TP53 mutations Are Associated with Mutated MYD88 and CXCR4, and Confer an Adverse Outcome in Waldenström Macroglobulinemia

Background: Whole genome sequencing identified highly recurrent somatic mutations in Waldenstrom macroglobulinemia (WM). Activating somatic mutations in MYD88 and CXCR4 are present in 90-95% and 30-40% of WM patients, respectively, and impact disease presentation, treatment outcome, and overall survival. In contrast, somatic mutations in the tumor suppressor gene TP53 occur less frequently (7%), and the clinical significance remains to be delineated. TP53 aberrations confer an adverse prognosis in related B-cell malignancies, such as chronic lymphocytic leukemia and multiple myeloma, predicting for treatment resistant and inferior survival. As such, we sought to characterize the clinical implications, as well as the clonal architecture, of somatic TP53 mutations in WM. Methods: We searched our database for WM patients with a TP53 mutation identified by a clinical next generation sequencing (NGS) assay using unsorted bone marrow (BM) samples. To validate the findings, CD19+ cells from BM aspirates were isolated, and DNA was extracted for mutational analysis. CD19-depleted peripheral blood (PB) mononuclear cells were used as normal paired samples. All samples were screened for MYD88, CXCR4, and TP53 mutations by Sanger sequencing, and zygosity was determined by establishing the ratio of mutant versus wile-type (WT) allele expression. TP53 copy number was determined using TaqMan Copy Number Assays (Applied Biosystems, Grand Island, NY, USA). Medical files were manually reviewed to gather pertinent clinical data. Results: Fourteen WM patients (14/265; 5.3%) had a TP53 mutation detected by a clinical NGS assay. Sanger sequencing identified somatic TP53 mutations within the WM clone in six patients, including one patient with two somatic mutations. Three patients had a TP53 mutation identified in both CD19+ and CD19- sorted samples, 4 patients were WT, and 1 patient did not have DNA for validation. No recurrent variants were identified. Among the six patients with validated somatic TP53 mutations, four (67%) had biallelic inactivation of TP53 . Three patients had a homozygous TP53 mutation determined by Sanger sequencing and one patient had deletion of TP53 . All six patients harbored both a CXCR4 mutation (4 nonsense, 2 frameshift) and the MYD88 L265P mutation, of which 4 patients (67%) had mutated homozygous MYD88 . At the time the TP53 mutation was detected, the median BM involvement and hemoglobin level was 80% and 9.2 g/dl, respectively. The median serum IgM level was 2,508 mg/dl, and two patients had symptomatic hyperviscosity. Five patients (83%) were high-risk according to the International Prognostic Scoring System for WM. Three patients were untreated, 2 patients were refractory to their most recent therapy (i.e. bortezomib, dexamethasone, rituximab), and 1 patient was relapsing. Four patients went on to receive systemic therapy, to which two patients were refractory. Two patients were treated with ibrutinib and both obtained a major response. After a median follow-up 7.7 months, 2 patients (33%) have died due to progressive disease; both patients had biallelic inactivation of TP53 . Conclusion: Somatic TP53 mutations occur concurrently with both MYD88 and CXCR4 mutations in WM patients and appear to confer an aggressive disease course. Our findings highlight the need to understand the cell-specific origin of mutations detected by clinical NGS assays. Disclosures Castillo: Pharmacyclics: Consultancy, Research Funding; Janssen: Consultancy, Research Funding; Millennium: Research Funding; Abbvie: Research Funding. Treon: Pharmacyclics: Consultancy, Research Funding.