and specificity persist. Hence, it is necessary to explore more sensitive and accurate techniques for detecting WM tumor burden in bone marrow in combination with serum IgM concentrations to evaluate efficacy.
Methods
Patients who were initially diagnosed with WM at Affiliated Hospital of Hebei University from January 1, 2017 to December 31, 2023 were included in this study. For patients with MYD88L265P mutation-positive detected by NGS, we detected the determination of MYD88L265P mutation fraction in bone marrow (BM) and peripheral blood(PB) ctDNA by ddPCR. Concurrently, flow cytometry and immunohistochemistry of BM pathology were conducted to quantify the percentage of small mature B lymphocytes, plasmacytoid lymphocytes, and plasma-cells(lymphoplasmacytic). The clinical characteristics of the patients, including IgM levels, extramedullary involvement, and clinical features, are collected.
Results
A total of 12 patients with MYD88L265P mutation were included, with a median of 70 years (55-76 years). The median IgM was 49.8 g/L (6.45-87.30g/L). Four patients exhibited extramedullary involvement, characterized by lymph node enlargement and splenomegaly. 50%(6/12) of the patients opted for BTKi-based treatment, while another 50% (6/12) chose PI-based treatment or traditional chemotherapy. According to the IWWM-6 criteria, ORR was 66.7% (8/12), VGPR+CR rates of 25% (3/12). According to the IWWM-11 Criteria, ORR was 66.7%(8/12), 41.7% (5/12) of the patients achieved VGPR+CR. The IWWM-11 Criteria, while simple and convenient, may lose accuracy. The median mutation load of BM MYD88L265P mutation, as determined by ddPCR, was 2.448% (0.30%-40.874%). The median mutation load of PB ctDNA MYD88L265P mutation was 2.493% (0.248%-20.238%). Additionally, nine patients underwent BM pathology and flow cytometry at initial diagnosis, with a median percentage of lymphoplasmacytic in BM pathology being 35% (5%-80%), and a median percentage of lymphoplasmacytic in BM flow cytometry being recorded as 3.81% (0.44%-64.15%).
The MYD88L265P mutation in BM and PB ctDNA was quantitatively measured by ddPCR every 3 and 6 months for the 12 patients to assess their mutation load. When considering mutation load as a continuous variable, the BM MDY88L265P mutation load exhibited a positive correlation with the PB ctDNA MDY88L265P mutation load (r=0.829, P=0.00&lt;0.01), pathological lymphoplasmacytic percentage (r=0.601, P=0.014&lt;0.05), and flow cytometry lymphoplasmacytic percentage (r=0.852, P=0.00&lt;0.01). However, it did not show significant correlations with IgM (r=-0.033, P=0.857&gt;0.05) or M protein (r=0.009，P=0.961＞0.05 ).The mutation load of PB ctDNA MYD88L265P was positively correlated with flow cytometry lymphoplasmacytic percentage (r=0.740, P=0.006＜0.01), but not significantly correlated with pathological lymphoplasmacytic percentage (r=0.304,P=0.393＞0.05), IgM concentration (r=0.109，P=0.677＞0.05) and M protein (r=0.217，P=0.403＞0.05).
Conclusion
The preliminary findings of this study indicate that there is no significant correlation between the MYD88L265P mutation burden in BM, the PB ctDNA MYD88L265P mutation burden by ddPCR and IgM, which could provide a more accurate tumour load and was not affected by tumour cell distribution. The ddPCR surveillance of MYD88L265P mutation load in PB ctDNA is a simpler and less invasive approach, warranting further investigation. The combination of MYD88L265P mutation load by ddPCR and IgM has the potential to offer a promising method for accurately assessing the efficacy of patients with WM，potentially serving as a valuable supplement to the IWWM - 11 Criteria.