Adenosine deaminases acting on RNA (ADARs) are involved in adenosine (A) - to - inosine (I) RNA editing and are implicated in development and diseases. Three ADAR gene family members (ADAR1-3) are found in mammals. ADAR1 and ADAR2 are widely expressed and catalytically active in mammals, while ADAR3 is considered to be inactive.

ADAR1 is an essential protein and ADAR1-deficient mice were embryonically lethal. Mutations in human ADAR1 gene were shown to be associated with Aicardi-Goutières syndrome, an early-onset encephalopathy that often results in severe and permanent neurological damage. This indicates that ADAR1 may play an important role during neural development in humans.  

Human ESCs (hESCs) lacking ADAR1 were first generated and then examined to gain insights into the role of ADAR1 in human embryonic stem cell (hESC) differentiation and neural induction. RNA-sequencing was further performed to systematically make a comparison of RNA editing, mRNA and miRNA changes between wild-type (WT) and ADAR1-deficient cells at several differentiation time points. 

A team of scientists led by Prof. CHEN Lingling from the Institute of Biochemistry and Cell Biology, Shanghai Institutes for Biological Sciences of Chinese Academy of Sciences, incollaboration with Prof. YANG Li from CAS-MPG Partner Institute for Computational Biology, found that ADAR1 deficiency in hESCs significantly affected hESC differentiation and neural induction with widespread changes in mRNA and miRNA expression, including up regulation of self-renewal-related miRNAs, such as miR302s.

Global editing analyses revealed that ADAR1 editing activity contributes little to the altered miRNA/mRNA expression in ADAR1-deficient hESCs upon neural induction. Genome-wide iCLIP studies identified that ADAR1 binds directly to pri-miRNAs to interfere with miRNA processing by acting as an RNA-binding protein. Aberrant expression of miRNAs and phenotypes observed in ADAR1-depleted hESCs upon neural differentiation could be reversed by an enzymatically inactive ADAR1 mutant, but not by the RNA-binding-null ADAR1 mutant. 

These findings reveal that ADAR1, but not its editing activity, is critical for hESC differentiation and neural induction by regulating miRNA biogenesis via direct RNA interaction.    

This work entitled “ADAR1 is required for differentiation and neural induction by regulating microRNA processing in a catalytically independent manner” has been published in Cell Research.    

This study was supported by the Chinese Academy of Sciences (the Strategic Priority Research Program), the Ministry of Science and Technology, and the National Natural Science Foundation of China. 

 Figure: A model for the non-enzymatic role of ADAR1 in hESC self-renewal and differentiation. (Image by Prof. CHEN Lingling's group)