Lack of IRF-1 expression in acute promyelocytic leukemia and in a subset of acute myeloid leukemias with del(5)(q31)

One allele of interferon regulatory factor-1 (IRF-1), a transcriptional act ivator of genes critical for growth suppression, differentiation, and apopt osis, is usually deleted in acute myeloid leukemias (AML) and myelodysplasi as (MDS) with deletion of chromosome 5q31. Accelerated exon skipping of IRF -1, resulting in transcripts lacking a translation initiation site, has bee n hypothesized as a means of functional inactivation of IRF-1 in AML/MDS. T o test this hypothesis, we developed quantitative competitive RT-PCR assays to measure levels of full length and exon-skipped IRF-1 transcripts and me asured IRF-1 proteins by Western blotting in a series of 45 samples of AML (13:-5/del5(q); II: t(15;17); 7: t(8;21); and 7: inv(16)), normal blood and marrow, and myeloid cell lines. In contrast to AMLs with inv(fs) or t(8;21 ), two AML samples with del(5q) had accelerated exon skipping and relativel y low levels of full-length transcripts, while a third sample had very low transcript levels; IRF-1 proteins were not expressed and could not be induc ed by interferon gamma (IFN gamma). An additional six AML cases with -5/del (5q) had moderate exon-skipping and lacked constitutive IRF-1 proteins; how ever IRF-1 proteins were IFN-gamma-inducible. Unexpectedly, all primary acu te promyelocytic leukemia (APL) samples lacked IRF-I protein and most exhib ited accelerated exon skipping; furthermore, IRF-1 could not be induced by IFN gamma or all-trans retinoic acid (ATRA) which both induce IRF-1 in the NB4 APL cell line. Thus, accelerated exon skipping results in a loss of IRF -I expression and function that cannot be overcome by exposure to inducing agents in a subset of AML patients with -5/del(5q) and in APL.