Female reproductive aging is characterized by progressive deterioration of ovarian function, yet the molecular mechanisms driving these changes remain incompletely understood. Here, we used long-read direct RNA-sequencing to map transcript isoform changes in mouse ovaries across reproductive age. Comparing young and aged mice after controlled gonadotropin stimulation, we identified widespread alternative splicing changes, including shifts in exon usage, splice site selection, and transcript boundaries. Aged ovaries exhibited increased isoform diversity, favoring distal start and end sites, and a significant rise in exon skipping and intron retention events. Many of these age-biased splicing events altered open reading frames, introduced premature stop codons, or disrupted conserved protein domains. Notably, several mitochondrial genes involved in the respiratory chain were affected. We highlight Ndufs4, a mitochondrial Complex I subunit, as a case in which aging promotes the alternative splicing of a short isoform lacking the canonical protein family (Pfam) domain. Structural modeling suggests this splice variant could impair Complex I function, resulting in increased reactive oxygen species production. Our data suggest a mechanistic link between splicing and mitochondrial dysfunction in the aging ovary. These findings support the model of the splicing-energy-aging axis in ovarian physiology, wherein declining mitochondrial function and adaptive or maladaptive splicing changes are intertwined. Our study reveals that alternative splicing is not merely a byproduct of aging but a dynamic, transcriptome-wide regulatory layer that may influence ovarian longevity. These insights open new avenues for investigating post-transcriptional mechanisms in reproductive aging and underscore the need to consider isoform-level regulation in models of ovarian decline.

Oocytes from women of advanced reproductive age have lower developmental potential, yet the underlying mechanisms of this phenomena are incompletely understood. Oocyte maturation is dependent upon translational control of stored maternal mRNA that were synthesized during oocyte growth. We observed that GC content of mRNA was negatively associated with half-life in oocytes from reproductively young women (< 30 years), contrastingly directly with oocytes from reproductively aged women (≥ 40 years) where mRNA half-lives were positively associated with GC nucleotide content. Additionally, we observed that mRNA half-lives were negatively associated with protein abundance in young oocytes, while GC content was positively associated with protein abundance in aged oocytes. Examination of codon composition during the GV-to-MII transition revealed that codons that facilitate rapid translation promoted mRNA stability and are considered optimal, while codons that slow translation destabilized mRNA, and are considered non-optimal. GC-containing codons were more optimal in reproductive aging, and also correlated positively with protein abundance. This study indicates that reproductive aging coincides with the stabilization of a subset of mRNA that have the potential to be over-translated during oocyte maturation, this is likely to lead to observed decreases in oocyte quality in older women. Because oocyte mRNA decay is translationally linked, this suggests that maternal aging causes defects in translation, which results in reduced translational efficiency and the retention of maternal mRNA that are normally degraded in oocytes from young women. In the case of oocytes, defects in translation can alter the RNA decay pathways and result in incorrect maternal mRNA dosage, which may negatively impact embryonic development.

In the original publication [...].

Supplementary Figure 1. Molecular characterization of fibroblasts. Supplemental Figure 2. Fibroblasts do not significantly affect metastasis of MCF10CA1d breast tumor xenografts. Supplemental Figure 3. CCR2 knockdown in 4T1 mammary carcinoma cells inhibits fibroblast mediated tumor growth. Supplementary Figure 4. CCL2 not affect AKT, SMAD3 or ERK1/2 phosphorylation in MCF10CA1d breast cancer cells. Supplemental Figure 5. Heatmap analysis of cell growth and cell cycle related proteins in CCR2 deficient breast cancer cells.

DOT1-like (DOT1L) histone methyltransferase is essential for mammalian erythropoiesis. Loss of DOT1L in knockout (Dot1l-KO) mouse embryos resulted in lethal anemia at midgestational age. The only recognized molecular function of DOT1L is its methylation of histone H3 lysine 79 (H3K79). We generated a Dot1l methyltransferase mutant (Dot1l-MM) mouse model to determine the role of DOT1L methyltransferase activity in early embryonic hematopoiesis. Dot1l-MM embryos failed to survive beyond embryonic day 13.5 (E13.5), similarly to Dot1l-KO mice. However, when examined at E10.5, Dot1l-MM embryos did not exhibit overt anemia like the Dot1l-KO. Vascularity and the presence of red blood cells in the Dot1l-MM yolk sacs as well as in the AGM region of Dot1l-MM embryos appeared to be similar to that of wildtype. In ex vivo cultures of yolk sac cells, Dot1l-MM primitive erythroblasts formed colonies comparable to those of the wildtype. Although ex vivo cultures of Dot1l-MM definitive erythroblasts formed relatively smaller colonies, inhibition of DOT1L methyltransferase activity in vivo by administration of EPZ-5676 minimally affected the erythropoiesis. Our results indicate that early embryonic erythropoiesis in mammals requires a DOT1L function that is independent of its intrinsic methyltransferase activity.

