AACR 2017 | Poster 3150/10 – PLCG2 C2-domain mutations co-occur with BTK and PLCG2 resistance mutations in Chronic Lymphocytic Leukemia undergoing ibrutinib therapy

At the American Association for Cancer Research (AACR) annual meeting in Washington, DC, USA, on Tuesday 4^th April, a poster session titled “Molecular Targeted Therapies 1” took place.

One of the posters on display (3150 / 10) was titled “PLCG2 C2-domain mutations co-occur with BTK and PLCG2 resistance mutations in chronic lymphocytic leukemia undergoing treatment with the BTK inhibitor ibrutinib” by Dan Jones from The Ohio State University, Columbus, OH, and colleagues.

The group analyzed all peripheral blood samples from CLL patients, with suitable depth of sequencing coverage, submitted to the Ohio State University James Polaris Molecular Laboratory.

Key Highlights:

• BTK-PLCG2 DNA sequencing carried out at 3-month intervals on 373 patients (1,231 samples)
• Treatments included ibrutinib monotherapy, or ibrutinib combined with ofatumumab and/or rituximab or venetoclax
• BTK codon 481 mutations observed above 1% variant allele fraction = 23.5% (88) patients
• Median time after ibrutinib initiation to BTK codon 481 mutation = 34.3 months
• Fifty-seven distinct recurrent/hotspot PLCG2 mutations observed in 9.7% (36) patients
• Median time after ibrutinib initiation to PLCG2 mutation = 35.1 months
• Formerly reported mutations in SH2/SH3 domains of PLCG2 were found: R665W, S707F, A708P, and L845F
• Novel mutations in the PLCG2 C2 domain were observed affecting codons 1140–1144, which code for the highly conserved aspartic acid residues that bind calcium and mediate membrane localization in other C2-domain containing proteins
• Patients harboring BTK mutations had a much higher incidence of recurrent mutations in PLCG2 (25/91, 27.4%) versus patients with wild-type BTK (11/282, 3.9%; P < 0.0001)
• PLCG2 C2 domain mutations were highly associated with other resistance mutations
• PLCG2 C2 domain mutations correlated with increasing CLL levels (clone size), whereas first identification of BTK codon 481 or SH2/SH3 PLCG2 mutations frequently came before hematologic relapse
• PLCG2 germline polymorphisms were mainly located to the EF-hand domain; R268W and H257L were two uncommon single nucleotide polymorphisms reported, correlating with presence or absence of BTK codon 481 mutations, respectively

The poster was summarized by stating that a high rate of co-occurrence of BTK codon 481 and PLCG2 mutations was found in samples from CLL patients unresponsive to ibrutinib. The group hypothesize that “multistep PLCG2 mutational activity may underlie resistance in many patients.” The authors also concluded by stating that they identified a novel mutation hotspot – the PLCG2 C2 domain – which often co-occurs with or after BTK codon 481 and/or PLCG2 SH2/SH3 mutations. Finally, it was also found that BTK mutations segregate with different PLCG2 germline polymorphisms.