Abstract
Bicoid interacting 3 domain containing RNA methyltransferase (BCDIN3D) is a member of the Bin3 methyltransferase family and is evolutionary conserved from worm to human. BCDIN3D is overexpressed in breast cancer, which is associated with poor prognosis of breast cancers. However, the biological functions and properties of BCDIN3D have been enigmatic. Recent studies have revealed that human BCDIN3D monomethylates 5′-monophsosphate of cytoplasmic tRNAHisin vivo and in vitro. BCDIN3D recognizes the unique and exceptional structural features of cytoplasmic tRNAHis and discriminates tRNAHis from other cytoplasmic tRNA species. Thus, BCDIN3D is a tRNAHis-specific 5′-monophosphate methyltransferase. Methylation of the 5′-phosphate group of tRNAHis does not significantly affect tRNAHis aminoacylation by histidyl-tRNA synthetase in vitro nor the steady state level or stability of tRNAHisin vivo. Hence, methylation of the 5′-phosphate group of tRNAHis by BCDIN3D or tRNAHis itself may be involved in certain unknown biological processes, beyond protein synthesis. This review discusses recent reports on BCDIN3D and the possible association between 5′-phosphate monomethylation of tRNAHis and the tumorigenic phenotype of breast cancer.
Introduction
Bicoid interacting 3 domain containing RNA methyltransferase (BCDIN3D) contains an S-(5′-adenosyl)-L-methionine (AdoMet) binding motif, and is homologous to a conserved family of eukaryotic protein methyltransferases acting on RNA-binding proteins (). The BCDIN3D is evolutionary conserved and has been identified in various animals from worms to human (), however, its biological properties and functions are unclear. BCDIN3D mRNA overexpression has been reported in human breast cancer cells, which is associated with cellular invasion and poor prognosis in triple-negative breast cancer (; ). The molecular basis of involvement of BCDIN3D in the tumorigenic phenotype of breast cancer has remained elusive. This review discusses recent studies on human BCDIN3D. We describe herein that a specific tRNA for histidine (tRNAHis) is now identified as a primary target of BCDIN3D and discuss the association between the tumorigenic phenotype of breast cancer and the methylation of tRNAHis by BCDIN3D.
How Does Human Bcdin3D Recognize Specific Rna?
reported that BCDIN3D catalyzes dimethylation of 5′-monophosphate of specific precursor microRNAs (pre-miRNAs) (), such as tumor suppressor miR145 and miR23b (; ; ; ), using AdoMet as a methyl-group donor. Dimethylation of the 5′-monophosphate of pre-miRNA nullifies the negative charge at the 5′-terminal of pre-miRNA. Since Dicer recognizes the negative charge at the 5′-terminal of pre-miRNAs for efficient and accurate cleavage (), the dimethylation of 5′-phosphate of pre-miRNA inhibits subsequent processing. Consequently, mature miRNAs are down-regulated. They also reported that the depletion of BCDIN3D mRNA by specific shRNAs suppressed the tumorigenic phenotype of MDA-MB231 breast cancer cells (). Therefore, it was proposed that BCDIN3D promotes the cellular invasion of breast cancer cells by downregulating tumor suppressor miRNAs through dimethylation of the 5′-phosphate group of the corresponding pre-miRNAs. However, there are no apparent common features including primary or secondary structures among the corresponding pre-miRNAs of downregulated miRNAs in breast cancer cells. Thus, the mechanisms by which BCDIN3D recognizes only a specific group of pre-miRNAs and downregulates mature miRNAs in breast cancer cells are unclear.
Cytoplasmic tRnaHis is Co-Purified With Bcdin3D and Contains a 5′-Monomethylmonophosphate Group
To identify other potential RNA substrates of BCDIN3D in vivo and to elucidate the mechanism underlying the recognition and regulation of specific RNAs by BCDIN3D, recently, BCDIN3D-binding RNAs in human HEK293T cells were analyzed (). When BCDIN3D, expressed in HEK293T cells, was purified from the cell extracts, a distinct 70–80-nucleotie-long RNA molecule was co-purified with BCDIN3D protein.
It was assumed that this co-purified RNA might be cytoplasmic tRNAHis (Figure 1A), since the nucleotide sequences of cytoplasmic tRNAHis from human and fruit fly reportedly contained a 5′-monomehtylphosphate group (; ). Analysis of the RNA co-purified with BCDIN3D via RT-PCR and sequencing confirmed that cytoplasmic tRNAHis is co-purified with BCDIN3D from the cell extracts, but not other tRNAs, such as tRNAPhe. Subsequent direct analysis of the RNA via liquid chromatography and mass spectrometry (LC-MS) revealed that this RNA is cytoplasmic tRNAHis. Moreover, the 5′-monophosphate of cytoplasmic tRNAHis was fully monomethylated, but not dimethylated at all. Furthermore, 5′-monophsophate of tRNAHis is reportedly fully monomethylated even under normal physiological conditions in HEK293T cells, as observed previously in cytoplasmic tRNAHis from HeLa cells ().