In eukaryotic cells, the homology-directed repair (HDR) and non-homologous end joining (NHEJ) pathways are required for the repair of DNA double strand breaks (DSB). The high-fidelity HDR pathway is particularly important for maintenance of genomic stability. In mammals, histone post-translational modifications and histone variant exchange into nucleosomes at sites of DSB generate an open chromatin state necessary for repair to take place. However, the specific contributions of histone modifications to histone variant exchange at DSB sites and the influence of these changes on the DNA repair process and genome stability are incompletely understood. Here we show that Dot1L-catalyzed methylation of H3 histone on lysine 79 (H3K79) is required for efficient HDR of DSB. In cells with DNA DSB either lacking Dot1L or expressing a methylation-dead Dot1L, there is altered kinetics of DNA repair factor recruitment, markedly decreased H2A.Z incorporation at DSB sites, and a specific and profound reduction in HDR, which results in significant genomic instability. These findings demonstrate a new role for Dot1L, identifying it as a critical regulator of the DNA repair process and a steward of genomic integrity. ### Competing Interest Statement The authors have declared no competing interest.

Immunoglobulin heavy chain (IgH) locus-associated G-rich long noncoding RNA (StiGLT) is important for physiological and pathological B cell DNA recombination. We demonstrate that the METTL3 enzyme-catalyzed N-6-methyladenosine (m(6)A) RNA modification drives recognition and 3' end processing of SgGLT by the RNA exosome, promoting class switch recombination (CSR) and suppressing chromosomal translocations. The recognition is driven by interaction of the MPP6 adaptor protein with nuclear m(6)A reader YTHDC1. MPP6 and YTHDC1 promote CSR by recruiting AID and the RNA exosome to actively transcribe StiGLT. Direct suppression of m(6)A modification of StiGLT or of m(6)A reader YTHDC1 reduces CSR. Moreover, METTL3, an essential gene for B cell development in the bone marrow and germinal center, suppresses IgH-associated aberrant DNA breaks and prevents genomic instability. Taken together, we propose coordinated and central roles for MPP6, m(6)A modification, and m(6)A reader proteins in controlling long noncoding RNA processing, DNA recombination, and development in B cells.

Mammalian oocytes must degrade maternal transcripts through a process called translational mRNA decay, in which maternal mRNA undergoes translational activation, followed by deadenylation and mRNA decay. Once a transcript is translationally activated, it becomes deadenylated by the CCR4-NOT complex. Knockout of CCR4-NOT Transcription Complex Subunit 6 Like (Cnot6l), a deadenylase within the CCR4-NOT complex, results in mRNA decay defects during metaphase I (MI) entry. Knockout of B-cell translocation gene-4 (Btg4), an adaptor protein of the CCR4-NOT complex, results in mRNA decay defects following fertilization. Therefore, mechanisms controlling mRNA turnover have significant impacts on oocyte competence and early embryonic development. Post-transcriptional inosine RNA modifications can impact mRNA stability, possibly through a translation mechanism. Here, we assessed inosine RNA modifications in oocytes, eggs, and embryos from Cnot6l-/- and Btg4-/- mice, which display stabilization of mRNA and over-translation of the stabilized transcripts. If inosine modifications have a role in modulating RNA stability, we hypothesize that in these mutant backgrounds, we would observe changes or a disruption in inosine mRNA modifications. To test this, we used a computational approach to identify inosine RNA modifications in total and polysomal RNA-seq data during meiotic maturation (GV, MI, and MII stages). We observed pronounced depletion of inosine mRNA modifications in samples from Cnot6l-/-, but not in Btg4-/- mice. Additionally, analysis of ribosome-associated RNA revealed clearance of inosine modified mRNA. These observations suggest a novel mechanism of mRNA clearance during oocyte maturation, in which inosine-containing transcripts decay in an independent, but parallel mechanism to CCR4-NOT deadenylation.

