N-terminus DUX4-immunohistochemistry is a reliable methodology for the diagnosis of DUX4-fused B-lymphoblastic leukemia/lymphoma (N-terminus DUX4 IHC for DUX4-fused B-ALL)

B-lymphoblastic leukemia/lymphoma (B-ALL) is the most common pediatric malignancy and the most commonly diagnosed adult lymphoblastic leukemia. Recent advances have broadened the spectrum of B-ALL, with DUX4 gene fusions implicated in a subclass occurring in adolescents and young adults and harboring a favorable prognosis. DUX4 fusions have been challenging to identify. We aimed to determine whether expression of the DUX4 oncoprotein, as detected by targeted immunohistochemistry, might serve as a surrogate for molecular detection of DUX4 fusions in B-ALL. A cohort of investigational B-ALLs was generated with enrichment for DUX4 fusions by the inclusion of cases with characteristic demographic features and immunophenotypic properties. B-ALLs with mutually exclusive cytogenetics were collected. Immunohistochemical staining by a monoclonal antibody raised against the N-terminus of the DUX4 protein was performed. N-DUX4 immunohistochemistry demonstrated strong, crisp nuclear staining in blasts of seven investigational cases, six of which had nucleic acid material available for molecular evaluation. Five of these cases demonstrated RNA-seq DUX4-fusion positivity. One N-DUX4 immunohistochemistry positive case lacked a definitive DUX4-fusion by RNA-seq, though demonstrated a gene expression profile characteristic of DUX4-rearranged B-ALLs, a CD2+ immunophenotype, and a lack of staining by C-terminus DUX4 antibody immunohistochemistry. At least 83.3% [5/6] positive predictive value. N-DUX4 immunohistochemistry was negative in blasts of three RNA-seq DUX4-fusion-negative cases (3/3; 100% negative predictive value). B-ALLs with mutually exclusive cytogenetic profiles were all N-DUX4 negative (0/10, specificity 100%). N-DUX4 immunohistochemistry is reliable for the distinction of DUX4-rearranged B-ALLs from other B-ALLs. We recommend its use for subclassification of B-ALLs in adolescents and young adults and in B-ALLs that remain "not otherwise specified."