Key Points: MRD assessment by sequencing is prognostic of TTP and OS in multiple myeloma patients. Among patients in complete response, MRD assessment by sequencing enables identification of 2 distinct subgroups with different TTP.

In this study, targeted sequencing to screen 50 multidrug refractory multiple myeloma (rMM) patients was performed by using the Multiple Myeloma Mutation Panel. Patients were pretreated with both immunomodulatory drugs (IMiDs) and proteasome inhibitors (PIs), and 88%, 78%, and 68% were refractory to an IMiD, a PI, or both, respectively. The majority of patients had progressive (82%) or refractory (78%) disease immediately before sampling, with 43% being IMiD refractory and 46% being PI refractory in the most recent line of therapy. Compared with newly diagnosed MM, an increased prevalence of mutations in the Ras pathway genes KRAS, NRAS, and/or BRAF (72%), as well as TP53 (26%), CRBN (12%), and CRBN pathway genes (10%) was observed. Longitudinal analyses performed in 3 patients with CRBN mutations at time of IMiD resistance confirmed that these mutations were undetectable at earlier, IMiD-sensitive time points. Furthermore, the functional introduction of these mutations in MM cells conferred lenalidomide resistance in vitro. These data indicate a differential genetic landscape in rMM associated with drug response. Key Points The incidence of mutations within the MAPK pathway, the CRBN pathway, and TP53 is significantly increased in drug-refractory MM. Mutations in CRBN might contribute to IMiD resistance in drug-refractory MM.

High-count monoclonal B-cell lymphocytosis (MBL) is an asymptomatic expansion of clonal B cells in the peripheral blood without other manifestations of chronic lymphocytic leukemia (CLL). Yearly, 1% of MBLs evolve to CLL requiring therapy; thus being critical to understand the biological events that determine which MBLs progress to intermediate/advanced CLL. In this study, we performed targeted deep sequencing on 48 high-count MBLs, 47 of them with 2–4 sequential samples analyzed, exploring the mutation status of 21 driver genes and evaluating clonal evolution. We found somatic non-synonymous mutations in 25 MBLs (52%) at the initial time point analyzed, including 12 (25%) with >1 mutated gene. In cases that subsequently progressed to CLL, mutations were detected 41 months (median) prior to progression. Excepting NOTCH1, TP53 and XPO1, which showed a lower incidence in MBL, genes were mutated with a similar prevalence to CLL, indicating the early origin of most driver mutations in the MBL/CLL continuum. MBLs with mutations at the initial time point analyzed were associated with shorter time-to-treatment (TTT). Furthermore, MBLs showing subclonal expansion of driver mutations on sequential evaluation had shorter progression time to CLL and shorter TTT. These findings support that clonal evolution has prognostic implications already at the pre-malignant MBL stage, anticipating which individuals will progress earlier to CLL.

Combined MEK‐BRAF inhibition is a well‐established treatment strategy in BRAF‐mutated cancer, most prominently in malignant melanoma with durable responses being achieved through this targeted therapy. However, a subset of patients face primary unresponsiveness despite presence of the activating mutation at position V600E, and others acquire resistance under treatment. Underlying resistance mechanisms are largely unknown, and diagnostic tests to predict tumor response to BRAF‐MEK inhibitor treatment are unavailable. Multiple myeloma represents the second most common hematologic malignancy, and point mutations in BRAF are detectable in about 10% of patients. Targeted inhibition has been successfully applied, with mixed responses observed in a substantial subset of patients mirroring the widespread spatial heterogeneity in this genomically complex disease. Central nervous system (CNS) involvement is an extremely rare, extramedullary form of multiple myeloma that can be diagnosed in less than 1% of patients. It is considered an ultimate high‐risk feature, associated with unfavorable cytogenetics, and, even with intense treatment applied, survival is short, reaching less than 12 months in most cases. Here we not only describe the first patient with an extramedullary CNS relapse responding to targeted dabrafenib and trametinib treatment, we furthermore provide evidence that a point mutation within the capicua transcriptional repressor (CIC) gene mediated the acquired resistance in this patient. Key Points. BRAF mutations constitute an attractive druggable target in multiple myeloma. This is the first genomic dissection of the central nervous system involvement in a multiple myeloma patient harboring a druggable BRAF^V600E mutation. Deep genomic characterization of the extramedullary lesion prompted a personalized therapeutic approach. Acquisition of CIC mutation confers a mechanism of BRAF‐MEK inhibitor drug resistance in multiple myeloma. The in silico interrogation of the CoMMpass clinical study revealed 10 patients with somatic mutations of CIC and its downregulation at gene expression level in multiple myeloma. CIC gene silencing decreases the sensitivity of multiple myeloma cells to BRAF‐MEK inhibition in vitro. The correlation between CIC downregulation and ETV4/5 nuclear factor expression in multiple myeloma BRAF‐mutant cells is shown for the first time. CIC mutation, its downregulation, and the related downstream effect on MMP24 support disseminative potential providing new clues in the extramedullary biology definition.