This study further elucidates the role of CYP33 in gene transcription by studying the effects of CYP33 overexpression and siRNA-mediated down-regulation on the expression of MLL target genes such as c-MYC, p21, p27, and HOXA9. CYP33 is a cis-trans prolyl isomerase (PPIase) which mediates repression of HOX gene expression after binding to the 3rd PHD finger of MLL through its own RRM domain. Upon binding to MLL, CYP33 increases recruitment of HDAC to the MLL repression domain which in turn leads to deacetylation of histone H3. This ultimately causes repression of MLL target genes (Xia et al. 2003). Chromosomal translocations involving the MLL gene (Mixed Lineage Leukemia) can lead to the production of fusion proteins with any of more than 60 different partner proteins and is implicated in the initiation of leukemia. Expression of MLL fusion proteins leads to increased expression of MLL target genes such as HOXA7, HOXA9 and MEIS1 (Hess et al., 2010). MLL-fusion proteins lack the 3rd PHD finger and therefore, lose their ability to bind CYP33; in the presence of these fusion proteins CYP33 can no longer repress MLL target genes. A mutation in CYP33's PPIase domain, leads to an enzymatically inactive CYP33 which can still bind to MLL. Upon overexpression of MLL and CYP33, HDAC is recruited to the MLL complex, as demonstrated by co-immunopreciptiation. However, when the CYP33 PPIase mutant is overexpressed, the lack of PPIase activity prevents the recruitment of HDAC to MLL. As a consequence the chromatin retains the acetylation mark on histone H3 and, although the CYP33 PPIase mutant binds MLL, no repression of MLL target genes is observed. Conversely, upon knockdown of wild-type CYP33, MLL target gene expression and H3K27Ac are increased. This study aids in the understanding of how MLL and CYP33 modulate gene function and helps to identify some of the mechanisms involved in hematopoietic development and leukemia.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4034. doi:1538-7445.AM2012-4034