Abstract 1978: SON and its splice variants regulate MLL1/2 complex-mediated H3K4me3 and transcription of leukemia-associated genes
Authors: Kim Jung-Hyun , Park Eun Young , Stone Joshua K. , Butler Thomas W. , Lim Steve , Ahn Eun-Young Erin 
Published: July 2016

SON is a poorly characterized nuclear protein that is particularly abundant in hematopoietic cells/organs and embryonic stem cells. This protein was recently identified as a splicing co-factor required for proper cell cycle progression and maintenance of stem cell pluripotency. Although SON's function in RNA splicing was recently highlighted, SON was originally identified as a DNA-binding protein and a potential regulator of transcription. However, the mechanism by SON controls transcription and its disease relevance are completely unknown.

To investigate SON's function in genome-wide gene regulation, we performed chromatin immunoprecipitation-sequencing (ChIP-seq) in K562 human leukemia cells using SON antibodies. SON ChIP-seq results demonstrated that most of SON-binding sites are located within the promoter of target genes which include signaling mediators, transcription factors and cell cycle regulators. Our ChIP-qPCR revealed that knockdown of SON causes enhanced recruitment of the mixed lineage leukemia (MLL)1/2 complexes, but not MLL3/4 and SET1A/B complexes, to the SON target chromatin, resulting in significant increase of tri-methylation of histone H3 lysine 4 (H3K4me3) levels. Surprisingly, the C-terminus of SON directly interacts with MLL-binding region of menin and interrupts menin interaction with MLL1/2. These findings demonstrate an inhibitory effect of menin-SON interaction on menin-MLL1/2 interaction, reducing H3K4me3 and MLL1/2 complex assembly.

In addition to full-length SON (SON F), two C-terminally truncated splice variants of SON (SON B and E) have been predicted in genome databases. To address the clinical significance of SON splice variants, we examined whether SON splice variants are differentially expressed in the condition of acute myeloid leukemia (AML). Interestingly, the expression levels of alternatively spliced SON isoforms, but not full-length SON, were significantly increased in human AML patient bone marrow/blood samples and mouse models of leukemia. The short isoforms of SON retain its DNA-binding ability, thereby competing with the full-length SON for target chromatin interaction. However, the short isoforms lack the menin-binding ability and could not inhibit MLL1/2 complex assembly. Importantly, overexpression of a short isoform of SON increased MLL complex-mediated H3K4me3 and markedly enhanced replating potential of hematopoietic progenitors.

Taken together, our study reveals that the MLL1/2 complex activity is competitively regulated by full-length SON and its alternatively spliced isoforms, and that target genes of MLL1/2-menin are aberrantly controlled by overexpressed “short SON” in AML patients. Furthermore, our findings strongly suggest the significant roles of SON splice variants in aberrant transcriptional initiation in leukemia and leukemic stem cell maintenance.

Citation Format: Jung-Hyun Kim, Eun Young Park, Joshua K. Stone, Thomas W. Butler, Steve Lim, Eun-Young Erin Ahn. SON and its splice variants regulate MLL1/2 complex-mediated H3K4me3 and transcription of leukemia-associated genes. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 1978.