Abstract SY43-02: RNA modifications and cell identity: The importance of forgetting the past to embrace the future
Authors: Batista Pedro J. 
Published: August 2018

Abstract
Impaired gene regulation lies at the heart of many disorders, including cancer. Gene expression is controlled through multiple mechanisms that are coordinated to ensure the proper and timely expression of each gene. From transcription to translation, every step of mRNA processing is controlled by interaction between the mRNA transcript and regulatory factors, such as RNA binding proteins or small noncoding RNAs. We now understand that these regulatory interactions can be modulated by RNA modifications. Presence of a modified nucleotide can create, or abrogate, sites for regulatory interactions by at least two distinct mechanisms: by inducing changes in RNA structure or through direct recognition by the RNA binding protein of the modified nucleotide. The RNA modification N6-methyladenosine (m6A) is present in several classes of RNAs, including mRNAs and lncRNAs, where it is the most abundant internal RNA modification. In mRNAs and lncRNAs, the m6A modification is present in a well-defined RNA motif, RRACH (R = A or G, H = A, C or U), and is enriched at specific transcript landmarks such as the last transcribed exon, near the STOP codon. The METTL3/METTL14 heterodimer is at the core of the large protein complex that catalyzes addition of m6A to mRNAs. To date, two enzymes capable of removing m6A have been identified: fat mass and obesity-associated protein (FTO) and alkB homolog 5 (ALKBH5).
Our studies on the role of m6A in ESCs revealed that thousands of messenger and long noncoding RNAs are m6A-modified, including transcripts encoding core pluripotency transcription factors such as Nanog, Sox2, and Myc. Our observations suggest that m6A marks unstable transcripts, including transcripts that need to be turned over upon differentiation. Loss of Mettl3 did not affect self-renewal, but resulted in impaired ESC exit from self-renewal towards differentiation to several lineages. Mettl3 KO cells did not differentiate into cardiomyocites or neurons in vitro, and subcutaneous injection of Mettl3 KO cells formed tumors consistent in morphology with teratomas that tended to be larger than tumors derived from wild-type cells. Furthermore, teratomas derived from Mettl3 KO cells were predominantly composed of poorly differentiated cells with very high mitotic indices and numerous apoptotic bodies, whereas wild-type cells differentiated predominantly into neuroectoderm. The role of m6A in facilitating transition between cell states is not limited to embryonic stem cells. The m6A interacting protein YTHDC2 is required for a clean transition from mitosis to meiosis in male and female germ cells. Ythdc2-/- male germ cells attempt to enter meiotic prophase but appear to have a mixed identity, maintaining expression of mitotic Cyclin A2 and failing to properly express many meiotic markers. Instead of executing meiotic prophase, the cells attempt an abnormal mitotic-like division, then quickly die. YTHDC2 is required during the early stages of meiotic entry in mice to both downregulate the previous mitotic program and facilitate proper expression of meiotic and differentiation genes. We propose that m6A makes the transition between cell states possible by facilitating a reset mechanism between stages, as occurs in ESCs and during gametogenesis. In contrast to epigenetic mechanisms that provide cellular memory of gene expression states, m6A enforces the transience of genetic information, helping cells to forget the past and thereby embrace the future.
Citation Format: Pedro J. Batista. RNA modifications and cell identity: The importance of forgetting the past to embrace the future [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr SY43-02.