<p>We analyzed clonal evolution in serial samples from five CLL patients who became resistant to the Bruton's tyrosine kinase (BTK) inhibitor ibrutinib, using whole-exome and deep targeted sequencing. We observe a BTK-C481S mutation in one of five patients, and multiple PLCG2 mutations in a second patient. The other patients had an expansion of clones harboring del(8p) carrying additional driver mutations (EP300, MLL2, EIF2A), with one patient developing trans-differentiation into CD19-negative histiocytic sarcoma. We calculated the growth kinetics of ibrutinib-resistant subclones and estimated the size of the resistant clones at treatment initiation, which we validated by droplet-microfluidic technology. Haplo-insufficiency of TRAIL-R, a consequence of del(8p), led to TRAIL insensitivity which may contribute to development of ibrutinib resistance. These findings demonstrate that ibrutinib therapy has the potential to lead to clonal selection and expansion of rare cell populations already present at the time of treatment initiation. They also provide insight into the heterogeneity of genetic changes associated with ibrutinib resistance, previously attributed solely to mutations in BTK and related pathway molecules.</p>

Clonal Evolution in Patients with Chronic Lymphocytic Leukemia