Approximately half of the human genome is comprised of transposable elements (TEs), which are genetic elements capable of amplifying themselves within the genome. Throughout the course of human life, TEs are expressed in germ cells, the preimplantation embryo, and the placenta but silenced elsewhere. However, the functions of TEs during embryonic development are poorly understood. Trophoblast stem (TS), embryonic stem (ES), and extraembryonic endoderm stem (XEN) cells are cell lineages derived from the preimplantation embryo and known to have different TE silencing mechanisms. Thus, it is likely distinct TEs are expressed in each lineage and that proteins coded by these TEs have lineage-specific functions. The purpose of this research was to determine which TEs are expressed in each of these stem cell lineages and to compare expression levels between lineages. Each lineage’s transcriptome was analyzed by quantifying TE expression in RNA-sequencing data from mouse stem cells. Expression data were then used for differential expression analyses performed between the cell types. It was found that certain families of TEs are distinctly expressed in certain lineages, suggesting expression of these families may be involved in the differentiation and development of each lineage, the understanding of which can lead to improved stem cell therapies and capacity to study human embryonic development. ### Competing Interest Statement The authors have declared no competing interest.

ABSTRACT Early mammalian erythropoiesis requires the DOT1L methyltransferase. We demonstrated that loss of DOT1L in mutant mice resulted in lethal anemia during midgestation. The molecular mechanisms by which DOT1L regulates embryonic erythropoiesis have not yet been elucidated. In this study, a methyltransferase mutant mouse line ( Dot1L -MM) was generated to determine whether the methyltransferase activity of DOT1L is essential for erythropoiesis. Dot1L- MM mice displayed embryonic lethality between embryonic days 10.5 and 13.5, similar to Dot1lL knockout ( Dot1L- KO) mice. However, when examined at E10.5, unlike the Dot1L- KO, Dot1L- MM embryos did not exhibit evidence of anemia. In ex vivo hematopoietic differentiation cultures, Dot1L- KO and Dot1L- MM yolk sac (YS) cells both formed reduced numbers of myeloid, and mixed hematopoietic colonies. Erythroid colonies were able to be formed in numbers equal to wildtype embryos. Extensively self-renewing erythroblast (ESRE) cultures were established using YS cells from E10.5 embryos. Dot1L- KO and Dot1L- MM cells expanded significantly less than wild-type cells and exhibited increased cell death. Strikingly, Dot1L- KO and Dot1L- MM cells of YS origin exhibited profound genomic instability, implicating DOT1L methyltransferase activity in maintenance of the genome as well as viability of hematopoietic progenitors. Our results indicate that the methyltransferase activity of DOT1L plays an important role early murine hematopoiesis.

Mammalian oocytes and eggs are transcriptionally quiescent and depend on post-transcriptional mechanisms for proper maturation. Post-transcriptional mRNA modifications comprise an important regulatory mechanism that can alter protein and miRNA recognition sites, splicing, stability, secondary structure, and protein coding. We discovered that fully grown mouse germinal vesicle oocytes and metaphase II eggs display abundant inosine mRNA modifications compared to growing oocytes from postnatal day 12 oocytes. These inosines were enriched in mRNA protein coding regions (CDS) and specifically located at the third codon base, or wobble position. Inosines, observed at lower frequencies in CDS of somatic tissues, were similarly enriched at the codon wobble position. In oocytes and eggs, inosine modifications lead primarily to synonymous changes in mRNA transcripts. Inosines may ultimately affect maternal mRNA stability by changing codon usage, thereby altering translational efficiency and translationally coupled mRNA degradation. These important observations advance our understanding of post-transcriptional mechanisms contributing to mammalian oocyte maturation.

