Characterization and Use of an Antibody Detecting the CBFβ-SMMHC Fusion Protein in inv(16)/t(16;16)-Associated Acute Myeloid Leukemias

Abstract

The inv(16)(p13q22) and t(16;16)(p13;q22) cytogenetic abnormalities occur commonly in acute myeloid leukemia (AML), typically associated with French-American-British (FAB) AML-M4Eo subtype. Reverse transcriptase-polymerase chain reaction (RT-PCR) techniques have been recently developed to detect the presence of several variants of the resultant CBFB-MYH11 fusion gene that encodes a CBFbeta-smooth muscle myosin heavy chain (SMMHC) fusion protein. We have now determined the clinical use of a polyclonal antibody [anti-inv(16) Ab] directed against a junctional epitope of the most common type of CBFbeta-SMMHC fusion protein (type A), which is present in 90% of inv(16)/t(16;16) AML cases. Using flow cytometry, reproducible methods were developed for detection of CBFbeta-SMMHC proteins in permeabilized cells; flow cytometric results were then correlated with cytogenetics and RT-PCR detection methods. In an analysis of 42 leukemia cases with various cytogenetic abnormalities and several normal controls, the anti-inv(16) Ab specifically detected all 23 cases that were cytogenetically positive for inv(16) or t(16;16), including a single AML case that was RT-PCR-negative. In addition to detecting all type A fusions, the anti-inv(16) Ab also unexpectedly identified the type C and type D CBFbeta-SMMHC fusion proteins. Molecular characterization of one RT-PCR-positive and Ab-positive t(16;16) case with a non-type A product showed a novel previously unreported CBFB-MYH11 fusion (CBFB nt 455-MYH11 nt 1893). Flow cytometric results were analyzed using the Kolmogorov-Smirnov statistic D-value and the median value for positive samples was 0.65 (range, 0.35 to 0.77) versus 0.07 (range, -0.21 to 0.18) in the negative group (P < .0001). The overall concordance between cytogenetics and RT-PCR was 97%, whereas the concordance between flow cytometry and cytogenetics was 100%. Thus, using the anti-inv(16) Ab, all cytogenetically positive and RT-PCR-positive AML cases with inv(16) or t(16;16) could be rapidly identified. This study demonstrates the use of this antibody as an investigational tool in inv(16)/t(16;16) AML and suggests that the development of such reagents may have potential clinical diagnostic use.