FIGURE 1
Cytoplasmic tRnaHis is Methylated by Bcdin3D in Vitro
The enzymatic activity of recombinant human BCDIN3D expressed in E. coli was examined using human cytoplasmic tRNAHis transcript as a substrate and S-(5′-adenosyl)-L- methionine (SAM) as a methyl-group donor in vitro (
BCDIN3D reportedly dimethylates pre-miR145 (
Cytoplasmic tRnaHis is Methylated by Bcdin3D in Vivo
BCDIN3D-knockout HEK293T cells, established via CRISPR/Cas9 editing, are viable, although they exhibit a slightly reduced growth rate than the parental cells (
A recent study using HEK293T cells reported that BCDIN3D-knockout or BCDIN3D overexpression do not alter mature miR145 expression levels (
Together with the recent results of in vitro methylation assays using recombinant BCDIN3D and tRNAHis transcript and the pre-miR145 transcript (
tRnaHis Recognition by Bcdin3D
Human cytoplasmic tRNAHis is matured through unique processes and has unique structural features among cytoplasmic tRNA species (
In vitro steady-state kinetics of methylation of mutant cytoplasmic tRNAHis transcripts by recombinant BCDIN3D revealed that BCIDN3D recognizes G-1, G-1:A73 mis-pairing at the top of the acceptor stem and 8-nucleotide-long extended acceptor helix. The minihelix of tRNAHis is also methylated efficiently by BCIN3D. Thus, BCDIN3D recognizes the unique structural features of cytoplasmic tRNAHis, especially in the top-half region of tRNAHis, and discriminates cytoplasmic tRHAHis from other tRNA species (
The structure of human BCDIN3D is still unclear. The amino acid sequence of BCDIN3D is homologous to that of the catalytic domain of methylphosphate capping enzyme (MePCE), which uses SAM to transfer a methyl group onto the γ-phosphate of the 5′-guanosine of 7SK RNA (
FIGURE 2

(A) A model of human Bicoid interacting 3 domain containing RNA methyltransferase (BCDIN3D)-tRNAHis complex structure. Human BCDIN3D structure was modeled using SWISS-MODEL (
The kinetics of methylation of tRNAHis mutants and the structural model of BCDIN3D and tRNAHis complex also suggest that human BCDIN3D is tRNAHis-specific 5′-monophosphate methyltransferase, and cytoplasmic tRNAHis is a primary target of human BCDIN3D.
The Biological Role of 5′-Methylation of Cytoplasmic tRnaHis
The biological role of 5′-monomethylphosphate of cytoplasmic tRNAHis remains unclear. While the 5′-monomethylphosphate of cytoplasmic tRNAHis decreases the affinity of tRNAHis toward histidyl-tRNA synthetase (
Perspective
The correlation between methylation of the 5′-monophosphate group of cytoplasmic tRNAHis and tumorigenic phenotype of breast cancer remains unknown (Figure 2B). tRNAs are involved in various biological process in cells (
In breast cancer cells, initiator tRNAMet is reportedly upregulated (
It would be noteworthy to assume that 5′-monomethylation of 5′-phosphate of tRNAHis regulates the expression of the tRNA half fragments and/or tRFs derived from tRNAHis in breast cancer cells or under specific biological or stress conditions. The production of tRNA half fragments and/or tRFs, in turn, might regulate the genes involved in tumorigenesis in breast cancers. Future studies are required to understand whether methylation of the 5′-phosphate group of tRNAHis by BCDIN3D is involved in the tumorigenic phenotype of breast cancer and other cancers and to potentially elucidate the unknown functions of tRNAs, beyond their established functions.
Statements
Author contributions
All authors listed have made a substantial, direct and intellectual contribution to the work, and approved it for publication.
Funding
The study in our laboratory was supported in part by grants to KT from the Funding Program for Next Generation World-Leading Researchers of JSPS, by Grants-in-Aid for Scientific Research (A), and Grant-in-Aid for Scientific Research on Innovative Areas from JSPS, Takeda Science Foundation, Japan Foundation for Applied Enzymology, Terumo Foundation for Life Science and Art, and Princess Takamatsu Cancer Research Foundation.
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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Summary
Keywords
Bicoid interacting 3 domain containing RNA methyltransferase, methylation, tRNA, breast cancer, protein synthesis
Citation
Tomita K and Liu Y (2018) Human BCDIN3D Is a Cytoplasmic tRNAHis-Specific 5′-Monophosphate Methyltransferase. Front. Genet. 9:305. doi: 10.3389/fgene.2018.00305
Received
21 June 2018
Accepted
18 July 2018
Published
03 August 2018
Volume
9 - 2018
Edited by
Akio Kanai, Keio University, Japan
Reviewed by
Naoki Shigi, National Institute of Advanced Industrial Science and Technology (AIST), Japan; Yohei Kirino, Thomas Jefferson University, United States
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© 2018 Tomita and Liu.
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*Correspondence: Kozo Tomita, kozo_tomita@cbms.k.u-tokyo.ac.jp; kozo-tomita@edu.k.u-tokyo.ac.jp
This article was submitted to RNA, a section of the journal Frontiers in Genetics
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