Mutations in Driver Genes and Changes in Clonal Dynamics Are Associated with Shorter Time to Treatment in MBL Cases

Introduction: Monoclonal B cell lymphocytosis (MBL) is an asymptomatic expansion of clonal CD19+/CD5+ B cells with less than 5x10⁹/L cells in the peripheral blood and without other manifestations of chronic lymphocytic leukemia (CLL). Approximately 1% of MBL evolves to CLL requiring therapy per year; thus it is critical to develop more precise tools to identify which MBL will progress to CLL and require treatment.

Patients and Methods: In this study, we performed targeted deep sequencing (TDS) on 49 high-count MBL individuals (median B-cell count 3.7x10⁹/L; range 0.8-4.9x10⁹/L) and explored the mutation status of 20 driver genes recurrently mutated in CLL. We analyzed the clonal evolution in 45 of these 49 MBLs by screening 2-4 sequential samples (average time between samples 56 months, range 10-119 months). At last follow-up, 19 cases (39%) had progressed to Rai>0, and 10 cases (20%) required treatment. Tumor and germ line DNAs were isolated from sorted CD5+/CD19+ and CD5-/CD19- cell populations, respectively. Overall, 154 samples from 49 MBL cases (105 tumor and 49 germ line) were screened using semiconductor sequencing technology. The latter genetic information was integrated with relevant clinical and biological parameters, and we evaluated the effect of driver mutations and clonal expansion on time to CLL progression and time to treatment (TTT).

Results and Discussion: Our cohort consisted in 17 women and 32 men, with a median age of 66 years (range: 44-80). Five cases presented secondary diseases, including melanoma, lung and bladder cancer. Clinical and biological parameters were collected, including IGHV mutation status (mutated 66%, unmutated 34%), ZAP70 and CD49 expression (25% each). At presentation, 46% of cases had del(13q), 27% trisomy 12, 6% del(11q), and 4% del(17p).

Overall, we found somatic non-synonymous mutations in 23 of 49 MBLs (47%) at the initial time point including 22% of cases with more than one mutated driver gene. The average depth of coverage was 730x, thus allowing the identification of small subclonal mutations. Recurrent mutations were found in most of the drivers: CHD2, DDX3X (8% of cases), FBXW7, NOTCH1, SF3B1 (6% each), ATM, BCOR, BIRC3, BRAF, KRAS, MED12, MYD88 and ZMYM3 (4% each). Furthermore, ITPKB, POT1, SAMHD1 and XPO1 were mutated in only one case, whereas no mutations were found in HIST1H1E, RIPK1 and TP53. In 4 individuals, we found two mutations in the same gene (BRAF, DDX3X, KRAS and SAMHD1). Genes that are known to be associated with disease progression in CLL were either mutated with significantly lower incidence (NOTCH1, SF3B1) or not mutated (TP53). Mutations were detected on average 45 months (range 9-73) prior to progression to CLL Rai>0 indicating the early origin of most driver gene mutations in the MBL/CLL continuum. The presence of driver mutations in MBL was associated with shorter TTT (median TTT: present: 96 months vs. not present: not reached, HR: 5.52, 95% CI: 1.2-26.2, P =0.015).

Next, we looked at clonal expansion of driver mutations over time (defined as >2-fold change in the allelic frequency of driver mutations between time points). Of 20 MBLs with mutations at baseline who had sequential samples available, 10 cases showed clonal expansion. Seven out of 10 MBLs who required therapy showed clonal expansion, which was detected on average 15 months (range 6-30 month) prior to treatment. Finally, the detection of clonal expansion was significantly associated with reduced TTT (median TTT: clonal expansion: 21 months vs. no clonal expansion: 84 months, HR: 7.79, 95% CI: 1.94-31.3, P <0.001).

Conclusion: We have confirmed the existence of recurrent mutations in most CLL putative driver genes at the premalignant MBL stage many years before progression to CLL. Furthermore, the early identification of driver mutations and its clonal expansion predicts a shorter TTT. Of note, clonal evolution under selective pressure has recently been linked to the onset of CLL progression after therapy. In this study, we characterized the clonal dynamics in the pre-malignant stages of the disease and underlined its impact on clinical outcome. Despite the relatively small size of the cohort, these findings suggest that the sequential monitoring of MBL individuals with a simple and reliable technique, such as TDS, will be at least of prognostic use and thus its incorporation in the disease stratification and clinical management should be further tested.