Abstract Basal-like breast cancers are an aggressive breast cancer subtype, which often lack estrogen receptor, progesterone receptor, and Her2 expression, and are resistant to antihormonal and targeted therapy, resulting in few treatment options. Understanding the underlying mechanisms that regulate progression of basal-like breast cancers would lead to new therapeutic targets and improved treatment strategies. Breast cancer progression is characterized by inflammatory responses, regulated in part by chemokines. The CCL2/CCR2 chemokine pathway is best known for regulating breast cancer progression through macrophage-dependent mechanisms. Here, we demonstrated important biological roles for CCL2/CCR2 signaling in breast cancer cells. Using the MCF10CA1d xenograft model of basal-like breast cancer, primary tumor growth was significantly increased with cotransplantation of patient-derived fibroblasts expressing high levels of CCL2, and was inhibited with CRISP/R gene ablation of stromal CCL2. CRISP/R gene ablation of CCR2 in MCF10CA1d breast cancer cells inhibited breast tumor growth and M2 macrophage recruitment and validated through CCR2 shRNA knockdown in the 4T1 model. Reverse phase protein array analysis revealed that cell-cycle protein expression was associated with CCR2 expression in basal-like breast cancer cells. CCL2 treatment of basal-like breast cancer cell lines increased proliferation and cell-cycle progression associated with SRC and PKC activation. Through pharmacologic approaches, we demonstrated that SRC and PKC negatively regulated expression of the cell-cycle inhibitor protein p27KIP1, and are necessary for CCL2-induced breast cancer cell proliferation. Implications: This report sheds novel light on CCL2/CCR2 chemokine signaling as a mitogenic pathway and cell-cycle regulator in breast cancer cells.

In eukaryotic cells, the homologous recombination (HR) and non-homologous end joining (NHEJ) pathways are required for the repair of DNA double strand breaks (DSB). In mammals, histone modification and histone variant exchange into nucleosomes at sites of DSB generate an open chromatin state necessary for repair to take place. How histone modifications contribute to histone variant exchange at DSB sites, and how this process results in DNA repair remain unresolved. Here we show that Disruptor of telomeric silencing -1 like (DOT1L) is required for H2A.Z histone variant exchange at DSB sites. Cells from Dot1L-/- mice have increased genomic instability and defects in DNA repair. Loss of either DOT1L or its methylation activity results in decreased H2A.Z incorporation at DSB sites, increased amounts of single strand DNA, and significantly reduced repair activity by homologous recombination.

Background Fully grown mammalian oocytes and eggs are transcriptionally quiescent, and therefore have a unique RNA environment in which cellular processes depend on post-transcriptional regulation. RNA editing of adenosines into inosines (A-to-I) by adenosine deaminases acting on RNA (ADARs) is a common post-transcriptional gene regulatory mechanism, yet it has not been systematically studied in oocytes. Results A genome-wide RNA editing analysis of transcriptionally active growing oocytes from postnatal day 12 (PND12) mice, fully grown germinal vesicle (GV) oocytes, and transcriptionally quiescent metaphase II (MII) eggs indicates an abundant amount of A-to-I editing of mRNA transcripts. Editing of mRNA was greatest in GV oocyte and MII eggs compared to the PND12 immature oocytes, this was consistent with ADAR1 levels within these cells. Compared to somatic tissues, oocytes exhibited a different pattern of RNA editing, with a high proportion of RNA edits occurring in the coding regions. These edits resulted in nucleotide substitutions that were enriched at the third nucleotide of the codon (wobble position). Codon usage can affect mRNA stability and translation efficiency. Conclusions RNA editing in mouse oocytes is distinct from RNA editing in somatic cells due to increased frequencies of coding sequence RNA edits. We provide evidence in support of a previously unreported phenomenon of selective ADAR1 editing of the codon wobble position. Editing of the wobble position has the potential to fine tune post-transcriptional gene regulation through altering codon usage. This important observation advances our current understanding of RNA editing in mammalian cells.

Background: The major form of autosomal dominant polycystic kidney disease is caused by heterozygous mutations in PKD1, the gene that encodes polycystin-1 (PC1). Unlike PKD1 genes in the mouse and most other mammals, human PKD1 is unusual in that it contains two long polypyrimidine tracts in introns 21 and 22 (2.5 kbp and 602 bp, respectively; 97% cytosine and thymine). Although these polypyrimidine tracts have been shown to form thermodynamically stable segments of triplex DNA that can cause DNA polymerase stalling and enhance the local mutation rate, the efficiency of transcription and splicing across these cytosine- and thymine-rich introns has been unexplored. Methods: We used RT-PCR and Western blotting (using an mAb to the N terminus) to probe splicing events over exons 20-24 in the mouse and human PKD1 genes as well as Nanopore sequencing to confirm the presence of multiple splice forms. Results: Analysis of PC1 indicates that humans, but not mice, have a smaller than expected protein product, which we call Trunc_PC1. The findings show that Trunc_PC1 is the protein product of abnormal differential splicing across introns 21 and 22 and that 28.8%-61.5% of PKD1 transcripts terminate early. Conclusions: The presence of polypyrimidine tracts decreases levels of full-length PKD1 mRNA from normal alleles. In heterozygous individuals, low levels of full-length PC1 may reduce polycystin signaling below a critical "cystogenic" threshold.

