Expression and functional characterization of CD33 transcript variants in human acute myeloid leukemia
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George S. Laszlo1, Kimberly H. Harrington1, Chelsea J. Gudgeon1, Mary E. Beddoe1, Matthew P. Fitzgibbon2, Rhonda E. Ries1, Jatinder K. Lamba3, Martin W. McIntosh2, Soheil Meshinchi1,4,5, Roland B. Walter1,6,7
1Clinical Research Division, Fred Hutchinson Cancer Research Center, Seattle, WA, USA
2Public Health Sciences Division, Fred Hutchinson Cancer Research Center, Seattle, WA, USA
3Department of Pharmacotherapy and Translational Research College of Pharmacy, University of Florida, Gainesville, FL, USA
4Children’s Oncology Group, Arcadia, CA, USA
5Department of Pediatrics, University of Washington, Seattle, WA, USA
6Department of Medicine, Division of Hematology, University of Washington, Seattle, WA, USA
7Department of Epidemiology, University of Washington, Seattle, WA, USA
Roland B. Walter, email: firstname.lastname@example.org
Keywords: acute myeloid leukemia, antigen, CD33, immunotherapy, splice variants
Received: April 28, 2016 Accepted: May 17, 2016 Published: May 27, 2016
With the demonstration of improved survival of some acute myeloid leukemia (AML) patients with the CD33 antibody-drug conjugate, gemtuzumab ozogamicin (GO), CD33 has been validated as a target for antigen-specific immunotherapy. Since previous studies identified a CD33 splice variant missing exon 2 (CD33∆E2) and, consequently, the immune-dominant membrane-distal V-set domain, we investigated the expression and functional characteristics of CD33 transcript variants in AML. In primary AML specimens, we not only found full-length CD33 (CD33FL) and CD33∆E2 but also corresponding variants containing an alternate exon 7 predicted to encode a CD33 protein lacking most of the intracellular domain (CD33E7a and, not previously described, CD33∆E2,E7a) in almost all cases. In acute leukemia cell sublines engineered to express individual CD33 splice variants, all splice variants had endocytic properties. CD33FL and CD33E7a mediated similar degrees of GO cytotoxicity, whereas CD33∆E2 and CD33∆E2,E7a could not serve as target for GO. Co-expression of CD33∆E2 did not interfere with CD33FL endocytosis and did not impact CD33FL-mediated GO cytotoxicity. Together, our findings document a greater-than-previously thought complexity of CD33 expression in human AML. They identify CD33 variants that lack exon 2 and are not recognized by current CD33-directed therapeutics as potential target for future unconjugated or conjugated antibodies.
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