CONTEXT: Targeted therapies such as the BTK inhibitor Ibrutinib profoundly changed the management of patients with CLL. Molecular mechanisms underlying disease progression on therapy are being actively explored, and mutations in BTK or PLCγ2 gene have been identified as an important one.

PATIENTS AND METHODS: We selected 37 CLL patients who were on continuous Ibrutinib therapy for relapsed refractory CLL and/or the presence of TP53 alterations. Median duration of Ibrutinib was 3 years. BTK and PLCG2 mutations were systematically investigated by NGS.

RESULTS: Only 8 patients showed clinical evidence of disease progression at the time of analysis and a lymphocytes count ranging from 3 to 180 G/L (median 32 G/L). The other 29 patients had no sign of progression, and a lymphocytes count between 2 to 121 G/L (median 9 G/L). 
We found one or several mutations in 57% patients (21/37). Among progressive patients, 5/8 had mutations, but 55% non-progressive patients (16/29) harbored mutations as well. Variant allele frequencies (VAF) ranged from 5 to 53% (median 32%) in progressive patients and from 0.2 to 72% (median 16%) in non-progressive patients. VAF was not correlated to the lymphocyte count. In 33% cases (7/21), there was a co-occurrence of several mutations (2 to 4 mutations) in BTK and/or PLCG2.
We identified a total of 25 mutations in BTK and 7 in PLCG2. Mutations of BTK were mostly located in the cysteine 481 hotspot, with 4 different mutations: p.C481S (c.1442G> C or c.1441T> A), p.C481R and p.C481Y. We found an association of 2 of these mutations in 4/21 patients. We identified 4 different mutations in PLCG2: p.L845F, p.D993H, p.R665W and p.C849R, with a combination of 2 mutations in 3/4 patients.

CONCLUSIONS: Mutations of BTK and PLCG2 are frequent on Ibrutinib therapy, and association of several mutations is common. VAF is very variable, whether patients appear in clinical response or progressive. These results raise the question of the clinical significance of these mutations and a follow-up is required to determine if all these mutations are actually predictive of disease progression.

12 / High frequency of mutations in BTK and PLCG2 in chronic lymphocytic leukemia (CLL) patients on Ibrutinib therapy