Abstract Ductal carcinoma in situ (DCIS) is the most common form of breast cancer, with 50,000 cases diagnosed every year in the United States. Overtreatment and undertreatment remain significant clinical challenges in patient care. Identifying key mechanisms associated with DCIS progression could uncover new biomarkers to better predict patient prognosis and improve guided treatment. Chemokines are small soluble molecules that regulate cellular homing through molecular gradients. CCL2-mediated recruitment of CCR2+ macrophages are a well-established mechanism for metastatic progression. Although the CCL2/CCR2 pathway is a therapeutic target of interest, little is known about the role of CCR2 expression in breast cancer. Here, using a mammary intraductal injection (MIND) model to mimic DCIS formation, the role of CCR2 was explored in minimally invasive SUM225 and highly invasive DCIS.com breast cancer cells. CCR2 overexpression increased SUM225 breast cancer survival and invasion associated with accumulation of CCL2 expressing fibroblasts. CCR2-deficient DCIS.com breast cancer cells formed fewer invasive lesions with fewer CCL2+ fibroblasts. Cografting CCL2-deficient fibroblasts with DCIS.com breast cancer cells in the subrenal capsule model inhibited tumor invasion and survival associated with decreased expression of aldehyde dehydrogenase (ALDH1), a proinvasive factor, and decreased expression of HTRA2, a proapoptotic serine protease. Through data mining analysis, high expression of CCR2 and ALDH1 and low HTRA2 expression were correlated with poor prognosis of breast cancer patients. Implications: This study demonstrates that CCR2 overexpression in breast cancer drives early-stage breast cancer progression through stromal-dependent expression of CCL2 with important insight into prognosis and treatment of DCIS. Mol Cancer Res; 16(2); 296–308. ©2017 AACR.

OBJECTIVE:Ovarian carcinomas that originate from fallopian epithelial cells are suggested to arise due to repeated exposure to ovulatory follicular fluid (FF). Mechanistic explanation(s) for how this occurs are unknown. Here, we sought to understand if FF exposure to fallopian epithelial cells could induce DNA damage and expression of a known family of DNA mutators, apolipoprotein B mRNA editing enzyme, catalytic polypeptide (APOBEC) cytidine deaminases.METHODS:Follicular fluid and matched patient plasma samples were obtained from donors. Fallopian epithelial cells (FT33-TAg, FT189, FT190, and FT194) were cultured with FF or plasma for 24h, and cell proliferation and DNA damage were assessed. Effects of FF on Apobec gene expression were determined by qRT-PCR and western blot analyses. Fallopian epithelial cells were transfected with an APOBEC3A expression vector and DNA damage was assessed.RESULTS:Follicular fluid exposure increased epithelial cell proliferation as measured by three independent methods, and DNA damage accumulation as assessed using three independent measures. This effect was specific to FF, as matched patient plasma did not have the same effects. Increased expression of Apobec3a was observed in fallopian epithelial cells following exposure to 5 of 8 patient FF samples, and transient overexpression of APOBEC3A was sufficient to induce double strand DNA breaks.CONCLUSIONS:Follicular fluid can induce cell proliferation and DNA damage accumulation in cultured fallopian epithelial cells. Increased expression of APOBEC3A, a known DNA mutator, may explain the high incidence of DNA damage after FF exposure. The role of Apobec3a in ovulation-induced inflammation warrants further investigation.

