Chronic lymphocytic leukemia (CLL) is characterized by constitutive activation of the B-cell receptor (BCR) signaling pathway, but variable responsiveness of the BCR to antigen ligation. Bruton's tyrosine kinase (BTK) shows constitutive activity in CLL and is the target of irreversible inhibition by ibrutinib, an orally bioavailable kinase inhibitor that has shown outstanding activity in CLL. Early clinical results in CLL with other reversible and irreversible BTK inhibitors have been less promising, however, raising the question of whether BTK kinase activity is an important target of ibrutinib and also in CLL. To determine the role of BTK in CLL, we used patient samples and the Em-TCL1 (TCL1) transgenic mouse model of CLL, which results in spontaneous leukemia development. Inhibition of BTK in primary human CLL cells by small interfering RNA promotes apoptosis. Inhibition of BTK kinase activity through either targeted genetic inactivation or ibrutinib in the TCL1 mouse significantly delays the development of CLL, demonstrating that BTK is a critical kinase for CLL development and expansion and thus an important target of ibrutinib. Collectively, our data confirm the importance of kinase-functional BTK in CLL. (Blood. 2014; 123(8):1207-1213
IntroductionChronic lymphocytic leukemia (CLL) is a common adult leukemia that is currently incurable outside of stem cell transplantation. Although response to IgM ligation is variable, the B-cell receptor (BCR) signaling pathway is aberrantly active in this disease, with antigendependent 1,2 or -independent autonomous activation, 3 leading to constitutive activation of kinases inducing cell survival and proliferation. [4][5][6][7] One BCR pathway kinase that is uniformly overexpressed at the transcript level 8 and constitutively phosphorylated in CLL is Bruton's tyrosine kinase (BTK). Ibrutinib, an orally bioavailable irreversible inhibitor of BTK, has recently been shown to have outstanding clinical activity in CLL with extended durable remissions in both untreated and relapsed disease. 9 BTK is a critical mediator of B-lymphocyte signaling and development. Mutations in various domains are responsible for X-linked agammaglobulinemia, 10,11 a disorder characterized by developmental arrest of B cells and profound humoral immune deficiency in humans. A point mutation in the Pleckstrin homology domain is responsible for the milder X-linked immunodeficiency (XID) phenotype in the mouse, 12,13 which is characterized by reduced numbers of circulating B cells and reduced serum immunoglobulins. BTK is also a critical mediator in B-cell signaling. It is recruited to the membrane-bound signalosome in the early stages of B-cell activation, and, following phosphorylation by Syk and Lyn, participates in the phosphorylation of phospholipase C, gamma 2 (PLCg2), which leads to production of the second messengers diacylglycerol and inositol-1,4,5-triphosphate. This pathway is amplified in CLL and leads to prosurvival signals through its effects on phosphatidylinositol 3-kinase (PI3K), PL...
