THAP-zinc finger protein THAP1
The THAP-zinc finger protein THAP1 is a transcriptional regulator of EC proliferation, which we cloned several years ago from an HEV EC cDNA library. THAP1 is the prototype of a large family of transcription factors (> 100 THAP proteins in the animal kingdom) defined by a novel protein motif, the THAP-zinc finger, a large C2CH module (CysX2-4CysX35-53CysX2His) with an atypical three–dimensional structure. We showed that the THAP-zinc finger of THAP1 functions as a novel zinc-dependent sequence-specific DNA binding motif.
THAP1 role in EC proliferation
In addition to HEV ECs, THAP1 is also expressed in other types of ECs, including primary human umbilical vein ECs in culture. Using this cellular system and an RNA interference strategy, we demonstrated that THAP1 is a critical regulator of EC proliferation and cell cycle progression. Using chromatin immunoprecipitation (ChIP) assays in proliferating ECs, we also showed that endogenous THAP1 associates in vivo with a consensus THAP1-binding site found in the promoter of the pRB/E2F cell cycle target gene RRM1 (a G1/S-regulated gene required for S-phase DNA synthesis), indicating that RRM1 is a direct transcriptional target of THAP1. In addition, we identified cell proliferation factor HCF-1 and O-GlcNAc transferase OGT as major cellular partners of THAP1. HCF-1 has previously been shown to be required for G1/S and G2/M cell cycle transitions, and interaction of THAP1 with HCF-1 may therefore play important roles in EC proliferation. Accordingly, we showed that endogenous THAP1 mediates the recruitment of HCF-1 to the RRM1 promoter during EC proliferation, and that endogenous HCF-1 is required for RRM1 mRNA expression in primary human ECs.
Analysis of THAP1 expression in human primary ECs ex vivo indicated that THAP1 is expressed at higher levels in proliferating ECs than in quiescent growth-arrested ECs. Interestingly, expression of endogenous THAP1 was found in ECs freshly purified from rheumatoid arthrtitis synovium, a tissue associated with high levels of angiogenesis and EC proliferation. THAP1 is thus a good candidate for regulation of EC proliferation in vivo, and one objective of our team is to investigate the role of THAP1 in EC proliferation during developmental, physiological and pathological angiogenesis.
THAP1 role in other cell types
THAP1 may play important roles in other cell types, in addition to ECs. These may include neurons in the brain, since Ozelius and colleagues reported that mutations in THAP1 cause DYT6 dystonia, a neurological disease characterized by twisting movements and abnormal postures (Nature Genetics 2009). In collaboration with this team, we recently identified the gene DYT1, encoding Torsin A (a member of the AAA-ATPase family of chaperones with multiple functions in the cell), as a direct target gene of THAP1, providing a direct link between causative genes of DYT1 and DYT6 dystonias. Physical association of THAP1 with the DYT1 promoter was observed in mouse brain but also in primary human ECs, suggesting regulation of DYT1 by THAP1 may be important in both neuronal and ECs.