Cancer associated fibroblasts are the most abundant stromal cells in breast cancer, but their function in cancer progression has not been fully understood. We previously identified that the chemokine CCL2 was highly expressed in breast cancer associated fibroblasts, and high stromal CCl2 expression predicted poor outcome in basal like breast cancer. CCL2 is known to recruit monocyte/macrophage and promotes cancer progression. We previously found that recombinant CCL2 can directly signal to breast cancer cells and promote cell survival and invasion in vitro. In this study, we aimed to determine the functional importance of CCL2 signaling mediated fibroblast-cancer cell interactions in breast cancer progression. We used a fibroblast and cancer cell co-graft mouse model as the main functional assay. We generated primary fibroblasts from mouse mammary tumor and human breast cancer, and confirmed most of they expressed high level of CCL2. When co-grafted with the human basal breast cancer cell line MCF10A-CA1D into nude mice, fibroblasts enhanced xenograft growth. Stable knockdown of CCL2 expression from fibroblasts significantly reduced its ability in tumor growth promotion, while knockdown CCL2 from cancer cells did not. Decreased CCL2 production from fibroblasts resulted in increased apoptosis and autophagy in tumor samples. To determine the importance of direct CCL2 signaling to cancer cell, we generated the CCL2 receptor CCR2 mutant CA1D cancer cell lines by CRISPR-Cas9 targeting technology. Mutation of CCR2 in cancer cell significantly reduced tumor growth when co-grafted with CCL2 secreting fibroblasts. Lastly, we tested continuous delivery of CCL2 neutralizing antibody in the co-graft tumor model, but observed minimal therapeutic effect. Further examination of blood CCL2 level revealed an increased production of CCL2 from fibroblasts and mouse host after antibody treatment, which may contribute to the lack of therapeutic effect. In summary, our studies demonstrated the importance of CCL2/CCR2 signaling mediated fibroblasts-cancer cell interaction in basal like breast cancer progression. The CCL2 signaling pathway can be potentially served as therapeutic target, but requires development of efficient targeting strategy. Citation Format: Min Yao, Wei Bin Fang, Fang Fan, Nehemiah Alvarez, Patrick E. Fields, Nikki Cheng. The chemokine CCL2/CCR2 signaling mediated fibroblasts-cancer cells crosstalk promotes basal like breast cancer progression [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 2984. doi:10.1158/1538-7445.AM2017-2984

D isruptor o f T elomere silencing 1 ‐ L ike (DOT1L), is a histone 3, lysine 79 (H3K79) methyltransferase. The enzyme has been implicated in multiple processes, including activation of transcription, regulation of the cell cycle, leukemogenesis, and mouse embryonic development. Previous studies in our lab found that Dot1L deficiency results in an erythropoietic defect, leading to lethal anemia at around mid‐gestation (Feng et al., 2010). The precise molecular mechanism(s) by which DOT1L regulates embryonic hematopoiesis has not yet been elucidated and is the overall objective of this study. Specifically, we sought to determine whether the methyltransferase activity of DOT1L is essential for hematopoiesis. DOT1L is a large protein (1540aa) and involved in several, diverse processes, but only its methyltransferase activity has been documented. Additional functional domains of the protein might be responsible for its diverse activities. We developed a murine embryonic stem cell (mESC)‐based culture system in which to examine the role of DOT1L in hematopoiesis, in vitro . Using the Cas9/CRISPR system, we created two different mutations in Dot1L in cultured mESCs: a Dot1L knockout and a Dot1L point mutant. Knockout mESCs contain a large, out‐of‐frame deletion in both alleles of Dot1L , while the point mutant contains a single amino acid change in the methyltransferase domain of Dot1L , thereby eliminating its methyltransferase activity, but preserving the rest of the protein. We then induced these Dot1L knockout and point mutant mESC clones, along with wildtype mESCs, to undergo hematopoiesis in culture. Our data showed that this protocol leads to similar erythropoietic defects as those observed in vivo . Dot1L knockout and point mutant mESCs are capable of forming the same types of hematopoietic colonies as wildtype mESCs (both erythroid and myeloid), but there is a decrease in colony size and number compared to wildtype. These results suggest that the methyltransferase activity of DOT1L plays a predominant role in the activity of the protein as a whole, and is entirely responsible for its function in facilitating early, murine hematopoiesis. Support or Funding Information R01 DK091277, Fields (PI), 04/01/12‐03/31/17NIH/NIDDK Role of the Histone Methyltransferase DOT1L in Embryonic Erythropoiesis