Abstract

Chromosomal translocations in a number of hematopoietic malignancies, including AML and MDS, result in the formation of NUP98 gene chimeras. Recently, fusion of NUP98 with a novel gene, PHF23 or plant homeodomain (PHD) finger 23, was reported in a cryptic translocation in a patient with AML (Reader et al., 2007). PHF23 is one of three known NUP98 fusion partners, associated with AML, which contain PHD finger motifs. In this study, we report a more detailed characterization of PHF23, which we originally identified by differential display PCR as an erythropoietin-induced gene in a murine model of terminal erythroid differentiation. In silico sequence analysis revealed a five exon gene containing a nuclear localization signal (NLS) as well as the C-terminal PHD motif. PHF23 mapped to chromosome 17p13.1, <450 kb from the TP53 gene, in a region frequently disrupted in cancer. Quantitative PCR analysis indicated Phf23 expression is not restricted to hematopoietic cells but demonstrates widespread tissue distribution, with increased expression levels in more proliferative tissues (∼6 fold increase in gene expression in the testis over mRNA levels in the liver). A ubiquitously expressed Phf23 splice variant (with mRNA concentrations ∼30–45% lower than full-length Phf23 gene expression levels) was generated using an alternative 3′ splice site within exon four, which exactly corresponded to the chr17 translocation breakpoint in the AML patient described. PHF23 is evolutionarily conserved, particularly within the PHD finger domain (>76% sequence identity between human and zebrafish PHD motifs), and is significantly related to another potentially oncogenic protein, PHF13/SPOC-1, associated with ovarian cancer. PHD motifs act as zinc fingers and the expanding family of PHD-containing proteins are implicated in chromatin regulation. Cellular localization studies using fluorescently tagged PHF23 indicated that both the full-length and spliced isoforms are located exclusively within the nucleus. We predicted that PHF23 has a similar structure to PHD motifs of known histone effector proteins, which influence transcription through recognition of specific epigenetic marks. This was demonstrated by immunoprecipitation and histone peptide binding assays showing that PHF23 associated with histone H3, primarily di- and tri-methylated H3K4, which are characteristic marks of active transcription. Microarray analysis of PHF23 mRNA expression levels in AML and MDS patient samples indicated no defined association with AML, however a positive correlation was demonstrated with a subset of MDS patients exhibiting erythroid dysplasia. This study suggests that PHF23, a novel histone effector protein, may play a role in aberrant transcription during defective hematopoiesis.

Author notes

Disclosure: No relevant conflicts of interest to declare.